Seminar Series

What is improved by visual attention?

Does selective attention do anything for low-level visual processing, or does it help only the higher cogntive processes like memory? Grindley and Townsend found that knowledge of where a shape would appear did not facilitate its recognition. Others have found that when the target is present with multiple distractors, knowledge of position does help shape analysis, but this could be due to the purely statistical effects of positional uncerntainty. I shall describe psychophysical studies of orientational acuity for a tilted gaussian target presented with multiple distractors. In a brief, masked expsoure the effects of distractors are greater than expected from spatial uncertainty. I shall suggest an explanation for this. In the absence of positional information about the target, the observer uses texture analysers to find the mean orientation of the ensemble. Predictions: (1) acuity will be lowered if the distractors have a subliminal tilt away from the target (2) acuity will be enhanced if the distractors have tilts in the same direction as the target. I shall describe some current experiments in which a single target amongst distractors is apparently invisible. But if an increasing number of distractors share the orientation of the target, orientation acuity improves. "Crowding" and texture perception are two sides of the same mechanism: spatial averaging of local information by higher-order collector units. References: Morgan, M.J., Castet, E. & Ward, R.M. (1998) Visual search for a tilted target: tests of the spatial uncertainty model. Quart.J. Exp. Psychol., 51, 347-371. Morgan, M.J. & Baldassi, S. (1997) How the human visual system encodes the orientation of a texture and why it makes mistakes. Current Biology, 7, 999-1002. Solomon, J. A., Lavie, N. & Morgan, M. J. (1997). Contrast discrimination function: spatial cuing effects. Journal of the Optical Society of America A, 9, 2443-2448.