Xiaokun Xu

Adaptation in the fusiform face area (FFA): Image or person?

Viewing a sequence of faces of two different people results in a greater Blood Oxygenation Level Dependent (BOLD) response in FFA compared to a sequence of identical faces. Changes in identity, however, necessarily involve changes in the image. Is the release from adaptation a result of a change in face identity, per se, or could it be an effect that would arise from any change in the image of a face? Subjects viewed a sequence of two faces that could be of the same or different person, and in the same or different orientation in depth. Critically, the physical similarity of view changes of the same person was scaled, by Gabor-jet differences, to be equivalent to that produced by an identity change. Both person and orientation changes produced equivalent releases from adaptation in FFA (relative to identical faces) suggesting that FFA is sensitive to the physical similarity of faces rather than to the individuals depicted in the images.

Loci of the release from fMRI adaptation for changes in facial expression, identity, and viewpoint

Face recognition involves collaboration of a distributed network of neural correlates. However, how different attributes of faces are represented has remained unclear. We used functional magnetic resonance imaging-adaptation (fMRIa) to investigate the representation of viewpoint, expression, and identity of faces in the fusiform face area (FFA) and the occipital face area (OFA). In an event-related experiment, subjects viewed sequences of two faces and judged whether they depicted the same person. The images could vary in viewpoint, expression and/or identity. Critically, the physical similarity between view-changed and between expression-changed faces of the same person were matched by the Gabor-jet metric, a measure that predicts almost perfectly the effects of image similarity on face discrimination performance. In FFA, changes of identity produced the largest release from adaptation followed by changes of expression; but the release caused by changes of viewpoint was smaller and not reliable. OFA was sensitive only to changes in identity, even when image changes produced by identity variations were matched to those of expression and orientation. These results suggest that FFA is involved in the perception of both identity and expression of faces, a result contrary to the hypothesis of independent processing of changeable and invariant attributes of faces in the face-processing network.

Ha Ha! Versus Aha! A Direct Comparison of Humor to Nonhumorous Insight for Determining the Neural Correlates of Mirth

While humor typically involves a surprising discovery, not all discoveries are perceived as humorous or lead to a feeling of mirth. Is there a difference in the neural signature of humorous versus nonhumorous discovery? Subjects viewed drawings that were uninterpretable until a caption was presented that provided either: 1) a nonhumorous interpretation (or insight) of an object from an unusual or partial view (UV) or 2) a humorous interpretation (HU) of the image achieved by linking remote and unexpected concepts. fMRI activation elicited by the UV captions was a subset of that elicited by the humorous HU captions, with only the latter showing activity in the temporal poles and temporo-occipital junction (linking remote concepts), and medial prefrontal cortex (unexpected reward). Mirth may be a consequence of the linking of remote ideas producing high-and unexpected-activation in association and classical reward areas. We suggest that this process is mediated by opioid activity as part of a system rewarding attention to novel information.

Developmental phonagnosia: Neural correlates and a behavioral marker.

A 20-year old female, AN, with no history of neurological events or detectable lesions, was markedly poorer than controls at identifying her most familiar celebrity voices. She was normal at face recognition and in discriminating which of two speakers uttered a particular sentence. She evidences normal fMRI sensitivity for human speech and non-speech sounds. AN, and two other phonagnosics, were unable to imagine the voices of highly familiar individuals. A region in the ventromedial prefrontal cortex (vmPFC) was differentially activated in controls when imagining familiar celebrity voices compared to imagining non-voice sounds. AN evidenced no differential activation in this area, which has been termed a person identity semantic system. Rather than a deficit in the representation of voice-individuating cues, AN may be unable to associate those cues to the identity of a familiar person. In this respect, the deficit in developmental phonagnosia may bear a striking parallel to developmental prosopagnosia.

A neurocomputational account of the face configural effect

A striking phenomenon in face perception is the configural effect in which a difference in a single part appears more distinct in the context of a face than it does by itself. The face context would be expected to increase search complexity, rendering discrimination more--not less--difficult. Remarkably, there has never been a biologically plausible explanation of this fundamental signature of face recognition.We show that the configural effect can be simply derived from a model composed of overlapping receptive fields (RFs) characteristic of early cortical simple-cell tuning but also present in face-selective areas. Because of the overlap in RFs, the difference in a single part is not only represented in the RFs centered on it but also propagated to larger RFs centered on distant parts of the face. Dissimilarity values computed from the model between pairs of faces and pairs of face parts closely matched the recognition accuracy of human observers who had learned a set of faces composed of composite parts and were tested on wholes (Which is Larry?) and parts (Which is Larry’s nose?). When stimuli were high versus low passed the contributions of different spatial frequency (SF) bands to the configural effect were largely comparable. Therefore, it was the larger RFs rather than the low SFs that accounted for most of the configural effect. The representation explains why, relative to objects, face recognition is so adversely affected by inversion and contrast reversal and why distinctions between similar faces are ineffable.

Neural Correlates of Face Detection

Although face detection likely played an essential adaptive role in our evolutionary past and in contemporary social interactions, there have been few rigorous studies investigating its neural correlates. MJH, a prosopagnosic with bilateral lesions to the ventral temporal-occipital cortices encompassing the posterior face areas (fusiform and occipital face areas), expresses no subjective difficulty in face detection, suggesting that these posterior face areas do not mediate face detection exclusively. Despite his normal contrast sensitivity and visual acuity in foveal vision, the present study nevertheless revealed significant face detection deficits in MJH. Compared with controls, MJH showed a lower tolerance to noise in the phase spectrum for faces (vs. cars), reflected in his higher detection threshold for faces. MJHs lesions in bilateral occipito-temporal cortices thus appear to have produced a deficit not only in face individuation, but also in face detection.

Phonagnosia: A voice homologue to prosopagnosia

Phonagnosia is the inability to individuate people on the basis of their voice and is thus a condition parallel to that of prosopagnosia (Van Lancker & Canter, 1982). We report a case of developmental phonagnosia, AN, the second known to scientists (Garrido et al., 2009), with an exploration of the parallels between her and developmental prosopagnosia. In either condition, the individual shows very poor recognition accuracy of familiar faces or voices, and cannot imagine a familiar face or voice. Whereas controls show greater activation of the ventral medial prefrontal cortex (vmPFC) when imagining voices (vs. nonvoice sounds), AN showed no such activation. This same region shows greater activation to well-known faces versus faces that have just been familiarized through repetition in the course of the experiment. Developmental phonagnosia, as well as developmental prosopagnosia, may thus be conditions in which a person identity node (PIN) (Bruce & Young, 1986) cannot be activated through voice or face input. At the time of testing, AN was a 20-year-old female student at the University of Southern California with high cognitive, conversational, social, and face recognition abilities, and no known neurological insults. When tested on a celebrity voice recognition task, however, she was markedly poorer than controls (a number of whom were perfect) at identifying her most familiar celebrity voices. On each trial of the celebrity voice recognition task, participants would view a display of 1 to 4 celebrity headshots with their names and listen to two 7 s voice clips, one of a celebrity and the other a noncelebrity. The clips were carefully chosen not to provide any individuating information in content. Participants then selected the clip they judged to be that of a celebrity and (for 2 or 4 choices)

A neurocomputational account of the magnitude of face composite effects.

Identical top halves of two faces are more likely to be perceived as different when their different bottom halves are aligned rather than offset. Here, we demonstrate that the magnitude of the offset effect for each face can be predicted from a model of overlapping receptive fields with tuning profiles similar to the hypercolumns of simple cells in V1, although the cells are likely in face-selective areas. Importantly, a single face part (e.g. the left eye) is coded by multiple large receptive fields centered at a distance from the face part (Fig. 1). When different bottom halves are aligned to the identical top halves of faces, the large receptive fields centered on the top half of the face will extend to the differing bottom halves, thus making the top halves of the faces more dissimilar. By offsetting the differing bottom halves from the identical top halves of two faces, the features of the bottom halves no longer activate the large receptive fields centered on the top half of the face, leading to more accurate judgments of the identical top halves as the same (Fig. 2). The retention of early-level visual coding (Yue, Tjan, & Biederman, 2006; Xu, Biederman, & Shah, 2014) and the retinotopic representation of a face template in FFA (de Haas et al., 2014) may explain why the offset effect is unique to faces rather than the parts-based representation of objects. Meeting abstract presented at VSS 2015.

Coding of Visual Stimuli for Size and Animacy

Blocked presentations of objects (Small-Large Crossed w. AnimalsObjects). Subjects performed an oddball (red bounding box aroud image) detection task. Mean referential size of was comparable between animals and objects (Object size from Konkle & Oliva, 2011; Animals fromWikipedia). Animacy: Animals activated ventral lateral temporal cortex, while inanimate objects activated more medial regions of the ventral temporal lobe, and dorsal occipital lobe.