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Tuned mechanisms or `channels' have been demonstrated in many aspects of human vision, and their characteristics span a continuum from a small set of broadly tuned channels (as in the spectral tuning of cone mechanisms) to a large array of narrow channels (as in the spatial tuning of cone mechanisms). The optimal number and tuning widths of channels for a given dimension depends on a trade-off between an economy of processor resources and the avoidance of metamerism. A small number of broad channels requires a small investment in processor resources and can support fine discriminations but is subject to metameric confusions. A large number of narrow channels requires a greater investment in processor resources but allows for the representation of multiple values on the tuning dimension (e.g., transparency). In the context of stereopsis and vergence control, single unit recordings have provided evidence that disparity tuned mechanisms cover the range from closely spaced, narrow channels (`tuned' cells) to widely spaced, broad channels (`near/far' cells). In principal, near/far mechanisms should be sufficient to control vergence and allow for fine stereoacuity right around the horopter. Tuned mechanisms might be required for fine disparity discriminations off the horopter and for the perception of stereo transparency. We have investigated the disparity tuning characteristics of binocular visual mechanisms which mediate (1) the psychophysical detection of surfaces in dynamic noise stimuli and (2) the involuntary oculomotor vergence responses to such surfaces. We have found evidence that both perceptual and oculomotor systems involve a large set of narrowly tuned mechanisms with inhibition between neighboring channels. A model is developed which clarifies the nonobvious relationship between measured tuning functions and characteristics of underlying channels.
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