Phd in PHD PROGRAM IN BEHAVIORAL NEUROSCIENCE

Thesis title: The behavioural and neural correlates underlying memory-based decision making studied by a transitive inference task

Understanding the ordinal relationships between items requires constructing a rank order supporting decision-making between options. This process depends on the ability to learn reciprocal relationships and to select the best option available when making a choice. The transitive inference task, which involves inferring a novel relationship (e.g., A greater than C from A greater than B and B greater than C) after learning a series of adjacent pairs (e.g., A greater than B, B grater than C), is an important paradigm for studying such type of decision-making. In this thesis we explored behavioural and the neural correlates that underly decision making using a transitive inference task. Study 1: The prefrontal cortex (PFC) plays a crucial role in encoding the relative value of alternatives as a decision is formed. Higher-order cognitive abilities are influenced by genetic factors that affect dopamine availability in PFC, potentially contributing to individual differences. Here, we examined the performance of 83 participants in a transitive inference task (TI), grouped by genotype based on the Val158Met single-nucleotide polymorphism in the Catechol-O-Methyltransferase (COMT) gene. The task included a learning phase in which participants acquired the reciprocal relationships among a set of hierarchically ranked items (A greater than B greater than C), followed by a test phase in which they were required to compare all possible item pairs and select the higher-ranked one. While genotype did not significantly influence test-phase performance, it did affect learning efficiency. Specifically, Val homozygotes took a longer learning procedure than both heterozygotes and Met homozygotes during the learning phase. Drift diffusion modelling (DDM) revealed that task performance was explained by the efficiency of evidence accumulation, which was lower in Val homozygotes, accounting for their poorer performance not only during initial learning but also when required to switch to a reversed hierarchical structure (A smaller than B smaller than C). These findings suggest that individual differences in inferential decision-making and cognitive flexibility may be partially driven by genetically determined variations in prefrontal dopamine availability. Study 2: Behavioural evidence, including the symbolic distance effect (SDE) and the serial position effect (SPE), suggests that subjects form a unified, spatial mental schema of the ranked set, with performance reflecting the ease of localizing items within this mental continuum. Neural correlates of TI consistently point to a central role for the hippocampus-prefrontal cortex (PFC) network in encoding the relational memory structure, while other regions like the posterior parietal cortex (PPC) are implicated in spatial representation and comparison. However, area specific roles and the temporal coordination among the distributed neural structures supporting this process remain open questions. To characterize the precise temporal dynamics and functional connectivity of the cortical areas involved in serial-order decision-making, we used electroencephalography (EEG). Participants performed a modified TI task where the encoding of individual items was temporally separated from the subsequent rank comparison and decision-making, allowing for the isolation of distinct cognitive processes. Our analysis focused of Event-related potentials (ERPs) (P200, N200, P300), time-frequency analysis of the alpha and beta desynchronization (ERSPs) as markers of working memory and executive control, and a weighted phase-lag index (wPLI) to quantify functional connectivity. The results suggest that the encoding and processing of different task variables are distributed across distinct cortical areas, with the degree of interaction between these regions dynamically modulated depending on the specific phase and the task.