Functional organization of the insula and inner perisylvian regions

Abstract

In the last few years, the insula has been the focus of many brain-imaging studies, mostly devoted to clarify its role in emotions and social communication. Physiological data, however, on which one may ground these correlative findings are almost totally lacking. Here, we investigated the functional properties of the insular cortex in behaving monkeys using intracortical microstimulation. Behavioral responses and heart rate changes were recorded. The results showed that the insula is functionally formed by two main subdivisions: (i) a sensorimotor field occupying the caudal-dorsal portion of the insula and appearing as an extension of the parietal lobe; and (ii) a mosaic of orofacial motor programs located in the anterior and centroventral insula sector. These programs show a progressive shift from dorsally located nonemotional motor programs (ingestive activity) to ventral ones laden with emotional and communicative content. The relationship between ingestive and other behaviors is discussed in an evolutionary perspective.

Fig. 1.

Unfolded view of the lateral sulcus of the left hemisphere of M1 depicting its upper and lower banks and the insula. Each dot indicates the entrance point of the electrode. Black arrows indicate the antero-posterior (AP) position of the central sulcus (C) and the intraparietal sulcus (IP). In all penetrations, several sites were stimulated every 500 mm below the entrance point and above the exit point. (A) Posterior dorsal field. Red dots, mouth movements; yellow dots, hand movements; green dots, face movements; gray dots, lower and upper limbs. (B) Anterior field. Red dots, ingestive behavior; blue dots, disgust behavior; green dots, movement inhibition. (C) Ventral field. Yellow dots, affiliative behavior. (D) Miscellaneous responses. Gray dots, discomfort reactions; purple dots, gaze–trunk contralateral displacement; yellow dots, twitch of the chest; red dots, tremors. (E) Unresponsive sites. (F) Percentage of sites per each category. Blue, sensorimotor sites; green, ingestive sites; yellow, disgust sites; orange, affiliative sites; red, movement inhibition sites; tan, miscellaneous responses sites; gray, unresponsive sites.