Oliver J Braddick | PubMed Health

PubMed2003

When does the Titchener Circles illusion exert an effect on grasping?. Two- and three-dimensional targets.

This study used the Titchener Circles illusion to investigate the functional dissociation of the dorsal visuomotor and ventral perceptual systems. In order to investigate the visual requirements for an action to be driven by the dorsal stream, two-dimensional (2D) and three-dimensional (3D) targets were compared. Thirteen subjects made visual open loop manual estimations or grasping actions towards 2D and 3D versions of the illusion. No illusion effect was found for immediate grasping, but the illusion did influence manual estimation, irrespective of whether the target was 2D or 3D. It is suggested that the underlying representations used to drive grasping actions towards 2D targets are fundamentally the same as those used to drive natural grasping actions. While stereoscopically specified depth may be important to the dorsal visuomotor system, it does not appear to be necessary in order for an action to be based on dorsal representations. It is suggested that an action must be goal directed in order for that action to be driven by dorsal stream processes.

Adolescent↗

PubMed2002

Directional performance in motion transparency.

Motion transparency provides a challenging test case for our understanding of how visual motion, and other attributes, are computed and represented in the brain. However, previous studies of visual transparency have used subjective criteria which do not confirm the existence of independent representations of the superimposed motions. We have developed measures of performance in motion transparency that require observers to extract information about two motions jointly, and therefore test the information that is simultaneously represented for each motion. Observers judged whether two motions were at 90 degrees to one another; the base direction was randomized so that neither motion taken alone was informative. The precision of performance was determined by the standard deviations (S.D.s) of probit functions fitted to the data. Observers also made judgments of orthogonal directions between a single motion stream and a line, for one of two transparent motions against a line and for two spatially segregated motions. The data show that direction judgments with transparency can be made with comparable accuracy to segregated (non-transparent) conditions, supporting the idea that transparency involves the equivalent representation of two global motions in the same region. The precision of this joint direction judgment is, however, 2-3 times poorer than that for a single motion stream. The precision in directional judgment for a single stream is reduced only by a factor of about 1.5 by superimposing a second stream. The major effect in performance, therefore, appears to be associated with the need to compute and compare two global representations of motion, rather than with interference between the dot streams per se. Experiment 2 tested the transparency of motions separated by a range of angles from 5 degrees to 180 degrees by requiring subjects to set a line matching the perceived direction of each motion. The S.D.s of these settings demonstrated that directions of transparent motions were represented independently for separations over 20 degrees. Increasing dot speeds from 1 to 10 deg/s improved directional performance but had no effect on transparency perception. Transparency was also unaffected by variations of density between 0.1 and 19 dots/deg(2)

Discrimination, Psychological↗