Distortion products in geniculate X-cells: a physiological basis for masking by spatially modulated gratings?

The responses of X-cells in cat lateral geniculate nucleus, to complex grating patterns moving across the receptive field, were recorded with microelectrodes. The patterns were multi-component gratings, composed by adding a low spatial-frequency sinusoidal "signal" to a high spatial frequency "mask" which was either unmodulated, contrast-modulated (AM) or quasi-frequency modulated (QFM) at the signal frequency. The response to AM and QFM gratings has a component at the same frequency as the response to the "signal". This low frequency component has the properties of a distortion product generated by a quadratic non-linearity in the LGN. These properties may account for the psychophysical masking which occurs between modulated high-spatial-frequency gratings and gratings of the modulation frequency [Henning, Hertz and Broadbent (1975) Vision Res. 15, 887-897].