Vision Flashcards
What is sensory transduction?
The process whereby the characteristics of a stimulus are converted into nerve impulses
Which stimulus is used for vision?
Electromagnetic energy (light waves)
Which wavelengths are we sensitive to and what is the difference between them?
Between 380 and 760 nanometres
Different wavelengths have different colours, e.g. 380 is violet and 760 is red
How are human eyes protected?
Our eyelids, eyelashes and eyebrows stop foreign objects entering the eyes
Our eyelids also close automatically at the sudden approach of an object
Where do light waves enter the eye?
The cornea
What is the pupil?
An adjustable opening behind the cornea that controls the amount of light that enters the eye
What does the iris do?
Contains muscles that cause dilation or constriction of the pupil in order to either allow more or less light to enter the eye
What is the lens?
A flexible structure that either becomes thinner (to focus on distant objects) or thicker (to focus on nearby objects)
It also causes images to be focused on the retina
What is accommodation?
The process where an image is focused directly and sharply onto the retina
It is what determines good all-round vision
What is myopia?
Shortsightedness / nearsightedness
What is myopia caused by?
The visual image is focused in front of the retina because the eyeball is longer than normal
What is hyperopia?
Longsightedness / farsightedness
What is hyperopia caused by?
The visual image is focused behind the retina because the lens doesn’t thicken enough
What is the retina?
A multi-layered light sensitive tissue at the rear of the eyeball
What are the two main types of light sensitive receptor cells that are found in the retina?
Rods
Cones
What are rods?
Black and white brightness receptors that function best in dim light
What are cones?
Colour and detail receptors that function best in bright illumination
What is the fovea?
A small area in the centre of the retina that contains no rods but many densely packed cones
It is responsible for our most detailed vision
What are the rods and cones synaptically connected to ?
The bipolar cells
What are the bipolar cells synaptically connected to?
The ganglion cells
What is the role of the horizontal cells?
They collect information from the rods and cones and compress it so it can be sent to the bipolar cells
What is the role of the amacrine cells?
They collect information from the bipolar cells and compress it so it can be sent to the ganglion cells
How is the optic nerve formed?
Axons of the ganglion cells are collected into a bundle
What creates the blind spot?
An absence of photoreceptors where the optic nerve exits the eye
What does the lens to the image it projects onto the retina?
It reverses the image from right to left and top to bottom
How is the image then translated to be the right way around?
Activity of the photoreceptors and their associated neurons send input to the brain which reconstructs it
What are photopigments?
Protein molecules that allow rods and cones to translate light waves into nerve impulses
What is the role of photopigments?
They produce a chemical reaction that increases the rate of neurotransmitter release, causing a stronger signal to be passed onto the bipolar and ganglion cells - this sends a message through the visual system
What is dark adaptation?
The progressive improvement in brightness sensitivity that occurs over time under conditions of low illumination
Explain the process of regeneration of photopigment molecules
Bright light causes a depletion of photopigments in the rods and cones, meaning we cannot see in the dark
Over time, they regenerate, allowing us to see in the dark
Cones regenerate fully within about 10 minutes as they are less sensitive in the dark, while rods regenerate fully within about 30 minutes since they are more sensitive in the dark
How does dark adaptation occur?
Through the regeneration of photopigment molecules
Where is the visual message sent once it reaches the retina?
The optic nerve sends it to the thalamus, from where it is routed to the primary visual cortex (found in the occipital lobe)
What are feature detectors?
Cells within the primary visual cortex that fire selectively in response to visual stimuli that have specific characteristics
What happens in the visual association cortex?
It combines and interprets the information
