Effects of attention on dichotic listening: An O-15-PET study

The present study investigated the effect of attention on brain activation in a dichotic listening situation. Dichotic listening is a technique to stu dy laterality effects in the auditory sensory modality. Two different stimu li were presented simultaneously, one in each ear. Twelve subjects listened to lists of consonant-vowel syllables, or short musical instrument passage s, with the task of detecting a "target" syllable or musical instrument by pressing a button. The target stimulus appeared an equal number of times in the left and right ear. The subjects were instructed to either concentrate on the stimuli presented in both ears, or only on the left or right ear st imulus. Brain activation was measured with O-15-PET, and significant change s in regional normalized counts (rNC) were evaluated using statistical para metric mapping (SPM96) software. Concentrating on either the right or left ear stimulus significantly decreased activity bilaterally in the temporal l obes compared to concentrating on both ear stimuli, at the expense of an in creased activation in the right posterior and inferior superior parietal lo be. The CV-syllables activated areas corresponding to the classic language areas of Broca and Wernicke. The musical instrument stimuli mainly activate d areas in visual association cortex, cerebellum, and the hippocampus. An i nterpretation of the findings is that attention has a facilitating effect f or auditory processing, causing reduced activation in the primary auditory cortex when attention is explicitly recruited. The observed activations in the parietal lobe during the focused attention conditions could be part of a modality non-specific "attentional network". (C) 2000 Wiley-Liss, Inc.