Patterns of brain activity during visual imagery of letters

Cortical signals associated with visual imagery of letters were recorded from 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 visually imagine the letter corresponding to the auditory stimulus and to examine its visuospatial properties: The associated brain activity was compared with activity evoked by the same stimuli when the subjects just detected the intervening tones. All subjects produced broad imagery-related responses over multiple cortical regions. After initial activation of the auditory cortices, the earliest imagery-related responses originated in the left prerolandic area 320 msec after the voice onset. They were followed within 70 msec by signals originating in the posterior parietal lobe close to midline (precuneus) and, 100 msec later, in the posterior superior temporal areas, predominantly in the left hemisphere. The activations were sustained and partially overlapping in time. Imagery-related activity in the left lateral occipital cortex was observed in two subjects, and weak late activity in the calcarine cortex in one subject. Real audiovisually presented letters activated multiple brain regions, and task-induced visuospatial processing of these stimuli further increased activity in some of these regions and activated additional areas: Some of these areas were activated during imagery as well. The results suggest that certain brain areas involved in high-level visual perception are activated during visual imagery and that the extent of imagery-related activity is dictated by the requirements of the stimuli and the task.