Texture is the sensory correlate of surface material and microgeometry. When we run our fingers across a surface, we may perceive the surface as being rough, like sandpaper, or smooth, like glass; the surface may also vary along other sensory continua, such as hardness (e.g., stone) vs. softness (e.g. moist sponge), or stickiness (e.g., tape) vs. slipperiness (e.g., soap). Our exquisite sensitivity to surface texture spans six orders of magnitude: We can discern elements measured from tens of nanometers to tens of millimeters. This ability relies on two different neural codes in the peripheral nerve: Coarse surface elements are encoded in a spatial pattern of activation in one population of tactile nerve fibers and fine surface elements are encoded in millisecond precision temporal spiking patterns in two other populations of nerve fibers. These two streams of information are reflected in the texture responses of neurons in anterior parietal cortex (APC), in which neurons fall along a continuum: On one end are neurons that preferentially encode coarse textures and are sensitive to spatial patterns of input, on the other end are neurons that preferentially encode fine textures and are sensitive to temporal patterns of input. Furthermore, our perception of texture is independent of how we touch a surface, but the neural representations in the nerve and in APC are dependent on the nature of exploratory movements. How tolerant representations of texture are achieved remains unknown. The goal of the present study, then, is to investigate the neural basis of tactile texture perception in lateral parietal cortex (LPC), the primary downstream target of APC, using a large and diverse set of artificial and natural textures across the range of behaviorally relevant exploratory conditions. We seek to (1) characterize the representation of texture in LPC and (2) assess whether this representation is tolerant to changes in exploratory conditions. Furthermore, neuronal responses in LPC have been shown to depend on the behavioral context and the task. Our experimental paradigm will allow us to probe this task-dependence. The proposed experiments will shed light on high level somatosensory representations, which to date have received little experimental attention.
We have an exquisite sensitivity to surface textures, allowing us to identify silk, from satin, from sandpaper, and these perceptual representations are robust across the speed, force, or location on the skin that contacts a texture. Currently, despite large advances in our understanding of peripheral and early somatosensory processing, little is known about how tactile signals are represented at later stages. Here, we aim to characterize the neural codes underlying texture perception in lateral parietal cortex, an effort that will lead to advances in our understanding of sensory processing and pave the way for more sophisticated neuroprosthetic applications.
