Craig Aaen-Stockdale

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. We use stimuli composed of spatial frequency bandpass elements, equated for visibility, to show that the global motion deficit in amblyopia for both fellow and amblyopic eyes is still present once impairments in low-level processing have been factored out. This residual deficit appears to be spatial scale invariant and the relative deficit between the eyes shows a dependence on stimulus speed. We believe that this rules out an explanation of the amblyopic global motion deficit based solely on local motion input. We suggest instead that, in addition to low-level deficits, motion processing in a broadband, extra-striate, global motion mechanism is impaired in amblyopia.

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. By combining a psychophysical task that employed plaid stimuli with 1 Hz offline repetitive transcranial magnetic stimulation (rTMS), we demonstrated a double dissociation between striate and extrastriate visual cortex in terms of their contributions to motion integration. rTMS over striate cortex increased coherent motion percepts whereas rTMS over extrastriate cortex had the opposite effect. These effects were robust directly after the stimulation administration and gradually returned to baseline within 15 minutes. This double dissociation is consistent with previous patient data and the recent hypothesis that both coherent and transparent motion percepts are supported by the visual system simultaneously and compete for perceptual dominance. Hum Brain Mapp 2009.

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. contrast-defined) stimuli. We used random dot kinematogram (RDK) stimuli to determine whether global processing of optic flow is as efficient as processing of global translational motion for both first- and second-order stimuli. For first-order stimuli, we found that coherence thresholds for radial and rotational motion were equivalent to thresholds for translational motion, supporting previous findings. For second-order stimuli we found, firstly, that given sufficient contrast, second-order optic flow can be processed as efficiently as first-order optic flow and, secondly, that rotational and translational second-order motion are processed with equal efficiency. This contradicts the suggestion that there is a loss of efficiency between integration of second-order global motion and second-order optic flow. The third interesting finding was that the processing of radial second-order motion appears to suffer from a deficit that is dependent upon both the contrast and spatial extent of the stimulus. Further experiments discounted the possibility that the observed deficit is caused by a centrifugal or centripetal bias, but demonstrated that a longer temporal integration period for radial second-order motion is responsible for the observed difference. For durations of approximately 850ms, all three types of motion are processed with equal efficiency.

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. Here we investigated motion direction discrimination for abruptly presented two-frame Gabor stimuli in a group of five strabismic amblyopes and five control observers. Motion direction discrimination for this stimulus is inherently noisy and relies on the signal/noise processing of motion detectors. We varied viewing condition (monocular vs. binocular), stimulus size (5.3-18.5°) and stimulus contrast (high vs. low) in order to assess the effects of binocular summation, spatial summation and contrast on task performance. No differences were found for the high contrast stimuli; however the low contrast stimuli revealed differences between the control and amblyopic groups and between fellow fixing and amblyopic eyes. Control participants exhibited pronounced binocular summation for this task (on average a factor of 3.7), whereas amblyopes showed no such effect. In addition, the spatial summation that occurred for control eyes and the fellow eye of amblyopes was significantly attenuated for the amblyopic eyes relative to fellow eyes. Our results support the hypothesis that pooling of local motion information from amblyopic eyes is abnormal and highly sensitive to noise.

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. We also show that direction-discrimination of normal, inverted and scrambled walkers follow the same pattern for second-order as that obtained with first-order stimuli. These results are consistent with biological motion being processed by a mechanism that is cue-invariant.

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). The neural loci of both distortions remain unknown. In the current study we use a psychophysical approach to establish their relative positioning within the sensory processing hierarchy. We show that audiovisual integration induces marked distortions of perceived visual duration. We proceed to use these distorted durations as visual adapting stimuli yet find subsequent visual duration aftereffects to be consistent with physical rather than perceived visual duration. Conversely, the concurrent presentation of adapted auditory durations with nonadapted visual durations results in multisensory integration patterns consistent with perceived, rather than physical, auditory duration. These results demonstrate that recent sensory history modifies human duration perception prior to the combination of temporal information across sensory modalities and provides support for adaptation mechanisms mediated by duration selective neurons situated in early areas of the visual and auditory nervous system (Aubie, Sayegh, & Faure, 2012; Duysens, Schaafsma, & Orban, 1996; Leary, Edwards, & Rose, 2008).

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. Percepts were measured using both short presentation durations, where both the percept and the direction of motion were reported, and long presentation durations where the bi-stability of the stimulus was directly measured. In addition, we measured the ability of amblyopic eyes to perceive globally coherent motion in a multiple aperture stimulus. We found a small increased tendency for both amblyopic and fellow-fixing eyes to perceive short duration plaid stimuli as coherent relative to control eyes, but no difference for long duration plaids. In addition, amblyopic eyes saw less coherence in multiple aperture stimuli than fellow-fixing eyes but were not reliably different from control eyes. We therefore conclude that the neural mechanisms underlying plaid perception are only subtly abnormal in amblyopia.

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. Thus, the efficiency of the global translational motion computation that occurs in extrastriate cortical areas does not vary across the visual field. This may not be directly applicable to global radial motion because there are known visual field anisotropies.

Perceived time is spatial frequency dependent

Highlights ► The perceived duration of visual stimuli varies with spatial frequency content. ► Subjective time expansion in response to oddball stimuli is not a general result. ► Sounds being perceived as longer than visual stimuli is not a general result.

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.