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

Abstract

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.

Early integration of bilateral touch in the primary somatosensory cortex

“…Similar to previous MEG studies, we found neuromagnetic activity in SI contralateral to the probe stimulus and in bilateral SII [Hari et al, ; Hoechstetter et al, ; Maldjian et al, ; Wühle et al, ], whereas no activity in the iSI was found, corroborating the notion of the contralateral vocation of SI and the bilateral nature of SII. When adaptor and probe stimulus were delivered on the same hand, unilateral stimulation response suppression was comparable in cSII and cSI.…”

“…Using a dipole source model we characterised the well‐known stimulus‐specific activation represented by activity in the cSI, cSII, and iSII (see Fig. ), as reported by previous studies [Wühle et al, ]. During the subsequent steps of the analysis, the RS of the probe stimulus was determined as a function of which adaptor stimuli preceded the probe.…”

“…To assess the validity of our approach, we started by examining RS when both stimuli were delivered within the same hand (i.e., unilateral stimulation). On the basis of previous reports on animals [Simons et al, ] and our own fMRI study [Tamè et al, ], we expected a change in RS recorded in the SI contralateral to the presentation of the probe stimulus side (cSI) when the adaptor was applied to the same finger as the probe, compared with when the adaptor was applied to the neighbouring finger [Tamè et al, ; Wühle et al, ]. To test this hypothesis, RS values evoked by the probe stimulus in cSI for unilateral hand stimulation were entered into a repeated measure ANOVA with D elay (short, long) and F inger (homologous, non‐homologous) as within participant factors.…”

“…B). Because the temporal integration window is short in SI [Mauguière et al, ] and long in SII [Wühle et al, ] any selective interaction for short delays is more compatible with interactions occurring within SI, rather than top–down modulations via higher level processing. In agreement with our hypothesis, this result is compatible with the notion that somatosensory inputs from opposite body sides can interact at early stages of tactile processing most likely through transcallosal pathways connecting SI in the two hemispheres (Fig.…”

“…The rationale for this approach was twofold. First, the RS paradigm already proved very sensitive for revealing neural populations representing the tactile feature of interest [Wühle et al, ] which might even not be visible by condition contrasting. Second, the high‐temporal resolution (i.e., in the order of milliseconds) of MEG, combined with its good spatial resolution, allows monitoring brain activity immediately after stimulus onset and provides insight in the information retention dynamics within certain brain areas.…”