A non-linear analysis of afferent modulatory activity in the cat somatosensory system

The effects of prior input on afferent activity in the somatosensory system of the cat have been studied using both twin-pulse experiments and random stimulus trains for which evoked activity is characterized by a set of mathematical functions known as kernels. The kernels quantitatively characterize transformational processes occurring in the system. The first-order kernel is analogous to a classical averaged evoked potential, but to temporally interacting stimuli; the second-order kernel may be likened to a generalized recovery function. Responses to superficial radial nerve stimulation were obtained at sensory (S1) cortex, thalamic n. ventralis posterolateralis (VPL), and the nucleus cuneatus of the dorsal column nuclei (DCN). Comparison of first-order kernels with averaged evoked potentials showed good agreement in chronic awake, chronic anesthetized, and acute anesthetized animals. Second-order kernels indicated complex interactions with an overall depression of subsequent response amplitudes. Comparison between second-order kernels and the results of the two-pulse experiments at both S1 cortex and VPL showed good agreement for early latency response components, but significant deviations for later components suggest higher than second-order non-linearities for this system.