Angelo Buizza

A non-linear model for visual-vestibular interaction during body rotation in man.

A mathematical model for visual-vestibular interaction during body rotation in an illuminated visual surround is obtained by combining a previous model of the optokinetic reflex (OKR) with a simplified model of the vestibulo-ocular reflex (VOR). OKR is activated by the slip of the image of the external world on the retina, and represents a negative feedback loop around VOR. For large retinal slip velocities OKR behaves as a basically non-linear system. The validity of the model is proved via computer simulation by comparing predicted responses with the experimental results obtained in man by Koenig et al. (1978) in different situations of visual-vestibular interaction.

Visual-vestibular interaction in the control of eye movement: Mathematical modelling and computer simulation

After a brief description of the main anatomical structures subserving the oculomotor responses during combined vestibular and optokinetic stimulations, a mathematical model is presented. With respect to a previous model by Schmid et al. (1980), a more accurate definition of the roles of the neural mechanisms involved in oculomotor control in different conditions of visual-vestibular interaction is given. The model is proved to be able to predict not only oculomotor responses, but also single unit average responses in the vestibular nuclei and in the vestibulo-cerebellum. Experimental data available in the litterature on monkeys and cats are used for model validation.

Quantification of vestibular nystagmus by an interactive computer program.

An interactive program for the analysis of nystagmus is presented, and some results obtained by processing postrotational responses are reported. The program is provided with a special subroutine which gives a quantification of the rhythm and regularity of the response through the histograms of the ratios between the duration of successive intersaccadic intervals and between the amplitudes of successive fast components. The consideration of these histograms has been proved to be highly significant in discriminating between normal and pathological responses.

Effects of Hydrostatic Pressure on Sensory Discharge in Frog Semicircular Canals

The effects of endolymphatic and perilymphatic pressure changes on resting and mechanically evoked responses were studied in isolated posterior semicircular canals of the frog. The results demonstrated that ampullar receptors are extremely sensitive to hydrostatic pressure changes (0.25 mm H2O were sufficient to produce distinct changes), being inhibited by endolymphatic pressure increases and facilitated by perilymphatic ones. Intracellular recordings from single afferent axons showed that the effects of hydrostatic pressure result from a modified transmitter release from the synaptic pole of the hair cells. Unlike resting activity, mechanically evoked activity was always depressed in the presence of a hydrostatic pressure. This indicates that the sensitivity of ampullar receptors to mechanical stimuli, i.e. the gain of the conversion process, is maximal when no pressure is present between the inner and the outer fluid. The possible action of hydrostatic pressure on vestibular receptors is discussed.

Automatic detection and removal of fast phases from nystagmographic recordings by optimal thresholding

Abstract Recording of ocular nystagmus during vestibular tests does not measure the true response of the vestibulo-ocular reflex (VOR), because the VOR response (so-called slow phase of nystagmus) is interrupted by resetting saccades (so-called fast phase of nystagmus). In order to extract the real VOR contribution, saccades must be removed. In most of the nystagmus processing algorithms, saccade removal requires a human operator to choose a suitable eye velocity threshold able to separate fast from slow nystagmus phases. In the present report a fully automatic removal system is presented which selects an optimal velocity threshold by computing the VOR frequency response and maximizing its coherence function.

Model interpretation of visual-vestibular interaction in patients with labyrinthine and cerebellar pathologies

Oculomotor responses to combined optokinetic and vestibular stimulations in labyrinthine and cerebellar defective patients are discussed in terms of parametric changes in a model describing the interaction between the vestibulo-ocular reflex (VOR) and the optokinetic reflex (OKR). By making a few reasonable hypotheses about model parameter variations in relation to the type of pathology, the experimental results obtained by several authors can correctly be predicted and explained by the model. The model can therefore be used to define a set of parameters giving an estimate of the state of the system subserving VOR-OKR interaction in the examined patients. The model is also shown to be a powerful tool to assess the validity and the diagnostic significance of the procedures used to test VOR-OKR interaction.

Effects of caloric stimuli on frog ampullar receptors

The observation that caloric nystagmus can be evoked even in microgravity conditions argues against Baranys convective theory. To justify this result, gravity-independent mechanisms (mainly endolymphatic volume changes and direct action of the temperature on vestibular sensors) are believed to contribute to caloric-induced activation of vestibular receptors. To define the importance of both gravity-dependent and gravity-independent mechanisms, the posterior semicircular canal of the frog was thermally stimulated by a microthermistor positioned close to the sensory organ. The stimulus produced a gravity-dependent transcupular pressure difference that, depending on the position of the heater, could result in either excitation or inhibition of ampullar receptor sensory discharge. When the heater was positioned on the ampulla, or when the canal rested on the horizontal plane, no responses could be evoked by thermal stimuli. These results suggest that, in our experimental conditions (DeltaT up to 1.5 degrees C), neither a thermally induced expansion of the endolymph nor a direct action of the temperature on vestibular sensors play any major role.

New experimental data on cat's optokinetic responses. Is there need to revise previous models of the optokinetic reflex?

New data on cats optokinetic reflex (OKR) provided by Godaux and Vanderkelen (1984) have been interpreted by using a nonlinear model of OKR previously proposed by the authors. A general agreement between experimental data and theoretical predictions was obtained. In particular, the steep decrease of OKR gain observed experimentally at high frequencies appeared as a straightforward consequence of the intrinsic nonlinearity of OKR. In contrast with a recent statement by Gillis et al. (1984), it was concluded that the new data seem to confirm, rather than disprove, previous models of cats OKR.

About the effects of velocity saturation on smooth pursuit.

Abstract: By means of simple simulations and based on experimental results from the literature, it is argued that correct consideration of the well known velocity saturation of the smooth pursuit eye movement system may suggest new insight into some intriguing aspects of this systems behavior.

Visual Stabilization During Head Movement

In order to obtain a clear absolute perception of the visual surround during head movements, the visual information aquired in retinotopic coordinates should be transformed into absolute coordinates by taking into account head movement. The aim of this paper is to show how an estimate of head absolute velocity is made available in the vestibular nuclei (VN) from their vestibular and visual inputs. An attempt is also made to link the output of VN to reflex eye movements, self-motion sensation, and visual perception.