CORTICAL MECHANISMS UNDERLYING TACTILE DISCRIMINATION IN THE MONKEY .1. ROLE OF PRIMARY SOMATOSENSORY CORTEX IN PASSIVE TEXTURE-DISCRIMINATION

1. The discharge patterns of 359 single neurons in the hand representa tion of primary somatosensory cortex (SI) of two monkeys (Maraca mulat ta) were recorded during the performance of a passive texture discrimi nation task with the contralateral hand (104 in area 3b, 149 in area 1 , and 106 in area 2). Three nyloprint surfaces were mounted on a drum that was rotated under the digit tips. One surface was entirely smooth , whereas the other two were smooth over the first half and rough over the second half (smooth/rough) (raised dots, 1 mm high and 1 mm diam, in a rectangular array; spatial period of 3 mm across the rows and co lumns for most recordings; 9 mm between columns for selected recording s). The monkeys were trained to distinguish between the smooth and smo oth/rough surfaces. After the surface presentation, the monkey indicat ed the texture of the second half of the surface by pushing or pulling , respectively, on a lever with the other arm. For most recordings an average tangential speed of 49 mm/s was tested. For selected recording s motor speed was incremented (63, 75, or 89 mm/s). 2. Two hundred eig hty-three neurons had a cutaneous receptive field (RF) on the hand (96 in area 3b, 120 in area 1, and 67 in area 2). Thirty-five neurons had a deep RF (4 in area 3b, 15 in area I, and 16 in area 2). Seven neuro ns had mixed cutaneous and deep RFs (4 in area 1, 3 in area 2). Thirty -four neurons had no identifiable RF (4 in area 3b, 10 in area I, and 20 in area 2). 3. The discharge of 185 of 359 neurons was significantl y modulated during the presentation of one or both surfaces compared w ith the discharge at rest. Cells with a cutaneous RF that included par t or all of the distal phalangeal pads of the digits used in the task (usually digits III and IV) were more likely to be modulated during su rface presentation (132 of 179, 74%) than those with a cutaneous RF no t in contact with the surfaces (24 of 104, 23%). The remaining neurons (mixed, deep, or no RF) were also infrequently modulated (29 of 76, 3 8%). 4. Of the 185 modulated units, 118 cells were classified as textu re related because there was a significant difference in the discharge rate evoked by the smooth/rough and smooth surfaces. Cells with a cut aneous RF that included the digital pads in contact with the surfaces were frequently texture related (100 of 132, 76%). Texture sensitivity was less frequently observed in the remaining modulated neurons (18 o f 53, 34%: cutaneous RF not in contact with the surfaces, deep RF, mix ed cutaneous and deep RF, no identifiable RF). 5. Texture-related neur ons were found in areas 3b, 1, and 2. Two patterns of texture-related responses were observed in the 100 cutaneous units with an RF in conta ct with the surfaces. Thirty-one units were classified as showing a ph asic response at the time the digits encountered the leading edge of t he rough half of the surface. Fifty-eight cells were classified as pha sic-tonic (or some times tonic at the slowest motor speeds) because th e response lasted for the duration of the presentation of the rough po rtion of the surface. The remaining 11 neurons could not be readily cl assified into one or the other category and. indeed, generally showed clear texture-related responses only at higher motor speeds (>49 mm/s, 9 of 11). 6. Speed sensitivity was systematically evaluated in 41 of 100 texture-related units with a cutaneous RF in contact with the surf aces. The discharge of 66% of the units (27 of 41) varied significantl y with the speed of surface presentation, with discharge increasing at higher speeds. Speed sensitivity was found in all three cytoarchitect onic areas (6 of 6 cells in area 3b, 11 of 22 in area 11 and 10 of 13 in area 2). 7. Contact force was also systematically monitored in thes e experiments (69 of 100 texture-related cells with a cutaneous RF in contact with the surfaces). Linear regression analyses indicated that 22% (15 of 69) of the texture-related units were sensitive to contact force (13 positive, 2 negative). For some of these units, however, dis charge also covaried with speed (8 of 10). Because contact force was n ot always clearly independent of speed. the relative importance of con tact force in determining the discharge pattern of neurons involved in the appreciation of surface texture remains unclear. 8. The present r esults suggest that texture is a distributed function across areas 3b, 1, and 2. Phasic-tonic units likely provide information about the cha racteristics of the scanned textures. whereas the phasic responses may represent a form of feature selectivity in SI. Neurons that signal di fferences in the texture and yet are invariant for speed may provide t he neuronal basis for the perceptual constancy of texture across a ran ge of different speeds.