P300 dynamics reflect hierarchical encoding and inferential reasoning in a transitive inference task

Making appropriate decisions often relies on looking back to past experiences, connecting them through common elements. This is the case of serial order decision, as inferential reasoning allows individuals to deduce A>C if it is known that A>B and B>C, since item B links the elements A and C. The Transitive inference (TI) task is an experimental approach developed to investigate this type of decision making. TI requires participants to identify the hierarchical structure of a set of arbitrarily rank ordered symbols (A>B>C>D>E>F) by learning (learning phase) the ordinal relationship between all the adjacent pairs (A>B, B>C, C>D, D>E, E>F) and later, in a test phase, to compute the relationship between non-adjacent pairs as B>D or B>E. Analysing the decision performance by grouping items’ pairs by their rank differences, it emerges that the choice accuracy gradually increases as the difference between items’ rank differences in a pair increases. This symbolic distance effect (SDE) is used as an independent variable to experimentally modulate the difficulty of pair comparisons in the task. Previous studies highlighted that parietal and frontal brain areas are engaged during this decision process. However, how this modulation is reflected in the neural dynamics during learning is poorly explored. Here we investigated how the degree of difficulty in this form of decision is reflected in EEG activity and how this modulation is shaped by the learning of the items’ ordinal relation. We recorded EEG activity of 32 participants from 64 scalp electrodes during the learning and test phases of a six-item TI task. The analysis of the behavioural performance detected that the decision accuracy increased with the progression of the learning relations between adjacent items and that a significant SDE emerged in the test phase. These two behavioural modulations were significantly reflected in the corresponding mean amplitude of P300 over the Centro-parietal scalp regions in both phases of the task (Anova p<0.05). Additionally, the Centro-parietal P300 was significantly modulated by the rank’s encoding of each item of the set. Collectively, these results complement previous findings on the neural basis of decision making by examining the time course of inference-based decisions during both the stimuli encoding and comparison with other stimuli.