Lecture 1 Flashcards
Cell order from retina to visual cortex
Rods/Cones -> Bipolar cells - > Ganglion cells -> Optic Nerve -> Optic Chiasm -> Lateral Geniculate Nucleus -> Visual Cortex
Rhodopsin/Photopsin
This is a protein which converts light into closing of the Na+ channels so that the membrane hyperpolarizes -> neural signal sent to bipolar cell and in turn the ganglion cell
Retinal colour blindness
Absence of a particular cone type.
3-8 out of 100 people have this.
Macula
Area around the fovea. Has the sharpest vision
Macular Degeneration
Symptoms: Blank spots, Distorted central vision, and Acuity loss
Causes: Old age, smoking, diet, genetic
Receptors (pigment epithelium) are lost due to toxic products
No treatment
Pigment epithelium
At the back of the retina and these cells prevent the light from scattering. Cats have reflecting PE which means better low light vision but unsharp image.
Glaucoma
- Increase pressure inside the eye
- Damage of nerve fibers of the RGC’s: optic nerve
- Loss of peripheral vision first
- Treatment: eyedrops, surgery
The need for visual data compression
The brain needs to compress visual data because there are 130M photoreceptors and only 1M nerve fibers
Photoreceptors responding to light and dark
When hit with light it hyperpolarizes (closing of Na+ channel).
When hit with dark it depolarizes (opening of Na+ channel).
ON and OFF system of bipolar and ganglion cells
Bipolar cells with ON systems have sign inverting synapses which turn hyperpolarization into depolarization and send this signal to an ON ganglion cell. These cells respond to light
Bipolar cells with OFF systems have sign conserving synapses which send the depolarization to the OFF ganglion cells. These cells respond to dark.
Glutamate is used to convert hyperpolarization to depolarization.
Horizontal cells
These cells provide negative feedback on the receptors.
Ganglion cells and contrast
Ganglion cells respond especially well to contrast and not so much diffuse light. Luminance is discarded.
This causes the perception of luminance to not always be correct. This is demonstrated in visual illusions like with the gray squares and shadows.
Rods connecting to bipolar cells
Rods are connected to Rod Bipolar Cells and those are then connected to cone driven Bipolar Cells via Amacrine Cells. This means that Bipolar and RGC’s have receptive fields from cone and rod inputs.
Dark adaptation
- Pupil dilation
- Cone to rod transition
- Bleaching of pigment in photoreceptors becomes undone
- Less receptor signal means less negative feedback from horizontal cells.
Retinitis Pigmentosa
- Genetic disorder
- Progressive degeneration of receptors: first rods then cones
- Pigment deposits at affected parts of retina, depigmentation at vulnerable sites
- Night blindness -> loss of peripheral vision -> tunnel vision - > full blindness
- No Cure
2 types of ganglion cell
- Midget cell (X-type)
- Parasol cell (Y-type)
Midget cells (X-type)
- Small receptive field
- Single cone center and surround: colour
- Slow sustained responses
- Tuned to high spatial frequencies
Parasol cells (Y-type)
- Large receptive field
- Many cones input to center and surround: not colour selective
- Fast transient response
- Tuned to low spatial frequencies
Colour coding in the retina
Ganglion cells have a group of cones that are Red vs Green, Green vs Red or Blue vs Yellow
High and low spatial frequency
High spatial frequency shows outlines of images very wel and has contrast but not a lot of the overall shape very well.
Low spatial frequency is not very well outlined but you see the overall image but just a vague version of it.
Spatial frequency sensitivity
This depends on the contrast and brightness.
Spatial frequency sensitivity of RGC’s
Center part of the RGC receptive field is sensitive from low to high SF’s. Surround from low to intermediate SF’s. When combined this gives the characteristic SF tuning of RGC’s.
RGC projection to the brain (X and Y)
Midget Cells -> Parvocellular layers of the LGN (M -> P)
Parasol Cells -> Magnocellular layers of the LGN (P -> M)
Navon task (hierarchical letter stimuli)
Subjects have to detect either the global target (large letter or shape) , or the local target (small items).
Global targets are detected quicker than local ones. Also congruent stimuli are faster than incongruent.
In addition, global info is faster in the right hemisphere and local info faster in the left.
