Truett Allison

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 as 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. Human 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.

Social perception from visual cues: role of the STS region

Social perception refers to initial stages in the processing of information that culminates in the accurate analysis of the dispositions and intentions of other individuals. Single-cell recordings in monkeys, and neurophysiological and neuroimaging studies in humans, reveal that cerebral cortex in and near the superior temporal sulcus (STS) region is an important component of this perceptual system. In monkeys and humans, the STS region is activated by movements of the eyes, mouth, hands and body, suggesting that it is involved in analysis of biological motion. However, it is also activated by static images of the face and body, suggesting that it is sensitive to implied motion and more generally to stimuli that signal the actions of another individual. Subsequent analysis of socially relevant stimuli is carried out in the amygdala and orbitofrontal cortex, which supports a three-structure model proposed by Brothers. The homology of human and monkey areas involved in social perception, and the functional interrelationships between the STS region and the ventral face area, are unresolved issues.

Face-specific processing in the human fusiform gyrus

The perception of faces is sometimes regarded as a specialized task involving discrete brain regions. In an attempt to identi

The scalp topography of human somatosensory and auditory evoked potentials

face-specific cortex, we used functional magnetic resonance imaging (fMRI) to measure activation evoked by faces presented in a continuously changing montage of common objects or in a similar montage of nonobjects. Bilateral regions of the posterior fusiform gyrus were activated by faces viewed among nonobjects, but when viewed among objects, faces activated only a focal right fusiform region. To determine whether this focal activation would occur for another category of familiar stimuli, subjects viewed flowers presented among nonobjects and objects. While flowers among nonobjects evoked bilateral fusiform activation, flowers among objects evoked no activation. These results demonstrate that both faces and flowers activate large and partially overlapping regions of inferior extrastriate cortex. A smaller region, located primarily in the right lateral fusiform gyrus, is activated specifically by faces.

Brain stem auditory, pattern-reversal visual, and short-latency somatosensory evoked potentials: Latencies in relation to age, sex, and brain and body size

The waveform and topography of components of the scalp recorded somatosensory evoked poal (AEP) to click stimulation of the right ear, were determined for scalp electrode locations of the 10-20 system and for locations at the eye, mastoids, and posterior neck. Twenty-one SEP and twenty-two AEP components were analyzed. Differentiation of neurogenic and myogenic components was attempted on the basis of localization and variability. Some components of extracranial origin, apparently originating in frontal musculature, were small in most experienced subjects and large in most experimentally naive subjects. These and other presumptive myogenic potentials can distort adjacent neurogenic components. These data should aid in predicting SEP and AEP characteristics and in assessing myogenic distortion of neurogenic components.

Recovery functions of somatosensory evoked responses in man

To determine standards of normality for auditory, somatosensory and visual evoked potentials commonly used in the assessment of neurological disease, 8 AEP, 1 VEP and 12 SEP components were recorded to stimulation of left and right ears, eyes, and median nerves in 286 normal subjects ranging in age from 4 to 95 years. Peak and interpeak latencies, and left-right differences in latency, were analyzed as a function of age, sex, and estimates of brain and body size. Major features of the results were: (1) Peak latencies of all components showed statistically significant increases in latency with age except that VEP P100 latency decreased significantly between 4 and 19 years and did not change between 20 and 59 years. (2) In adults the peak latencies of all components were significantly later in males than in females. For AEPs and VEPs these differences were explained by sex differences in brain size, and for adult SEPs were explained by sex differences in arm and shoulder dimensions. No significant sex differences in VEP and SEP latencies were seen in children. (3) Most interpeak latencies showed significant differences in relation to age or sex. (4) Age and sex are useful predictors of latency for nearly all peak and interpeak latencies; in addition, height is a useful predictor of SEP peak latencies. (5) Left-right latency differences showed little age-related, and no sex-related, change. The interlaboratory use of these or other normative data was discussed. It was concluded that these AEP and SEP norms can probably be used in other laboratories if stimulating and recording conditions are similar. However, VEP results are difficult to transfer due to the poorly understood effects of variation in stimulus conditions. Some issues regarding the optimal characterization of norms were also discussed.

Eyes first! Eye processing develops before face processing in children

Abstract In human subjects the cerebral response evoked by a brief shock to median nerve is a complex potential lasting approximately 500 msec. Recovery functions of various components of the response were obtained in five subjects. A special-purpose analog computer extracted average evoked responses from the ongoing activity and separated conditioning and test responses which would normally overlap at brief interstimulus intervals. Comparison of these results with previous results in animal and man leads to the following analysis of somatosensory evoked response components: 1. 1. Component 1, a brief diphasic potential appearing 20 msec after the stimulus, exhibits considerable recovery as early as 50 msec and is essentially recovered in 200 msec. Its recovery and other criteria indicate that it is a thalamo-cortical radiation response. 2. 2. The behavior of component 2 during recovery suggests that it may be, or may contain, the classical primary positive response. 3. 3. Response components designated 3 and 4 require at least 1 sec for complete recovery. Although the present results are inconclusive, a tentative hypothesis would relate 3 to the “association” response and 4 to the “secondary” response previously described in cat and monkey. 4. 4. Component 5, a long-latency diphasic potential, requires several seconds for complete recovery. This and other characteristics of the response clearly relate it to the “non-specific” response or “V potential” previously described in man. 5. 5. Click-shock recovery functions showed that only component 5 was affected by a conditioning click. The degree of subnormality was roughly half that for a conditioning shock, indicating that this component is only partially “non-specific” to a given modality of sensory stimulation.

A comparative analysis of short-latency somatosensory evoked potentials in man, monkey, cat, and rat

Faces and eyes are critical social stimuli which adults process with ease, but how this expertise develops is not yet understood. Neural changes associated with face and eye processing were investigated developmentally using ERPs (N170), in 128 subjects (4–15 year olds and adults). Stimuli included upright faces to assess configural processing, eyes and inverted faces to assess feature-based processing. N170 was present in the youngest children with similar patterns of face sensitivity seen in adults. Development of N170 to upright faces continued until adulthood, suggesting slow maturation of configural processing. In contrast, N170 was shorter latency and much larger to eyes than faces in children and was mature by 11 years, suggesting the early presence of an eye detector, with a rapid maturational course.

The scalp topography of human visual evoked potentials

Abstract The spatial and temporal properties of the early portion of the somatosensory evoked potential (SEP) were assessed in normal human subjects and in monkeys, cats, and rats. When recorded from comparable loci, the SEP evoked by median nerve stimulation was similar in waveform and topography in all species, suggesting direct correspondences between human and animal components. To assess the neural origins of this activity, simultaneous surface and depth recordings were obtained from cats and monkeys, and the effects of lethal doses of sodium pentobarbital were studied in all animals. Components identifiable in most humans and in the animals tested, and the structures thought to be their primary generators, are: N10, peripheral nerve at the level of the brachial plexus; N12a and N12b, primary afferent fibers at caudal and rostral levels of the cervical cord; N13a, dorsal horn; N13b, cuneate nucleus; N14, medial lemniscus; P15, n. ventralis posterolateralis; P16 and P18, uncertain; N20 or P20, primary somatosensory cortex.

Erps evoked by viewing facial movements.

Abstract The topography of all identifiable components of the scalp-recorded visual evoked potential (VEP) to unpatterned centrally-viewed light flashes presented in Maxwellian view to the right eye was determined for all scalp electrode locations of the 10–20 system and for locations at the right eye, mastoids, and posterior neck. Twenty-two components were analyzed. Of these, six were regarded as electroretinographic, one as myogenic, and the rest as neurogenic. Supplementary analysis suggested that VEP components and their topography are similar whether evoked by unpatterned flashes presented in Maxwellian view, by unpatterned stroboscopic flashes, or by patterned, flashes, but not by pattern reversal. Potentials thought to originate in periorbital musculature were evoked by somatosensory and auditory as well as visual stimuli. They were larger in inexperienced than in experienced subjects, were larger to unpredictable than to predictable stimuli, but did not differ whether the subject was in a Maxwellian view apparatus or seated in a comfortable chair. These potentials can distort recordings of presumptive neurogenic components in the same latency range. These data should aid in predicting VEP characteristics and in assessing myogenic contamination of neurogenic activity. Components thought to be analogous in the somatosensory, auditory and visual modalities were discussed.