The Influence of Eye Closure on Somatosensory Discrimination: A Trade-off Between Simple Perception and Discrimination.

We often close our eyes to improve perception. Recent results have shown a decrease of perception thresholds accompanied by an increase in somatosensory activity after eye closure. However, does somatosensory spatial discrimination also benefit from eye closure? We previously showed that spatial discrimination is accompanied by a reduction of somatosensory activity. Using magnetoencephalography, we analyzed the magnitude of primary somatosensory (somatosensory P50m) and primary auditory activity (auditory P50m) during a one-back discrimination task in 21 healthy volunteers. In complete darkness, participants were requested to pay attention to either the somatosensory or auditory stimulation and asked to open or close their eyes every 6.5 min. Somatosensory P50m was reduced during a task requiring the distinguishing of stimulus location changes at the distal phalanges of different fingers. The somatosensory P50m was further reduced and detection performance was higher during eyes open. A similar reduction was found for the auditory P50m during a task requiring the distinguishing of changing tones. The function of eye closure is more than controlling visual input. It might be advantageous for perception because it is an effective way to reduce interference from other modalities, but disadvantageous for spatial discrimination because it requires at least one top-down processing stage.

Modality-independent reduction mechanisms of primary sensory evoked fields in a one-back task.

Attentional modulation of early, primary sensory components is still a topic of debate, as studies have produced conflicting results concerning the existence of a modulation within the primary somatosensory cortex and its direction. We previously showed that attention to tactile stimuli in a stream with visual stimuli leads to a reduction of primary somatosensory components when discrimination of different stimulus locations is requested. The question arises whether this effect is universal and independent from the distracting or attended modality. To test this, we compared the magnitude of primary somatosensory evoked fields (somatosensory P50m) in a one-back task after tactile finger stimulation during attention to tactile stimuli vs. auditory distraction in 28 volunteers. In comparison to acoustic distraction, we found a significantly decreased primary somatosensory activity when attending to tactile stimuli. Strikingly, similar results were produced within the auditory modality: when attention was focused on acoustic targets, primary auditory (auditory P50m) fields were lower as compared to the situation when attention was directed to the tactile stimulation. Our results clearly indicate that the type of task, independent from the modality, is actually the crucial factor for the direction of modulation of early sensory components by attention. Therefore, our finding of reduced primary sensory components in a discrimination task represents a universal effect independent from the distracting or attended modality.

Thalamocortical impulse propagation and information transfer in EEG and MEG.

Measures of functional connectivity and information transfer between the thalamus and the cortex can provide detailed insight into brain function. Employing magnetoencephalography and electrical median nerve stimulation, it has been recently proposed that impulse propagation along the thalamocortical fiber tract can be described by a single moving dipole source. Other studies, however, using electroencephalography observed dipole clustering in the thalamus and the cortex. To assess the source of these conflicting results, we simultaneously recorded somatosensory evoked potentials and fields in 12 healthy volunteers. Using a single dipole model for the time interval of 10 to 30 milliseconds after stimulus onset, we found continuous thalamocortical dipole movement in 10 volunteers and dipole clustering in the precortical near thalamic and cortical regions in 2 volunteers. Thus, independent of the recording method, both clustering and movement can be observed. The degree of temporal overlap between the precortical near thalamic and the cortical activity distinguished the volunteers exhibiting clustering and those exhibiting movement. In a subsequent simulation study, we could show that both dipole clustering and dipole movement can occur, depending on the temporal overlap of the precortical and cortical activities. In conclusion, we propose a two-dipole model to better account for precortical and cortical activity and information transfer.