CONTRAST GAIN-CONTROL IN FIRST-ORDER AND 2ND-ORDER MOTION PERCEPTION

A novel pedestal-plus-test paradigm is used to determine the nonlinear gain-control properties of the first-order (luminance) and the second -order (texture-contrast) motion systems, that is, how these systems' responses to motion stimuli are reduced by pedestals and other masking stimuli. Motion-direction thresholds were measured for test stimuli c onsisting of drifting luminance and texture-contrast-modulation stimul i superimposed on pedestals of various amplitudes. (A pedestal is a st atic sine-wave grating of the same type and same spatial frequency as the moving test grating.) It was found that first-order motion-directi on thresholds are unaffected by small pedestals, but at pedestal contr asts above 1:2% (5-10x pedestal threshold), motion thresholds increase proportionally to pedestal amplitude (a Weber law). For first-order s timuli, pedestal masking is specific to the spatial frequency of the t est. On the other hand, motion-direction thresholds for texture-contra st stimuli are independent of pedestal amplitude (no gain control what ever) throughout the accessible pedestal amplitude range (from 0 to 40 %). However, when baseline carrier contrast increases (with constant p edestal modulation amplitude), motion thresholds increase, showing tha t gain control in second-order motion is determined not by the modulat or (as in first-order motion) but by the carrier. Note that baseline c ontrast of the carrier is inherently independent of spatial frequency of the modulator. The drastically different gain-control properties of the two motion systems and prior observations of motion masking and m otion saturation are all encompassed in a functional theory. The stimu lus inputs to both first- and second-order motion process are normaliz ed by feedforward, shunting gain control. The different properties ari se because the modulator is used to control the first-order gain and t he carrier is used to control the second-order gain. (C) 1996 Optical Society of America.