Center/surround relationships of magnocellular, parvocellular, and koniocellular relay cells in primate lateral...

As in other primates, the lateral geniculate nucleus (LGN) of the prosimian primate, bush baby (Galago crassicaudatus), contains three morphologically and physiologically distinct cell classes [magnocellular (M), parvocellular (P), and koniocellular (K)] (Norton & Casagrande, 1982; Casagrande & Norton, 1991). The present study examined quantitatively the center/surround relationships of cells in all three classes. Estimates of receptive-field center size (Rc) and sensitivity (Kc) and of surround size (Rs) and sensitivity (Ks) were obtained from 47 LGN relay cells by fitting a difference of Gaussians function to contrast-sensitivity data. For M and P cells, center size (Rc) increases with eccentricity but is about two times larger for M than for P cells at a given eccentricity. Surround size (Rs) increases with eccentricity for P but not for M or K cells. The center sensitivity (Kc) is inversely related to center size (Rc) and surround sensitivity (Ks) is inversely related to surround size (Rs) for cells in all classes, a result consistent with the sensitivity regulation that is produced by light adaptation. High spatial-frequency cutoff (acuity) is inversely related to center size (Rc). However, the peak contrast sensitivity is relatively independent of Rc. The ratio of the integrated strength (volume) of the surround to the volume of the center remains relatively constant (median, 0.87) across all three cell classes. This ratio is an excellent predictor of a cell's rolloff in contrast sensitivity at low spatial frequencies: cells with a low surround/center ratio have less low-frequency rolloff. Although M, P, and K cells generally display similar center/surround relationships, differences in center size and the other parameters between the classes distinguish most M, P, and K cells. These findings demonstrate that both similarities and differences in the visual-response properties of primate LGN cells in these three parallel afferent pathways can be explained by basic center/surround relationships.