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Some data have been taken as evidence that after prolonged viewing, near-vertical orientations "normalize" to appear more vertical than they did previously. After almost a century of research, the existence of tilt normalization remains controversial. The most recent evidence for tilt normalization comes from data suggesting a measurable "perceptual drift" of near-vertical adaptors toward vertical, which can be nulled by a slight physical rotation away from vertical (Müller, Schillinger, Do, & Leopold, 2009). We argue that biases in estimates of perceptual stasis could, however, result from the anisotropic organization of orientation-selective neurons in V1, with vertically-selective cells being more narrowly tuned than obliquely-selective cells. We describe a neurophysiologically plausible model that predicts greater sensitivity to orientation displacements toward than away from vertical. We demonstrate the predicted asymmetric pattern of sensitivity in human observers by determining threshold speeds for detecting rotation direction (Experiment 1), and by determining orientation discrimination thresholds for brief static stimuli (Experiment 2). Results imply that data suggesting a perceptual drift toward vertical instead result from greater discrimination sensitivity around cardinal than oblique orientations (the oblique effect), and thus do not constitute evidence for tilt normalization., (© 2015 ARVO.)

Humans are experts at face recognition. The mechanisms underlying this complex capacity are not fully understood. Recently, it has been proposed that face recognition is supported by a coarse-scale analysis of visual information contained in horizontal bands of contrast distributed along the vertical image axis-a biological facial "barcode" (Dakin & Watt, 2009). A critical prediction of the facial barcode hypothesis is that the distribution of image contrast along the vertical axis will be more important for face recognition than image distributions along the horizontal axis. Using a novel paradigm involving dynamic image distortions, a series of experiments are presented examining famous face recognition impairments from selectively disrupting image distributions along the vertical or horizontal image axes. Results show that disrupting the image distribution along the vertical image axis is more disruptive for recognition than matched distortions along the horizontal axis. Consistent with the facial barcode hypothesis, these results suggest that human face recognition relies disproportionately on appropriately scaled distributions of image contrast along the vertical image axis., (© 2014 ARVO.)

When a unique "oddball" stimulus is embedded in a train of repeated standard stimuli, its duration can seem relatively exaggerated (V. Pariyadath & D. Eagleman, 2007; P. U. Tse, J. Intriligator, J. Rivest, & P. Cavanagh, 2004). We explored the possibility of a link between this and signal intensity reductions at low levels of visual processing. In Experiment 1, we used Troxler fading as a metric of signal intensity-the apparent fading of a stimulus with prolonged viewing (I. P. V. Troxler, 1804). Fading was exaggerated by presenting oddball and standard stimuli to different eyes. However, there was no fading difference when standard stimuli were presented persistently or intermittently. These results contrast with oddball effects, which were insensitive to eye of origin, and which were contingent on intermittent standard stimuli. In Experiment 2, we show that oddball effects can be elicited with oddballs that are less intense versions of repetitive stimuli, and in Experiment 3, we show that oddball effects can scale with the discrepancy between repeated and oddball stimuli. These observations discredit any oddball effect explanation predicated on low-level neural response magnitudes to individual stimuli. Instead, our data support the view that oddball effects are driven by predictive coding (V. Pariyadath & D. Eagleman, 2007), reflecting the discrepancy between expected and actual inputs.

When discrepant images are shown to the two eyes, each can intermittently disappear. This is known as binocular rivalry (BR). The causes of BR are debated. One view is that BR is driven by a low-level visual process, characterized by competition between monocular channels. Another is that BR is driven by higher level processes involved in interpreting ambiguous input. This would link BR to other phenomena, wherein perception changes without input changes. We reasoned that if this were true, the timing of BR changes might be related to the timing of changes in other multi-stable stimuli. We tested this using combinations of simple (orthogonal gratings) and complex (pictures of houses and faces) stimuli. We also presented simple stimuli in conjunction with a stimulus that induced an ambiguous direction of rotation. We found that the timing of simple BR changes was unrelated to the timing of either complex BR changes or to direction changes within an ambiguous rotation. However, the timings of changes within proximate BR stimuli, both simple and complex, were related, but only when similar images were encoded in the same monocular channels. These observations emphasize the importance of monocular channel interactions in determining the timing of binocular rivalry changes.

When different images are presented to the two eyes, each can intermittently disappear, leaving the other to dominate perception. This is called binocular rivalry. When using radial gratings, focal contrast increments can trigger a traveling wave of perceptual dominance change, originating at the locus of the contrast increment and circling the stimulus. This has been linked to a sweep of activity through V1 that can be traced via fMRI. The dominance of more complex images, like human faces, has been linked to higher level processing structures characterized by more holistic object centered properties. We therefore decided to assess how dominance would spread through more complex images. Using Kanisza squares and human faces we found that dominance tended to spread gradually away from the locus of the contrast increment, often along real or illusory contours. We also found that perceptual dominance was slow to spread between facial regions encoded by different monocular channels. These data are consistent with low-level monocular mechanisms, like those found in V1, playing a determinant role in the spread of perceptual dominance through complex images during binocular rivalry.

Humans exist in an environment wherein many unrelated events occur in close spatial and temporal proximity. Audio-visual timing experiments, however, have often examined only isolated pairs of sensory events. We therefore decided to assess how audio-visual timing perception would be shaped by the presence of an additional audio or visual event. We found that the point of subjective synchrony for a sensory event can be shifted away from the presence of other temporally proximate events. These interactions made audio-visual pairs seem unrelated, or asynchronous, at timings at which they had seemed synchronous when presented in isolation. This shows that the interval across which humans are insensitive to audio-visual asynchrony is not fixed, but dynamic, shaped by interactions between multiple sensory events. Importantly, we establish that these interactions can enhance the sensitivity of timing judgments. These interactions could therefore help to segregate unrelated sensory events across time. Such effects are likely to be common in the cluttered environments in which humans exist.

When a brief static flash is presented in physical alignment with a moving target, the position of the flash can seem to lag behind that of the moving target. Various explanations exist. One of the most popular is the differential latency hypothesis. This assumes that neural latencies for moving stimuli are shorter than those for static stimuli. Accordingly, the flash lag would occur because perception is temporally fragmented, with moving stimuli seen before static. To test this, observers were asked to make different judgments concerning the same stimulus. Observers either judged if color changes in moving and static stimulus sections were synchronous, or whether the different stimulus sections were aligned at the time of a color change. If the flash-lag were driven by a simple differential latency, we would expect both judgments to be marked by a temporal advantage for moving stimuli. Our results suggest the contrary, as only the later judgment was marked by a flash-lag effect. The apparent timing of moving and static color changes was veridical. However, when we introduced a systematic differential latency, by modulating image contrast, both judgments were affected. Our data therefore suggest that a simple differential can modulate flash-lag type effects, but they do not cause the phenomenon.

Synchronization is thought to have a role in linking disparate components into neural assemblies. However, the particular frequency of the synchronization is generally considered to be incidental to its functional role. Here we report a link between enhanced alpha activations and an illusory jitter of the same frequency. We measured perceived jitter rates and the magnetoencephalography during presentations of a stimulus wherein red squares and superimposed vertical green bars moved together across a black background. The green bars were either darker, equiluminant with, or brighter than the red squares. We established that the illusory jitter rate, robustly seen only in the equiluminant condition, was approximately 10 Hz. Crucially, neural oscillations around 10 Hz were enhanced in this condition. Surprisingly, approximately 10 Hz oscillations were also enhanced during illusory jitter perception relative to a moving stimulus that contained physical 10 Hz jitter. This suggests that the enhanced synchronization is associated with illusory jitter generation rather than with jitter perception. Since the stimulus eliciting illusory jitter moves smoothly and rigidly, both the percept and enhanced neural synchrony must be generated within the visual system. Our data therefore indicate a match between the dynamics of synchronous neural activity and the dynamics of a sensory experience offering the intriguing possibility of a common cause.