neural nets and light adaptation (TR)

[Dr. Josef Skrzypek]

	  NEURAL NETWORK CONTRIBUTION TO LIGHT ADAPTATION:
	     FEEDBACK FROM HORIZONTAL CELLS TO CONES


		       JOSEF SKRZYPEK

   Machine Perception Laboratory, Computer Science Department
	                    and
	  CRUMP Institute of Medical Engineering.
			    UCLA


			  SUMMARY

Vertebrate cones respond to a stepwise increase in localized light
intensity with a graded potential change of corresponding amplitude.
This S-shaped intensity-response (I-R) relation is limited to 3 log
units of the stimulating light and yet, cone vision remains functional
between twilight and the brightest time of day.  This is in part due to
light adaptation mechanism localized in the outer segment of a cone.
The phenomenon of light adaptation can be described as a resetting of
the system's response-generation mechanism to a new intensity domain
that reflects the ambient illumination. In this project we examined
spatial effects of annular illumination on resetting of I-R relation by
measuring intracellular photoresponses in cones. Our results suggest
that peripheral illumination contributes to the cellular mechanism of
adaptation. This is done by a neural network involving feedback synapse
from horizontal cell to cones. The effect is to unsaturate the membrane
potential of a fully hyperpolarized cone, by "instantaneously" shifting
cone's I-R curves along intensity axis to be in register with ambient
light level of the periphery. An equivalent electrical circuit with
three different transmembrane channels leakage, photocurrent and
feedback was used to model static behavior of a cone. SPICE simulation
showed that interactions between feedback synapse and the light
sensitive conductance in the outer segment can shift the I-R curves
along the intensity domain, provided that phototransduction mechanism is
not saturated during maximally hyperpolarized light response.

Key words: adaptation, feedback, cones, retina, lateral interactions

Josef Skrzypek
Computer Science Department
3532D Boelter Hall
UCLA Los Angeles, California 90024
INTERNET: SKRZYPEK at CS.UCLA.EDU