Saccade response modulation in areas MT/MTC across cell types and layers

To maintain a stable percept of the world around us, the information during saccades is suppressed to avoid retinal blur, termed saccadic suppression. Extra-retinal motor signals controlling saccadic suppression could originate from oculomotor areas such as area FEF and the Superior Colliculus (Sommer and Wurtz 2008), but less is known about how the signals are integrated locally. Previously we have found that area MT and MTC neurons vary considerably in their timing and amplitude of saccadic suppression as well as the latency of the post-saccadic excitatory rebound response after suppression. In particular, we found a higher proportion of neurons in MTC that exhibited an early latency excitatory response that could reflect extra-retinal motor feedback during saccades (Bucklaew et al., VSS, 2023). By recording from neurons in both areas as two monkeys freely viewed either natural images or a blank screen, we observed that area MTC showed a distinct peak in the early response interval (<50ms), which remained significant for the blank screen condition and likely reflects extra-retinal feedback signals. This early peak in response is driven by a subpopulation of cells (18%) that had strong modulations on blank screens, which are seen in both MT and MTC. Here we sought to examine in detail the distribution of these neurons across cortical cell types and layers in MT/MTC. We found no major distinctions in the laminar distribution of early peak neurons as a function of cortical layer. However, units with initial positive modulation were more likely to have narrow shaped spike waveforms while units with initial suppressive modulation were more likely to have broad spike waveforms. These patterns held true across all cortical layers, with broad spiking cells having a lower mean modulation compared to narrow spiking cells, and suggests distinct pathways for motor feedback to influence saccadic suppression.