Models of Simple and Complex Cells

The receptive field properties of neurons in the retina and LGN are fairly similar both in their spatial center-surround organization and in their temporal band-pass structure. What happens at the level of primary visual cortex where LGN axons project? The first systematic investigations of cortical visual responses were made by Hubel and Wiesel at the end of the 1950s and early 1960s, taking advantage of newly engineered tungsten electrodes that permitted stable extracellular recordings from nerve cells over long periods of time in awake animals. Similar electrodes have been used over more than sixty years for extracellular recordings in cats and monkeys. Hubel and Wiesel soon discovered that the responses of cortical neurons are much richer than what had been described at earlier stages of visual processing and reported the existence of two major types of cells called simple and complex in the primary visual cortex of the cat. They went on to characterize many other aspects of the representation of visual information by cortical neurons and cell assemblies in primary visual cortex. We will focus here on describing the linear and non-linear properties of simple and complex cell receptive fields as well as some aspects of the transformation of neural signals occurring at the cortical level.