Bidirectional modulation of primary visual cortex excitability: a combined tDCS and rTMS study.

Area of Science:

• Neuroscience
• Neurophysiology
• Brain Stimulation

Background:

• Transcranial direct current stimulation (tDCS) can prime motor cortex (M1) excitability for subsequent repetitive transcranial magnetic stimulation (rTMS).
• The modifiability of the visual cortex compared to the motor cortex is not well understood.

Purpose of the Study:

• To investigate if tDCS can prime the human visual cortex response to rTMS.
• To compare the modifiability of the visual cortex with that of the motor cortex.

Main Methods:

• tDCS was applied to the occipital cortex in nine healthy subjects.
• A 5 Hz rTMS train followed tDCS, with phosphene threshold (PT) measured before and after stimulation.
• Control experiments assessed rTMS alone on PT.

Main Results:

• Anodal tDCS transiently decreased PT; subsequent rTMS hastened its return to baseline.
• Cathodal tDCS transiently increased PT, but rTMS did not alter this effect.
• rTMS alone at 20 seconds did not change PT, while 60 seconds decreased PT similar to anodal tDCS.

Conclusions:

• tDCS and rTMS induce only short-lasting excitability changes in the visual cortex.
• Priming effects of tDCS on rTMS in the visual cortex are modest.
• These results indicate significant differences in the modifiability between human motor and visual cortices.