The application of TMS in Vision
Transcranial magnetic stimulation (TMS) can be used to mask visual stimuli, disrupt visual task performance or prevent visual awareness (de Graaf et al., 2011). TMS has already done much to elucidate visual pathways and investigate visual processing at the cortical level. The anatomical position of the visual cortex, consisting of the primary visual cortex (the striate cortex or V1 - one of the best-explored areas of the visual cortex) and extrastriate visual cortical areas (including V2, V3, V4 and V5), makes it easily accessible for accurate TMS investigations.
Ongoing research
In 1989 Amassian et al. first clearly showed that (single-pulse) TMS can disrupt visual processing. The authors tested themselves in recognising trigrams of alphabetical letters and showed that single pulse TMS increases difficulty in discriminating between them when applied to V1. Since then, many studies have contributed to the visual suppression effects of TMS. Longer lasting, more intense and cumulative effects can be seen with rTMS.
In 2010 de Graaf et al., published results suggesting that, with sensitive TMS masking, further research might be able to reveal visual processes in early visual cortex previously unreported. Based on the premise that TMS can induce masking by interfering with ongoing neural activity in early visual cortex, the Authors claim the study goes beyond previous work in two regards; First, by exploring both objective vision (an orientation discrimination task) and subjective vision (a stimulus visibility rating on a four-point scale), across a wide range of time windows with high temporal resolution. Second, by using a very sensitive TMS-masking paradigm: stimulation was at relatively low TMS intensities, with a figure-8 coil, and the small stimulus was difficult to discriminate already at baseline level. They reported an interesting masking curve; within the classical-masking time window, previously reported to encompass broad SOAs anywhere between 60 and 120 ms, they saw at least two dips in objective performance, with no masking in-between. The subjective measure of vision did not mirror this pattern.
Other areas that TMS is currently being used to investigate include:
- Visual perceptual priming
- Visual neglect
- Visual search – encompassing the parietal cortex and the frontal eye fields (FEF)
- Synchronised cortical discharge
References
- de Graaf et al., Conscious Cogn, 2011
- de Graaf et al., Experimental Brain Research, 2010
- Oxford Handbook of Transcranial Stimulation
- Amassian et al. 1989
