Unit responses of the auditory cortex of waking cats at rest and after defensive conditioning

In chronic experiments with glass microelectrodes responses of 288 spontaneously active neurons in the auditory cortex were investigated in cats at rest (123 neurons) and after defensive conditioning to sound (165 neurons). In the first situation 43% of neurons did not respond to acoustic stimulation. Most (about 60%) responses of the reacting neurons showed marked inhibition. Conditioning caused an increase (up to 72%) in the number of neurons responding to acoustic stimulation, the appearance of tonic responses, a severalfold increase in the amplitude of the responses, an increase in the number of responses of activation type, and stabilization of their form. The results point to increased excitability of neurons in this cortical area.Translated from Neirofiziologiya, Vol. 11, No. 1, pp. 25–34, January–February, 1979.     

Dynamics of single unit activity in the association cortex of waking cats during defensive conditioning

Responses of neurons in association area 5 during defensive conditioning to acoustic stimulation were studied in chronic experiments on cats. As a rule the neurons responded by excitation to presentation of conditioned and unconditioned stimuli. During the conditioned reflex unit responses usually appeared in the first 50 msec after the beginning of acoustic stimulation,i.e., they were connected with the action of the conditioned stimulus and not with manifestations of conditioned-reflex motion. The most significant changes in responses of cortical association units were observed in the initial period of conditioning. During stabilization of the conditioned reflex, responses of some neurons became stabilized, whereas in other neurons the spontaneous activity and intensity of responses increased, and in a third group the response to one of the stimuli disappeared. This last result indicates a switch during conditioning from polysensory unit responses to monosensory specialized responses. Extinctive inhibition was found to consist of a gradual decrease in the level of the spike discharge and its approximation to spontaneous activity, i.e., to be passive in character.Translated from Neirofiziologiya, Vol. 10, No. 6, pp. 563–572, November–December, 1978.     

Pervasive synchronization of local neural networks in the secondary somatosensory cortex of cats during focal cutaneous stimulation

Extracellular discharges were recorded from 205 neurons in the secondary somatosensory (SII) cortex of isoflurane-anesthetized cats. Cross-correlation analysis was used to characterize the temporal coordination of SII neurons recorded during cutaneous stimulation with a focal air jet that moved back-and-forth across the distal forelimb. Over two-thirds of the recorded neuron pairs (n=357) displayed significant levels of synchronized activity during one or both directions of air-jet movement. The probability of detecting correlated activity varied according to the distance separating the neurons. Whereas synchronized responses were observed in 82.3% of the pairs in which the neurons were separated by 200–300 μm, the incidence of synchronization declined to 52.3% for neurons that were separated by 600–800 μm. The distance between neurons also had a significant effect on the temporal precision of correlated activity. For neurons that were separated by 200–300 μm, synchronized responses in the cross-correlograms (CCGs) were characterized by narrow (0.5–1.0 ms) peaks at time zero. For SII neurons that were more widely separated, the peak half-widths were substantially broader and more likely to be displaced from time zero. Analysis of directional sensitivity indicated that only 14.2% of the correlated neurons displayed a directional preference for synchronized activity. By comparison, 63.4% of the neurons displayed a directional preference in their discharge rate. These results indicate that stimulus-induced synchronization is a prominent feature among local populations of SII neurons, but synchronization does not appear to play a critical role in coding the direction of stimulus movement. A comparison of these results with those obtained from similar experiments conducted in primary somatosensory (SI) cortex indicates that neuronal synchronization is more likely in SII cortex. This finding is discussed with respect to the known functional differences between the SI and SII cortical areas. Electronic Publication     

Hemispheric mapping of secondary somatosensory cortex in the rat

This study used high-resolution hemispheric mapping of somatosensory evoked potentials to determine the number and organization of secondary somatosensory areas (SII) in rat cortex. Two areas, referred to as SII and PV (parietoventral), revealed complete (SII) or nearly complete (PV) body maps. The vibrissa and somatic representation of SII was upright, rostrally oriented, and immediately lateral to primary somatosensory cortex (SI), with a dominant face representation. Vibrissa representations in SII were highly organized, with the rows staggered rostrally along the mediolateral axis. Area PV was approximately one fifth the size of SII, and located rostral and lateral to auditory cortex. PV had a rostrally oriented and inverted body representation that was dominated by the distal extremities, with little representation of the face or vibrissae. These data support the conclusion that in the rat, as in other species, SII and PV represent anatomically and functionally distinct areas of secondary somatosensory cortex.     

Two distinct regions of secondary somatosensory cortex in the rat: topographical organization and multisensory responses

In rodents, as in other species, regions of secondary somatosensory cortex (SII) may be distinguished from primary cortex (SI) both anatomically and electrophysiologically. However, the number of rodent SII subregions, their somatotopic organization, and their function are poorly understood. The presence of multisensory responsive neurons in some areas of SII suggests that one of its roles may be in the integration of somatosensory information with information from other sensory modalities. In this study, we used auditory, somatosensory, or combined auditory/somatosensory stimuli, and high-resolution epipial-evoked potential maps of rat SII to identify the number of spatially discrete subregions, estimate their somatotopic organization, and delineate regions with multisensory response properties. Maps revealed two distinct subregions within SII, one rostral and the other caudal, which were situated lateral to the posteromedial barrel subfield. Distinct somatotopies were evident at both SII loci, and analysis of evoked responses within both areas indicated multisensory interactions. These data are consistent with the presence of classically defined rostral SII regions and provide functional evidence for a lesser known, but distinct, caudal SII area. Furthermore, evidence for multisensory interactions within SII suggests that both secondary areas may process features specifically associated with multisensory integration in parallel with unimodal processing in primary areas.     

Neuronal correlates of decision-making in secondary somatosensory cortex

The ability to discriminate between two sequential stimuli requires evaluation of current sensory information in reference to stored information. Where and how does this evaluation occur? We trained monkeys to compare two mechanical vibrations applied sequentially to the fingertips and to report which of the two had the higher frequency. We recorded single neurons in secondary somatosensory cortex (S2) while the monkeys performed the task. During the first stimulus period, the firing rate of S2 neurons encoded the stimulus frequency. During the second stimulus period, however, some S2 neurons did not merely encode the stimulus frequency. The responses of these neurons were a function of both the remembered (first) and current (second) stimulus. Moreover, a few hundred milliseconds after the presentation of the second stimulus, these responses were correlated with the monkey's decision. This suggests that some S2 neurons may combine past and present sensory information for decision-making.