Layer dependent multiplicative effects of spatial attention on contrast responses in human early visual cortex
Attention mechanisms at different cortical layers of human visual cortex remain poorly understood. Using submillimeter resolution fMRI at 7T, we investigated the effects of top down spatial attention on the contrast responses across different cortical depths in human early visual cortex. Gradient echo (GE) T2* weighted BOLD signal showed an additive effect of attention on contrast responses across cortical depths. Compared to the middle cortical depth, attention modulation was stronger in the superficial and deep depths of V1, and also stronger in the superficial depth of V2 and V3. Using ultra high resolution (0.3mm in plane) balanced steady state free precession (bSSFP) fMRI, a multiplicative scaling effect of attention was found in the superficial and deep layers, but not in the middle layer of V1. Attention modulation of low contrast response was strongest in the middle cortical depths, indicating baseline enhancement or contrast gain of attention modulation on feedforward input. Finally, the additive effect of attention on T2* BOLD can be explained by strong nonlinearity of BOLD signals from large blood vessels, suggesting multiplicative effect of attention on neural activity. These findings support that top down spatial attention mainly operates through feedback connections from higher order cortical areas, and a distinct mechanism of attention may also be associated with feedforward input through subcortical pathway. Highlights Response or activity gain of spatial attention in superficial and deep layers Contrast gain or baseline shift of attention in V1 middle layer Nonlinearity of large blood vessel causes additive effect of attention on T2* BOLD