TRANSFORMATIONS OF TASK-DEPENDENT PLASTICITY FROM A1 TO HIGHER-ORDER AUDITORY CORTEX

Everyday, humans and animals navigate complex acoustic environments, where multiple sound sources overlap. Somehow, they effortlessly perform an acoustic scene analysis and extract relevant signals from background noise. Constant updating of the behavioral relevance of ambient sounds requires the representation and integration of incoming acoustical information with internal representations such as behavioral goals, expectations and memories of previous sound-meaning associations. Rapid plasticity of auditory representations may contribute to our ability to attend and focus on relevant sounds. In order to better understand how auditory representations are transformed in the brain to incorporate behavioral contextual information, we explored task-dependent plasticity in neural responses recorded at four levels of the auditory cortical processing hierarchy of ferrets: the primary auditory cortex (A1), two higher-order auditory areas (dorsal PEG and ventral-anterior PEG) and dorso-lateral frontal cortex. In one study we explored the laminar profile of rapid-task related plasticity in A1 and found that plasticity occurred at all depths, but was greatest in supragranular layers. This result suggests that rapid task-related plasticity in A1 derives primarily from intracortical modulation of neural selectivity. In two other studies we explored task-dependent plasticity in two higher-order areas of the ferret auditory cortex that may correspond to belt (secondary) and parabelt (tertiary) auditory areas. We found that representations of behaviorally-relevant sounds are progressively enhanced during performance of auditory tasks. These selective enhancement effects became progressively larger as you ascend the auditory cortical hierarchy. We also observed neuronal responses to non-auditory, task-related information (reward timing, expectations) in the parabelt area that were very similar to responses previously described in frontal cortex. These results suggests that auditory representations in the brain are transformed from the more veridical spectrotemporal information encoded in earlier auditory stages to a more abstract representation encoding sound behavioral meaning in higher-order auditory areas and dorso-lateral frontal cortex.

The Effects of Insulin-Induced Moderate Hypoglycemia on Hippocampal Plasticity

Self-regulation of blood glucose in diabetics via insulin administration introduces the risk of hypoglycemia. Previous studies have shown hypoglycemia damages the dentate gyrus, an area of the hippocampus associated with anxiety- and depressive-like behavior. To date, only depressive-like behaviors have been observed following moderate hypoglycemia. This study sought to examine whether acute moderate hypoglycemia induces both behaviors due to high clinical comorbidity. One episode of moderate hypoglycemia was induced in a male Sprague-Dawley rat. Twenty-four hours later, hippocampal function was evaluated via the elevated plus maze and the forced swim test to assess anxiety-like and depressive-like behavior. Results, though not statistically significant, suggested that acute moderate hypoglycemia may increase anxiety- and depressive-like behavior. These findings may elucidate hypoglycemia-related behavioral changes.