Epilepsy, consciousness and neurostimulation

Consciousness is often disrupted in epilepsy. This may involve altered responsiveness or changes in awareness of self and subjective experiences. Subcortical arousal systems and paralimbic fronto-parietal association cortices are thought to underpin current concepts of consciousness. The Network Inhibition Hypothesis proposes a common neuroanatomical substrate for impaired consciousness during absence, complex partial and tonic-clonic seizures. Neurostimulation in epilepsy remains in its infancy with vagal nerve stimulation (VNS) as the only firmly established technique and a series of other methods under investigation including deep brain stimulation (DBS), intracranial cortical stimulation and repetitive transcranial magnetic stimulation (rTMS). Many of these systems impact on the neural systems thought to be involved in consciousness as a continuous duty cycle although some adaptive (seizure triggered) techniques have been developed. Theoretically, fixed duty cycle neurostimulation could have profound effects on responsiveness, awareness of self and subjective experience. Animal studies suggest vagal nerve stimulation positively influences hippocampal long term potentiation. In humans, a chronic effect of increased alertness in VNS implanted subjects and acute effect on memory consolidation have been reported but convincing data on either improvements or deterioration in attention and memory is lacking. Thalamic deep brain stimulation (DBS) is perhaps the most interesting neurostimulation technique in the context of consciousness. Neither bilateral anterior or centromedian thalamic nucleus DBS seem to affect cognition. Unilateral globus pallidus internus DBS caused transient wakefulness in an anaesthetised individual. As intracranial neurostimulation, particularly thalamic DBS, becomes more established as a clinical intervention, the effects on consciousness and cognition with variations in stimulus parameters will need to be studied to understand whether these secondary effects of neurostimulation make a significant positive (or adverse) contribution to quality of life.