ASPIRATION LESIONS OF THE AMYGDALA DISRUPT THE RHINAL CORTICOTHALAMICPROJECTION SYSTEM IN RHESUS-MONKEYS

In macaque monkeys, aspiration but not excitotoxic lesions of the medi al temporal lobe limbic structures, the amygdala and hippocampus, prod uce a severe impairment in visual recognition memory. Furthermore, cer tain ventromedial cortical regions, namely the rhinal (i.e., entorhina l and perirhinal) cortex, are now known to be critical for visual reco gnition memory. Because the route taken by temporal cortical efferent fibers, especially perirhinar efferents, passes nearby the amygdala, i t is possible that inadvertent damage to these fibers is produced by t he aspirative but not the excitotoxic process, thereby accounting at l east in part for the different behavioral outcomes of the two types of lesion. To test this idea, we assessed the integrity of the rhinal co rticothalamic projection system after aspiration lesions of the amygda la. Three rhesus monkeys with unilateral amygdala removals received bi laterally symmetrical injections of a retrograde fluorescent tracer in to the medial portion of the mediodorsal nucleus of the thalamus. Retr ogradely labeled cells were identified using conventional fluorescence microscopy techniques. In all three cases, the rhinal cortex of the i ntact hemispheres contained moderate numbers of retrogradely labeled c ells. By contrast, the rhinal cortex of the amygdalectomized hemispher es consistently contained few retrogradely labeled cells, and a direct comparison of the two hemispheres showed this difference to be statis tically significant. A similar asymmetric pattern was observed for are a TE but not for the cortex lining the dorsal bank of the superior tem poral sulcus, nor for the rostral cingulate motor area, which was exam ined as a control. The results indicate that aspiration lesions of the amygdala not only remove the cell bodies of the amygdala, as intended , but also inadvertently disrupt projection fibers arising from cells in the rhinal cortex and area TE that pass nearby or through the amygd ala en route to the thalamus. Behavioral studies examining the effects of aspiration lesions of the amygdala in nonhuman primates need to ta ke these findings into consideration.