Humans use internal models to estimate gravity and linear acceleration

Because sensory systems often provide ambiguous information, neural process es must exist to resolve these ambiguities. It is likely that similar neura l processes are used by different sensory systems. For example, many tasks require neural processing to distinguish linear acceleration from gravity(1 ), but Einstein's equivalence principle states that all linear acceleromete rs must measure both linear acceleration and gravity. Here we investigate w hether the brain uses internal models, defined as neural systems that mimic physical principles, to help estimate linear acceleration and gravity(2-4) . Internal models may be used in motor control(5-7), sensorimotor integrati on(8-10) and sensory processing(11-14), but direct experimental evidence fo r such models is limited. To determine how humans process ambiguous gravity and linear acceleration cues, subjects were tilted after being rotated at a constant velocity about an Earth-vertical axis. We show that the eye move ments evoked by this post-rotational tilt include a response component that compensates for the estimated Linear acceleration even when no actual line ar acceleration occurs. These measured responses are consistent with our in ternal model predictions that the nervous system can develop a non-zero est imate of linear acceleration even when no true linear acceleration is prese nt.