Direction Selective Simple Cell

A direction selective cell responds differently depending on the direction of motion of the stimulus. Many, if not most, visual cells in the cortex are direction selective, although for simplicity we have only supplied you with a single example in our simulation.

Our direction-selective model is a Barlow-Levick null-inhibition type of model. Two odd-type simple cells drive the direction-selective (DS) simple cell. One excites the DS cell, the other inhibits it. The inhibitory input is spatially offset. When the stimulus comes from this null direction, the DS cell is inhibited by the excitation of the non DS simple cell. When the stimulus comes from the preferred direction, the DS cell responds because the inhibitory input to the DS cell has not yet been excited. The inhibitory input to the DS cell lasts longer because it is more lowpass filtered.

Real direction selective simple cells are thought by some people to arise from a correlation-type mechanism. The DS cell correlates a direct input with a delayed, spatially offset input. In addition, DS cells combine inputs of various phases and symmetries to result in a realistic response. People are still arguing about the mechanism of direction selectivity in early visual cortex, and the mechanism that results in the very long delays that seem to be necessary to explain direction selectivity for very small speeds.are not yet understood.