Seeing and the Retina

Overview-Review of Retinal Physiology-Seeing Edges with the Retina-Retinal Implants

Review of the Retina

-Photoreceptors-Rods-Cones

-Bipolar Cells-Ganglion Cells-Optic Nerve

Seeing Edges with Receptive Fields in the Retina

Receptive Fields of Bipolar CellsCenter Surround Receptive

FieldsOn-Center Off-SurroundOff-Center On-Surround

This example leaves out horizontal cells, which serve as lateral connections between receptors and bipolar cells

In this model the sum of excitation and inhibition in each receptive field adds to zero, so as to have no charge when not on an edge in the visual field

Convolution Array of Ganglion CellsIn the convolution array

of the bipolar cells, each cell is connected to the receptor cell directly above it as well as the cells directly surrounding that receptor cell

The charge of the cell in the convolution array is affected by the sum of the excitation and inhibition in the receptive field above it

Edges and Receptive Fields

Receptive Fields not on an edge have zero excitation due to sum of center and surround cancelling out

However, receptive fields on the edge of the image get a non-zero charge due to different light gradients on either side of the receptive field

Zero Crossing & the AND Detector

Zero Crossing on edge due to two different charges on each side

It is hypothesized that there is a specific AND detector that looks for this zero crossing to see edges

How Off-Center On-Surround Receptive

Fields HelpCorresponding Off-

Center On-Surround Receptive Field for every On-Center Off-Surround field

With no negative charge in visual cells, AND cells look for charge in both kinds of receptive feilds

SummaryBy looking for charge in receptive fields of

receptor cells, cells in the convolution array are able to see edges in our visual field

Since visual neurons can’t have negative charges, in order to see edges on receptive fields that have more inhibition than excitation, off-center on-surround receptive fields are also present to provide positive charge in these places

Thusly edges are detected in our visual field on the convolution array of the bipolar cells where corresponding receptive fields of the receptor cells provide charge unequal to zero

NoteAs said in the book, this model for edge

detection is highly simplified as to make it easier to understand

Retinal Implants

Epiretinal Implants

Subretinal Implants

Sachs, H.G., V.P. Gabel, “Retinal replacement—the development of microelectronic retinal prostheses— experience with subretinal implants and new aspects” Springer-Verlag 2004

Frisby, John P., & James V. Stone, Seeing, 2nd ed. MIT Press, Cambridge, 2010