Abstract
Phosphenes are commonly evoked by transcranial magnetic stimulation (TMS) to study the functional organization, connectivity, and excitability of the human visual brain. For years, phosphenes have been documented only from stimulating early visual areas (V1–V3) and a handful of specialized visual regions (V4, V5/MT+) in occipital cortex. Recently, phosphenes were reported after applying TMS to a region of posterior parietal cortex involved in the top-down modulation of visuo-spatial processing. In the present study, we systematically characterized parietal phosphenes to determine if they are generated directly by local mechanisms or emerge through indirect activation of other visual areas. Using technology developed in-house to record the subjective features of phosphenes, we found no systematic differences in the size, shape, location, or frame-of-reference of parietal phosphenes when compared to their occipital counterparts. In a second experiment, discrete deactivation by 1 Hz repetitive TMS yielded a double dissociation: phosphene thresholds increased at the deactivated site without producing a corresponding change at the non-deactivated location. Overall, the commonalities of parietal and occipital phosphenes, and our ability to independently modulate their excitability thresholds, lead us to conclude that they share a common neural basis that is separate from either of the stimulated regions.
Highlights
Phosphenes are brief visual percepts caused by mechanically or electrically induced depolarization of cells in the retina or visual brain [1]
While occipital phosphenes tended to be larger than parietal ones, no significant group-level differences were observed with regard to either measure of phosphene size, area or length
A qualitative examination of drawings made by participants using the Laser Tracking and Painting (LTaP) system [52] revealed that parietal phosphenes, like their occipital counterparts, were perceived in the visual field contralateral to the transcranial magnetic stimulation (TMS) location, typically below the horizontal meridian
Summary
Phosphenes are brief visual percepts caused by mechanically or electrically induced depolarization of cells in the retina or visual brain [1]. Evoked phosphenes were first elicited in humans by applying alternating electrical currents through scalp electrodes [2]. Transcranial magnetic stimulation (TMS) has been used to safely, painlessly and noninvasively evoke phosphenes in the human occipital cortex [3]. TMS works on the principles of electromagnetic induction [4]: current is passed briefly through a coil held against the scalp, generating a rapidly changing electromagnetic field; this noninvasive field induces a brief focal electrical current in the underlying cortex, which in turn produces a synchronous depolarization of neurons in the target region (for a review, see [5]). Phosphenes have proved instrumental in demonstrating the state-dependent nature of neurostimulation methods [15,16,17]
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