Abstract
The avian nidopallium caudolaterale (NCL) is considered homologous to the primate prefrontal cortex (PFC). Although the role of the PFC in executive functions is well established, the role of the NCL remains relatively unknown. The aim of the current thesis was to clarify the role of the NCL in two executive functions. The first was working memory, which is the ability to maintain and manipulate information across time. The second was serial-order behaviour, which is the execution of a series of behaviours in a sequential order.
The role of the NCL in working memory was examined by recording neurons from the NCL of pigeons trained on delayed matching-to-sample (DMS) tasks across two experiments, with either 3, 6, or 12 sec delay periods. In the first experiment we found delay activity represents a neural correlate of the to-be-remembered stimulus, however activity waned across the period. In the second experiment we examined the possibility that delay activity represents a neural correlate of temporal information, as well as the functional significance of such activity. We found no evidence of either temporal coding or a functional relationship between delay activity and behavioural performance.
The role of the NCL in serial-order behaviour, was examined by using tetrodotoxin (TTX) to temporarily lesion the region in pigeons trained on a four-item externally ordered (EO) serial-order task. Performance decreased significantly following TTX infusions, however the behavioural deficit could not be explained by a breakdown in sequencing or serial-order behaviour, and was more likely the result of a more generalised deficit such as decreased inhibition.
To clarify the role of the NCL in serial-order behaviour we then trained pigeons to complete three-item EO or internally ordered (IO) tasks, and recorded activity from NCL neurons to determine whether the NCL codes ordinal knowledge, a mechanism crucial for successful serial-order behaviour. We found many neurons that represented a neural correlate for ordinal position during the IO but not EO task, supporting the view that the NCL is involved in serial-order behaviour, at least with respect to IO tasks. The absence of ordinal coding during the EO task could be explained by the different strategies that pigeons adopt between EO and IO tasks, therefore we cannot rule out the possibility that the NCL codes ordinal knowledge in an EO task.
Taken together, we show the NCL is critical for both working memory and serial-order behaviour, at least with respect to IO sequences. Although it is possible that the NCL codes specific task information across a multitude of tasks, it is much more likely that the NCL codes underlying mechanisms that can account for the involvement across many tasks. Much like the primate PFC, the NCL may process information in a top-down manner by focusing attention on task relevant information for peripheral regions to code. To fully understand the role of NCL in various complex cognitive tasks, multiple brain regions will need to be examined simultaneously, enabling a complete analysis of what, how, and where information is maintained across the avian brain.