Linear and non-linear components in the centre-surround response of X- and Y-cells in the cat lateral geniculate...

Thalamocortical projection cells of cat dorsal lateral geniculate nucleus (dLGN) have been described to exhibit different types of non-linear integration of spatial contrast in addition to the linear integration inside their classical receptive field (CRF). We analysed whether a single mechanism might generate the two harmonic distortions of a linear response elicited inside the CRF and the shift effect elicited from regions outside the CRF. Therefore, both non-linear response types were investigated with identical stimulus conditions in the same cell. A quantitative analysis revealed that both response types can be elicited in nearly all Y-cells and in at least 50% of the X-cells. With blockade of GABA(A) inhibition by bicuculline methiodide (BICU) the number of X-cells with shift effect (SE) and second harmonics (2H) increased to more than 80%. Both, SE and 2H exhibited significantly correlated variations in their response-amplitude and -latency and in the frequency of their occurrence with changes in stimulus parameters (contrast, spatial frequency, area) and during BICU application. We assume that both non-linear contrast responses, the SE and the 2H might depend on the same (most probably retinal) mechanisms. We further suggest that the principal organisation of X- and Y-cell receptive fields might be very similar and that the differing spatial contrast responses may result from the different spatial resolution of their CRF subunits.