The relation between single-cell activity and fMRI activations in posterior parietal cortex

Abstract

To investigate the relation between fMRI activations and single-cell responses in parietal cortex we recorded single-unit (SUA), multi-unit (MUA) and local field potential (LFP) activity in the Anterior Intraparietal area (AIP) in voxels that were significantly more activated by curved surfaces than by flat surfaces at different disparities. We scanned two macaque monkeys in a 3T Siemens MR scanner with an 8-channel phased-array coil (MION, 1.25 mm isotropic voxels). Depth structure sensitivity was assessed by the interaction between the factors binocular disparity (disparity vs. no-disparity control condition) and depth order (zero-order disparity vs. second-order disparity). The region in AIP and part of the Lateral Intraparietal area (LIP) that was significantly more activated (corrected FWE=0,05) by curved surfaces than by flat surfaces at different disparities extended 9 mm in both the anterior-posterior and the medio-lateral direction. With the same stimulus set we recorded extracellular responses (48% SUA, 52% MUA) and LFPs in 315 AIP sites in two monkeys and in 14 grid positions (spacing 1 mm), covering 5 mm from posterior to anterior and 4 mm from medial to lateral. Eighty-nine percent (262 sites) of the recording sites was responsive, either early (0-450 ms after stimulus onset) excitatory (50%), early inhibitory (10%) or late (450-1000 ms after stimulus onset) responsive (29%). Averaged across all recording sites and all grid positions, both spiking activity and high gamma power (80-150 Hz) were significantly stronger for curved surfaces than for flat surfaces at different disparities. However, merely 18% of the responsive sites (48/262) preserved their 3D-shape selectivity across positions in depth indicating higher-order disparity selectivity. These higher-order disparity selective sites were mainly concentrated in 2 grid positions in monkey K. and 3 grid positions in monkey M. The main effect of binocular disparity was significant in 33% of the responsive sites in 13 out of 14 grid positions. We measured significantly stronger LFP responses to curved compared to flat surfaces in the gamma (7/14 sites), beta (4/14 sites) and alpha bands (4/14 sites). The magnitude of the interaction between depth order and binocular disparity in the spiking or LFP activity did not correlate with the interaction effect in percent signal change of the fMRI study. Thus in AIP, unlike in premotor cortex (Theys et al., 2009. Program No. 852.9. Neuroscience 2009. Online), the region activated (fMRI) by curved surfaces consists of a heterogeneous population of neurons and is much more extensive than the 3D-shape selective area as measured with single-cell recordings.status: publishe