Effect of insulin on the brain activity obtained during visual and memory tasks in healthy human subjects

Insulin receptors are found throughout the brain, particularly in the hippocampus, although the impact of insulin on memory is unclear. The purpose of this study was to examine the effect of insulin on event-related potentials in response to a standard memory task and visual evoked potentials (VEPs) during exposure to a reversing checkerboard. We hypothesized that insulin would decrease P300 magnitude and latency during the presentation of previously observed stimuli, but would have no effect on VEPs. Sixteen humans participated in two euglycemic clamp studies with somatostatin performed in random order in which serum insulin levels were either suppressed (14 +/- 1 pmol/l) or increased by insulin infusion (433 +/- 40 pmol/l). At steady state, event-related potentials and then VEPs were collected using a 32-electrode cap. The major finding was that the P300 amplitude measured during the identification of an object as old was significantly smaller over parietal regions when insulin was infused than when no insulin was provided. Insulin was without effect on the VEPs. We conclude that insulin has region- and task-specific effects on neuronal activation. While the P300 amplitude measured during the presentation of an old object was reduced during insulin infusion, the hormone was without effect on VEPs.