Transcranial magnetic stimulation in the visual system. I. The psychophysics of visual suppression
When applied over the occipital pole, transcranial magnetic stimulation (TMS) disrupts visual perception and induces phosphenes. Both the underlying mechanisms and the brain structures involved are still unclear. The first part of the study characterizes the suppressive effect of TMS by psychophysical methods. Luminance increment thresholds for orientation discrimination were determined in four subjects using an adaptive staircase procedure. Coil position was controlled with a stereotactic positioning device. Threshold values were modulated by TMS, reaching a maximum effect at a stimulus onset asynchrony (SOA) of approx. 100 ms after visual target presentation. Stronger TMS pulses increased the maximum threshold while decreasing the SOA producing the maximum effect. Slopes of the psychometric function were flattened with TMS masking by a factor of 2, compared to control experiments in the absence of TMS. No change in steepness was observed in experiments using a light flash as the mask instead of TMS. Together with the finding that at higher TMS intensities, threshold elevation occurs even with shorter SOAs, this suggests lasting inhibitory processes as masking mechanisms, contradicting the assumption that the phosphene as excitatory equivalent causes masking. In the companion contribution to this one we present perimetric measurements and phosphene forms as a function of the stimulation site in the brain and discuss the putative generator structures.
KeywordsOccipital cortex Visual masking Threshold modulation Psychometric function Slope
We thank Sandra Beck, Kuno Kirschfeld, and Hans-Günther Nusseck for support and for many fruitful discussions.
- Bolz J, Rosner G, Wässle H (1982) Response latency of brisk-sustained (X) and brisk-transient (Y) cells in the cat retina. J Physiol (Lond) 382:171–190Google Scholar
- Dorner-Schandl F, Durst W, Kolling G, Leo-Kottler B (1993) Rasterperimetrie mit dem Tübinger Automatik Perimeter. Universitäts-Augenklinik, TübingenGoogle Scholar
- Inghilleri M, Berardelli A, Cruccu G, Manfredi M (1993) Silent period evoked by transcranial stimulation of the human cortex and cervicomedullary junction. J Physiol (Lond) 466:521–534Google Scholar
- Kammer T, Lehr L, Kirschfeld K (1999) Cortical visual processing is temporally dispersed by luminance in human subjects. Neurosci Lett 263:133–136Google Scholar
- Kammer T, Puls K, Erb M, Grodd W (2004) TMS in the visual system. II. Characterization of induced phosphenes and scotomas. Exp Brain Res (accepted for publication)Google Scholar
- Kesten H (1958) Accelerated stochastic-approximation. Ann Math Stat 29:41–59Google Scholar
- Moliadze V, Zhao Y, Eysel UT, Funke K (2003) Effect of transcranial magnetic stimulation on single-unit activity in the cat primary visual cortex. J Physiol (Lond) 553:665–679Google Scholar