Week 2: Neural processing and brain substrates Flashcards
(35 cards)
Pathway for visual processing
Vision is the most important function for humans
Light –> eye –> retina –> optic nerve –> optic chiasm –> lateral geniculate nucleus –> optic radiation –> the 1st visual cortex (V1)
The image is focused on the retina, which lines the back of the eye.
Visual receptors
Rods and cones (outer segment):
- contain light-sensitive chemicals
- visual pigments that react to light and trigger electrical signals
- distributed differently across the retina
- more rods than cones (120 - 6 million)
- peripheral retina
- optic disk –> none –> blind spot
- Fovea has many cones and no rods
Rod monochromats
Person without a cone (a genetic defect)
Macular degeneration
A condition where the fovea and surrounding are degenerate so the person cannot see whatever they are looking at.
Retinitis pigmentosa
A condition where the peripheral retina initially degenerates and causes loss of vision in the periphery (tunnel vision)
Focusing
Two-element optical system –> the cornea and the lens
If fails –> Myopia (focus before the retina; near-sightedness) and Hyperopia (focus behind retina; far-sightedness)
The cornea
The transparent covering of the front of the eye
~80 percent of the eyes focusing power
it is fixed in place so it can’t adjust it’s focus
The lens
20 percent of the eye’s focusing power
can change its shape to adjust the eye’s focus for objects located at different distances
Cilliary muscles - change the thickness of the lens
Transduction
When the light is absorbed by light-sensitive visual pigment molecules in the receptors
Visual pigments (retinal + opsins)
transduction –> fundamental for sensation and perception
act like filters
our perception is limited by these filters
More intact pigments –> more sensitive –> can see better in the dark
Izomerization
Isomerization of the retinal creates a chemical chain reaction –> activates thousands of charged molecules to create electrical signals in receptors –> activation of the receptor
Bleaching
Is the seperation of the retinal from the opsin –> visual pigment regeneration (when not in use)
Regeneration much faster in cones
Dark adaptation
A lighted place –> a dark place
- Hard to see at first –> see better after some time
Dark adaptation curves for rods and cones
- the cones are more sensitive than the rods at the beginning of dark adaptation
Spectral sensitivity
Rods and cones differ in responding to light in different parts of the visual spectrum - different wavelengths
The rods are more sensitive to short-wavelength light than are the cones
rods –> 500nm
cones –> 560nm
Rods are also more sensitive in the dark and the cones in the light.
By measuring the spectral sensitivity curve
- presents one wavelength at a time
- measures the subject’s sensitivity to each wavelength
Properties of the rod and cone visual pigments
Determine:
- the increase in sensitivity that occurs in the dark (dark adaptation)
- light sensitivity
- The sensitivity to different wavelengths across the spectrum
- spectral sensitivity
Neural convergence
126 million receptors but only 1 million ganglion cells
more convergences of rods than cones
R to G ratio = 126 to 1 (rod) and 6-1 (cone)
Convergence causes the rods to be more sensitive (to the light) than the cones
Cones, instead, have better acuity (more details)
Lateral inhibition and sensation
The capacity of an excited neuron to reduce the activity of its neighbours usually using inhibitory interneurons
This creates a contrast in stimulation that allows increased sensory perception –> clearer signals
Lateral inhibition –> modulating the amount/intensity of incoming information about a specific stimulus
The Chevreul illusion, Mach illusion, Hermann Grid Illusion
Mach bands
Light and dark bands create fuzzy borders called mach bands
Hermann grid illusion
Four squares of the grid and five receptors under the pattern.
Receptor a is located at the intersection and B, C, D and E have a black square on either side.
The receptors connect to bipolar cells. The response of each of the five receptors are 100. The initial response of the bipolar cells matches the response of the receptors.
Lateral inhibition (-10) travels to bipolar cell A and makes the signal go from 100–> 60.
Receptive field
H. Keffer Hartline (1938, 1940)
Nobel prize (1967)
Recording from a single optic fiber of the frog
* Activation of an optic fiber (ganglion cell)→by a specific area of retina
(hundreds ~ thousands of receptors)
* Receptive fields are overlapping each other (due to inter-cells)
* Activation of one single receptor→activations of several ganglion neurons
Centre-surround receptive field
On a receptive field, center and surround area respond differently to the light.
Excitatory centre, inhibitory surround
Inhibitory center and excitatory surround
Contextual modulation
Interesting phenomenon of neurons during the visual processing
Enhanced neuronal responses to stimuli placed within the classical receptive field (CRF) that are modulated by stimuli placed outside of the CRF
- condition –> the CRF must be activated before the ERF has any influence
Function –> orientation pop-up, texture-boundary segmentation, detexturization, gain control, recalibration, second-order processing, feature selectivity, color constancy and increased coding efficiency for border extraction.
LGN (lateral geniculate nucleus)
Receptive fields were found
- excitatory and inhibitory areas
One of the purposes of the LGN is to regulate neural information as it flows from the retina to the cortex.
The LGN receives more signals from the cortex than from the retina
- feedback to regulate the flow of information that is sent by the LGN to the cortex –> sensory gating and better perception?
Visual Cortex
The striate cortex (area V1)
Cells found in the striate cortex with receptive fields
- excitatory and inhibitory areas
- each cell has a specific type of stimulus for the best response (tuning curve)
Retinotoptic organisation
Simple cell vs complex cell
Columnar organisation
