Low level Vision Flashcards
Define the visual system
It is hierarchically organised. It is passed on from:
1. Retina
2. LGN (lateral geniculate nucleus)
3. Primary visual cortex
4. Through other cortical revisions specially for vision
What is perception?
A selective representation of reality= created by the brain + constrained by particular neural systems + sensory organs
Examples of direct methods to study perception
Brain imaging and electrophysiology
Example of indirect methods to study perception
Psychophysics
What do direct and indirect methods to study perceptions help us to do?
Study structure and function of visual systems
What does the retina do?
Converts light reflected from objects into neural signals through rods and cones
Where can phototransduction be found?
Within the outer segments of rods and cones
What is phototransduction?
Photosensitive pigment molecules that can change shape in response to light to start intracellular signalling cascade which causes an electrical signal
What is a bipolar cell?
Provides main pathways from photoreceptors to ganglion cells.
They cause an opposite electrical response in the bipolar cell.
They do this through:
Direct connections to surrounding photoreceptors w/ small no. of photoreceptors
Indirect connections via horizontal cells
Describe the retinal circuity
- Cones and rods connect to the bipolar cells
- Bipolar cells connect to the output cells of the retina (retinal ganglion)
- Horizontal cells and amacrine cells process visual info horizontally, laterally across the retina
Define a neuron’s receptive field
An area on the retina. This causes a response in the neuron when photoreceptors in that area are stimulate by light.
What does the retina circuitry look like?
Retina = backwards
Rods and cones = bottom
Dip = optic disc part of the fovea to get ↑ light
Other processing layers = towards the surface of the retina
How do bipolar cells get light?
Bc other processing layers, bipolar cells rely on direct connections from the cones above it + indirect connections horizontally
Describe the process of on-centre bipolar cells
Light on photoreceptors found in centre of receptive field = excitatory response
Light on photoceptors found in surrounding receptive field = inhibitory response
What of on-centre bipolar cells preferred stimulus?
bright spot of light in dark background
- Off centre bipolar cells
Light on photoreceptors found in centre of receptive field = inhibitory response
Light on photoceptors found in surrounding receptive field = excitatory response
What is off-centre bipolar cells preferred stimulus?
- Preferred stimulus = dark spot on a bright background
What is a receptive field?
Area on the retina + causes a response in neuron when photoreceptors in that area = stimulated by light.
What is a retinal ganglion
The output cells of the retina. They send action potentials to the brain.
Describe the fading example
FADING EXAMPLE
If you fixated on a blue dot, the fuzzy blob fades from perception = invisible.
Fuzzy blob = poor stimulus for retinal ganglion cell
Equal amounts of light = received by centre + surrounding receptive field
Causes excitatory + inhibitory responses to cancel + ganglion cell ≠ signal presence of stimulus
What are the two classes of the retinal ganglion cell?
Midget cell
Parasol
Represent the start of 2 independent parallel processing streams = remain segregated in what + where pathways all the way through the visual processing hierarchy
What is a midget cell?
a. Small receptive fields
b. Not sensitive to fast flicker
c. Red-green colour selective
d. ‘What’ stream
What is a parasol stream?
a. Large receptive fields
b. Sensitive to fast flickers
c. Not colour selective
d. ‘Where’ stream
What does edge information mean?
Signalled by retinal ganglion cells.
Info sent from the brain = distorted version of the info in the retinal image.
What does edge information show about perception?
Shows perception not just pixel mapping of retinal image but + reconstructing of external world from distorted input signals.
Define lateral geniculate nucleus (LGN)
Part of the thalamus.
Relay station, sending visual signals from retina to cortex.
Neurons have a centre-surround receptive field structure similar to retinal ganglion cells
Define the Lateral geniculate nucleus
Separate visual streams from the retina are separated to diff. anatomical layers in the LGN.
Define the Primary visual cortex
Discovered by Hubel and Wiesel.
Structure of receptive fields of neurons in V1 is elongated + preferentially respond to orientate lines/ edges
What are simple cells ?
They are orientation selective cells in V1.
The receptive fields of V1 (primary visual cortex) cells = elongated bc they wire together retinal (+ LGN) cells w/ neighbouring receptive cells.
What are grandmother cells?
A neuron that responds only to a highly specific stimulus
What is a criticism of the evidence supporting grandmother cells?
Quiroga et al. (2005) found cells in human temporal cortex responded selectively to images of one person or object
Only presented 16 diff. objects to each P = statistically unlikely to hit grandmother cell
Results = consistent w/ a sparse code = a network of neurons represent each concept, w/ connections between the networks of associated concepts.
What does retinotpy mean?
The primary map in the visual cortex.
What are operation channels?
Information about diff. orientations – signalled by diff. pops of neurons
What does spatial frequency coding mean?
Diff. pops of V1 neurons = sensitive to diff. spatial freq.
Human sensitivity to contrast varies w/ spatial freq.
if the visula system is adapted by exposure to a particular freq. = reduction in contrast specificity
if there is a reduction, does sensitivity to other spatial frequencies change?
No
What type of neurons show a reduction in contrast sensitivity?
Adapted neurons
What does contrast sensitivity function mean?
Describes human it as a function of spatial freq. and peaks at medium spatial freq.
Caused by combined responses of no. of diff. neutrons that are sensitive to diff. ranges of spatial frequency
How does the receptive field structure increase in complexity?
Builds through the visual processing hierarchy via wiring together neurons at lower levels
Theoretical pinnacle = grandmother cells w/ receptive fields that are spec. for 1 indvdl object
Cells in area TEO (near IT) in the ventral stream = moderately complex receptive fields
How are neutrons in the visual cortical areas mapped?
Retinotopically mapped
a. Neighbouring neurons respond to neighbouring regions on the retina (or visual field)
Similar to other systematic organisations in the cortex e.g
a. Somatotopy
b. Tonotopy
How is spatial coding and population linked
It is encoded by a population code = over a pop. of neuronns sensitive to different spatial. freq
What are spatial frequency channels?
Neurons selective for low spatial frequencies encode broad patterns of light and dark,
while neurons selective for high spatial frequencies encode finer detail.
What is V1?
Primary visual cortex
Where is motion processed?
Cortical area (V5)
What is the waterfall illusion?
Motion is encoded via an opponent code. After
adaptation in one direction, illusory motion is perceived in the opposite direction.
What are Reichardt detectors?
Detecting motion in the fly, and motion in humans is probably encoded via a similar mechanism.
Describe the process of Reichardt detectors starting from location 1
Location 1 on the retina is connected to neuron A via a delay + location 2 is directly
connected to neuron A. If a stimulus moves from location 1 to location 2, a response =
triggered.
Combined inputs from both connections, neuron A is active = excite the output neuron.
Describe the process of Reichardt detectors from location 2
Neuron B has connections of the opposite pattern and is not activated when a stimulus
moves from location 1 to location 2.
If a stimulus moves from location 2 to location 1, neuron B receives the converging
inputs.
Neuron B activated = inhibits the output neuron.
opponent code = waterfall illusion – after exposure to rightward motion neuron A adapts and its response fatigues. When no motion, inputs to the output neuron = unbalanced + overall inhibition from neuron B, consistent with leftward motion. Illusory leftward motion is perceived.
