Effect of Attention on Prestimulus Neural Noise

Attending to a target sound increases the number of cortical resources allotted towards processing the target stimuli, leading to larger response amplitudes for the cortical auditory evoked potentials (CAEPs). However, the effect of attention on the neural noise, as well its definition, is still not clear. Having defined neural noise as the neural activity immediately preceding a stimulus, we aimed to explore the effects of attention on the prestimulus activity when measured using CAEPs. Using a 256-channel montage, we compared the global RMS amplitudes of the prestimulus (PreRMS), poststimulus (PostRMS), and the difference between PostRMS and PreRMS (DiffRMS) measured under active attention and passive attention conditions. Paired t-tests revealed a significant attention-related increase in the amplitudes of all three measures. We suppose that the attention-related excitation of target-relevant cortical pathways as well as the inhibition of target-irrelevant mechanisms, in combination, resulted in an increase in the overall neural activity in the three measures. Higher prestimulus activity can, therefore, be used as an objective index of attention and is likely to indicate anticipatory cortical preparation. Our results further validate the supposition that prestimulus activity is not merely neural noise, but indicates the neurophysiological activity associated with complex sensory and/or cognitive functions.