NEURONAL ENCODING OF TEXTURE CHANGES IN THE PRIMARY AND THE SECONDARYSOMATOSENSORY CORTICAL AREAS OF MONKEYS DURING PASSIVE TEXTURE-DISCRIMINATION

Two rhesus monkeys wire trained to discriminate, with the use of passi ve touch, a standard surface [rectangular arrays of raised dots with a spatial period (SP) of 2 mm across the rows and columns] from three m odified surfaces in which the SP between rows was increased to 3, 4, o r 5 mm over the second half of the surface. After the surface presenta tion (to digit tips 3 and 3 of one hand) the monkeys indicated the pre sence or absence of a change in texture by pulling or pushing a lever, respectively, with the opposite hand. Of 193 neurons recorded from pr imary somatosensory cortex (SI, 3 hemispheres) and 94 neurons from sec ondary somatosensory cortex (SII, 1 hemisphere), all contralateral to the stimulated hand, the discharge of 51 SI and 19 SII neurons was cla ssified as texture related. Two types of texture-related responses wer e obtained. Graded neurons showed a linear relationship between mean d ischarge frequency and SP; nongraded neurons showed a significant chan ge in discharge over the modified half of the surfaces but the dischar ge did not distinguish between the three modified surfaces. The distri bution of these texture responses was significantly different in SI an d 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 nong raded. The results were interpreted as suggesting that the nongraded r esponses reflect feature extraction in SII, signaling the presence of a change in texture but not its magnitude, and so support the notion t hat texture signals art processed sequentially, first in SI and then i n SII.