Contributions of stimulus encoding and memory search to right hemisphere superiority in face recognition: behavioural and electrophysiological evidence

Personality in Autism Spectrum Disorder: Associations With Face Memory Deficit and Theory of Mind

OBJECTIVE

To examine the personality profiles of adults with autism spectrum disorder (ASD) using a standard personality assessment and to investigate the association between personality, ASD-related face memory deficit (FMD), and theory of mind (ToM). In a broader context, to examine whether there are distinct clinical phenotypes in the ASD population that have implications for personality development and treatment.

METHOD

Fifty-five adults with ASD and 22 neurotypical (NT) adults underwent a battery of neuropsychological tests, including measures of personality, face memory, and ToM. We compared ASD and NT groups in terms of their Personality Assessment Inventory (PAI) profiles. Additional analyses focused on the association between specific PAI scales and FMD. Performance on the Eyes Test was compared across groups and was examined in relation to FMD.

RESULTS

Adults with ASD demonstrated significant elevations on several PAI scales compared with NT adults. The presence of FMD was associated with differing PAI profiles among the ASD adults. The ASD adults with FMD scored significantly higher on scales that are sensitive to positive impression management and treatment rejection and significantly lower on scales that are sensitive to borderline personality, anxiety, depression, schizophrenia, and stress. There was a significant association between performance on the Eyes Test and FMD in the ASD group.

CONCLUSION

Adults with ASD have a unique personality profile. Further, ASD adults with FMD have reduced insight into their difficulties with emotional processing and may not be as sensitive as ASD adults without FMD to the emotions of others.

Speaker Variations Influence Speechreading Speed for Dynamic Faces

We investigated the influence of task-irrelevant speaker variations on speechreading performance. In three experiments with video digitised faces presented either in dynamic, static-sequential, or static mode, participants performed speeded classifications on vowel utterances (German vowels /u/ and /i/). A Garner interference paradigm was used, in which speaker identity was task-irrelevant but could be either correlated, constant, or orthogonal to the vowel uttered. Reaction times for facial speech classifications were slowed by task-irrelevant speaker variations for dynamic stimuli. The results are discussed with reference to distributed models of face perception (Haxby et al, 2000 Trends in Cognitive Sciences4 223–233) and the relevance of both dynamic information and speaker characteristics for speechreading.

Neuronal generator patterns at scalp elicited by lateralized aversive pictures reveal consecutive stages of motivated attention

Event-related potential (ERP) studies have provided evidence for an allocation of attentional resources to enhance perceptual processing of motivationally salient stimuli. Emotional modulation affects several consecutive components associated with stages of affective-cognitive processing, beginning as early as 100-200ms after stimulus onset. In agreement with the notion that the right parietotemporal region is critically involved during the perception of arousing affective stimuli, some ERP studies have reported asymmetric emotional ERP effects. However, it is difficult to separate emotional from non-emotional effects because differences in stimulus content unrelated to affective salience or task demands may also be associated with lateralized function or promote cognitive processing. Other concerns pertain to the operational definition and statistical independence of ERP component measures, their dependence on an EEG reference, and spatial smearing due to volume conduction, all of which impede the identification of distinct scalp activation patterns associated with affective processing. Building on prior research using a visual half-field paradigm with highly controlled emotional stimuli (pictures of cosmetic surgery patients showing disordered [negative] or healed [neutral] facial areas before or after treatment), 72-channel ERPs recorded from 152 individuals (ages 13-68years; 81 female) were transformed into reference-free current source density (CSD) waveforms and submitted to temporal principal components analysis (PCA) to identify their underlying neuronal generator patterns. Using both nonparametric randomization tests and repeated measures ANOVA, robust effects of emotional content were found over parietooccipital regions for CSD factors corresponding to N2 sink (212ms peak latency), P3 source (385ms) and a late centroparietal source (630ms), all indicative of greater positivity for negative than neutral stimuli. For the N2 sink, emotional effects were right-lateralized and modulated by hemifield, with larger amplitude and asymmetry for left hemifield (right hemisphere) presentations. For all three factors, more positive amplitudes at parietooccipital sites were associated with increased ratings of negative valence and greater arousal. Distributed inverse solutions of the CSD-PCA-based emotional effects implicated a sequence of maximal activations in right occipitotemporal cortex, bilateral posterior cingulate cortex, and bilateral inferior temporal cortex. These findings are consistent with hierarchical activations of the ventral visual pathway reflecting subsequent processing stages in response to motivationally salient stimuli.

Right-Hemisphere Memory Superiority: Studies of a Split-Brain Patient

Six experiments explored hemispheric memory differences in a patient who had undergone complete corpus callosum resection The right hemisphere was better able than the left to reject new events similar to originally presented materials of several types, including abstract visual forms, faces, and categorized lists of words Although the left hemisphere is capable of mental manipulation, imagination, semantic priming, and complex language production, these functions are apparently linked to memory confusions—confusions less apparent in the more literal right hemisphere Differences between the left and right hemispheres in memory for new schematically consistent or categorically related events may provide a source of information allowing people to distinguish between what they actually witnessed and what they only inferred

ERP evidence for hemispheric asymmetries in abstract but not exemplar-specific repetition priming

Implicit memory retrieval is thought to be exemplar-specific in the right hemisphere (RH) but abstract in the left hemisphere (LH). Yet, conflicting behavioral priming results illustrate that the level at which asymmetries take effect is difficult to pinpoint. In the present divided visual field experiment, we tried to address this issue by analyzing ERPs in addition to behavioral measures. Participants made a natural/artificial decision on lateralized visual objects that were either new, identical repetitions, or different exemplars of studied items. Hemispheric asymmetries did not emerge in either behavioral or late positive complex (LPC) priming effects, but did affect the process of implicit memory retrieval proper as indexed by an early frontal negativity (N350/(F)N400). Whereas exemplar-specific N350/(F)N400 priming effects emerged irrespective of presentation side, abstract implicit memory retrieval of different exemplars was contingent on right visual field presentation and the ensuing initial stimulus processing by the LH.

ERP evidence for hemispheric asymmetries in exemplar-specific explicit memory access

The right cerebral hemisphere (RH) appears to be more effective in representing visual objects as distinct exemplars than the left hemisphere (LH) which is presumably biased towards coding objects at the level of abstract prototypes. As of yet, relatively little is known about the role that asymmetries in exemplar-specificity play at the level of explicit memory retrieval. In the present study, we addressed this issue by examining hemispheric asymmetries in the putative event-related potential (ERP) correlates of familiarity (FN400) and recollection (LPC). In an incidental study phase, pictures of familiar objects were presented centrally. At test, participants performed a memory inclusion task on identical repetitions and different exemplars of study items as well as new items which were presented in only one visual hemifield using the divided visual field technique. With respect to familiarity, we observed exemplar-specific FN400 old/new effects that were more pronounced for identical repetitions than different exemplars, irrespective of the hemisphere governing initial stimulus processing. In contrast, LPC old/new effects were subject to some hemispheric asymmetries indicating that exemplar-specific recollection was more extensive in the RH than in the LH. This further corroborates the idea that hemispheric asymmetries should not be generalized but need to be distinguished not only in different domains but also at different levels of processing.

Differential Lateralization for Words and Faces: Category or Psychophysics?

This set of three experiments assessed the influence of different psychophysical factors on the lateralization of the N170 event-related potential (ERP) component to words and faces. In all experiments, words elicited a left-lateralized N170, whereas faces elicited a right-lateralized or nonlateralized N170 depending on presentation conditions. Experiment 1 showed that lateralization for words (but not for faces) was influenced by spatial frequency. Experiment 2 showed that stimulus presentation time influenced N170 lateralization independently of spatial frequency composition. Finally, Experiment 3 showed that stimulus size and resolution did not influence N170 lateralization, but did influence N170 amplitude, albeit differentially for words and faces. These findings suggest that differential lateralization for words and faces, at least as measured by the N170, is influenced by spatial frequency (words), stimulus presentation time, and category.

Configurational factors in the perception of faces and non-facial objects: an ERP study

During the viewing of human faces, it is easier to recognize detailed features if the face is presented in an unusual configuration; for example, a split face. The present study used electroencephalography to investigate the brain activity elicited in response to a neutral, inverted, and split face and compared this to the activity produced in response to a non-facial stimulus (a clock). Results showed that the N170 response amplitude was larger and its latency longer during recognition of split and inverted faces as compared to a normal face, whereas no amplitude change was seen for the different clock configurations. However, for the P300 component, larger amplitudes were observed in both the face and the object condition. Taken together, the results suggest that unusual human face presentations are detected at earlier stages than unusual object presentations.

Working memory load for faces modulates P300, N170, and N250r

We used event-related potential (ERP) methodology to examine neural activity associated with visual working memory (WM) for faces. There were two main goals. First, to extend previous findings of P300 load modulation to WM for faces. Second, to examine whether N170 and N250r are also influenced by WM load. Between one and four unfamiliar faces were simultaneously presented for memory encoding. After a 1-sec delay, a target face appeared, and participants had to judge whether this face was part of the previous face array. P300 amplitude decreased as WM load increased, and this P300 suppression was observed at both encoding and retrieval. WM load was also found to modulate other ERPs. The amplitude of the N170 elicited by the target face decreased with load, and this N170 decrease leveled off at load 2, reflecting the behavioral WM capacity of around two faces. In addition, the N250r, observed as an ERP difference for target faces that were present in the encoding array relative to target faces that were absent, was also reduced for higher WM loads. These findings extend previous work by showing that P300 modulation by WM load also occurs for faces. Furthermore, we show, for the first time, that WM load affects the N250r and the early visual N170 component. This suggests that higher visual areas play an important role in WM for faces.

Hemispheric asymmetries in image-specific and abstractive priming of famous faces: evidence from reaction times and event-related brain potentials

We investigated hemispheric differences in image-specific and abstractive immediate repetition priming of famous faces. Participants performed speeded familiarity decisions for centrally presented famous and unfamiliar target faces. Target faces were preceded by lateralized primes (150 ms), presented either in the left or right visual field (LVF or RVF). Primes were either an identical photograph of the famous target face (image-specific priming), a different image of the famous target face (abstractive priming) or a different familiar face (unprimed condition). Reaction times (RTs) revealed significant effects of priming for both image-specific and abstractive priming overall. In addition, image-specific priming was more than twice the magnitude for targets following LVF primes as compared to RVF primes. By contrast, no hemispheric differences emerged for abstractive face priming across different images. Whereas ERPs revealed no evidence that priming affected the N170 component, both image-specific and abstractive priming significantly modulated the amplitudes of a right temporal N250r and a parietal N400 component. Behavioural and electrophysiological evidence for hemispheric differences in image-specific and abstractive face priming are discussed with respect to current theories of how the human left and right ventral temporal cortices represent abstractive and form-specific visual information.

Neural correlates of intentional and incidental recognition of famous faces

Event-related potentials (ERPs) were used to study the relationship between intentional and incidental recognition of famous faces. Intentional and incidental recognition were operationally defined as repeated presentations of targets and nontargets within a modified Sternberg task. These repetitions elicited temporally and topographically distinct ERP modulations. A repetition effect around 300 ms (ERE/N250r) and a preceding modulation did not differ between intentional and incidental recognition, whereas a following repetition effect (LRE/N400) around 500 ms showed differences between incidental and intentional recognition. These results show that during the first few hundred milliseconds intentional and incidental face recognition relate to similar processing, indicating that familiar faces are recognized even when their identification is not required.

The peculiarity of the right-hemisphere function in depression: solving the paradoxes

Depression is characterized by functional insufficiency of the right hemisphere combined with its physiological overactivation. This paradox can be solved in the frame of the general concept of brain laterality. According to the present assumption, the left hemisphere organizes any information in an unambiguous monosemantic context, and this process requires an additional activation of the brain cortex in order to restrict natural relationships between objects and events. On the contrary, the right hemisphere organizes any information in the polysemantic context based on the simultaneous capture of the numerous natural relationships between elements of information. In healthy creative subjects this process does not require additional physiological activation of the cortex. In depression the physiological overactivation of the right hemisphere reflects the unsuccessful effort to overcome its functional insufficiency.

Human brain potential correlates of repetition priming in face and name recognition

We investigated repetition priming in the recognition of famous people by recording event-related brain potentials (ERPs) and reaction times (RTs). Participants performed speeded two-choice responses depending on whether or not a stimulus showed a famous person. In Experiment 1, a facilitation was found in RTs to famous (but not to unfamiliar) faces when primed by the same face shown in an earlier priming phase of the experiment. In ERPs, an influence of repetition priming was observed neither for the N170 nor for a temporal N250 component which in previous studies had been shown to be sensitive to immediate face repetitions. ERPs to primed unfamiliar faces were more negative over right occipitotemporal areas than those to unprimed faces, but this effect was specific for repetitions of the same image, consistent with recent findings. In contrast, ERPs to primed familiar faces were more positive than those to unprimed faces at parietal sites from 500-600 ms after face onset, and these priming effects were comparable regardless of whether the same or a different image of the celebrity had served as prime. In Experiment 2, similar results were found for name recognition-a facilitation in RTs to primed familiar but not unfamiliar names, and a parietal positivity to primed names around 500-600 ms. ERP repetition effects showed comparable topographies for faces and names, consistent with the idea of a common underlying source. With reference to current models of face recognition, we suggest that these ERP repetition effects for familiar stimuli reflect a change in post-perceptual representations for people, rather than a neural correlate of recognition at a perceptual level.

A serial test of the laterality of familiar face recognition

The purpose of the present study was to address the issue of laterality of familiar face recognition. Seventy-two participants judged familiar faces presented laterally or centrally for their "faceness," familiarity, occupation, and name (which represent four stages of familiar face processing) using one of three response modes-verbal, manual, or combined. The pattern of reaction times (RTs) implied a serial process of familiar face recognition. Centrally presented stimuli were recognized faster than laterally presented stimuli. No RT differences were found between the left and right visual fields (VFs) across all judgments and response modes. The findings were interpreted as supporting the notion that there are no significant hemispheric differences in familiar face recognition.

Interhemispheric cooperation for familiar but not unfamiliar face processing

Evidence for interhemispheric cooperation during language processing has been demonstrated for words, but not for meaningless pseudowords. Specifically, responses were found to be faster and more accurate when identical copies of a word were presented bilaterally to both hemispheres, relative to unilateral single presentations. This bilateral advantage for words seems to be a robust effect in normals. The present study addressed the question of whether the bilateral advantage is restricted to lexical material or whether it is a more global phenomenon occurring for meaningful material in general. Thirty healthy participants performed a familiarity decision in which one copy of familiar and unfamiliar faces was presented tachistoscopically to the right visual hemifield (RVF), the left visual hemifield (LVF) or simultaneously to both visual hemifields (bilateral condition, BVF). We obtained a highly significant familiarity by visual field interaction(P < 0.006) showing that only for familiar faces, a bilateral advantage was obtained. Unfamiliar face processing did not yield a bilateral advantage. We conclude that interhemispheric cooperation only occurs for meaningful material for which cortical representations can be assumed.

Age-related slowing in face and name recognition: evidence from event-related brain potentials

Age-related slowing in recognizing famous names and faces was investigated with event-related brain potentials (ERPs). In a group of young adults, item repetition induced early (220-340 ms) and late (400-700 ms) ERP modulations, apparently signaling the access to, respectively, domain-specific representations of faces and names and domain-general semantic knowledge about the persons. These repetition effects and other ERP components were then used as process-specific time markers in middle-aged and elderly participants. For both faces and names, the elderly participants' responses were slowed, but repetition priming in reaction times was not. The ERP latencies suggested that most of the age-related slowing occurred in the access to domain-specific representations and during response decision, whereas sensory and perceptual processing was largely spared.

Personal names and the human right hemisphere: an illusory link?

Names are thought to be represented in the brain differently from common nouns. Although this idea is supported by both theoretical and empirical arguments, the brain areas that are relevant for the recognition of personal names-and in particular the extent of right hemisphere involvement-remain controversial. We investigated the hypothesis that, unlike common nouns, personal names are represented preferentially by the right hemisphere (D. Van Lancker, 1991; D. Van Lancker et al., 1991; C. Ohnesorge & D. Van Lancker, 1999). Participants performed lexical decisions to common nouns and pseudowords (Experiment 1) or familiarity decisions to personal names (Experiment 2), which were presented briefly to the left or right visual fields. Asymmetries were small or absent for both pseudowords and unfamiliar names. For familiar names, both reaction times and error rates revealed strong advantages for the right visual field/left hemisphere (RVF/LH), which were comparable to the asymmetries for nouns. Familiar personal names may be represented by brain systems that differ from those representing common nouns, but current evidence does not suggest a distinct contribution of the right hemisphere to these brain systems.

Face and Word Recognition in Patients with Left and Right Hemispheric Lesions: Evidence from Reaction Times and ERPs

Abstract: We recorded reaction times (RTs) and event-related potentials (ERPs) in patients with unilateral lesions during a memory search task. Participants memorized faces or abstract words, which were then recognized among new ones. The RT deficit found in patients with left brain damage (LBD) for words increased with memory set size, suggesting that their problem relates to memory search. In contrast, the RT deficit found in patients with RBD for faces was apparently related to perceptual encoding, a conclusion also supported by their reduced P100 ERP component. A late slow wave (720-1720 ms) was enhanced in patients, particularly to words in patients with LBD, and to faces in patients with RBD. Thus, the slow wave was largest in the conditions with most pronounced performance deficits, suggesting that it reflects deficit-related resource recruitment.

Implicit perception in patients with visual neglect: lexical specificity in repetition priming

Twelve patients with left-sided neglect, five patients with left-sided hemianopia, and 16 matched controls performed lexical decisions for foveally presented target words or pseudowords. Target words or pseudowords could be preceded by the same stimuli, presented for 150 ms as primes in either the left (LVF) or the right (RVF) visual hemifield. Primes in the RVF caused similar levels of priming across the three groups. In contrast, whereas primes in the affected LVF did not cause priming in hemianopic patients, patients with neglect showed highly significant priming from LVF primes, and the level of priming from these neglected primes was increased relative to normal control levels. This priming effect was attributed to lexical access, because no repetition priming was observed for pronounceable pseudowords. In striking contrast to the priming results, neglect patients' explicit recognition performance for these prime stimuli was at chance for the left hemifield, although it was near perfect for the right hemifield. This demonstrates that the observed priming effects in the left hemifield were indeed caused by stimuli that were not explicitly perceived.

Neuropsychological impairments in the recognition of faces, voices, and personal names

In order to determine the dissociability of face, voice, and personal name recognition, we studied the performance of 36 brain-lesioned patients and 20 control subjects. Participants performed familiarity decisions for portraits, voice samples, and written names of celebrities and unfamiliar people. In those patients who displayed significant impairments in any of these tests, the specificity of these impairments was tested using corresponding object recognition tests (with pictures of objects, environmental sounds, or written common words as stimuli). The results showed that 58% of the patients were significantly impaired in at least one test of person recognition. Moreover, 28% of the patients showed impairments that appeared to be specific for people (i.e., performance was preserved in the corresponding object recognition test). Three patients showed a deficit that appeared to be confined to the recognition of familiar voices, a pattern that was not described previously. Results were generally consistent with the assumption that impairments in face, voice, and name recognition are dissociable from one another. In contrast, there was no clear evidence for a dissociability between deficits in face and voice naming. The results further suggest that (a) impairments in person recognition after brain lesions may be more common than was thought previously and (b) the patterns of impairment that were observed can be interpreted using current cognitive models of person recognition (Bruce & Young, 1986; Burton, Bruce, & Johnston, 1990).

III. Electrophysiological studies of face processing in Williams syndrome

Williams Syndrome (WMS) is a genetically based disorder characterized by pronounced variability in performance across different domains of cognitive functioning. This study examined brain activity linked to face-processing abilities, which are typically spared in individuals with WMS. Subjects watched photographic pairs of upright or inverted faces and indicated if the second face matched or did not match the first face. Results from a previous study with normal adults showed dramatic differences in the timing and distribution of ERP effects linked to recognition of upright and inverted faces. In normal adults, upright faces elicited ERP differences to matched vs. mismatched faces at approximately 320 msec (N320) after the onset of the second stimulus. This "N320" effect was largest over anterior regions of the right hemisphere. In contrast, the mismatch/match effect for inverted faces consisted of a large positive component between 400 and 1000 msec (P500) that was largest over parietal regions and was symmetrical. In contrast to normal adults, WMS subjects showed an N320-mismatch effect for both upright and inverted faces. Additionally, the WMS subjects did not display the N320 right-hemisphere asymmetry observed in the normal adults. WMS subjects also displayed an abnormally small negativity at 100 msec (N100) and an abnormally large negativity at 200 msec (N200) to both upright and inverted faces. This ERP pattern was observed in all subjects with WMS but was not observed in the normal controls. These results may be linked to increased attention to faces in subjects with WMS and might be specific to the disorder. These results were consistent with our ERP studies of language processing in WMS, which suggested abnormal cerebral specialization for spared cognitive functions in individuals with WMS.

Handedness and P300 from auditory stimuli

The P3(00) event-related potential (ERP) was elicited in 20 left- and 20 right-handed normal young adult male subjects using a simple auditory stimulus discrimination task. P3 amplitude from the target stimuli was larger at anterior electrode sites for left- compared to right-handed subjects. P3 latency from the standard stimuli was shorter for left- compared to right-handers. The N1, P2, and N2 components generally demonstrated similar handedness effects. The relationship of ERP amplitude and handedness to anatomical variables and cognitive factors is discussed.

Event-related potential (ERP) asymmetries to emotional stimuli in a visual half-field paradigm

To investigate the hypothesis of a right hemispheric superiority in negative emotional processing, event-related potentials (ERPs) were recorded from 17 sites (Fz, Cz, Pz, F3/4, F7/8, C3/4, T7/8, P3/4, P7/8, O1/2) in a visual half-field paradigm. While maintaining fixation, right-handed women viewed pictures of patients with dermatological diseases before (negative) and after (neutral) cosmetic surgery. A principal components analysis with Varimax rotation performed on ERPs revealed factors identified as N1, N2, early P3, late P3, and slow wave. Repeated measures analyses of variance performed on factor scores revealed a significant effect of emotional content for all factors except for N1. However, asymmetries in emotional processing were restricted to N2 and early P3, with maximal effects over the right parietal region. N2-P3 amplitude was augmented for negative and reduced for neutral stimuli over right hemisphere regions. Visual field presentation interacted with these asymmetries in enhancing amplitudes contralaterally for early but ipsilaterally for late ERP components. Overall, findings for N2 and P3 support theories of an asymmetry in emotional processing.

On the utility of P3 latency as an index of mental chronometry

The stimulus evaluation view on P3 latency holds that P3 latency mainly reflects stimulus-processing time, in contrast to response-processing time. A review of the experimental evidence, however, leads to the conclusion that P3 is not a sensitive tool for separating between stimulus- and response-related processes. Rather, it appears that P3 latency is a sensitive index of any response-time changes when response times in the fast condition are brief, with its sensitivity decreasing when response times in the fast condition get longer. This regularity was confirmed by a detailed analysis of the published evidence from Sternberg's task and was not attributable to speed-accuracy trade-off or to different methods of parametrization. The structures generating the scalp P3b are involved both in stimulus processing and in response selection. Response selection may exert its effect on P3 in one of two ways; either directly, fully delaying P3 latency, or affecting a second P3 component (P-CR) only, thus having an attenuated effect on P3 latency.

Electrophysiological Studies of Face Perception in Humans

Event-related potentials (ERPs) associated with face perception were recorded with scalp electrodes from normal volunteers. Subjects performed a visual target detection task in which they mentally counted the number of occurrences of pictorial stimuli from a designated category such us butterflies. In separate experiments, target stimuli were embedded within a series of other stimuli including unfamiliar human faces and isolated face components, inverted faces, distorted faces, animal faces, and other nonface stimuli. Unman faces evoked a negative potential at 172 msec (N170), which was absent from the ERPs elicited by other animate and inanimate nonface stimuli. N170 was largest over the posterior temporal scalp and was larger over the right than the left hemisphere. N170 was delayed when faces were presented upside-down, but its amplitude did not change. When presented in isolation, eyes elicited an N170 that was significantly larger than that elicited by whole faces, while noses and lips elicited small negative ERPs about 50 msec later than N170. Distorted human faces, in which the locations of inner face components were altered, elicited an N170 similar in amplitude to that elicited by normal faces. However, faces of animals, human hands, cars, and items of furniture did not evoke N170. N170 may reflect the operation of a neural mechanism tuned to detect (as opposed to identify) human faces, similar to the "structural encoder" suggested by Bruce and Young (1986). A similar function has been proposed for the face-selective N200 ERP recorded from the middle fusiform and posterior inferior temporal gyri using subdural electrodes in humans (Allison, McCarthy, Nobre, Puce, & Belger, 1994c). However, the differential sensitivity of N170 to eyes in isolation suggests that N170 may reflect the activation of an eye-sensitive region of cortex. The voltage distribution of N170 over the scalp is consistent with a neural generator located in the occipitotemporal sulcus lateral to the fusiform/inferior temporal region that generates N200.

Event-related brain potentials in a varied-set memory search task: a reconsideration

Event-related potentials (ERPs) were recorded in 19 healthy subjects as they completed two Sternberg (1969, American Scientist, 57, 421-457) memory tests. In separate sessions, either single digits (i.e., 0-9) or 10 abstract figures were used as stimuli. In both sessions, memory set sizes were 1 (M1), 2 (M2), or 4 (M4). The amplitude and latency of the parietal P400 and the frontocentral negativity preceding P400 varied significantly with set size, but only between M1 and M2, whereas reaction time increased dramatically from M1 to M2 and from M2 to M4. These findings challenge previous assertions that the ERPs reflect aspects of the exhaustive serial search proposed by Sternberg. A late parietal positivity (P620), which failed to vary with set size, was larger in response to figures than to digits and may represent the search for, or utilization of, semantic traces of the stimuli.

Visual acuity and hemispheric asymmetries in pigeons

Pigeons and domestic chicks tested under monocular conditions in pattern distinction tasks generally show higher discrimination performances with the right eye seeing. At least two different mechanisms could mediate this asymmetry: the dominance of the right eye could either be due to hemispheric differences in the cognitive operations performed during these tasks, or may reflect a lateralization in the spatial frequency discrimination capacity of the left and the right eye system. The aim of the present study was to decide between these two hypotheses. Therefore nine adult homing pigeons (Columba livia) were tested with the left or the right eye seeing in a visual acuity task using high-contrast square wave gratings. Visual acuity, defined as the spatial frequency at which the psychometric function crossed the 75% correct line was virtually identical for the two eyes with 6.6 for the left and 6.4 c/deg for the right eye. Thus, visual lateralization as demonstrated in various pattern discrimination tasks seems not to depend on asymmetries in acuity but probably reflects hemispheric differences in the visual processing mode.

Event-related potentials and models of performance asymmetries in face and word recognition

Performance asymmetries in divided visual field studies may be ascribed either to hemispheric differences in processing efficiency or to the costs of interhemispheric transfer towards the superior hemisphere. In order to distinguish between these alternatives, event-related potentials (ERPs) were recorded while subjects had to recognize laterally presented faces or words. As expected, behavioural left- and right-field advantages were observed for faces and words, respectively. Regardless of stimulus type, the ERPs displayed a sustained temporo-parietal negativity over the hemisphere stimulated directly via the contralateral hemifield. Both this hemifield-dependent negativity (HDN) and the performance asymmetries diminished to insignificance when the same stimuli were presented but subjects simply made a left-right decision about stimulus location. We conclude that the HDN is no obligatory, stimulus-bound ERP component but depends on lateralized cognitive processing. The stimulus-unspecific and time-invariant topography of the HDN might indicate that it relates to the allocation of processing resources to the directly stimulated hemisphere. The findings suggest that both faces and words were processed predominantly in the directly stimulated hemisphere, supporting an efficiency explanation of the performance asymmetries.

Memory search for faces and digits in patients with unilateral brain lesions

To clarify whether the impairment of right-brain-damaged (RBD) patients in face recognition is related to perceptual or mnestic processing stages, we tested unilaterally lesioned patients and controls in a Sternberg-type memory search task. Subjects had to memorize sets of 1, 2, or 3 faces (or, in a control condition, digits) and were then to recognize these memorized stimuli among new ones by speeded choice reactions. In this task, deficits in stimulus encoding and memory search should show up in increased intercepts and slopes, respectively, of the RT function over memory set size. A face-specific impairment of the RBD patient group, consisting in longer reaction times and higher error rates, was confirmed but could not be unequivocally attributed to either stimulus encoding or memory search. However, inspection of individual data suggested that (1) some RBD patients are virtually unimpaired in face recognition and (2) if impairment after right hemisphere damage is present, it may selectively affect either stimulus encoding or memory search.