Photoreceptor

You can view the locations of the photoreceptors. Choose View/Photoreceptor Locations from the menu.

Receptive Field

The photoreceptor is the first stage of processing in the retina. A photoreceptor responds to the brightness at a certain location on the screen. It has a steady-state (DC) response and a transient response. The gain of the transient response is higher than that of the steady state response by a factor of 2 or 3. The transient response adapts away after a second or so.

In this simulation we have not modeled the amazing gain-control of real photoreceptors that let them function over many decades of illumination. (There's no need for this in the controlled environment of a virtual tangent screen). We have also not modeled the known feedback from the horizontal cells to the photoreceptor, via the interesting triad synapse. However, we have modeled the photoreceptor's temporal adapatation properties using a linear highpass filter. The photoreceptors response is the sum

R= ACGain*Highpass(input)+DCGain*input

We can compute the photoreceptor response to the simple stimuli you can use in the simulation because people have developed very fast algorithms for computing the intersections of convex polygons. If we needed to do a general convolution of the shape of the photoreceptor and an arbitrary stimulu,s the simulation would probably not run in real time. These polygon intersections are shown in the output from one of the test classes below.