Physiology of Vision Flashcards
Describe the direct (vertical) for signal transmission of the retina.
Photoreceptors > Bipolar cells > Ganglion cells.
Describe the role of horizontal cells in the retina.
- Receive input from photoreceptors and project to other photoreceptors and bipolar cells.
Describe the role of amacrine cells in the retina.
- Receive input from bipolar cells and project to: ganglion cells, bipolar cells and other amacrine cells.
Light converts 11-cis-Retinal to what?
all-trans-Retinal (activated form).
Rhodopsin is made up of what?
Opsin + 11-cis-Retinal.
Another name for all-trans-RetinOl?
Vitamin A.
What facilitates seeing in dim light?
Rods.
What facilitates seeing in normal day light?
Cones.
Convergence in the rod system results in what?
- Increasing sensitivity to light.
- Decreasing visual acuity.
What forms the basis of colour vision?
- Cones.
- Different opsins for discrete wavelengths.
Short-wavelength cones see which colour?
Blue.
Medium-wavelength cones see which colour?
Green.
Long-wavelength cones see which colour?
Red.
Rods see which colour?
None.
Describe rods of the retina.
- Achromatic.
- Peripheral retina.
- High convergence.
- High light sensitivity.
- Low visual acuity.
Describe cones of the retina.
- Chromatic.
- Central retina: fovea.
- Low convergence.
- Low light sensitivity.
- High visual acuity.
Describe lateral inhibition.
- Increases contrast and sharpness in visual response by exaggerating the difference in detected stimulus intensity by adjacent neurons.
- Aids in localisation.
Lateral inhibition improves what?
Stimulus localisation.
Horizontal cells connect “surround” neurons around the centre of each ganglion. This samples excitation in the surround and responds by releasing what?
GABA.
- If low surround, less GABA released.
What function does centre-surround organisation and lateral inhibition serve?
Emphasises areas of contrast i.e. sharpening boundary between objects of different luminance.
Our visual system detects local differences in light what?
Intensity, not absolute amounts of light.
Lateral inhibition modifies receptive fields of ganglion cells to have what?
Centre-surround organisation.
Describe parallel processing of retinal output.
Simultaneous input from both eyes relays information to be compared in the cortex to allow for judging the depth and distance of an object.
Describe “on-centre” cells of the retinal ganglia.
Stimulated when centre of its receptive field is exposed to light. Inhibited when the surround cells are exposed to light.
Describe “off-centre” cells of the retinal ganglia.
- Stimulated when surround cells are exposed to light.
- Inhibited when centre of receptive field is exposed to light.
Each eye sees what part of the visual space?
- Monocular visual field (+/- 45o).
NB the visual fields overlap to create a binocular visual field (+/-45o).
The retina is divided in half relative to what?
The fovea.
The retina is divided into what?
- Nasal and temporal hemiretina.
Nerve fibres from the nasal half of each retina cross over where?
The optic chiasm.
Which optic tracts cross over at the optic chiasm?
Nasal (60%).
The temporal (40%) do not.
Around half of the visual cortex is dedicated to information from what?
The fovea.
The visual cortex is said to have what kind of organisation?
Visuotopic.
With regards to input to the Striate cortex.
Magnocellular LGN neurons project to where?
Layer IVC-alpha.
With regards to input to the Striate cortex.
Parvocellular LGN neurons project to where?
Layer IVC-beta.
With regards to input to the Striate cortex.
Koniocellular LGN axons project to where?
Bypass layer IV to synapse in layers II and III.
How may the synapses of koniocellular LGN axons in layers II and III of the striate cortex be observed?
As “blobs” following cytochrome oxidase staining.
The first binocular neurons are found where?
In the striate cortex - most layer III neurons are binocular (but not layer IV).
Cortical receptive fields contain what type of neurons?
- Orientation selective neurons.
Cortical receptive fields respond to what?
Light/dark bars or edges - ONLY if in a particular orientation.
Peak frequency of Cortical receptive fields depends on what?
Angle and preferred orientation.
Retinal synapses in LGN are not affected by deprivation, why?
Because they are monocular.
In the cortex, monocular deprivation during a “critical period” in development results in what?
Active afferents in one eye and lower activity from the other.
Sensory deprivation early in life can alter the structure of what?
The cerebral cortex.
