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Rhinal cortex lesions produce mild deficits in visual discrimination learning for an auditory secondary reinforcer in
M G Baxter1, W S Hadfield, E A Murray
1Laboratory of Neuropsychology, National Institute of Mental Health, USA. mbaxter@wjh.harvard.edu
Abstract:
Aspiration, but not neurotoxic, lesions of the amygdala impair performance on a visual discrimination learning task in which an auditory secondary reinforcer signals which of 2 stimuli will be reinforced with food. Because aspiration lesions of the amygdala interrupt projections of the rhinal cortex traveling close to the amygdala, it was hypothesized that damage to the rhinal cortex would severely impair learning in this task. Rhesus monkeys (Macaca mulatta) were trained to solve visual discrimination problems based on an auditory secondary reinforcer, were given lesions of the rhinal cortex or the perirhinal cortex alone, and were then retested. The monkeys displayed a reliable, albeit mild, deficit in postoperative performance. It is concluded that the aspiration lesions of the amygdala that produced a severe impairment did so because they interrupted connections of temporal cortical fields beyond the rhinal cortex that are also involved in learning in this task.
Insights
Lesioning the amygdala severely impairs visual learning, but rhinal cortex damage causes only mild deficits. This suggests amygdala lesions affect learning by damaging broader temporal cortical areas beyond the rhinal cortex.
Area of Science:
- Neuroscience
- Cognitive Neuroscience
- Animal Behavior
Background:
- The amygdala plays a crucial role in learning and memory.
- Previous studies showed aspiration lesions of the amygdala impair visual discrimination learning.
- These lesions are known to damage adjacent rhinal cortex projections.
Purpose of the Study:
- To investigate the specific role of the rhinal cortex in visual discrimination learning.
- To determine if rhinal cortex damage accounts for the severe impairment observed after amygdala aspiration lesions.
Main Methods:
- Rhesus monkeys were trained on a visual discrimination task with an auditory secondary reinforcer.
- Lesions were made to the rhinal cortex or perirhinal cortex alone.
- Postoperative performance was assessed on the same learning task.
Main Results:
- Monkeys with rhinal cortex or perirhinal cortex lesions showed a mild deficit in postoperative performance.
- The impairment was less severe than that observed with amygdala aspiration lesions.
- This indicates that the rhinal cortex is not solely responsible for the severe learning impairment.
Conclusions:
- Amygdala aspiration lesions impair visual learning primarily by affecting temporal cortical areas beyond the rhinal cortex.
- The rhinal cortex contributes to, but is not the sole cause of, severe learning deficits after amygdala damage.
- These findings refine our understanding of the neural circuitry underlying visual learning and memory.
