explore the models and carry out simulations by obtaining a Macintosh computer program over Internet (13). Movshon and coworkers proposed that each cortical area conducts calculations having the same basic form but using distinctive inputs.

William Newsome (14) presented experiments that probe the neural basis of decision making. What are the cognitive links between sensation and action? Neural responses in the lateral intraparietal area (LIP) of the cortex were monitored while alert monkeys discriminated the direction of visual motion. A monkey was required to judge the direction of coherent motion in a dynamic random dot pattern in which only a fraction of the dots moved consistently in one of two directions. After a delay of about a second, the monkey reported the direction of coherent motion by making an eye movement to one of two visual targets. The significant finding was that neurons in LIP generate signals that predict the decision a monkey will make. Newsome views neural activity in LIP as a window on the decision-making process in which weak, slowly arriving sensory information is integrated. Two fascinating questions arise: (a) What is the neural circuitry that links sensation to decision making to motor activity? (b) Which elements of this circuitry continue to be utilized when monkeys make decisions based on a different sensory attribute, such as the color of a random dot pattern?

In the final talk, Ken Nakayama (15) showed that stereoscopic vision plays a key role in the perception of surfaces. Because two-dimensional surfaces are often only partially visible, a three-dimensional interpretation is needed before two-dimensional information can be fully evaluated. The task is to distinguish between true boundaries and spurious ones caused by occlusion, to determine border ownership. First, binocular disparity (the differential angular separation between pairs of image points in the two retinas) is used to sort edges and determine their ownership. Second, half-visible points (image points in one eye having no counterpart in the other because of occlusion) provide complementary information. The identity of the eye receiving the visual input needs to be known to form the correct image. This essential eye-oforigin information probably resides in the striate cortex (V1). The reader can explore relations between stereopsis and the perception of surfaces by looking at Nakayama 's vivid stereoscopic illustrations (15).

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