Szonya Durant

Shifts of criteria or neural timing? The assumptions underlying timing perception studies

In timing perception studies, the timing of one event is usually manipulated relative to another, and participants are asked to judge if the two events were synchronous, or to judge which of the two events occurred first. Responses are analyzed to determine a measure of central tendency, which is taken as an estimate of the timing at which the two events are perceptually synchronous. When these estimates do not coincide with physical synchrony, it is often assumed that the sensory signals are asynchronous, as though the transfer of information concerning one input has been accelerated or decelerated relative to the other. Here we show that, while this is a viable interpretation, it is equally plausible that such effects are driven by shifts in the criteria used to differentiate simultaneous from asynchronous inputs. Our analyses expose important ambiguities concerning the interpretation of simultaneity judgement data, which have hitherto been underappreciated.

Temporal dependence of local motion induced shifts in perceived position

It has been shown that a moving visual pattern can influence the perceived position of outlying, briefly flashed objects. Using a rotating bar as an inducing stimulus we observed a shift, in the direction of motion, of the perceived position of small bars flashed together on either side of the moving bar. The greatest shift occurred when the 13 ms flashes were presented 60 ms before the rotating bar came closest to their locations. By varying rotation speed we showed that the peak effect was determined by the temporal rather than the spatial interval. The motion induced shift could be attenuated by introducing background flickering dots. The perceived shift decreased with distance from motion when the eccentricity of the flashes was kept constant. We conclude that the shift reflects feedback to primary visual cortex from motion selective cells in extrastriate cortex with receptive fields that overlap the retinal location of the flash.

Characterizing contrast adaptation in a population of cat primary visual cortical neurons using Fisher information

When cat V1/V2 cells are adapted to contrast at their optimal orientation, a reduction in gain and/or a shift in the contrast response function is found. We investigated how these factors combine at the population level to affect the accuracy for detecting variations in contrast. Using the contrast response function parameters from a physiologically measured population, we model the population accuracy (using Fisher information) for contrast discrimination. Adaptation at 16%, 32%, and 100% contrast causes a shift in peak accuracy. Despite an overall drop in firing rate over the whole population, accuracy is enhanced around the adapted contrast and at higher contrasts, leading to greater efficiency of contrast coding at these levels. The estimated contrast discrimination threshold curve becomes elevated and shifted toward higher contrasts after adaptation, as has been found previously in human psychophysical experiments.

Latency differences and the flash-lag effect.

The tendency for briefly flashed stimuli to appear to lag behind the spatial position of physically aligned moving stimuli is known as the flash-lag effect. Possibly the simplest explanation for this phenomenon is that transient stimuli are processed more slowly than moving stimuli. We tested this proposal using a task based upon the simultaneous tilt illusion. When an oriented stimulus is surrounded by another oriented stimulus, the inner stimulus can appear to be rotated away from the orientation of the surround. By flashing central static sinewave gratings at specific phases of an annular gratings rotation cycle, we were able to determine the temporal dependence of the tilt illusion. Our results suggest a small, approximately 20 ms, processing advantage for the rotating stimulus relative to the flashed stimulus. Such a small advantage, if due to differential latencies, is insufficient to account for the flash-lag effect.

Dynamics of the influence of segmentation cues on orientation perception

Contextual effects abound in vision. The tilt illusion (TI) is an example-a tilted surrounding annulus causes a vertical central pattern to appear rotated away from the surround. We investigate the dynamics of this effect by presenting components of the stimulus asynchronously. At equal contrast, the largest illusion occurs when centre and surround are presented simultaneously. We vary the spatial gap between centre and surround, the relative contrast and depth and find that these segmentation cues result in a reduced TI upon simultaneous presentation, but not all other times. This reveals the dynamics of orientation and other segmentation cue interactions.

The role of glial cells and the complement system in retinal diseases and Alzheimer’s disease: common neural degeneration mechanisms

Abstract Many age-related degenerative diseases of the central nervous system (CNS) increasingly appear to have similarities in their underlying causes. By applying knowledge between disorders, and in particular between degenerative diseases of different components of the CNS (e.g. the eye and the brain), we can begin to elucidate general mechanisms of neural degeneration. Age-related macular degeneration and glaucoma, two diseases of retinal neurons, which have recently been discussed in view of their common mechanisms with Alzheimer’s disease, highlight this perspective. This review discusses the common roles of the complement system (an immunological system) and glial cells (providing, amongst other functions, trophic support to neurons) in these three disorders. A number of facets of these systems would seem to be involved in the mechanisms of degeneration in at least two of the three diseases considered here. Regulatory proteins of the complement system (such as factor H), neurotrophin levels, and the interaction of microglia with the complement system in particular may be general to all three presentations of neural degeneration. Investigating the functioning of these fundamental systems across different diseases exemplifies the importance of considering advances in knowledge across a wider base than specific disease pathology. This may give insights both for understanding the function of these supporting systems and providing an avenue for developing future therapeutic targets general to neural degenerative diseases.

Moving from spatially segregated to transparent motion: a modelling approach

Motion transparency, in which patterns of moving elements group together to give the impression of lacy overlapping surfaces, provides an important challenge to models of motion perception. It has been suggested that we perceive transparent motion when the shape of the velocity histogram of the stimulus is bimodal. To investigate this further, random-dot kinematogram motion sequences were created to simulate segregated (perceptually spatially separated) and transparent (perceptually overlapping) motion. The motion sequences were analysed using the multi-channel gradient model (McGM) to obtain the speed and direction at every pixel of each frame of the motion sequences. The velocity histograms obtained were found to be quantitatively similar and all were bimodal. However, the spatial and temporal properties of the velocity field differed between segregated and transparent stimuli. Transparent stimuli produced patches of rightward and leftward motion that varied in location over time. This demonstrates that we can successfully differentiate between these two types of motion on the basis of the time varying local velocity field. However, the percept of motion transparency cannot be based simply on the presence of a bimodal velocity histogram.

Brain mechanisms of recovery from pure alexia: A single case study with multiple longitudinal scans

Pure alexia is an acquired reading disorder, typically due to a left occipito-temporal lesion affecting the Visual Word Form Area (VWFA). It is unclear whether the VWFA acts as a unique bottleneck for reading, or whether alternative routes are available for recovery. Here, we address this issue through the single-case longitudinal study of a neuroscientist who experienced pure alexia and participated in 17 behavioral, 9 anatomical, and 9 fMRI assessment sessions over a period of two years. The origin of the impairment was assigned to a small left fusiform lesion, accompanied by a loss of VWFA responsivity and by the degeneracy of the associated white matter pathways. fMRI experiments allowed us to image longitudinally the visual perception of words, as compared to other classes of stimuli, as well as the mechanisms of letter-by-letter reading. The progressive improvement of reading was not associated with the re-emergence of a new area selective to words, but with increasing responses in spared occipital cortex posterior to the lesion and in contralateral right occipital cortex. Those regions showed a non-specific increase of activations over time and an increase in functional correlation with distant language areas. Those results confirm the existence of an alternative occipital route for reading, bypassing the VWFA, but they also point to its key limitation: the patient remained a slow letter-by-letter reader, thus supporting the critical importance of the VWFA for the efficient parallel recognition of written words.

Combining direction and speed for the localisation of visual motion defined contours.

Detecting discontinuities in motion signal distributions is an essential operation of visual systems, contributing to perception and visuo-motor control. Discontinuities can be signalled by a difference in speed, direction or both. We measured how localisation accuracy for a motion defined contour depends on the velocity differences that define it. A vertical motion contour was defined by two fields of random dots with systematically varied combinations of speed and direction. We find that our data is best explained by assuming that localisation precision is inversely proportional to direction and speed differences that are linearly summed and weighted according to reliability, the optimal solution for combining independent estimates.

Variation in the local motion statistics of real-life optic flow scenes

Optic flow motion patterns can be a rich source of information about our own movement and about the structure of the environment we are moving in. We investigate the information available to the brain under real operating conditions by analyzing video sequences generated by physically moving a camera through various typical human environments. We consider to what extent the motion signal maps generated by a biologically plausible, two-dimensional array of correlation-based motion detectors (2DMD) not only depend on egomotion, but also reflect the spatial setup of such environments. We analyzed the local motion outputs by extracting the relative amounts of detected directions and comparing the spatial distribution of the motion signals to that of idealized optic flow. Using a simple template matching estimation technique, we are able to extract the focus of expansion and find relatively small errors that are distributed in characteristic patterns in different scenes. This shows that all types of scenes provide suitable motion information for extracting ego motion despite the substantial levels of noise affecting the motion signal distributions, attributed to the sparse nature of optic flow and the presence of camera jitter. However, there are large differences in the shape of the direction distributions between different types of scenes; in particular, man-made office scenes are heavily dominated by directions in the cardinal axes, which is much less apparent in outdoor forest scenes. Further examination of motion magnitudes at different scales and the location of motion information in a scene revealed different patterns across different scene categories. This suggests that self-motion patterns are not only relevant for deducing heading direction and speed but also provide a rich information source for scene structure and could be important for the rapid formation of the gist of a scene under normal human locomotion.