Cortical image density determines the probability of target discovery during active search. 
An analysis of target detection as a function of target eccentricity was made on eye movement data collected from three monkey subjects during active visual search.  Target detection probability was invariant across array set size and eccentricity conditions when the cortical density of relevant stimuli surrounding the target was held constant.  When target color was used to guide search, the effective cortical density was the density of stimuli that shared the target's color.  Thus the passive constraint of cortical magnification in combination with an active selection for a stimulus attribute, in this case color, sets the spatial framework for detection of the target.