The velocity storage time constant: Balancing between accuracy and precision.

Abstract

The velocity storage mechanism is often described in terms of the exponential decay in eye velocity in an upright subject who experiences a constant velocity yaw rotation after a rapid acceleration. The velocity storage time constant for this decay is roughly 6-30s, which means that for low-frequency head rotations, eye velocity and perceptions have large errors compared to actual motion. One may wonder if there would be benefits to having a longer time constant, which would improve accuracy. In this paper, simulations are used to highlight that improved accuracy may come at the cost of increased noise-i.e., reduced precision. Specifically, since the velocity storage mechanism extends the 5.7s time constant of the semicircular canal, it must be performing an integration process over a certain frequency range. In fact, all mathematical models of velocity storage include an integration. This integration would also integrate neural noise. Thus, increasing the velocity storage time constant would lead to integration over a wider range of frequencies, resulting in more noise in the brain's estimate of motion. Simulation results show this accuracy-precision tradeoff. Recent evidence is also reviewed supporting the hypothesis that the brain optimizes the velocity storage time constant to resolve this accuracy-precision tradeoff during aging and with variations in stimulus amplitude.