Lecture 5 Flashcards
List the sensory hierarchy
Association cortex Secondary sensory cortex Primary sensory cortex Thalamic nuclei Receptors The process begins at the receptors and each layer adds more information or analysis, making the signal more specific and complex.
How is the sensory hierarchy divided?
It’s divided into sensation and perception. Sensation involves the first 3 levels of the hierarchy (receptors to PVC) and it’s defined as detecting of the presence of a stimulus. Perception involves the last 2 levels of the hierarchy and it’s defined as integrating, recognising and interpreting the sensation. It’s a high order process. One is able to sense and not perceive and vice versa.
What are the 6 main parts of the eye?
Retina, blind spot, pupil/iris, lens, fovea and the optic nerve.
Describe the process of vision in the eye
Light enters through the pupil and the iris (a contractile tissue) regulates the size of the pupil. The iris contracts when there is a lot of light and it gives your eye the characteristic colour, e.g. blue eyes. Once it has entered the pupil, it goes through the lens. This focuses the incoming light and turns it all upside down so light from the bottom is projected to the top of the retina and left to right is also opposite. The retina contains the specific receptor cells as well as 4 types of neurons. The retina has a blind spot, where the signals leave the eye and this is filled in by the brain.
Describe the layout of the retina
There are 6 layers, the top layers are transparent and the back layers process the light. The light firstly hits the retinal ganglion cells which leaves the eye in one bundle called the optic nerve (where the blind spot is), then there’s the amacrine cells, followed by the bipolar cells, then the horizontal cells. After this is the cone receptors and the rod receptors. When the light reaches the receptors a chain of processes occur resulting in an action potential in the ganglia.
Where is the fovea and what does it do?
It’s an area in the centre of the retina and it is specialised for high acuity vision.
What do the rod receptors do?
They respond best to faint light and there are 20 times more receptors here compared to cone receptors. Collections of rods converge to one ganglion cell.
What do the cone receptors do?
They’re found in and around the fovea. They respond best to bright light and are essential for colour vision. Each cone receptor in the fovea converges to one ganglion cell.
Discuss how rods and cones process light
They contain photopigments which release energy. The energy activates secondary messengers which inhibit the receptor cells. This initiates signal transduction across neurons to the brain.
Discuss the signal transmission from the retina to the brain
About 90% of the ganglion cells are part of the retina-geniculate-striate pathway. Light from the left visual field in each eye reaches the right visual cortex and vice versa. The axons after the eye cross near the nose, the optic chiasma, apart from the temporal parts, this segregates the visual fields, left visual field in each eye to the right hemisphere and vice versa, the part closest to the nose cross. The axons then travel through the thalamus in the lateral geniculate nucleus to the primary visual cortex, also called V1 or striate cortex. Each level of the system is organised like the retina in that the top of the visual field hits the lower part of the retina and this is maintained throughout the system as different neurons are excited.
What’s another term for edge perception?
How do we see edges?
Contrast perception
Lateral inhibition in the retina allows us to see edges. This occurs when areas of light and dark hit the retina. When the horizontal cells are activated by the areas of light, they excite the bipolar cells which inhibits the local area of horizontal cells, therefore inhibiting the bipolar cell and those adjacent to it. This means that the last cells in the area of light are most activated as they only receive inhibition from one adjacent bipolar cell. Also the first cells in the area of dark aren’t excited and are inhibited by the adjacent cells so they’re least activated. This contrast in activation has a heightened action potential because of the inhibited cells, allowing us to perceive edges.
Why do we need to move our eyes?
The eye moves about three times per second and these movements are called saccades. It’s essential for a wide angle colour perception. If our eyes don’t move then the retinal images will disappear after a few seconds. This happens because the neurons in the retina respond to signal change so if there is no change, there is no response.
How do we still see our environment as stable even though we have saccades?
This is because the visual system temporally integrates the information between saccades. This allows us to have high quality perception and ensures the world doesn’t disappear when we blink. If you fix your eye on one point then your wider vision will decrease in quality because the saccades aren’t occurring.
How do we perceive colour?
We are able to see wavelengths between 400 and 700 nm. We perceive colour because of component and opponent processing. Colour is perceived in the cones and different wavelengths correspond to different colours. Component processing is also called trichromatic theory, it involves 3 types of cones. The photopigment in each cone is specialised for either short, medium or long wavelengths. The perception of colour depends on the activity of the cone. There are three cones as one cone would confuse the processing. Opponent processing involves the neurons in the system. Complementary colours cannot exist together (red-green) because the neurons respond antagonistically. For example, a red wavelength would excite one neuron and inhibit the neuron which responds to green.
Discuss colour blindness
It’s when you cannot distinguish between certain colours due to the absence of photopigments which respond to a certain wavelength. It’s usually red or green. It’s found on the X chromosome and is therefore mainly found in men. To test for this you can use the Ishihara colour test.
