| Summary: | One of the most important means we use to interpret and interact with our environment is the process of assigning incoming visual stimuli into categories. While some categories may be innate or instinctual, many categories are developed through experience with stimuli. A major goal of modern neuroscience is to understand the neural mechanisms behind how the brain transforms basic sensory processing into such abstract categories. Previous neurophysiological studies have found that upstream visual areas such as the middle temporal (MT) area mainly encode information about visual features, rather than information about categories. These same studies found that, after training on tasks requiring categorization of visual stimuli, neural representations in downstream areas such as posterior parietal cortex (PPC) reflect the learned category membership of these stimuli. However, the neural mechanisms behind how categories are learned and stored in short-term memory are poorly understood.
To understand how learning new behavioral tasks impacts underlying neuronal memory representations, we recorded from posterior parietal cortex (PPC) both before and after training on a visual motion categorization task. Here we show that categorization training influenced both visual and cognitive encoding in PPC, with a marked enhancement of memory-related delay-period encoding during the categorization task which was not observed during a motion discrimination task prior to categorization training. In contrast, the prefrontal cortex (PFC) exhibited strong delay-period encoding during both discrimination and categorization tasks. This reveals a dissociation between PFC's and PPC's roles in short term memory, with generalized engagement of PFC across a wide range of tasks, in contrast with more task-specific and training dependent memory encoding in PPC.
We examined the behavioral and neuronal dynamics of category learning in the lateral intraparietal (LIP) area by recording from LIP neurons during three distinct training stages: before, during, and after categorization training. Prior to training on the categorization task, monkeys were trained on a motion matching task which required keeping a sample stimulus in memory prior to making a decision about whether the test stimulus that followed was an identical match to the sample. The monkeys were then trained on a categorization task which had a similar structure but required the monkeys to decide whether sample and test stimuli were in the same category, as defined by a learned category boundary. Comparison of neuronal activity across these three stages of learning revealed that individual neurons do not represent category information during training, but rather change tuning during training to reflect category information at the population level. Since PPC does not encode categories early in the category learning process, this suggests that other areas such as prefrontal cortex (PFC) may play an important role in early learning. Furthermore, neural responses in LIP are modulated by decision prior to the decision epoch. This suggests that LIP may be critically involved in learning abstract information such as categories, in addition to being involved in making decisions about incoming visual stimuli.
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