Reduced perceptual dimensionality in extrafoveal vision.

The classification behaviour of human observers with respect to compound Gabor signals is tested at foveal and extrafoveal retinal positions. Classification performance is analysed in terms of a probabilistic classification model recently proposed by Rentschler, Jüttner and Caelli [(1994) Vision Research, 34, 669-687]. The analysis allows inferences about structure and dimensionality of the individual internal representations underlying the classification task and their temporal evolution during the learning process. Using this technique it is found that the internal representations of direct and eccentric viewing are intrinsically incommensurable, in the sense that extrafoveal pattern representations are characterized by a lower perceptual dimension in feature space relative to the corresponding physical input signals, whereas foveal representations are not. The observed deficits cannot be renormalized by size scaling (cortical magnification); however, they can be partially reduced by learning although the learning progress strongly depends on the observer's practice. The structural incommensurability between foveal and extrafoveal representations poses constraints on possible forms of foveal-extrafoveal interaction, which might have implications on related perceptual phenomena such as visual stability across saccadic eye movements.