Vestibulo-ocular reflex and optokinetic nystagmus in adult cats reared in stroboscopic illumination

Role of Motor and Visual Experience During Development of Bipedal Locomotion in Chicks

The purpose of this research was to investigate the role of motor and visual experience during the development of locomotion in chicks. We have previously demonstrated that when locomotor activity is restricted immediately posthatching, chicks walk with shorter stride lengths and attenuated head bobbing movements. Head bobbing is an optokinetic response in birds, driven by the movement of the visual world across the retina (i.e., optic flow). During locomotion, optic flow is generated by forward translation, and we have shown that the magnitude of head bobbing movements and stride lengths are moderately correlated in walking chicks. In the present study, we investigated this relationship more closely by examining whether imposed changes in stride length could affect head excursions during head bobbing. We manipulated stride length by hobbling chicks immediately after hatching and subsequently quantified kinematic parameters, including step timing and head excursions, during walking. Imposition of shorter stride lengths induced chicks to take more frequent steps, spend less time in contact with the ground, and shortened head excursions during head bobbing. Nevertheless, the developmental changes in head excursions were not fully accounted for by altered stride lengths, so in a separate experiment, we investigated whether the development of head bobbing relies on the normal experience of optic flow. We raised chicks under stroboscopic illumination to eliminate chicks' experience of optic flow but found that this did not significantly alter head bobbing. These results are discussed along with related findings in other species and the possible neural and biomechanical constraints underlying development of walking and head bobbing in birds.

Influence of visual experience on developmental shift from long-term depression to long-term potentiation in the rat medial vestibular nuclei

The influence of visual experience deprivation on changes in synaptic plasticity during postnatal development was studied in the ventral part of the rat medial vestibular nuclei (vMVN). We analysed the differences in the occurrence, expressed as a percentage, of long-term depression (LTD) and long-term potentiation (LTP) induced by high frequency stimulation (HFS) of the primary vestibular afferents in rats reared in the light (LR) and those in the dark (DR). In LR rats, HFS only induced LTD in the early stages of development, but the occurrence of LTD progressively decreased to zero before their eyes opened, while that of LTP enhanced from zero to about 50%. Once the rats' eyes had opened, LTD was no longer inducible while LTP occurrence gradually reached the normal adult value (70%). In DR rats, a similar shift from LTD to LTP was observed before their eyes opened, showing only a slightly slower LTD decay and LTP growth, and the LTD annulment was delayed by 1 day. By contrast, the time courses of LTD and LTP development in DR and LR rats showed remarkable differences following eye opening. In fact, LTD occurrence increased to about 50% in a short period of time and remained high until the adult stage. In addition, the occurrence of LTP slowly decreased to less than 20%. The effect of light-deprivation was reversible, since the exposure of DR rats to light, 5 days after eye opening, caused a sudden disappearance of LTD and a partial recover of LTP occurrence. In addition, we observed that a week of light deprivation in LR adult rats did not affect the normal adult LTP occurrence. These results provide evidence that in a critical period of development visual input plays a crucial role in shaping synaptic plasticity of the vMVN, and suggest that the visual guided shift from LTD to LTP during development may be necessary to refine and consolidate vestibular circuitry.

Visual influences on the development and recovery of the vestibuloocular reflex in the chicken

Whenever the head turns, the vestibuloocular reflex (VOR) produces compensatory eye movements to help stabilize the image of the visual world on the retina. Uncompensated slip of the visual world across the retina results in a gradual change in VOR gain to minimize the image motion. VOR gain changes naturally during normal development and during recovery from neuronal damage. We ask here whether visual slip is necessary for the development of the chicken VOR (as in other species) and whether it is required for the recovery of the VOR after hair cell loss and regeneration. In the first experiment, chickens were reared under stroboscopic illumination, which eliminated visual slip. The horizontal and vertical VORs (h- and vVORs) were measured at different ages and compared with those of chickens reared in normal light. Strobe-rearing prevented the normal development of both h- and vVORs. After 8 wk of strobe-rearing, 3 days of exposure to normal light caused the VORs to recover partially but not to normal values. In the second experiment, 1-wk-old chicks were treated with streptomycin, which destroys most vestibular hair cells and reduces hVOR gain to zero. In birds, vestibular hair cells regenerate so that after 8 wk in normal illumination they appear normal and hVOR gain returns to values that are normal for birds of that age. The treated birds in this study recovered in either normal or stroboscopic illumination. Their hVOR and vVOR and vestibulocollic reflexes (VCR) were measured and compared with those of untreated, age-matched controls at 8 wk posthatch, when hair cell regeneration is known to be complete. As in previous studies, the gain of the VOR decreased immediately to zero after streptomycin treatment. After 8 wk of recovery under normal light, the hVOR was normal, but vVOR gain was less than normal. After 8 wk of recovery under stroboscopic illumination, hVOR gain was less than normal at all frequencies. VCR recovery was not affected by the strobe environment. When streptomycin-treated, strobe-recovered birds were then placed in normal light for 2 days, hVOR gain returned to normal. Taken together, the results of these experiments suggest that continuous visual feedback can adjust VOR gain. In the absence of appropriate visual stimuli, however, there is a default VOR gain and phase to which birds recover or revert, regardless of age. Thus an 8-wk-old chicken raised in a strobe environment from hatch would have the same gain as a streptomycin-treated chicken that recovers in a strobe environment.

Visual and vestibular reflexes that stabilize gaze in the chameleon

Spontaneous eye movements as well as visual, vestibular, and proprioceptive cervical reflexes which contribute to gaze stabilization were investigated in the chameleon using the magnetic search-coil technique. The oculomotor range of each eye was very large (180 deg horizontally x 80 deg vertically). Spontaneous ocular saccades were independent in the two eyes and could have very large amplitudes. The fast phases of nystagmus during the stabilization reflexes were also independent in the eyes. In the head-restrained condition, optokinetic nystagmus (OKN) had a low gain in both horizontal and vertical planes (0.35 at 5 deg/s) and showed little binocular interaction. The vestibulo-ocular reflex (VOR) exhibited a low gain (0.2-0.3 from 0.05-1 Hz) and a high-phase lead at low frequency (140 deg at 0.05 Hz). Rotation of the animal in the presence of a visible surround increased the overall gain of gaze stabilization to 0.4-0.5 (P < 0.01) and considerably reduced the phase lead (38 deg at 0.05 Hz). In the head-free condition, head and eye reflexes were active simultaneously during both optokinetic and vestibular stimulation, but nystagmic head movements appeared only occasionally with a rather loose eye-head coordination. During optokinetic stimulation, eye movements contributed more than head movements to gaze stabilization, whereas, during vestibular or visuo-vestibular stimulation, the relative contribution of eye and head responses varied with stimulus frequency. When the head was freed, overall gain for gaze stabilization increased from 0.35 to 0.45 (P < 0.05) for optokinetic stimulation at 5 deg/s and from 0.2-0.3 to 0.4-0.75 (P < 0.001) for vestibular stimulation at 0.05-1 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

Early sensory determinants of locomotor speed in adult cats: II. Effects of strobe rearing on vestibular functions

Cats raised under stroboscopic illumination are known to exhibit oculomotor and visuomotor deficits, but little is known about their locomotor abilities. Four strobe-reared cats with intact labyrinths were tested in a locomotor test involving various walking surfaces and various illumination conditions. Apart from their general slowness under all the experimental conditions, these strobe cats showed no special deficit on narrow rails, which indicates that their dynamic balancing abilities were normal. In these subjects, the decrease in the use of kinetic visual cues was roughly compensated for by an increase in the use of position cues. When tested after chronic bilateral labyrinthectomy, the strobe-reared cats' locomotor speeds were identical to those of control labyrinthectomized cats, except on wide platforms involving orientation towards a visual goal. These results show that in the absence of motion-vision, vestibular control of dynamic balance can mature normally, but they suggest that other aspects of locomotion involving the processing of vestibular and kinesthetic inputs may be impaired.

Optokinetic nystagmus in the ferret: including selected comparisons with the cat

The aim of this study was to evaluate the functional significance of similarities observed in the anatomy and the physiology of cat and ferret visual systems. Optokinetic nystagmus (OKN) in response to movement of the entire visual field, and optokinetic after nystagmus (OKAN) were measured in 8 ferrets with binocular stimulation. A shift of the beating field in the same direction as the fast phase of eye movements was observed both in ferret and cat. The absence of a fast rise in slow phase velocity (SPV) and similarities in the time constant to reach the steady state OKN gain, using step velocity stimuli are noted. As in the cat, primary OKAN was observed with a gradual decrease in its SPV. Following termination of stimulation, no sudden fall in SPV was noted for either species. However, for the ferret, the decrease was more rapid. With monocular stimulation, small differences were observed in OKN gain when responses to temporonasal and nasotemporal directions of the stimulus were compared in the two species. In contrast, the ferret displays a OKN gain which is approximatively twice that of the cat at stimulus velocities of 100 degrees/sec. Even at 200 degrees/sec., visual movement still induces a discernable OKN response (gain .0.07). Secondary OKAN, always present in the cat, was observed in only 43% of ferret records. Taken together with other considerations, these findings recommend the ferret as an alternative to the cat for the study of OKN and of other visuo-motor capacities in carnivores.

Stabilizing gaze reflexes in the pigeon (Columba livia). II. Vestibulo-ocular (VOR) and vestibulo-collic (closed-loop VCR) reflexes

The vestibulo-ocular reflex (VOR) and the closed-loop vestibulo-collic reflex (CL-VCR) were investigated in the pigeon. The animals, placed either in the fixed-head condition (VOR) or in the free-head condition (CL-VCR) were rotated in darkness (vestibular responses) or in the presence of visual surroundings (visuo-vestibular responses). The linear range of the reflexes were determined both in the frequency and in the velocity domains. Results show that: 1. Pigeons develop a strong VOR, which presents the same asymmetry observed with the OKN, the gain being higher when the slow-phase occurs in the T-N direction. This asymmetry persists in the light (VOR + OKN). In the free-head condition, both the eye and the head display a synchronized nystagmus whose effects are additive. The head reflex (CL-VCR) contributes about 80% of the gaze stabilization. 2. In the medium-low frequency range, the head response (CL-VCR) has a lower gain than the VOR (head-fixed), but the gain of both reflexes increases with frequency, up to about 1 at 0.6-1 Hz. The gaze response (eye + head) presents an optimal gain above 0.06 Hz. The phase lead is higher for the VOR than for the CL-VCR (40 degrees and 32 degrees respectively at 0.03 Hz), but both phases also become nul around 1 Hz. The time constants are 6.5 s for the VOR, 8.5 s for the CL-VCR and 9.6 s for the gaze response (VOR + CL-VCR). 3. While the VOR gain shows a saturation at peak stimulation velocities (PV) higher than 20 degrees/s (at 0.3 Hz), the CL-VCR gain is linear at least up to 60 degrees/s (the highest PV used). However, the phase lead declines when the PV is greater than 20 degrees/s, both for the VOR and the CL-VCR. 4. When the vestibular stimulation is delivered in the light (visuo-vestibular stimulation), there is no phase shift. The VOR gain (fixed-head) is optimal and linear over the entire frequency range, but it saturates for PV higher than 40 degrees/s. In the free-head condition, while the gaze gain is linear and close to 1 in both the frequency and the velocity domains, the head response gain (CL-VCR) remains lower especially in the low frequency and in the low velocity ranges.

The horizontal optokinetic nystagmus in the cat

Horizontal optokinetic eye nystagmus (OKN) and after nystagmus (OKAN) were recorded in the alert cat (head restrained) in response to velocity steps and sinusoidal optokinetic stimuli. A strong dependency of OKN performance on stimulus pattern was found: responses were most regular and gain was high over a large range of stimulus velocities when the stimulus consisted of a high-contrast random dot pattern. Following velocity steps, OKN showed a small amplitude fast rise in slow phase velocity (SPV) which was followed by a slow build-up to steady state. The amplitude of the initial jump in SPV increased with stimulus amplitude up to 30 degrees/s and saturated afterwards. The plateau level of initial SPV ranged from 5 to 15 degrees/s. The slow build-up of SPV showed non-linearities, i.e. the time to steady state increased with stimulus amplitude and the slow rise of SPV was irregular. In most animals steady state SPV showed no signs of response saturation for step amplitudes up to 60-80 degrees/s or more. The open-loop gain (steady state SPV/retinal slip velocity) depended on retinal slip velocity and decreased from 46 at 0.5 degrees/s to 0.4 at about 60 degrees/s. OKAN I and II were consistently observed and occasionally OKAN III was noted. OKAN I durations (mean 13.8 +/- 5.1 s) and OKAN II amplitudes were independent of stimulus magnitude. Initial SPV of OKAN I was typically the same as that of OKN, i.e. no fast fall was observed. Cessation of pattern rotation in light, however, produced a fast initial decay of SPV. A least square fitting of OKAN time course was performed with various time functions. The SPV of OKAN I and II was best fitted with a damped sine wave, indicating that cat optokinetic system behaves like a second order underdamped system. Sinusoidal stimuli produced strong response non-linearities. At a given frequency gain decreased with increasing stimulus amplitudes. Gain correlated best with stimulus acceleration. In addition, strong stimuli produced characteristic response distortions. In the visual-vestibular conflict situation vectorial summation of VOR and OKN was observed only with small stimuli.