Principles of intrinsic motor cortex connectivity in primates

Nicholas S. Card, Omar Gharbawie

Abstract:

The forelimb representation in motor cortex (M1) is an important model system in contemporary neuroscience. Efforts to
understand the organization of the M1 forelimb representation in monkeys have focused on inputs and outputs. In contrast,
intrinsic M1 connections remain mostly unexplored, which is surprising given that intra-areal connections universally out-
number extrinsic connections. To address this knowledge gap, we first mapped the M1 forelimb representation with intra-
cortical microstimulation (ICMS) in male squirrel monkeys. Next, we determined the connectivity of individual M1 sites
with ICMS 1 intrinsic signal optical imaging (ISOI). Every stimulation site activated a distinctive pattern of patches (;0.25
to 1.0 mm radius) that we quantified in relation to the motor map. Arm sites activated patches that were mostly in arm
zones. Hand sites followed the same principle, but to a lesser extent. The results collectively indicate that preferential connec-
tivity between functionally matched patches is a prominent organizational principle in M1. Connectivity patterns for a given
site were conserved across a range of current amplitudes, train durations, pulse frequencies, and microelectrode depths. In
addition, we found close correspondence in somatosensory cortex between connectivity that we revealed with ICMS1ISOI
and connections known from tracers. ICMS1ISOI is therefore an effective tool for mapping cortical connectivity and is par-
ticularly advantageous for sampling large numbers of sites. This feature was instrumental in revealing the spatial specificity
of intrinsic M1 connections, which appear to be woven into the somatotopic organization of the forelimb representation.
Such a framework invokes the modular organization well-established for sensory cortical areas.