Jodie Davies-Thompson

Internal and External Features of the Face Are Represented Holistically in Face-Selective Regions of Visual Cortex

The perception and recognition of familiar faces depends critically on an analysis of the internal features of the face (eyes, nose, mouth). We therefore contrasted how information about the internal and external (hair, chin, face outline) features of familiar and unfamiliar faces is represented in face-selective regions. There was a significant response to both the internal and external features of the face when presented in isolation. However, the response to the internal features was greater than the response to the external features. There was significant adaptation to repeated images of either the internal or external features of the face in the fusiform face area (FFA). However, the magnitude of this adaptation was greater for the internal features of familiar faces. Next, we asked whether the internal features of the face are represented independently from the external features. There was a release from adaptation in the FFA to composite images in which the internal features were varied but the external features were unchanged, or when the internal features were unchanged but the external features varied, demonstrating a holistic response. Finally, we asked whether the holistic response to faces could be influenced by the context in which the face was presented. We found that adaptation was still evident to composite images in which the face was unchanged but body features were varied. Together, these findings show that although internal features are important in the neural representation of familiar faces, the faces internal and external features are represented holistically in face-selective regions of the human brain.

Intra- and interhemispheric connectivity between face-selective regions in the human brain.

Neuroimaging studies have revealed a number of regions in the human brain that respond to faces. However, the way these regions interact is a matter of current debate. The aim of this study was to use functional MRI to define face-selective regions in the human brain and then determine how these regions interact in a large population of subjects (n = 72). We found consistent face selectivity in the core face regions of the occipital and temporal lobes: the fusiform face area (FFA), occipital face area (OFA), and superior temporal sulcus (STS). Face selectivity extended into the intraparietal sulcus (IPS), precuneus (PCu), superior colliculus (SC), amygdala (AMG), and inferior frontal gyrus (IFG). We found evidence for significant functional connectivity between the core face-selective regions, particularly between the OFA and FFA. However, we found that the covariation in activity between corresponding face regions in different hemispheres (e.g., right and left FFA) was higher than between different face regions in the same hemisphere (e.g., right OFA and right FFA). Although functional connectivity was evident between regions in the core and extended network, there were significant differences in the magnitude of the connectivity between regions. Activity in the OFA and FFA were most correlated with the IPS, PCu, and SC. In contrast, activity in the STS was most correlated with the AMG and IFG. Correlations between the extended regions suggest strong functional connectivity between the IPS, PCu, and SC. In contrast, the IFG was only correlated with the AMG. This study reveals that interhemispheric as well as intrahemispheric connections play an important role in face perception.

An image-dependent representation of familiar and unfamiliar faces in the human ventral stream

People are extremely proficient at recognizing faces that are familiar to them, but are much worse at matching unfamiliar faces. We used fMR-adaptation to ask whether this difference in recognition might be reflected by an image-invariant representation for familiar faces in face-selective regions of the human ventral visual processing stream. Consistent with models of face processing, we found adaptation to repeated images of the same face image in the fusiform face area (FFA), but not in the superior-temporal face region (STS). To establish if the neural representation in the FFA was invariant to changes in view, we presented different images of the same face. Contrary to our hypothesis, we found that the response in the FFA to different images of the same person was the same as the response to images of different people. A group analysis showed a distributed pattern of adaptation to the same image of a face, which extended beyond the face-selective areas, including other regions of the ventral visual stream. However, this analysis failed to reveal any regions showing significant image-invariant adaptation. These results suggest that information about faces is represented in a distributed network using an image-dependent neural code.

Image-Invariant Responses in Face-Selective Regions Do Not Explain the Perceptual Advantage for Familiar Face Recognition

The ability to recognize familiar faces across different viewing conditions contrasts with the inherent difficulty in the perception of unfamiliar faces across similar image manipulations. It is widely believed that this difference in perception and recognition is based on the neural representation for familiar faces being less sensitive to changes in the image than it is for unfamiliar faces. Here, we used an functional magnetic resonance-adaptation paradigm to investigate image invariance in face-selective regions of the human brain. We found clear evidence for a degree of image-invariant adaptation to facial identity in face-selective regions, such as the fusiform face area. However, contrary to the predictions of models of face processing, comparable levels of image invariance were evident for both familiar and unfamiliar faces. This suggests that the marked differences in the perception of familiar and unfamiliar faces may not depend on differences in the way multiple images are represented in core face-selective regions of the human brain.

Functional organisation of visual pathways in a patient with no optic chiasm.

Congenital achiasma offers a rare opportunity to study reorganization and inter-hemispheric communication in the face of anomalous inputs to striate cortex. We report neuroimaging studies of a patient with seesaw nystagmus, achiasma, and full visual fields. The subject underwent structural magnetic resonance imaging (MRI), diffusion tensor imaging (DTI) studies, and functional MRI (fMRI) using monocular stimulation with checkerboards, motion, objects and faces, as well as retinotopic quadrantic mapping. Structural MRI confirmed the absence of an optic chiasm, which was corroborated by DTI tractography. Lack of a functioning decussation was confirmed by fMRI that showed activation of only ipsilateral medial occipital cortex by monocular stimulation. The corpus callosum was normal in size and anterior and posterior commissures were identifiable. In terms of the hierarchy of visual areas, V5 was the lowest level region to be activated binocularly, as were regions in the fusiform gyri responding to faces and objects. The retinotopic organization of striate cortex was studied with quadrantic stimulation. This showed that, in support of recent findings, rather than projecting to an ectopic location contiguous with the normal retinotopic map of the ipsilateral temporal hemi-retina, the nasal hemi-retinas representation overlapped that of the temporal hemi-retina. These findings show that congenital achiasma can be an isolated midline crossing defect, that information transfer does not occur in early occipital cortex but at intermediate and higher levels of the visual hierarchy, and that the functional reorganisation of striate cortex in this condition is consistent with normal axon guidance by a chemoaffinity gradient.

Neuroanatomic correlates of the feature-salience hierarchy in face processing: An fMRI -adaptation study

Previous fMRI studies suggest that faces are represented holistically in human face processing regions. On the other hand, behavioral studies have also shown that some facial features are more salient than others for face recognition: the neural basis of this feature-salience hierarchy is not known. We used fMRI-adaptation together with a behavioral discrimination task and an ideal observer analysis to ask (1) whether different face parts contribute different amounts to the neural signal in face responsive regions, and (2) whether this response correlates more with the behavioral performance of human subjects or with the physical properties of the face stimuli. Twenty-three subjects performed a same/different discrimination experiment to characterize their ability to detect changes to different face parts. The same subjects underwent an fMRI-adaptation study, in which limited portions of the faces were repeated or changed between alternating stimuli. The behavioral study showed high efficiency in identity discrimination when the whole face, top half, or eyes changed, and low efficiency when the bottom half, nose, or mouth changed. During fMRI, there was a release of adaptation in the right and left fusiform face area (FFA) with changes to the whole face, top face-half, or the eyes. Changes to the bottom half, nose or mouth did not result in a significant release of adaptation in the right FFA, although bottom-half changes resulted in a release of adaptation in the left FFA. Adaptation in the right and left FFA and the right pSTS was correlated with human perceptual efficiency but not with ideal observer measures of the physical image differences between face parts. The feature-salience hierarchy of human face perception is therefore reflected in the activity in the right and left FFA and right pSTS, further supporting the key role of these structures in our perceptual experience of faces.

Representation of visual symbols in the visual word processing network

BACKGROUND Previous studies have shown that word processing involves a predominantly left-sided occipitotemporal network. Words are a form of symbolic representation, in that they are arbitrary perceptual stimuli that represent other objects, actions or concepts. Lesions of parts of the visual word processing network can cause alexia, which can be associated with difficulty processing other types of symbols such as musical notation or road signs. OBJECTIVE We investigated whether components of the visual word processing network were also activated by other types of symbols. METHOD In 16 music-literate subjects, we defined the visual word network using fMRI and examined responses to four symbolic categories: visual words, musical notation, instructive symbols (e.g. traffic signs), and flags and logos. For each category we compared responses not only to scrambled stimuli, but also to similar stimuli that lacked symbolic meaning. RESULTS The left visual word form area and a homologous right fusiform region responded similarly to all four categories, but equally to both symbolic and non-symbolic equivalents. Greater response to symbolic than non-symbolic stimuli occurred only in the left inferior frontal and middle temporal gyri, but only for words, and in the case of the left inferior frontal gyri, also for musical notation. A whole-brain analysis comparing symbolic versus non-symbolic stimuli revealed a distributed network of inferior temporooccipital and parietal regions that differed for different symbols. CONCLUSION The fusiform gyri are involved in processing the form of many symbolic stimuli, but not specifically for stimuli with symbolic content. Selectivity for stimuli with symbolic content only emerges in the visual word network at the level of the middle temporal and inferior frontal gyri, but is specific for words and musical notation.

Reduced Myelin Water in the White Matter Tracts of Patients with Niemann-Pick Disease Type C.

SUMMARY: Previous studies using diffusion tensor imaging to examine white matter in Niemann-Pick disease type C have produced mixed results. However, diffusion tensor imaging does not directly measure myelin and may be affected by other structural changes. We used myelin water imaging to more directly examine demyelination in 2 patients with Niemann-Pick disease type C. The results suggest that this technique may be useful for identifying regional changes in myelination in this condition.

Erasing the face after-effect

Perceptual after-effects decay over time at a rate that depends on several factors, such as the duration of adaptation and the duration of the test stimuli. Whether this decay is accelerated by exposure to other faces after adaptation is not known. Our goal was to determine if the appearance of other faces during a delay period after adaptation affected the face identity after-effect. In the first experiment we investigated whether, in the perception of ambiguous stimuli created by morphing between two faces, the repulsive after-effects from adaptation to one face were reduced by brief presentation of the second face in a delay period. We found no effect; however, this may have been confounded by a small attractive after-effect from the interference face. In the second experiment, the interference stimuli were faces unrelated to those used as adaptation stimuli, and we examined after-effects at three different delay periods. This showed a decline in after-effects as the time since adaptation increased, and an enhancement of this decline by the presentation of intervening faces. An exponential model estimated that the intervening faces caused an 85% reduction in the time constant of the after-effect decay. In conclusion, we confirm that face after-effects decline rapidly after adaptation and that exposure to other faces hastens the re-setting of the system.

Perceptual learning of faces: A rehabilitative study of acquired prosopagnosia

Despite many studies of acquired prosopagnosia, there have been only a few attempts at its rehabilitation, all in single cases, with a variety of mnemonic or perceptual approaches, and of variable efficacy. In a cohort with acquired prosopagnosia, we evaluated a perceptual learning program that incorporated variations in view and expression, which was aimed at training perceptual stages of face processing with an emphasis on ecological validity. Ten patients undertook an 11-week face training program and an 11-week control task. Training required shape discrimination between morphed facial images, whose similarity was manipulated by a staircase procedure to keep training near a perceptual threshold. Training progressed from blocks of neutral faces in frontal view through increasing variations in view and expression. Whereas the control task did not change perception, training improved perceptual sensitivity for the trained faces and generalized to new untrained expressions and views of those faces. There was also a significant transfer to new faces. Benefits were maintained over a 3-month period. Training efficacy was greater for those with more perceptual deficits at baseline. We conclude that perceptual learning can lead to persistent improvements in face discrimination in acquired prosopagnosia. This reflects both acquisition of new skills that can be applied to new faces as well as a degree of overlearning of the stimulus set at the level of 3-D expression-invariant representations.