The role of the thalamus in "top down" modulation of attention to sound.

By correlating rCBF with rate of presentation of tones we used PET to identify brain regions where auditory signals elicited a transient neural response. In one condition volunteers were asked to attend to the tones and ignore visual signals, while in the second condition they were asked to attend to the visual signals and ignore the tones. Activity in primary auditory cortex and adjacent areas was strongly correlated with rate of tone presentation, but this relationship was not affected by the direction of attention. In only one area, the right midthalamus, was the response to tones modulated by attention. In this area responses to tones occurred when attention was directed to sound, but not when attention was directed to visual stimuli. There is considerable evidence that the EEG evoked response to tones (N100/Nd response) is strongly modulated by attention and arises in auditory cortex. The ERP is the sum of activity from many sources. The amplitude of this response reflects not only the amount of activity in these sources, but also the degree of synchrony between them. The difference between these typical ERP results and our result from PET could be resolved if we assume that, in our paradigm, attention did not increase the amount of neural activity in auditory cortex, but rather the degree of synchrony between many sources. The signal in the thalamus, which we observed only when volunteers were attending to the tones, might provide the basis for this synchrony.