Magnetic Coil Suppression of Visual Perception at an Extracalcarine Site

Abstract

Perception of extrafoveal visual targets can be suppressed by magnetic stimulation over the occipital lobes, but the site of interference for this and similar phenomena has not been well defined. We modified a previously used technique to determine the locus of magnetic activation. Using butterfly stimulus coils of different sizes and electric field profiles, we determined a scalp position of minimum threshold and a level of stimulator output that produced 50% error rates for each coil. Intersection of the corresponding electric field profiles in air and in a saline model head was similar and identified superficial occipital cortex rather than the primary visual area as the site of perceptual suppression. Less direct analyses involving the distribution of induced electric fields produced the same conclusion. These results suggest specific hypotheses about the effects of magnetic stimulation on visual physiology.

Manifestation of scotomas created by transcranial magnetic stimulation of human visual cortex

“…2). Note that this technique allows determination of a suppressed region with high resolution with only a single trial, whereas many trials are required for characterization based on performance in discriminating or identifying small visual targets at various locations 2,4,7 . When the coil was shifted to the contralateral, symmetrically positioned spot (0 to 1 in Fig.…”

“…In human visual cortex, documented effects are mostly suppressive [2][3][4] , but a small number of studies report TMS-induced phosphenes [5][6][7] . Suppression is typically characterized by deteriorated performance in discrimination or identification tasks that use briefly presented, small visual targets such as letters followed by a single magnetic pulse 2,3 .…”

Simultaneous TMS-fMRI of the Visual Cortex Reveals Functional Network, Even in Absence of Phosphene Sensation

“…However, the origin of phosphene generation is unknown, and its induction with single-pulse TMS is irregular [4-7]. Apparently, phosphenes are generated in V1, the primary visual cortex, and V2/3, the extrastriate visual-cortical areas [2, 5,6], indicating that the primary visual cortex is one fundamental area for phosphene perception [7,8]; nevertheless, others studies point to its subcortical origin (e.g., optic radiation) [9]. Meister and colleagues [10] found remarkable differences in fMRI-activation during visual stimulation (checkerboard paradigm) between people perceiving phosphene and those lacking this perception, suggesting that this disparity might reflect inter-individual functional differences in visual neuronal networks.…”

Mechanisms underlying rapid experience-dependent plasticity in the human visual cortex

“…T ranscranial magnetic stimulation (TMS) delivered to occipital sites (1,2) can elicit phosphenes, originating close to the cortical surface (3). The minimum TMS intensity required to elicit phosphenes is defined as phosphene threshold (PT) and represents a measure of visual cortex excitability (4,5).…”