Theory of spatial-frequency filtering by the human visual system. II. Performance limited by video noise

A detailed theory of the detection of sinusoidal gratings displayed with suprathreshold luminous fluctuations is developed by employing a previous model of the visual and decision-making systems. An important feature of the model is the organization of the photoreceptors and decision-making system into a set of parallel, independent photoreceptive field (PRF)-decision center channels that function like a set of parallel spatial-frequency filters, each associated with an independent threshold detector. A technique is proposed for determining the modulation sensitivity functions (MSFs) of single detection channels by obtaining threshold modulation (MTN) data at a fixed sinusoidal grating frequency (v) while varying the center frequency (vc) of narrow-band luminous fluctuations caused by video noise (VN). The theory predicts that the ratio, at a given v, of MTN obtained as a function of vc to the MTN obtained without VN is proportional to the MSF of the particular channel for which the widths of the excitatory and inhibitory regions of the PRF equal a half-period of v. Good agreement between theoretical curves and experimental data appearing in the literature provides strong corroboration of the theory.