Functional organization of primate cortical areas SI, SII, MI and PPC

Abstract

The goals of this study were: (i) to determine whether primary somatosensory (SI), secondary somatosensory (SII) and posterior parietal (PPC) areas are initially activated in parallel or in serial, (ii) to investigate the patterns of ipsilateral evoked activity in primary motor area (MI), SI and PPC, and (iii) to determine the contribution of SII and PPC to long-latency components of surface-recorded activity.;Somatosensory evoked potentials (SEPs) and concomitant multiunit activity (MUA) were collected from cortex of awake, behaving monkeys (Macaca fascicularis) using a low-impedance, linear-array, multicontact electrode to record from multiple laminae simultaneously. Data were analyzed using techniques sensitive to both supra-threshold unit activity, and excitatory and inhibitory post-synaptic potentials.;In the first study, we hypothesized that the anatomically-defined, subcortical inputs to SI, SII and PPC are activated in parallel. To test this, we compared the onset latencies of initial activity in each of these areas. We found that the mean initial onset latencies in these areas were 6.8 ({dollar}\pm{dollar}1.3), 7.4 ({dollar}\pm{dollar}1.2) and 7.0 ({dollar}\pm{dollar}1.4) ms, respectively, supporting initial parallel activation across SI, SII and PPC.;In the second study, we hypothesized that ipsilateral activity in SI, MI and PPC is related to the role that each area plays in integrating contralateral and ipsilateral information. To test this, we analyzed the temporal and spatial patterns of intracortical ipsilateral evoked activity in these areas. Our results suggest that (a) in MI, cortical processing of ipsilateral inputs may contribute to segregating contralateral from ipsilateral afferents, (b) in PPC, conical processing of ipsilateral afferents may dominate the activity of a cortical column, and (c) in SI, there are no normally expressed or tonically suppressed ipsilateral inputs.;In the third study, we hypothesized that the long-latency components of the surface SEP represent activity generated in non-primary somatosensory areas. Three long-latency complexes were distinguished from the epidural distribution: (i) the {dollar}\rm N\sb{lcub}45{rcub}/P\sb{lcub}90{rcub},{dollar} overlying the central sulcus, (ii) the {dollar}\rm N\sb{lcub}55{rcub}/P\sb{lcub}100{rcub},{dollar} overlying the crown of the intraparietal gyrus, and (iii) the {dollar}\rm N\sb{lcub}75{rcub}/P\sb{lcub}90{rcub}{dollar} overlying medial and posterior portions of the parietal lobe. Although both SII and PPC are characterized by large amplitude inverting potentials and coincident transmembrane current flow, only that from PPC could be associated with the surface response.