Topic 7: visual processing Flashcards
where is the retina and what does it contain?
back of the eye, contains photoreceptors specialised to convert light energy into neural activity
describe the properties of light
-electromagnetic radiation visible to our eyes
-wave of energy, has:
…wavelength –> distance between successive peaks and troughs,
…and has frequencies –> the number of waves per second
…and amplitude, the difference between wave trough and peak
the energy content of electromagnetic radiation is proportional to its…
frequency
radiation emitted at a high frequency (short wavelength) has the lowest energy content, true or false?
False, it has the highest energy content, examples are gamma radiation emitted by some radioactive materials, wavelengths less than 1nm
radiation emitted at lower frequencies (longer wavelengths) has less energy, true or false?
True, examples are radar and radio waves, wavelengths greater than 1nm
What is the part of the electromagnetic spectrum that is detectable by our visual system?
wavelengths of 400-700nm
What is the pupil?
the opening that allows light to enter the eye and reach the retina, appears dark because of light-absorbing pigments in the retina
What is the iris?
surrounds the pupil, pigmentation provides what we call the eye’s colour, the iris contains 2 muscles that can vary the size of the pupil, one makes it smaller when contracts the other larger
What is the cornea?
covers pupil and iris, glassy transparent external surface of the eye, continuous with the sclera
What is the sclera?
the “white of the eye”, which forms the tough wall of the eyeball
where does the eye ball sit?
sits in a bony eye socket in the skull, also called the eye’s orbit
What are the extraocular muscles?
-inserted into the sclera, move the eyeball in the orbit, not visible because they lie behind the conjunctiva
What is the conjunctiva?
membrane that folds back from the inside of the eyelids and attaches to the sclera
What is the optic nerve?
Carry axons from the retina, exits the back of the eye, passes through the orbit, and reaches the base of the brain near the pituitary gland
What is the ophthalmoscope?
a device that enables one to peer into the eye through the pupil to the retina, can see blood vessels on its surface
where do blood vessels in the eye originate?
from a pale circular region called the optic disc, which is also where the optic nerve fibres exit the retina
what doesnt occur at the optic disk and why?
sensation of light, because there are no photoreceptors there
What doesnt occur at the place where large blood vessel exit the eyes?
sensation of light, because vessels cast shadows on the retina
What is the macula?
middle of each retina, dark-coloured region with a yellow-ish hue, part of the retina for central vision (as opposed to peripheral vision). distinguished also by the relative absence of large blood vessels
What is the fovea?
a dark spot about 2mm in diameter, retina thinner in fovea than elsewhere, centre of retina
What is aqueous humour?
nourishment by fluid for the cornea is provided by the aqueous humour (lies between cornea and lens)
what is the lens?
located behind the iris, suspended by ligaments (called zonule fibres), attached to ciliary muscle, which forms rings inside the eye
What is vitreous humour?
serves to keep eyeball spherical, lies between the lens and retina
What light does the eye collect? and how does it focus images?
-The light rays emitted by or reflected off objects in the environment, and focuses them onto the retina to form images
-bringing objects into focus involves the combined refractive powers of the cornea and lens –> cornea is site where most refractive power of the eye is
What does the eye use in order to see a single point?
refraction to focus the light to one point of the retina, light passes into a medium where its speed is slower and it will bend toward a line that is perpendicular to the border, or interface, between the media
What is the focal distance?
The distance from the refractive surface to the point where parallel light rays converge
What does the focal distance depend on in the eye?
curvature of the cornea, the tighter the curve, the shorter the focal distance
What is the refractive power?
reciprocal of the focal point, measure in measurements called “diopter” (cornea has refractive power of roughly 42 diopters)
What is the refractive power of the lens and cornea?
lens = roughly 6 diopters
cornea = roughly 42 diopters
what is the lens more involved with?
-in forming crisp mages of objects located closer than 9m from the eye
-With near objects, the light rays originating at a point are no longer parallel. Rather, these rays diverge from a light source or a point on an object, and greater refractive power is required to bring them into focus on the retina.
-This additional focusing power is provided by changing the shape of the lens, a process known as accommodation
What happens during accommodation?
-During accommodation, the ciliary muscle contracts and swells in size, thereby making the area inside the ring smaller and decreasing the tension in the suspensory ligaments.
-Consequently, the lens becomes rounder and thicker because of its natural elasticity. This rounding increases the curvature of the lens surfaces, thereby increasing their refractive power.
-Conversely, relaxation of the ciliary muscle increases the tension in the suspensory ligaments, and the lens is stretched into a flatter shape.
what is the pupillary light reflex?
-involves connections between the retina and neurons in the brainstem that control the muscles that constrict the pupils –> when lights increases
-this is consensual, shining light on one eye causes constriction of both pupils
What is the benefits of the pupillary light reflex?
increase in depth of focus
Describe the visual field
-The left visual field is images on the right side of the retina and the right visual field on the left side of the retina
-similarly upper visual field is image on bottom of retina and vice versa
What is visual acuity?
The ability of the eye to distinguish two points near each other
What does visual acuity depend on?
several factors but especially on spacing of photoreceptors in retina and precision of eye’s refraction
What is the viusal angle?
Distance across the retina, ability of eye to revolve points that are separated by a certain number of degree
What is the pathway of visual information before exiting the eye?
photoreceptors to bipolar cells to ganglion cells
what do photoreceptors do?
respond to light, they influence membrane potential of the bipolar cells connected to them
What do the ganglion cells do?
fire action potential in response to light, and these impulses propagate along the optic nerve to the rest of the brain