Bipolar Cell

The bipolar cells compute the rectified difference between the photoreceptor and horizontal cell outputs.

Rectification manifests itself in two types of bipolar cells, on and off type.  The on type responds when the photoreceptor sees a stimulus brighter than the surround, while the off type responds when the stimulus to the photoreceptor is darker than the surround. The resting output of bipolar cells is low--most of the output dynamic range is used to represent only a particular sign of contrast, either on-ness or off-ness.

On-Center RF

 

Off-Center RF

We modeled this computation as the simple sigmoidal function shown below that applied to the difference between a particular photoreceptor and the single horizontal cell. The sigmoid is shifted so that the output for zero differential input is near zero. In reality, this computation probably occurs in the triad synapse formed by the photoreceptor, horizontal cell, and bipolar cell.

Transfer function of bipolar cell