Investigating a network model of word generation with positron emission tomography.

By using positron emission tomography (PET) we examined the biological validity of a network model describing changes in cerebral activity associated with intrinsic and extrinsic word generation. The production of words not specified by an extrinsic stimulus constitutes willed or intrinsic generation. Perceiving a heard word is an example of extrinsic generation. The model incorporates three neuronal systems: a pool that stores word representations in a distributed fashion, an afferent system conveying sensory input to the pool and a modulating system that alters the responsivity of neurons in the pool. Simulations based on the model suggested that intrinsic generation would be associated with low activity in the pool, consequent on reduced modulation, and extrinsic generation with high activity. We measured cerebral activity with PET during intrinsic (verbal fluency) and extrinsic (responding to heard words) word generation and found this pattern of changes in the left superior temporal region. We were able to designate this region the site of the distributed word store and implicate the left dorsolateral prefrontal cortex (DLPFC) as the source of modulation. The relation between the superior temporal gyrus and DLPFC was shown by examining the correlation between the two regions in terms of cerebral activity. We conclude that the left DLPFC is responsible for modulating the responsivity of a neural system in the superior temporal gyrus and is the probable mediator of changes in attentional and intentional states that underlies the intrinsic generation of words.