BASIC AND CLINICAL ASPECTS OF VERTIGO AND DIZZINESS Estimating the Time Constants of the rVOR A Model-Based Study S. Ramat and G. Bertolini Dipartimento di Informatica e Sistemistica, Universit ` a di Pavia, Pavia, Italy Single-unit recordings of vestibular afferents from the semicircular canals of squirrel monkeys have shown that the cupular time constant (T c ) is between 5 and 6 sec. Such recordings obviously cannot be performed in humans, and the corresponding values have thus been inferred to be somewhat longer based on their size and on the cupula- endolymph system. The ocular motor response of the rotational vestibulo-ocular reflex (rVOR) is characterized by longer time constants, typically between 15 and 20 sec, due to the so-called velocity storage mechanism (VSM), which prolongs the time constant of the afferents through central processing. Recent studies have attempted to determine the time constant of the cupula by fitting the slow phase velocity (SPV) of the response to postrotational stimuli using a mathematical model of the rVOR processing. To this goal they considered the processing of head velocity due to the peripheral vestibular organs and to the VSM. The resulting estimates of T c are lower than expected, averaging about 4 sec. These modeling approaches, though, neglect both the processing of the final common pathway and the adaptation shown by the discharge of primary vestibular afferents. Here we argue that such an approach may be bound to underestimate the duration of the rVOR time constants. Key words: rotational vestibulo-ocular reflex; semicircular canals; mathematical mod- eling; VOR Introduction The rotational vestibulo-ocular reflex (rVOR) produces eye movements aimed at stabilizing the image of the external world on the retina in response to head rotations. These are sensed by the semicircular canals (SCC), which are stimulated by head angular acceleration, and their afferents project to the vestibular nuclei (VN). The mechanical properties of the cupula- endolymph system act so that over the range of frequencies of natural head movements (0.05– 5 Hz) the firing rate of the canal afferents is proportional to head angular velocity. 1,2 An extensive study published by Fernandez and Address for correspondence: Stefano Ramat, Universit` a di Pavia– Informatica e Sistemistica, Via Ferrata, 1 PAVIA PV 27100, Italy. steram@bioing.unipv.it Goldberg in 1971 on the physiology of the af- ferents from the semicircular canals in squir- rel monkeys, based on single-unit recordings in 45 animals, found that the main cupu- lar time constant (T c ) is about 5.7 sec. The same approach was then applied to such other species as rhesus monkey, rabbit, gerbil, cat, and pigeon. Although similar recordings cannot be per- formed in humans and therefore the time con- stant of the cupula cannot be directly measured, many attempts have been made to either ex- trapolate it from the physical characteristics of human canals-endolymph system or to estimate it from rVOR responses. These approaches typ- ically provided an estimate of T c between 6 and 7 sec. 3,4 Yet recent studies based on model- ing the ocular motor response to postrotatory stimuli have suggested values of the human T c around 4 sec, 5,6 which are generally lower than previously thought. Basic and Clinical Aspects of Vertigo and Dizziness: Ann. N.Y. Acad. Sci. 1164: 140–146 (2009). doi: 10.1111/j.1749-6632.2009.03855.x C  2009 New York Academy of Sciences. 140