Vision Flashcards
Characterisitcs of light
wavelength and intensity
Intensity
Number of photons hitting ur eyes per second
This change is perceived as a change in brightness
What is optic array
What determines the optic array
Is the spatial distribution of light
It is detemiened by the position of sources of light (eg sun) and position of the reflectors of light (eg any visible objects)
Pathway from light hitting obejct to perception and recognition of object
Light –> eye/retina –> Optic Chiasm –> LGN –> Striate cortex –> extrastriate cortex –> Perception of object
Important function of the visual system
Convert pattern of light on the retina into a perception of a three dimensional world
(eg another function is control of action)
Computational approach and key issue of this approach
Perception as information processing
treating image as information where there are areas of high (eg eye) and low (blurred solid background) information
Key question: how and where is information encoded at different levels of visual system
Principle of Least commitment (Marr & Barlow)
Dont throw away any information because you might need it later
Principle of graceful degradation (Marr & Barlow)
If a system were to break down it should still be useable eg if one eye breaks u still have another one, same with ears
Principle of least redundancy (Marr & Barlow)
Encode informrion in the most efficient way possible, because we have limited cpaapcity and dont want to waste energy storing info that is not iportant or necessary
Strucutre of visual system in the eye:
Retina -> blood vessels -> Rods and cones activated -> signal goes to bipolar cells -> ganglion cells -> optic nerve -> leaves eye
Phototransduction
converting light into electircal signal to
Absorb photons of light –> exposure to light changes the visual pigment molecuels shape which acts as a switch alowing fir chenical reaction
Metamers
Different distrituons of light but activate the cones in the same relative amounts
Receptive field
Region of retina that when stimulate influences the firing rate of the neuron
A region of space to which a particular sensory neuron responds.
Spectral sensitivity
Different photorecptora are sensitve to different wavelenghts (S, M,L)
Visual Electrophysiology
Tiny electrodes placed in or close to visual neurons
Visual stimuli presented to animal, and electrical signals/action potentials are recorded (spike)
Center-surround structure
found in ganglion cell
if in center then excitatory but if around htr center then inhibitroy to neuron firing
Signal more likely to fire if its in the center bit of the structure
So where there is high information (eg eyes) there will be increased firing rate –> finds intensity differences in image ur looking at
Finds intensity edges- the edge between high info and low info centers
Key- they emphasize edges
Color-opponenet ganglion cells
Black-white
Blue-yellow
Red-green
One color will exicte the neuron while the other will be inhibited
Lateral Geniculate Nucleus (LGN)
2 LGN (1 in each henmispehre)
Information coming from ipsilateral (same side) and contralateral (opposite side) eye
Retinoscoptic organization
Where light is hitting retina
This is mapped out and organized by there light is hitting retina
Layers of LGN
6 layers encompassed by:
Parvocellular layers(outermost) - red and green cones
Koniocellular layers - blue/yellow
Magnocellular layers(innermost) - black and white
Parallel processing
Fine detail, red-green colour (P pathway)
Blue-yellow colour (K pathway)
Fast temporal changes (M pathway)
Striate Cortex aka V1
Stripey (striate)
More refined than LGN but purpose is still debated
We are blind without it
I closest to skull, VI furthest from skull
porbbaly not seat of vision
Simple Cells (Hubel & Wiesel, 1981)
In striate cortex
Elongated receptive field (not circular) responsive to bar shape
Orientation tuning - as you rotate bar of light the reponses of neuron changes
But unlike the LGN cells, they have orientation selectivity rather than center-surround visual fields.
Hubel and Wiesel (1959) found that elongated stimuli that looked like bars seemed to be particularly effective stimuli for these cells. Indeed, they found that some cells wanted a dark bar on a light background and others that responded to a white bar on a dark background.
Complex cells (Hubel & Wiesel, 1981)
In striate cortex
Complex cells are also neurons in V1 that respond optimally to a stimulus with a particular orientation. But, unlike simple cells, they respond to a variety of stimuli across different locations. For example, a complex cell will respond to a dark bar on a light background and a light bar on a dark background. In contrast, a simple cell only responds to one but not the other.
Complex cells respond best to moving stimuli.
End-stopped cells (Hubel & Wiesel, 1981)
In striate cortex
Why is there a belief there might be a hierarchal structure in the visual system?
Center surround structure in ganglion and LDN—> tie them together and get simple cell –> tie simple cells together to get complex cells
Extrastriate Cortex
Beyond striate cortex/ v1, receptibve fields get bigger and more specialized
As receptoive feield gets bigger, precise retinotopic position becomes less relevant
Lots of other parts of brain reposnidn to visual input
Depth perception in V1
Having 2 eyes = axons from both meet at a certain place and our brain takes that into account and this gives us depth perception. Missing one eye means we will lack depth perception
