Shirvalkar et al implanted electrodes into the central lateral nucleus in the central thalamus of rats. A stereotaxic device was used to place the electrodes in the desired location, and the animals were left to recover for 1 week after electrode implantation. After this time had elapsed, the researchers applied continuous stimulation at a frequency of 100 Hz to the rats’ brains, and investigated the effect of the stimulation on the animals’ performance in a cognitive task.
The cognitive performance of rats was measured using an object recognition task which exploits the innate tendency of the animals to explore novel stimuli. Rats were first shown one of two unique pairs of objects; 2 hours later, they were shown the first object along with a novel one. Rats whose brains had been stimulated spent more time exploring the novel object than control rats that had not been stimulated.
Using immunohistochemistry, the authors then showed that expression of the immediate early genes c-fos and zif268 was up-regulated throughout layers II-IV the neocortex and in the dentate gyrus of the hippocampus. c-fos has been repeatedly localized to the nucleus of activated neurons and is thought to be an early marker for neuronal activity; zif268 is expressed in the same regions as c-fos and is known to be up-regulated during associative learning which induces long-term potentiation. In these experiments, the up-regulation of both genes was observed 2 hours after stimulation.
The central thalamic nucleus has been implicated in maintaining the alert waking state and facilitating behaviours related to attention; neurons in this area of the brain connect brainstem neurons involved in arousal to cells in the basal ganglia and cerebral cortex.
Electrical stimulation is already being used to treat various neuropsychiatric disorders, and stimulation of the central thalamus has been suggested as a way of restoring the impaired cognitive abilities in such conditions. Deep brain stimulation is also being used to alleviate symptoms of Parkinson’s disease and depression. However, it would be unethical for the method to be used to attempt to enhance the cognitive abilities of healthy people.