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Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - Nov 2020)
The anterior cingulate cortex (ACC) has been implicated in a myriad of different functions. Converging evidence suggests that the ACC continuously monitors and evaluates actions and their consequences. Such functions are essential in representing action sequences which are the building blocks of all complex behaviors. This dissertation seeks to delineate how ACC neuronal ensembles represent different types of information with special emphasis on action sequences. Chapter 2 shows that the ACC ensembles represents different action sequences via unique activity patterns that change if the order of the actions are altered or if the locations of the actions is changed. Interestingly such shifts are achieved when overall levels of activity remain fixed. Chapter 3 reveals a very different arrangement in which progression through a sequence of actions towards a goal is associated with a change in the overall level of neural activity without a significant change in the patterns of activity. Specifically, ACC ensembles display a smooth progressive change in overall activity over three lever press actions that culminate in a reward. In contrast, the dorsal striatal (DS) ensembles recorded simultaneously from the same animals display fluctuations in activity level that are tightly linked to each action. Together these two chapters show that the ACC may use two different firing rate-related codes to convey categorical versus continuous forms of information.Chapter 4 provides a further examination of the mechanisms which allows the ACC ensembles to encode multiple types of categorical information. While the DS neurons encode both the sequence and the location of the levers in a somewhat synchronized fashion, ACC neurons encoded both of these types of information but kept them functionally segregated. As a result, even though ACC single neurons were no better than the DS in sequence decoding, sequence decoding by ACC ensembles was far superior to DS ensembles. The last chapter attempts to produce a unified theory of ACC function based on its coding properties. I will argue that the ACC monitors many aspects of experience while evaluating the current state with reference to a goal. Its multiple coding schemes efficiently serve both monitoring and evaluating functions.
Master's Student Supervision (2010 - 2018)
Specialized brain structures encode spatial locations and movements, yet there is growing evidence that this information is also represented in the rodent medial prefrontal cortex (mPFC). Disambiguating such information from the encoding of other types of task-relevant information has proven challenging. In order to determine the extent to which movement and location information is relevant to mPFC neurons, tetrodes were used to record neuronal activity while limb positions, poses (i.e. recurring constellations of limb positions), velocity and spatial locations were simultaneously recorded with two cameras every 200ms as rats freely roamed in an experimental enclosure. Regression analyses using Generalized Linear Models revealed that over half of the individual mPFC neurons were significantly responsive to at least one of the factors and many were responsive to more than one. On the other hand, each factor accounted for only a very small portion of the total spike count variance of any given neuron (
The neonatal ventral hippocampal lesion (NVHL) is the most well-characterized neurodevelopmental animal model of schizophrenia. NVHL animals are known to display marked deficits in cognitive flexibility and working memory (WM), which are largely reminiscent of cognitive deficits seen in human patients. Though WM deficits are a well-characterized feature of the NVHL model, our study was the first to use magnetic resonance imaging (MRI), on live rats, to determine the relationship between lesion extent and the WM deficit in a variable delayed non-match to sample (vDNMS) task. Similar to the existing literature, NVHL animals showed a significant deficit in performance when compared to sham operated animals. Interestingly, however, the magnitude of deficit in WM performance in NVHL animals was stable, regardless of delay length. We suggest that this delay-independent WM deficit reflects inefficiency during the encoding stage of WM processes in NVHL animals. Additionally, we found no evidence of a relationship between ventral hippocampal (VH) lesion volume and the magnitude of WM deficit, suggesting that there may be a threshold level of VH damage, beyond which no further WM impairment is produced.