Abstract
Purpose :
Knowing how primary visual cortex (V1) neurons integrate cone photoreceptor inputs is central to understanding how form and color are represented within V1. Basic knowledge on how signals from individual cones modulate V1 neural firing is unknown. We examined how signals originating from identified cone photoreceptors are summed within the receptive field of V1 neurons.
Methods :
We recorded extracellularly from 5 neurons in one anesthetized macaque, with receptive fields located at ~2° eccentricity from the fovea. We used an adaptive optics scanning laser ophthalmoscope to image and present stimuli to the retina. An 842±25 nm channel was used for retinal imaging, optical correction, eye tracking and cone selection. We used spatiotemporal noise movies to map receptive fields via spike-triggered averaging. Cone-targeted stimulation entailed 10 increment stimulus intensities delivered with a 543±11 nm channel, a wavelength that minimized the sensitivity difference between L and M cones. Stimuli subtending 0.75 arcmin (≈2.5 µm) were flashed pseudorandomly 10 times to single cones or cone pairs at a rate of 3 Hz and ranged logarithmically over 2 orders of magnitude in intensity. Chromatic aberrations between imaging and stimulus channels were measured and corrected. Spikes were sorted offline, and we used spike rates to compute intensity response functions.
Results :
We tested 15 individual cones providing input to the receptive fields of 5 V1 cells; 10 yielded intensity-dependent responses. The pair-wise summed responses of these cones were then compared to the linear addition of their individual responses. In one cell, summation of cone inputs was supralinear at low intensities and sublinear at high intensities compared to the linear prediction, while two cells produced the opposite behavior. We also observed responses from two other cells that were sublinear at all intensities, but with increasing suppression at the highest intensities. For the 5 cones that did not yield responses alone, stimulating them in pairs generated 11 intensity response relationships.
Conclusions :
Our preliminary results suggest that single cone-targeted stimulation can drive responses in V1 cells. Occasionally, cones need to be stimulated in pairs before responses could be produced in some cells. V1 cell responses to paired cone stimulation depend on stimulus intensity and are not readily predictable from the sum of their cone inputs.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.