Cortical processing of hyperacuity tasks.

Spatial discrimination thresholds were determined by having subjects make comparisons between stimuli presented successively at the same or at different locations in the visual field. Two tasks were employed, discrimination of line orientation and discrimination of the distance between two parallel lines (spatial interval discrimination). We find that discrimination thresholds based on the comparison of features in spatially-separated stimuli are comparable to those based on the comparison of features in two stimuli presented successively at the same location. This holds even when the stimuli are presented in nonhomologous positions in the visual field or are presented in a manner such that they activate cells in opposite cortical hemispheres in the early visual areas. This shows that discriminability is not determined solely by differences in the firing rates of striate or pre-striate neurons. Rather, it appears that the attributes of visual stimuli are precisely coded and available for comparison at higher levels of visual processing. Implications of this finding for models of hyperacuity are discussed.