Sensory system: Vision Flashcards
What does it mean to say that vision is based on interrelationship?
Just like in a melody, we recognize not the sequence of notes, but there interrelationship.
What is meant by a ‘winner-take-all’ perceptual strategy?
only one part of the image can be selected as the focus of attention
Why is vision called as a creative process?
Gestalt psychology: What we see represents the organization of sensations by the brain.
Max Wertheimer: The brain makes certain assumptions about what is to be seen in the world, expectations that seem to derive in part from experience and in part from the built-in neural wiring for vision.
What are the assumptions about the visual objects?
1) We fill in what is not there
2) Muller-Lyer illusion: We judge the size from the shape
3) Object recognition: Filling in the missing details
How is the size of an object perceived?
The size depends on other objects in the visual field.
What is meant by occlusion?
Making sense of the patterns
What are rods and cones?
Rods: sensitive to light, low spatial resolution, slow recovery of current, many rods connect to one bipolar cell
•Cones: very high spatial resolution (visual acuity) and not sensitive to light; mediate color, have a sharp response, one-to-one connection to bipolar cells.
What are the simillarities of rods and cones?
- contain photoreceptors (activated by light)
What are the differences of rods and cones?
- size and shape
- range of luminance: rods more sensitive than cones
- fovea: distribution of cones and rods
- connections to bipolar cells (Cones are responsible for visual acuity: one-to-one connection to bipolar cells. But many rods on one bipolar cell)
How does phototransduction of dark current work?
In the absence of light, cation channels in rods and cones are kept open by intracellular cGMP and conduct an inward current, carried largely by Na+.
What are the principal steps of phototransduction?
(1) Light is absorbed by and activates pigment molecules (opsin or rhodopsin in rods)
(2) The activated pigment stimulates a G protein (transducin), which in turn activates cGMP phosphodiesterase. This enzyme catalyzes the breakdown of cGMP to 5-GMP.
(3) As the cGMP concentration is lowered, the cGMP-gated channels close, reducing the inward current and causing the photoreceptor to hyperpolarize.
How do rods and cones respond to light?
by graded hyperpolarizing response
How is color vision established?
3 kinds of cone opsins sensitive to different light wavelengths
When are you color blinded? (dichromacy)
red-green color blindness due to the absence of either M or L cones
Name the aspects of S, M and L cones.
S cones: 5-10% , absent in fovea
•Ratio of M to L cones differs largely from individual to individual (no impact on color perception)
How does the cone system represent colour?
- 3 kinds of cone opsins sensitive to different light wavelengths – short (S cones) medium (M cones) and long wavelengths (L cones)
- Not exactly the same as blue, green and red colours but very close to these colour ranges
- Combined activation of different cones and higher level processing in the brain allow us to see multiple colours
How can we detect luminance contrasts?
on-center and off-center ganglion cells detect luminance contrasts
What is receptive field?
part of visual field that one cell monitors – can be seen as having a center and surround.
What cells constitute retinal circuitry?
rods and cones bipolar cells ganglion cell amacrine cells horizontal cells
What happens to the cells that constitute retinal circuitry when the light falls on the center of their receptive field?
Center cone hyperpolarizes, bipolar on-center cell depolarizes, bipolar off-center cell hyperpolarizes, on-center ganglion cell fires action potentials, off-center ganglion cells shut down
What happens to the cells that constitute retinal circuitry when the light falls on the receptive field’s surround?
Center cone depolarizes, bipolar on-center cell hyperpolarizes, bipolar off-center cell depolarizes, on-center ganglion cell shuts down, off-center ganglion cells fires action potentials
What is the role of glutamate on mGluR6
-> - depolarisation
What is the role of glutamate on AMPA kainate?
-> + hyperpolarisation
What is the main usefulness of center-surround receptive fields?
emphasize edges
What are the function of horizontal cells?
Center-surround antagonism in light adaptation
Light on center: Horizontal cells
horizontal cells release an inhibitory transmitter that maintains cones in a slightly hyperpolarized state (light transduction)
Light off center
hyperpolarization of the horizontal cell reduces the amount of inhibitory transmitter and these cones become depolarized (opposite of light transduction)
Which cells are responsible for light adaptation?
Horizontal cells
How do … cells achieve light adaptation?
Off-center cones influence the signal in on-center cones through inhibition by horizontal cells.
Light off-center deactivates inhibition in horizontal cells (through hyperpolarization in off-center cones). This reduces the amount of inhibitory transmitter and these cones become depolarized (opposite of light transduction)
Summary
Rods and cones contain photoreceptors activated by light
•Dark current: cation channels gated by cGMP are open, cells are depolarized
•Light: reduction in cGMP through G-coupled photoreceptors, closing of cation channels, cells are hyperpolarised
•Rods: sensitive to light, low spatial resolution, slow recovery of current, many rods connect to one bipolar cell
•Cones: very high special resolution and not sensitive to light (visual acuity), color, sharp response, one-to-one connection to bipolar cells.
•On and off-center cells for contrast detection
•Light adaptation: the intensity of spot illumination depends on the background illumination. Much of the surround antagonism arises via lateral connections by horizontal cells
