Abstract
We revisit the quantitative evidence for the existence of discrete simple and complex cell classes in V1. The dichotomy, introduced by Hubel and Wiesel (1962, 1968), was initially based on qualitative measurements of receptive field properties. Later, Skottun et al (1991) provided quantitative support for this dichotomy by showing that, when neurons are stimulated with the optimal drifting sinusoidal grating, the ratio between the first harmonic of the spike rate and its mean (F1/DC) has a bimodal distribution in V1. Furthermore, the resulting classification corresponded well with Hubel and Wiesel's criteria.
Here, we show that the bimodal distribution of F1/DC, and the location of the dip at F1/DC=1, is predicted by a half-rectification model where the distributions of the intracellular first harmonic and mean responses are unimodal. This demonstrates the existence of a simple system that, with unimodally distributed physical parameters, generates F1/DC distributions that are statistically indistinguishable from the real data. Thus, surprisingly, bimodality of the F1/DC distribution does not necessarily imply the existence of two discrete classes of neurons. In addition, we tested published distributions of a large number of V1 characteristics (including sub-field segregation, orientation selectivity, phase sensitivity, and color tuning, among others) for the significance of bimodality and found none that supported the existence of distinct “simple” and “complex” cell classes in V1.
Future research may be able to demonstrate a dichotomy in a suitable feature space. Alternatively, our results, consistent with theoretical ideas put forward by Chance and Abbott (1999) and others, suggest that “simple” and “complex” cells may represent the extremes of a continuum of neuronal properties. Such a continuum could be generated by a rather uniform intracortical microcircuitry as opposed to the simple-to-complex hierarchy of the original model of V1 architecture.
Support: NIH EY-9314 (FM), NIH EY-12816 (DLR) and NSF-IBN-9720305 (DRL)