Notes

Both simple and complex cells are orientation selective. In the case of the simple cells, presentation of the bars in different areas of the receptive fields have differential effects due to on/off regions, while this doesn't occur in complex cells that show a constant response across the whole receptive field.

Simple vs Complex Cells Responses The figure above is interesting to compare the responses of simple cells and complex cells to sinusoidal stimuli. In the case of a simple cell, we obtain a rectified sinewave as we have also seen before. In the case of the complex cell, that receives inputs from two simple cells with opposite receptive fields, we have a doubled frequency of the sinewave, frequency doubling.

Could Simple and Complex Cells be Extremes of a Continuum?

Simple cells in V1 have segregated On/Off subregions in their receptive fields, while complex cells have overlapping On/Off subregions. These two cell types form the extremes at each end of a continuum of receptive field types. Hubel & Wiesel suggested a hierarchical scheme of processing whereby spatially offset simple cells drive complex cells. In their pioneering studies of V1, Hubel & Wiesel described the existence of two classes of cells, which they termed "simple" and "complex". The original classification scheme was based on a number of partly subjective tests of linear spatial summation. Later, investigators adopted an objective classification method based on the ration between the amplitude of the first harmonic of the response and the mean spike rate (or the F1/F0 ratio) when the neuron is stimulated with drifting sinusoidal gratings. This measure is bimodally distributed over the population and divides neurons into two classes that correspond closely to the classical definition by Hubel & Wiesel. Here we show that a simple rectification model can predict the observed bimodal distribution of F1/F0 in V1 when the distributions of the intracellular response modulation and mean are unimodal. Thus, contrary to common belief, the bimodality of F1/F0 does not necessarily imply the existence of two discrete cell classes.