Fig. 1.

Selectivity of sensory systems. (A) Somatosensory homunculus: a neuronal map of the skin and epithelia in the human cerebral cortex. The size of neuronal representation of skin regions correlates with sensitivity to tactile stimuli (2). (B) Distribution of the peaks of spatial frequency tuning functions for neurons in macaque primary visual cortex. The cell tuning was measured for low temporal frequencies, corresponding to a section of the contrast sensitivity function in C. As in A, the size of neuronal representation of a stimulus correlates with the sensitivity to that stimulus. (C) Human spatiotemporal contrast sensitivity function. The varying height of the surface represents the varying contrast threshold: the amount of luminance contrast (“modulation”) that makes the stimulus just visible; the smaller the modulation at the threshold the higher the sensitivity. (D) A contour plot of the sensitivity function from C. The level curves are the isosensitivity contours, here plotted for five magnitudes of contrast threshold, from 0.005 to 0.08. The ratios of temporal frequency to spatial frequency are stimulus speeds, notated on top right. Stimulus conditions that correspond to the same speed form a diagonal line. Such constant-speed lines for different speeds are parallel to one another in the logarithmic coordinates. The lines are plotted for nine speeds spanning a 500-fold range of speed. The thicker curve labeled “max” connects points of maximal sensitivity across speeds. [A is reproduced from ref. 1; C and D are adapted from ref. 3; B is reprinted from ref. 4: Vision Research, 22/5, De Valois et al., Spatial frequency selectivity of cells in macaque visual cortex. Copyright 1982 with permission from Elsevier.]