Chapter 9

Chapter summary

Vision at different light levels can be divided into three zones: scotopic (darkness; rod-mediated); twilight (rod- and cone-mediated); and daylight (cone-mediated). The shift from scotopic to photopic vision takes 30 minutes.

The spatial contrast sensitivity function shows the visual system’s relative sensitivity details at different scales or spatial frequencies, from fine or high frequency (individual leaves of a tree, for example) to coarse or low frequency (overall tree shape). The temporal contrast sensitivity function shows relative sensitivity to light variation over time (flicker or temporal frequency). Sensitivity is optimal at medium spatial and temporal frequencies, and the shape of the function can be explained by the properties of neurons in the visual pathway.

Natural images contain information at many different spatial frequencies, and Fourier theory provides a rigorous mathematical procedure of analysing the spatial frequency content of images. Psychophysical and neurophysiological research shows that the visual system processes images using channels that are selectively tuned to specific ranges of spatial frequency.

These patterns of response in spatial frequency channels contributes to several computations performed by the visual system, including edge location, texture analysis, assessment of the gist of an image, and analysis of depth and motion.