Parallel processing in rabbit first (SI) and second (SII) somatosensory cortical areas: effects of reversible inactivation by cooling of SI on responses in SII.

1. Previous observations on the effect of ablation or inactivation of the primary somatosensory cortex (SI) on the responses of neurons within the second somatosensory area (SII) to tactile stimuli point to profound differences between monkeys and certain other mammals in the organization of thalamocortical systems. In the cat, for example, tactile information appears to be conveyed in parallel from the thalamus to both SI and SII, whereas, in macaque and marmoset monkeys, it is conveyed in a serial (or hierarchical) scheme from the thalamus to SI and thence to SII. The present study examined the responses of individual SII neurons during reversible, cooling-induced inactivation of SI in another nonprimate placental mammal, the rabbit, to obtain further evidence on whether the above differences might reflect a fundamental distinction between simian primates and other mammalian species. 2. When the temperature at the face of a silver cooling block over the forepaw and hindpaw regions of SI was lowered to 5-13 degrees C, the SI surface potentials evoked by brief tactile stimuli were abolished (indicative of SI inactivation), whereas SII potentials remained intact. 3. The responses of 25 SII neurons to controlled tactile stimuli (consisting of 1- to 1.5-s trains of vibration or rectangular mechanical pulses) were studied before, during, and after inactivation of SI. The effects on the spontaneous activity of a further three SII neurons that lacked identified receptive fields were also studied. 4. The response or activity levels of 26 of the 28 SII neurons examined (93%) were unaffected by SI inactivation.(ABSTRACT TRUNCATED AT 250 WORDS)