Brain's compensatory response to drug-induced cognitive impairment

Topiramate (TPM), a frequently prescribed antiseizure medication, can cause severe cognitive side-effects. Though these side-effects have been studied behaviorally, the underlying neural mechanisms are unknown. In a double-blind, randomized, placebo-controlled, crossover study of TPM's impact on cognition, nine healthy volunteers completed three study sessions: a no-drug baseline session and two sessions during which they received either TPM or placebo. Electroencephalogram was recorded during each session while subjects performed a working-memory task with three memory-loads.

Comparing TPM with baseline we found the following results. (a) TPM administration led to declines in behavioral performance. (b) Fronto-central event-related potentials (ERP) elicited by probe stimuli, representing the primary task network activity, showed strong memory-load modulations at baseline, but the magnitude of these load-dependent modulations was significantly reduced during TPM session, suggesting drug-induced impairments of the primary task network. (c) ERP responses over bilateral fronto-temporal electrodes, which were not load sensitive at baseline, showed significant memory-load modulations after TPM administration, suggesting the drug-related recruitment of additional neural resources. (d) At fronto-central scalp sites, there was significant increase in response amplitude for low memory-load during TPM session compared to baseline, and the amplitude increase was dependent on TPM plasma concentration, suggesting that the primary task network became less efficient under TPM impact. (e) At bilateral fronto-temporal electrodes, there were no ERP differences when comparing low memory-load trials, but TPM administration led to an increase in ERP responses to high load, the magnitude of which was positively correlated with task performance, suggesting that the recruited neural resources were beneficial for task performance. Placebo-TPM comparison yielded similar effects albeit with generally reduced significance and effect sizes.

Our findings support the hypothesis that TPM impairs the primary task network by reducing its efficiency, which triggers compensatory recruitment of additional resources to maintain task performance.