Temporal analysis of visual responses from the frog retina.
thesisposted on 19.11.2015, 08:43 by Kenneth Allan Francis. Gration
1) The effects of sine-wave and step inputs of light upon the frog (R. temporaria) electroretinogram were examined at different mean intensities. At all mean intensities tested (3 - 90 lux) the E.R.C. evoked by small amplitude stimuli is approximately linear, although inherent non-linearities are apparent. With larger amplitude stimuli the non-linearities become increasingly evident. At a mean intensity of 3 lux the b-wave evoked by small amplitude stimuli is approximately fitted by the equation for a linear filter with 4 R.C. elements, but the non-linearities have to be accounted for. An increase in mean intensity reduces b-wave sensitivity and shortens the time constants of the response. At a mean intensity of 90 lux the simple filter model does not describe the b-wave response characteristics. However, a reasonable fit to the experimental data may be obtained by closing feedback loop around the transfer function of the simple filter. It is tentatively suggested that a mechanism analogous to feedback may account for the shortening of the b-wave time constants caused by light adaptation. 2) The response characteristics of off-type ganglion cells were studied in response to sine-wave and step inputs of light. Upon the basis of receptive field size, shape and sensitivity distribu tion, three classes of receptive field are identified. Responses evoked by stimulation at different positions within a receptive field are compared and found to differ: peripherally evoked responses having longer time constants than FSP evoked responses. A model, based upon the known anatomy of the retina, is proposed to explain the position dependent changes in ganglion cell response time constants.