When we interact with an object, much information about the object is conveyed through signals from the hand. Information about the shape of the object, its texture, its compliance, and its thermal properties is carried in the pattern of activity evoked in a variety of receptors embedded in the skin, the joints, and the muscles. We can often recognize an object simply on the basis of sensory signals emanating from our hands. Without this information, manipulating objects would be slow, clumsy, and effortful.

Our goal is to characterize the sensory information originating from the hand and understand how this information is transformed in successive stages of processing. Our approach involves combining psychophysics, peripheral and cortical neurophysiology, and computational modeling. The goal is to discover the aspect of the neural response that accounts qualitatively and quantitatively for behavior at each stage of perceptual processing.

This lab is currently following four lines of inquiry:

Sensory feedback for upper limb neuroprostheses

Tactile sensation is critical for effective object manipulation, but current prosthetic upper limbs make no provision for delivering haptic feedback to the user. For individuals who require use of prosthetic limbs, this lack of feedback transforms a mundane task into one that requires herculean concentration and effort. Although vibrotactile motors and sensory substitution devices can be used to convey gross sensations, a direct neural interface is required to provide detailed and intuitive sensory feedback. In view of this, the new generation of neuroprostheses will enable electrical stimulation of somatosensory neurons in the peripheral or central nervous system.  With this in mind, we develop and test approaches to conveying the tactile information required for basic object manipulation through electrical stimulation of somatosensory neurons.

The neural basis of tactile texture perception 

Texture is the sensory correlate of surface material and microgeometry. Though textural information can be obtained both visually and auditorily, touch yields much finer and more complex textural information than do the other sensory modalities. When we run our fingers across a textured surface, small vibrations are produced in the skin. These vibrations are then transduced by specialized receptors embedded in the skin that convey information about the microgeometry of the surface. Indeed, when these receptors and their associated afferent fibers are desensitized, the perception of surface microgeometry is severely impaired or abolished. These vibrations have been shown to be spectrally complex, leading to the suggestion that the perception of texture may be analogous to that of auditory timbre. With this in mind, we seek to understand how spectrally complex vibrations and, by extension, surface texture, are encoded in the responses of neurons in the peripheral nerve and in the somatosensory cortex.

Spectrograms of the responses evoked in a neuron in area 3b by sinusoidal stimuli at 50, 100 and 200 Hz. While there is clear temporal patterning in the neuronal responses at frequencies up to about 100Hz, responses to 200Hz sinusoids are devoid of any temporal patterning.

The neural basis of proprioception 

Interacting with our environment requires us to resolve spatial relationships. Accordingly, the brain maintains multiple representations of space, each at different scales and in different coordinate systems, all of which must interact intimately to guide action. Information about our body in space, proprioception, is key because all other neural representations of space must ultimately interface with proprioceptive representations for us to act efficiently upon objects. The movements of patients who have lost proprioceptive feedback, and thus must rely solely on vision, are consequently very slow, poorly coordinated, and require great concentration. In addition to its function in motor control, the awareness of our body and its position in space is an essential component of our sense of self. Furthermore, the hand is our most important organ for grasping, tool use, and haptic exploration. Accordingly, we seek to characterize the neural representation of hand position and hand movements in areas of the brain known to receive proprioceptive signals (i.e., areas 3a and 2). To this end, we measure the neuronal responses evoked in the brain during natural grasping movements and seek to determine the aspects of hand movements to which individual neurons are sensitive. Finally, we wish to describe how the conformation of the entire hand is represented in the activity of populations of somatosensory neurons.

-- Sliman Bensmaia, Ph.D.
       Principal Investigator