Concurrent use of somatotopic and external reference frames in a tactile mislocalization task.

Localizing tactile stimuli on our body requires sensory information to be represented in multiple frames of reference along the sensory pathways. These reference frames include the representation of sensory information in skin coordinates, in which the spatial relationship of skin regions is maintained. The organization of the primary somatosensory cortex matches such somatotopic reference frame. In contrast, higher-order representations are based on external coordinates, in which body posture and gaze direction are taken into account in order to localise touch in other meaningful ways according to task demands. Dominance of one representation or the other, or the use of multiple representations with different weights, is thought to depend on contextual factors of cognitive and/or sensory origins. However, it is unclear under which situations a reference frame takes over another or when different reference frames are jointly used at the same time. The study of tactile mislocalizations at the fingers has shown a key role of the somatotopic frame of reference, both when touches are delivered unilaterally to a single hand, and when they are delivered bilaterally to both hands. Here, we took advantage of a well-established tactile mislocalization paradigm to investigate whether the reference frame used to integrate bilateral tactile stimuli can change as a function of the spatial relationship between the two hands. Specifically, supra-threshold interference stimuli were applied to the index or little fingers of the left hand 200ms prior to the application of a test stimulus on a finger of the right hand. Crucially, different hands postures were adopted (uncrossed or crossed). Results show that introducing a change in hand-posture triggered the concurrent use of somatotopic and external reference frames when processing bilateral touch at the fingers. This demonstrates that both somatotopic and external reference frames can be concurrently used to localise tactile stimuli on the fingers.

Prestimulus oscillatory power and connectivity patterns predispose conscious somatosensory perception.

Which aspects of our sensory environment enter conscious awareness does not only depend on physical features of the stimulus, but also critically on the so-called current brain state. Results from magnetoencephalography/EEG studies using near-threshold stimuli have consistently pointed to reduced levels of α- (8-12 Hz) power in relevant sensory areas to predict whether a stimulus will be consciously perceived or not. These findings have been mainly interpreted in strictly "local" terms of enhanced excitability of neuronal ensembles in respective cortical regions. The present study aims to introduce a framework that complements this rather local perspective, by stating that the functional connectivity architecture before stimulation will predetermine information flow. Thus, information computed at a local level will be distributed throughout a network, thereby becoming consciously accessible. Data from a previously published experiment on conscious somatosensory near-threshold perception was reanalyzed focusing on the prestimulus period. Analysis of spectral power showed reduced α-power mainly in the contralateral S2 and middle frontal gyrus to precede hits, thus overall supporting the current literature. Furthermore, differences between hits and misses were obtained on global network (graph theoretical) features in the same interval. Most importantly, in accordance with our framework, we could show that the somatosensory cortex is "more efficiently" integrated into a distributed network in the prestimulus period. This finding means that when a relevant sensory stimulus impinges upon the system, it will encounter preestablished pathways for information flow. In this sense, prestimulus functional connectivity patterns form "windows" to conscious perception.

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.

Mislocalization of near-threshold tactile stimuli in humans: a central or peripheral phenomenon?

Principles of brain function can be disclosed by studying their limits during performance. Tactile stimuli with near-threshold intensities have been used to assess features of somatosensory processing. When stimulating fingers of one hand using near-threshold intensities, localization errors are observed that deviate significantly from responses obtained by guessing - incorrectly located stimuli are attributed more often to fingers neighbouring the stimulated one than to more distant fingers. Two hypotheses to explain the findings are proposed. The 'central hypothesis' posits that the degree of overlap of cortical tactile representations depends on stimulus intensity, with representations less separated for near-threshold stimuli than for suprathreshold stimuli. The 'peripheral hypothesis' assumes that systematic mislocalizations are due to activation of different sets of skin receptors with specific thresholds. The present experiments were designed to decide between the two hypotheses. Taking advantage of the frequency tuning of somatosensory receptors, their contribution to systematic misclocalizations was studied. In the first experiment, mislocalization profiles were investigated using vibratory stimuli with frequencies of 10, 20 and 100 Hz. Unambiguous mislocalization effects were only obtained for the 10-Hz stimulation, precluding the involvement of Pacinian corpuscles in systematic mislocalization. In the second experiment, Pacinian corpuscles were functionally eliminated by applying a constant 100-Hz vibratory masking stimulus together with near-threshold pulses. Despite masking, systematic mislocation patterns were observed rendering the involvement of Pacinian corpuscles unlikely. The results of both experiments are in favor of the 'central hypothesis' assuming that the extent of overlap in somatosensory representations is modulated by stimulus intensity.

Effects of motor activity on the organization of primary somatosensory cortex.

Recent studies have shown that adaptation of representational maps within the primary somatosensory cortex can be induced by task-related motor activity. Here, we explore the relationship between the complexity of the motor task and the extent of task-specific adaptation within the primary somatosensory cortex. We hypothesized that the extent of adaptation increases with the complexity of the motor task. Using neuromagnetic source imaging based on electrical stimulation of the thumb and ring finger, we demonstrate that cortical finger representations are more distant during performance of the pinch finger grip than in a rest condition. Our data suggest that somatosensory cortical maps undergo rapid modulation depending on the task-specific involvement of somatosensory feedback in movements.

Context-dependent impact of presuppositions on early magnetic brain responses during speech perception.

Discourse structure enables us to generate expectations based upon linguistic material that has already been introduced. The present magnetoencephalography (MEG) study addresses auditory perception of test sentences in which discourse coherence was manipulated by using presuppositions (PSP) that either correspond or fail to correspond to items in preceding context sentences with respect to uniqueness and existence. Context violations yielded delayed auditory M50 and enhanced auditory M200 cross-correlation responses to syllable onsets within an analysis window of 1.5s following the PSP trigger words. Furthermore, discourse incoherence yielded suppression of spectral power within an expanded alpha band ranging from 6 to 16Hz. This effect showed a bimodal temporal distribution, being significant in an early time window of 0.0-0.5s following the PSP trigger and a late interval of 2.0-2.5s. These findings indicate anticipatory top-down mechanisms interacting with various aspects of bottom-up processing during speech perception.

Cortical processing of near-threshold tactile stimuli: an MEG study.

In the present study we tested the applicability of a paired-stimulus paradigm for the investigation of near-threshold (NT) stimulus processing in the somatosensory system using magnetoencephalography. Cortical processing of the NT stimuli was studied indirectly by investigating the impact of NT stimuli on the source activity of succeeding suprathreshold test stimuli. We hypothesized that cortical responses evoked by test stimuli are reduced due to the preactivation of the same finger representation by the preceding NT stimulus. We observed attenuation of the magnetic responses in the secondary somatosensory (SII) cortex, with stronger decreases for perceived than for missed NT stimuli. Our data suggest that processing in the primary somatosensory cortex including recovery lasts for <200 ms. Conversely, the occupancy of SII lasts >/=500 ms, which points to its role in temporal integration and conscious perception of sensory input.

The right hand knows what the left hand is feeling.

The mislocalization profile, describing incorrect localization of faint tactile stimuli to different regions of the body, has been shown to provide insight into the processing of tactile stimuli. Interhemispheric somatosensory processing was examined in 15 subjects by studying the interference of left-hand stimulation on right-hand perception. In different conditions supra-threshold interference stimuli were applied to the left thumb or little finger either 200 or 500 ms prior to the application of a test stimulus on the right hand. Data show that interference stimuli applied to the left hand massively altered localization responses for stimuli applied to the right side. Stimulating the left thumb yielded an increased number of mislocalizations to the right thumb. Similarly, stimulating the left little finger caused a shift in localization responses towards the right ring finger. Results support the hypothesis that interaction of somatosensory information originating from different sides of the body follows a somatotopic organization.