Title: Divisive Inhibition as a Solution to the Correspondence Problem
Author: Chien-Chung Chen
Affiliation: Department of Psychology, National Taiwan University, Taiwan.
To perceive an object in a scene, the visual system needs to integrate local image elements together for a global percept. However, in any sufficiently complex scene, there are multiple possible ways to organize local elements. Hence, it is a challenge for the visual system to find the right correspondence among local elements. For instance, to perceive symmetry, the visual system needs to find correspondence between image elements across a symmetry axis. However, if the location and orientation of the symmetry axis are unknown, the midpoint between any pairs of image elements is a candidate for a symmetry axis. We measured symmetry detection under various contexts and different amount of axis-orientation uncertainty. Our result was best described by a multiple channel model in which each channel tunes to a specific axis orientation. The response of each channel is the number of corresponding elements consistent with the tuned symmetry axis divided by an inhibition signal from other channels. Similar computation principle also found in Glass pattern perception and Ebbinghaus size illusion. Thus, divisive inhibition, which was originally proposed to explain phenomena in the contrast domain, is ubiquitous in perceptual grouping. It serves to suppress unwanted groupings and ensure the emergence of the right ones.
Title: Early Spatial Vision: A View through Two Eyes
Author: Mark A. Georgeson
Affiliation: School of Life & Health Sciences, Aston University, Birmingham, UK.
Simple features such as edges are the building blocks of spatial vision. How are visual features and their properties (location, blur, contrast) derived from spatial filter responses in early vision? And how are these visual signals combined across the two eyes? Our psychophysical evidence from blur-matching experiments supports a multi-scale model where edges are found at spatial peaks of response of odd-symmetric receptive fields (gradient operators), and edge blur corresponds to the spatial scale of the most active operator. This model correctly predicts perceived blur for many different luminance profiles, and explains some surprising effects: blurred edges look sharper at low contrast, and at shorter lengths. Binocular combination of early signals certainly involves binocular summation and interocular suppression, but our understanding remains incomplete. Linear summation, for example, predicts that fused edges should look more blurred and lower contrast with increasing disparity, but in experiments edge blur and contrast appear constant across all disparities, whether fused or diplopic. In an effort to unify a variety of effects, I shall describe recent developments in modelling, where binocular summation is highly nonlinear, is shaped by the relative contrasts in the two eyes, and where monocular signals may make a direct contribution to perception.
Title: Curiosities in Spatial Vision
Author: Tim S. Meese
Affiliation: School of Life & Health Sciences, Aston University, Birmingham, UK.
Spatial vision is one of the great success stories in the computational modelling of psychophysical data in human vision. However, along the way, and following the contributions of many other researchers, I have found several curiosities that remain to be properly understood. These include: (a) the presence of distinct bumps in the handles of dipper functions; (b) a change of dipper handle slopes from 0.6 to 1.00 (i.e. Weber’s Law) when the spatial frequency is low and the temporal frequency is high and (c) curious lawful features of contrast-matching functions which include the implication that spatial filtering is absent (or equivalent to a delta/Dirac pulse) in suprathreshold conditions. I shall discuss these (largely, unpublished) findings and offer some discussion of the possible underlying processes and their implications.
Title: The Psychophysical function for contrast
Author: Joshua A. Solomon
Affiliation: School of Health Sciences, City University, London, UK.
Accurate derivation of the psychophysical (a.k.a. transducer) function from just-noticeable differences requires accurate knowledge of the relationship between the mean and variance of apparent intensities. Alternatively, the psychophysical function can be inferred from estimates of the average between easily discriminable intensities. Such estimates are unlikely to be biased by the aforementioned variance, but they are notoriously variable and may stem from decisional processes that are more cognitive than sensory. To circumvent cognitive pollution, I used small, densely packed stimuli of varying contrast. Estimates of average intensity became increasingly variable as size and spacing increased, but average estimates of average intensity were always closer to the veridical mean power (i.e. contrast^2) than they were to the mean contrast or the mean contrast^4.