[Motion detection, optokinetic nystagmus and moving retinal images. The influence of visual attention on 2 mechanisms of motion detection]

Optokinetic response and visual suppression of the vestibulo-ocular reflex in the open loop condition in the cat

Optokinetic nystagmus (OKN), vestibulo-ocular reflex (VOR) and the visual suppression of the VOR were investigated before and after elimination of retinal image motion following monocular surgical paralysis. Monocular paralysis gave rise to an instability of eye position in the dark which was abolished during the illumination of the mobile but not the immobile eye. In the open-loop condition there was a substantial increase in the monocular OKN gain and a large asymmetry with respect to temporo-nasal and naso-temporal stimulation. Viewing a head-fixed world through a completely paralysed eye suppressed the VOR. This visual suppression was asymmetric and the asymmetry was consistent with that found in the open-loop OKN. These results imply that in the cat visual system, cancellation models of visual suppression of the VOR need to be expanded to accommodate signals generated by stable retinal images.

Visual following during stimulation of an immobile eye (the open loop condition)

We had the rare opportunity to examine a patient with a unilateral complete ophthalmoplegia in an eye with normal visual acuity. We recorded the movements of the covered, sound eye of the patient during full-field optokinetic stimulation of his seeing, immobile eye (open loop condition). The open loop gain for low velocity stimuli (less than 1 deg/sec) was high (greater than 50) but progressively fell with faster stimuli. This finding is inconsistent with the representation of the optokinetic system as a linear velocity servo. In a second experiment, the patient foveally fixated, with the immobile eye, a head-fixed target during sinusoidal, en block rotation. The vestibular movements of the sound eye were suppressed, even though no retinal image motion occurred in the seeing eye. This suggests that visual attention to a stable retinal image can be used to suppress the vestibulo-ocular reflex. Our results were from one patient only and any generalizations must be made cautiously. Nevertheless, our findings are comparable with previous "open loop" studies in normal human beings and patients.

Human optokinetic responses under quasi-open and closed loop conditions

The human optokinetic response to a horizontally moving stripped pattern surrounding the subject was investigated under quasi-open and closed loop conditions. Open loop conditions were produced by the addition of an external signal from measured slow phase eye velocity to stripe velocity. A comparison of open and closed loop responses to step and sinusoidal changes of stripe velocity indicates that the central nervous system controlling slow phase optokinetic following can be described as a simple first order lag (Ka/(s + a)) where K is 4.7 and the time constant, 1/a, is 1.25 s.

The Aubert-Fleischl phenomenon: a temporal frequency effect on perceived velocity in afferent motion perception

Apparent velocities of moving visual stimuli are known to be different depending on whether the subject pursues the stimulus (efferently controlled motion perception) or whether the eye is stationary and the image moves across the retina (afferent motion perception). Afferent motion perception of a periodic pattern or a moving single object causes overestimation of velocity (magnitude estimations) as compared to smooth pursuit. This socalled Aubert-Fleischl phenomenon is shown to depend on local temporal frequency stimulation on the retina caused by the repetitive passage of contrast borders of the moving periodic pattern. This is evidenced by the fact that for a given stimulus speed the amount of overestimation is a function of the spatial frequency of the pattern (or the angular subtend of a single moving object) and that the Aubert-Fleischl phenomenon is not observed if a single edge moves. Background characteristics seem not to influence the apparent velocity during smooth pursuit.