Griffiths, Carla Seoyun; (2024) Learning Rewires the Brain: Investigating the Neural Correlates of Perceptual Constancy in Ferret Auditory Cortex. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Perceptual constancy, the act of recognising auditory objects over identity-preserving variation, is an everyday component of normal hearing that we often take for granted. In this thesis, I first establish a behavioural paradigm in which ferrets recognised a complex target stimulus (the word `instruments') in the presence of 54 other non-target English word tokens, where the mean fundamental frequency (F0) was shifted upwards or downwards within and across the trial duration. Having established that animals can be trained to do this task and demonstrate behaviour consistent with pitch-constancy I then went on to examine in more detail using gradient-boosted regression trees. I find that, like humans, ferrets incorporate pitch in their everyday decision-making during the task. By recording from the auditory cortex of these same animals I then went on to explore whether single auditory cortical neurons displayed pitch constancy. Using neural decoding, we found that trained neurons could discriminate the target word from other non-target words at the single cell level at higher rates than four naive animals. This indicates that units from trained animals represent the target word more robustly against non-target probe words. We also found that these cells were more robust to changes in mean F0 and the probe word they were paired with during decoding relative to the naïve cells, suggesting that auditory cortex cells maintain the hallmarks of generalisation towards the target word via training. We also found that the temporal profile of cells that had good performance and high generalizability had a consistent temporal profile across different distractor words compared to the naive animals, which had scores over time that were more variable, consistent with encoding spectrotemporal information. This study is one of the first of its kind due to the volume of behavioural and neural data collected and the rigorous computational approaches to strive towards unbiased, parametric-free modelling approaches where possible. My findings suggest that training leads to the unique representation of the target word in many auditory cortex cells during task engagement.

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