Anterior parietal cortical topographic organization in macaque monkey: a reevaluation

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Abstract
Extracellular single-neuron recordings are carried out within the forearm and hand representations of the anterior parietal cortex in unanesthetized Macaca mulatta and M. fascicularis monkeys maintained under neuromuscular block. Natural stimuli are used to map the receptive field (RF) and submodality properties of a large number of single neurons. Planar maps depicting the relations between anterior parietal functional organization and cytoarchitecture were constructed from the observations. The topographical relations revealed by the RF data obtained in individual microelectrode penetrations and by the summary planar maps indicate that the anterior parietal cortex of M. mulatta or M. fascicularis monkeys contains a continuous and topographically complex representation of the forelimb. The representation consists of a large and predominantly cutaneous core zone (extending across areas 3b, 1 and 2) principally devoted to the representation of the forelimb digits, and a surrounding border zone, which receives both cutaneous and deep input from spatially extensive regions on the forelimb. The submodality composition within the anterior parietal forelimb representation varies with the anteroposterior gradient in cytoarchitecture and with proximity to the digital core zone. While the planar topographic map indicates that a given region on the forelimb provides input to multiple and sometimes widely spaced cell columns, its characteristics are not consistent with the proposal that each cytoarchitectonic field contains a complete, separate and functionally independent map of the forelimb. A limited sample of anterior parietal neurons whose RF and response properties were determined are restudied after systemic administration of a general anesthetic drug. All of the general anesthetic drugs utilized (barbiturates, ketamine and nitrous oxide) reduced RF dimensions and frequently altered RF configuration. The dosage levels required to produce these effects are below or equal to those necessary to induce general anesthesia; the effects are proportional to dosage for all agents studied. The prominent effects of general anesthetic drugs on RF size and configuration that if the topographical organization of the anterior parietal cortex of the same animal were mapped several times, each time using a different general anesthetic or anesthetic level, the maps would differ substantially. The anesthetic effects demonstrated introduce prominent topographic discontinuities into the anterior parietal body representation and alter the dimensions of territories representing different body regions. They do not satisfactorily account for the discrepancies between existing accounts of the distribution of anterior parietal neurons belonging to distinguishable submodality groups, or the neighborhood relations between forelimb regions represented on opposite sides of a given cytoarchitectonic boundary. The features of topographical organization revealed by RF data obtained from unanesthetized subjects are consistent with the stimulus-evoked pattern of anterior parietal activation demonstrated by the [14C]2-deoxyglucose ([14C]2-DG) metabolic mapping method and with the pattern of distribution of interhemispheric connections demonstrated using anterograde or retrograde tracing procedures.

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