Patterns of brain activity during visual imagery of letters

Cortical signals associated with visual imagery of letters were recorded fr om 10 healthy adults with a whole-scalp 122-channel neuromagnetometer. The auditory stimulus sequence consisted of 20 different phonemes corresponding to single letters of the Roman alphabet and of tone pips (17%), delivered once every 1.5 sec in a random order. The subjects were instructed to visua lly imagine the letter corresponding to the auditory stimulus and to examin e its visuospatial properties: The associated brain activity was compared w ith activity evoked by the same stimuli when the subjects just detected the intervening tones. All subjects produced broad imagery-related responses o ver multiple cortical regions. After initial activation of the auditory cor tices, the earliest imagery-related responses originated in the left prerol andic area 320 msec after the voice onset. They were followed within 70 mse c by signals originating in the posterior parietal lobe close to midline (p recuneus) and, 100 msec later, in the posterior superior temporal areas, pr edominantly in the left hemisphere. The activations were sustained and part ially overlapping in time. Imagery-related activity in the left lateral occ ipital cortex was observed in two subjects, and weak late activity in the c alcarine cortex in one subject. Real audiovisually presented letters activa ted multiple brain regions, and task-induced visuospatial processing of the se stimuli further increased activity in some of these regions and activate d additional areas: Some of these areas were activated during imagery as we ll. The results suggest that certain brain areas involved in high-level vis ual perception are activated during visual imagery and that the extent of i magery-related activity is dictated by the requirements of the stimuli and the task.