From qualia to quantia: A system to document and quantify phosphene percepts elicited by non-invasive neurostimulation of the human occipital cortex

The stimulation of the occipital cortex induces transient visual percepts, known as phosphenes. The characterization and analysis of the features of these visual qualia can provide a window into the physiology and neuroanatomy of cerebral visual networks of humans. Phosphenes can be reliably elicited in humans by a variety of invasive and non-invasive techniques that depolarize visual cortex neurons. Nonetheless both research into their neural basis and categorization of their features are ultimately reliant on subjective self-reports. A variety of methods have been employed to provide a more objective means of recording the localization and morphology of neurostimulation-induced phosphenes. In spite of these attempts, phosphenes remain difficult to measure. A standard technique able to both document the myriad of features characterizing phosphenes in a flexible manner and allow a systematic quantitative comparison across groups or repeated measures is lacking. We hereby provide detailed instructions on how to use off-the-shelf components to construct and implement the LTaP (laser tracking and painting system) system for a relatively objective and real-time documentation of the presence, shape, area, spatial location and distribution of phosphenes in visual space. We further provide experimental data demonstrating the feasibility and reliability of the LTaP system to accurately capture established features of phosphenes induced by transcranial magnetic stimulation (TMS) of occipital cortex. (C) 2011 Elsevier B.V. All rights reserved.