Second-order processes in vision: introduction.

led to a radical reconceptualization ofthe nature of visual processing within the framework oflinear systems theory. Pattern adaptation and thresholdpattern-discrimination experiments provided dramaticevidence that human vision filters the retinal input into anumber of channels selectively tuned for spatiotemporalfrequency and orientation. These channels were concep-tualized as approximately linear, shift-invariant transfor-mations of the time-varying retinal input. It was hy-pothesized that these filters were realized byretinotopically organized arrays of neurons. The neu-rons in a given such array were all assumed to have thesame receptive field profile (typically modeled as a Gaborfunction of some specific spatial frequency and orienta-tion), and to monitor different locations in the stimulusfield: one neuron for each location. Thus the responsesof these neurons in a given channel array composed a‘‘neural image’’