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When observers must discriminate a weak sensory signal in noise, early sensory areas seem to reflect the instantaneous strength of the sensory signal. In contrast, high-level parietal and prefrontal areas appear to integrate these signals over time with activity peaking at the time of the observer's decision. Here, we used functional magnetic resonance imaging to investigate how the brain forms perceptual decisions about complex visual forms in a challenging task, requiring the discrimination of ambiguous 2-tone Mooney faces and visually similar nonface images. Face-selective areas in the ventral visual cortex showed greater activity when subjects reported perceiving a face as compared with a nonface, even on error trials. More important, activity was closely related to the time of the subject's decision for face judgments, even on individual trials, and resembled the time course of activity in motor cortex corresponding to the subject's behavioral report. These results indicate that perceptual decisions about ambiguous face-like stimuli are reflected early in the sensorimotor pathway, in face-selective regions of the ventral visual cortex. Activity in these areas may represent a potential rate-limiting step in the pathway from sensation to action when subjects must reach a decision about ambiguous face-like stimuli.

. Behavioral studies have shown that object recognition becomes severely impaired at fast presentation rates, indicating a limitation in temporal processing capacity. Here, we studied whether this behavioral limit in object recognition reflects limitations in the temporal processing capacity of early visual areas tuned to basic features or high-level areas tuned to complex objects. We used functional MRI (fMRI) to measure the temporal processing capacity of multiple areas along the ventral visual pathway progressing from the primary visual cortex (V1) to high-level object-selective regions, specifically the fusiform face area (FFA) and parahippocampal place area (PPA). Subjects viewed successive images of faces or houses at presentation rates varying from 2.3 to 37.5 items/s while performing an object discrimination task. Measures of the temporal frequency response profile of each visual area revealed a systematic decline in peak tuning across the visual hierarchy. Areas V1-V3 showed peak activity at rapid presentation rates of 18 -25 items/s, area V4v peaked at intermediate rates (9 items/s), and the FFA and PPA peaked at the slowest temporal rates (4 -5 items/s). Our results reveal a progressive loss in the temporal processing capacity of the human visual system as information is transferred from early visual areas to higher areas. These data suggest that temporal limitations in object recognition likely result from the limited processing capacity of high-level object-selective areas rather than that of earlier stages of visual processing.

The current study investigated the sensitivity of face recognition to two changes of the stimulus, a rotation in depth and an inversion, by comparing the performance of two prosopagnosic patients, RN and CR, with non-neurological control subjects on a face-matching task. The control subjects showed an effect of depth rotation, with errors and reaction times increasing systematically with rotation angle, and the traditional inversion effect, with errors and reaction times increasing under inverted conditions. In contrast, RN showed no effect of rotation or inversion on his error data but did show a less sensitively graded effect of rotation and the traditional inversion effect on reaction times. CR did not show a graded effect of rotation on his errors or reaction times. Although CR showed the traditional inversion effect on his error data, he displayed an inversion superiority effect on his reaction time data, which supports the claim that the damaged holistic processing systems continue to dominate face processing in prosopagnosia even though they are malfunctioning. These results suggest that the damage that occurs to the ventral temporal cortex in prosopagnosia may have forced the patients to rely on sources of information that are not dependent on the view of the face and, moreover, cannot be adapted to deal with rotated faces under both upright and inverted conditions.

Prior work suggests that nonface objects of expertise can interfere with the perception of faces when the two categories are alternately presented, suggesting competition for shared perceptual resources. Here, we ask whether task-irrelevant distractors from a category of expertise compete when faces are presented in a standard visual search task. Participants searched for a target (face or sofa) in an array containing both relevant and irrelevant distractors. The number of distractors from the target category (face or sofa) remained constant, whereas the number of distractors from the irrelevant category (cars) varied. Search slopes, calculated as a function of the number of irrelevant cars, were correlated with car expertise. The effect was not due to car distractors grabbing attention, because they did not compete with sofa targets. Objects of expertise interfere with face perception even when they are task irrelevant, visually distinct, and separated in space from faces.

Agnosia, the impairment in object and face recognition despite intact vision and intelligence, is one of the most intriguing and debilitating neuropsychological deficits. The goal of this study was to determine whether S.M., an individual with longstanding visual agnosia and concomitant prosopagnosia, can be retrained to perform visual object recognition and, if so, what neural substrates mediate this reacquisition. Additionally, of interest is the extent to which training on one type of visual stimulus generalizes to other visual stimuli, as this informs our understanding of the organization of ventral visual cortex. Greebles were chosen as the stimuli for retraining given that, in neurologically normal individuals, these stimuli can engage the fusiform face area. Posttraining, S.M. showed significant improvement in recognizing Greebles, although he did not attain normal levels of performance. He was also able to recognize untrained Greebles and showed improvement in recognizing common objects. Surprisingly, his performance on face recognition, albeit poor initially, was even more impaired following training. A comparison of pre- and postintervention functional neuroimaging data mirrored the behavioral findings: Face-selective voxels in the fusiform gyrus prior to training were no longer so and were, in fact, more Greeble-selective. The findings indicate potential for experience-dependent dynamic reorganization in agnosia with the possibility that residual neural tissue, with limited capacity, will compete for representations.

Recent studies indicate that expertise with objects can interfere with face processing. Although competition occurs between faces and objects of expertise, it remains unclear whether this reflects an expertise-specific bottleneck or the fact that objects of expertise grab attention and thereby consume more central resources. We investigated the perceptual costs of expertise by measuring visual thresholds for identifying targets embedded within RSVP sequences presented at varying temporal rates. Car experts and novices searched for face targets among face and car distractors, or watch targets among watch and car distractors. Remarkably, car experts were slower than novices at identifying faces among task-irrelevant cars, yet faster than novices at identifying watches among cars. This suggests that car expertise leads to greater functional overlap between cars and faces while reducing the functional overlap between cars and objects, a result incompatible with the notion of an encapsulated module for exclusive processing of faces.

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