A neural hierarchy for illusions of time: duration adaptation precedes multisensory integration.

Perceived time is inherently malleable. For example, adaptation to relatively long or short sensory events leads to a repulsive aftereffect such that subsequent events appear to be contracted or expanded (duration adaptation). Perceived visual duration can also be distorted via concurrent presentation of discrepant auditory durations (multisensory integration).

Interaction of first- and second-order signals in the extraction of global-motion and optic-flow.

The intention of this series of experiments was to determine the extent to which the pathways sensitive to first-order and second-order motion are independent of one another at, and above, the level of global motion integration. We used translational, radial and rotational motion stimuli containing luminance-modulated dots, contrast-modulated dots, or a mixture of both. Our results show that the two classes of motion stimuli interact perceptually in a global motion coherence task, and the extent of this interaction is governed by whether the two varieties of local motion signal produce an equivalent response in the pathways that encode each type of motion.

Duration channels mediate human time perception.

The task of deciding how long sensory events seem to last is one that the human nervous system appears to perform rapidly and, for sub-second intervals, seemingly without conscious effort. That these estimates can be performed within and between multiple sensory and motor domains suggest time perception forms one of the core, fundamental processes of our perception of the world around us. Given this significance, the current paucity in our understanding of how this process operates is surprising.

Impaired spatial and binocular summation for motion direction discrimination in strabismic amblyopia.

Amblyopia is characterised by visual deficits in both spatial vision and motion perception. While the spatial deficits are thought to result from deficient processing at both low and higher level stages of visual processing, the deficits in motion perception appear to result primarily from deficits involving higher level processing. Specifically, it has been argued that the motion deficit in amblyopia occurs when local motion information is pooled spatially and that this process is abnormally susceptible to the presence of noise elements in the stimulus.

Co-operative interactions between first- and second-order mechanisms in the processing of structure from motion.

Structure from motion (SFM) is the ability to perceive three-dimensional structure from stimuli containing only two-dimensional motion signals and this ability seems to be a result of high-level cortical processes. It has long been thought that local motion signals defined by second-order cues only weakly contribute to perception of SFM since performance on purely second-order SFM tasks is poor, relative to first-order stimuli. We hypothesized that the mechanisms responsible for deriving SFM were insensitive to low-level stimulus attributes such as the first- or second-order nature of the dots composing the stimulus, in other words: that they were "cue-invariant", but that large differences in sensitivity to local first- and second-order motions were responsible for previous findings.

Low-level mechanisms may contribute to paradoxical motion percepts.

A recent series of experiments demonstrated a surprising deterioration of visual motion discrimination with increasing stimulus size for stimuli of high contrast. This counterintuitive finding was explained as a result of surround suppression in visual area V5. Equally paradoxical was the finding that older observers showed better performance than younger observers.

A double dissociation between striate and extrastriate visual cortex for pattern motion perception revealed using rTMS.

The neural mechanisms underlying the integration and segregation of motion signals are often studied using plaid stimuli. These stimuli consist of two spatially coincident dynamic gratings of differing orientations, which are either perceived to move in two unique directions or are integrated by the visual system to elicit the percept of a checkerboard moving in a single direction. Computations pertaining to the motion of the individual component gratings are thought to take place in striate cortex (V1) whereas motion integration is thought to involve neurons in dorsal stream extrastriate visual areas, particularly V5/MT.

Biological motion perception is cue-invariant.

Previous work investigating whether biological motion is supported by local second-order motion has been contradictory, with different groups finding either a difference or no difference in performance compared to that obtained with first-order stimuli. Here we show psychophysically, using randomized-polarity and contrast-modulated stimuli, that detection of second-order biological motion walkers is worse for stimuli defined by second-order cues, but this difference is explained by a difference in visibility of the local motion in the stimuli. By mixing first-order and second-order dots within the same stimulus, we show that, when the two types of dot are equally visible, first-order noise dots can mask a second-order walker, and vice-versa.

The amblyopic deficit for global motion is spatial scale invariant.

Humans with amblyopia display anomalous performance for global motion discrimination. Attempts have been made to rule out an explanation based solely on the visibility loss in lower visual areas. However, it remains a possibility that the altered scale over which local motion is processed in V1 might lead to reduced efficiency of global motion processing in extra-striate cortex.

Ibn al-Haytham and psychophysics.

Persian scholar Ibn al-Haytham ('Alhazen') has rightly been credited with many advances in optics and vision science, but recent spurious claims that he is the 'founder of psychophysics' rest upon unsupported assertions, a conflation of psychophysics with the wider discipline of psychology, and semantic arguments over what it is to 'found' a school of thought.

Global motion processing: The effect of spatial scale and eccentricity.

Here we examine how global translational motion sensitivity varies with the spatial frequency of the elements in local motion and on the eccentricity of stimulation. Using DC-balanced, spatially narrowband elements (radial log Gabors) matched in terms of multiples above contrast threshold, we show that global translational motion sensitivity is best at mid high spatial frequencies and worst at low spatial frequencies. Furthermore, we show that the lower global motion sensitivity of the periphery is due to differences in spatial scale/contrast that can be attributed to lower reaches of the visual pathway where the local motion signal is transduced.

Plaid perception is only subtly impaired in strabismic amblyopia.

Amblyopes exhibit a global motion anomaly that implicates processing beyond the local motion analysis of V1 possibly involving areas MT and MST in the extra-striate cortex. Here, we sought to further investigate this deficit by measuring the perception of moving plaid stimuli by amblyopic observers, since there is good physiological evidence that the motion of such stimuli is determined by processes beyond V1. The conditions under which the two moving components constituting the plaids were seen to cohere or move transparently over one another were investigated by manipulating their relative spatial frequencies.

Second-order optic flow deficits in amblyopia.

Purpose: Amblyopic observers show deficits for global motion discrimination that cannot be accounted for by their contrast sensitivity impairment. The processing of first- and second-order translational global motion is deficient, as is the processing of first-order optic flow, suggesting that cortical function in extrastriate areas is impaired. The authors sought to determine whether amblyopes show impairment in the processing of optic flow defined by second-order motion, whether these deficits are comparable in the two eyes, and whether these deficits are correlated with first-order deficits.

Second-order optic flow processing.

Optic flow-large-field rotational and radial motion-is processed as efficiently as translational motion for first-order (luminance-defined) stimuli. However, it has been suggested recently that the same pattern does not hold for second-order (e.g.

Motion-detection thresholds for first- and second-order gratings and plaids.

The two-stage decomposition-recombination model of 2D motion perception has been criticised on the basis that the direction of plaid stimuli can be accurately discriminated at speeds so low that the direction of their Fourier components is not discriminable. The nature of this gap in performance between gratings and plaids was investigated across a range of spatial frequencies and durations for first- and second-order stimuli. Motion-detection thresholds were obtained using a 2AFC, constant stimuli procedure and it was found that although thresholds for detection of plaid motion were often lower than those for gratings, the gap in performance between first-order plaids and gratings was unreliable, varying in magnitude and occasionally direction with the spatial frequency of the stimulus, presentation duration and observer.