A Multidimensional Model of the Effect of Gravity on the Spatial Orientation of the Monkey

TLDR

The model builds on observer theory, optimal observer theory, and coordinate‑rotation mathematics. The study develops a sensory‑conflict model of spatial orientation, hypothesizing that the squirrel monkey’s CNS integrates body and sensory dynamics to form an internal model. The model compares expected to actual sensory afference to compute sensory conflict, which then drives central estimates of velocity, gravity, and acceleration toward accuracy. The model accurately predicts velocity storage during earth‑vertical rotation, reduced horizontal VOR time constants with eye‑rotation axis shifting toward gravity after postrotatory tilt, and bias modulation and decay observed in off‑vertical axis rotations.

Abstract

A “sensory conflict” model of spatial orientation was developed. This mathematical model was based on concepts derived from observer theory, optimal observer theory, and the mathematical properties of coordinate rotations. The primary hypothesis is that the central nervous system of the squirrel monkey incorporates information about body dynamics and sensory dynamics to develop an internal model. The output of this central model (expected sensory afference) is compared to the actual sensory afference, with the difference defined as “sensory conflict”. The sensory conflict information is, in turn, used to drive central estimates of angular velocity (“velocity storage”), gravity (“gravity storage”), and linear acceleration (“acceleration storage”) toward more accurate values. The model successfully predicts “velocity storage” during rotation about an earth-vertical axis. The model also successfully predicts that the time constant of the horizontal vestibulo-ocular reflex is reduced and that the axis of eye rotation shifts toward alignment with gravity following postrotatory tilt. Finally, the model predicts the bias modulation, and decay components that have been observed during off-vertical axis rotations (OVAR).