تبعیض از حالات عاطفی چهره در یک کار عجیب و غریب بصری: یک مطالعه ERP

Abstract Several ERP studies have shown an orienting complex, the N2/P3a, associated to the detection of stimulus novelty. Its role consists in preparing the organism to process and react to biologically prepotent stimuli. Whether this N2/P3a: (1) could be obtained with complex visual stimuli, such as with emotional facial expressions; and (2) could take part in a complex discrimination process has yet to be determined. To investigate this issue, event-related potentials were recorded in response to repetitions of a particular facial expression (e.g. sadness) and in response to two different deviant (rare) stimuli, one depicting the same emotion as the frequent stimulus, while the other depicted a different facial expression (e.g. fear). As expected, deviant stimuli evoked an N2/P3a complex of larger amplitude than frequent stimuli. But more interestingly, when the deviant stimulus depicted the same emotion as the frequent stimulus the N2/P3a was delayed compared to the response elicited by the different-emotion deviant. The N2/P3a was thus implicated in the detection of physical facial changes, with a higher sensitivity to changes related to a new different emotional content, perhaps leading to faster adaptive reactions.

. Introduction Most event-related brain potential (ERP) studies have used an ‘oddball paradigm’, in which subjects have to detect, amongst a series of standard stimuli, an infrequent deviant one (Garcia-Larrea et al., 1992). The detection of stimulus change may play a role in turning attention to events of biological importance (Halgren and Marinkovic, 1995). This has been indexed by three main ERP components, in the auditory as well as in the visual modalities. First, when subjects are placed in inattentive conditions, deviant stimuli in a homogenous stimulus sequence elicit a specific negative deflection, called mismatch negativity (MMN) ( Näätänen et al., 1978 for audition; Tales et al., 1999 for vision), which reflects an automatic (attention-independent) neural mechanism underlying the perception of stimulus change ( Näätänen et al., 1993). 1 Second, when subjects are placed in attentive conditions, deviant stimuli evoke a series of field potentials, the N2/P3a, overlapping the MMN activity described above, and called by Halgren and Marinkovic (1995) the orienting complex, because it subserves attention. Indeed, the orienting complex is defined as the mobilization of cerebral and somatic resources in order to effectively cope with a biologically important event. Third, a P3b component, recorded maximally at parietal sites and functionally related to the conscious detection of change leading subjects to respond to deviant stimuli, has also been recorded ( Bentin et al., 1999 and Campanella et al., 2000). In the present study, we employed a variation of the visual oddball paradigm2 that did not manipulate attention. Our hypotheses were focused on the N2/P3a complex and the P3b component, given that the MMN is classically described in inattentive conditions. As suggested above, the N2/P3a complex could reflect the afferent (preparation-to-process) and efferent (preparation-to-respond) functions of the orienting complex (Halgren and Marinkovic, 1995), whereas the P3b component is possibly related to the conscious subjects' responses (Bentin et al., 1999). Therefore, it is plausible to think that: (1) similar N2/P3a and P3b components could be obtained in response to deviant complex visual stimuli, such as emotional facial expressions (due, for instance, to their high importance in social communication); and (2) these components could be modulated, in latency and/or in amplitude, by the categorical nature of frequent and deviant stimuli. By using a morphing procedure, it is possible to generate continua of different morphed faces moving linearly from one facial expression (e.g. sadness) to another one (e.g. fear). Several studies have shown that two different morphed faces, perceived as sharing the same emotion (WITHIN-categorical differences), are harder to discriminate than two different morphed faces perceived as two different emotions (BETWEEN-categorical differences), even if the physical distance inside each pair is identical ( Etcoff and Magee, 1992, Calder et al., 1996 and Young et al., 1997). The main purpose of the present study was to show that the visual N2/P3a complex elicited in response to deviant stimuli, would be modulated (in amplitude and/or in latency) whether or not deviant stimuli shared the same facial expression as the frequent one. We hypothesized that this modulation would be more important when the rare stimulus does not share the same informational content then the frequent stimulus. This is of the greatest relevance: (1) as expressive faces are very relevant social signals, so that we have to pay attention to them, particularly when a change in a facial emotional expression has to be detected; and (2) as it would index, in the N2/P3a, a phenomenon taking part in an active discrimination process for emotional facial expressions, in order that subjects could cope efficiently with particular (e.g. threatening) facial expressions. Indeed, in such a case, it would be necessary to analyze what can be done about this situation, in order to furnish fast and adaptive reactions ( Bradley et al., 1999 and Mogg et al., 2000). Moreover, as expressive morphed faces were used in the present study, we expected that the present results could lead us to learn more about the neurophysiological processes involved: (1) in facial expression analysis; and (2) in the phenomenon of categorical perception of emotion.

. Conclusions The findings of the present study leads to three main considerations. First, by using a complex visual oddball task with faces, the N2/P3a is associated with deviants and can be modulated by the categorical origin of the frequent and rare stimuli. This is of particular interest, because it suggests that the visual N2/P3a activity does not only reflect an automatic detection of the rarity of a visual event (in order to switch attention), but is also engaged in an automatic discrimination of the rare stimuli, so that rare stimuli presenting a high degree of novelty (rare BETWEEN presents a new facial expression) were processed more rapidly than rare stimuli presenting a lower informational content (identical facial expression). This latency effect may have a primordial adaptative value, given that failing to allocate special attention to such stimulus change would result in no coping or in inappropriate coping. With this in mind, we suggest that a similar paradigm could help us to investigate more precisely a number of processes implicated in psychopathology (e.g. anxiety disorders). Clark (1999) has proposed several mechanisms—one of them directly pertaining to the deployment of attention—that can account for the persistence of anxiety. According to the sustained vigilance model ( Beck et al., 1985), anxious individuals would be characterized by an overactive fear cognitive representation, leading them to over-perceive threat in their environment. An alternative view is proposed by the sustained avoidance model ( Clark and Wells, 1995), suggesting that anxious individuals are actively avoiding threat stimuli when such a cognitive avoidance might provide them an escape from the feared situation. We suggest that, if anxious individuals were confronted with the design used in the present study (and compared with non-anxious-ones), the latency effect found on the N2 component (referring to the allocation of attention to stimulus categorical change) would be more important to rare BETWEEN stimuli showing a fearful expression than for rare stimuli showing a lesser degree of threatening information, but only if the sustained vigilance model was right (due to the fact that they would over-perceive threatening cues). Conversely, if the sustained avoidance model was correct, we suggest that the N2 latency effect—between threatening and non-threatening stimuli—should disappear, because anxious individuals would not focus their attention more to threatening cues than to non-threatening ones. Establishing which model is correct by using this kind of experiment would be of particular importance, both for theoretical reasons and for applied concerns. In fact, while the sustained vigilance model implies that anxious individuals should be trained to redirect their attention on non-threatening information in order to have a less biased perception of reality, the sustained avoidance model suggests that anxious individuals should focus their attention on stimuli initially perceived as threatening in order to disconfirm this initial impression, or to develop their abilities to confront anxious situations. Such experiments are underway in our laboratory. Second, several prior studies found that emotion-modulated ERP components occurred considerably later than they did in the present study. Subjects of Potter and Parker (1989) had to decide whether the second face of a pair matched the first one in terms of expression. The ERPs showed a difference in the 490–540 ms time range, only for a right parietal site. Accordingly, Hautecoeur et al. (1993) showed a modulation of a parietal P400 when subjects were asked to look for emotional expression of the face (smiling or non-smiling) in comparison with a recognition task (known or unknown). By using intracranial recordings, Halgren and Marinkovic (1995) showed that significant differentiation among waveforms evoked by different facial emotions appears frontocentrally in the 400–600 ms latency range. In the present study, we showed that the discrimination process between different emotional facial expressions could be neurophysiologically indexed at an earlier latency (around 270 ms) than previously shown. Third, we give what may be the first neurophysiological account for a categorical perception effect based on facial expressions (higher correct response latencies for WITHIN differences as compared to BETWEEN ones). Indeed, categorical perception of facial expressions has received strong empirical support at a behavioral level (Etcoff and Magee, 1992, Calder et al., 1996 and Young et al., 1997), suggesting that the discrimination performance is affected by category membership rather than by objective physical distance. According to Tanaka et al. (1998), we have in long-term memory different prototypes (exemplars) of all the different facial emotional expressions, so that the morphed faces of BETWEEN-categorical pairs are ‘attracted’ by two different stored representations, while for the WITHIN-categorical pairs, the two faces are ‘attracted’ by a single representation. Within-categorical pairs are thus more difficult to discriminate because the two different morphed faces activate a same stored facial representation. Then, the two different stimuli lead to an early perceptual similarity that has to be inhibited to take into account the existing physical differences. In the present study, we showed, at a neurophysiological level, that: (1) this latency effect was not specifically related to a motor response effect (which would have been indexed by a unique delayed P3b component); (2) it began at an earlier stage (around 270 ms), as marked by the delayed N2 component; (3) it probably found its origin on supplementary visual areas (the latency effect being present over all posterior electrodes); and (4) it was correlated to the behavioral delay in response latencies, indexed by a delayed P3b component.