Memory and assimilation to context in delayed matching-to-sample

Psychology Science, Volume 48, 2006 (1), p. 17-38

Memory and assimilation to context in delayed matching-to-sample ACHIM ELFERING1, VIKTOR SARRIS

Abstract This paper reports effects of short-term memory and context stimuli on recognition of visual stimuli. After presentation of a square as the target stimulus, participants had to store the target during a variable delay, until they had to identify the target within a sample of seven squares that differed systematically in size (context variation). Marked context effects (“shifts”) that occurred as responses to the test series were obtained when sets of comparison stimuli were arranged asymmetrically with respect to targets. Participants overestimated the size of the target in a set of larger comparison stimuli by choosing a larger stimulus to match the target, and vice versa (Experiment 1). This assimilation effect increased with longer delays between target offset and the onset of comparison stimuli (Experiment 2). Briefer target exposure also induced stronger assimilation (Experiment 3). The results indicated that visual short-term memory modulates (contextual) stimulus integration in delayed matching to sample. A working model of memory and contextual effects in matching is discussed. Key words: context; assimilation; memory psychophysics; delayed matching to sample

1

Achim Elfering, Department of Psychology, University of Berne, Switzerland, Viktor Sarris, Department of Psychology, University of Frankfurt, Germany. These experiments are part of the doctoral dissertation of Achim Elfering. The authors thank Hans-Georg Geissler, Joseph McGrath, and Richard Moreland for their helpful comments on earlier versions of the manuscript. Correspondence concerning this article should be addressed to Achim Elfering, Department of Psychology, University of Berne, Muesmattstr.45, 3000 Berne 9, Switzerland. Electronic mail to: [email protected].

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Memory and assimilation in delayed matching-to-sample All judgments are context bound. Moreover, in most situations, stimuli and contexts are changing in time, hence judgments depend on encoding, storage, and retrieval of focussed and contextual information (Bouton, Nelson, & Rosas, 1999). Thus, memory retrieval should depend on the change in contextual information, and effects of context on psychophysical judgments should also depend on memory for previous stimuli and judgments. Whereas memory researchers paid attention to the influence of contextual information on retrieval performance for some decades (Balsam, 1985), context theories in psychophysics did not systematically address the influence of memory processes on context effects for a long time (e.g., Parducci & Wedell, 1986), but recently memory processes are conceptualised as basically involved in category rating and absolute identification (e.g., Petrov & Anderson, 2005). Nevertheless, research on the effects of context within psychophysics rarely exploited procedures previously developed in the study of the effects of context on memory. This study is an attempt to demonstrate the systematic differences in psychophysical context effects associated with visual memory. The simple task used here, delayed matching to sample, is appropriate to concurrently study recognition memory and context effects within the same trial (e.g., Parr, 1992). Before moving on, however, we must first offer some theoretical background.

Perceptual relativity At the beginning of the last century, Hollingworth (1910) showed that perceptual judgements shift depending on changes in the range of stimuli. The central tendency effect later on was described as a common category effect and has been frequently reported in the literature as the tendency for estimates of individual stimuli to be biased toward the central value of the presented set of stimuli (e.g., Poulton, 1989). Hollingworth attributed the central tendency effect to immediate perception, not to reconstruction of stimulus properties: “In all estimates of stimuli belonging to a given range or group we tend to form our judgments around the median value of the series - toward this mean each judgment is shifted by virtue of a mental set corresponding to the particular range in question. This central tendency is not a ‘law of sense memory’. It is a law of immediate perception and disappears as the experiment becomes a memory test" (p. 462). In the same sense Koffka (1935) characterized a subjective stimulus scale as an internal frame of reference that functions as the background against which individual stimuli are judged. Helson (1947) later provided the first quantifiable model of this effect. He postulated that the frame of reference ("adaptation level", AL) is one-dimensional and bipolar in structure. The subjective size of a stimulus was supposed to be a function of its objective size and the adaptation-level (for a more recent account of this AL approach see, e.g., Thomas, 1993).

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Psychophysical context effects and memory Hollingworth (1910) saw the central tendency effect as a law of stimulus integration that proceeded in time but Hollingworth denied a systematic influence of memory on the central tendency effect. For a long period psychophysical context research viewed effects of time to cause noise to the general principles of judgment rather than to reflect substantial mechanisms involved in perceptual and judgmental processing per se. For instance, Helson distinguished between time-order errors and time errors. The order of stimulus presentations is crucial in time-order errors. The shorter the interval between stimulus presentations, the stronger are time-order errors (for example, the stimulus response function in rapid judgments of increasing stimuli differs from stimulus response functions when stimuli are judged in ascending or descending order, a phenomenon called hysteresis, see Stevens, 1957). In contrast, time errors were only supposed to appear, for example when inter-stimulusintervals were longer than three seconds (Helson, 1964), or when the duration of stimulus exposure was restricted (Helson & Kozaki, 1968; for a discussion of time errors see Hellström, 1985, 2003). The critical step was to change from the “error” point of view towards studying effects of time, because they potentially reflect substantial judgmental processes (Hellström, 2003). Memory research triggered criticism that within an experimental session Helson assumed all stimuli to be of equal weight in information processing (Krantz & Campbell, 1961). For example, if stimuli are all integrated with equal weight throughout an experiment, typical position effects, such as primacy effects (long-term memory) and recency effects (short-term memory) would not occur. However, recency effects within context effects were observed frequently (Campbell, Lewis & Hunt, 1958; Krantz & Campbell, 1961; Parducci, 1954, 1959). Moreover, Anderson (1971) showed that recency effects depended on the position of stimuli in the sequence of presentation, but not on contrast effects, as proposed by Helson (1964). Noteworthy not only are all weights not equal, but the weight of a given element changes across time (e.g., after 20 stimuli in a sequence have been presented, stimuli 16-20 have a clear recency "weighting", but after 40 stimuli in a sequence have been presented, stimuli 16-20 have a much weaker weighting). Thomas (1993) adopted Helson’s AL model to generalization testing, postulating systematic changes in the prevailing AL across trials through a weighted integration of the initial training AL and a succeeding test-series AL. In his model, recency effects may appear as a function of relative weighting. A weaker encoding of the training AL (by drawing participants’ attention in training to a stimulus attribute not varied in subsequent generalization testing) led to less weighting of the training AL and a stronger shift in generalization tests (for a critical test of the model see Sander & Sarris, 1997). Working memory and psychophysical context in stimulus rating. In many judgments, working memory demand involves the storage of stimulus properties and the storage of previous judgments. Thus, much research on context effects that includes judgments on a verbal response scale resembles a memory test for paired associations between stimuli and response categories, even when no memory instructions are given (Haubensak, 1992; Johnson & Mullally, 1969; Siegel & Siegel, 1972, Wedell, 1990). In his range-frequency theory, Parducci (1965) argues that judgments are a compromise between two principles. First, participants assign differences between stimuli into corresponding differences of response scores (range tendency). Second, they tend to use all categories equally often (frequency

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tendency). Parducci and Wedell (1986) showed that working memory may modulate the effects of context on judgments. The effects from showing some stimuli more often than other stimuli of a stimulus set (frequency effect) becomes stronger if fewer response categories (category effect) and more experimental stimuli (stimulus effect) are used. Limited capacity in working memory may cause the category and stimulus effects. Wedell and Parducci (1985) found that the search span of past trials is limited to12 preceding stimulus-response elements. When there are only few response categories, memory information is rather complete for the frequency principle, and when the set of stimuli is small, working memory is complete with regard to the range principle. Therefore, the number of response categories and the number of distinct stimuli in the stimulus set alter the strength of context effects even when contextual manipulations (e.g., a change in stimulus range) remain unchanged. According to the multiple standards model of Petzold and Haubensak (2004), stimulus ratings are a joint outcome of short-term and long-term internal standards for judgment. Generally, individuals try to judge stimuli consistently under consideration of previous stimulus response trials (short-term internal standard) and the stimulus range (long-term internal standard). The multiple standards model also addresses how memory capacity may account for range and frequency context phenomena, and the effects of stimulus discriminability on absolute judgments. Meanwhile, there is need for clarifying the influence of memory on the most basic process of stimulus integration, i.e. stimulus recognition. Working memory and psychophysical context in recognition performance. Addressing shape constancy, Thouless (1931) reported an experiment in which he exposed a circular disc at various angles and asked observers to judge its shape each time. The observers did so by selecting a matching disc from a series of circular and elliptical ones which they had been given. When the disc was directly in front of them and in a vertical plane the judgement was easy, but the task was more difficult when the disc was rotated away from the observer so that it appeared elliptical. Judgements of shape reflected a compromise between the shape as displayed at an angle (an ellipse) and the actual shape of the object (the circle). Therefore, observers did not see the shape as it would be on the retina but instead exhibited a 'phenomenal regression' - the phenomenal or apparent shape was inbetween the tilted shape and the vertical shape. This has been called a 'perceptual compromise'. Recently Geissler (2004) reanalysed data from various experiments on recognition of geometrical stimulus structures, carried out until the end of the last century. After Geissler, stimulus properties and task demands were understood to jointly determine the interplay of perceptual and memory processes. In complex structural recognition, memory functions to simplify processing in order to keep rapid decision-making possible. Geissler and coworker called this top-down control of processes “memory-guided inference”. Thus, assimilation to stimulus dimensions in a particular task is seen as part of memory guided inference to reduce cognitive demand. In recognition of distinct geometrical pattern assimilation to mid values of a stimulus dimension was observed (“shrinkage”) that were irrelevant for object recognition. Participants’ RT were lowest for mid values of shrinkage, and consistently higher for the non-distorted stimuli. This assimilative process evolved quickly after transition from training trials to testing trials (Geissler, 2004). One important conclusion Geissler made is that memory guided inference is based on the same sensory information that is involved in perception of structured objects. Noteworthy he also tried to explain the shift of RT to mid serial values of shrinkage with an AL conception, that supposes various standards for training and test values, whose weights change with experimental time and experience. However, as Geissler stated, although fitting

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the average response curves rather well, such an approach could not explain the immediate shift that is complete within first responses to the test series. The weighting of training and test standards that change over time would suggest a gradual change with experimental testing time and experience with test stimuli, because these are the parameters that change the weights and cause the shift. Indeed, gradual change is what Thomas (1993) reported from his experiments studying recognition in a “go-no-go” task. In “go-no-go” training human or animal subjects are instructed to respond to one stimulus value (S+) and to ignore another one (S-). In generalization tests after training, responses typically show a shift of the maximum response probability as the peak of responding from the S+ in training to a stimulus value that is farther removed from the S- value. Thereby, gradual change of the matching choice is typical when every single stimulus is presented in sequence and is supposed to represent gradual change in ALs of training and test stimuli. Short-term and long-term processes of memory are probably involved in gradual change of responses, as models on dynamics in operant conditioning suggest (e.g., Dragoi & Staddon, 1999). Unfortunately, most go–no-go studies included successive stimulus presentations, and therefore memory aspects as decay of training stimulus traces and stimulus interference become superimposed. Apparently, it would be promising to study psychophysical context effects within tasks from memory research that are extensively studied for their cognitive processes involved in performance. The basic hypothesis of three experiments reported in this paper is that assimilation as a context effect that is memory guided would emerge also in a delayed matching to sample task (DMTS), that allows systematic variation of memory demand. In DMTS, a target stimulus is presented that has to be identified in a set of comparison stimuli. We expect to induce context effects with comparison stimuli that are arranged asymmetrically around the target’s size. The presentation of comparison stimuli that are mostly smaller or larger than the target should systematically bias the likelihood of comparison stimuli to match the target. After showing context effects in DMTS, the second step is the systematic variation of memory demand. Memory demand can be increased selectively, either by inserting a time delay between target offset and the onset of the comparison stimuli, or by shortening target exposure time. A delay between target offset and the onset of comparison stimuli and brief exposure of the target should increase the context effects.

Experiment 1: Context effects in a delayed matching-to-sample task In MTS a target stimulus has to be identified in a choice of comparison stimuli. Matching a target to a set of comparison stimuli that are more extreme in a single direction should evoke a response shift towards this direction to the medium stimulus value of the comparison stimuli. Contextual choices should appear as assimilation effects; the targets size is underestimated within a set of smaller comparison stimuli and vice versa. The point of subjective indifference (PSI) should significantly differ between context series. The PSI in MTS was computed as the asymptotic median of the cumulative response curve, indicating the (theoretical) stimulus size that was most similar to the target (because smaller and larger stimuli were chosen by chance). If context effects appeared, then we wanted to know whether they would change across experimental trials. We did not expect systematic context-induced "shifts" across trials, because trials in DMTS can be seen as "closed" events, – a single target

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has to be stored in mind to make a choice among the samples, but afterwards, a completely new trial starts (this is in sharp contrast to the dynamic change in context effects during transition at the beginning of generalization tests, see Elfering, 1997 for psychophysical context effects from the same set of stimuli in an operant learning study). Making a choice "completes" a trial, without necessary further influence on the next trial. Comparably low proactive interference across trials would be an important advantage compared to other tasks. Proactive interference in DMTS would mean that accuracy in matching the target to samples (i.e. hit rate) would depend on preceding target presentations (Capaldi & Neath, 1995). Furthermore, hit rate should be best in first trials and decrease with testing. We tested proactive interference in regression of choice in each individual trial on current target, contextual conditions and preceding target presentations, the latter predictor variable would indicate proactive interference. Change in hit rate was tested across first 3, 5 and first 10 trials with Cochrans Q – Test.

Method Participants. Twenty undergraduate psychology majors served as volunteers (11 women and 9 men, mean age = 32 years, SD = 7 years). All participants had normal vision or wore corrective glasses and were unaware of our hypotheses. Each participant received credits, toward the completion of a course research requirement.

Table 1: Targets and sets of comparison stimuli used in experiments 1, 2, and 3

Stimulus size Small Experiment 1, 2, 3a: Control C0 Contextual C1 Sets C2

Smaller target X X X X X X

Large

X X X

X X X X

X X X

X

X

X

X

X

X

X

X X

X X

X X X

X X X X

X X X

Larger target Control Contextual Sets

C0 C1 C2

X

X

X

X

X

X

X

Note. aStimuli varied in steps of 1 mm length of sides (Experiment 1, and 2), except in Experiment 3 stimuli were varied in steps of 2 mm length of sides. Comparison stimuli were arranged either symmetrically (context control condition, C0), or asymmetrically (context conditions) around the two target squares. With reference to the targets size, the asymmetric context test series were composed either with predominantly smaller comparison stimuli (C1), or with predominantly larger comparison stimuli (C2).

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Apparatus and materials. Stimuli were presented on a 15" touch-sensitive monitor (HLseries, Microsystems Inc.). Participants responded to stimuli by touching the surface of this monitor. The stimuli shown in one trial comprised an initial target square, and then the same target square was still presented and surrounded by 7 comparison squares, whose sides varied in steps of 1 mm (Table 1). Tests included four different conditions with two targets (the 17mm-target, and the 21mm-target) and two contextual comparison sets each. One contextual comparison set comprised more smaller squares (C1) and the other comparison set consisted of more larger squares (C2). The squares were projected on a grey background. Participants sat 60 cm in front of the screen, so the resulting angle of each stimulus was between 1.15 ° and 2.48 °. In a pilot study, we found that sizes of stimuli were above difference thresholds (Elfering, 1997). Procedure. Participants completed a 10-trial training session with acoustic feedback to become familiar with the DMTS task. During training, only three comparison stimuli were presented. After finishing the training participants were told to expect an increase in task difficulty because seven comparison stimuli would be used and no feedback would be offered. Participants were asked to carefully inspect all comparison stimuli. There were no speed instructions, and target inspection time was self-controlled. Comparison stimuli were presented with a delay of 2000 ms after the target disappeared (Figure 2B). The comparison stimuli were arranged in a circle around the position where the target stimulus was presented before. During tests participants did 56 trials including the four different context conditions. The sequence of the two targets across trials was quasi-random with the restraint that only two repetitions of the same target occurred. The monitor positions of the seven comparison stimuli sizes were counterbalanced, so that every comparison stimulus size appeared twice at each monitor position. Intertrial intervals were 5 sec, so the test session lasted about 25 min.

Results Different contextual comparison sets clearly affected participants’ choices (cf. Figure 1). Choices in contextual sets with predominantly smaller stimuli (C1) were smaller and choices with predominantly larger stimuli were larger (C2). Context effects were strong for the 17mm-target (mean difference between context series of 1.2 mm, or 7 % of size) and for the 21mm-target (1.1 mm, or 5.2 % of size). Context determined 43 % of the variance in the 17mm-target judgments (F(1, 39) = 62.33, p < .01), and 47 % of the variance in the 21mmtarget judgments (F(1, 39) = 102.22, p < .01; separated ANOVAs for the smaller and larger target with context (C1, C2) as IV and the PSI that was aggregated across 14 serial trials as the dependent variable). As Figure 1 shows, systematic shifts in context effects across trials were not evident. Trend analyses in 2(context) X 14(trials) ANOVAs showed that variance across the trials was always small. Eta-square ranged between 4 % in C2 for the 17mm-target, and 12 % in C1, which was the only significant effect of trials (F(13, 247) = 2.69, p < .01). This effect consisted mostly of a significant linear term that explained 20 % of the trial variance, (F(1, 247) = 6.62, p < .05), due to a decrease in context effects after the first two trials. There was no evidence of systematic changes across trials, aside from a slight tendency for larger context effects to occur in early trials. Variability across trials was small compared to variance

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Size of Matched Comparison Stimulus [mm]

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22

21mm-Target

21

C2

20

C1

19 17mm-Target

18

C2 17

C1

16 15

M ± 1 SEM 1

2

3

4

5

6

7

8

9 10 11 12 13 14

Trial Number Figure 1 : Assimilation in delayed matching to sample (Experiment 1). Mean size of the comparison stimuli that were chosen to match the 17mm-target (lower two trends) and the 21mm-target (upper two trends) across 14 experimental test trials.

between participants. Further analyses of the effects of sequence and interval between repeated context presentations did not reveal any significant results. In testing the proactive interference effect, multilevel linear regression showed no significant influence of the preceding target presentation (B = .001, SE = .016, ns) or the second earlier target presentation (B = .008, SE = .016, ns). The hit rate in first ten presentations was moderate (25%, 10%, 30%, 35%, 15%, 20%, 25%, 25%, 20%, 40%) and Cochran’s Test on change in binary variables did not indicate significant change when testing the first three presentations (Q (2) = 2.36, ns), the first five presentations (Q (4) = 4.53, ns), or the sequence of ten presentations (Q (9) = 7.45, ns).

Discussion Experiment 1 showed that meaningful context effects could be induced in DMTS. Recognition of the same target differed by contextual comparison stimuli and the same context had different effects depending upon the relationship between a target and that context. Thus, as a starting point, the phenomenon of assimilation could be studied within this task

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known from memory research. Moreover, context effects were strong. They accounted for a substantial part of variation in the data. Finally, there is good reason to assume that trials in DMTS are more independent events than in other tasks. In other words, evidence for proactive interference was low. This is in line with previous results from a number of studies that have examined proactive interference in DMTS by assessing accuracy on the current trial as a function of the stimuli presented on the prior trial or a stimulus presented prior to the sample, a pre-example stimuli. The proactive interference in DMTS was typically small (e.g., Medin, 1980; Zentall & Hogan, 1974). Because experiment 1 demonstrated both successful induction of assimilation effects and low proactive interference, the next steps were to make formal predictions about the size of context effects in DMTS, and to test for the influences of short-term memory.

Experiment 2: the effect of delay on context effects By applying the MTS paradigm to context research, it should be possible to study some basic memory effects involved in context processing. The aim in experiment 2 was to test for a moderating impact of visual short-term memory in context effects within DMTS. There were two steps used in modelling. First, the model of Sarris and Zoeke (1985) – that predicted context effects in an operant learning task was applied to DMTS. Briefly, the model predicts the bias in matches as a function of the targets’ size and the asymmetry of the comparison stimuli (cf. Equation 1). Second, the impact of the visual short-term memory during delay was modelled through a simple logarithmic weighting of the target. We used logarithmic weighting for two reasons. Logarithmic weighting was one of the best identified in a meta-analysis of perceptual memory models by Laming and Scheiwiller (1985). These models involved the lengths of lines and the areas of circles (see also Laming and Laming (1992). Power functions and logarithmic retention models also fitted the time course of forgetting in more complex stimulus material. The logarithmic function was chosen for adapting the working model of Sarris and Zoeke (1985) to the DMTS task. The full model equation is: PSI(Match) = (1/log delay * SizeTarget + Midsized-StimulusComparison Set) / (1 + 1/log delay),

(1)

whereas the equation predicts the match of a target stimulus within comparison stimuli to depend on the delay - measured in ms - between the offset of the target and the onset of comparison stimuli, the size of the target stimulus, and the set of comparison stimuli. When comparison stimuli are equally spaced, the midsized stimulus (stimulus # 4 out of the seven comparison stimuli) represents the context variation. For the condition of simultaneous presentation of the target and the comparison stimuli, a delay value of 10 ms was inserted into the equation to avoid a division by zero and to account for minimum delay in comparison caused by saccades of the eye. In experiment 2 we expected to replicate context effects that are systematic deviations of the match from the targets’ size to occur in DMTS. Second, we expected these effects to increase with memory demands. That is, we expected context effects to grow as the time between the offset of the target and the onset of comparison stimuli (delay) increased. Context effects should be larger with longer durations of time that the target has to be held in memory.

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Method Participants. Eighty undergraduate psychology majors volunteered to participate (63 women and 17 men, mean age = 29.7 years, SD = 8 years). All participants had normal vision or wore corrective glasses and were unaware of our hypotheses. Their participation was rewarded by credits toward a requirement for their degrees.

A. Control (No Delay): Simultaneous Presentation of Target and Comparison Stimuli, for 2000 ms each

“Touch“ “Touch“

Target

Time

B. Delay: Successive Presentation of Target and Comparison Stim uli

“Touch“ Delay Target

“Touch“

Time

Figure 2: Procedure for the delayed matching-to-sample task (DMTS) in experiment 2. Under the control condition (A. Control), the target and comparison stimuli were shown simultaneously for 2 s (visual search). In delay conditions (B. Delay), there was a delay between target offset and comparison stimuli onset of 500, 2000, or 5000 ms (between design)

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Materials and procedure. Stimuli were the same as in Experiment 1. Target stimuli and context series were the same. Two contextual control series were introduced. The mid stimulus of the two contextual control series equalled the target, so that there were three comparison stimuli that were smaller than the target, and three stimuli that were larger than the target. In other words, in the contextual control conditions, the seven comparison stimuli were symmetrically composed to the targets. The context series were composed asymmetrically to the targets, because in context series five out of seven comparison stimuli were either smaller (C1) or larger (C2) than the target. Control series were necessary to estimate the general tendency to underestimate the target with increasing delay as described in research on negative time error (Hellström, 2003). As in Experiment 1, context was manipulated within participants. Delay included values of 0, 500, 2000, and 5000 ms and was manipulated between participants. In the condition without delay, the target was still shown simultaneously with some comparison stimuli for 2 s to allow visual search (see Figure 2). Afterwards, the target disappeared. The training procedure was unchanged from Experiment 1. The test trials always started with a context control condition. As in Experiment 1, the sequence of the two targets in trials was quasi-random with the restraint that only two repetitions of the same target occurred. The monitor positions of the seven stimuli in each set were counterbalanced, so that every stimulus appeared once at each position. Intertrial intervals were 5 sec. A block of 42 trials lasted between 10 and 25 min depending on the delay condition. Data analysis. According to the model prediction of change in PSI, all analyses referred to PSI as the unit of interest. Context effects, the effects of delay on context were analysed by ANOVA of PSI data.

Results In context series C1 and C2, we expected participants to choose a matching stimulus that deviated from the targets’ size in the direction of the series asymmetry. In other words, we expected participants to under- or over-estimate the target depending on the set of comparison stimuli and accordingly to assimilate judgments towards the centre of stimulus series. The graphs in Figure 3 show that the PSI for the context series differed from the PSI of the context-free, i.e. symmetrical contextual control curves (C0). Separated 2(context) X 4(delay) ANOVAs for each target were executed with contextual PSI values that were standardized on the contextual control PSI and aggregated across 7 serial trials as the dependent variable. Analyses revealed no main effects for delay (17mm-target: F(3, 76) = 0.84, p = .47, η2 = .03; 21mm-target: F(3, 76) = 0.65, p = .59, η2 = .03). There were significant context effects for both the 17mm-target, F(1, 76) = 60.28, p < .001, and the 21mm-target, F(1, 76) = 89.36, p < .001. Context conditions determined 44 % of the variance in the 17mm-target, and 54 % of the variance in the 21mm-target (partial eta squared). As Figure 3 shows, context effects without delay were generally smaller than those in the delay conditions, and longer delay intervals produced larger context effects in DMTS. Meanwhile, the interaction of both factors was only significant in the 21mm-target (17mm-target: F(3, 76) = 2.03, p = .12, η2 = .07; 21mm-target: F(3, 76) = 4.03, p < .01, η2 = .14).

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+2

A. Prediction C2

PSI [mm]

+1

C0

0

-1

C1 0 +2

1000

2000

3000

4000

5000

B. Results 21mm-Target

PSI [mm]

+1

C2 0

C0 C1

-1

+2

M ± 1 SEM 17mm-Target

PSI [mm]

+1

C2 C0

0

C1 -1

0

1000

2000

3000

4000

5000

Inter-Stimulus Delay [ms]

Figure 3: A. Prediction of the PSI (left); B. Mean experimental PSI-trends (right) as a function of delay (Experiment 2). - The curves for the contextual series for the smaller target (lower trends) and the larger target (upper trends); the contextual control-series lines (C0) are dotted (with respect to the targets the context series C1 consisted of smaller stimuli, the context series C2 of larger stimuli). - (The vertical bars represent one standard error of the mean (SEM) beneath and above the PSI)

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Discussion Context effects repeatedly were shown in DMTS and the predicted effects of delay on context strength were observed in both targets. Logarithmic weighting of delay resulted in predicted monotonic trends that showed most dynamic change up to a second of delay, but little further increase in change for more extended periods of delay. This corresponds to a time constant of approx. 100 ms in a general decay of the target representation (target representation loss = exp(-delay/100)). Processes therefore took place in a storage system with short lifetime and – potentially – high susceptibility to interference, because interference is a likely process by which context stimuli evoked assimilation. Massaro and Loftus (1996) distinguished two storage systems of sensory information, one with more rapid decay and susceptible to interference they named sensory storage and a longer lasting perceptual storage with more limited capacity. With respect to this rather established categorization the results probably relate more to processes within the sensory storage than to processes within perceptual storage. If so, a second limiting factor for memory storage of the target that alters the effect of context should be short presentation times of the target. Experiment 3 therefore investigated the potential influence of target presentation time on context effects in DMTS.

Experiment 3: limited target presentation and context effects Limited presentation times – especially in critical ranges – should restrict visual maintenance. Memory research suggests the repeated inspection of visual material functions like ‘rehearsal’ in auditory working memory: "such a mechanism might involve some form of active maintenance, but on the other hand it could depend on the inhibition of competing excitation, hence minimising interference" (Baddeley, 1986, p.121). There are few studies that addressed the duration of stimulus presentation within a psychophysical context setting. Evidence from those few studies however, indicates that the strength of contextual influence depends on presentation times of both, experimental stimuli that are to be judged or identified, and contextual stimuli. For instance, in research on adaptation theory, it was shown that the impact of a stimulus on the prevailing adaptation level depends on how long the stimulus was presented. In an operant learning study, a brief exposure of the training stimulus showed larger context-induced shifts in following generalization tests (Giurintano, 1972, reported in Thomas, 1974). Thus, brief exposure of (training) stimuli was associated with less impact of these particular stimuli in generalization testing. Most studies on psychophysical context involve judgment of a set of contextual stimuli with verbal rating scale. Helson and coworker showed in this typical task that comparably longer presentation of some stimulus values within stimulus series resulted in a correspondent shift of the AL in the direction of those stimuli (Helson & Kozaki, 1968). Longer presentation of a stimulus increased the impact or the stimulus in stimulus integration relative to other stimuli. In other words, presenting the larger stimuli of a series comparably longer than the smaller stimuli resulted in an increase of adaptation level beyond the value that could be expected from the mere size of the stimuli. Accordingly, how long targets in DMTS are presented should affect context strength. Shorter presentation of targets should correspond to stronger impact of contextual comparison stimuli that should result in stronger context effects. Therefore, we expected duration of target presentation to be inversely related to context effects. The briefer a target’s presentation, the

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stronger the contextual impact of comparison stimuli should be (see the prediction in Figure 4). The model predicts larger context effects when the target presentation is short and smaller context effects when the target presentation is long. In the model, time of target presentation was modelled as weight to the targets influence on the matching decision: PSI(Match) = (log duration * SizeTarget + Midsized-StimulusComparison Set) / (1 + log duration),

(2)

The equation states that the size of the matching stimulus, calculated as the asymptotic stimulus that is indifferent from the target (PSI(Match)), depends on the duration of target presentation (ms), the size of the target (mm), and the set of comparison stimuli (mm). As in formula (1), the midsized stimulus of the comparison stimuli represents the strength of context variation.

Method Participants. Each block included 20 participants. Forty undergraduate psychology majors volunteered to participate (27 women and 13 men, mean age = 27.1 years, SD = 6 years). All participants had normal vision or wore corrective glasses and were unaware of our hypotheses. Their participation was rewarded by credits toward a requirement for their degrees. Materials. The stimuli again were 15 differently sized red squares (cf. Table 1), but these now varied in length in steps of 2 mm, from 12 to 40 mm with a 22mm-target and a 30mmtarget. Again, the squares were projected on a grey monitor background. Visual angles were between 1.15 ° and 3.82 °. The doubling of stimulus steps should prevent potential floor and ceiling effects. The rationale is that without reducing stimulus similarity, it is not likely for context effects to increase with brief target presentation, when they were already very strong with unrestricted presentation times in experiment 1 and experiment 2. Thus, we made stimuli more distinct to generally reduce context effects strength, but to permit substantial variation in context effect strength due to how long the targets were shown. Context variation remained unchanged, except that the control series was presented with longest target exposures (block 1: 100 ms; block 2: 1000 ms). Procedure. The training was unchanged from Experiment 2. In test sessions, the delay between target offset and the onset of comparison stimuli was always 300 ms. The context variation was a within-factor. In experiment 3, we studied two ranges of exposure times in two blocks (block 1 : 25 to 100 ms; block 2 : 125 to 1000 ms). Within a block it was a completely within-factorial experiment. Each participant was assigned to one experimental block. There were 126 experimental trials in all. Intertrial intervals were unchanged from Experiment 2, so the 126 trials lasted between 20 and 30 min. Data analysis. Data were analysed separately for each block. Analysis of data was unchanged from Experiment 2. The test for context effects with ANOVA included only the longest target presentations within each block. This was to show that context effects did occur in conditions with modest memory demand.

Memory and assimilation

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A. Prediction Block 2

Block 1 +2

PSI [mm]

C2 0

C1 -2

25

50

75 100

125 250

500

750

1000

Target Duration [ms]

B. Results 32 30mm-Target

M ± 1 SEM

30

C2

28

PSI [mm]

C1 26 24 22mm-Target 22

C2 C1

20

25

50

75 100

125 250

500

750

1000

Target Duration [ms] Figure 4: A. Prediction of the point of subjective indifference (PSI, left); B. Mean PSI-trends (right) as a function of target exposure time (Experiment 3). The curves for the contextual test series refer to the smaller target (lower trends) and the larger target (upper trends). This experiment was conducted in two blocks of four presentation times each (block 1: 25 - 100 ms, block 2: 125 - 1000 ms). - (The vertical bars represent one standard error of the mean (SEM) beneath and above the PSI)

32

A. Elfering, V. Sarris

Results First, the ANOVA context series with 1000 ms of target presentations in block 2 revealed significant context effects for both the 22mm-target, F(2, 38) = 8.28, p < .01, η2 = .30 and the 30mm-target, F(2, 38) = 6.25, p < .01, η2 = .25. In block 1 with 100 ms target exposure, context effects were larger, F(2, 38) = 19.96, p < .001, η2 = .51, and F(2, 38) = 17.77, p < .001, η2 = .48 for the 22mm-target and 30mm-target, respectively. Moreover, context effects were increased by reducing the presentation periods. Figure 4 shows that differences between curves for smaller (C1) and larger (C2) series increased with shorter presentations of targets. The linear increase of context effects for shorter presentation periods was more evident in block 1 with periods between 25 and 100 ms. In this block, all linear trends were significant, F(1,19) between 8.20 and 23.48, all p < .01, η2 between .30 and .55. In block 2, only series C2 for the 30mm-target, F(1,19) = 4.13, p < .01, η2 = .18, showed a significant linear trend.

Discussion Context effects appeared within the longest target presentations, and context effects occurred in sets of stimuli that were less similar than in the first and second experiment. Context effects increased with brief exposure of the targets, and this trend was more obvious in very brief presentation times (block 1) than in more moderate presentation times (block 2).

General discussion The paper reports three experiments showing that (1) context effects judgments of visual size in a DMTS task, (2) context effects were larger the longer the delay between target and comparison set, and (3) context effects were larger the shorter the time of target exposure. Participants choice of a larger comparison in the larger set and a smaller comparison in the smaller did indicate an assimilation process, i.e., the larger set caused the target to appear larger than it really was (overestimation, since its memory representation matched a larger comparison stimulus), and the smaller set had the opposite effect. Assimilation became more intense with increased memory demand in DMTS. Besides the need for further replication and extension, the quantitative formal prediction of assimilation by visual short-term memory received basic support by the major findings. The coherent suggestion is that working memory should be explicitly included in psychophysical theories of context effects. The reported experiments examined a common category effect that has been reported in the literature: the central tendency effect. Estimates of individual stimuli were biased toward the central value of the presented set of comparison stimuli. In the literature both encoding and reconstruction accounts of this central-tendency effect were considered (e.g., Crawford, Huttenlocher, & Engebretson, 2000). Crawford et al. (2000) studied judgments on comparison of plain vertical lines with vertical lines embedded in the Müller-Lyer illusion. Both stimuli were estimated while still in view or from memory. Although bias due to the MüllerLyer illusion remained constant across the two conditions, bias due to the context set (category) occurred only when stimuli were estimated from memory. The results suggest that the

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33

category bias occurs at a later stage of processing within the Müller-Lyer effect and offer support for a reconstruction account of category effects on stimulus estimation. Contrast and assimilation. Considering the realisation of the DMTS task the reader might get reminded to the Ebbinghaus or Titchener illusions, where a centre circle is surrounded by larger or smaller circles that are all of the same size. In Ebbinghaus or Titchener illusions psychophysical contrast occurs. Two circles of identical diameter appear different when one circle “A” is surrounded with larger circles and another circle “B” is surrounded with smaller circles. In this case “A” is looking smaller than “B”. This would be a simultaneous contrast effect. Indeed, the second author demonstrated contrast effects in the Ebbinghaus illusion to depend on the size of contextual stimuli and on the distance between centre and surrounding stimuli (“Contour – Distance Model of Relative Size Contrast”, Sarris, 1986). On the other hand, there is important conceptual difference between the Ebbinghaus or Titchener illusions and DMTS in that (1) only DMTS demands for a judgment as the central stimulus has to be compared with surrounding stimuli, (2) only in DMTS the central stimulus has to be stored in mind for later comparison, and (3) only in DMTS the surrounding stimuli are of different size and create a multitude of stimulus impressions. Nevertheless it is important to note that any reference to the Ebbinghaus or Titchener illusions in explanation of context effects in DMTS would expect contrast effects to occur. The target should look smaller when most comparison stimuli are larger than the target compared to when most comparison stimuli are smaller than the target. Accordingly when most comparison stimuli are larger than the target, the comparison stimulus that is picked out to match the target should be smaller than the target. However, the results in DMTS did not confirm contrast effects but assimilation effects when the retention interval after target presentation was increased. And there was also a lack of contrast effects in conditions when there was no delay, but target and comparison stimuli were shown at the same time. Therefore, - as in the Crawfort et al. study (2000), assimilation in DMTS as shown in the central tendency effect could be attributed to reconstruction processes rather than to perception while the contrast effect in the Ebbinghaus or Titchener illusions is due to perception as Müller-Lyer effect is. With reference to Hollingworth (1910), the results confirmed his view on the central tendency effect with one important exception: The central tendency effect does not disappear but increases when stimulus judgments become a memory test. Three experiments showed DMTS to be a promising way to study memory and context effects. The task has three important procedural advantages that are (a) its lack of carry-over effects across trials when intertrial-intervals are not too short, (b) the feasibility to vary memory demand within a single trial, and (c) the ease of the task. Another advantage of DMTS is that it has become standard in neurosciences, and therefore there is some knowledge about the physiological structures and processes involved (e.g., Tsutsui, Jiang, Sakata, & Taira, 2003; Zhou & Fuster, 1997). Psychophysical experiments using DMTS therefore have the potential to contribute to memory psychophysics (or mnemophysics), a discipline that up to now focused on psychophysical scaling. Memory psychophysics particularly addressed the change of Stimulus-Response (S-R) function when judgments based on stimuli stored in memory, e.g., psychophysical functions become „flatter“ when memorized stimuli are judged in comparison to the rating of present stimuli (e.g., Algom, 1991; Hubbard, 1994; Petrusic, Baranski & Kennedy, 1998, Ward, Armstrong, & Golestani, 1999), but until now widely ignored context effects.

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Memory effects: change in judgmental heuristic or simple noise? It is a critical question on the nature of the observed phenomena whether short-term memory plays a more specific role in context processing than a simple change in judgmental heuristic or simple noise modulation, or both. Briefly, with decreasing capacity in working memory, it seems to be a good heuristic to choose the middle stimulus within a set of comparison stimuli, especially when the representation of the target becomes vague (noise modulation). A post-hoc analysis of contextual control trials from experiment 2 showed that the peak of judgmental choice is in mid-range of the set of comparison stimuli, however with increasing delay there is no corresponding increase in frequency of the mid-stimulus as the matching stimulus. Indeed the peak of choices is shifted to the next smaller stimulus. Thus, there was little evidence for the simple heuristic to chose a mid-ranged value when memory load is increasing. A test whether memory is simple noise modulation is to compare generalization across conditions of delay. Therefore generalization slopes of the contextual control conditions of Experiment 2 were analysed (for the test on change in generalization see Heinemann, Avin, Sullivan & Chase, 1969, p.218). ANOVAs with the slope as the dependent variable and the four conditions of delay as the independent variable showed no effect of delay on generalization (smaller target: F(3,79) = 0.63, ns; larger target: F(3,79) = 0.37, ns). Altogether, posthoc analyses revealed no evidence that memory-related change in judgmental heuristics and/or increased noise could explain the pattern of results. The memory-related change of context strength is probably not the result of response bias at the recognition test, but memory-based comparison of stimuli may give rise to context effects (for a recent model approach in scaling and identification, cf. Petrov & Anderson, 2005) .

Table 2: Relative frequency of comparison stimuli as matching the targets by conditions of delay: data of the contextual control series (C0) (Experiment 2)

Si [mm] Delay 0 ms (Control) 500 ms 2000 ms 5000 ms 0 ms (Control) 500 ms 2000 ms 5000 ms

14 15 16 Smaller Target: 5.7 10.7 28.6 7.1 21.4 29.3 7.1 22.9 25.7 5.7 22.9 30.0

17 X 36.4 24.3 26.4 25.0

Series-stimuli (C0) 18 19 20

21

22

23

24

12.9 5.7 11.4 5.0 1.4 12.9 2.9 2.1 12.1 3.6 0.7 Larger Target: 4.3 13.6 28.6 12.1 19.3 27.9 12.1 22.9 30.7 8.6 17.9 32.9

X 36.4 30.0 21.4 25.0

10.0 7.9 7.1 11.4

5.0 2.1 4.3 2.9

2.1 0.7 1.4 1.4

Note. T.1, T.2 =Target stimuli in the matching-to-sample task.

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Limitations Two methodological constraints might be seen in experiment three. The first is that smallest durations in experiment three fall within the period of temporal integration and may be seen as weak signals versus strong signals. However the trends did not indicate a qualitative change in results between smallest and more moderate presentation times. Second, because of the lack of masking to prevent afterimages, we could not preclude afterimages to play a role in matching results. The potential effect of afterimages should vary with how long a target was presented, but not with contextual stimuli. When target exposure time is heavily restricted, we would expect afterimages to increase hits in matching when compared with a masking condition. In other words, afterimages should reduce context effects when target exposure time is heavily restricted. Therefore, afterimages could not cause the context effect, and if afterimages did influence the context effect, they would diminish the effect and not cause it. Results of experiment 3 therefore represent a lower bound of memory driven effect size estimate. Results of PSI in experiment 2 confirmed the prediction in that the PSI did not substantially change when the delay changed from 500 ms to 5000 ms. Most change occurred between 0 and 500 ms, but in experiment 2 there was no systematic variation of delay within this time interval. A replication of experiment 2 should also include a 100, 200, 300, 400 milliseconds’ delay. Short-term retention of information serves to coordinate perception and action. Note that it is of considerable importance in DMTS to include motor system responses into the task, i.e. pointing and touching (see Fuster, 1995). That is why participants had to touch the target and the matching stimulus on the screen. Thus, DMTS as a cognitive psychophysics approach in the light of a frame-of-reference paradigm might help to uncover the transphenomenal neuronal substrates of the interplay between sensation and memory function (Ehrenstein, Spillmann, & Sarris, 2003). Delayed matching to sample seems to be a promising paradigm in (memory) psychophysics, because it is in line with the search for a more process-oriented approach that takes into account the temporal and spatial realities that can moderate perceptual relativity (Baird, 1997; Cangöz, 1999; Laming, 1997; Lockhead, 1994, 2004; Marks & Algom, 1998, Sarris, 2004). The nonverbal response modus in DMTS is an important advantage when developmental and comparative perspectives within psychophysical context research are considered (e.g., Sarris, in press; Sarris, Sander, & Elfering, 1995). Finally, the matching approach has much to offer for psychophysical scaling in general. Laming (1997) concluded in his book on the measurement of sensation that there is no absolute judgment of stimulus magnitude and, equally, there is no absolute judgment of differences in magnitude or of ratios. In the end Laming asked how, then, one should measure sensation. In the concluding chapter Laming arrived at the proposal: “. . . subjective sensation might be measured as physical intensity of the matching stimulus.” Laming clearly indicated that the value of such judgments depends on the context, and therefore might be limited. The value of the experiments reported in this contribution is to confirm the contextual influence while making a further step towards their understanding.

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