Altered motor cortex excitability by colour perception as a hint for cross-modal plasticity

Aims: Colour affects not only the emotional state, but also cognitive performance and perception of other sensory modalities such as tone or smell. However, very little is known about the neuronal mechanisms which account for these cross-modal interactions. To explore neurobiological mechanisms of these synaesthetic phenomena, we used transcranial magnetic stimulation (TMS) to assess the impact of colour perception on motor cortex excitability.

Material and Methods: Transcranial magnetic stimulation allows to assess non-invasively the integrity and excitability patterns of cortical and subcortical networks. Using this method we studied ten healthy volunteers under three different conditions (no visual stimulation, stimulation with red light, stimulation with green light). Red and green light was matched for colour intensity and brightness. The colours were presented in a randomized order by video goggles. Measurements of cortical excitability started after 5 minutes adaptation and included the assessment of motor threshold (MT), cortical silent period (CSP), paired-pulse inhibition (PPI) and paired-pulse facilitation (PPF).

Results: Intracortical inhibition (PPI) was significantly increased during green light stimulation as compared with the baseline condition, while red light had no significant effect on PPI. All other parameters of cortical excitability (PPF, MT, CSP) remained unchanged during red and green light stimulation.

Conclusions: These data give first evidence that colour perception may lead to changes in motor cortex excitability as a direct hint for cross modal plasticity.