Aim: 

Sensory stimuli of any given sensory modality elicit a number of transient brain responses, some of which are detected in the ongoing EEG as event-related potentials (ERPs). The larger part of ERPs is a negative-positive biphasic wave (N1-P2), maximal at the scalp vertex. The N1-P2 response has been extensively used to gain knowledge about the neural basis of sensory perception in humans. However, much of this knowledge relies on a critical assumption: that the N1-P2 response reflects neuronal activities that are primarily modality-specific (i.e. unique for the sensory modality of the eliciting stimulus).

Methods: 

Here we apply a novel blind source separation algorithm (Probabilistic ICA) to ERPs elicited by a random sequence of auditory, innocuous somatosensory, noxious somatosensory, and visual stimuli in healthy volunteers.

Results: 

We show that the assumption that the N1-P2 response reflects neuronal activities that are primarily modality-specific is incorrect. The N1-P2 response is predominantly explained by neuronal activities that are multimodal (i.e. independent of the sensory modality of the eliciting stimulus), and largely related to the saliency of the eliciting stimulus (i.e. its ability to attract attention).

Conclusion: 

By showing (i) that it is incorrect to assume that the N1-P2 response reflects neural processes that are modality-specific, and (ii) that the greater part of the N1-P2 is related to stimulus saliency, our findings considerably undermine the current knowledge of how the brain processes sensory input, and urge for novel experimental designs that allow dissecting the modality-specific or amodal nature of observed experimental effects.