Rod-rod interaction in the retina of the turtle

Intracellular responses were recorded from rods in isolated eye-cups of the snapping turtle, Chelydra serpentina. Responses to small and large diameter spots of 500 nm light were studied. 1. The peak amplitudes of responses smaller than approximately 2 mV were directly proportional to irradiance. Small spots (less than 100 mum diameter) produced approximately 30 muV/rhodopsin molecule bleached. Increasing stimulus diameter to 400-500 mum increased this five to seven times to about 200 muV/rhodopsin molecule bleached in the impaled receptor. The difference is attributed to a neural "enhancement" produced by stimulating neighbouring rods. 2. Enlarging the diameter of a spot altered the shape of responses produced by very dim lights. 3. The variance of responses to a small spot was only slightly less than the mean. The variance of responses to a large spot was much less than the mean. 4. Responses evoked by a small spot of dim light obeyed the superposition principle in that the response to a very dim step of light was the integral of the response of a very dim flash. Responses evoked by a large spot did not obey the superposition principle. The response to a step of dim light covering a spot of large diameter was less than predicted from the integral of the response to a flash. The difference is attributed to a neural "disenhancement" produced by stimulating neighbouring rods. 5. The time course of this disenhancement could be observed by presenting two large diameter, dim flashes within a short interval. The time course of disenhancement did not coincide with that of the voltage response but was delayed such that its maximum occurred after the peak amplitude. 6. Dim background lights of different diameter, which delivered the same quantity of light to the impaled cell but very different quantities of light to neighbouring cells, left the response produced by a small diameter test spot unaltered. It is concluded that rod-rod interaction can modify the intracellular responses of rods in two ways; it produces an early enhancement which increases response amplitude nearly tenfold and also a delayed disenhancement which replaces the wave of enhancement that follows a flash.