Introduction: Studying the mechanism of eliciting subjective visual sensations called “phosphenes” is one way to understand the neural correlates of visual perception. Non-invasive measurements of cortical excitability during transcranial magnetic stimulation (TMS) of human primary visual cortex suggest the involvement of V1 excitation in producing phosphenes. However, electrophysiological mechanism of phosphene perception is poorly characterized by these non-invasive methods. We propose that phosphene thresholds obtained by invasive cortical stimulation of V1 provide a more reliable representation of V1 excitation and may inherit the basic characteristics of neuronal excitation, specifically the strength-duration curve.

Methods: Two parallel cortical strip leads (each with 4 contacts) was implanted over the right medial occipital lobe of a blind volunteer to assess the feasibility of a visual cortical prosthesis. Phosphene thresholds in the form of stimulation current was measured for a range of stimulation pulse-widths and frequencies in multiple time points after implantation. Current thresholds were plotted against the stimulation Pulse-widths and frequencies.

Results: Current-pulse width and current-frequency curves share the two basic characteristics of the strength-duration curve of neuronal excitation: i) a rheobase current (the minimal amplitude of infinite pulse width) is present in both curves and ii) for the lowest amount of pulse-width or frequency, a minimum charge is required to elicit a phosphene and the extra charge increases linearly as pulse-width or frequency increase.

Conclusions: The similar attributes of phosphene threshold curves and strength-duration curve of neuronal excitation suggests that V1 may be directly involved in phosphene perception. Therefore our current knowledge of neuronal excitation can be employed to generate visual perception especially in the case of visual cortical prostheses.

Patient Care: This research provides an understanding of neural correlates of phosphene perception which can be employed to design systems that are able to generate visual perception especifically visual cortical prostheses.

Learning Objectives: - Phosphene thresholds provide an index of V1 excitation. - V1 is directly involved in phosphene perception. - Neuronal excitation understandings can be employed in generating visual perception.

References: Terhune, D. B., Murray, E., Near, J., Stagg, C. J., Cowey, A., & Cohen Kadosh, R. (2015). Phosphene perception relates to visual cortex glutamate levels and covaries with atypical visuospatial awareness. Cerebral cortex, 25(11), 4341-4350. Stewart, L. M., Walsh, V., & Rothwell, J. C. (2001). Motor and phosphene thresholds: a transcranial magnetic stimulation correlation study. Neuropsychologia, 39(4), 415-419. Antal, A., Kincses, T.Z., Nitsche, M.A. et al. Exp Brain Res (2003) 150: 375. https://doi.org/10.1007/s00221-003-1459-8