Daniel D. Dilks

Differential selectivity for dynamic versus static information in face-selective cortical regions

Neuroimaging studies have identified multiple face-selective regions in human cortex but the functional division of labor between these regions is not yet clear. A central hypothesis, with some empirical support, is that face-selective regions in the superior temporal sulcus (STS) are particularly responsive to dynamic information in faces, whereas the fusiform face area (FFA) computes the static or invariant properties of faces. Here we directly tested this hypothesis by measuring the magnitude of response in each region to both dynamic and static stimuli. Consistent with the hypothesis, we found that the response to movies of faces was not significantly different from the response to static images of faces from these same movies in the right FFA and right occipital face area (OFA). By contrast the face-selective region in the right posterior STS (pSTS) responded nearly three times as strongly to dynamic faces as to static faces, and a face-selective region in the right anterior STS (aSTS) responded to dynamic faces only. Both of these regions also responded more strongly to moving faces than to moving bodies, indicating that they are preferentially engaged in processing dynamic information from faces, not in more general processing of any dynamic social stimuli. The response to dynamic and static faces was not significantly different in a third face-selective region in the posterior continuation of the STS (pcSTS). The strong selectivity of face-selective regions in the pSTS and aSTS, but not the FFA, OFA or pcSTS, for dynamic face information demonstrates a clear functional dissociation between different face-selective regions, and provides further clues into their function.

The Occipital Place Area Is Causally and Selectively Involved in Scene Perception

Functional magnetic resonance imaging has revealed a set of regions selectively engaged in visual scene processing: the parahippocampal place area (PPA), the retrosplenial complex (RSC), and a region around the transverse occipital sulcus (previously known as “TOS”), here renamed the “occipital place area” (OPA). Are these regions not only preferentially activated by, but also causally involved in scene perception? Although past neuropsychological data imply a causal role in scene processing for PPA and RSC, no such evidence exists for OPA. Thus, to test the causal role of OPA in human adults, we delivered transcranial magnetic stimulation (TMS) to the right OPA (rOPA) or the nearby face-selective right occipital face area (rOFA) while participants performed fine-grained perceptual discrimination tasks on scenes or faces. TMS over rOPA impaired discrimination of scenes but not faces, while TMS over rOFA impaired discrimination of faces but not scenes. In a second experiment, we delivered TMS to rOPA, or the object-selective right lateral occipital complex (rLOC), while participants performed categorization tasks involving scenes and objects. TMS over rOPA impaired categorization accuracy of scenes but not objects, while TMS over rLOC impaired categorization accuracy of objects but not scenes. These findings provide the first evidence that OPA is causally involved in scene processing, and further show that this causal role is selective for scene perception. Our findings illuminate the functional architecture of the scene perception system, and also argue against the “distributed coding” view in which each category-selective region participates in the representation of all objects.

A critical review of the development of face recognition: Experience is less important than previously believed

Historically, it has been argued that face individuation develops very slowly, not reaching adult levels until adolescence, with experience being the driving force behind this protracted improvement. Here, we challenge this view based on extensive review of behavioural and neural findings. Results demonstrate qualitative presence of all key phenomena related to face individuation (encoding of novel faces, holistic processing effects, face-space effects, face-selective responses in neuroimaging) at the earliest ages tested, typically 3–5 years of age and in many cases even infancy. Results further argue for quantitative maturity by early childhood, based on an increasing number of behavioural studies that have avoided the common methodological problem of restriction of range, as well as event-related potential (ERP), but not functional magnetic resonance imaging (fMRI) studies. We raise a new possibility that could account for the discrepant fMRI findings—namely, the use of adult-sized head coils on child-sized heads. We review genetic and innate contributions to face individuation (twin studies, neonates, visually deprived monkeys, critical periods, perceptual narrowing). We conclude that the role of experience in the development of the mechanisms of face identification has been overestimated. The emerging picture is that the mechanisms supporting face individuation are mature early, consistent with the social needs of children for reliable person identification in everyday life, and are also driven to an important extent by our evolutionary history.

Resting-State Neural Activity across Face-Selective Cortical Regions Is Behaviorally Relevant

Interest has increased recently in correlations across brain regions in the resting-state fMRI blood oxygen level-dependent (BOLD) response, but little is known about the functional significance of these correlations. Here we directly test the behavioral relevance of the resting-state correlation between two face-selective regions in human brain, the occipital face area (OFA) and the fusiform face area (FFA). We found that the magnitude of the resting-state correlation, henceforth called functional connectivity (FC), between OFA and FFA correlates with an individuals performance on a number of face-processing tasks, not non-face tasks. Further, we found that the behavioral significance of the OFA/FFA FC is independent of the functional activation and the anatomical size of either the OFA or FFA, suggesting that face processing depends not only on the functionality of individual face-selective regions, but also on the synchronized spontaneous neural activity between them. Together, these findings provide strong evidence that the functional correlations in the BOLD response observed at rest reveal functionally significant properties of cortical processing.

Mirror-Image Sensitivity and Invariance in Object and Scene Processing Pathways

Electrophysiological and behavioral studies in many species have demonstrated mirror-image confusion for objects, perhaps because many objects are vertically symmetric (e.g., a cup is the same cup when seen in left or right profile). In contrast, the navigability of a scene changes when it is mirror reversed, and behavioral studies reveal high sensitivity to this change. Thus, we predicted that representations in object-selective cortex will be unaffected by mirror reversals, whereas representations in scene-selective cortex will be sensitive to such reversals. To test this hypothesis, we ran an event-related functional magnetic resonance imaging adaptation experiment in human adults. Consistent with our prediction, we found tolerance to mirror reversals in one object-selective region, the posterior fusiform sulcus, and a strong sensitivity to these reversals in two scene-selective regions, the transverse occipital sulcus and the retrosplenial complex. However, a more posterior object-selective region, the lateral occipital sulcus, showed sensitivity to mirror reversals, suggesting that the sense information that distinguishes mirror images is represented at earlier stages in the object-processing hierarchy. Moreover, one scene-selective region (the parahippocampal place area or PPA) was tolerant to mirror reversals. This last finding challenges the hypothesis that the PPA is involved in navigation and reorientation and suggests instead that scenes, like objects, are processed by distinct pathways guiding recognition and action.

Human Adult Cortical Reorganization and Consequent Visual Distortion

Neural and behavioral evidence for cortical reorganization in the adult somatosensory system after loss of sensory input (e.g., amputation) has been well documented. In contrast, evidence for reorganization in the adult visual system is far less clear: neural evidence is the subject of controversy, behavioral evidence is sparse, and studies combining neural and behavioral evidence have not previously been reported. Here, we report converging behavioral and neuroimaging evidence from a stroke patient (B.L.) in support of cortical reorganization in the adult human visual system. B.L.s stroke spared the primary visual cortex (V1), but destroyed fibers that normally provide input to V1 from the upper left visual field (LVF). As a consequence, B.L. is blind in the upper LVF, and exhibits distorted perception in the lower LVF: stimuli appear vertically elongated, toward and into the blind upper LVF. For example, a square presented in the lower LVF is perceived as a rectangle extending upward. We hypothesized that the perceptual distortion was a consequence of cortical reorganization in V1. Extensive behavioral testing supported our hypothesis, and functional magnetic resonance imaging (fMRI) confirmed V1 reorganization. Together, the behavioral and fMRI data show that loss of input to V1 after a stroke leads to cortical reorganization in the adult human visual system, and provide the first evidence that reorganization of the adult visual system affects visual perception. These findings contribute to our understanding of the human adult brains capacity to change and has implications for topics ranging from learning to recovery from brain damage.

Reorganization of Visual Processing in Macular Degeneration Is Not Specific to the “Preferred Retinal Locus”

Recent work has shown that foveal cortex, deprived of its normal bottom-up input as a result of macular degeneration (MD), begins responding to stimuli presented to a peripheral retinal location. However, these studies have only presented stimuli to the “preferred retinal location,” or PRL, a spared part of the peripheral retina used by individuals with MD for fixating, face recognition, reading, and other visual tasks. Thus, previous research has not yet answered a question critical for understanding the mechanisms underlying this reorganization: Does formerly foveal cortex respond only to stimuli presented at the PRL, or does it also respond to other peripheral locations of similar eccentricity? If foveal cortex responds to stimuli at PRL because it is the long-term habitual use of this region as a functional fovea that drives the formerly foveal cortex to respond to stimuli presented at the PRL (the “use-dependent reorganization” hypothesis), then foveal cortex will not respond to stimuli presented at other locations. Alternatively, it may be that foveal cortex responds to any peripheral retinal input, independent of whether input at that retinal location has been chronically attended for months or years (the “use-independent reorganization” hypothesis). Using fMRI, we found clear activation of formerly foveal cortex to stimuli presented at either the PRL or an isoeccentric non-PRL location in two individuals with MD, supporting the use-independent reorganization hypothesis. This finding suggests that reorganization is driven by passive, not use-dependent mechanisms.

Size-optimized 32-Channel Brain Arrays for 3 T Pediatric Imaging

Size‐optimized 32‐channel receive array coils were developed for five age groups, neonates, 6 months old, 1 year old, 4 years old, and 7 years old, and evaluated for pediatric brain imaging. The array consisted of overlapping circular surface coils laid out on a close‐fitting coil‐former. The two‐section coil former design was obtained from surface contours of aligned three‐dimensional MRI scans of each age group. Signal‐to‐noise ratio and noise amplification for parallel imaging were evaluated and compared to two coils routinely used for pediatric brain imaging; a commercially available 32‐channel adult head coil and a pediatric‐sized birdcage coil. Phantom measurements using the neonate, 6‐month‐old, 1‐year‐old, 4‐year‐old, and 7‐year‐old coils showed signal‐to‐noise ratio increases at all locations within the brain over the comparison coils. Within the brain cortex the five dedicated pediatric arrays increased signal‐to‐noise ratio by up to 3.6‐, 3.0‐, 2.6‐, 2.3‐, and 1.7‐fold, respectively, compared to the 32‐channel adult coil, as well as improved G‐factor maps for accelerated imaging. This study suggests that a size‐tailored approach can provide significant sensitivity gains for accelerated and unaccelerated pediatric brain imaging. Magn Reson Med, 2011.

Organization of high-level visual cortex in human infants.

How much of the structure of the human mind and brain is already specified at birth, and how much arises from experience? In this article, we consider the test case of extrastriate visual cortex, where a highly systematic functional organization is present in virtually every normal adult, including regions preferring behaviourally significant stimulus categories, such as faces, bodies, and scenes. Novel methods were developed to scan awake infants with fMRI, while they viewed multiple categories of visual stimuli. Here we report that the visual cortex of 4–6-month-old infants contains regions that respond preferentially to abstract categories (faces and scenes), with a spatial organization similar to adults. However, precise response profiles and patterns of activity across multiple visual categories differ between infants and adults. These results demonstrate that the large-scale organization of category preferences in visual cortex is adult-like within a few months after birth, but is subsequently refined through development.

Awake fMRI reveals a specialized region in dog temporal cortex for face processing

Recent behavioral evidence suggests that dogs, like humans and monkeys, are capable of visual face recognition. But do dogs also exhibit specialized cortical face regions similar to humans and monkeys? Using functional magnetic resonance imaging (fMRI) in six dogs trained to remain motionless during scanning without restraint or sedation, we found a region in the canine temporal lobe that responded significantly more to movies of human faces than to movies of everyday objects. Next, using a new stimulus set to investigate face selectivity in this predefined candidate dog face area, we found that this region responded similarly to images of human faces and dog faces, yet significantly more to both human and dog faces than to images of objects. Such face selectivity was not found in dog primary visual cortex. Taken together, these findings: (1) provide the first evidence for a face-selective region in the temporal cortex of dogs, which cannot be explained by simple low-level visual feature extraction; (2) reveal that neural machinery dedicated to face processing is not unique to primates; and (3) may help explain dogs’ exquisite sensitivity to human social cues.