Cortical processing of near-threshold tactile stimuli in a paired-stimulus paradigm – an MEG study

In the present magnetoencephalography study, we applied a paired-stimulus paradigm to study the weak cortical responses evoked by near-threshold tactile prime stimuli by means of their attenuating effect on the cortical responses evoked by subsequently applied above-threshold test stimuli. In stimulus pairs with adequate interstimulus intervals (ISIs), the extent of test stimulus response attenuation is related to the amplitude of prime stimulus responses, and the duration of the attenuating effect indicates how long memory traces of a prime stimulus reside in cortical areas. We hypothesized that the attenuation of test stimulus responses, studied for ISIs of 30, 60 and 150 ms, would provide insight into the temporal dynamics of near-threshold stimulus processing in primary (SI) and secondary somatosensory cortex (SII), and reveal differences in response amplitude due to conscious perception. Attenuation of test stimulus responses in SI was observed for ISIs up to 60 ms, whereas in SII the effect outlasted the ISI of 150 ms. Differences due to conscious perception of the near-threshold stimuli were only observed in SII with stronger attenuation for perceived than for missed near-threshold stimuli. Applying this indirect approach to near-threshold stimulus processing, we could show that the extent and duration of response attenuation is related to prime stimulus processing and differential temporal and functional characteristics of near-threshold stimulus information processing in SI and SII: transient processing of basic stimulus information not sufficient for conscious perception in SI and long-lasting activations involving conscious perception in SII.