“High-resolution fMRI measurements of orientation-dependent suppression in primary visual cortex”
Dr. Cheryl A. Olman
Associate Professor, University of Minnesota
Date/Time
Tuesday, November 1 2022, 13:30-14:20
Abstract
In recent years, cortical depth-resolved functional MRI has become possible, allowing neuroscientists to distinguish responses in the middle (input) layers of the gray matter (GM) of the brain from responses in superficial and deep layers, which are involved in relaying responses to other brain regions and making internal predictions about the state of the external world. In primary visual cortex (V1) this depth-dependent segregation of fMRI signals allows us to study contextual modulation of neural responses. In V1, initial (input) responses to a small, simple pattern (grating stimulus) should be weakly selective to the orientation of the stimulus; local crosstalk between neighboring populations should refine this response so superficial and deep layers show greater orientation selectivity. If a target stimulus is embedded in a larger image, the response should be suppressed when the stimulus is camouflaged. The degree of suppression should be modulated by context, with a release from suppression occurring when orientation contrast or some other cue signifies a boundary. Furthermore, contextual modulation of responses should be stronger in deep and superficial layers than in middle layers. I will present 0.6 mm isotropic GE BOLD measurements from fMRI experiments in human observers that were designed to test these predictions about the refinement of orientation preference and orientation-dependent surround suppression throughout the GM depth. Our findings are mixed: while there is evidence that fMRI correctly reflects the laminar profile of feedforward orientation selectivity in V1, and we do see robust effects of feedback-mediated figure/ground segmentation in V1, we see little evidence in univariate analyses for the orientation-dependent suppression mechanisms that are expected to be strong and intrinsic to V1. This failure to observe orientation-dependent surround suppression is surprising, but it can be understood as the consequence of a neural population code that simultaneously represents multiple probable visual interpretations of the external world in multiplexed pathways. We will end the presentation with discussion of the implications of signal multiplexing for fMRI experiment design and analysis.