Human finger somatotopy in areas 3b, 1, and 2: A 7T fMRI study using a natural stimulus

To study the properties of human primary somatosensory (S1) cortex as well as its role in cognitive and social processes, it is necessary to noninvasively localize the cortical representations of the body. Being arguably the most relevant body parts for tactile exploration, cortical representations of fingers are of particular interest. The aim of the present study was to investigate the cortical representation of individual fingers (D1-D5), using human touch as a stimulus. Utilizing the high BOLD sensitivity and spatial resolution at 7T, we found that each finger is represented within three subregions of S1 in the postcentral gyrus. Within each of these three areas, the fingers are sequentially organized (from D1 to D5) in a somatotopic manner. Therefore, these finger representations likely reflect distinct activations of BAs 3b, 1, and 2, similar to those described in electrophysiological work in non-human primates. Quantitative analysis of the local BOLD responses revealed that within BA3b, each finger representation is specific to its own stimulation without any cross-finger responsiveness. This finger response selectivity was less prominent in BA 1 and in BA 2. A test-retest procedure highlighted the reproducibility of the results and the robustness of the method for BA 3b. Finally, the representation of the thumb was enlarged compared to the other fingers within BAs 1 and 2. These findings extend previous human electrophysiological and neuroimaging data but also reveal differences in the functional organization of S1 in human and nonhuman primates. Hum Brain Mapp, 2012. © 2012 Wiley Periodicals, Inc.

Anatomical and functional properties of the foot and leg representation in areas 3b, 1 and 2 of primary somatosensory cortex in humans: A 7T fMRI study

Michel Akselrod, Olaf Blanke, Roger Gassert, Roberto Martuzzi, Andrea Serino, Wietske Van der Zwaag

Primary somatosensory cortex (S1) processes somatosensory information and is composed of multiple subregions. In particular, tactile information from the skin is encoded in three subregions, namely Brodmann areas (BAs) 3b, 1 and 2, with each area representing a complete map of the contralateral body. Although, much is known about the somatotopic organization of the hand in human S1, less research has been carried out regarding the somatotopic maps of the foot and leg in S1. Moreover, a latero-medial S1 organization along the superior part of the postcentral gyrus has been reported when moving from hip to toes, yet to date there is no study investigating leg/foot maps within the different subregions of S1. Using ultra-high field MRI (7T), we mapped six cortical representations of the lower limb (hip to toes) at the single subject level and performed this analysis separately for BAs 3b, 1 and 2. Analyzing the BOLD responses associated with tactile stimulations of the mapped foot and leg regions on each side, we quantified the extent and the strength of activation to determine somatotopic organization. In addition, we investigated whether each mapped representation also responded to the stimulation of other body parts (i.e. response selectivity) and conducted dissimilarity analysis relating these anatomical and functional properties of S1 to the physical structure of the lower limbs. Our data reveal somatotopy for the leg, but not for the foot in all investigated BAs, with large inter-subject variability. We found only minor differences between the properties of the three investigated BAs, suggesting that S1 maps for the lower limbs differ from those described for the hand. We also describe greater extent/strength of S1 activation for the big toe representation (compared to the other mapped representations) within all BAs, suggesting a possible homology between the first digit of upper and lower extremity in humans, and report different patterns of selectivity in the foot representations (i.e. lower selectivity) compared to the other leg representations (i.e. greater selectivity). These data provide a detailed description of human S1 subregions for the foot and leg, highlight the importance of high-resolution mapping studies and of single subject analysis, and indicate potential differences between the lower and the upper limb.

2017

Integration of a virtual reality based arm in primary somatosensory cortex

Hannes Bleuler, Miguel Nicolelis, Solaiman Shokur

Abstract: Recent advances in brain-machine interfaces (BMIs) have demonstrated the possibility of motor neuroprosthetics directly controlled by brain activity. Ideally neuroprosthetic limbs should be integrated in the body schema of the subject. To explore the ways to enhance such incorporation, we recorded modulations of neuronal ensemble activity in the primary somatosensory (S1) cortex during tactile stimulation simulated in virtual reality (VR) under conditions known to evoke a rubber-hand illusion. A realistic 3D mesh represented monkey body in VR. The monkey’s arms were hidden by an opaque plate and virtual arms projected on the plate. A robotic brush, also hidden from the monkey, touched various locations on forearms of the monkey and was synchronized with a virtual brush touching the projected VR arms. Additionally, we implemented tactile stimulation with air puffs. We have tested various combination of tactile (physical touch), visual (VR arm being touched) and sound (robotic brush touching the arm) inputs: synchronous tactile and visual (T-VR), tactile without visual (T), and visual only (VR). Neuronal ensemble activity was recorded from S1 and primary motor cortex (M1). We found differences in both S1 and M1 activities across the stimulation types. In particular S1 responses to T-VR were stronger than for T. Moreover, S1 neurons were modulated during visual stimulation without touch (VR), suggesting S1 activation as a neuronal mechanism of the rubber-hand illusion. Further, we decoded stimulation parameters from the activity of large neuronal populations. These results suggest a flexible and distributed representation of somatosensory information in the cortex, which can be modified by visual feedback from the body and/or artificial actuators.

2010

Detection of Optogenetic Stimulation in Somatosensory Cortex by Non-Human Primates - Towards Artificial Tactile Sensation

David Allenson Borton, Fabien Bertrand Paul Wagner

Neuroprosthesis research aims to enable communication between the brain and external assistive devices while restoring lost functionality such as occurs from stroke, spinal cord injury or neurodegenerative diseases. In future closed-loop sensorimotor prostheses, one approach is to use neuromodulation as direct stimulus to the brain to compensate for a lost sensory function and help the brain to integrate relevant information for commanding external devices via, e.g. movement intention. Current neuromodulation techniques rely mainly of electrical stimulation. Here we focus specifically on the question of eliciting a biomimetically relevant sense of touch by direct stimulus of the somatosensory cortex by introducing optogenetic techniques as an alternative to electrical stimulation. We demonstrate that light activated opsins can be introduced to target neurons in the somatosensory cortex of non-human primates and be optically activated to create a reliably detected sensation which the animal learns to interpret as a tactile sensation localized within the hand. The accomplishment highlighted here shows how optical stimulation of a relatively small group of mostly excitatory somatosensory neurons in the nonhuman primate brain is sufficient for eliciting a useful sensation from data acquired by simultaneous electrophysiology and from behavioral metrics. In this first report to date on optically neuromodulated behavior in the somatosensory cortex of nonhuman primates we do not yet dissect the details of the sensation the animals exerience or contrast it to those evoked by electrical stimulation, issues of considerable future interest.

Public Library of Science2014

Anatomical and functional properties of the foot and leg representation in areas 3b, 1 and 2 of primary somatosensory cortex in humans: A 7T fMRI study

Michel Akselrod, Olaf Blanke, Roger Gassert, Roberto Martuzzi, Andrea Serino, Wietske Van der Zwaag

2017