The Visual System Flashcards
Feature detection
The brain has specific neurons or circuits of neurons specialised for detection of particular features of the sensory world
Adaptation
Neural signals in features which remain constant are dampened down, the brain is mostly interested in changes in the environment
What is the goal of the visual system?
Build a predictive model of the external world based on incident light
_____ radiation is converted into ______ impulses
Electromagnetic, neural
What controls the amount of light entering the eye?
The iris and the pupil
Pupillary constriction/dilation
- The iris adjusts the size of the pupil
- This is controlled by how much light enters the eye
Pupil in the dark
Iris muscle relaxes and pupil dilates
More light enters the eye so sensitivity is improved, but acuity is poorer
Pupil in the light
Iris muscle contracts and pupil gets smaller
Less light enters but the image on retina is sharper and acuity is improved
Why less acuity with greater light?
When more light enters, each and every point projects to a larger area on the retina, and these areas overlap, creating blurring
Cornea
Transparent cover for the front of the eye
Helps to focus incoming light
Fixed focus
Lens
Sits behind the pupil
Can change shape through the process of accommodation
What holds the lens in place?
Suspensory ligaments called zonules
Accommodation in close vision
- Ciliary muscle contracts
- Ciliary body moves closer to the lens
- Tension in zonula fibres is reduced and the lens rounds up
- Reduces the focal distance
Accommodation in distant vision
- Ciliary muscle relaxes
- Ciliary body moves away from the lens
- Tension of zonula fibres increases and the lens flattens
- The focal length increases
Retina
Thin, light sensitive tissue lining the back of the eye
Contains a layer of photoreceptive cells that convert light into neural signals
Macula
Centre of the retina
Contains very high concentration of photoreceptors
Macular degeneration - loss of central vision with age
Fovea
Centre of the macula
Site of our sharpest vision
Why does red eye occur?
Flash of the camera reflects off the blood in the choroid and back through the pupil
What is reflective eye?
Many nocturnal animals have a layer of reflective tissue, called tapetum lucidum, which reflects light back through the retina
Sclera
Tough protective layer of connective tissue (white of the eye)
Choroid
Layer of tissue between the retina and sclera
Contains many blood vessels and is critical for providing oxygen and glucose for retinal cells
Why is the retina inverted?
Theory that it is a space saving solution, eyes would need to be larger to enable light to be focused on photoreceptors
Photoreceptors
Convert light into neural signals via visual transduction
Types of photoreceptors
Rods and cones
Rods
- Scotopic (dim light) vision
- Operate in low lighting
- High convergence - many rods connected to one bipolar cell - poor acuity but high sensitivity
- One type so monochromatic
- Peripheral vision
Cones
- Photopic vision (well-lit)
- Only operate in good lighting
- Low convergence - each cone connects to one bipolar cells - good acuity but poor sensitivity
- Three types (red, green, blue) so responsible for colour
- Central vision
The vast majority of cones are crammed into the fovea, giving it high ______, _____ vision
Acuity, colour
Rods in the dark
- Rhodopsin inactive
- Sodium channels are kept open by cyclic GMP
- Cell depolarised
- Rods continually release glutamate
Rods in the light
- Rhodopsin active
- Cyclic GMP broken down and sodium channels close
- Cells hyperpolarised
- Glutamate release is reduced
Bipolar cells
- Process input from photoreceptors and output to the retinal ganglion cells
- Allow some low-level signal processing to occur in the retina
- Photoreceptors converge via bipolar cells onto retinal ganglion cells
Few-to-one conversion
Maintains excellent resolution (cones)
Many-to-one
Maintains excellent sensitivity (rods)
Retinal ganglion cells
Wired up to bipolar cells and photoreceptors
Facilitate the detection of the edges in images
What cells are present in the fovea?
Only cones, no rods
Where are the blood vessels in the fovea?
Displaced to the side along with other cells, minimising the distortion of the light hitting the photoreceptors
Around ____ of the nerve fibres in the optic nerve are supplied by the fovea
50%
Blind spot
Ganglion cell axons leave the retina in the optic nerve where there are no photoreceptors
Completion
The brain fills in the gap where the blind spot is
Why does the blind spot appear white?
Axons are covered in a myelin sheath, a white fatty substance
Why does the eye move so much?
A brain that provides foveal vision at all locations would weigh 10 tonnes?
Therefore, the eye continually scans the visual field with the fovea
Visual system integrates this information to produce wide-angled, high acuity, coloured perception
How many fixations does the eye make per second?
Three
