Patterns of Brain Activity during Visual Imagery of Letters

Abstract

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.

Composition of brain oscillations and their functions in the maintenance of auditory, visual and audio–visual speech percepts: an exploratory study

“…4). This finding is in line with those of Raij (1999) and Sakowitz et al (2001), according to whom cortical activation following audio-visual stimuli involves the activation of multiple cortical regions. Also, it was shown that even simple visual stimuli activate a large number of distinct occipital, temporal, and parietal cortical areas (Felleman and Van Essen 1991).…”

Gray matter volume changes following reading intervention in dyslexic children

“…The precuneus has been implicated in various functions including visuo-spatial imagery and memory retrieval (for a review see Cavanna and Trimble, 2006). The left precuneus has been implicated in visual imagery of letters, specifically the visuo-spatial aspects of the imagery (Raij, 1999). Thompson et al (2009) found bilateral precuneus activation during a spatial location task compared to a spatial-transformation task.…”

Dissociable Influences of Auditory Object vs. Spatial Attention on Visual System Oscillatory Activity

“…, [68]) and auditory [8]–[10] spatial attention tasks. As suggested by non-human primate neurophysiological [69] as well as human fMRI [70], [71] and MEG [72] studies, the precuneus is central for complex spatial processes that require combining information from different modalities and spatial frames of references. Such processes include navigation [70], [71], updating object-position information during observer motion [73], and linking motor goals to visuospatial representations [74], [75].…”