Week 2 Lecture 4: Visual Processing Flashcards
Where are photoreceptors and where are they found?
Photoreceptors are light-sensitive neurons that convert light energy into electrical energy in cell (phototransduction) and can be found in the retina
What are the two types of photoreceptors in the retina?
Rods (low light) and cones (bright light and colour vision) (more rods than cone)
Do rods and cones fire action potential?
No, although they are neurons, they do not fire action potentials but instead respond to stimuli with graded membrane potentials
What is the structure of rods and cones?
- In the outer segment, the membrane folds into a disk-like layers that contain visual pigments that respond to light
- In the inner segment, there is the nucleus and organelles for protein synthesis
- In a basal layer, there is a synapse that releases glutamate
How do photoreceptors detect light?
Using membrane-bound visual pigments, pigment molecules change shape when light hits them, this conformational change starts a signalling cascade that hyperpolarizes the cell, reducing its release of glutamate
When are photoreceptors most active?
In darkness photoreceptors produce more glutamate
What pigment is present in rods?
Rhodopsin
Where are photoreceptors most densely packed?
The macula, a central disk, especially in its central pit the fovea
What is the fovea used for?
Detailed vision
Are there photoreceptors distributed all over the retina?
No, there are no photoreceptors in the blindspot, the hole where the axons carrying visual information leave the eye (optic nerve)
Which are more sensitive, cones or rods?
Rods are more sensitive, can detect single photons.
Do cones only operate in bright light conditions? Do rods only operate in dim conditions?
Yes for both, for rods specifically, in daylight, rhodopsin is broken down so light cannot be sensed. When light go dim, rod dark adapt (they rebuild their rhodopsin stores)
How rods and cones distributed in the retina?
Cones are found primarily in the fovea and rods are found in the peripheral retina (not the fovea)
What are the three layers of neurons in the retina?
Photoreceptors, bipolar neurons, ganglion cells.
Where is convergence greatest in retina? Where is it least prevalent?
Convergence is greatest in the peripheral retina and least in the fovea where there are some receptors that synapse 1:1 with bipolar neurons
What are receptive fields?
the region of the retina where light affects the cell’s activity
What are center-surround receptive fields?
Property of bipolar neurons and ganglion cells, light affects either round center region or doughnut-shaped surround
What is the difference between on-center and off-centre receptive fields
On-center cells are excited by light in the center of their field and inhibited by light in their surround while off-center cells are excited by light in their surround and inhibited by light in their center
What do bipolar cell respond strongly to?
Contrast, when the light is uniform, the effects of the center and surround cancel each other out, leaving the cell at its resting level of activity therefore contrast is required
How do bipolar cells respond to contrast?
With graded potentials (do not fire action potentials)
What are ganglion cells?
Bipolar cells synapse onto them, they fire action potentials and have center-surround receptive fields (detect contrast)
What can be inferred from the Chevruel illusion?
That the accentuation of contrast perceived is a result of the bipolar and ganglion cells that sense contrast
How does the size of receptive fields of ganglion cells differ in different parts of the retina?
The receptive field of ganglion cells near the fovea are smaller than receptive fields of ganglion cells in the periphery because less bipolar receptors project onto them
What is the consequences of differing receptive fields of ganglion cells across the retina?
Ganglion cells in the periphery are sensitive to light (rods are in the periphery) and poor at reporting spatial detail since the receptive field are larger, blends info from many receptors
Ganglion cells in the fovea are NOT sensitive to light but have decent spatial resolution since the receptive field of ganglion cells is smaller
How are ganglion cells classified?
Based on how their signals are used in the brain
What are the classifications of ganglion cells?
Magnocellular: large, provide info that is used to infer movement, phasic
Parvocellular: provide info that is used to infer form and fine detail, phasic-tonic
Melanopsin: photoreceptors, with their own visual pigment (melanopsin), projects to suprachiasmatic nucleus (center for circadian rhythms)
How does visual information leave the retinas?
The axons of ganglion cells form the optic nerve (cranial nerve 2) which leaves the eye through the blind spot
What happens at the optic chiasm?
Nerves from the nasal half of each retina crosses over to other side of the brain
Why do nerves cross over?
It is necessary for all the info from the respective visual hemifield to come together in the respective hemisphere
What are the nerve bundles that emerge from the optic chiasm called?
Optic tracts
Where do the optic tracts end up?
In the two lateral geniculate nuclei (LGN) in the thalamus which project via optic radiation to the primary visual cortex (V1)
How many neurons are in the LGN and why is it significant?
There are 2 million neuron in the two LGN. This is significant because it is the same as the number of ganglion cells
Where is the V1 located?
The occipital lobe
How are visual area of the brain organized?
Retinotopically, neurons close together in visual areas (LGN, V1) get info from close-together regions in the retina
