Evoked response amplitudes from somatosensory cortices do not determine reaction times to tactile stimuli.

Sensory events cause changes in brain activity, which underlie the perception of and behavioural responses to sensory stimuli. Evoked cortical responses are an important measure of these stimulus-evoked changes in brain activity. However, evidence on the relationship between behavioural responses and evoked responses is inconsistent. Therefore, we used magnetoencephalography to reinvestigate the relationship between evoked responses from somatosensory cortices and behavioural responses to somatosensory stimuli. We characterized modulations of somatosensory-evoked responses exerted by preceding painful and tactile conditioning stimuli (CS), and related these modulations of evoked responses to modulations of reaction times. Our results show that painful CS yield a long-lasting (> 4 s) facilitation of evoked responses, whereas tactile CS result in a shorter lasting (1-2 s) suppression of evoked responses to tactile stimuli. These contrary physiological effects were both associated with a significant shortening of reaction times. These findings indicate that the conditioning effects of painful and tactile stimuli represent essentially different modulatory mechanisms. Moreover, our results show that amplitudes of evoked responses from somatosensory cortices do not determine reaction times to tactile stimuli.