Vision Flashcards
What are 3 functions of the eye?
- Provide information about the shape and colour of objects to allow recognition of what the object is
- Provide information about where the object is and how it is moving in 3D space
- Provide information about changes in absolute levels of illumination, which enables the animals to synchronise their circadian and circannual rhythms with changes in the environment.
What features of the cornea and lens are essential to vision?
For the eye to function effectively, the cornea and lens have to be transparent and devoid of blood vessels that would scatter the light passing through them and degrade the quality of the retinal image.
If the lens and cornea are devoid of blood vessels, how are they supplied?
- Composed of living cell that require oxygen and nourishment and removal of waste products.
- Provided for by continual circulation of aqueous humor, that fills that anterior eye chamber.
- Produced by filtration of blood through the walls of the blood capillaries in the pupil to form the aqueous humor.
How is aqueous humor reabsorbed?
Reabsorption of fluid occurs into a venous plexus around the edge of the anterior chamber.
Describe the balance of fluid filtration and absorption of aqueous humor.
Balance is vital for eye health.
A slight positive pressure is important for maintaining the shape of the eyeball, but too high an intraocular pressure can impair vascular perfusion of the retina, leading to ischaemic damage and physical damage, increasing the risk of a detached retina.
Describe the pupil in dim light.
Sympathetic efferent activity causes the radial muscle fibres of the pupil to contract increasing its diameter. This is mydriasis and allows more light into the eye in dim lighting conditions to maximise sensitivity of the visual system.
Describe the pupil in bright light.
Parasympathetic efferent activity contracts the circular muscle constricting the size of the pupil and reducing the amount of light entering the eye. This is known as miosis and is important to prevent the temporary saturation of the photoreceptors, for example by dazzling bright light when emerging from a burrow or moving from deep shade to sunlight.
What happens to ganglion cells as light level increases?
- As light levels increase, the activity of melanopsin containing retinal ganglion cells that project to the pretectal nucleus of the midbrain increase.
- Neurones in the pretectal nucleus then project to the parasympathetic motor nucleus of the oculomotor nerve, which is part of the Erdinger Westphal nucleus.
- This conveys excitation via parasympathetic preganglionic fibre to the ciliary ganglion and then via postganglionic parasympathetic fibres to constrict the iris.
Describe the pupillary light reflex.
Bilateral. Direct response in the eye that receiving light stimulation is more prominent than the consensual response in the contralateral eye, due to decussation at the optic chiasm and then contralateral projections from the pretectal nucleus to the parasympathetic nucleus of the oculomotor nerve. Important diagnostic test that can distinguish dysfunction of afferent or efferent components of the pathway.
What is indicated in the presence of consensual response but absence of direct response?
Indicates that the sensory component and pretectal nucleus on the stimulated side is intact, but that there is a dysfunction in the efferent component from the parasympathetic nucleus of the oculomotor nerve to the pupil.
What is indicated in the absence of both consensual and direct responses?
When light stimulates 1 eye but a normal response with stimulation of the other indicates the efferent pathway to the unresponsive eye is intact and that the problem is in the sensory pathway.
Describe the focusing power of the eye.
Most of the focussing power of the eye is due to refraction at the cornea due to the large difference in the refractive indices of air and water. But the power of the cornea is fixed.
How does the eye focus on distant objects?
Change in the shape of the lens:
- The lens is suspended by zonular fibres from the ciliary body, which contains the ring of the ciliary muscle.
- Relaxation of the muscle increases the diameter of the ciliary body exerting tension on the zonular fires.
- This stretches and flattens the lens, allowing it to focus on distant objects.
How does the eye focus on near objects?
Change in the shape of the lens:
- The ciliary muscle is contracted via parasympathetic control, which reduces the tension on the zonular fibres and allows the elasticity of the lens to accommodate to a more rounded and thicker profile.
- This focuses the more divergent light rays from the closer point in the visual field.
What is the other roles of the pupil?
Pupil constriction increases the depth of field of the image on the retina.
Pupil constriction improves the quality of the visual image by reducing the aberration of the optical image caused by rays of light passing through the periphery of the lens.
Distinguish chromatic and spherical aberration.
Chromatic aberration results from different wavelengths pf light being refracted to different extents and therefore brought into focus at different distances.
Spherical aberration results from the shape of the lens and again means that light passing through the peripheral lens is focussed at different distances to light passing through the central region, which degrades image quality.
Compare the fields of vision of predators and prey species.
- Primates ad predators have forward facing eyes with a high degree of binocular vision where the visual fields of the left and right eyes overlap.
- Comparison of the visual images from the 2 eyes enable the perception of depth for objects in the near distance.
- Important for predators that need to catch prey or primates who live in arboreal environments and need to judge distances accurately.
- Depth perception also requires the coordinated movement of the eyes, whereas animals with little binocular overlap have more ability to move their eyes independently.
What are 3 visual cues used in binocular vision?
Relative size of objects learned from experience.
Overlap of images.
Haze - caused by light scattering and a different spectral prolife due to absorption of certain wavelengths by the intervening atmosphere.
What are the photoreceptors present in different species types?
Predominantly rods in nocturnal species and rod and cones in diurnal species.
Describe the density of photoreceptors across the eye.
- The density of photoreceptors is lower in the peripheral retina and increases towards a maximum in a central area called the area centralis.
- The higher density of photoreceptors is mirrored by a higher density of retinal ganglion cells in the area centralis that transmit the information to the brain.
- This higher density of receptors is associated with higher visual acuity – a better ability to distinguish fine spatial detail.
What is the area centralis?
In diurnal species, the area centralis is packed with cones and provide the most acute vision in good light levels.
In most mammalian species, possibly excepting cats, the area centralis cannot be directly visualised using an ophthalmoscope.
What is a fovea?
Species with particularly high visual acuity, like humans, have a fovea. This appears as a depression in the retina, where the cell layers overlying the photoreceptors are displaced to the side to decrease light scattering.
What is the ‘blind spot’?
There are no photoreceptors in the optic disc, which is where the optic nerve leaves the eye.
What is the visual streak?
While predators have area centralis, many herbivores, such as cattle and horses, instead have an elongated strip of higher photoreceptor density called the visual streak. The orientation of the streak is horizontal and enables a high acuity image for horizon scanning to detect predators. Even when horses are grazing, their eye rotates so that the visual streak remains aligned to the horizon.
Define vergence eye movements.
Species with a high degree of binocular overlap rely on vergence movements of the eyes to move the area centralis of each eye to investigate objects at different distances.
Distinguish convergence and divergence eye movements.
Shifting the point of fixation to a closer object requires a convergence movement to swivel the eyes medially in order to maintain the image on the area centralis in each eye.
Conversely, shifting the point of fixation to a more distant image requires a divergence movement in which the eyes move laterally.
Describe velocity and purpose of convergence eye movements.
- 25˚ per second, relatively slow.
- This is slow enough for ongoing control of the eye movements in response to sensory feedback.
- The velocity of movement is initially rapid, but then slows down as the eyes get closer to the new fixation point.
- Convergence of the eyes onto a closer fixation point is associated with accommodation to change the focus of the lens and pupil constriction to increase the depth of field of the retinal image, so that the focus does not have to be precise.
What is saccadic eye movement and which species does it occur in?
- Species with an area centralis tend to make greater use of eye movements than species with a visual streak.
- Saccadic eye movements are used to move the area centralis from 1 part of the visual field to another.
- In these species, the eyes are continually and subconsciously moving around the visual image in what are known as microsaccades.
Compare avian visual field with other animals.
There is a motor loop via the basal ganglia and frontal eye fields that enables to animals to consciously move their point of fixation to explore the visual field. But there is great species variation and head movements are equally important in many species for moving their area of highest acuity around the visual field. Most birds have a relatively little eye movement but a great range of head movement, more than 180˚.
Describe the speed of saccadic eye movements.
- Saccadic eye movements are very rapid, around 500 ˚ per second, far too fast for any ongoing feedback control.
- Instead these are ballistic movements.
- Rapid movements completed very rapidly using motor neurone activity.
Describe the sensory feedback involved in saccadic eye movements.
- If movement is inaccurate, sensory feedback from the eye will drive a correcting microsaccade, after delay while feedback is processed.
- Sensory feedback will also provide parametric feedback to adapt the motor model to ensure the saccade is performed more accurately in the future.
- System works well for saccadic eye movements, as load on the eye does not normally vary, so the outcomes of the movement is highly predictable.
What is the optokinetic reflex of the eye?
Slow steady movements, such as those that track objects in the visual field.
- Tracking movement is followed by a rapid saccade in the opposite direction to rest the position of the eye.
- For objects that leave the visual field, there is no head movement then tracking stops after the saccade.
- If a larger scale movement, sawtooth pattern of the eyes known as nystagmus. The direction of the nystagmus is defined by the direction of the rapid saccades.
Describe the vestibulocular reflex as another form of tracking eye movement.
- Sensory input from the vestibular system moves the eyes in the opposite direction to the head to stabilise the retinal image during head movement,
- If the head keeps rotating, then nystagmus is induced with the same direction as the head rotation.
- This response reduces and eventually disappears if the head continues, due to adaptation of the vestibular system.
- But if movement is stopped, this will again stimulate the vestibular system and induce nystagmus in the opposite direction to the original head movement.
How can optokinetic and vestibulocular reflexes be tested?
Can sit a large dog in an office chair and rotate it to test the vestibulocular reflex.
It is easier for a handler to hold smaller dogs whilst spinning around with them and then get an observer to look for the physiological nystagmus when the movement stops.
Define pathological nystagmus.
Pathological nystagmus, which can either be spontaneous or positional (elicited when the head is held in a certain position). Pathological nystagmus is a symptom of a dysfunction in the neural pathways underlying these tracking responses.
Compare the size and shape of avian eyes.
Avian eyes are typically larger than a similarly sized mammal.
Variety of eyeball shape exists in birds. Eagles and other birds of prey have tubular shapes, the eyes can be moved and accommodated independently in many birds.
Describe the position of the avian eyes.
Predators typically have forward facing eyes, enforcing binocular vision. But eye movements are not so important in birds as they are in mammals, as the avian head is so much more mobile.
Describe the avian nictating membrane.
Birds possess a nictating membrane as do many mammals, but in some migrating birds, it is transparent and protects the eyeball from drying during long migrations.
Describe the avian area centralis and fovea.
Eagles and birds of prey have 5 times the cone density in the area centralis than the human eye, providing extraordinarily high visual acuity.
Many species of bird have 2 foveas, 1 for lateral monocular vision and the other for frontal binocular vision.
What allows birds to hunt from great heights and maintain a focused retinal image of prey in a steep dive?
Contain striated ciliary muscles and the lens is directly suspended from the muscle fibres, which makes the lens shape changed during accommodation far more rapid than in mammals.
