The left frontal lobe is critical for the AH4 fluid intelligence test

The frontal lobes are thought to make a fundamental contribution to fluid intelligence. However, evidence that fluid intelligence is impaired following focal frontal lobe lesions is surprisingly sparse and based on non-verbal tests of fluid intelligence. We investigated performance on Part 1 of the Alice Heim 4 (AH4–1), a verbal test of fluid intelligence, in a sample of 35 patients with focal, unilateral, left or right, frontal brain tumours and 54 healthy controls. We analysed the following variables: overall number of correct AH4–1 answers, overall AH4–1 accuracy and accuracy on four selected categories of AH4–1 questions that assess abilities previously linked to the frontal lobes, namely: synonyms, verbal analogies, numerical series and multistage calculations. We found several significant frontal effects. Thus, in comparison to healthy controls, frontal patients had a significantly lower overall number of AH4–1 answers, had significantly lower overall AH4–1 accuracy and had significantly poorer performance on verbal analogies and multistage calculations. We also found several significant lateralised left frontal effects. Thus, in comparison to healthy controls, left, but not right, frontal patents had significantly lower overall AH4–1 accuracy and poorer performance on synonyms, numerical series and multistage calculation questions. This suggests that the left frontal lobe plays a critical role in AH4–1 performance. Moreover, left frontal patients had significantly lower overall AH4–1 accuracy and poorer performance on multistage calculations than right frontal patients. These results suggest that a left lateralised frontal network is critically involved in some aspects of fluid intelligence and, in particular, multistage calculations.