Contribution of frontal cortex to the spatial representation of number

Neuropsychological and neurophysiological studies have suggested that prefrontal cortex may be involved in non-verbal number processing, when relevant for current behavioural goals. More precisely, it has been suggested that an intact right inferior frontal gyrus (rIFG) in humans may be necessary to the use of a spatial representation of numbers, also known as mental number line. In a popular model of spatial functions (e.g., Corbetta et al., 2008), rIFG is part of a right-lateralised ventral fronto-parietal network that conveys signals to a dorsal network supporting attentional orienting in contralateral space. Within the dorsal network, the frontal eye fields (FEF) are known to contribute to visual scene analysis and visual conjunction search tasks when eye movement commands are not required. In the present study, we hypothesised they might also be involved in exploring a conceptual space, such as the mental number line. We examined the proposed functions of the human rIFG and right Frontal Eye Field (rFEF) by interfering with their normal functioning with repetitive Transcranial Magnetic Stimulation (rTMS) while participants performed numerical tasks. The results suggest that, when number magnitude is relevant to the task, rIFG supports orienting to the entire mental number line while rFEF are crucial for contralateral orienting (that is towards small numbers).