Retinal Prosthesis Dissertation - Student Home Pages

Figure 28 Receptive field example
3.6.3 Commonality of sub retinal and epi retinal approaches
In Carver Meads 1990 invited paper “[200]” which refers to Mahowald’s “Silicon
Retina” [201] a description is given of each node in the retina (photoreceptor) and
the hexagonal resistive network between them. The effect of this network is to
compute a spatially weighted average of photoreceptor inputs meaning the output
from the circuit is the difference between the resistive network and that node. So this
sub retinal approach using photodiodes as receptors replicates the centre surround
processing of the vertebrae retina[202]. This centre surround processing is also used
in epi-retinal approaches by using the particular edge detection method described
below to process the received image from an external camera [40, 111, 200, 201,
203, 204]. The receptive field concept is described by Difference of Gaussians
(DOG) which for the centered two-dimensional case is,
( )
( ) ( ) ( ) ( )
(1)
Where x and y are the pixel co-ordinates, σ (standard deviation); pragmatically the
diameter of the centre, and β (space constant) is the ratio of outer diameter to inner
diameter of the receptive field function. To define the difference of Gaussian (DOG)
i.e. f(x,y,σ)from the above description in image processing terms, that part of the
equation relating to the inner circle i.e. the kernel is defined as g inner (x,y,σ) =
( ) ( )
( ) (2)
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