The two-dimentional shape of spatial interaction zones in the parafovea

The spatial analysis of a target may be strongly degraded by the simultaneous presentation of nearby pattern elements. The present study investigated the shape and extent of the region of interaction as a function of retinal location. The stimuli consisted of 3 collinear Ts which were randomly oriented up (T) or down. The task was to discriminate the orientation of the middle T. The retinal locations studied were at 0, 2.5, 5 and 10°, on the lower vertical meridian and on the nasal halves of both the horizontal and the 45° diagonal visual field meridians. The extent of the interaction region was defined as the separation between the midpoint of two adjacent Ts that resulted in 75% correct discrimination. The shape of the interaction region was determined by using several orientations (horizontal, vertical, left diagonal and right diagonal) for the virtual line joining the 3 Ts. Our results show that the size of the interaction regions varies linearly with eccentricity as does the size of a just resolved individual T. However, the size of the interaction region varies much more rapidly than does the resolution threshold for an individual T. The spatial interaction zones appear to be elongated radially, so that they have an elliptical shape. The size of the major axis is about 2-3 times the size of the minor axis. The major axis is along the meridian through the central visual field (i.e. it is oriented radially) while the minor axis is oriented tangentially (i.e. isoeccentrically).