Midterm 2: Lectures Flashcards
How to calculate the probability of two events
P(A&B) = P (B|A) * P (A)
P (A&B) - Probability that A and B both happened
P (B|A) - Probability that B happens, given that A happened
What is the minimum number of quanta per rod is needed to produce a meaningful visual signal?
1 quantum per rod
input signals arrive at ____________
dendrites
Output signal travels down the ________
axon
Inputs integrated by the __________
neuron
_____________ at the end of the axon
provide input to other neurons
Synapses
Neurons transmit __________ electrical signals
wet
- cause depolarization
- Increases the likelihood of an action potential
Excitatory transmitters
- cause hyperpolarization
- Decreases the likelihood of an action potential
Inhibitory transmitters
The first cells in the visual pathways that fire action potentials are the retinal ________________.
ganglion cells
What are retinal ganglion cells tuned for?
Spot size
What is a visual neuron’s receptive field?
the area of the retina where presenting light can affect the cell’s firing
How are Mach bands explained by retinal ganglion cells?
lateral inhibition; Retinal ganglion cells are sensitive to changes in light intensity due to their center-surround receptive fields, which are organized as either on-center, off-surround or off-center, on-surround configurations. This arrangement allows the cells to enhance contrast by responding more vigorously to edges where light and dark areas meet. When light falls on the receptive field’s center, it activates the cell, while light on the surrounding area inhibits it. At the edges of different luminance, the cells on the lighter side receive more stimulation in the center with less inhibition from the surround, while the cells on the darker side experience more inhibition. This differential activation causes an exaggeration of brightness at the light edge and darkness at the dark edge, leading to the Mach band effect.
Smaller RFs, slower responses
Midget ganglion cells
Larger Rfs, faster responses
Parasol ganglion cells
Why are rods and cones important?
- Anyone cell type can’t handle the full range of vision
- bandwidth: need multiple classes to divide and conquer all the visual info
- Energy efficiency: more energy, the faster you send the single
Pathway of visual fields
retina -> optic nerve -> optic chiasm (where crossing occurs) -> LGN -> visual cortex
Which part of the visual field do you see in your left eye?
Temporal retina stays the same -> left hemisphere
Nasal retina crosses -> right hemisphere
Which part of the visual field do you see in your right eye?
Temporal retina stays the same -> right hemisphere
Nasal retina crosses -> left hemisphere
Which hemisphere is the right visual field processed?
Left hemisphere
Which hemisphere is the left visual field processed?
Right hemisphere
left and right cut
Saggital
front and back cut
coronal
top and bottom cut
Axial
inability to recognize faces
Prosopagnosia
a partial or total absence of color vision
Achromatopsia
DIfficulty in perceiving movement
Akinetopsia
Inability to read/see words
Alexia
Deals with pattern identity, the WHAT pathway
Ventral
deals with spatial relationships and motion, the WHERE pathway
Dorsal
causes a person to have a deficit in awareness and attention on one side of their space, damage to the Dorsal pathway
Hemineglect
What hemisphere is affected by hemineglect?
RIGHT hemisphere
Hypercolum
- A single location column
- A complete set of orientation columns
(0 to 180 degrees) - Left and right dominance columns
When presented with an annulus (a ring of light), a retinal ganglion cell will typically respond by?
inhibiting its firing rate if the annulus stimulates the “surround” area of its receptive field
The idea that what the spiking activity of a neuron means in terms of its eRect on perception is determined not by the nature of the activity itself (which is basically the same for all neurons) but rather by how the neuron is
connected to sensory input and other neurons.
Muller’s Law of Specific Nerve Energies
A more specific version of Muller’s Law above. If you could stimulate
just a single neuron and no others, the percept would be just what that neuron is responsible for representing in the external world - the stimulus that best drives a response from the neuron.
Neuron doctrine
A period during development during which abnormal visual experience has a long-term effect on visual function or cortical properties. Not all aspects of vision have critical periods - the
photoreceptors develop normally without visual experience.
Critical period for a visual function
What are the advantages of color vision?
Finding objects, segmenting one part of scenes from another, judging object properties including fruit ripeness, emotion from facial color, signalling (traffic lights, poison frogs). We also discussed disadvantages (tradeoffs with rods and nocturnal vision, might take more energy for the brain to process color information).
Behavioral trichromacy/color matching experiment
Humans with normal color vision
are able to match any color light presented by combining three primary colors together with
appropriate intensities of each. We simulated this process using a spreadsheet, both in a
later lecture and on a HW problem. Two primaries is not enough.
There are three classes of cones with different spectral sensitivities. These are the long (L), middle (M), and short (S) wavelength sensitive cones. Each class has a different spectral sensitivity. When the isomerizations of the three classes of cones to two different lights are the same, the lights match
Biological trichromacy
Most non-primate mammals are dichromats along the lines of
human red/green color blindness. A few are monochromats. Mammals likely had tetrachromatic ancesters, but lost two photopigment types and became dichromatic during nocturnal phase of their evolution. However, some primates re evolved trichromacy later on.
Mammalian color vision
What are some factors that might drive the variation in color vision across species
Depth in water (affects range of wavelengths where there is substantial light power), need to find
specific foods (for primates, fruit), whether you are nocturnal or not (harder to support
color vision processing at low light levels)
one way we think that the visual system achieves color and lightness constancy
Simultaneous contrast
- the structure and vocabulary of a language influence how its speakers perceive and think about the world.
- people who speak different languages may experience the world differently due to the linguistic structures available to them.
Linguistic Relativity (Whorf hypothesis)
Explain who Patient MM is
Blind at age 3 - lost one eye and damage to cornea. Reports no visual
memories or imagery. Stem cell transplant in the remaining eye at age 43 restored sight. (2003 paper). Like SB, MM had deficits also along the Gestalt Laws (continuity, transparency, etc). Did not improve much after retesting 10 years later. We have measurements of MM’s contrast sensitivity function which reveals he had reduce acuity, and measurements of his brain response in V1 which indicated he did not have cortical magnification of the foveal representation. The latter may be part of the reason his acuity is low.
Describe Patient MM’s ventral pathway
MM’s ventral pathway does not contain normal face and object-selective regions. This may underly his deficits in his ability to recognize faces and facial expressions, judge gender, and recognise objects.
Probabilities sum to 1
P(x) + P(~x)=1
Conditional Probablity
P(y|x)=P(x,y)/P(x)
so P(x,y)=P(y|x)p(x)
Bayes Rule
P(x|y)=P(y|x)P(x) /P(y)
Where P(x) is the prior, P(x|y) the posterior, and P(y|x) the likelihood and x is generally a hypothesis and y is data or percepts.
Independence Assumption
P(x,y) = P(x)*P(y) if and only if x and y are independent
Conditional independence assumption
P(x,y|z) = P(x|z)P(y|z) if and only if x and y are conditionally independent given z
- the idea that all languages share a common underlying structure, or universal grammar.
- Language is shaped by human cognition
- linguistic differences are surface phenomena that don’t affect the brain’s cognitive processes.
Linguistic Universalism (Chomsky)
Marginalization
P(x) = P(x|y)P(y) + P(x|~y)P(~y)
