Neuronal encoding of texture changes in the primary and the secondary somatosensory cortical areas of monkeys during passive texture discrimination

Two rhesus monkeys wire trained to discriminate, with the use of passive touch, a standard surface [rectangular arrays of raised dots with a spatial period (SP) of 2 mm across the rows and columns] from three modified surfaces in which the SP between rows was increased to 3, 4, or 5 mm over the second half of the surface. After the surface presentation (to digit tips 3 and 3 of one hand) the monkeys indicated the presence or absence of a change in texture by pulling or pushing a lever, respectively, with the opposite hand. Of 193 neurons recorded from primary somatosensory cortex (SI, 3 hemispheres) and 94 neurons from secondary somatosensory cortex (SII, 1 hemisphere), all contralateral to the stimulated hand, the discharge of 51 SI and 19 SII neurons was classified as texture related. Two types of texture-related responses were obtained. Graded neurons showed a linear relationship between mean discharge frequency and SP; nongraded neurons showed a significant change in discharge over the modified half of the surfaces but the discharge did not distinguish between the three modified surfaces. The distribution of these texture responses was significantly different in SI and SII: whereas most of the texture-related neurons in SI (44 of 51, 86%) were graded, the majority of those in SII (12 of 19, 63%) were nongraded. The results were interpreted as suggesting that the nongraded responses reflect feature extraction in SII, signaling the presence of a change in texture but not its magnitude, and so support the notion that texture signals art processed sequentially, first in SI and then in SII.