Superior Parietal Lobule: A Role in Relative Localization of Multiple Different Elements.

Simultanagnosia is an impairment in processing multiple visual elements simultaneously consecutive to bilateral posterior parietal damage, and neuroimaging data have specifically implicated the superior parietal lobule (SPL) in multiple element processing. We previously reported that a patient with focal and bilateral lesions of the SPL performed slower than controls in visual search but only for stimuli consisting of separable lines. Here, we further explored this patient's visual processing of plain object (colored disk) versus object consisting of separable lines (letter), presented in isolation (single object) versus in triplets. Identification of objects was normal in isolation but dropped to chance level when surrounded by distracters, irrespective of eccentricity and spacing. We speculate that this poor performance reflects a deficit in processing objects' relative locations within the triplet (for colored disks), aggravated by a deficit in processing the relative location of each separable line (for letters). Confirming this, performance improved when the patient just had to detect the presence of a specific colored disk within the triplets (visual search instruction), while the inability to identify the middle letter was alleviated when the distracters were identical letters that could be grouped, thereby reducing the number of ways individual lines could be bound.

Visual search for orientation among heterogeneous distractors: experimental results and implications for signal-detection theory models of search.

Three experiments tested a signal-detection theory (SDT) model of visual search (e.g., as described in J. Palmer, C. T. Ames, & D. T. Lindsey, 1993). In Experiment 1, participants searched for a 0 degrees line among distractors at (a) 30 degrees; (b) 1/3 at 30 degrees, 2/3 at 50 degrees; (c) 1/3 at 30 degrees, 50 degrees, and 70 degrees; and (d) 1/3 at 30 degrees, 2/3 at 70 degrees. The SDT model predicts improved performance in the more heterogeneous conditions, as some distractors are more discriminable from the target. In contrast, in Experiment 1 human performance degraded in the more heterogeneous conditions (c and d). In Experiment 2, sparser displays improved the performance of the SDT model. In Experiment 3, search for theta degrees among homogeneous theta + 20 degrees distractors was compared with search for theta degrees among theta+/-20 degrees distractors. Performance in the latter condition was often worse, relative to performance in the homogeneous condition, than predicted by the SDT model; however, this depended greatly on the identity of the target.

A simple saliency model predicts a number of motion popout phenomena.

Visual search for a moving target among stationary distractors is more efficient than searching for a stationary target among moving distractors, and searching for a fast target among slow distractors is more efficient than vice versa. This indicates that the ease of search for a target with a particular motion is not determined simply by the difference between target and distractor velocities. We suggest a simple model for predicting ease of search for a unique motion, based upon a quantitative measure of target saliency. Essentially, search will be easier the more the target motion deviates from the general pattern of velocities in the scene. Our model predicts a number of well-known motion search phenomena, and suggests that one control for target saliency as well as target discriminability when drawing conclusions about visual system mechanisms from search experiments.

Surface orientation from texture: isotropy or homogeneity (or both)?

We examine two models for human perception of shape from texture, based on two assumptions about the surface texture: isotropy and homogeneity. Observers made orientation judgments on planar textured surfaces. Surface textures were either isotropic or anisotropically stretched or compressed. If subjects used an isotropy assumption, they would make biased orientation estimates for the anisotropic textures. In some conditions some observers showed no bias for the anisotropic textures relative to the isotropic textures. In general, even when the observers showed bias, the biases were significantly less than those predicted if the observer used only deviation from isotropy as a cue. Observers appear to use both the deviation from isotropy and a texture gradient or affine texture distortion cue for shape from texture.

Search asymmetries? What search asymmetries?

In order to establish a search asymmetry, one must run an experiment with a symmetric design and get asymmetric results. Given an asymmetric design, one expects asymmetric results, and such results do not imply an asymmetry in the search mechanisms. In this paper, I argue that a number of experiments purporting to show search asymmetries contain built-in design asymmetries. A saliency model of visual search predicts the results of these experiments, using only a simple measure of target-distractor similarity, without reliance on asymmetric search mechanisms. These results have implications for search mechanisms and for other experiments purporting to show search asymmetries.

A computational model for shape from texture.

Shape from texture is best analysed in a two-stage framework analogous to stereopsis and structure from motion: (a) computing the 'texture distortion' from the image; and (b) interpreting the 'texture distortion' to infer the orientation and shape of the scene surface in 3D. We model the texture distortion at a point in any particular direction on the image plane as an affine transformation and derive the relationship between the parameters of the affine transformation and the surface shape and orientation. We have developed a technique for estimating affine transforms between nearby image patches which is based on solving a system of linear constraints derived from a differential analysis. It is not necessary explicitly to identify texels or make restrictive assumptions about the nature of the image texture like isotropy. We have developed two different models for recovering surface orientation (slant and tilt) and shape (principal curvatures and directions) based on the estimated affine transforms in a number of directions. Experimental results are presented on images of planar and curved surfaces under perspective projection.