INVOLUNTARY ATTENTION SWITCH WITH DIFFERENT LEVELS OF DISTRACTOR STRENGTH Stefan Berti and Erich Schröger Institut für Allgemeine Psychologie, Universität Leipzig, Seeburgstr. 14-20, D-04103 Leipzig Email:
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Abstract Sinusoidal tones with 200 and 400 ms duration were presented binaurally with equal probability every 1300 ms. Subjects had to indicate by a button press whether a short or a long tone was presented in the current trial. The stimuli were presented with standard (p = 0.84; 1000 Hz) or deviant (p = 0.16) pitch. These occasional pitch changes were task-irrelevant. Pitch changes consisting in a frequency increase/decrease of 1%, 3%, 5%, or 10% resulted in prolonged reaction times in the duration discrimination task and MMN, P3a, and RON, components in the event-related brain potential that are elicited in the context of distraction. These measures being sensitive to task-irrelevant pitch changes did increase as a function of pitch deviancy. The findings demonstrate that rather small task-irrelevant changes in a repetitive sound can cause distraction on a behavioral and electrophysiological level. Moreover, the psychometric functions supports the view that distraction is not an allor-none-phenomenon and confirms the hypothesis that these event-related brain potential measures are related to each other.
The present study aimed at investigating how involuntary attention switch develops with increasing distractor strength. Obviously, large distractors like novel stimuli, abrupt onsets of events, or stimuli largely differing from the context result in an involuntarily switch of attention usually measured as decrease in performance in a primary task. However, with most distraction paradigms one can hardly compare different degrees of distractor strength since large distractors are required to obtain reliable distraction effects and since the distractor strength can hardly be controlled for with simple physical parameters. A new auditory distraction paradigm recently introduced by Schröger & Wolff (1998a) circumvents these disadvantages: It yields stable distraction effects in the behavioral and electrophysiological data even with small distractors and it permits to continuously vary distractor size on a single stimulus dimension. In this paradigm, subjects have to discriminate the duration of tones that are of short or long duration equiprobably and that can be of frequent standard pitch or of infrequent deviant pitch. These task-irrelevant deviations in pitch cause distraction on an electrophysiological and on a behavioral level. Behaviorally they prolong reaction time in the duration discrimination task and elicit a characteristic series of deflections in the event-related brain potential (ERP): MMN, P3a, and RON, respectively, which are most likely related to different stages in the processing of distracting events: Mismatch negativity (MMN) consisting in a frontocentral prominent negativity occurring between 150 to 250 ms after onset of the deviancy indicates the pre-attentive sound change detection. The change detection process underlying MMN generation uses the sensory representation of the acoustic regularities extracted from the sound sequence. Sounds that do not match with the neural trace of these regularities elicit MMN, whether or not attention is
focused on the sounds (for reviews see Schröger, 1997; Näätänen & Winkler, 1999; Picton, Alain, Otten, Ritter, & Achim, 2000). P3a, a positivity which follows the MMN in time at around 300 ms from stimulus onset, is taken to reflect involuntary attention switching to a distracting event. Usually, rather salient deviancies are needed in order to elicit P3a. With the present paradigm, however, which makes it difficult to disentangle task-irrelevant aspects of stimulation from task-relevant ones as both are embedded in the same perceptual object, even small task-irrelevant deviances elicit P3a. It is thought to reflect the action of attention switching, whereas the MMN is thought to reflect the registration of the deviance (for reviews see Friedman, Cycowicz, Gaeta, 2001; Knight & Scabini, 1998; Woods, 1990). Reorienting negativity (RON), a later negative deflection (occurring around 500 ms from stimulus onset), is elicited in situations where turning attention back to the primary task after being oriented away is required (Ahveninen et al., 2000; Berti & Schröger, 2001;Schröger & Wolff, 1998b; Schröger, Giard, & Wolff, 2000; Yago et al., 1999). We studied the effect of the degree of the difference between the standard and deviant pitch on the behavioral and electrophysiological distraction effects (RT costs, MMN, P3a, RON). One aim of the study was to determine whether behavioral and/or electrophysiological distraction effects can be obtained with task-irrelevant pitch changes approaching threshold. Usually, the pitch change used in this paradigm is between 5% and 10%, the smallest pitch change in the present study was 1%. Another aim of the study was to evaluate the effects of the degree of deviancy (distractor strength) on the different measures being sensitive to taskirrelevant pitch changes. That is, the psychometric functions should be determined and compared.
Method Ten subjects (three male; age range 18-24 years; mean age 21 years) performed an auditory duration discrimination task: Sinusoidal tones with 200 and 400 ms duration were presented binaurally (via headphone) with equal probability every 1300 ms. Subjects had to indicate by a button press whether a short or a long tone was presented in the current trial. The stimuli were presented as standard or deviant stimuli. Standard stimuli (84% of the trials) had a frequency of 1000 Hz. Additionally, four types of deviant stimuli with different degrees of deviation to the standard frequency were realized: a 1% deviation (frequency: 990 Hz or 1010 Hz; 4% of the trials), a 3% deviation (frequency: 970 Hz or 1030 Hz; 4% of the trials), a 5% deviation (frequency: 950 Hz or 1050 Hz; 4% of the trials), and a 10% deviation (frequency: 900 Hz or 1100 Hz; 4% of the trials). These occasional pitch changes were irrelevant for the duration discrimination task and subjects were instructed to attend to the duration information only. Mean reaction times were computed separately for the standard and the four types of deviant stimuli relative to the point in time when the decision can be drawn, i.e. 200 ms after stimulus onset. For the computation of the reaction times on standard trials the first standard after a deviant was excluded from the computation. Only correct responses within a time window between 150 and 900 ms were included in the computation of the reaction times. Correspondingly, the overall percentage of correct responses was computed. During the experiment the EEG was recorded from 9 electrodes of the 10-20 system and from the right (RM) and left mastoids (LM). The reference electrode was placed at the tip of the nose. In addition the horizontal and the vertical electro-occulogram was recorded to control for eye-movements. The EEG was filtered during the recording with a 0.1-40 Hz bandpass
and a 50 Hz notch. Offline, the EEG was filtered with a 1-20 Hz bandpass. The ERPs were computed separately for the standard and the four types of deviant stimuli within a timewindow from –200 and 800 ms after stimulus onset (all epochs with extensive eyemovements were rejected from the ERP computation). A 100 ms time interval from -200 to 100 ms before stimulus onset served as a baseline. For computing the ERPs of standard stimuli the first event after a deviant trial was excluded. In addition, difference waves were computed separately for all types of deviation (1%, 3%, 5% and 10%) by subtracting the ERPs elicited by the standard stimuli from the ERPs elicited by the deviant stimuli. For all four types of deviation the maximum amplitudes of the MMN, P3a, and RON component were measured from these difference waves.
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Figure 1. a. Distraction effect in the behavioral measurement (RT to the deviant stimuli minus RT to the standard stimuli) as a function of deviancy. All types of deviant stimuli resulted in prolongation of the RT in the primary task. Moreover, this prolongation increases with increasing deviation of the stimuli. b.-d. Amplitude of the MMN (b.), P3a (c.) and RON (d.) (measured from the deviant minus standard difference waves) as a function of deviancy. The amplitudes increase with increasing size of the deviancy.
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1% deviation 3% deviation 5% deviation 10% deviation Figure 2. Deviant related effects in the event-related difference waves (ERPs elicited by deviant stimuli minus ERPs elicited by the standard stimuli; 10 Hz filtered) at FZ for the different types of deviations (1%, 3%, 5%, and 10%). All kinds of deviants elicited a sequence of MMN-P3a-RON, which is clearly visible with supra-threshold deviations (3%, 5%, and 10%) and which is rather small with the smallest deviation (1%). With increasing deviation the MMN, P3a, and RON show increasing amplitude. The RON seems to saturate earlier than MMN and P3a.
Results Overall performance in the duration discrimination task for all types of stimuli is better than 85% correct responses: standard trials: 92% correct responses; 1% deviation: 91% correct responses; 3% deviation: 90% correct responses; 5% deviation: 91% correct responses; 10% deviation: 87% correct responses. The RT-data show a clear effect of the type of stimulus, assessed by a repeated measurement ANOVA with the factor stimulus type (5 levels; the standard and four deviant stimuli): F4,36 = 6.17; p < 0.01. The distraction effect (RTdeviants RTstandards) is displayed in Figure 1a, separately for the different types of deviation. All types of deviant stimuli result in a RT-prolongation which is numerically increasing with increasing
degree of deviation (from 18 ms for 1% to 25 ms for 10% deviating stimuli). However, a subsequent ANOVA for the distraction effect (factor: degree of deviation; 4 levels) failed in confirming this increase; F3,27 = 0.62. Figure 2 shows the difference waves for the four different types of deviant stimuli at the FZ. All deviant stimuli elicit an early negative peak around 200 ms (MMN) followed by a positive deflection peaking between 300 and 400 ms (P3a) and a late negative deflection with a maximum between 500 and 600 ms (RON). In the 1% deviation condition, the MMN, P3a, and RON components are strongly reduced and show prolonged latencies compared with the ERPs of the other stimulus types. Moreover, all components show increasing amplitudes with increasing deviancy. In contrast, the relative increase of the amplitude of RON is smaller and visible only in the early RON time window (500-550 ms). In the late RON time window (550600 ms) there seems to be no difference between the different deviant trials. A series of ANOVAs (factor: degree of deviation; 4 levels) for the different components confirmed these observations. An effect of the factor degree of deviation is present for the MMN component (time window 120-170 ms), F3,27 = 17.98, p < 0.0001, for the P3a component (time window 300-350 ms), F3,27 = 36.33, p < 0.0001, and for the early RON component (time window 500550 ms), F3,27 = 18.86, p < 0.0001, but not for the late RON component (time window 550600 ms), F3,27 = 0.36.
Discussion The present experiment reveals a clear dependency of the processing of task-irrelevant pitch changes from the degree of the physical difference between standard and deviant pitch. The effect of the degree of the deviation is especially reflected in the event-related brain potentials. The increase in amplitude in MMN and P3a with increasing deviancy is consistent with previous research (e.g. Schröger, 1996; Woods, 1990). This finding is consistent with the hypothesis that distraction depends on distractor strength and inconsistent with the alternative hypothesis that distraction is an all-or-none phenomenon. Also RON did partly vary as a function of deviancy. This association strengthens the view that MMN, P3a, and RON are related effects being elicited in the context of distraction. Although reaction times did numerically also increase with increasing deviancy, this effect was not statistically significant. This lack of a statistically significant effect may be due to insufficient statistical power. Currently we increase the number of subjects to test this possibility. It seems also possible that the behavioral consequences of the distraction can be compensated by the time the behavioral response is triggered or that there is a ceiling effect, that is, the smallest deviancy condition may already be too effective on a behavioral level. More research is required to clarify this issue. Interestingly, there are different effects in the two RON latency ranges suggesting that RON can be divided in two sub-components, an early and a late one. While the early RON shows a clear dependency on the degree of deviation the late part was unaffected by the degree of the deviation. Moreover, contrary to the MMN and P3a, the early RON seems to show a saturation in amplitude increase as there is only a small amplitude difference between the 5% and 10% deviation conditions. It may be speculated that that the early part of RON may reflect the general recovery from the distraction (that is not that much related to the task but merely depends on the degree deviation of the deviant stimuli from the standard stimuli) and the late RON may reflect the re-focusing of the task relevant information (which does not that much depend on the degree of deviation but merely is in the service of the primary task).
In sum, the present study shows that pitch changes as small as 1% or 3% may result in a robust distraction effect at behavioral and electrophysiological levels (see also Schröger & Berti, 2000). Moreover, these distraction effects turned out to partly depend on the degree of deviancy (i.e. on distractor strength) supporting the hypothesis that they are related to distraction. However, also these measures correlated well with distractor strength there were also distinct differences in the respective psychometric functions which can be resolved by a serial model of distraction consisting of different stages revealing a partly differential sensitivity to distractor strength: Pre-attentive deviance-detection (indicated by MMN), elicitation of involuntary attention switch (indicated by P3a), attempt to recover from distraction (early RON) and re-focus on task-relevant stimulus information (late RON). Studies by Berti & Schröger (2001) and Yago et al. (1999) showed that the present distraction measures are not limited to the auditory modality but may also be utilized for studying visual distraction phenomena. Therefore, the present distraction paradigm may help to investigate the cognitive and neuronal processes underlying involuntary attention switching more detailed. In other words, both inner and outer psychophysics of distraction may be successfully pursued with this approach.
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