Spatial localization of colour and luminance stimuli in human peripheral vision

A variety of studies suggest the localization of objects in three-dimensional space is predominantly the task of the magnocellular (M) system, and conversely that the parvocellular (P) system plays little or no role in localization. However, there are conflicting reports and the goal of this paper was to determine whether spatial localization is predominantly accomplished by one or the other visual system. Both manual pointing and three-target alignment protocols were used to measure localization accuracy for eccentrically presented patches of a sinewave grating. Two general approaches were adopted to activate preferentially one or the other pathway: (1) we varied the spatio-temporal frequency, contrast and chromatic properties of the stimulus to conform with the physiological response properties of either M or P cells; and (2) some measurements were made both with steady fixation and during large saccades, as the latter have been reported to cause selective suppression of the M system [Burr, Morrone & Ross (1994). Nature, 371, 511-513]. Each stimulus was presented at or near its detection contrast threshold, which was determined separately for each visual field location using forced-choice procedures. Using manual pointing, both M- and P-type stimuli were localized to within about 1.3 degrees at retinal eccentricities near 10 degrees. This accuracy was not affected by distractor targets in the peripheral field or temporal uncertainty in stimulus presentation, but was reduced by a similar amount for each stimulus type during saccadic eye movements. Using the alignment task, localization accuracy remained at about 1.3 degrees for P-type stimuli but improved to 0.5 degrees for M-type stimuli. We conclude that both M and P systems play an equally important role in localizing peripheral targets for the purpose of visuo-motor tasks such as pointing, but that the M system may offer an advantage over the P system for the perceptual task of localizing a stimulus relative to nearby targets.