Representation of orientation in the somatosensory system

In this paper we discuss how orientation is represented and transformed in the somatosensory system. Information about stimulus orientation plays an important role in sensory processing. In touch it provides critical information about how stimuli are positioned on the hand, which is important for grasping and lifting objects. It also provides important information about tactile shape. Psychophysical studies show that humans have a high capacity to discriminate the orientation of shapes and gratings indented into the finger pad. Further, these studies demonstrate that orientation discrimination is a reliable and stable method for assessing tactile spatial acuity. Neurophysiological studies suggest that orientation information is processed by the slowly adapting type 1 (SA1) afferent system. While orientation is poorly represented in the responses of individual afferent fibers, it is well represented in the population response properties of peripheral SA1 afferents and in the responses of central neurons in the primary (S1) and secondary (S2) somatosensory cortex. In S2, neurons with orientation selective and orientation non-selective responses tend to have large receptive fields that span multiple pads on multiple digits. Neurons in S2 that are orientation selective have similar tuning functions on different finger pads. These neurons may provide position-invariant responses or may be responsible for integrating features across hands, which is important for haptic object recognition of large shapes from the hand. Neurophysiological studies in trained animals show that the responses of about 85% of the neurons in S2 are affected by the animals focus of attention and that attention to the orientation of a bar modifies both the mean firing rate (i.e. gain) of neurons encoding orientation information and the degree of synchronous firing between pairs of neurons.