Visual Priming Within and Across Symbolic Format Using a Tachistoscopic Picture Identification Task: A PET Study Âatrice Desgranges1, Brigitte Landeau1, Karine Lebreton1, Be Jean-Claude Baron1,2, and Francis Eustache1
Abstract & The present work was aimed at characterizing picture priming effects from two complementary behavioral and functional neuroimaging (positron emission tomography, PET) studies. In two experiments, we used the same line drawings of common living/nonliving objects in a tachistoscopic identification task to contrast two forms of priming. In the within-format priming condition (picture±picture), subjects were instructed to perform a perceptual encoding task in the study phase, whereas in the cross-format priming condition (word±picture), they were instructed to perform a semantic encoding task. In Experiment 1, we showed significant priming effects in both priming conditions. However, the magnitude of priming effects in the same-format/perceptual encoding condition was higher than that in the different-format/semantic encoding condition, while the recognition performance did not differ between the two conditions. This finding supports the existence of two forms of priming that may be subserved by different systems. Consistent with these behavioral findings, the PET data for Experiment 2 revealed distinct priming-related
INTRODUCTION Many researchers now consider that memory is not a unitary entity but consists of several separate forms and systems (Tulving, 1995; Schacter & Tulving, 1994; Squire, 1992). In particular, a distinction that has been studied systematically for 20 years is that between the implicit and the explicit expressions of memory (Graf & Schacter, 1985). Explicit (or declarative) memory refers to the conscious recollection of prior experiences, as measured by standard tests of recall and recognition that require intentional retrieval of acquired information. Implicit (or nondeclarative) memory refers to the nonconscious influence of previous experiences on subsequent performance or behavior, assessed with tasks that do not involve intentional retrieval of previously processed information. Those solely ``descriptive'' concepts encompass a broad and heterogeneous group of various abilities (Roediger, 1990; Schacter, 1987). 1 2
INSERM U 320, GIP Cyceron, Universite de Caen, France, University of Cambridge, UK
D 2001 Massachusetts Institute of Technology
patterns of regional cerebral blood flow (rCBF) decreases for the two priming conditions when primed items were compared to unprimed items. The same-format priming condition involved reductions in cerebral activity particularly in the right extrastriate cortex and left cerebellum, while the differentformat priming condition was associated with rCBF decreases in the left inferior temporo-occipital cortex, left frontal regions, and the right cerebellum. These results suggest that the extrastriate cortex may subserve general aspects of perceptual priming, independent of the kind of stimuli, and that the right part of this cortex could underlie the same-format-specific system for pictures. These data also support the idea that the cross-format/semantic encoding priming for pictures represents a form of lexico-semantic priming subserved by a semantic neural network extending from left temporo-occipital cortex to left frontal regions. These results reinforce the distinction between perceptual and conceptual priming for pictures, indicating that different cerebral processes and systems are implicated in these two forms of picture priming. &
Priming represents the measure of unconscious recollection that has attracted most interest. It is defined as a facilitating effect in processing an item based on a prior encounter with this item or a similar stimulus (Tulving & Schacter, 1990). Perceptual and Conceptual Priming: Experimental and Neuropsychological Data For the last 30 years, priming has been extensively studied by psycholinguists, experimental psychologists and neuropsychologists. A lot of evidence from experimental and neuropsychological dissociations supports the distinction between explicit and implicit memory as well as different forms of priming. Tulving and Schacter (1990) proposed to distinguish perceptual priming and semantic (or conceptual) priming. Perceptual priming is based on the physical or perceptual representations of stimuli, while conceptual priming is based on the meaning properties of items. Several priming paradigms have been developed using various tasks and materials. The main methods used to assess picture perceptual priJournal of Cognitive Neuroscience 13:5, pp. 670±686
ming are picture naming, briefly flashed or degraded picture identification, picture fragment completion, and object decision. Priming is documented when subjects identify or complete with greater accuracy or reduced response time previously experienced items as compared to novel items. The tests employed in the investigation of conceptual priming typically use words or sentences, such as the tasks of answering general knowledge questions (Blaxton, 1989) or category instance production (Srinivas & Roediger, 1990). The distinction between perceptual and conceptual priming rests on two major converging lines of evidence. Firstly, the magnitude of perceptual and conceptual priming is affected differentially by various experimental manipulations (Roediger & McDermott, 1993). In particular, changing the physical or perceptual features of the stimuli, such as the modality (auditory/visual), the surface attributes (case of letters, font, or other) or symbolic format of stimuli (word/ picture), reduces perceptual priming but rarely affects conceptual priming. Conversely, conceptual priming is sensitive to levels of encoding depth (i.e., deep or shallow) whereas perceptual priming is minimally or much less strongly affected by this factor. Secondly, patients with dementia and focal cortical lesions show distinct performance patterns on tests assessing these two forms of priming, suggesting a neuropsychological dissociation (Fleischman et al., 1997; Gershberg, 1997; Srinivas, Breedin, Coslett, & Saffran 1997; Swick & Knight, 1996; Gabrieli et al., 1994; Gabrieli, Fleischman, Keane, Reminger, & Morrell, 1995; Keane, Gabrieli, Fennema, Growdon, & Corkin, 1991; Keane, Gabrieli, Mapstone, Johnson, & Corkin, 1995). It has been postulated that perceptual and conceptual priming reflect the operation of different brain systems. Tulving and Schacter (1990) suggested that perceptual priming is mediated by a cortically based, presemantic perceptual representation system (PRS) that processes and represents information about the form and structure, but not about the meaning and associative properties, of words, objects, and other kinds of stimuli. The PRS may be composed of several domain-specific subsystems: a visual word-form system, an auditory word-form system, and a structural description system, implicated in visual word priming, auditory word priming, and visual object priming, respectively (Ochsner, Chiu, & Schacter, 1994; Schacter, 1992, 1994; Schacter, Chiu, & Ochsner, 1993). The PRS may not be implicated in conceptual priming, which may rather involve the modification or addition of information in semantic memory. Perceptual and Conceptual Priming: Functional Brain Imaging Data Recently, studies using functional neuroimaging techniques such as positron emission tomography (PET) and
functional magnetic resonance imaging (fMRI) brought anatomico-functional arguments in favor of the distinction between perceptual and conceptual priming (for a review see Cabeza & Nyberg, 2000; Buckner & Koutstaal, 1998; Schacter & Buckner, 1998; Wiggs & Martin, 1998). The major finding that emerged from these works suggests a neural correlate of priming different from that observed in explicit memory retrieval and characterized by significant decreases in the amount of activation present in specific brain areas. Concerning perceptual priming, these studies examined essentially visual or auditory priming for words using the stem-completion paradigm. It was found that within-modality priming was associated with decreases of regional cerebral blood flow (rCBF) in extrastriate occipital cortex, when the primed and unprimed words were compared (Badgaiyan, Schacter, & Alpert, 1999; BaÈckman et al., 1997; Schacter, Alpert, Savage, Rauch, & Albert, 1996; Schacter, Badgaiyan, & Alpert, 1999; Buckner et al., 1995; Squire et al., 1992). Similar results have also been reported using word fragment completion (Blaxton et al., 1996). Activation, linked to priming, has also been found in various structures (Beauregard, Gold, Evans, & Chertkow, 1998; Blaxton et al., 1996). Notably, Squire et al. (1992) observed an increase of cerebral activity in the right hippocampal region reflecting the likely contamination of the word stem completion task by explicit retrieval (see also, Buckner et al., 1995, and for a discussion of this point, Beauregard et al., 1998). This hypothesis is supported by the findings of Schacter et al. (1996), who failed to find hippocampal activation when methodological steps were taken to lessen the probable contribution of explicit memory. Recently, it has been shown that distinct cerebral mechanisms may mediate within- and cross-modality priming in the visual/ auditory word stem completion task (Badgaiyan et al., 1999; Schacter et al., 1999). Cross-modality priming was associated with increased rCBF in prefrontal cortex and decreased rCBF in the left angular gyrus, which suggests the involvement in this task of explicit retrieval and lexical processes, respectively. Finally, regarding perceptual priming for nonverbal material, using pictures of common objects and a conceptual classification task Buckner et al. (1998) observed reductions in activity in the occipital cortex, which may reflect picture perceptual priming. Recently, Henson, Shallice, and Dolan (2000) showed priming-related reductions in the right fusiform gyrus on brief visual presentation of familiar faces and symbols. Several neuroimaging studies have investigated conceptual priming revealing reductions in cerebral activity for primed items in higher-order prefrontal regions in tasks requiring semantic processing. For instance, conceptual priming measured using the verb generation task involved decreases in left prefrontal cortex compared to novel nouns (Raichle et al., 1994). Studies using fMRI showed consistently similar prefrontal decreases Lebreton et al.
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associated with tasks such as concrete/abstract word classification (Gabrieli et al., 1996; Demb et al., 1995), word generation in response to visual or aural cues (Buckner, Koutstaal, Schacter, & Rosen, 2000), living/ nonliving classification of words and objects (Wagner, Desmond, Demb, Glover, & Gabrieli, 1997), and classification of objects as capable of moving on their own or not (Buckner et al., 1998). Overall, these data suggest an anatomico-functional dissociation between perceptual and conceptual priming: Some posterior cortical regions may specifically subserve perceptual priming, whereas conceptual priming may involve prefrontal regions. It remains unclear, however, whether some occipital regions are associated with specific object types (i.e., words, faces, and symbols) or with general aspects of perceptual priming, independently of the kind of stimuli. Notably, it has been suggested that object priming might depend on a structural description subsystem that involves the temporal regions (Schacter, 1990, 1994; Schacter et al., 1995). Functional neuroimaging may help to unveil the neural substrates of perceptual priming for pictures. To assess visual perceptual priming for pictures with functional neuroimaging, several paradigms are possible. Behavioral paradigms using briefly flashed or degraded picture identification and picture fragment completion have revealed robust priming effects (Berry, Banbury, & Henry, 1997; Brown, Jones, & Mitchell, 1996; Srinivas, 1993; Weldon & Roediger, 1987; Warren & Morton, 1982). Some studies have also showed a reduction, but not an elimination, of priming effects when the symbolic format of stimuli (word/picture) varied between the study and the test phase suggesting that this residual priming did not depend on physical or perceptual features of stimuli (for review, see Roediger & McDermott, 1993). Hirshman, Snodgrass, Mindes, and Feenan (1990) proposed that cross-format priming is mediated by semantic processing (for another concept, see Kirsner, Milech, & Standen, 1983). This interpretation was supported by their results, which showed cross-format priming effects only when the words had been previously generated rather than read on picture fragment completion. Berry et al. (1997), through the use of a semantic encoding of words, observed cross-format priming effects on picture fragment completion, while Weldon and Roediger (1987), using a perceptual encoding task, did not find priming in degraded picture identification. These discrepancies may however be explained by the different nature of the tasks employed. Additional studies that assess picture priming on other tasks are necessary to identify the nature of the processes involved in cross-format priming. In particular, the use of the briefly flashed picture (or picture tachistoscopic) identification task, which represents a relatively ``pure'' perceptual priming paradigm, may be very pertinent to address this issue. 672
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Rationale of the Study With the aim of extending our knowledge about neural substrates and systems subserving priming and characterizing the properties and processes of picture priming, we performed two complementary experiments using the tachistoscopic identification task for achromatic line drawings of living/nonliving common objects. Priming effects on this task would be evidenced when the identification (or naming) performance of subjects was more accurate for the primed picture than for the unprimed picture. Our aim was to contrast, in a single study and with the same task and material, two forms of priming through the change of symbolic format between the study and test phases, under conditions where the type of encoding was covaried with this manipulation. The first experiment was a behavioral study, which had the major aim to reveal two forms of priming effects. In the within-format priming condition (picture±picture), subjects were instructed to perform a physical processing of pictures (perceptual encoding) in the study phase, whereas in the cross-format priming condition (word±picture), they were instructed to perform an elaborate processing of words (semantic encoding). Furthermore, we have used a recognition test varying the same condition (i.e., same-format/perceptual encoding vs. different-format/semantic encoding) in order to detect the expected dissociation between implicit and explicit memory performance. The second experiment was a functional neuroimaging study using PET to unveil specific neural substrates of each of these two forms of priming, and to reveal distinct cerebral activity patterns. More specifically, this PET study should bring to light the areas involved in perceptual priming for figurative drawings, which has not been examined to date. The data should also help us to clarify the role of the occipital cortex in priming effects, a region that has been involved in priming for words, faces, and symbols. If the occipital cortex subserves general aspects of visual perceptual priming, common to all symbolic formats, we should observe priming-related rCBF decreases in occipital cortex measured with our same-format priming condition for pictures. Based on the hypothesis that our crossformat priming condition may reflect a form of conceptual priming, we should find rCBF decreases in frontal regions (which have been previously reported with conceptual priming tasks) associated with this condition, when comparing the primed and unprimed items.
RESULTS Experiment 1 Identification Scores Analysis of data revealed a significant priming effect on the two priming conditions (Table 1). In the sameformat condition, the percentages correct responses were 61.81% ( 9.63) and 31.67% ( 8.9) for the Volume 13, Number 5
target drawings and the control drawings, respectively; the corresponding figures for the different-format condition were 49.99% ( 11.59) and 32.08% ( 9.62), respectively (mean SD). The 2 (Condition: sameformat, different-format) 2 (Item Type: target, control) mixed ANOVA with repeated measures showed a significant main effect of the item type across the conditions, F(1,46) = 121.204, p < .0001; a significant effect of the condition, F(1,46) = 9.15, p = .0041; and a significant interaction, F(1,46) = 10.53; p = .0022. In the two conditions, the successful identification performance was more frequent for the studied than the nonstudied items and this difference was significantly larger in the same-format than the different-format condition. Recognition Scores The percentage of correct recognition in the sameformat condition was 88.47% ( 10.68) and in the different-format condition it was 86.11% ( 10.93); the percentage of false alarms (FAs) was 5.14% ( 5.81) and 4.44% ( 5.87), respectively. The 2 (Condition: sameformat, different-format) 2 (Item Type: hits, FAs) mixed ANOVA showed a significant main effect of the item type, F(1,46) = 2159.56, p < .0001; no effect of the condition, F(1,46) = 0.74, p = .39; and no interaction, F(1,46) = 0.22, p = .64. The studied items were most often recognized than the never seen before items both
in the same- and different-format condition and there was no significant difference in performances between the two conditions. Comparison Between the Identification and Recognition Scores In order to show that the performances on the implicit and explicit tests were independent, we have carried out a correlation analysis based on corrected scores. The priming scores were obtained by subtracting the correct identification percentage of control items from that of the target items. The recognition scores were the recognition percentage of hits minus FAs. The r test revealed no significant correlation between the two scores of memory for the same-format condition (r = .196), nor for the different-format condition (r = .119), and there was no significant difference between the two correlation measures (p = .79). This result shows that the processes that subserve the performance on the recognition task cannot account for the facilitation in stimuli identification in the two priming conditions, and vice versa. Experiment 2 Behavioral Results The analysis of behavioral data revealed a significant priming effect for both priming conditions (Table 1). The mean percentage of correct responses was 67.96%
Table 1. Mean Percentage ( SD) of Correct Identification on the Priming Tasks for the Primed (Target) and the Unprimed (Control) Items and Mean Recognition Scores for the Studied (Hits) and Nonstudied Items (FAs) as a Function of the Two Conditions in the Behavioral Experiment, and Mean Percentage of Correct Responses Across the Three Scan Conditions in the PET Experiment Behavioral Study
Priming task
Recognition task
Conditions
Target (%)
Control (%)
Priming (%)
same-format different-format
61.81 ( 9.63) 49.99 ( 11.59)
31.67 ( 8.9) 32.08 ( 9.62)
30.14 ( 8.31) 17.78 ( 10.34)
Conditions
Hits (%)
FAs (%)
Recognition (%)
same-format different-format
88.47 ( 10.68) 86.11 ( 10.93)
5.14 ( 5.81) 4.44 ( 5.87)
83.34 ( 11.08) 81.68 ( 14.55)
Conditions
Correct Responses (%)
Priming (%)
Same-format Different-format Baseline
67.96 ( 7.01) 47.22 ( 8.62) 29.63 ( 6)
38.33 ( 8.70) 23.33 ( 14.16)
PET Study
Scan Conditions
In the behavioral study, the memory scores (priming and recognition) were calculated as the differences between the target and control items and between hits and FAs, respectively. In the PET study, the priming scores were the differences between the identification accuracy scores for each of the priming conditions and the baseline condition.
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Table 2. Foci of Significant Normalized rCBF Increases in the ``Baseline'' as Compared to the Rest Condition Baseline Minus Rest Coordinates (mm) Anatomical Location of Maximum Voxel
x
y
z
Z Score
L ant. insula
30
22
6
3.71
R ant. insula
26
20
8
3.67
R inf. frontal gyrus
18
36
26
6.81
L inf. frontal gyrus
24
38
24
6.47
L inf. frontal gyrus (post. part)
50
6
30
4.15
R ant. cingulate gyrus (post. part)
8
18
38
3.91
L ant. cingulate gyrus (post. part)
8
6
50
3.54
R sup. temporal sulcus
58
24
0
3.84
L sup. temporal sulcus
68
14
6
3.19
R parahippocampal cortex
34
34
22
3.82
L parahippocampal cortex
36
30
16
3.51
R primary visual cortex
18
96
6
6.09
R lingual gyrus
10
58
0
5.91
R fusiform gyrus
34
84
6
4.78
R inf./med. occipital gyrus
40
56
26
5.12
0
76
18
5.98
L primary visual cortex
28
90
14
5.33
L fusiform gyrus
34
88
6
5.62
L inf./med. occipital gyrus
46
62
14
6.03
36
58
26
5.12
2
60
28
4.89
8 16
64 56
6 14
5.32 5.44
48
52
28
5.39
4
44
14
4.37
L thalamus
6
2
4
4.86
R thalamus
10
14
10
4.47
2
28
12
3.38
Insular cortex
Frontal cortex
Cingulate cortex
Temporal cortex
Occipital cortex
Lingual gyrus
Cerebellum R cerebellar cortex R cerebellum L cerebellar cortex
L cerebellum (vermis)
R mesencephalon
The data are local maxima detected with the SPM 96 software (see Methods). Within these regions, the anatomical localization of the maximum Z scores voxels is based on both the MNI template and Talairach and Tournoux's (1988) stereotactic atlas. Uncorrected significance level was set at p < .001 (Z score > 3.09; R = right; L = left; inf. = inferior; sup. = superior; ant. = anterior; post. = posterior; med. = median).
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Table 3. Foci of Significant Normalized rCBF Increases in the ``Same-Format Priming'' Condition as Compared to the Rest Condition Same-Format Priming Minus Rest Coordinates (mm) Anatomical Location of Maximum Voxel
x
y
z
Z Score
32
18
8
4.15
R inf. frontal gyrus
18
36
26
6.83
L inf. frontal gyrus
24
38
24
6.51
L inf. frontal gyrus (post. part)
52
8
40
3.47
4
6
52
3.83
R sup. temporal sulcus
62
24
2
3.74
L sup. temporal gyrus
68
14
4
3.80
18
98
8
6.08
6
24
4
3.68
29
87
8
4.58
0
76
16
5.89
R inf./med. occipital gyrus
40
54
24
5.26
L primary visual cortex
20
94
12
4.14
6
32
0
3.43
L lingual gyrus
14
82
12
3.09
L fusiform gyrus
36
88
2
4.16
L inf./med. occipital gyrus
48
62
16
6.42
24
56
16
4.50
0
60
28
5.16
8 18
63 56
28 14
5.15 3.70
48
52
26
5.27
2
42
16
3.93
12
14
10
3.52
6
2
4
3.69
Insular cortex L anterior insula Frontal cortex
Cingulate cortex L anterior cingulate gyrus (post. part) Temporal cortex
Occipital cortex R primary visual cortex R sup. colliculus R lingual gyrus Lingual gyrus (BA 18)
L sup. colliculus
Cerebellum R cerebellar cortex Cerebellum L cerebellar cortex
L cerebellum (vermis) Thalamus R thalamus (ventrolateral nucleus) L thalamus (anterior nucleus)
The data are local maxima detected with the SPM 96 software (see Methods). Within these regions, the anatomical localization of the maximum Z scores voxels is based on both the MNI template and Talairach and Tournoux's (1988) stereotactic atlas. Uncorrected significance level was set at p < .001 (Z score > 3.09; R = right; L = left; inf. = inferior; sup. = superior; ant. = anterior; post. = posterior; med. = median).
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Table 4. Foci of Significant Normalized rCBF Increases in the Different-Format Priming Condition as Compared to the Rest Condition Different-Format Priming Minus Rest Coordinates (mm) Anatomical Location of Maximum Voxel
x
y
z
Z Score
R ant. insula
34
18
10
3.09
L ant. insula
34
22
4
4.39
R inf. frontal gyrus
18
36
26
6.92
L inf. frontal gyrus
24
38
24
6.35
L inf. frontal gyrus
52
4
56
3.22
0
8
52
3.53
R med. temporal sulcus
62
36
0
4.29
L med. temporal gyrus
60
32
6
3.72
L sup. temporal gyrus
68
14
4
4.85
L parahippocampal cortex
34
32
18
3.86
R primary visual cortex
18
98
6
6.16
R lingual gyrus
26
86
18
4.58
R fusiform gyrus
58
28
0
4.29
R fusiform gyrus
20
81
4
4.13
R inf./med. occipital gyrus
40
54
26
4.34
L primary visual cortex
22
94
14
5.19
L lingual gyrus
14
80
10
3.60
L fusiform gyrus
32
90
2
4.26
L fusiform
28
70
12
3.63
L inf./med. occipital cortex
46
62
16
5.58
10
68
10
4.07
Cerebellum (vermis)
0
76
16
6.35
L cerebellum (vermis)
2
42
12
3.31
L cerebellar cortex
8
66
8
5.59
18
60
20
4.70
46
50
24
5.28
8
20
4
4.22
10
6
14
3.56
Insular cortex
Frontal cortex
Cingulate cortex Ant. cingulate gyrus (posterior part) Temporal cortex
Occipital cortex
Cerebellum R cerebellar cortex
L red nucleus L caudate nucleus
The data are local maxima detected with the SPM 96 software (see Methods). Within these regions, the anatomical localization of the maximum Z scores voxels is based on both the MNI template and Talairach and Tournoux's (1988) stereotactic atlas. Uncorrected significance level was set at p < .001 (Z score > 3.09; R = right; L = left; inf. = inferior; sup. = superior; ant. = anterior; post. = posterior; med. = median).
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Table 5. Foci of Significant Normalized rCBF Decreases in the Priming Conditions as Compared to the Baseline Condition Coordinates (mm) Anatomical Location of Maximum Voxel
x
y
z
Z Score
Same-Format Priming Versus Baseline (Decreases) R lingual gyrus
12
54
4
4.83
R sup. temporal gyrus
48
20
2
3.99
L cerebellum (dentate nucleus)
16
52
34
3.52
8
48
12
3.32
18
36
4
3.42
L cerebellar vermis L parahippocampal gyrus
Different-Format Priming Versus Baseline (Decreases) R sup. frontal gyrus
18
24
28
4.71
L inf. frontal gyrus
48
12
28
3.92
R sup. temporal gyrus
52
18
2
3.53
R cerebellum (dentate nucleus)
30
46
46
3.42
L inf./med. occipital gyrus
44
62
6
3.37
6
24
14
3.72
R mesencephalon
The data are local maxima detected with the SPM 96 software (see Methods). Within these regions, the anatomical localization of the maximum Z scores voxels is based on both the MNI template and Talairach and Tournoux's (1988) stereotactic atlas. Uncorrected significance level was set at p < .001 (Z score > 3.09; R = right; L = left; inf. = inferior; sup. = superior).
( 7.01), 47.22% ( 8.62), and 29.63% ( 5.99) in the same-format priming, different-format priming, and baseline conditions, respectively. One-way ANOVA showed a significant main effect of the scan condition, F(2,17) = 72.33, p < .0001. In both priming conditions, the subjects identified more drawings than in the baseline condition (t = 10.68, p < .0001, for same-format vs. baseline, and t = 7.65, p < .0001, for different-format vs. baseline, paired t test), and the successful identification percentage was larger in the same-format condition than the different-format (t = 5.89, p < .0001, paired t test). In order to estimate that the repetition of blocks did not affect the identification performances, a 3 (Scan Condition: same-format priming, different-format priming, baseline) 2 (Block: block1, block2) mixed ANOVA was done. This analysis revealed a significant main effect of the block, F(1,16) = 5.201, p < .04; a significant effect of the scan condition, F(2,16) = 105.45, p < .0001; and a significant interaction, F(2,16) = 8.78, p = .0009. Post hoc t tests showed no significant difference between the two blocks for the different-format priming condition, nor for the baseline condition (t = 1.38, p = .21, and t = 1.45, p = .19, respectively, paired t test), but found one for the same-format priming condition (t = 9.55, p < .0001, paired t test). This result may be attributable to the task that involved an important factor of ``proceduralization'' of the tachistoscopic identification procedure. Because the physical features of the material used were maintained in the same-format priming condition, this factor may have played a greater role
in this condition than in the different-format priming condition. PET Results Experimental conditions minus rest. Relative to the rest condition, the ``baseline'' condition produced several large bilateral rCBF increases in the orbito-frontal gyrus, the occipital lobe, the temporal cortex, the cerebellum, the anterior insula, and the anterior cingulate cortex. Activation peaks were also observed in the bilateral thalamus and right mesencephalon. In the occipital lobe, major bilateral activation was detected in the primary visual cortex (BA 17), the lingual and fusiform gyri (BA 18, 19), extending to the posterior part of the inferior temporal cortex and the posterior part of left hippocampal region. Another bilateral activation was found in the superior temporal sulcus (Table 2). As shown in Table 3 and Table 4, essentially similar foci of activation were observed in the two priming conditions compared to the rest condition. The only minor differences were as follows: (1) when the ``sameformat priming'' condition was contrasted with the rest condition, rCBF increases in the anterior cingulate cortex and the anterior insula were observed on the left side only and no activation was found in the left parahippocampal cortex or the right mesencephalon; and (2) in the ``different-format priming'' condition versus rest comparison no significant activation locus was detected in the right mesencephalon and the thalamus. Lebreton et al.
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Figure 1. Statistical parametric map showing the significant rCBF decreases (left) when subjects performed the ``same-format priming'' task compared to the baseline condition. The volumes are thresholded at Z = 3.09 (p < .001, uncorrected for multiple comparison) and projected in three orthogonal directions (sagittal, coronal, and transverse). Stereotactic coordinates of local maxima are given in Table 5. On the right is show the anatomical localization of the significant peak in the right extrastriate cortex onto a normal MRI set transformed into Talairach's template (note that the anterior-most cluster did not correspond clearly to the gray matter and is not listed in Table 5). L = left; R = right.
Priming conditions versus baseline. When we compared the ``same-format priming'' condition with the baseline condition, according to the masking procedure described in the method, we found that the priming effects were associated with significant decreases in rCBF. These deactivations were in the right extrastriate occipital cortex in the region of the lingual gyrus (BA 19), in the left cerebellum and vermis, in the right
temporal gyrus (BA 42), and in the left parahippocampal gyrus. No specific significant increase in blood flow was observed (Table 5 and Figure 1). Relative to the baseline condition, the ``different-format priming'' condition was accompanied by rCBF decreases in the right frontal gyrus, the right temporal gyrus (BA 22/42), the left middle occipital gyrus (BA 19/ 37), the right cerebellum, and the mesencephalon
Figure 2. Statistical parametric map showing the significant rCBF decreases (left), when subjects performed the ``different-format priming'' task compared to the baseline condition. The volumes are thresholded at Z = 3.09 (p < .001, uncorrected for multiple comparison) and projected in three orthogonal directions (sagittal, coronal, and transverse). Stereotactic coordinates of local maxima are given in Table 5. On the right is show the anatomical localization of the significant peak in the left occipital cortex onto a normal MRI set transformed into Talairach's template (as in Figure 1, the anterior-most cluster did not correspond clearly to the gray matter and is not listed in Table 5). L = left; R = right.
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(Table 5 and Figure 2). Only one activation was observed in the left cerebellum.
DISCUSSION Experiment 1 Our results revealed priming effects on the two priming conditions. Moreover, we observed a superiority of the within-format priming effects relative to the cross-format priming effects. We will discuss these two issues consecutively. Regarding the first result, the perceptual priming effects within a symbolic format found in our picture tachistoscopic identification task are consistent with the published literature in which a similar perceptual identification paradigm for pictures (Brown et al., 1996; Warren & Morton, 1982) or for words (Weldon, 1991; Kirsner, Milech, & Stumpfel, 1986; Jacoby & Dallas, 1981) was used, and with some other studies that assessed picture perceptual priming on different tasks (Berry et al., 1997; Hirshman et al., 1990; Weldon & Roediger, 1987). The observation of significant cross-format priming effects is also in agreement with other studies that showed priming effects across the symbolic format on picture fragment completion (Berry et al., 1997; Hirshman et al., 1990). On the contrary, Weldon and Roediger (1987) did not find cross-format priming on degraded picture identification. This single discrepant finding may be attributable to methodological differences, as these authors employed a perceptual encoding of target items whereas we used a semantic encoding. Nonetheless, Hirshman et al. (1990) showed cross-format priming effects only when the words had been previously generated rather than read. This difference of results between the studies of Hirshman et al. (1990) and Weldon and Roediger (1987) may be also due to the distinct operation type implicated in the tasks employed in these studies (identification vs. production of stimuli). Our results provided evidence that this hypothesis is unlikely because our task consists also in the identification of stimuli. However, Weldon and Roediger (1987) tested degraded stimuli with a single degradation level, when we used undegraded pictures. Furthermore, Snodgrass and Feenan (1990) and Snodgrass and Hirshman (1994) observed that priming effects for degraded pictures differed as a function of picture fragmentation level. Thus, the performance of subjects in the Weldon and Roediger's study was constrained by the choice of perceptual cues and by the number of likely responses. Consequently, their task was perhaps too difficult to reveal cross-format priming effects. Consistent with this assumption, the same-format priming magnitude in our study is larger than they reported (30% and 17%, respectively). Regarding our second finding, our results concur with previous reports that modification of the physical features of stimuli between the study and test phases affects perceptual priming (Berry et al., 1997; Rajaram & Roe-
diger, 1993; Hirshman et al., 1990; Roediger & Blaxton, 1987; Roediger, Weldon, Stadler, & Riegler, 1992; Weldon & Roediger, 1987; Weldon, Roediger, & Challis, 1989; Weldon, 1991; Kirsner et al., 1986). We found that the symbolic format change, coupled with a change in the encoding task, yielded a reduction, but not an elimination of priming on the picture tachistoscopic identification task. This result is congruent with the data of Berry et al. (1997) and Hirshman et al. (1990) who used the picture fragment completion task crossed with a semantic encoding. The use of a semantic encoding of target items allows one to reveal cross-format priming on this task. It could seem surprising that cross-format priming effects were lower than within-format priming effects, given that subjects performed a semantic encoding task in the crossformat priming condition. However, regarding the strong perceptual component present in our tachistoscopic identification task, it seems unlikely that the harmful effect of the change of format could be counteracted by any semantic encoding. Finally, we showed distinct performance patterns on implicit and explicit tests. The manipulated conditions affected priming effects but not recognition scores, such that priming effects were higher in the within- than in the cross-format condition, whereas recognition scores did not differ in the same conditions. Furthermore, we found no significant correlation between these memory scores. The evidence provided by this dissociation is inconsistent with a contamination of priming effects by explicit retrieval strategies in the priming conditions. On the other hand, contrary to prior studies, we found that recognition scores were not enhanced by the semantic encoding on the cross-format condition. The usual explicit memory advantage for semantically encoded words could have been counteracted by the study-test perceptual mismatch of item in the semantic encoding conditionÐindeed, the performance in explicit memory was enhanced for the seen pictures rather than words as in the within-format condition. The data of our behavioral study also revealed that our tachistoscopic identification paradigm is relevant to assess two forms of priming for pictures. The different magnitude of priming effects (i.e., same-format > differentformat) suggests that the within-format priming effects may depend on a picture format-specific system, while the picture cross-format priming effects may be mediated by other processes and systems. The fact that the subjects performed a semantic encoding of words underlines the possibility that our cross-format priming effects might reflect a form of priming of a conceptual nature. This hypothesis is discussed below with reference to the anatomico-functional data from the second experiment. Experiment 2 First, the baseline versus rest condition comparison revealed large and major activation foci in the primary Lebreton et al.
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and associative visual cortex, extending to the posterior part of the inferior temporal and parahippocampal cortices. These results are consistent with several previous reports dealing with visual object recognition and object naming (Moore & Price, 1999; Martin, Wiggs, Ungerleider, & Haxby, 1996; Menard, Kosslyn, Thompson, Alpert, & Rauch, 1996; Price, Moore, Humphreys, Frackowiak, & Friston, 1996; Bookheimer, Zeffiro, Blaxton, Gaillard, & Theodore, 1995; Haxby et al., 1991, 1994). We also found activation in the left anterior insula and the right cerebellar cortex, consistent with neuroimaging studies of picture naming (Etard et al., 2000; Bookheimer et al., 1995). The involvement of the cingulate cortex may have to do with attention processes required by our identification task (Corbetta, Miezin, Dobmeyer, Shulman, & Petersen, 1991). The orbitofrontal increases in rCBF may be associated with visual attention and motivation (Nobre, Coull, Frith, & Mesulam, 1999; Rogers et al., 1999). As expected, essentially similar activation patterns were found when the other two behavioral tasks were compared with rest. Secondly, we found that priming effects mainly involve rCBF decreases for the primed items compared to the unprimed items, consistent with earlier studies (see Cabeza & Nyberg, 2000; Schacter & Buckner, 1998; Wiggs & Martin, 1998). This observation reinforces the hypothesis of a ``cerebral economy principle'' associated with priming effects: Cerebral activity required for the processing of previously experienced items is less intensive than for the novel items (Squire et al., 1992). This interpretation is also consistent with the cognitive concept of priming effects that postulates that the prior study of stimuli enhances subsequent processing of these same stimuli. Furthermore, the absence of activation associated with priming effects in regions that have been observed in explicit memory retrieval provide an argument against the involvement of explicit memory processes in the priming conditions. These rCBF decreases involved some but not all regions that were activated in the baseline condition. This result suggests a functional brain specificity of priming effects. Consistent with our working hypothesis, we identified distinct patterns of rCBF decreases as a function of the priming condition. Relative to the baseline condition, the same-format priming condition produced particularly rCBF decreases in the ``right'' posterior occipital cortex (lingual gyrus region, BA 19) and left cerebellum, whereas the different-format priming condition implicated a region of occipital cortex that was more anterior, extending to the posterior part of the inferior temporal gyrus (BA 19/ 37), and lateralized to the ``left'' hemisphere, left frontal regions, and the right cerebellum. This dissociation in the rCBF decrease patterns provides evidence that the two priming conditions assessed in this study represent two distinct forms of priming. The involvement of the extrastriate cortex in perceptual priming is consistent with early imaging studies, which 680
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assessed word priming with the stem-completion task within visual and auditory modalities (Badgaiyan et al., 1999; Schacter et al., 1996, 1999; Buckner et al., 1995; Squire et al., 1992). Such studies suggested that this cortex could be the specific neural substrate of a system that subserves word form-specific perceptual priming. Our results revealed that this cerebral area also plays a role in visual format-specific perceptual priming for living/ nonliving entity pictures that have preexistent representations in memory. The preferential role of the right hemisphere in format-specific effects has been reported in divided visual field studies of word-stem completion priming (Marsolek, Kosslyn, & Squire, 1992; Marsolek, Squire, Kosslyn, & Lulenski, 1994). These latter studies indicated that the within-modality (visual) and withinform (letter case) priming magnitude was greater for words and word-stems presented in the left than the right visual field. Similar results have been found with the identification task of words (Burgund & Marsolek, 1997), and letter-like forms (Marsolek, 1995). Recently, using the object identification task, Marsolek (1999) has shown that same-exemplar priming was greater than different-exemplar priming and different-format priming when the objects were presented in the left visual field/ right hemisphere. Functional neuroimaging studies of within-modality word-stem perceptual priming showed priming-related bilateral reductions in the occipital cortex. The lack of a single right-sided deactivation in these studies may be attributable to methodological features. In particular, the perceptual nature of the word-stem completion paradigm is less ``pure'' than the tachistoscopic identification task and they proposed often a semantic encoding of words (Badgaiyan et al., 1999; Schacter et al., 1996; Buckner et al., 1995; Squire et al., 1992). Employing the brief visual presentation task, Henson et al. (2000) found rCBF decreases in the right fusiform gyrus linked to familiar symbol and faces priming effects. The right hemispheric specificity may also reflect the use of nonverbal material. Thus, in addition to indicating that the extrastriate cortex subserves general aspects of perceptual priming, independently of the nature of stimuli, our results also suggest that the right lateral part of this cortex could specifically underlie a format-specific system that processes and represents the physical and surface attributes of figurative drawings. Others studies varying the physical features of pictures between the study and test phases (e.g., the left or right orientation) will be necessary to extend our knowledge about this point. On the contrary, cross-format priming was associated with a rCBF decrease in the left lateral occipito-temporal region. Several interpretations of this difference in laterality may be proposed. First, this involvement of the left hemisphere might reflect the symbolic format change of the stimuli. Burgund and Marsolek (1997) and Marsolek et al. (1992, 1994) postulated that word cross-modality and cross-form priming on word-stem completion does not depend on a word form-specific (or concrete form) Volume 13, Number 5
system, which may be localized in the right hemisphere, but depends on a word visual abstract form system in either both or the left hemisphere(s). Recently, Marsolek (1999) postulated also that dissociable neural subsystems operate in parallel to underlie visual object recognition: A specific-exemplar subsystem operating more effectively than an abstract-category subsystem in the right hemisphere and an abstract-category subsystem operating more effectively than a specific-exemplar subsystem in the left hemisphere. Our results suggest that a similar hemisphere dissociation might underlie the within- and cross-format priming effects on picture tachistoscopic identification. Thus, the visual picture cross-format priming might depend on a left cerebral system that involves higher level occipital regions than the regions that subserve the picture format-specific system. Secondly, these differences in laterality may also be attributed to the change in encoding task. More specifically, the semantic encoding task in the different-format condition involved lexical access or processing, then semantic categorization and the generation of semantic attributes of words, whereas the perceptual encoding task did not require lexical processing of or access to the objects' names. The left occipito-temporal decrease associated with cross-format priming in our study could reflect a prior lexical access to the tested pictures' names. Finally, this result argues also in favor of the conceptual nature of our cross-format priming condition. Indeed, activation in the left occipito-temporal region has been previously reported with picture semantic processing tasks (Ricci et al., 1999; Vandenberghe, Price, Wise, Josephs, & Frackowiak, 1996). This region is part of the semantic network that extends from the left superior occipital gyrus through the middle and inferior temporal cortex to the inferior frontal gyrus. Activation in the right cerebellum and the midbrain associated with this network were also reported. The left occipito-temporal cortex may subserve specifically a visual semantic system that represents the visual properties of objects (Vandenberghe et al., 1996). In accordance with such studies, our different-format priming condition involved rCBF decreases in the left frontal cortex, the right cerebellum, and mesencephalon, relative to baseline. Thus, these deactivations associated with the cross-format priming condition could reflect visual conceptual priming effects, which would agree with imaging studies of conceptual priming (Buckner et al., 2000; Wagner et al., 1997; Gabrieli et al., 1996; Demb et al., 1995). Right cerebellum deactivation related to conceptual priming effects have also been reported in previous studies (Buckner et al., 1998; Blaxton et al., 1996). This interpretation is also consistent with the psychological concept of cross-format priming proposed by Hirshman et al. (1990) and with the idea that underlined the elaboration of this paradigm. The two priming conditions also involved rCBF decreases in the right auditory cortex (Heschl gyrus, BA
42). The involvement of this region in cross-format priming may be related with the fact that the subjects were instructed to read words during the study phase and to name the drawings that corresponded to these words, such that deactivation of this region may reflect an auditory repetition priming effect. In the same-format condition, the subjects did not name aloud the pictures during the study phase but they could have done it silently, in an unintentional way. Within-format priming was also associated with rCBF decreases in the left parahippocampal gyrus. This region is known to be implicated in the spatial processing of visual objects (Haxby et al., 1991, 1994). During the study phase of the same-format priming condition, the subjects saw pictures that are likely to activate this region. Consequently, less cerebral activity in this region may have been necessary to explore again these same pictures during the test phase. The fact that this deactivation was not found in the different-format priming is consistent with this idea because the subjects had previously studied words rather than pictures. Finally, the same-format priming condition involved a primingrelated decrease in the left cerebellum, whereas in the different-format priming condition, a decrease in the right lateral cerebellum was noted, an inverse dissociation from that found for the occipital cortex (see above). This result is coherent with the existence of crossed anatomical pathways between these two structures. Moreover, Blaxton et al. (1996) also found an inverse cerebellar deactivation pattern associated with perceptual and conceptual priming effects on the word fragment completion task.
METHODS The subjects of both experiments were informed that they were taking part in a study about the perceptual processing of drawings. They were blinded to the fact that the study involved memory. Experiment 1 Subjects Forty-eight healthy university students volunteered to participate in this study. Half of them performed the memory tasks (priming and recognition) with the same-format condition (19 females, 5 males, mean age = 21.5 2.5 years) and the others with the different-format condition (12 females, 12 males, mean age = 23.3 2.5 years). General Design The subjects were individually tested and each of them performed a priming task followed by a recognition task, each task being performed with different stimuli. The two Lebreton et al.
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memory tasks were made up of two phases (study and test) separated by a 2-min filler task consisting of a backwards counting task. In both of these tasks, two conditions were manipulated across subjects: In the sameformat condition, the stimuli had the same symbolic format between the study and test phases (drawing/ drawing); while in the different-format condition, it was different (word/drawing). The priming paradigm was a tachistoscopic identification task, which consisted in naming aloud briefly flashed drawings of living/nonliving common objects. For each subject, the presentation time was previously chosen from a selection session so that their baseline performance (i.e., for drawings never seen before) was between 20% and 40% correct naming. The procedure for this session consisted in the successive presentation of eight lists of 20 drawings. The presentation time of each list varied as a function of the performance of the subject (the initial time was 50 msec). The experimental features for the identification task were the same as those used in the priming condition (see below), except there was no study phase. The mean time and percentage of successful identification ( SD) were 40.42 msec ( 13.98) and 28.51% ( 4.21) for the same-format and 34.58 msec ( 11.41) and 29.14% ( 5.92) for the different-format condition. There was no significant difference for each of these scores between the two conditions, F(1,23) = 2.29, p = .14, for the mean time, and F(1,23) = 0.36, p = .56, for the percentage (t tests). Procedure In the study phase of the same-format condition, the subjects carried out a perceptual processing on 35 drawings (30 targets and 5 fillers), which were presented for 3 sec with 500-msec interstimulus interval. They were instructed to decide if the drawing was facing right or left or faced directly forward (neutral position). In the study phase of the different-format condition, the subjects saw for 5 sec a total of 35 words (30 targets and 5 fillers; one every 500 msec) on which they carried out a semantic processing. They were instructed to read the words aloud, to indicate the semantic category, and to give one or several feature(s) of the referent of the word, preferably functional with the hope of discouraging imagery. In the test phase of the priming tasks, 60 drawings were shown successively on the center of the screen for 30 msec. Each drawing was followed by a pattern mask (meaningless figure) presented for 500 msec, thereafter a white screen appeared for 2.5 sec. The subjects named drawings aloud or said the word ``pass.'' Among the drawings, 30 were studied items (target) and 30 were nonstudied drawings (control). In the same-format condition, the target drawings were the previously processed drawings and in the different-format condition, the target items were the drawings that corresponded to the studied words. 682
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In the recognition test, 60 drawings were presented for 3 sec with a 500-msec interstimulus interval. The recognition test was a yes/no forced-choice task. For the priming tasks, the scores of identification were the mean percentage of correct naming for the target and control drawings. Priming was present if this score was significantly larger for the target items than the control items. For the recognition tests, the scores were the percentage of the target (hits) and control (FAs) drawings that were recognized as being in the study list. Material The stimuli comprised 310 achromatic line drawings of living/nonliving common objects selected from the drawings of Gaillard, Girard, Lemarchand, Eustache, and Hannequin (1998) and Snodgrass and Vanderwart (1980). The naming agreement of drawings was greater than 74%. Of the 310 items, 160 were used for the exposure selection session (eight lists of 20 items), 10 were used for training of study instructions, 120 were used for the memory tasks (four lists of 30 items each), and 10 were used as filler items to avoid primacy and recency effects (three at the beginning and two at the end of each study list). The 120 memory-task items were divided into four lists of 30 items each in such a way that they were equated on their levels of visual complexity (mean SD, 2.95 0.92), F(3,29)= 0.74 10 5, p > .99, and image agreement (3.83 0.51), F(3,29) = 0.007, p > .99. The four lists of 30 drawings were counterbalanced across subjects so that each list was used equally often as a target and control list in each of the priming and recognition conditions. The size of the drawings did not exceed 80 100 pixels. Three achromatic pattern masks of meaningless tangled lines were created through the use of the MacDraw Pro software with a 100 100 pixels size. The words were presented in a 32-point Helvetica font. The stimuli were presented using SuperLab 1.68 software (Cedrus) on a Power Macintosh 6222/75. The stimuli were displayed on a 14-in. Apple color monitor screen that was positioned 40 cm away from the subjects. During the identification task, the three masks were seen equally often for each series of 10 drawings and never repeated more than twice successively. In the priming and recognition tasks, all target and control drawings were mixed in such a way that the number of the successive target or control drawings did not exceed three, and the order of target drawings varied between the study and test phases for each series of 10 successive stimuli.
Experiment 2 Subjects Nine healthy male volunteers were recruited among undergraduate students of the University of Caen and Volume 13, Number 5
financially compensated for their participation in the study. The study was approved by the Regional Ethics Committee, and subjects gave informed written consent after all procedures had been fully explained. All subjects were right-handed (as measured by Edinburgh handedness inventory), between the ages of 20 and 28 (mean = 21.1 years), unmedicated, and free from central nervous system disease or injury and had no abnormality on their T1-weighted high-resolution magnetic resonance images (MRIs). They were selected on the basis of their performance in the tachistoscopic identification task (see below). On the day of the PET experiment, while lying on the couch, each subject went through a training session of the different experimental instructions. At the end of the PET session, a standardized debriefing was undertaken. Experimental Procedure The priming tasks used were the same as those described for Experiment 1 except for minor adaptations to adjust for the constraints of PET. The behavioral procedure in Experiment 2 differed from that in Experiment 1 on two major aspects. First, to constrain the time course of the experimental tasks, the presentation time of drawings in the identification task was identical for all subjects, whereas it was variable across subjects in Experiment 1. We worked it out from the results of the selection session of Experiment 1, choosing the most frequently selected time across the 48 subjects, which was 30 msec. Thus, at the PET study selection phase, we recruited only subjects whose performance was between 20% and 40% hit-rate on the tachistoscopic identification task of never-seen drawings presented for 30 msec. Secondly, to isolate processing related to same- and different-format priming effects, the performances for the target and control items were measured separately in the priming and baseline conditions, respectively, whereas these two types of items were mixed in Experiment 1. Thus, the priming tasks were preceded by a study phase, whereas the baseline task was not. The design consisted of four conditions (replicated twice for each subject) in two separated blocks: ``sameformat priming,'' ``different-format priming,'' ``baseline'' tasks, and ``rest.'' Thus, each subject underwent eight scans, two for each of the four conditions. During the interscan intervals of 8 min, the subjects carried out the study phases followed by the filler task in the priming conditions, whereas they were only invited to relax in the baseline condition. During PET scanning, the subjects performed the drawings tachistoscopic identification task. In the three experimental tasks (priming and baseline), 40 drawings were shown successively on the center of the screen for 30 msec each. In the priming conditions, 30 were targets and 10 were fillers, which were used to limit the involvement of explicit memory.
In the baseline condition, the 40 drawings had not been seen before, but 30 served only as controls, and 10 were ``fillers.'' The 90-sec PET scanning was obtained during the presentation of 30 items (targets or controls) mixed with 3 fillers (1 for 10 consecutive drawings). The seven other fillers were shown before (N = 4) or after (N = 3) PET data acquisition. During the rest condition, no instruction was given to the subjects except not to move and to keep their eyes closed. The rest condition was always placed at the beginning of each block, but the order of the three experimental conditions and of the two blocks was counterbalanced across subjects. Material The stimuli were the same as those described in Experiment 1. The 310 line drawings were divided among several lists: (1) five lists of 10 items for the selection and learning of instructions phases; and (2) six lists of 30 target and control items, six lists of 10 filler items, and 20 items to avoid primacy recency effects, for the three experimental tasks. The features of the computerized material were identical to those used in Experiment 1, except that the stimuli were displayed on a Gateway, ev700, 17-in. monitor screen. The responses were recorded on a tape recorder (Eiki model 3279). PET Scanning Techniques Subjects were installed within the tomograph around which a black tent was set up. A mirror was positioned above the subject's head, which allowed them to see the center of the monitor screen, which itself was placed in front of the tomograph. Laser beams were used to verify head alignment, which was along the cantho-meatal line. Prior to PET acquisition, a 68Ge transmission scan was obtained for attenuation correction. Each subject underwent eight PET acquisition periods over a 1.5-hr period. Scanning was obtained from an ECAT EXACT HR + PET device operating in 3-D mode with the acquisition of sixty-three 2.425-mm thick contiguous brain slices (total axial field-of-views: 152 mm). Brain activity was monitored as changes in relative rCBF using 15O-labeled water (half-life of 123 sec) method, administered as an intravenous 6±8 mCi bolus injection. Each rCBF measurement was acquired sequentially during 90 sec and the between-scan interval was 8 min. The tasks were started 30 sec before the scans. Data Analysis The scans were reconstructed, including a correction for head attenuation using the measured transmission scan, with a Hanning filter of 0.5 mm 1 cut-off frequency and a pixel size of 2 2 mm2. The data were analyzed with Statistical Parametric Mapping (SPM) implemented in Matlab (Friston et al., 1995). Following automatic scan Lebreton et al.
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realignment with AIR (Woods, Grafton, Holmes, Cherry, & Mazziotta, 1997), the original brain images were transformed into the standard stereotactic space of Talairach and Tournoux (1988) using the MNI (Montreal Neurological Institute) template. The images were smoothed with a Gaussian filter of 12 mm. Global differences in rCBF were removed by scaling and statistical parametric maps corresponding to comparisons between the ``rest,'' ``same-format priming,'' ``different-format priming,'' and ``baseline'' conditions were generated using the SPM 96 software package (Friston et al., 1995). In order to examine relevant changes in rCBF, data from the two blocks of three experimental tasks were combined to yield a single baseline condition, sameformat priming condition, and different-format priming condition. A four-level task factor ANOVA was performed at p = .001, uncorrected for multiple comparisons. Post hoc t statistic maps were then generated for (1) each experimental condition versus rest, (2) sameformat priming versus baseline, and (3) different-format priming versus baseline contrasts. The corresponding Z volumes were thresholded at Z = 3.09 (p = .001). We used the ``masking'' routine of SPM 96 (function ``inclusive'') to search only for rCBF decreases related to the priming effects (i.e., baseline vs. each priming condition) in the areas found to be activated in the ``baseline minus'' comparison. This procedure allowed us specifically to unveil priming-related rCBF decreases eliminating the rCBF decreases in areas that were not significantly activated in the baseline condition as compared to the rest condition. In all analyses, a cluster size > 1 voxel was used as threshold. Anatomical identification of the significant peaks was performed directly on the MNI template renderings, and with Talairach's coordinates based on classic linear transforms. Acknowledgments The authors thank their colleagues V. Beaudoin, O. Thirel, M. Chajari, and G. Perchey for their invaluable help in tracer production and data acquisition, Dr. K. Benali for statistical assistance, and the members of Groupe d'Imagerie Neurofonctionnelle for their availability and help. Reprint requests should be sent to Dr. F. Eustache, Laboratoire de Neuropsychologie, C.H.U. Co à te de Nacre, 14033 Caen Cedex, France. E-mail:
[email protected].
Note This work was presented in part at the Fifth International Congress of Human Brain Mapping in Du Èsseldorf, June 1999.
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