SPATIAL SCALE INTERACTIONS AND VISUAL-TRACKING PERFORMANCE

It has previously been observed that low spatial frequencies (less tha n or equal to 1.0 cycles deg(-1)) tend to dominate high spatial freque ncies (greater than or equal to 5.0 cycles deg(-1)) in several types o f visual-information-processing tasks. This earlier work employed reac tion times as the primary performance measure and the present experime nts address the possibility of low-frequency dominance by evaluating v isually guided performance of a completely different response system: the control of slow-pursuit eye movements. Slow-pursuit gains (eye vel ocity/stimulus velocity) were obtained while observers attempted to tr ack the motion of a sine-wave grating. The drifting gratings were pres ented on three types of background: a uniform background, a background consisting of a stationary grating, or a flickering background. Low-f requency dominance was evident over a wide range of velocities, in tha t a stationary high-frequency component produced little disruption in the pursuit of a drifting low spatial frequency, but a stationary low frequency interfered substantially with the tracking of a moving high spatial frequency. Pursuit was unaffected by temporal modulation of th e background, suggesting that these effects are due to the spatial cha racteristics of the stationary grating. Similar asymmetries were obser ved with respect to the stability of fixation: active fixation was les s stable in the presence of a drifting low frequency than in the prese nce of a drifting high frequency.