Stabilizing gaze reflexes in the pigeon (Columba livia)

Summary A quantitative study of horizontal and vertical optokinetic nystagmus (OKN) and optocollic reflex (OCR) has been performed in the pigeon using the search-coil technique. The reflexes were analysed in response to either velocity steps or sinusoidal stimulation. Results show that: 1. In response to a velocity step stimulation, the slow phase velocity of both OKN and OCR increases gradually to reach a steady state level. When the stimulation stops in the dark, After Responses (OKAN-I, OKAR-I) occur. Time constants of the OKN charge (or OCR charge) and of the After Responses are lower for vertical than for horizontal responses. 2. In the free-head condition, both the head and the eye display a synchronized nystagmus which add their effects. However, the head reflex (OCR) accounts for about 80–90% of the entire linear gaze response (head + eye), except for the vertical steady state responses which are wholly accomplished by the head (OCR). 3. Both closed-loop and open-loop gains of steady state responses are higher for horizontal than for vertical reflexes. Vertical OCR, horizontal OKN and vertical OKN show properties of binocular integration, the response gain being higher for binocular than for monocular stimulations. By contrast, the horizontal OCR shows little binocular integration but displays a higher response gain for monocular stimulation, compared to horizontal OKN. 4. The horizontal OKN elicited by both monocular and binocular stimulation is asymmetrical, the gain being higher when the eye is driven by a temporo-nasal stimulation. In contrast, both vertical OKN and vertical OCR are practically symmetrical. 5. While both the gain of horizontal OKN and its linear range (up to 20°/s) are improved when the head is free (gaze gain close to 1 up to 40°/s), the vertical OKN and the vertical OCR have similar gain profiles and similar domains of linearity (up to 10°/s). 6. In response to increasing the frequency of a sinusoidal stimulation at constant peak velocity, all the reflexes display a drop in gain and a strong increase of phase lag. The phase increase is greater for horizontal than for vertical reflexes. On the other hand, both gain and phase are higher for OCR than for OKN, both in the horizontal plane as well as in the vertical plane. 7. For sinusoidal stimulations, when the peak velocity (PV) is increased at a constant frequency (0.03 Hz), nonlinearities appear (drop in gain, phase increase) both for OKN and OCR. While the most important parameter which determines the phase is the stimulation frequency, both peak velocity and peak acceleration are involved in the gain saturation.