Relationship of intrinsic connections to forelimb movement representations in monkey motor cortex: a correlative anatomic and physiological study.

1. Intracortical microstimulation (ICMS) and horseradish peroxidase (HRP) histochemistry were combined to examine the relationship between intrinsic connections and intracortical microstimulation sites eliciting evoked movements in the forelimb representation of adult macaque monkey motor cortex. 2. The distribution of sites from which stimulation-evoked movements about individual forelimb joints were elicited under anesthesia varied considerably among animals. Identical movements could often be elicited from multiple, noncontiguous sites. 3. After single, small extracellular HRP injections at sites from which thumb movement was evoked, small groups of retrogradely labeled cells and dense patches of axon terminations were found scattered across a wide area of the forelimb representation. Terminal patches were discontinuous and arose from horizontal, intracortical axons. 4. Correlating the HRP labeling with the physiologically defined movement maps revealed a profuse set of intrinsic, bidirectional connections that connect digit representations and representations of movements about the wrist, elbow, and shoulder. 5. HRP injections placed in the forelimb representation close to the physiologically defined face representation resulted in virtually no retrogradely labeled cells or terminal fiber labeling that crossed into the face representation. A patch of anterolaterally placed label that was present may be the dissociated rostrolateral arm area of other authors. 6. Taken together, these data suggest that extensive, horizontally oriented, intrinsic axon collaterals provide inputs to many different forelimb movement representations and may be recruited during complex movements to coordinate the activity of motor cortical zones whose predominant output is to forelimb muscle groups acting synchronously.