Neural basis of spontaneous thought processes

Studies examining thought processes have focused upon the deliberate, goal-directed mental processes occurring during complex cognitive tasks. Spontaneously occurring thought processes have, on the other hand, received much less attention. Such spontaneous thought processes occur frequently when no task is present or when task demands are low. Although their existence has been recognised, their study has been difficult due to lack of direct behavioural measures. Nevertheless, a number of behavioural methods based on subjects' verbal reports have been developed. Findings derived using such behavioural methods suggest that spontaneous thought processes share common cognitive mechanisms with purposeful, task-related thought processes. Furthermore, evidence from neuroimaging observations is accumulating suggesting similar conclusions about the neural basis of spontaneous thought processes. These neuroimaging findings demonstrate an overlap in the pattern of activation between various cognitive tasks and rest, with a number of higher cortical regions activated in common, including visual areas, medial temporal lobe, and lateral cortical association areas. Many of these observations have, however, been based upon comparisons between rest and tasks posing relatively high cognitive demands. In contrast, here we report an fMRI study in which rest was compared to a simple left/right response task of minimal cognitive demands. Rest was associated with greater activation in temporopolar cortex, parahippocampus, rostrolateral prefrontal cortex, parietal and visual cortical areas. Activation of temporal lobe structures was particularly extensive and robust, suggesting that long-term memory processes may form the core of spontaneous thought. By considering such long-term memory processes as an essential part of thought mechanisms, it may be possible to gain better understanding into spontaneous thought phenomena that have remained unaccounted for until now.