The Effects of Localized Inactivation of Somatosensory Cortex, Area 3a, on Area 2 in Cats

Abstract

In cats and primates, area 3a of the somatosensory cortex is the primary recipient of proprioceptive input (Phillips et al., 1971). Neurons in area 3a project to area 2 (Pons and Kaas, 1986; Porter, 1991), where somatic input relayed from the cortex and the thalamus may be integrated (Iwamura and Tanaka, 1978a,c). The goal of the present study was to determine the effects of area 3a input on neuronal activity in area 2 of cats. Extracellular recording techniques were used to identify neurons in area 2 that responded to deep stimulation of the contralateral forepaw. Neurons in area 3a that responded to the same receptive field modality and location as those in area 2 were also isolated. Single-unit or multiunit responses and evoked potentials to electrical stimulation of the shared peripheral receptive field and spontaneous activity were recorded from areas 2 and 3a. Lidocaine, a local anesthetic, was injected at the area 3a recording site to block neuronal activity. Spontaneous activity and receptive fields were abolished and evoked potentials were considerably diminished at the injection site, immediately after lidocaine was administered. Changes in unit responses, spontaneous activity, and evoked potentials in area 2 were monitored following inactivation of the somatotopically "matched" site in area 3a. Unit activity was recorded at 15 matched sites. In area 2, changes in unit responses to the peripheral stimulation and/or in spontaneous activity were observed at most of the recording sites following inactivation of area 3a. Spontaneous activity rates changed at 63% of the sites (mean change = 85%). Unit responses to the peripheral stimulation changed at 57% of the recording sites (mean change = 47%). The remaining sites in area 2 did not show lidocaine-induced changes. These sites may not have been connected with the matched sites in area 3a. Spontaneous activity and unit responses were not always similarly altered at a given site; sometimes one increased while the other decreased. Decreases in unit responses and spontaneous activity following inactivation of area 3a input were the predominant effects, indicating that area 3a has a facilitatory effect on neuronal activity observed in some regions of area 2. However, increases in neuronal activity at some sites indicated that the effects of area 3a input on area 2 are nonuniform. Evoked potentials were recorded at 19 matched sites, before and after injection of lidocaine. Evoked potentials also changed at some area 2 recording sites following area 3a inactivation.(ABSTRACT TRUNCATED AT 400 WORDS)