Emotional Context Modulates the Response to Somatosensory Stimuli Within 20 milliseconds

The processing of discrete stimuli is modulated by the emotional context in which they appear. According to the limited data available, this modulation could occur from the initial levels of processing. Our aim was to find the first trace of emotional context modulation in the neural response to discrete stimuli presented within that context. We focused on the somatosensory domain, characterized by faster and better localized responses within the ascending pathway than the more widely studied visual modality. Early components of somatosensory evoked potentials (SEPs), particularly those reflecting the thalamic and primary somatosensory cortex activity occurring within the first 20 milliseconds from stimulus onset, were analyzed to test whether they are already affected by the emotional context. Participants (n = 37) received somatosensory stimulation via electrical pulses over the median nerve of the right arm while viewing 12 stereoscopic (3D) images of animals categorized as emotionally positive, negative, or neutral to generate different affective contexts. SEPs elicited by somatosensory stimuli-which were identical in all contexts- were recorded through a bipolar CP3-CP4 channel, revealing a P17 (originating in the thalamus) and an N20 (primary somatosensory cortex or S1) component. The negative contexts evoked significantly smaller P17-N20 peak-to-peak amplitudes than positive and neutral contexts, an attenuation of somatosensory perception probably reflecting a rapid defensive mechanism. These results reveal the earliest emotion-modulated brain response described to date in humans (17-20 milliseconds and thalamic/S1 origin) and point to the somatosensory ascending pathway as an active emotional processor.