Visual and vestibular reflexes that stabilize gaze in the chameleon

Visual acuity in the flying snake, Chrysopelea paradisi

Visual control during high-speed aerial locomotion requires a visual system adapted for such behaviors. Flying snakes (genus: Chrysopelea) are capable of gliding at speeds up to 11 m s-1 and perform visual assessments before take-off. Determining nuances of control requires a closed-loop experimental system, such as immersive virtual arenas. To characterize vision in the flying snake Chrysopelea paradisi, we used digitally reconstructed models of the head to determine a 3D field of vision. We also used optokinetic drum experiments and compared slowphase optokinetic nystagmus (OKN) speeds to calculate visual acuity and conducted preliminary experiments to determine whether snakes would respond to closed-loop virtual stimuli. Visual characterization showed that C. paradisi likely has a large field of view (308.5 ± 6.5° azimuthal range), with a considerable binocular region (33.0 ± 11.0° azimuthal width) that extends overhead. Their visual systems are broadly tuned and motion-sensitive, with peak OKN response gains of 0.50 ± 0.11 seen at 46.06 ± 11.08 Hz, and a low spatial acuity, with peak gain of 0.92 ± 0.41 seen at 2.89 ± 0.16 cpd (cycles per degree). These characteristics were used to inform settings in an immersive virtual arena, including framerate, brightness, and stimulus size. In turn, the immersive virtual arena was used to reproduce the optokinetic drum experiments. We elicited OKN in open-loop experiments, with a mean gain of 0.21 ± 0.9 seen at 0.019 ± 6x10-5 cpd and 1.79 ± 0.01 Hz. In closed-loop experiments, snakes did not exhibit OKN, but held the image fixed, indicating visual stabilization. These results demonstrate for that C. paradisi responds to visual stimuli in a digital virtual arena. The accessibility and adaptability of the virtual setup make it suitable for future studies of visual control in snakes and other animals in an unconstrained setting.

Ocular Kinematics Measured by In Vitro Stimulation of the Cranial Nerves in the Turtle

After animals are euthanized, their tissues begin to die. Turtles offer an advantage because of a longer survival time of their tissues, especially when compared to warm-blooded vertebrates. Because of this, in vitro experiments in turtles can be performed for extended periods of time to investigate the neural signals and control of their target actions. Using an isolated head preparation, we measured the kinematics of eye movements in turtles, and their modulation by electrical signals carried by cranial nerves. After the brain was removed from the skull, leaving the cranial nerves intact, the dissected head was placed in a gimbal to calibrate eye movements. Glass electrodes were attached to cranial nerves (oculomotor, trochlear, and abducens) and stimulated with currents to evoke eye movements. We monitored eye movements with an infrared video tracking system and quantified rotations of the eyes. Current pulses with a range of amplitudes, frequencies, and train durations were used to observe effects on responses. Because the preparation is separated from the brain, the efferent pathway going to muscle targets can be examined in isolation to investigate neural signaling in the absence of centrally processed sensory information.

Visual resolution and contrast sensitivity in two benthic sharks

Sharks have long been described as having 'poor' vision. They are cone monochromats and anatomical estimates suggest they have low spatial resolution. However, there are no direct behavioural measurements of spatial resolution or contrast sensitivity. This study estimates contrast sensitivity and spatial resolution of two species of benthic sharks, the Port Jackson shark, Heterodontus portusjacksoni, and the brown-banded bamboo shark, Chiloscyllium punctatum, by recording eye movements in response to optokinetic stimuli. Both species tracked moving low spatial frequency gratings with weak but consistent eye movements. Eye movements ceased at 0.38 cycles per degree, even for high contrasts, suggesting low spatial resolution. However, at lower spatial frequencies, eye movements were elicited by low contrast gratings, 1.3% and 2.9% contrast in H portusjacksoni and C. punctatum, respectively. Contrast sensitivity was higher than in other vertebrates with a similar spatial resolving power, which may reflect an adaptation to the relatively low contrast encountered in aquatic environments. Optokinetic gain was consistently low and neither species stabilised the gratings on their retina. To check whether restraining the animals affected their optokinetic responses, we also analysed eye movements in free-swimming C. punctatum We found no eye movements that could compensate for body rotations, suggesting that vision may pass through phases of stabilisation and blur during swimming. As C. punctatum is a sedentary benthic species, gaze stabilisation during swimming may not be essential. Our results suggest that vision in sharks is not 'poor' as previously suggested, but optimised for contrast detection rather than spatial resolution.

Variation in the position of the jugal medial ridge among lizards (reptilia: squamata): its functional and taxonomic significance

The course of the medial ridge in the lizard jugal shows considerable morphological variation. There are four basic configurations: (1) the medial ridge is located ventral to mid-height on the suborbital process and anterior to mid-length on the postorbital process; (2) the medial ridge is located ventrally on the suborbital process (as above), but posteriorly on the postorbital process; (3) the medial ridge is located dorsally on the suborbital process and anteriorly on the postorbital process; and (4) the medial ridge is centrally located along the entire length of the jugal. Ancestral character state reconstruction shows that type 1 is plesiomorphic for Squamata regardless of the broad-scale phylogenetic topology. Type 3 is present in chamaeleonids and convergently in Anolis barbatus. Type 3 is a synapomorphy of the chamaeleonids. Type 2 is considered plesiomorphic for Anguidae, Heloderma and Xenosaurus, although it is independently modified in some extant members. These taxa form a clade in molecular phylogenies of Squamata, and the course of the medial ridge of the jugal therefore provides some measure of morphological support for this arrangement. The course of the medial ridge may be best explained by the position of the eye and by the angle of the jugal; its relations with other bony orbital structures (supraocular osteoderms, palpebral, supraorbital flanges) and the posterior extent of the maxilla are also discussed.

Role of the trochlear nerve in eye abduction and frontal vision of the red-eared slider turtle (Trachemys scripta elegans)

Horizontal head rotation evokes significant responses from trochlear motoneurons of turtle that suggests they have a functional role in abduction of the eyes like that in frontal-eyed mammals. The finding is unexpected given that the turtle is generally considered lateral-eyed and assumed to have eye movements instead like that of lateral-eyed mammals, in which innervation of the superior oblique muscle by the trochlear nerve (nIV) produces intorsion, elevation, and adduction (not abduction). Using an isolated turtle head preparation with the brain removed, glass suction electrodes were used to stimulate nIV with trains of current pulses. Eyes were monitored via an infrared camera with the head placed in a gimble to quantify eye rotations and their directions. Stimulations of nIV evoked intorsion, elevation, and abduction. Dissection of the superior oblique muscle identified lines of action and a location of insertion on the eye, which supported kinematics evoked by nIV stimulation. Eye positions in alert behaving turtles with their head extended were compared with that when their heads were retracted in the carapace. When the head was retracted, there was a reduction in interpupillary distance and an increase in binocular overlap. Occlusion of peripheral fields by the carapace forces the turtle to a more frontal-eyed state, perhaps the reason for the action of abduction by the superior oblique muscle. These findings support why trochlear motoneurons in turtle respond in the same way as abducens motoneurons to horizontal rotations, an unusual characteristic of vestibulo-ocular physiology in comparison with other mammalian lateral-eyed species.

The neural control of fast vs. slow vergence eye movements

When looking between targets located in three-dimensional space, information about relative depth is sent from the visual cortex to the motor control centers in the brainstem, which are responsible for generating appropriate motor commands to move the eyes. Surprisingly, how the neurons in the brainstem use the depth information supplied by the visual cortex to precisely aim each eye on a visual target remains highly controversial. This review will consider the results of recent studies that have focused on determining how individual neurons contribute to realigning gaze when we look between objects located at different depths. In particular, the results of new experiments provide compelling evidence that the majority of saccadic neurons dynamically encode the movement of an individual eye, and show that the time-varying discharge of the saccadic neuron population encodes the drive required to account for vergence facilitation during disconjugate saccades. Notably, these results suggest that an additional input (i.e. from a separate vergence subsystem) is not required to shape the activity of motoneurons during disconjugate saccades. Furthermore, whereas motoneurons drive both fast and slow vergence movements, saccadic neurons discharge only during fast vergence movements, emphasizing the existence of distinct premotor pathways for controlling fast vs. slow vergence. Taken together, these recent findings contradict the traditional view that the brain is circuited with independent pathways for conjugate and vergence control, and thus provide an important new insight into how the brain controls three-dimensional gaze shifts.

Eye movements of laterally eyed birds are not independent

Most birds have laterally placed eyes with two largely separated visual fields. According to studies in pigeons laterally eyed birds move their eyes independently in most situations, eye coordination just occurred during converging saccades towards frontal stimuli. Here we demonstrate for the first time that laterally eyed zebra finches show coordinated eye movements, regarding direction and amplitude. Spontaneous and visually elicited movements of the two eyes were recorded simultaneously, using a newly developed eye tracking system. We found that, if one eye moves in a certain direction, the other eye simultaneously performs a counter-movement in the opposite direction. Based on these data we developed a hypothesis of how laterally eyed birds cope with the situation in which the left and right eye simultaneously obtain images with different content. We suggest that the counter-movements maintain the spatial relationship of the two visual fields. ;Oculospatial constancy', as we call it, facilitates the combination of the left and right visual percept on the level of peripheral or unattended viewing, and the localization of appearing stimuli within the whole visual field. As soon as two visual stimuli simultaneously appear in the left and right visual field, the birds decide on one stimulus and direct the fovea of the appropriate eye towards it for high resolution analysis, the other eye simultaneously performing a counter-saccade. This leads to the assumption that, in contrast to simultaneous peripheral perception with two eyes, the processing of foveal information is possible only for one eye at one time.

Canal and otolith contributions to compensatory tilt responses in pigeons

Gaze-stabilizing eye and head responses compensate more effectively for low-frequency rotational motion when such motion stimulates the otolith organs, as during earth-horizontal axis rotations. However, the nature of the otolith signal responsible for this improvement in performance has not been previously determined. In this study, we used combinations of earth-horizontal axis rotational and translational motion to manipulate the magnitude of net linear acceleration experienced by pigeons, under both head-fixed and head-free conditions. We show that phase enhancement of eye and head responses to low-frequency rotational motion was causally related to the magnitude of dynamic net linear acceleration and not the gravitational acceleration component. We also show that canal-driven and otolith-driven eye responses were both spatially and temporally appropriate to combine linearly, and that a simple linear model combining canal- and otolith-driven components predicted eye responses to complex motion that were consistent with our experimental observations. However, the same model did not predict the observed head responses, which were spatially but not temporally appropriate to combine according to the same linear scheme. These results suggest that distinct vestibular processing substrates exist for eye and head responses in pigeons and that these are likely different from the vestibular processing substrates observed in primates.

Responses to Moving Visual Stimuli in Pretectal Neurons of the Small-Spotted Dogfish (Scyliorhinus canicula)

Single-unit recordings were performed from a retinorecipient pretectal area (corpus geniculatum laterale) in Scyliorhinus canicula. The function and homology of this nucleus has not been clarified so far. During visual stimulation with a random dot pattern, 45 (35%) neurons were found to be direction selective, 10 (8%) were axis selective (best neuronal responses to rotations in both directions around one particular stimulus axis), and 75 (58%) were movement sensitive. Direction-selective responses were found to the following stimulus directions (in retinal coordinates): temporonasal and nasotemporal horizontal movements, up- and downward vertical movements, and oblique movements. All directions of motion were represented equally by our sample of pretectal neurons. Additionally we tested the responses of 58 of the 130 neurons to random dot patterns rotating around the semicircular canal or body axes to investigate whether direction-selective visual information is mapped into vestibular coordinates in pretectal neurons of this chondrichthyan species. Again all rotational directions were represented equally, which argues against a direct transformation from a retinal to a vestibular reference frame. If a complete transformation had occurred, responses to rotational axes corresponding to the axes of the semicircular canals should have been overrepresented. In conclusion, the recorded direction-selective neurons in the Cgl are plausible detectors for retinal slip created by body rotations in all directions.

Vestibular gaze stabilization: different behavioral strategies for arboreal and terrestrial avians

In birds, it is thought that head movements play a major role in the reflexive stabilization of gaze and vision. In this study, we investigated the contributions of the eye and head to gaze stabilization during rotations under both head-fixed [vestibuloocular (VOR)] and head-free conditions in two avian species: pigeons and quails. These two species differ both in ocular anatomy (the pigeon has 2 distinct foveal regions), as well as in behavioral repertoires. Pigeons are arboreal, fly extended distances, and can navigate. Quails are primarily engrossed in terrestrial niches and fly only short distances. Unlike the head-fixed VOR gains that were under-compensatory for both species, gaze gains under head-free conditions were completely compensatory at high frequencies. This compensation was achieved primarily with head movements in pigeons, but with combined head and eye-in-head contributions in the quail. In contrast, eye-in-head motion, which was significantly reduced for head-free compared with head-fixed conditions, contributed very little to overall gaze stability in pigeons. These results suggest that disparity between the stabilization strategies employed by these two birds may be attributed to differences in species-specific behavior and anatomy.

Three-dimensional vestibular eye and head reflexes of the chameleon: characteristics of gain and phase and effects of eye position on orientation of ocular rotation axes during stimulation in yaw direction

We investigated gaze-stabilizing reflexes in the chameleon using the three-dimensional search-coil technique. Animals were rotated sinusoidally around an earth-vertical axis under head-fixed and head-free conditions, in the dark and in the light. Gain, phase and the influence of eye position on vestibulo-ocular reflex rotation axes were studied. During head-restrained stimulation in the dark, vestibulo-ocular reflex gaze gains were low (0.1-0.3) and phase lead decreased with increasing frequencies (from 100 degrees at 0.04 Hz to < 30 degrees at 1 Hz). Gaze gains were larger during stimulation in the light (0.1-0.8) with a smaller phase lead (< 30 degrees) and were close to unity during the head-free conditions (around 0.6 in the dark, around 0.8 in the light) with small phase leads. These results confirm earlier findings that chameleons have a low vestibulo-ocular reflex gain during head-fixed conditions and stimulation in the dark and higher gains during head-free stimulation in the light. Vestibulo-ocular reflex eye rotation axes were roughly aligned with the head's rotation axis and did not systematically tilt when the animals were looking eccentrically, up- or downward (as predicted by Listing's Law). Therefore, vestibulo-ocular reflex responses in the chameleon follow a strategy, which optimally stabilizes the entire retinal images, a result previously found in non-human primates.

Independent and conjugate eye movements during optokinesis in teleost fish

In response to movements involving a large part of the visual field, the eyes of vertebrates typically show an optokinetic nystagmus, a response in which both eyes are tightly yoked. Using a comparative approach, this study sets out to establish whether fish with independent spontaneous eye movements show independent optokinetic nystagmus in each eye. Two fish with independent spontaneous eye movements, the pipefish Corythoichthyes intestinalisand the sandlance Limnichthyes fasciatus were compared with the butterflyfish Chaetodon rainfordi, which exhibits tightly yoked eye movements. In the butterflyfish a single whole-field stimulus elicits conjugate optokinesis, whereas the sandlance and pipefish show asynchronous optokinetic movements. In a split drum experiment, when both eyes were stimulated in opposite directions with different speeds, both the sandlance and the pipefish compensated independently with each eye. The optokinetic response in the butterflyfish showed some disconjugacy but was generally confused. When one eye was occluded, the seeing eye was capable of driving the occluded eye in both the butterflyfish and the pipefish but not in the sandlance. Monocular occlusion therefore unmasks a link between the two eyes in the pipefish, which is overridden when both eyes receive visual input. The sandlance never showed any correlation between the eyes during optokinesis in all stimulus conditions. This suggests that there are different levels of linkage between the two eyes in the oculomotor system of teleosts, depending on the visual input.

Ocular biology and diseases of Old World chameleons

For centuries, Old World chameleons (Chamaeleonidae family) have been collected and studied for their unusual biology and features, which are unique among lizards and other vertebrates. They have advanced mechanisms for capturing prey with their tongue, but have a primitive mechanism for hearing. Chameleons have the most studied ocular system because of their highly adapted yet primitive biology. This system has specific features that are susceptible to new diseases, which may require novel therapies.

New ideas about binocular coordination of eye movements: is there a chameleon in the primate family tree?

Many animals with laterally placed eyes, such as chameleons, move their eyes independently of one another. In contrast, primates with frontally placed eyes and binocular vision must move them together so that both eyes are aimed at the same point in visual space. Is binocular coordination an innate feature of how our brains are wired, or have we simply learned to move our eyes together? This question sparked a controversy in the 19(th) century between two eminent German scientists, Ewald Hering and Hermann von Helmholtz. Hering took the position that binocular coordination was innate and vigorously challenged von Helmholtz's view that it was learned. Hering won the argument and his hypothesis, known as Hering's Law of Equal Innervation, became generally accepted. New evidence suggests, however, that similar to chameleons, primates may program movements of each eye independently. Binocular coordination is achieved by a neural network at the motor periphery comprised of motoneurons and specialized interneurons located near or in the cranial nerve nuclei that innervate the extraocular muscles. It is assumed that this network must be trained and calibrated during infancy and probably throughout life in order to maintain the precise binocular coordination characteristic of primate eye movements despite growth, aging effects, and injuries to the eye movement neuromuscular system. Malfunction of this network or its ability to adaptively learn may be a contributing cause of strabismus.

Quantitative and topographical study of retinal ganglion cells in the chameleon (Chameleo chameleon)

Chameleons display a number of well-described physiological peculiarities of their visual system, but there is no information on the topography of the retinal ganglion cell layer. In the present study, ganglion cell density of the chameleon retina was constructed from whole mounts of the retina stained with cresyl violet. For the identification of ganglion cells, these latter cells were labelled retrogradely with horseradish peroxidase applied to the optic nerve. Using this criterion, the proportion of ganglion cells was estimated to represent 80% of retinal cells, while glial cells and amacrine cells represented 14 and 6%, respectively, of the total cell population of the retina. As for the main features of the retinal map, first, ganglion cells were distributed inhomogeneously within the ganglion cell layer, and revealed the existence of a putative area centralis. Second, a horizontal visual streak, which showed two peak density areas, was identified. These features point out the degree of specialisation of the chameleon retina and the complexity of its visual system.

Characteristics of cervico-ocular responses in the chameleon

The cervico-ocular reflex (COR) was investigated in the chameleon. Two kinds of responses were observed by oscillating the body (sine-wave stimuli) in the fixed-head animal: a "smooth response" of very low gain (around 0.08) and a saccadic response composed of 1-12 saccades per cycle of stimulation (depending on the stimulation frequency). Both responses were elicited in the compensatory direction (same direction as the stimulation) and exhibited a frequency dependence with low-pass properties. The saccadic response was especially developed and displayed a higher gain (up to 0.4) than the smooth response. In darkness, the saccades were triggered near the zero point (head-body alignment), whereas in the presence of a fixed visual surround they were elicited more regularly throughout the stimulation cycle. The amplitude of saccades was increased in the light. Consequently, the gain and the phase lag of the saccadic response were enhanced by the visual input. No visuo-cervical interaction was observed for the smooth response. Oscillating the body at a constant velocity (seesaw or ramp stimuli) revealed a frequency effect on the number of saccades (during a cycle of stimulation), but not on the gain of the response. Increasing the amplitude of oscillations augmented only very slightly the amplitude of saccades and consequently decreased the gain. Hence, the best working range of the saccadic response corresponds to body or head movements of low amplitude (up to +/- 20 deg) and low frequency (up to 0.25 Hz), and is improved by a visual input. These properties are discussed on a comparative point of view. It is proposed that, in chameleons, the saccadic response could contribute to gaze stabilization and add to the vestibulo-ocular and the optokinetic responses.

Distribution of substance P-like immunoreactivity in the chameleon brain

The distribution of substance P-like immunoreactivity in the chameleon brain and spinal cord was studied with immunohistochemical methods using polyclonal antibodies against substance P. In the telencephalon, immunoreactive cell bodies and fibers were located primarily in the striatum and in the globus pallidus. In addition, few substance P-like fibers were observed in the cortical areas, in the septum, and in the amygdala. In the diencephalon, a high density of immunostained neurons and fibers were seen in the periventricular and ventrolateral hypothalamus. Another group of cell bodies was located in the optic tectum and particularly in the stratum griseum central. A large number of immunoreactive fibers were also detected in the thalamic nuclei and in the median eminence. In the mesencephalon, few immunoreactive neurons were observed in the ventral tegmental area, in the substantia nigra, and in the nucleus reticularis isthmi. These latter nuclei, the periventricular area, the posterior commissure, the nucleus lentiformis mesencephali, the oculomotor nucleus, and the raphe nuclei contained a dense plexus of substance P immunoreactive fibers. No immunoreactive cell bodies were observed in raphe nuclei. In the spinal cord, no substance P-like immunoreactive neurons were observed, but a large number of substance P immunostained fibers were seen in the dorsal and lateral part of the dorsal horn and surrounding the dorsal parts of the central canal. The results of the present study are discussed with respect to those obtained in other species of reptiles, the main differences concerning the lateral septum, the habenula, the area of the paraventricular organ, and the raphe nuclei.