Field-of-view information for self-motion percepion
Psychology Press eBooks · 2014
There are other differences between the two modes. The focal mode is shown by Leibowitz, Shupert-Rodemer, and Dichgans (1979) to be very sensitive to changes in luminance, while the ambient mode seems to oper ate in an all-or-none fashion. The focal mode suffers a systematic decrease in performance as the luminance level is lowered, while ambient functions are performed adequately as long as the source of stimulation is visible. Refractive error also seems to have a differential effect on the two modes. Leibowitz et al. (1979) found that the ambient function is maximally ef ficient as long as a minimal pattern is available, but even slight refractive distortion is sufficient to disrupt focal functions such as reading or iden tification. The fact that retinal location, luminance, and refractive error have different effects on the two modes seems to imply that the modes are sensitive to distinct spatial frequencies, i.e., the number of luminance changes per degree of visual angle. The resolution of high frequencies is degraded by blur (Regan, Silver, & Murray, 1977) and reduced luminance, while these factors have little effect on low frequencies. This suggests that low frequencies might be sufficient for the ambient function while higher frequencies might be needed for focal recognition and discrimination.