3: Visual System Flashcards
1.
What is the visual system?
The visual system is the part of the central nervous system (CNS) which gives organisms the ability to process visual detail, as well as enabling the formation of several non-image photo response functions. It detects and interprets information from visible light to build a representation of the surrounding environment.
2.
Describe the Retina:
The retina is a layered structure with several layers of neurons interconnected by synapses. The only neurons that are directly sensitive to light are the photoreceptor cells.
3.
Describe photoreceptors.
- The photoreceptors (the first link in visual processing) are situated behind the neurons that connect them to the rest of the brain.
- The two classic cells that make up phororeceptors are the rods and cones.
4.
Describe the Rods
- Rods mediate nocturnal vision
- Convergent pathway to ganglion cells
- High sensitivity but poor spatial resolution(acuity)
- Rods are concentrated at the outer edges of the retina and are used in peripheral vision.
- On average, there are approximately 90 million rod cells in the human retina.
- More sensitive than cone cells, rod cells are almost entirely responsible for night vision.
Rods are far right of image -
5,
Describe Cones
- Cones mediate colour vision
- Densely packed in the fovea centralis, a 0.3 mm diameter rod-free area with very thin, densely packed cones which quickly reduce in number towards the periphery of the retina.
- Direct pathway to ganglion cells
- Low sensitivity but good spatial resolution
- about six to seven million cones in the human eye
6.
Describe the Fovea
- The fovea is a small, central pit composed of closely packed cones in the eye
- The fovea is responsible for sharp central vision (also called foveal vision), which is necessary in humans for activities where visual detail is of primary importance, such as reading and driving.
7.
List 6 steps from Light to Ganglion Cells
- Light enters eye
- rhodopsin and iodopsin (light- absorbing molecules) split apart
- change in membrane permeability of the outer segment of rods and cones (decrease of sodium NA+)
- gradual electrical potential in the photoreceptors
- bipolar cells (red in image)
- ganglion cells
8.
Ganglion cells: there are size differences - how many and what do they account for?
Ganglion cells differ in type cells (large, medium and small) and concentration (greater in the fovea)
These size differences account for:
1) processing different visual information and relay it to different parts of the brain
2) the representation of particular points in the retina
9.
Where do the axon of the ganglion cells stream towards?
axon of ganglion cells stream towards the optic disc
- exit the retina
- become myelinated
- form the optic nerve
10.
Macula cieca – blind spot what causes it?
It is the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on the optic disc of the retina where the optic nerve passes through the optic disc.
Since there are no cells to detect light on the optic disc, a part of the field of vision is not perceived. The brain interpolates the blind spot based on surrounding detail and information from the other eye, so the blind spot is not normally perceived.
11.
What is a receptive field?
The group of photoreceptors that converge onto a single ganglion cell represent its receptive field.
For example, the receptive field of a ganglion cell in the retina of the eye is composed of input from all of the photoreceptors which synapse with it, and a group of ganglion cells in turn forms the receptive field for a cell in the brain. This process is called convergence.
12.
Ganglion Receptive Fields
Receptive fields:
- Are circular & size varies from small (in the fovea) to large (in the periphery)
- Contain two zones: OFF and ON, which produce different neural activity in response to retinal stimulation.
- Two parallel pathways ON and OFF cells (crucial for contrast)
13.
Discuss receptive field centers.
Receptive fields centers may be either ON or OFF while the surround is the opposite.
ON-CENTER -> central excitatory zone and an inhibitory surround.
OFF-CENTER -> central inhibitory zone and an excitatory surround.
14.
What is simultaneous brightness contrast?
Example image below: This is a visual illusion; the two central gray squares are physically identical, one surrounded by white and the other surrounded by black. This illusion can be explained by what we know about the visual processing in the retina.
- Retinal responses depend on the local average image intensity. On the left, the background is black so the average intensity there is pretty small. Here we divide by a small number yielding a brighter percept. On the right, however, the background is white so the average intensity there is pretty large. Here we divide by a large number yielding a darker percept.
15.
Simultaneous brightness contrast:
What is the difference in firing rate between neurons in rececptive field centers of dark and light?
- Firing rate of neurons whose receptive fields intersect a contrast boundary will differ from those of neurons whose receptive fields fall entirely on either side of the boundary.
- Neurons whose receptive field centres lie just within the target on the dark background centres will fire at a higher rate than neurons whose receptive field centres lie just within the target on the light background.
- The darker are less inhibited by their oppositely disposed receptive field surrounds than the lighter.
16.
What is the central part of our visual field?
The binocular field.
The fixation point is at the center.
17.
What is the optic tract?
- The optic tract carries retinal information relating to the whole visual field.
- Specifically, the left optic tract corresponds to the right visual field, while the right optic tract corresponds to the left visual field.
18.
Is the image focused onto the retina the right way up and where is the optic chiasm?
No. The light and corresponding image is actually focused upside down onto the retina.
The superior (top) half of the visual field is processed by the inferior (ventral) retina and the inferior (bottom) half of the visual field vise versa.
The optic chiasm is the part of the brain where the optic nerves (CN II) partially cross. The optic chiasm is located at the bottom of the brain immediately below the hypothalamus
- Visual Pathways
Where is the the lateral geniculate nucleus (LGN) and what is it’s role?
Information from the left visual field travels in the right optic tract and terminates in the lateral geniculate nucleus (LGN) in the thalamus
LGN is a relay center in the thalamus for the visual pathway. It receives a major sensory input from the retina.
20.
What makes up the posterior visual pathway?
The posterior visual pathway refers to post-geniculate structures (after the lateral geniculate nucleus).
- The optic radiations, one on each side of the brain, carry information from the thalamic lateral geniculate nucleus to layer 4 of the visual cortex.
- The visual cortex - the region that receives information directly from the LGN is called the primary visual cortex, (also called V1 and striate cortex)
- Form of visual stimuli.
What are M and P cells?
M cell (Magnocellular): neurons located within the magnocellular layer of the lateral geniculate nucleus of the thalamus. Large cell body and rich dendritic ramifiaction.
P cell (parvocellular): small cell body, and limited dendritic ramification
M and P cells consist of ON-centre and OFF-centre cells
M cell have large receptive fields and track large objects, and movement
P cell have small receptive fields and track small objects, details and colour. Parvocellular cells have greater spatial resolution, but lower temporal resolution, than the magnocellular cells.
22.
How are the 6 layers of the lateral geniculate nuclei (LGN) divided up? Draw them out.
a) Three layers (2, 3 and 5) receive input from ipsilateral (same side) retina, other three (1, 4 and 6) from the contralateral (opposite) retina
b) Each layer has a retinotopic representation. A stimulus in a certain position in the space will activate cells within each layer that fall along a line perpendicular to the LGN’s surface.
c) Each layer has different cytoarchitecture (cellular composition). Axons in the lower two layers (magnocellular – M) are larger than in the other four layers (parvocellular – P).
23.
_____________ in lateral geniculate nucleus are quite similar to those found in the _________.
Receptive fields in lateral geniculate nucleus are quite similar to those found in the retina.
24.
Describe LGN receptive fields for colour and draw the combinations:
- Yellow on, Blue Off
- Blue on, Yellow Off
When a portion of the receptive field is illuminated with the colour shown, the cell’s rate of firing increases. When a portion is illuminated with the complementary colour, the cell’s rate of firing decreases.
Concentric single opponent cells -> one type of cone activates the centre and the other type of cone has the opposite effect on the surround
