To combine information from different sensory modalities, the brain must deal with considerable temporal uncertainty. In natural environments, an external event may produce simultaneous auditory and visual signals yet they will invariably activate the brain asynchronously due to different propagation speeds for light and sound, and different neural response latencies once the signals reach the receptors. One strategy the brain uses to deal with audiovisual timing variation is to adapt to a prevailing asynchrony to help realign the signals. Here, using psychophysical methods in human subjects, we investigate audiovisual recalibration and show that it takes place extremely rapidly without explicit periods of adaptation. Our results demonstrate that exposure to a single, brief asynchrony is sufficient to produce strong recalibration effects. Recalibration occurs regardless of whether the preceding trial was perceived as synchronous, and regardless of whether a response was required. We propose that this rapid recalibration is a fast-acting sensory effect, rather than a higher-level cognitiveprocess.Anaccountintermsofresponsebiasisunlikelyduetoastrongasymmetrywherebystimuliwithvisionleadingproducebigger recalibrations than audition leading. A fast-acting recalibration mechanism provides a means for overcoming inevitable audiovisual timing variation and serves to rapidly realign signals at onset to maximize the perceptual benefits of audiovisual integration.

Summary When viewing a different stimulus with each eye we experience the remarkable phenomenon of binocular rivalry: alternations in consciousness between the stimuli [1, 2]. According to a popular theory first proposed in 1901, neurons encoding the two stimuli engage in reciprocal inhibition [3-8] so that those processing one stimulus inhibit those processing the other, yielding consciousness of one, dominant, stimulus at any moment, the other being suppressed. Also according to the theory, neurons encoding the dominant stimulus adapt, weakening their activity and the inhibition they can exert, while neurons encoding the suppressed stimulus recover from adaptation, until the balance of activity reverses, triggering an alternation in consciousness. Despite its popularity, this theory has one glaring inconsistency with data: during an episode of suppression, visual sensitivity to brief probe stimuli in the dominant eye should decrease over time, and should increase in the suppressed eye, yet sensitivity appears constant [9, 10]. Using more appropriate probe stimuli (Experiment 1) in conjunction with a new method (Experiment 2) we found that sensitivities in dominance and suppression do show the predicted complementary changes. Highlights We devised a new method to probe contrast sensitivity during rivalry episodes Sensitivity of the dominant eye declines; sensitivity of the suppressed eye improves Sensitivities are similar just prior to a switch of perceptual dominance This confirms predictions from reciprocal-inhibition theory of binocular rivalry

BackgroundA prevailing view is that audiovisual integration requires temporally coincident signals. However, a recent study failed to find any evidence for audiovisual integration in visual search even when using synchronized audiovisual events. An important question is what information is critical to observe audiovisual integration.Methodology/Principal FindingsHere we demonstrate that temporal coincidence (i.e., synchrony) of auditory and visual components can trigger audiovisual interaction in cluttered displays and consequently produce very fast and efficient target identification. In visual search experiments, subjects found a modulating visual target vastly more efficiently when it was paired with a synchronous auditory signal. By manipulating the kind of temporal modulation (sine wave vs. square wave vs. difference wave; harmonic sine-wave synthesis; gradient of onset/offset ramps) we show that abrupt visual events are required for this search efficiency to occur, and that sinusoidal audiovisual modulations do not support efficient search.Conclusions/SignificanceThus, audiovisual temporal alignment will only lead to benefits in visual search if the changes in the component signals are both synchronized and transient. We propose that transient signals are necessary in synchrony-driven binding to avoid spurious interactions with unrelated signals when these occur close together in time.

Previous studies have generally estimated that two independent channels underlie human temporal vision: one broad and low-pass, the other high, and band-pass. We confirm this with iso-oriented targets and masks. With orthogonal masks, the same high-frequency channel emerges but no low-pass channel is observed, indicating the high-frequency channel is orientation invariant, and possibly pre-cortical in origin. In contrast, orientation dependence for low frequencies suggests a cortical origin. Subsequent masking experiments using unoriented spatiotemporal-filtered noise demonstrated that high-frequency masks (>8Hz) suppress low-frequency targets (1 and 4Hz), but low frequencies do not suppress high frequencies. This asymmetry challenges the traditional assumption of channel independence. To explain this, we propose a two-channel model in which a non-orientation-selective high-frequency channel suppresses an orientation-tuned low-frequency channel. This asymmetry may: (i) equalise the over-representation of low temporal-frequency energy in natural stimuli (1/f power spectrum); (ii) contribute to motion deblurring.

We investigated how crowding-a breakdown in object recognition that occurs in the presence of nearby distracting clutter-works for complex letter-like stimuli. Subjects reported the orientation (up/down/left/right) of a T target, abutted by a single flanker composed of randomly positioned horizontal and vertical bars. In addition to familiar retinotopic anisotropies (e.g., more crowding from more eccentric flankers), we report three object-centered anisotropies. First, inversions of the target element were rare: errors included twice as many ±90° as 180° target rotations. Second, flankers were twice as intrusive when they lay above or below (end-flanking) compared to left or right (side-flanking) of an upright T target (an effect that holds under global rotation of the target-flanker pair). Third, end flankers induce subjects to make erroneous reports that resemble the flanker (producing a structured pattern of errors), but errors induced by side flankers do not (instead producing random errors). A model based on probabilistic weighted averaging of the feature positions within contours can account for these effects. Thus, we demonstrate a set of seemingly "high-level" object-centered crowding effects that can arise from "low-level" interactions between the features of letter-like elements.

It has been proposed that visual crowding—the breakdown in recognition that occurs when objects are presented in cluttered scenes—reflects a limit imposed by visual attention. We examined this idea in the context of an orientation averaging task, having subjects judge the mean orientation of a set of oriented signal elements either in isolation, or “crowded” by nearby randomly oriented elements. In some conditions, subjects also had to perform an attentionally demanding secondary task. By measuring performance at different levels of signal orientation variability, we show that crowding increases subjects’ local uncertainty (about the orientation of individual elements) but that diverting attention reduces their global efficiency (the effective number of elements they can average over). Furthermore, performance with the same stimulus-sequence, presented multiple times, reveals that crowding does not induce more stimulus-independent variability (as would be predicted by some accounts based on attention). We conclude that crowding and attentional load have dissociable perceptual consequences for orientation averaging, suggesting distinct neural mechanisms for both. For the task we examined, attention can modulate the effects of crowding by changing the efficiency with which information is analyzed by the visual system but since crowding changes local uncertainty, not efficiency, crowding does not reflect an attentional limit.

Combining signals across the senses improves precision and speed of perception, although this multisensory benefit declines for asynchronous signals. Multisensory events may produce synchronized stimuli at source but asynchronies inevitably arise due to distance, intensity, attention and neural latencies. Temporal recalibration is an adaptive phenomenon that serves to perceptually realign physically asynchronous signals. Recently, it was discovered that temporal recalibration occurs far more rapidly than previously thought and does not require minutes of adaptation. Using a classical audiovisual simultaneity task and a series of brief flashes and tones varying in onset asynchrony, perceived simultaneity on a given trial was found to shift in the direction of the preceding trial’s asynchrony. Here we examine whether this inter-trial recalibration reflects the same process as prolonged adaptation by combining both paradigms: participants adapted to a fixed temporal lag for several minutes followed by a rapid series of test trials requiring a synchrony judgment. Interestingly, we find evidence of recalibration from prolonged adaptation and inter-trial recalibration within a single experiment. We show a dissociation in which sustained adaptation produces a large but decaying recalibration effect whilst inter-trial recalibration produces large transient effects whose sign matches that of the previous trial.

It is well established that activity of striate neurons may be either facilitated or suppressed by visual stimuli presented outside of their classical receptive field (CRF) limits. Whilst two general mechanisms have been identified as candidates for these contextual effects; those based on extra-striate feedback and long-range horizontal striate connections; the physiological data supporting these models is both ambiguous and inconsistent. Here we investigate psychophysically the phenomenon of collinear facilitation, in which contrast detection thresholds for foveally presented Gabor stimuli are reduced via concurrent presentation of remote collinear flankers. Using backward noise masking, we demonstrate that the minimum exposure duration required to induce facilitation increases monotonically with greater target-flanker separation. The inferred cortical propagation velocities of this process (0.10-0.23 ms(-1)) closely correspond with depolarising activity observed to travel across striate cortex of several species. These dynamics strongly suggest that contrast facilitation is mediated via long-range horizontal striate connections. This conclusion complements a recent suggestion that collinear induced long-range suppressive dynamics depend on extra-striate feedback.