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Aspiration lesions of the amygdala disrupt the rhinal corticothalamic projection system in rhesus monkeys
S Goulet1, F Y Doré, E A Murray
1Ecole de Psychologie, Université Laval, Quebec, Canada.
Abstract:
In macaque monkeys, aspiration but not excitotoxic lesions of the medial temporal lobe limbic structures, the amygdala and hippocampus, produce a severe impairment in visual recognition memory. Furthermore, certain ventromedial cortical regions, namely the rhinal (i.e., entorhinal and perirhinal) cortex, are now known to be critical for visual recognition memory. Because the route taken by temporal cortical efferent fibers, especially perirhinal efferents, passes nearby the amygdala, it is possible that inadvertent damage to these fibers is produced by the aspirative but not the excitotoxic process, thereby accounting at least in part for the different behavioral outcomes of the two types of lesion. To test this idea, we assessed the integrity of the rhinal corticothalamic projection system after aspiration lesions of the amygdala. Three rhesus monkeys with unilateral amygdala removals received bilaterally symmetrical injections of a retrograde fluorescent tracer into the medial portion of the mediodorsal nucleus of the thalamus. Retrogradely labeled cells were identified using conventional fluorescence microscopy techniques. In all three cases, the rhinal cortex of the intact hemispheres contained moderate numbers of retrogradely labeled cells. By contrast, the rhinal cortex of the amygdalectomized hemispheres consistently contained few retrogradely labeled cells, and a direct comparison of the two hemispheres showed this difference to be statistically significant. A similar asymmetric pattern was observed for area TE but not for the cortex lining the dorsal bank of the superior temporal sulcus, nor for the rostral cingulate motor area, which was examined 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 amygdala en route to the thalamus. Behavioral studies examining the effects of aspiration lesions of the amygdala in nonhuman primates need to take these findings into consideration.
Insights
Aspiration lesions of the amygdala in monkeys damage nearby rhinal cortex fibers crucial for visual memory. This unintended fiber damage explains why aspiration, unlike excitotoxic lesions, impairs recognition memory.
Area of Science:
- Neuroscience
- Primate Cognition
- Memory Research
Background:
- Medial temporal lobe lesions, specifically amygdala and hippocampus, impair visual recognition memory in macaques.
- Rhinal cortex (entorhinal and perirhinal) is critical for visual recognition memory.
- Aspiration lesions differ from excitotoxic lesions in their impact on surrounding neural pathways.
Purpose of the Study:
- To investigate whether aspiration lesions of the amygdala inadvertently damage adjacent temporal cortical projection fibers.
- To determine if this unintended damage to rhinal corticothalamic projections explains behavioral deficits in visual recognition memory.
Main Methods:
- Three rhesus monkeys underwent unilateral amygdala aspiration lesions.
- Retrograde fluorescent tracer was injected into the mediodorsal nucleus of the thalamus.
- Retrogradely labeled cells in the rhinal cortex were quantified using fluorescence microscopy.
Main Results:
- The rhinal cortex in amygdalectomized hemispheres showed significantly fewer retrogradely labeled cells compared to intact hemispheres.
- This asymmetric pattern of fiber disruption was also observed in area TE.
- Control areas, including the superior temporal sulcus cortex and rostral cingulate motor area, were unaffected.
Conclusions:
- Aspiration lesions of the amygdala cause unintended damage to projection fibers from the rhinal cortex and area TE.
- This disruption of corticothalamic pathways is a critical factor in the visual recognition memory impairments observed after aspiration lesions.
- Future behavioral studies using aspiration lesions must account for this inadvertent fiber damage.