Laminar alignment

Orthogonal array penetrations were confirmed online through a reverse-correlation RF mapping procedure (Nandy et al., 2017; Westerberg et al., 2019; Cox et al., 2019a; Cox et al., 2019b; Dougherty et al., 2019; Figure 2—figure supplement 1). RFs were found to represent portions of visual space consistent with previous reports of V4 (Gattass et al., 1988). Positions of recording sites relative to V4 layers were determined using CSD (Schroeder et al., 1998; Nandy et al., 2017). Current sinks following visual stimulation first appear in the granular input layers of cortex, then ascend to the supragranular compartment. Previously described observations of laminar V4 CSD include a sink in the infragranular layers following the ascent to the supragranular layers (Nandy et al., 2017), an observation we do not observe in our data. This is likely because we used task-evoked CSD for alignment with stimulus presentation persisting throughout the measurement interval whereas the descending sink observation was found with very brief stimulus presentations. It is likely that the strength of the persistent supragranular sink is masking the previously reported infragranular sink (Mitzdorf, 1985). We computed CSD and identified the granular input sink session-wise. Sessions were aligned by this input sink. ‘L4’ refers to granular input layer, ‘L2/3’ – supragranular layers, and ‘L5/6’ – infragranular layers. Each laminar compartment was assigned the same number of recording sites to alleviate biases during analysis.