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It is debated whether different forms of bistable perception result from common or separate neural mechanisms. Binocular rivalry involves perceptual alternations between competing monocular images, whereas ambiguous figures such as the Necker cube lead to alternations between two possible pictorial interpretations. Previous studies have shown that observers can voluntarily control the alternation rate of both rivalry and Necker cube reversal, perhaps suggesting that bistable perception results from a common mechanism of top-down selection. However, according to the biased competition model of selective attention, attention should be able to enhance the attended percept and suppress the unattended percept. Here, we investigated selective attentional modulation of dominance durations in bistable perception. Observers consistently showed much weaker selective attentional control for rivalry than for Necker cube reversal, even for rivalry displays that maximized the opportunities for feature-, object-, or space-based attentional selection. In contrast, nonselective control of alternation rate was comparably strong for both forms of bistable perception and corresponded poorly with estimates of selective attentional control. Our results support the notion that binocular rivalry involves a more automatic, stimulus-driven form of visual competition than Necker cube reversal, and as a consequence, is less easily biased by selective attention.

Neuronal oscillations are ubiquitous in the brain and contribute to perception and attention. However, most associated evidence derives from post hoc correlations between brain dynamics and behavior. Although a few recent studies demonstrate rhythms in behavior, it remains largely unknown whether behavioral performances manifest spectrotemporal dynamics in a neurophysiologically relevant manner (e.g., the temporal modulation of ongoing oscillations, the cross-frequency coupling). To investigate the issue, we examined fine spectrotemporal dynamics of behavioral time courses in a large sample of human participants (n ϭ 49), by taking a high time-resolved psychophysical measurement in a precuing attentional task. We observed compelling dynamic oscillatory patterns directly in behavior. First, typical attentional effects are demonstrated in low-pass (0 -2 Hz) filtered time courses of behavioral responses. Second, an uninformative peripheral cue elicits recurring ␣-band (8 -20 Hz) pulses in behavioral performances, and the elicited ␣ pulses for cued and uncued conditions are in a temporally alternating relationship. Finally, ongoing ␣-band power is phase locked to ongoing -bands (3-5 Hz) in behavioral time courses. Our findings constitute manifestation of oscillations at physiologically relevant rhythms and powerphase locking, as widely observed in neurophysiological recordings, in behavior. The findings suggest that behavioral performance actually consists of rich dynamic information and may reflect underlying neuronal oscillatory substrates. Our data also speak to a neural mechanism for item attention based on successive cycles () of a sequential attentional sampling (␣) process.

Are visual face processing mechanisms the same in the left and right cerebral hemispheres? The possibility of such 'duplicated processing' seems puzzling in terms of neural resource usage, and we currently lack a precise characterization of the lateral differences in face processing. To address this need, we have undertaken a three-pronged approach. Using functional magnetic resonance imaging, we assessed cortical sensitivity to facial semblance, the modulatory effects of context and temporal response dynamics. Results on all three fronts revealed systematic hemispheric differences. We found that: (i) activation patterns in the left fusiform gyrus correlate with image-level face-semblance, while those in the right correlate with categorical face/non-face judgements. (ii) Context exerts significant excitatory/inhibitory influence in the left, but has limited effect on the right. (iii) Face-selectivity persists in the right even after activity on the left has returned to baseline. These results provide important clues regarding the functional architecture of face processing, suggesting that the left hemisphere is involved in processing 'low-level' face semblance, and perhaps is a precursor to categorical 'deep' analyses on the right.

What aspects of facial information do we use to recognize individuals? One way to address this fundamental question is to study image transformations that compromise facial recognizability. The goal would be to identify factors that underlie the recognition decrement and, by extension, are likely constituents of facial encoding. To this end, we focus here on the contrast negation transformation. Contrast negated faces are remarkably difficult to recognize for reasons that are currently unclear. The dominant proposals so far are based either on negative faces' seemingly unusual pigmentation, or incorrectly computed 3D shape. Both of these explanations have been challenged by recent results. Here, we propose an alternative account based on 2D ordinal relationships, which encode local contrast polarity between a few regions of the face. Using a novel set of facial stimuli that incorporate both positive and negative contrast, we demonstrate that ordinal relationships around the eyes are major determinants of facial recognizability. Our behavioral studies suggest that destruction of these relationships in negatives likely underlies the observed recognition impairments, and our neuro-imaging data show that these relationships strongly modulate brain responses to facial images. Besides offering a potential explanation for why negative faces are hard to recognize, these results have implications for the representational vocabulary the visual system uses to encode faces.contrast negation ͉ face perception ͉ fMRI ͉ neural representation ͉ object recognition I n principle, a contrast negated image is exactly as informative as its positive counterpart; negation perfectly preserves an image's 2D geometric and spectral structure. However, as anyone who has had to search through a roll of negatives for a snapshot of a particular person knows, this simple operation has dramatically adverse consequences on our ability to identify faces, as illustrated in Fig. 1 (1-6). Exploring the causes of this phenomenon is important for understanding the broader issue of the nature of information the visual system uses for face identification.Several researchers have hypothesized that negated face-images are hard to recognize because of the unnatural shading cues in negatives, which compromise shape from shading processes (7-11). The resulting problems in recovering veridical 3D facial shape are believed to impair recognition performance. Although plausible, it is unclear whether this explanation is a sufficient one, especially in light of experimental results showing preserved recognition performance in the absence of shading gradients (12), and theories of face recognition that are based on the use of 2D intensity patterns rather than recovered 3D shapes (13,14). Another prominent hypothesis is that negation causes faces to have unusual pigmentation (15, 16). However, the adequacy of this ''pigmentation hypothesis'' has been challenged by data showing that hue negation, which also results in unnatural pigmentation (making the entire face l...

The constructive nature of perception can be revealed under viewing conditions that lead to vivid subjective impressions in the absence of direct input. When a low-contrast moving grating is divided by a large gap, observers report seeing a 'visual phantom' of the real grating extending through the blank gap region. Here, we report fMRI evidence showing that visual phantoms lead to enhanced activity in early visual areas that specifically represent the blank gap region. We found that neural filling-in effects occurred automatically in areas V1 and V2, independent of where the subject attended. Moreover, when phantom-inducing gratings were paired with competing stimuli in a binocular rivalry display, subjects reported spontaneous fluctuations in conscious perception of the phantom that were accompanied by tightly coupled changes in early visual activity. Our results indicate that phantom visual experiences are closely linked to automatic filling-in of activity at the earliest stages of cortical processing. Keywordsvisual perception; visual cortex; illusions; attention; awareness; consciousness A particularly vivid and powerful form of perceptual completion involves the formation of moving visual phantoms. When a low-contrast moving grating is divided by an orthogonal gap, subjects typically perceive a dimmer version of the surrounding dynamic pattern continuing across the blank gap region ( Fig. 1a). Visual phantoms are greatly enhanced by motion of the surrounding inducers, and can occur anywhere in the normal visual field across gaps as large as 10 degrees 1 . These illusory phantoms appear to match the pattern, motion, color, and texture of the physically surrounding inducers, and remarkably, can mimic the perceptual effects of real stimuli. For example, moving phantoms can induce local motion aftereffects, suggesting that phantom impressions are actively represented in the brain 2 . However, the neural basis of visual phantoms has not been studied previously. Such knowledge is important for understanding how the brain fills-in gaps in sensory information and forms representations of subjective perceptual content in the absence of direct input.We used functional magnetic resonance imaging (fMRI) to measure neural responses to visual phantoms in corresponding regions of the human visual cortex. Subjects maintained central fixation while low-contrast oscillating gratings were presented in the upper and lower left visual field, separated by a large 7° x 7° gap. The gratings were presented either vertically ( Fig. 1a), leading to the perception of a phantom, or horizontally ( Fig. 1b), to serve as a no-phantom control condition. A sequence of letters was presented concurrently at fixation, and subjects were cued prior to each fMRI trial to perform an attentionally demanding task involving either the central letters or the peripheral gratings. This manipulation of spatial attention served as an important control, as it is known that focal attention can activate corresponding regions of visual cortex even wh...

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