The evidence we have examined indicates that meaningful neural events may entail activity in a distributed network in such a manner that at each of the cortical loci involved there is coherent activation of neurons in an area extending over several millimetres. One corollary of this is that the patterns of activation are accessible to macroscopic imaging techniques that measure local changes in regional cerebral blood flow (rCBF) associated with changes in the metabolic requirements of active neurons. Techniques such as PET and fMRI can be used to provide images of rCBF with a spatial resolution of several millimetres. SPET provides images of rCBF with somewhat less sensitivity and resolution. It is likely that the demand for increased rCBF during neuronal firing will be greatest in the vicinity of the synapses, since synaptic processes consume a substantial amount of energy.
In principle, the activation of either excitatory neurons or inhibitory neurons can cause an increase in rCBF. In practice, it is likely that whenever neurons in a particular cortical location are engaged in meaningful processing, local circuits that include both excitatory and inhibitory neurons will be active, since a combination of excitatory and inhibitory neurotransmission would generally be required to produce meaningful temporal and spatial patterns of neural activity.
None the less, because the major long-range fibres that bring specific information from a distant site to the local circuit are excitatory glutamatergic fibres, and these incoming fibres usually form synapses with many local neurons it is reasonable to assume that an increase in local CBF reflects an increase in excitatory input to that location, while a decrease in local CBF reflects a decrease in excitatory input to that location.
Studies using PET confirm that mental processes are associated with activation at a distributed set of cerebral loci, with relatively extensive activation at each locus. For example, in studies of word generation carried out in collaboration with colleagues at Hammersmith Hospital (Frith et al, 1991a, 1991b), we found that during the internal generation of words beginning with a specified letter, there is activation of extensive regions in the left dorsolateral and medial frontal
cortex, posterior cingulate and thalamus, while there is a diminution of activity in the superior temporal gyrus. The extent of activation in these regions was substantially greater than would be accounted for by smoothing arising from sources such as the limited spatial resolution of the PET camera. In these studies, the pattern of cerebral activity associated with the internal generation of words was determined by subtracting the pattern of activity during the articulation of a list of words provided by the experimenter from the pattern of activity during the articulation of words generated by the subject.
In principle, the activation of either excitatory neurons or inhibitory neurons can cause an increase in rCBF. In practice, it is likely that whenever neurons in a particular cortical location are engaged in meaningful processing, local circuits that include both excitatory and inhibitory neurons will be active, since a combination of excitatory and inhibitory neurotransmission would generally be required to produce meaningful temporal and spatial patterns of neural activity.
None the less, because the major long-range fibres that bring specific information from a distant site to the local circuit are excitatory glutamatergic fibres, and these incoming fibres usually form synapses with many local neurons it is reasonable to assume that an increase in local CBF reflects an increase in excitatory input to that location, while a decrease in local CBF reflects a decrease in excitatory input to that location.
Studies using PET confirm that mental processes are associated with activation at a distributed set of cerebral loci, with relatively extensive activation at each locus. For example, in studies of word generation carried out in collaboration with colleagues at Hammersmith Hospital (Frith et al, 1991a, 1991b), we found that during the internal generation of words beginning with a specified letter, there is activation of extensive regions in the left dorsolateral and medial frontal
cortex, posterior cingulate and thalamus, while there is a diminution of activity in the superior temporal gyrus. The extent of activation in these regions was substantially greater than would be accounted for by smoothing arising from sources such as the limited spatial resolution of the PET camera. In these studies, the pattern of cerebral activity associated with the internal generation of words was determined by subtracting the pattern of activity during the articulation of a list of words provided by the experimenter from the pattern of activity during the articulation of words generated by the subject.
Get the RSS feed