Inter- and intra-individual differences in fluid reasoning show distinct cortical responses

Fluid intelligence is a general cognitive ability associated with problem solving in the absence of task-specific knowledge. Neuroscientific studies of fluid intelligence have studied both fluid intelligence tasks of varying difficulty and individual differences in fluid intelligence ability, but have failed to appropriately distinguish the two dimensions. Here we use task-based fMRI (N=34) to show that within and between subject dimensions show both partial overlap and widespread differences. Individuals with higher ability showed widespread increased activity including bilateral frontoparietal systems, whereas more difficult items were associated with more focal activity increases in middle frontal gyri, frontal poles and superior frontal poles. Finally, we show that when difficulty is equated across individuals, those with higher ability tend to show more fronto-parietal activity, whereas low fluid intelligence individuals tend to show greater activity in higher visual areas. The fMRI and behavioural data for our paper are freely available in online repositories.