RANDOM DOT STIMULI SELECTIVELY ENGAGE A DISTINCT POPULATION OF VISUAL CORTICAL NEURONS

The restoration of visual function through cortical stimulation necessitates stimulation paradigms that evoke coherent percepts. While primary visual cortical (V1) neurons are classically characterized using oriented gratings, electrical stimulation of these circuits produces dynamic clouds of dots known as phosphenes. The origin of these peculiar percepts at the level of neural circuit activity remains unknown. To address this, we performed laminar single-unit recordings using high-density arrays in V1 of the tree shrew, a diurnal mammal whose visual system closely resembles that of primates. Animals were presented with full-field drifting gratings of varying orientation and phosphene-like dot displays parametrically varied in size, contrast (black to white), and speed.
As expected, many units were driven only by the gratings or by both the gratings and dots. However, an unexpected population responded uniquely to the dot stimuli. Additionally, firing rate changes evoked by the dots were on average larger than those evoked by the drifting gratings, and this included those neurons responsive to both classes. Finally, while individual units behaved as expected in terms of orientation tuning, equivalent numbers tuned to each grating orientation, responses to the dots showed tuning at the population level with significant effects of size, velocity, and contrast.
These findings reveal a distinct V1 subpopulation preferentially responsive to punctate visual stimuli, with potential relevance for phosphene-like percepts during cortical stimulation.