Cortical Area 8 and Its Thalamic Projection in Macaca Mulatta.

The frontal eyefields were discovered in 1874 by Ferrier and though considerable research has subsequently been done on physiological, anatomical and clinical as­ pects of this area in man and monkey (Smith, ’49), the specific role of the fron­ tal cortex in the complex organization of oculomotor activity is still virtually un­ known. Even the elucidation of anatom­ ical problems has made little progress since Brodmann ( ’05) characterized the area of the frontal eyefield as a specific cortical area (area 8), though histological maps were repeatedly challenged by elec­ tro-physiological localization charts. Un­ certainty also exists about the thalamic nucleus of origin of afferent projections to area 8. Yet, thalamo-cortical relation­ ship is of fundamental importance for the understanding of the anatomical and functional organization of a given cortical field, and they have proved to be uniquely suited for comparative studies of mam­ malian cortex (Akert, ’63). For example the Orbitofrontal region (i.e. frontal gran­ ular cortex) has been defined as the re­ ceiving area of the nucleus medialis dor­ salis in several species (Rose and Woolsey, ’48), and the precentral motor cortex (Brodmann’s area 4 ) has been shown to receive essential thalamic afferents from nucleus ventralis lateralis (Akert and Woolsey, ’53). Since area 8 is situated between the precentral motor and the Orbitofrontal regions its thalamic counter­ part could not have been predicted with certainty. To Hassler ( ’49) it seemed more reasonable to assume that area 8 receives afferents from nucleus ventralis lateralis (the most medial portion) be­ cause he considered the eyefields primarily a motor cortex comparable to the fields of origin of the pyramidal system. Seem­ ingly, there is supporting evidence for this hypothesis from human cases with lesions of the frontal eyefields (Stern, ’42; Nor­ man, ’45; Freeman and Watts, ’47). On the other hand, there are experimental observations in the Rhesus monkey by Walker ( ’40b) and Mettler ( ’47) as well as by Pribram, Chow and Semmes ( ’53) who have found that the afferents to area 8 arise from nucleus medialis dorsalis (the most lateral portion). A decision in this dilemma with the aid of ablation-degeneration techniques is dif­ ficult because in the higher primates a considerable portion of area 8 is situated in the depth of a sulcus. In the species Macaca mulatta, area 8 is partially situ­ ated in the depth of sulcus arcuatus, and its surgical removal may entail unintended damage to Subcortical fiber systems which innervate cortical areas other than area 8. The same source of errors exists in most if not all human cases with damage to area 8. This can lead to errors in the interpretation of retrograde thalamic changes and contrasting conclusions may be drawn by different investigators. In order to eliminate error factors of this kind control cases have been included in this study in which the areas rostrally as well as caudally to area 8 were ablated without injuring area 8 itself. The results of the test and control series in respect to retrograde thalamic degeneration were consistent and have shown that area 8 receives projections from the paralamellar portion of nucleus medialis dorsalis.