Impedance changes in cerebral tissue accompanying a learned discriminative performance in the cat

Abstract

Electrical impedance in electrodes chronically implanted in cortical and subcortical structures has been measured in a series of five cats, using tissue signals with an amplitude of 15 to 20 μv at 1000 cycle/sec. The current distribution around the asymmetric dipole at the lower end of the coaxial electrode system was examined in a large scale model, and the relative magnitudes of resistive and reactive components in the observed impedance were measured daily for as long as 6 months proir to initiation of training. Training involved discriminative approach in a modified T-maze to a food reward on the basis of a visual cue. With initial performance at around chance levels, separate computed averages of impedance records from hippocampal structures during correct and incorrect responses showed only irregular deviations around the baseline. At intermediate performance levels, a brief transient fall in impedance appeared in the computed average on presentation of the discriminative task, followed by a longer-lasting rise. Considerable differences were noted in the performance levels at which major consistent impedance deviations first appeared, in comparison of different animals, and between different brain regions in the same animal. At high performance levels, a deeper transient fall in hippocampal impedance of 2.0 to 6.0 per cent of the baseline value immediately followed presentation of the test situation and persisted beyond the completion of the task. It was followed by a slow rise which exceeded 8.0 per cent of baseline impedance in some cases. This “evoked” impedance change persisted undiminished with considerable overtraining. Extinction of the learned habit abolished these brief evoked impedance changes, which reappeared with retraining. No baseline impedance shifts were seen in hippocampal structures during acquisition or extinction of the discriminative habit. Possible mechanisms underlying these impedance changes are discussed, such as a redistribution of ionic material in intraneuronal, intraglial and extracellular compartments, with the possibility that they may occur partly in the glial compartment. A model of the cerebral system is discussed relating changing impedance loads imposed by glial tissue on electrotonic generators in dendritic structures to changing characteristics of electrotonic wave processes in cerebral tissue. Since these wave processes may be concerned in the physicochemical changes underlying the storage of information, the possible significance of the interface between glial and neuronal tissue in these structural alterations is discussed.