Effects of Regional Neural Architecture on Signal Processing in the Macaque Retina

PROJECT SUMMARY
The retina employs a multitude of parallel neural circuits to encode the many aspects of vision. At all
light levels, and in all circuits, cellular noise threatens the accurate encoding of visual stimuli. To combat noise,
the retina uses circuit and synaptic mechanisms to amplify signal and filter out noise. Two examples of this are
nonlinear convergence, the filtering and subsequent pooling of many neural signals into one neuron, and
divergence, the spreading of a single signal into many neurons. Two important neural circuits that emphasize
the presence, or lack, of circuit mechanisms, are the rod pathway and foveal midget pathway, respectively. The
rod pathway uses high degrees of nonlinear convergence to amplify single photon signals enabling vision in
extremely dark environments. The foveal midget pathway encodes hyperfine spatial detail by doing away with
circuit mechanisms such as convergence and divergence in exchange for 1:1 connections between neurons.
The relative density of neurons in the retina varies across retinal regions influencing the degree of
convergence/divergence, and thus the magnitude of noise in neural circuits. While rod pathway sensitivity has
been studied in peripheral macaque retina, where convergence is high and cell density is low, the question of
how cellular connection augments rod pathway sensitivity across regions has yet to be answered. This is all
the more salient due to recent literature which has pointed to regions with lower convergence but higher cell
density as having the highest dim light sensitivity. My project proposes to look at single cell metrics of rod
pathway sensitivity in these retinal regions.
The foveal midget pathway is capable of responding to minute variations in contrasts to encode the fine
spatial detail of our foveal vision. It manages to do so in the absence of convergence or divergence to amplify
signal. The question remains: “how has signal processing in the foveal midget pathway adapted to a lack of
key circuit mechanisms?” My project proposes to determine if regularity in synaptic release is the mechanism
by which the foveal midget circuitry encodes information in the absence of prominent circuit mechanisms like
convergence and divergence.
This project will bridge the gap between our mechanistic understanding of rod pathway sensitivity and
the regional sensitivity indicated by psychophysical studies – providing a more complete understanding of how
varied cellular connectivity affects our most sensitive retinal pathway. Given the importance of our high-acuity
foveal vision and the relative lack of understanding of foveal signal processing, this project will determine key
mechanisms enabling foveal vision to operate with hyperfine spatial acuity. The pursuit of this project will
enhance our understanding of how the neural code changes as a consequence of varied cellular connectivity.

Grant Number: 1F31EY037192-01
NIH Institute/Center: NIH
Principal Investigator: Theodore Bucci