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Neel T. Dhruv, Samuel G. Solomon, Jonathan W. Peirce; Profound Contrast Adaptation Early in the Visual Pathway. Journal of Vision 2004;4(8):225. doi: 10.1167/4.8.225.
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Prolonged viewing of a pattern leads to a change in the appearance of that and similar patterns. Earlier physiological research found no contrast adaptation at sub-cortical levels but profound contrast adaptation in cortical neurons. However, some recent studies suggest that slow contrast adaptation occurs in retinal ganglion cells. We asked if contrast adaptation occurs in neurons of the lateral geniculate nucleus (LGN) of primates. In five macaques prepared for acute recording we measured contrast response curves for drifting gratings of optimal spatial and temporal frequency before and after presentation of a full-contrast adapting grating. Adaptation substantially reduced the sensitivity of magnocellular (M) cells to low contrasts. For our population of 22 cells, the average contrast detection threshold increased by 404%. Similar measurements on 21 parvocellular (P) cells and 5 cells with S-cone input revealed weak contrast adaptation or none at all. Contrast adaptation might be beneficial if it brings normally high (saturating) contrast levels into a cell's operating range. We found that adaptation in M cells significantly increased the discriminability of gratings of high contrast. The contrast adaptation in M cells, which appears to be present in their retinal afferents, differs in important ways from that observed in cortical neurons. Unlike in cortical neurons, adaptation in M cells was not specific for spatial frequency or orientation; it could be induced with high (at least up to 45 Hz) but not low temporal frequencies (ca. 1 Hz). Our result suggests a new tool for determining the perceptual role of the magnocellular pathway: one could measure an observer's sensitivity to stimuli before and after adaptation to high-contrast gratings displayed at imperceptibly fast speeds.
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