Subjective Characteristics of TMS-Induced Phosphenes Originating in Human V1 and V2

Abstract

One way to study the neural correlates of visual consciousness is to localize the cortical areas whose stimulation generates subjective visual sensations, called phosphenes. While there is support for the view that the stimulation of several different visual areas in the occipital lobe may produce phosphenes, it is not clear what the contribution of each area is. Here, we studied the roles of the primary visual cortex (V1) and the adjacent area V2 in eliciting phosphenes by using functional magnetic resonance imaging-guided transcranial magnetic stimulation (TMS) combined with spherical modeling of the TMS-induced electric field. Reports of the subjective visual features of phosphenes were systematically collected and analyzed. We found that selective stimulation of V1 and V2 are equally capable of generating phosphenes, as demonstrated by comparable phosphene thresholds and similar characteristics of phosphene shape, color, and texture. However, the phosphenes induced by V1 stimulation were systematically perceived as brighter than the phosphenes induced by the stimulation of V2. Thus, these results suggest that V1 and V2 have a similar capability to produce conscious percepts. Nevertheless, V1 and V2 contribute differently to brightness: neural activation originating in V1 generates a more intense sensation of brightness than similar activation originating in V2.

Mapping the visual brain areas susceptible to phosphene induction through brain stimulation

“…In this study, we predicted a location of the maximum induced current to describe TMS-related effects relative to functional areas (Okamoto and Dan 2005). We found that stimulation of early visual areas produces phosphenes reliably (V1 40.8 %, V2d 45.4 %, V2v 41.7 % of stimulations) as previously reported (Abrahamyan et al 2011; Salminen-Vaparanta et al 2014). However, our results suggest that TMS induces a percept most frequently when aimed at dorsal visual areas (V3d 60 %, V3a 56.4 % of stimulations).…”

“…However, in a previous study, participants reported slightly smaller phosphenes for stimulation of the dorsal visual cortex (V3d and V3a) compared to primary visual cortex V1 (Kammer et al 2005). In general, previous phosphene studies have reported that the overall appearance of phosphenes does not change significantly when different areas of the brain are stimulated (Kammer et al 2005; Salminen-Vaparanta et al 2014). This makes it unlikely that phosphene appearance systematically affected the stimulation outcome of this experiment.…”

“…In the main experiment, stimulation at each grid target was set at 110 % of the estimated phosphene threshold (Salminen-Vaparanta et al 2014). For some targets, stimulation intensity suggested by the adjustment algorithm exceeded 90 % stimulator output which we used as an upper limit on stimulation intensity.…”

“…Salminen-Vaparanta et al (2014) used detailed retinotopic maps and current modelling to show that separate stimulation of both V1 and V2d is equally capable of inducing phosphenes. Their approach underlines that we need to have knowledge of the individual functional structure of the visual cortex if we want to understand where in the brain phosphenes can be induced.…”

Phosphene perception is due to the ultra-weak photon emission produced in various parts of the visual system: glutamate in the focus

“…TMS inhibition possibly reflects the activity of GABAergic interneurons, but facilitation depends on the activation of intracortical fibers by the subthreshold stimulus, inducing the local release of glutamate (Oliveri and Caltagirone, 2006). TMS-induced electric field could induce phosphenes in the striate cortex (V1) of the macaque (Tehovnik and Slocum, 2007) and selective stimulated V1 and V2 (Salminen-Vaparanta et al, 2014) that were able to generate phosphene perception to a similar degree. It seems that TMS-induced phosphenes are due to the glutamate-related UPE.…”