High-density optical mapping of the human somatosensory cortex to vibrotactile stimulation
|S.P. Koch1, J. Menert1,2, C. Schmitz1,3, S. Holtze1, A. Villringer1,2,4, H. Obrig1,5
1Berlin NeuroImaging Center, Charité, Berlin, Germany/2Max-Planck Institute for Human Cognitive and Brain Science, Leipzig, Germany/3NIRx Medizintechnik GmbH, Berlin, Germany/4Berlin School of Mind and Brain, Berlin, Germany/5Day Clinic for Cognitive Neurology, University Hospital, Leipzig, Germany
Optical imaging is considered a versatile functional imaging tool in studies which limit the application of fMRI due to experimental/technical or ethical/clinical reasons. The major shortcoming of the methodology is its low spatial resolution. Recently, however, Zeff and co-workers  showed retinotopic activations to excentric stimuli in the human visual cortex by high-density optical tomography. The current study was performed to find out whether high-resolution optical topography allows to demonstrate homuncular somatotopic representation in the somatosensory cortex.
In 8 subjects the thumb and the 5th finger of the left hand were stimulated pseuso-randomely with PC-controlled electrical toothbrushes integrated in a glove. Vibrotacitle stimulation periods (4s) were separated by jittered baseline periods (12-16s). To functionally localize the sensori-motor cortex subjects additionally performed blocks of randomized self-paced finger tapping of the left hand (thumb against all other fingers for 4s). Task performance was guided by the acoustic sound of the toothbrushes, thus minimizing differences between the somatosensory and the motor task. Optical topography was performed over a rectangular array of 30 sources and 30 detectors (fibre optics) fixated over the sensorimotor cortex contralateral to the stimulated hand. Because read out for each source is realized simultaneously for all detectors, the setup provides 900 measuring channels allowing for a multi distance based depth resolution rendering a rough optical tomography. To align optically measured functional activation with anatomical structures, each subject equipped with fiducial markers encompassing the optical array underwent an anatomical MR-scan.
Results: GLM-based analysis revealed a functional activation during both finger tapping and vibrotactile stimulation (focal decrease in deoxy-hemoglobin concentration compared to baseline). In a first topographical approach images of layers based on the relationship between source-detector distances and estimated optical penetration depth show an activation pattern which was spatially different for vibrotactile stimulation of the thumb compared to the 5th finger. Vibrotactile stimulation of the thumb caused a spatially broader activity compared to the 5th finger. However, in 6 of 8 subjects both conditions also differed spatially with respect to the condition-specific maximum of activation. The results correspond to the somatosensory organization, with the thumb laterally and the 5th finger medially localized. Presently, the first tomographical results also reveal condition-specific activations with spatially different activations. This latter result was confirmed by permutation tests within the individual subject.
Conclusions: The results show that optically based high-density imaging is feasible to localize and discern somatopic activations to vibrotactile stimulation of different fingers. As expected by homuncular organisation of the somatosensory cortex the hemodynamic response to vibrotactile stimulation of the thumb was localized more laterally compared to the 5th finger. This result is in good agreement with fMRI studies that have investigated the human somatosensory system [2,3,4] and prove that functional optical techniques can yield high-resolution maps of functional cortical anatomy.
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