Quantitative analysis of retinal ganglion cell classifications.

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Abstract
The classification of cat retinal ganglion cells as X or Y on the basis of linearity or nonlinearity of spatial summation was confirmed and extended. Recordings were taken from optic tract fibers of anesthetized, paralyzed cats. When an alternating phase sine wave grating was used as a stimulus, X cells had null positions and Y cells responded at all positions of the grating. These results did not depend on the temporal wave form or the temporal frequency of pattern alternation over a wide range. At high spatial frequencies for the particular cell, a Y cell gave a big on-off response, or frequency doubling, at all positions of the grating, while an X cell did not. The use of contrast sensitivity vs. spatial phase also served to differentiate the 2 cell types. With an alternating sine grating stimulus X cells had a sinusoidal dependence on spatial phase, while each Y cell''s sensitivity depended in a complicated manner on spatial phase. Sensitivity versus spatial phase for different Fourier components of the neural response also separated the 2 classes of cells. Significant 2nd harmonic distortion was present in Y cells. The 2nd harmonic component was spatial phase insensitive, and became dominant at high spatial frequencies. The maximum of the 2nd/1st harmonic ratio was taken as an index of nonlinearity. X cells always had a nonlinearity index less than 1 while in Y cells this index always exceeded 1. Response to spots, diffuse light and drifting gratings were compared to the nonlinearity index as a basis for classifying cells. The nonlinearity index was most reliable because it was least dependent on retinal eccentricity.