4The behavioral contrast sensitivity function reflects the combined neural contrast sensitivity of many cortical neurons, each tuned to a particular range of frequencies. Similarly (but not shown), behavioral contrast sensitivity as a function of orientation reflects the combined neural contrast sensitivity of many cortical neurons each tuned to a particular range of orientations. This “multiple channels” hypothesis is plausible if the neurophysiological description of primary visual cortex is accurate, but is there any independent behavioral evidence for this hypothesis?5Effect of adaptation to a particular spatial frequency on the contrast sensitivity function. Blakemore, C. B. and F. W. CAmpbell (1969). “On the existence of neurones in the human visual system selectively sensitive to the orientation and size of retinal images.” Journal of Physiology, London 203: 237-260.67One line of evidence comes from pattern adaptation studies. Here is an example of an orientation adaptation effect. There are similar demonstrations of spatial-frequency adaptation effects.8Here is how the orientation adaptation effect is predicted by the multiple channel hypothesis.9Another line of evidence comes from pattern masking studies. Here is a single vertical (0 deg tilt) grating.10Here a low contrast grating, tilted 90-deg, has been added to the high contrast vertical grating. Note that the low contrast grating is very visible.11Here a low contrast grating tilted 5 deg to the left has been added to the high contrast grating. Note that it is nearly invisible. The multiple channel hypothesis predicts this result.12Here a low contrast 5-deg tilted grating has been added, but it has a higher spatial frequency. Note that the low contrast grating becomes visible again. This result is also predicted by the multiple channel hypothesis.13Here are results from a careful behavioral study show the effect of changing the contrast, orientation, and spatial frequency of a masking grating on threshold for detecting a 2 cycle per degree target grating.14Simultaneous brightness contrast. Simultaneous contrast contrast. Chubb, C., G. Sperling, et al. (1989). “Texture interactions determine perceived constrast.” Proc. Natl. Acad. Sci. 86: 9631-9635.15A masking effect not easily explained by the properties of cortical neurons.16Example of texture pair that should be easy to segment.17Example of a texture pair that should be harder to segment.18The gray scale (luminance) is the same for the two squares in the checkerboard; in fact, it is exactly the same gray shown at the tails of the arrows. Illustrates how perception may reflect the complex properties of the environment. Figure by Edward Adelson.19Visual areas in the visual cortex of the macaque monkey.Felleman, D. J. and D. C. Van Essen (1991). “Distributed hierarchical processing in the primate cerebral cortex.” Cerebral Cortex 1: 1-47.20Major connections between visual cortical areas. The width of the connection is proportional to the number of axon fibers. The top side (warm colors) is the so-called “where pathway” and the bottom side (cool colors) is the so-called “what