lecture 1 & 2 Flashcards
Muscular degeneration of the eye
Deterioration of the retina in the macula (fovea)
Blindness, loss of central vision
The optic disk
Location of the eye where the optic nerve leaves the eye. Blind spot where there are no photoreceptive retina, blind spot not registered as the two eyes combined eliminate this effect
Wavelength
The distance between two peaks (spectrum from blue to red with increasing wavelength)
What is the first transformation of visual process
Transformation of light (reflected by an object) onto a retinal image (retinal object representation)
How do we focus on an object?
Ciliary muscles tighten and relax to change the thickness of the lens (bends light to fall on fovea)
Main problem with focusing on an object?
There is a fixed distance between the lens and retina
Accommodation
The process by which the eye changes optical power to focus on an object as its distance varies
Accommodation to a near target
Tightened ciliary muscles. Thick lens (more curvature( so light is bent a lot. Otherwise the focus point would be before the fovea = blurred image.
Accommodation to a far target
Relaxed ciliary muscles, lens becomes slim so has little curvature. Light is bent a little. Otherwise the focus point would be behind the retina =blurred image
Far point
The maximum distance of an object from the eye for which a clear image of the object can be seen
Near point
The minimum distance of an object from the eye for which a clear image of the object can be seen.
Myopia
Far objects are out of focus because the lens is too thick (bends the light too much) or the eyeball is too long. Concave correction lenses which diverge the light before it enters the eye,
Hyperopia
Near objects are out of focus as the eyeball is too short. Convex correction glasses converge
Transduction
When an image on the retina is transformed into electrical activity. Photoreceptors convert images we see to electrical signals
Bipolar cells
Vertical connectors
How does light move through the eye?
Light moves through all layers of the retina, reaches the photoreceptors, the rods/cones transfer information to an electrical signal by synapsis in the photoreceptors. This information is supplied to the ganglion cells.
What happens to light sensitive chemicals when hit by light?
Light sensitive chemicals change to cause a reaction that causes an electrical signal to travel to the ganglion cells through the receptors axons and synapses and the bipolar cells as an action potential.
Where is information from the ganglion passed.
To the optic nerve,
How many photoreceptors are in the optic disk
None- blind spot
How do the rods and cones differ?
Density/distribution Ability to dark adaptation Absolute sensitivity Acuity Spectral sensitivity Colour visions
Number of rods & cones
Rods: 120 million
Cones: 6 millions
Absolute sensitivity
One ganglion receives input from 120 rods and 6 cones. There is more convergence of rods than cones. Absolute threshold is the smallest level of energy required by an external stimuli to be detectable by senses.
Dark adaptation
In the day there is a lot of light needed for sensation. In the night there is little needed for sensation.
Rod and cone dark adaptation
Cones will adapt very quickly to darkness. Rods will adapt after cones and this takes more time.
Rod cone break
When the rods take over dark adaptation to complete adaptation.
Compare the absolute sensitivity and visual acuity in rods and cones.
Absolute sensitivity is higher in rods than cones in the dark adapted eye.
Visual acuity is better with cones than rod vision in the light adapted eye.
Spectral sensitivity in rods and cones
Rods: more sensitive to shorter wavelengths, maximum at 500nm.
Cones: More sensitive to longer wavelengths, maximum at 560nm
Rod colour vision
Dark adapted vision, operates at low luminance. No colour sensation,
Cone colour vision
Light adapted vision, operates high luminance. S cones (respond to short wavelengths, blue) M cones (respond to medium wavelengths, green) L cones (Respond to long wavelength, red)
Neural convergence
A description of the number of neurons that synapse into a single neuron.
Increases the sensitivity of rods (ability to detect light)
Assumptions of neural convergence
Any receptor that detects a light, fires at a rate of 2.
Firing rates sum up during convergence.
Purkinje shift
Increased sensitivity to short wavelengths in dark adapted eye
Photopic vision
cone dominated, foveal and peripheral vision, light adapted vision, operates at high luminance, visual acuity, most sensitivity to long wavelengths, basis of colour vision
Scotopic vision
Rod dominated, peripheral vision, dark adapted vision, low visual acuity, sensitive to short wavelengths, no colour sensation
Mesopic vision
Rod and cone vision together
Stimulating ganglion cells
Each ganglion cell receives input from 126 photoreceptors. Input is not added but excited or inhibited depending on where the stimulus is in the ganglions receptive field
Receptive field
The receptive field of a sensory neuron is the region in the visual field in which a stimulus can modify the firing rate of this neuron.
IPL
The inner plexiform layer.
Ganglion cells dendrites extend in the IPL. Ganglion cells receive inputs from bipolar cells and retina amacrine cells to convey signals from photoreceptors to the IPL.
What does luminance discontinuity cause
Activation of 3 main types of ganglion cells: magnocellular, parvocellular and koniocellular
Magnocellular cells
Input from rods, not colour specific
Parvocellular cells
Input from single M or L cones, colour specific (green or red on/off)
Koniocellular cells
Excitatory input from S cones, inhibitory input from M and L cones (blue on)
Lateralisation in the retino-geniculo-striate pathway
Lateralisation at the optic chasm in the nasal axons cross over to the other side of the brain, the temporal axons stay on the same side.
Where are visual fields represented to due lateralisation.
In the centrolateral hemisphere (left visual field in the right hemisphere)
What makes up the primary visual cortex
V1 (visual area 1)
Striate visual cortex
Extrastriate visual areas
V2. V3. V4, V5, IT
What happens as visual information travels through the cortex
From the primary visual cortex to the extrastriate visual area there is ongoing neural convergence: The neurons receptive fields increase and visual information gets more complex.
V1
Magno and Parvo layers are preserved in the V1 input layer but then merge. Some V1 neurons are orientation selective, motion direction selective, colour selective or brightness selective.
V2
V2 receptive fields are twice as large as V1 receptive fields.
V2 neurons respond to more complex features: length, angles, arcs, shapes
Two separate steams in vision
Parietal (dorsal) stream processes object locations.
Temporal (ventral) stream processes object identities.
Evidence for two separate vision streams.
Monkeys with a parietal lesion could distinguish a cube from a triangle (object discrimination) while monkeys with temporal lesions could not.
Monkeys with temporal lesions could learn the position of an object (location discrimination) but monkeys with parietal lesions could not.
V3
Receptive fields are five times larger in the V3 and eight times larger in the middle temporal area than in the V1. These integrate information over a larger area of the retina and are ideal for motion perception
V4
Receptive fields are 5 times larger in V4 than in V1.
V4 neurons respond to object defining features (colour, orientation, complex shape, texture)
