Visual System Flashcards
How does physiological coding of sensory experience work?
stronger stimulus evokes more action potentials, and as you push MORE, the number goes up linearly, stronger stimulus is perceived as stronger
How do low level processes get converted into complex processes? What are some examples?
Low level:
- orientation
- colour
- contrast
- disparity
- movement
Complex:
- contour integration
- surface properties
- shape discrimination
- surface depth
- surface segmentation
- object motion/shape from cues
What is the wavelength of light our eyes can detect?
400-750 nm roughly (purple-red)
Heat emits a longer wavelength of light that we can’t see but some animals can!
What are the various parts of your eye? What detects light in the retina?
Retina: detector like in a camera
Photoreceptors are scattered across the whole thing to detect light.
Lens: focuses light onto fovea
Bipolar and ganglion cells: bring information to the optic nerve, must be moved (pushed to the side) in order to access photoreceptors at the fovea
What is a receptive feild? Tell me about them.
Region of space (somatosensory, tonal, visual, chemical) that elicits change in firing
- packed tighter in the fovea
- periphery cells are more sparce
- circular patches are feilds OF a particular ganglion cell: light hits one of the photoreceptors and the info converges onto the ganglion cell
-receptive fields are smaller in the fovea
A photon of light, anywhere it hits activates the same ganglion cell in that receptive field
Same as for skin!
what are the degrees for receptive fields in periphery vs. fovea?
periphery: 10 degrees
fovea: 0.1 degrees
What are the two major classes of photoreceptors?
RODS AND CONES
Cones: fovea primarily, higher resolution, requires more light
Rods: periphery, rod based vision is lower resolution but better night vision.
Higher convergence of photoreceptors in Rods (more rods per ganglion cell, allows for smaller stimulus to help spatially sum and reach threshold)
What is the distribution of rods/cones in the eye?
6 million cones
100 million rods
ALl bundle togetehr and go through the optic nerve through optic disk
What is your blind spot?
the optic disk/where the optic nerve bundles together and goes out !!
When you have both eyes open this blind spot is covered for
Support the statement “different cones are sensitive to different wavelengths”
S cones, M cones, L cones etc. All have different pigments that absorb different wavelengths of light
Simultaneous activity in these cones is what colour vision is!
POPULATION CODING
- if you only had one cone type you couldn’t differentiate colours, but when you have them at the same time, the cone will be able to tell because it will absorb more of one wavelength and less of another!
Absorbing this light promotes changes in NTs and neurological activity
How does night vision work?
exclusively rod based, therefor colour is not perceived, it is all grey
maximally sensitive to light around 500 nm
What are on-center receptive fields and how do they work?
Region of the receptive field that is sensitive/non sensitive to light.
If you fire on the “on” area, you will see action potentials
If you fire on the off area, there will be no potentials, but as soon as you REMOVE the stimulus from the ‘off’ section, APs will fire because of RELATIVITY: removal of the light is like administering darkness
If you cover the whole thing with light: nothing happens becuase you need to see contrast. a lot of light/lot of darkness is essentially the same
If you cover half of both areas, you will get unsteady firing, a bit of imbalance
What is contrast enhancement and how does it work?
When viewing colours next to each other that are slightly different, there is a phenomenon known as “mach bands” and they are spots where light is a little more bright/dark on either side of the contrast. This is due to LATERAL INHIBITION
-sharpens attention and makes it more clear
The intensity is the same for most of the colour (same level of inhibiton from neighbouring neurons) HOWEVER when next to another colour, the inhibition is higher/lower than usual so it exaggerates that receptor’s response to the light because it is more/less inhibited
This was initially measured in crabs
Ask becca about that slide in visual systems that is highlighed “
contrast enhancement via. lateral inhibiton
What is the difference between on center and off center receptive fields when a dark object appears? why is this important?
on center: when a dark object appears, APs fire at a lower frequency than baseline: the brain will realize that there is a signal MISSING from baseline firing but takes about 100 ms to detect (reflexes)
off center: when a dark object appears it fires at a higher frequency, this allows you to detect change faster!
Beneficial: you can detect changes in contrast quickly, more activity isn’t always ‘better’ the brain cares about changes in activity
you want to know if there is change happening in front of you ASAP
