Local and global limitations on direction integration assessed using equivalent noise analysis

Abstract

We used an equivalent noise (EN) paradigm to examine how the human visual system pools local estimates of direction across space in order to encode global direction. Observers estimated the mean direction (clockwise or counter-clockwise of vertical) of a field of moving band-pass elements whose directions were drawn from a wrapped normal distribution. By measuring discrimination thresholds for mean direction as a function of directional variance, we were able to infer both the precision of observers' representation of each element's direction (i.e., local noise) as well as how many of these estimates they were averaging (i.e., global pooling). We estimated EN for various numbers of moving elements occupying regions of various sizes. We report that both local and global limits on direction integration are determined by the number of elements present in the display (irrespective of their density or the size of region they occupy), and we go on to show how this dependence can be understood in terms of neural noise. Specifically, we use Monte Carlo simulations to show that a maximum-likelihood operator, operating on pooled directional signals from visual cortex corrupted by Poisson noise, accounts for psychophysical data across all conditions tested, as well as motion coherence thresholds (collected under similar experimental conditions). A population vector-averaging scheme (essentially a special case of ML estimation) produces similar predictions but out-performs subjects at high levels of directional variability and fails to predict motion coherence thresholds.

Brief Report: Coherent Motion Processing in Autism: Is Dot Lifetime an Important Parameter?

“…Nonetheless, the results of the current study add to the mixed pattern of motion coherence findings in the literature and suggest that children with autism do not have general, pervasive difficulties with coherent motion perception. The lack of a clear-cut difference in motion coherence sensitivity between individuals with and without autism may reflect the fact that integration and segregation demands are confounded in motion coherence tasks (Dakin et al, 2005). Indeed, these factors may cancel each other out in children with autism, who may exhibit a profile of enhanced direction integration and reduced segregation of signal from noise compared to TD children (Manning et al, submitted).…”

Orientation coherence sensitivity

“…For both dorsal and ventral global tasks, performance depended on the ''visibility'' of the stimulus; in other words, stimulus suprathreshold contrast (Dakin & Bex, 2001a;Hess & Zaharia, 2010). Both tasks display scale invariance (Dakin & Bex, 2001a;) and a lack of dependence on aperture size (Dakin, 2001;Dakin et al, 2005;Downing & Movshon, 1989;Watamaniuk & Sekuler, 1992; although see Jones et al, 2003). This suggests that information processing in dorsal and ventral pathways is either collapsed across spatial frequency or processed similarly across scale and that it is only contrast relative to threshold that is important.…”

“…And second: Can coherence detection be thought of as a purely integrative process? To answer the first question, we investigated a number of key properties for global form for which we have comparable global motion data, for example, the spatial (spatial frequency and orientation) (Dakin, Mareschal, & Bex, 2005;, contrast Simmers, Ledgeway, Hess, & McGraw, 2003); eccentricity (Hess & AaenStockdale, 2008;Hess & Zaharia, 2010), and areal summation (Dakin, 2001;Downing & Movshon, 1989;Watamaniuk & Sekuler, 1992;Williams & Sekuler, 1984) dependencies. To answer the second question, we modify our task so that there is a clear prediction if performance is solely determined by integrative mechanisms.…”

Disentangling signal and noise in autism spectrum disorder

“…Most CM studies use random dot kinematograms (RDKs), for which participants are required to discriminate the overall dominant motion direction of a field of dots, while a certain percentage of the dots moves randomly across the display. However, rather than being a direct test for global processing capacities, as frequently thought, CM tasks actually confound the capacity to integrate local inputs into global percepts with one's sensitivity to noise (Dakin, Mareschal, & Bex, 2005;Manning, Dakin, Tibber, & Pellicano, 2014). While RDK performance in ASD is predominantly interpreted in relation to possible global integration deficits (or weak central coherence), the results could also be interpreted in terms of the participants' ability to cope with noise inherently present in the displays.…”