The laminar profile of spatial attention in macaque V1 and V4

PhD ThesisSpatial attention allows processing to be prioritised for one or more locations in the visual field, even in the presence of other distracting or irrelevant stimuli. Previous work has shown that attention modulates the activity of the brain at the level of spiking activity, local field potentials and coherence between and within neuronal groups. However currently little is known about how these attentional modulations differ between groups of neurons in different cortical layers and areas. We trained two adult male rhesus macaques to perform a covert visuospatial attention task whilst we recorded simultaneously from V1 and V4. Recordings were taken with multichannel laminar electrodes allowing recording from supragranular, granular and infragranular cells within the same cortical microcolumns. We used current source density analysis to align our recording contacts to the cortical laminar profile (layers). The receptive fields of the V1 and V4 cells we recorded from were overlapping which meant they could be driven by the same stimulus in the task. To measure the attentional modulation of information flow between different groups of neurons we calculated field coherence, Granger causality and spike-rate correlations. Attention increased firing rates for all of the cell types, layers and areas in our study. We also show that variability as measured by gain variance and noise correlations is reduced by attention. Although we find differences between the two monkeys regarding LFP power changes and regarding coherence measures within and between the areas investigated, we find that attention consistently increased the Granger causality in the gamma frequency band between V1 and V4. We demonstrate that the flow of information in the alpha/beta and gamma bands follows expected interareal feedback and feedforward patterns between V1 and V4. We also provide evidence that feedforward gamma oscillations are generated, contrary to expectations, in the infragranular layers of V1