The nature of the inputs to cortical motion detectors

Recently, Jagadeesh, Wheat and Ferster [(1993) Science, 262, 1901–1904] presented intracellular recordings from direction-selective simple cells in primary visual cortex and provided an analysis to support the idea that synaptic summation in simple cells is linear. New analysis presented in this study reveals that: (1) the number of subunits contributing to the analyzed simple cell inputs is two; (2) the subunits are nonlinear in the time domain; (3) each subunit linearly integrates the luminance across the receptive field being, thus, linear to local contrast; (4) the waveforms of the subunit signals are linearly modulated by local contrast at the subunit loci unless the contrast changes its sign; (5) the synaptic summation in the simple cell is linear; (6) nonlinearity of even harmonics has sufficient information for retrieving of relative spatial phase of the subunits and reconstruction of the exact temporal profiles of the subunit signals.