Thalamic and cortical mechanisms of attention suggested by recent positron emission tomographic experiments.

An attention mechanism in the pulvinar nucleus of the thalamus appears to be involved in the filtering of a location in a cluttered visual field, according to recent PET data. The mechanism in the pulvinar is assumed to be a particular type of circuit that reciprocally connects thalamic relay cells to cortical cells. This circuit, which is characteristic of thalamic nuclei in general, appears to embody an algorithm that enhances firing in a target set of cells while inhibiting firing in the surrounding set of cells. Recent PET studies have also identified specific areas of cortex that show increased blood flow when humans expect to process particular aspects of visual objects, such as location, shape, color, and movement velocity. An expectation for a particular feature can be regarded as a peaked activity distribution across cortical synaptic space that increases the effectiveness (accuracy and speed) of the perception of that feature and features similar to that feature. The expectation process is assumed to be generated by a thalamic enhancement circuit, which in turn is driven by a cognitive procedure, apparently located in the anterior cingulate area of the cortex. The selection of a particular cognitive procedure is assumed to depend on the momentary relative motivational value associated with it by means of connections to deeper limbic structures. Under usual conditions, the procedure having the dominant motivational value is the one selected.