Changes in visual cortex excitability in blind subjects as demonstrated by transcranial magnetic stimulation

Abstract

Any attempt to restore visual functions in blind subjects with pregeniculate lesions provokes the question of the extent to which deafferented visual cortex is still able to generate conscious visual experience. As a simple approach to assessing activation of the visual cortex, subjects can be asked to report conscious subjective light sensations (phosphenes) elicited by focal transcranial magnetic stimulation (TMS) over the occiput. We hypothesized that such induction of phosphenes can be used as an indicator of residual function of the visual cortex and studied 35 registered blind subjects after partial or complete long-term (>10 years) deafferentation of the visual cortex due to pregeniculate lesions. TMS was applied over the visual cortex in 10 blind subjects with some residual vision (visual acuity <20/400; Group 1), 15 blind subjects with very poor residual vision (only perception of movement or light; Group 2), 10 blind subjects without any residual vision (Group 3) and 10 healthy controls. A stimulation mapping procedure was performed on a 1 x 1 cm skull surface grid with 130 stimulation points overlying the occipital skull. We analysed the occurrence of phosphenes at each stimulation point with regard to frequency and location of phosphenes in the visual field. Previous experiments have shown that repetitive TMS reliably elicits brief flashes of white or coloured patches of light. Therefore, stimulation was performed with short trains of seven consecutive 15 Hz stimuli applied with an intensity of 1.3 times the motor threshold. Under such conditions, phosphenes occurred in 100% of subjects in Group 1, in 60% of Group 2 and in 20% of Group 3. Phosphene thresholds were normal, but the number of effective stimulation sites was significantly reduced in Groups 2 and 3. The results indicate that in blind subjects there is alteration in TMS-induced activation of the deafferented visual cortex or processes engaged in bringing the artificial cortex input to consciousness. The ability to elicit phosphenes is reduced in subjects with a high degree of visual deafferentation, especially in those without previous visual experience.

Transcranial magnetic stimulation of the visual cortex induces somatotopically organized qualia in blind subjects

“…In contrast, 9 of 10 SC subjects described phosphenes after occipital TMS (mean intensity threshold, 69 Ϯ 12% of maximal stimulator intensity). Attribution of phosphene locations varied with stimulation sites, in accordance with other studies (20,21).…”

Visual Cortex Activity in Early and Late Blind People

“…Thus, similar TMS levels evoke visual experience of phosphenes when applied over occipital cortex in some congenitally blind people, those who are blind but retain some light or motion sensitivity, and sighted subjects (Gothe et al, 2002). However, TMS to occipital cortex elicits no phosphenes in most early blind subjects who have no previous visual function (Gothe et al, 2002). Nevertheless, another study demonstrated interference with Braille reading when TMS was applied to occipital visual areas in early blind subjects (Cohen et al, 1997(Cohen et al, , 1999, which indicates that occipital cortex contributes some function in these people.…”

“…Thus, similar TMS levels evoke visual experience of phosphenes when applied over occipital cortex in some congenitally blind people, those who are blind but retain some light or motion sensitivity, and sighted subjects (Gothe et al, 2002). However, TMS to occipital cortex elicits no phosphenes in most early blind subjects who have no previous visual function (Gothe et al, 2002).…”

A Novel Approach for Documenting Phosphenes Induced by Transcranial Magnetic Stimulation

“…Phosphenes were elicited by delivering individual pulses of TMS to the right occipital cortex adjacent to the occipital pole. Phosphene threshold values provide a valuable indication of relative cortical excitability; and have been used both as an outcome measure and as a means of determining the appropriate intensity of TMS in variety of studies of human visual perception [8][9][10][11][12] .…”