Amygdalofugal and amygdalopetal connections with modality-specific visual cortical areas in macaques (Macaca fuscata, M. mulatta, and M. fascicularis).

The origins and terminations of the amygdaloid connections with the modality-specific visual cortical areas TEa (anterior TE area), TEp (posterior TE area), TEO, V4, V2, MST (medial superior temporal visual area), MT (middle temporal visual area), and V1 were studied in macaques. These were compared with the amygdaloid connections of a vision-related polysensory area TG by making cortical injections of horseradish peroxidase (HRP) and incubating the sections with tetramethylbenzidine (TMB) as the chromogen. Both areas TEa and TEp receive a major projection from the lateral basal nucleus and a minor one from the accessory basal nucleus of the amygdaloid complex, whereas these areas send a major projection to the lateral nucleus and a minor one to the lateral basal nucleus. Areas TEO, V4, V2, MST, MT, and V1 receive projections only from the lateral basal nucleus; none of them project to any amygdaloid nucleus. Thus, the amygdalofugal projections are more widespread and more complex than the amygdalopetal projections. These findings indicate that the connections between the amygdaloid nuclei and the visual areas are generally nonreciprocal and underlie the importance of a feedback mechanism from the amygdala to the visual cortical areas in visual information processing. There appears to be a caudorostral (occipitotemporal) gradient in the distribution and density of the amygdaloid projections, which become progressively more widespread and heavier among the progressively more rostral visual areas (from area V1 to area TEa). The amygdaloid connections with area TG are distinctly different from the connections with the visual areas. Area TG is reciprocally connected mainly with the periamygdaloid cortex, and with the lateral, accessory basal, and medial basal nuclei of the amygdala as well.