Neurons with unusual response and receptive-field properties in upper laminae of cat SI cortex.

High-impedance micropipettes are used to record (both extra- and intracellularly) the electrical activity of neural elements located 550 micron or less from the pial surface of cerebral cortical areas 3a, 3b, 1, and 2 in unanesthetized cats. These elements are designated as "upper-layer SI units" and most frequently are sampled within the arm and forelimb digit sectors of areas 3b and 1. Mechanical stimulation of the skin is employed to determine the receptive field (RF) and response properties of the upper-layer units sampled. Single-shock electrical stimulation of the skin is used to obtain estimates of the minimal latency for eliciting spike discharge. Intracellular iontophoretic injection of horseradish peroxidase (HRP) is used to determine the laminar locations of the somata of the neural elements from which recordings are obtained. The receptive field (RF) and response properties of most upper-layer units sampled in areas 3b and 1 differ substantially from those of units recorded at depths greater than 550 micron from the pial surface in the same cortical fields. The members of one group of upper-layer area 3b and 1 units (U units) respond best to infrequently repeated (typically less than 0.5/s), slowly moving (1-5 cm/s) tactile stimuli. For the same units, repetitive application of slow-moving tactile stimuli to the RF typically leads to an enhancement of responsiveness accompanied by an elevation of spontaneous activity. In contrast, repetitive stimuli delivered at high velocity and at short interstimulus intervals lead to a decrease in unit responsiveness and to an absence of spontaneous activity. The members of a second group of upper-layer units (R units) respond best to moving stimuli delivered at higher velocities (5-20 cm/s), respond reliably at stimulus repetition rates well in excess of 0.5/s, and do not exhibit pronounced changes in responsiveness to repeated stimulation. The RFs of most upper-layer units (both R and U units) involve restricted regions on the contralateral upper limb, but the RFs of U units have poorly defined borders. In addition, the distribution of sensitivity within the RF of at least some U units is nonuniform and, frequently, discontinuous. Contralateral as well as ipsilateral body regions are included within the RFs for 12% of the upper-layer neurons sampled; the remainder (8%) have RFs restricted to the contralateral body.(ABSTRACT TRUNCATED AT 400 WORDS)