Cognitive and Computational Neuroscience

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Brain Size to body size ratio

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Brain Size to body size ratio

  • Homo Sapiens have the largest brain size to body size ratio

  • Porpoise (and dolphin) are very close

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Brain size increase in hominids

  • During evolution, humans have experienced an exponential increase in brain size

  • In a simple life form: every position and function of a nerve cell is determined by genes

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Bigger Brains

  • A single byte (or 8 bits) can represent 4 DNA base pairs. In order to represent the entire diploid human genome in terms of bytes, we can perform the following calculations:

  • 6×10^9 base pairs/diploid genome x 1 byte/4 base pairs = 1,5×10^9 bytes

  • 1,5×10^9 bytes = 1.5 Gigabytes

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Number of Neurons

100 billion (10^11)

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Number of synapses

10 000 per neuron (10^15 synapses)

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Hypothesis

  • First cells divide symmetrically: they keep the same function (Symmetrical division)

  • After this they divide asymmetrically: one cell gets a different function (ie. Neuron) (Asymmetrical division)

  • The cortex is formed from an inside-out matter

  • New neurons migrate radially, you get exponentially more neurons

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Radial Unit Hypothesis

  • First cells divide symmetrically in the ventricular Zone (VZ): keep the same function (Symmetrical division)

  • After this, they divide asymmetrically: one cell gets a different function (ie. Neuron) and the differentiated cell migrates to the Cortical Plate (Asymmetrical division)

  • The cortex is formed from an inside-out manner

  • From the ventricular zone (VZ) to the cortical plate (CP), deeper layers first

  • Neuron Migration happens along the radial glial cells

  • Radial glial cells form radial units from the ventricular zone through the intermediate zone to the cortical plate

  • These maintain their topography (relative locations)

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Symmetrical division

  • for the first five to six weeks of gestation, the cells in the subventricular zone divide in a symmetrical fashion

  • the result is exponential growth in the number of precursor cells

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Asymmetrical division

  • At the end of six weeks, when there is a stockpile of precursor cells, asymmetrical division begins

  • After every cell division

  • One of the two cells formed becomes a migratory cell destined to be part of another layer

  • The other cell of the two cells remains in the subventricular zone

  • It continues to divide asymmetrically in the subventricular zone

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Radial glia cells

  • Stretch from the subventricular zone to the surface of the developing cortex

  • Their work does not end with development

  • Radial glia cells are transformed into astrocytes in the adult brain

  • Helping to form part of the blood-brain barrier

  • Neurons migrate along the radial glia cells that form a pathway for the neurons

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Radial migration

  • Cortical neurons are born and how they migrate radially from the ventricular zone toward the surface of the developing cortex

  • Neurons migrate along the radial glial cells

  • Radial glial highway is organized in a straight line from the ventricular zone to the cortical surface

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Radial unit Hypothesis

  • The fact that by prolonging the division, we can exponentially increase the number of neurons

  • Provides the simplest explanation for our enormous increase in brain size

  • Explains why the cortex is organized in columns

  • Each unit is not enlarged, instead the number of units increases (**)

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Cortical column

  • A principle unit of organization that has functional consequences and a development history

  • The cortical columns that arise from (**) these groupings have functional and anatomical consequences in the adult

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Inside-out formation of cortex

  • In which each cohort of neurons migrates past its cortical plate predecessors to form a more superficial layer

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Ventricular zone

  • The cortical neurons that form other parts of the brain arise from precursor cells in the ventricular zone

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Subventricular Zone

  • The cortical neurons arise from the subventricular zone

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Intermediate Zone

  • Located between the ventricular zone and the cortical plate

  • The white matter in this area is where neurons (created in the ventricular zone) migrate through in order to reach the cortical plate

  • This zone is only present during carcinogenesis

  • Eventually transform into adult white matter

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Cortical Plate

  • In humans and many other species, the fetal brain is well developed and shows cortical layers, neuronal connectivity and myelination

  • The fetal brain is already extremely complex, but far from completely being developed

  • The first migrating neurons approach the surface of the developing cortex – point known as cortical plate

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Marginal Zone

  • Predecessor for layer one of the cortex

  • Marginal zone + cortical zone = 6 layers that form the cortex

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Synaptogenesis

  • Substantial amount of that growth comes from synaptogenesis

  • The formation of synapses and the growth of dendritic trees

  • Early – in the deeper cortical layers

  • Later – in more superficial layers

  • Synaptogenesis – followed by synapse elimination (also called pruning)

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Synapse elimination/pruning

  • Synaptic pruning is a competitive process

  • Allows for learning development of higher cognitive functions

  • The elimination of some synaptic contacts between neurons during development, including postnatally

  • Eliminating the interconnections between neurons that are redundant, unused or do not remain functional

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Myelination

Differential timing in human development

  • Myelination of axons extends even further into life

  • Axons in different cortical areas myelinate at different times

  • Sensory and motor areas first

  • Frontal and parietal areas last

  • Myelination of frontal cortex continues way into adulthood :)

  • Until age 25-30

  • Use it now (in your 20s) or never get it

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Ontogeny = Phylogeny

  • Ontogeny recapitulates phylogeny

  • Delayed maturation of the frontal lobes is an example of ontogeny following phylogeny

  • A late addition in evolution means late development

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Brain Plasticity

The brain also displays enormous potential for plasticity after complete development

  • !!! your brain constantly changes!!!

  • The brain can recruit other areas that are/were used for something else (Areas from the somatosensory cortex that are not used anymore are recruited by other parts)

  • Reason why blind people can hear and feel better

  • Term that refers to the brains ability to change and adapt as a result of experience Learning

  • Mostly connectivity: synaptic changes Reorganization

  • The brain can repair or reorganize after an accident

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Phantom limbs

  • the ability to feel sensations and even pain in a limb or limbs that no longer exist

  • Non-painful sensation: perception of movement and perception of external sensations (touch, temperature, pressure, vibration, itch)

  • Pain sensations: range from burning and shooting pains to feelings of tingling “pins and needles”

  • Only occurs in amputees

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Chromosomes

  • My chromosomes make me an individual

  • Each chromosome consists of genes

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Genes

  • A gene encodes a functional element (protein)

  • A gene consists of a code (triplets of nuclei acids)

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DNA

  • DNA is deoxyribonucleic Acid

  • Consist of four bases: cytosine (C), guanine (G), adenine (A), or thymine (T)

  • Base pairs: C – G and A – T

  • When we reproduce, our DNA is combined with that of our partner

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Genotype

  • The genes

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Phenotype

  • How they are expressed

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Recombination and cross-over

  • Combine two individuals

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Mutation

  • Randomly alter a bit (usually low)

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Genotype strings and individuals

  • A genetic algorithm has a large number of strings/genotypes

  • Each genotype is also called an individual

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Populations

  • A collection of individuals is called a population

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Fitness value/function

  • Each string/genotype/individual is assigned a fitness value according to phenotype

  • This value can be normalized to a range between 0 and 1

  • You can use anything you like to calculate your fitness value

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Cross-over

  • You have various forms of cross-over

  • The simples form is single-point cross-over

  • 00110110 x 11001101 = 11000110 x 00111101

  • It is also possible to have multiple cross-over points

  • 00110110 x 11001101 = 00101110 x 11010101

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Inversion

  • we select a subset of genes and invert the entire string in the subset

  • 0123456789 x 0165432789

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Mutation

  • There are various types of mutation, depending on your representation

  • Binary mutation: 001100 = 001110

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Elitist selection

  • Order individuals by fitness

  • Select top -10, -20, … fittest individuals

  • Pro: very straightforward to implement, very fast

  • Con: each generation loses a lot of genetic information, “genetic degeneration”

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Roulette wheel selection

  • The wheel is rotated

  • The weakest individual has smallest share of the roulette wheel

  • The fittest individual has largest share of the roulette wheel

  • Pro: better than elitist selection

  • Con: comes very close to elitist selection if fitness values are very unequa

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Tournament selection

  • Select random k individuals (k=tournament size)

  • From this selection: select the best individual

  • Pro: Very straightforward; does not have problems of roulette wheel selection

  • Con: a large k boils to elitist selection

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Genetic Algorithms Pros

  • Widely acceptable

  • Easy to implement

  • Easy to parallelize

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Genetic Algorithms Cons

  • Same local minima problem as with back-prop, but genetic algorithms are better suited to escape local minima

  • Lots of variation (e.g. Selection, parameters)

  • Sometimes difficult to find right encoding

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Genetic Algorithms applications

  • Scheduling problems

  • Financing

  • Complex mathematical problems

  • Engineering: NASA, Intel

  • Chemistry

  • Compilers

  • Exoskeleton

  • Airforce Strategy Advisor

  • Airbus

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Cognitive Control

  • Allows us to use our perceptions, knowledge, and goals to bias the selection of action and thoughts from a multitude of possibilities

  • Allow us to override automatic thoughts and behavior and step out of the realm of habitual responses

  • Give us cognitive flexibility, letting us think and act in novel and creative ways

  • By being able to suppress some thoughts and activate others, we can simulate plans and consider the consequences of those plans

  • We can plan for the future and troubleshoot problems

  • Essential for purposeful goal- oriented behavior and decision making

you might want to stop at the doughnut store when heading to work in the morning, cognitive control mechanisms can override that sugary urge, allowing you to stop by the café for a healthier breakfast

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Anatomical Orientation picture:

  • first, which includes the lateral pre- frontal cortex and frontal pole

  • supports goal-oriented behavior

  • involved with planning, simulating consequences, and initiating, inhibiting, and shifting behavior

  • second control system, which includes the medial frontal cortex

  • essential role in guiding and monitoring behavior

  • works in tandem with the prefrontal cortex

  • monitoring ongoing activity to modulate the degree of cognitive control needed to keep behavior in line with goals

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Visible light spectrum

  • segment of electromagnetic spectrum that the human eye can view

  • human eye can detect wavelengths from 380 to 700 nanometers Longer wavelengths (red) Shorter wavelengths (blue)

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Iris

  • the colored part

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Pupil

  • the black circular opening in the iris

  • let’s light in

  • pupil size is adjusted to filter the amount of light

  • smaller for bright light

  • larger for low light

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Cornea

  • a clear dome over the iris

  • protective outer layer

  • serves as barrier against dirt, germs, and other things that can cause damage

  • also filters out some of the sun’s ultraviolet light

  • as light enters your eye, it gets refracted by the corneas curved edge

  • helps determine how well you can focus on objects close-up and far away

  • Three main layers

  • Epithelium

  • Stops outside matter from getting into your eye

  • Also absorbs oxygen and nutrients from tears

  • Stroma

  • Middle (thickest) layer lies behind epithelium

  • Made up mostly of water and protein that give it an elastic but solid form

  • Endothelium

  • Single layer of cells on the very back of the stroma

  • Stroma absorbs excess liquid and the endothelium pulls it out

  • Without this function the stroma would become waterlogged

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Lens

  • Focuses light rays onto the retina

  • Lens is transparent

  • Can be replaced if necessary

  • Lens deteriorates as we age – result – needing reading glasses

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Retina

  • Nerve layer lining the back of the eye

  • Retina senses light and creates electrical impulses

  • These impulses are sent through the optic nerve to the brain

  • Sees pictures upside down

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Optic nerve

  • Bundle of more than a million nerve fibers carrying visual messages from the retina to the brain

  • In order to see we must have light and our eyes must be connected to the brain

  • The brain controls what you see, since it combines images

  • Retina sees pictures upside down – brain turns images right side up

  • Reversal of the images that we see – like mirror in a camera

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Inverted projection on retina

  • Images of the world that surrounds us are projected upside down onto our retina

  • Image reversal – allows us tremendous peripheral vision and ability to see objects larger

  • Without reversal – limited view of our world (similar to viewing the world through a drinking straw

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Blind Spot

  • Small portion of the visual field of each eye

  • Corresponds to the position of the optic disk within the retina

  • No photoreceptors (rods or cones) in optic disk

  • Therefore, no image detection in this area

  • Blind spot of the right eye is located to the right of the center of vision

  • Blind spot of the left eye is located to the left of the center of vision

  • With both eyes open – blind spots not perceived because the visual fields of the two eyes overlap

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Optic Disk

  • Can be seen in the back of the eye with an ophthalmoscope

  • Located on the nasal side of the macula lutea

  • Oval shape

  • 1.5 mm in diameter

  • Entry point into the eye for major blood vessels that serve the retina

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Fovea

  • Small, central pit composed of closely packed cones in the eye

  • Located in the center of the macula lutea of the retina

  • Responsible for sharp central vision

  • Necessary in humans for activities for which visual detail – primary importance

  • Surrounded by parafovea belt and perifovea outer region

  • Employed for accurate vision in the direction where it is pointed

  • Compromises less than 1% of the retinal size

  • Takes up over 50% of the visual cortex

  • Sees only the central two degrees of the visual field

  • The farther away from the fovea the fewer receptor

  • Central region of the retina that is densely packed with cone cells and provides high resolution visual information

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Rods

  • Responsible for vision at low light levels (scotopic vision)

  • Usually located around the boundary of the retina

  • About 120 million photoreceptors out of the total 125 million photoreceptors in the human eye

  • Outer segment is cylindrical – contain rhodopsin pigment (made up of vitamin A)

  • Do not give color vision

  • Do not have any differentiation

  • If lack of the pigment in the rods = night blindness

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Cones

  • Fewer in number and are of cone shape

  • Located in the center of the retina

  • 5 million photoreceptors out of 125 million

  • Outer segment is conical of cones (contain iodopsin pigment)

  • Give color vision

  • If lack of pigment – cause color blindness

  • Chromatic (three types of pigment):

  • Short wavelength

  • The blue part of the spectrum

  • Medium wavelength

  • The greenish region

  • Long wavelength

  • The reddish region

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Ganglion cells

  • Final output neurons of the vertebrate retina

  • Collect information about the visual world from bipolar cells and amacrine cells

  • Process visual information

  • Begins as light entering the eye

  • Ganglion cells transmit it to the brain via their axons

  • We have only 2 million ganglion cells to telegraph information from the retina

  • Many rods feed into a single ganglion cell

  • Each ganglion cell is innervated by only a few cones

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Bipolar cell

  • Part of the retina

  • Exist between photoreceptors (rod cells and cone cells) and ganglion cells

  • They act directly or indirectly

  • To transmit signals from the photoreceptors to the ganglion cells

  • Receive synaptic input from either rods or cones, or both

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Horizontal cell

  • Laterally interconnecting neurons having cell bodies in the inner nuclear layer of the retina

  • Help integrate and regulate the input from multiple photoreceptor cells

  • Receive input from multiple photoreceptor cells

  • use that input to integrate signaling from different populations of photoreceptor cells

  • Adjust the signals that will be sent to bipolar cells

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Amacrine cells

  • Lie in the inner retina

  • Make connections with bipolar cells and ganglion cells

  • Create functional subunits within the receptive fields of many ganglion cells

  • contribute to vertical communication within retinal layers

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Lateral inhibition

  • The phenomenon in which a neurons response to a stimulus is inhibited by the excitation of a neighboring neuron

  • Observed in the retina and the lateral geniculate nucleus (lgn) of organisms

  • Makes neurons more sensitive to spatially varying of stimulus than to spatially uniform stimulus

  • Helps refine some somatosensory information

  • Only the neurons that are most stimulated and least inhibited respond

  • Plays an important role in visual perception by increasing the contrast and resolution of visual stimuli

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Contrast enhancement

  • Defined as the manipulation and redistributing the image pixels in a linear or non-linear fashion

  • Improve the separation of obscured structural variations in pixel intensity into a more visually differentiable structural distribution

  • Required to increase the quality of low contrast images by expanding the dynamic range of input gray level contrast enhancement without disturbing other parameters of the image is one of the difficult tasks in image processing

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Visual system

  • Comprises the sensory organ (eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex)

  • Gives organisms the sense of sight

  • Enabling the formation of several non-image photo response functions Figure 5.23 shows how visual information is conveyed from the eyes to the central nervous system. Before entering the brain, each optic nerve splits into two parts. The temporal (lateral) branch continues to traverse along the ipsilateral side. The nasal (medial) branch crosses over to project to the contralateral side; this crossover place is called the optic chiasm.

  • Detects and interprets information from the optical spectrum perceptible to that species to “build a representation” of the surrounding environment

  • Carries out number of complex tasks

  • Reception of light and the formation of monocular neural representation

  • Color vision

  • The neural mechanisms underlying stereopsis and assessment of distances and between objects

  • Identification of particular object of interest

  • Motion perception

  • Analysis and integration of visual information

  • Pattern recognition

  • Accurate motor coordination under visual guidance

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Eye

  • Light entering the eye is refracted

  • As it passes through the cornea

  • Then passes through the pupil (controlled by the iris)

  • Further refracted by the lens

  • Cornea and lens act together as a compound lens

  • To project an inverted image onto the retina

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Optic chiasm

  • Optic nerves from both eyes meet and cross at the optic chiasm

  • At the base of the hypothalamus of the brain

  • The information coming from both eyes is combined

  • Then splits according it the visual field

  • Corresponding halves of the field of view (left & right)

  • Sent to the left and right halves of the brain to be processed

  • The right side of primary visual cortex deals with the left half of the field of view from both eyes (similarly for the left brain)

  • Small region in the center of the field of view is processed redundantly by both halves of the brain

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lgn

  • Lateral geniculate nucleus

  • Sensory relay nucleus in the thalamus of the brain

  • Consists of six layers in humans and other primates starting from catarhinias (including apes)

  • One type of ganglion cell (m cell) sends output to the bottom two layers

  • Another type of ganglion cell (p cell) projects to the top four layers

  • Visual information reaching the cortex has been processed by at least four distinct neurons

  • Photoreceptors

  • Bipolar cells

  • Ganglion cells

  • Lgn cells

  • Lgn cells have receptive fields responding

  • If the stimulus falls within a very limited region of space (one degree of visual angle)

  • Layers 1, 4, 6 – correspond to information from the contralateral (crossed) fibers of the nasal retina (temporal visual field)

  • Layers 2, 3, 5 – correspond to information from the ipsilateral (uncrossed) fibers of the temporal retina (nasal visual field)

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Visual cortex

  • Largest system in the human brain

  • Responsible for processing the visual image

  • Lies at the rear of the brain above the cerebellum (highlighted in the photo)

  • Primary visual cortex (v1) - region that receives information directly from the lgn

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V1

  • Initial cortical processing area for vision

  • Located in the most posterior portion of the occipital lobe (brodmann area 17)

  • Most studied visual area in the brain

  • In mammals it is located in the posterior pole of the occipital lobe

  • Simplest, earliest cortical visual area

  • Highly specialized for processing information about static and moving objects

  • Excellent in pattern recognition

  • Equivalent to the striate cortex (brodmann area 17 – defined by its anatomical location)

  • Cells in v1 have slightly larger receptive fields

  • This magnification process continues through the visual system

  • Cells in the temporal lobe have receptive fields that may encompass an entire hemifield

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simple cells

  • mainly in layers 4 and 6

  • Have distinct excitatory and inhibitory regions

  • Cell that responds primarily to oriented edges and gratings

  • Such cells are tuned to different frequencies and orientations

  • Calculate edges

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Complex cells

  • Can be found in the v1, v2, v3

  • Will respond primarily to oriented edges and gratings

  • However, it has a degree of spatial invariance

  • Means – receptive field cannot be mapped into fixed excitatory and inhibitory zones

  • Some respond to patterns of light in a certain orientation within a large receptive field

  • Regardless of the exact location

  • Some respond optimally only to movement in a certain direction

  • In v2

  • Use the information from many simple cells to represent corners and edge terminations

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End-stopped cells

  • Are thought to detect singularities like line and edge crossings, vertices and line endings

  • Neuron in any visual area of the cerebral cortex

  • Maximally responsive to a line of a certain length or to a corner of a larger stimulus

  • Reduced or absent response when the line or corner is extended beyond a certain point

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Orientation columns

  • Organized regions of neurons

  • Excited by visual line stimuli of varying angles

  • Columns are located in the primary visual cortex (v1) and span multiple cortical layers

  • Neurons with similar properties are arranged in columns perpendicular to the surface of the cortex

  • Range the six cortical layers until they reach the white matter

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Contralateral projection

  • Between hemisphere and the correspondent side of the body

  • Example: moving the right hand will make the left hemispheres activity go up

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Upside-down projection

  • 'Images' in your brain are just collections of neural activations, and not actual pictures

  • They cannot have an orientation

  • brain is capable of flipping your visual field if required as measured through perceptual adaptation experiments using inversion glasses

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Center-surround receptive field

  • Allows ganglion cells to transmit information

  • Whether photoreceptor cells are exposed to light

  • About the differences in firing rates of cells in the center and surround

  • This allows the ganglion cells to transmit information about contrast

  • Size of the receptive field governs the spatial frequency of the information

  • Small receptive fields

  • Stimulated by high spatial frequencies (fine detail)

  • Large receptive fields

  • Stimulated by low spatial frequencies (coarse detail)

  • Retinal ganglion cell receptive fields convey information about:

  • Discontinuities in the distribution of light falling on the retina

  • Often specify the edges of objects

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Retinotopy / retinal mapping / retinally mapped

  • Mapping of visual input from the retina to neurons

  • Particularly those neurons within the visual stream

  • Retinotopy mapping in humans is done with functional magnetic resonance imaging (fmri)

  • Subject inside the fmri machine focuses on a point

  • Then retina – stimulated with a circular image or angled lines about focus point

  • Radial map displays the distance from the center of vision

  • Angular map shows angular location using rays angled about the center of vision

  • Combining both maps – you can see separate regions of the visual cortex

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Magnocellular

  • Also called M-cells

  • Neurons located within the Adina magnocellular layer of the lateral geniculate nucleus (LGN) of the thalamus

  • Part of the visual system

  • Characterized by their relatively large size compared to parvocellular cells

  • Receive input from parasol ganglion cells

  • Cannot provide finely detailed or colored information, but still provides useful static, depth and motion information

  • High light/dark contrast detection

  • More sensitive at low spatial frequencies than high spatial frequencies

  • Important for providing information about the location of objects

  • Can detect the orientation and position of objects in space, information that is sent

  • Information = important – detecting the difference in positions of objects on the retina of each eye (important tool in binocular depth perception)

  • Cells in the M pathway – ability to detect high temporal frequencies and can detects quick changes in the positions of an object

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Parvocellular

  • Also called P-cells

  • Neurons located within the parvocellular layers of the lateral geniculate nucleus (LGN) of the thalamus

  • More modern than M-cells

  • Receive their input from midget cells (type of ganglion cell) (axons of midget cell are exiting the optic tract)

  • Information from each eye is kept separate at this point and continues to be segregated – until processing in the visual cortex

  • Sensitive to color and capable of discriminating fine details than the M cell

  • Greater spatial resolution than M cells

  • Lower temporal resolution than M cells

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Koniocellular

  • Neuron with a small cell body

  • Located in the koniocellular layer of the LGN (in primates and humans)

  • Quantity of neurons = number of M cells

  • Present between the layers (picture)

  • Koniocellular layers are much thinner due to their size

  • Neurochemically and anatomically distinct from M and. P cells

  • Three proteins

  • Calbindin

  • Alpha subunit of type II calmodulin-dependent protein kinase

  • Gamma subunit of protein kinase C

  • Some cells respond to color

  • Some reacts to achromatic gratings

  • Contribute to brightness contrast information and color contrast in species with color vision

  • Contribute to eye movement-related signals

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After V2 - Dorsal (‘where’ or ‘how’)

  • Involved in guidance of actions (eg. Reaching) and recognizing where objects are in space

  • Also known as parietal stream, where stream, how stream

  • Stretches from the primary visual cortex (V1) in the occipital lobe forward into the parietal lobe

  • Interconnected with the parallel ventral stream (what stream)

  • Commences with purely visual functions in the occipital lobe

  • Gradually transferring to spatial awareness

  • Termination in parietal lobe

  • Posterior parietal cortex = essential for “the perception and interpretation of spatial relationships, accurate body image, learning of tasks involving coordination of the body in space”

  • Contains individually functioning lobules

  • Lateral intraparietal sulcus (LIP) contains neurons that produce enhanced activation when attention is moved onto the stimulus

  • Ventral intraparietal sulcus (VIP) where visual and somatosensory information are integrated

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Ventral (‘what’)

Associated with object recognition and form representation

  • Described as “what” stream

  • Strong connections to

  • The medial temporal lobe (stores long-term memories)

  • The limbic system (controls emotions)

  • Dorsal stream (deals with object locations and motion)

  • Gets main input from the parvocellular layer of the LGN of the thalamus

  • These neurons project to

  • V1 sublayers 4Cß, 4A, 3B, 2/3a

  • From there the ventral pathway goes through V2 and V4 to areas of the inferior temporal lobe

  • Areas of the inferior temporal lobe

  • Posterior inferotemporal (PIT)

  • Central inferotemporal (CIT)

  • Anterior inferotemporal (AIT)

  • Each visual area contains a full representation of visual space

  • It contains neurons whose receptive fields together represent the entire visual field

  • Visual information enters the ventral stream through the primary visual cortex

  • Travels through the rest of the areas in sequence

  • All areas in the ventral stream – influenced by extraretinal factors

  • Attention

  • Working memory

  • Salience

  • Damage to ventral stream – inability to recognize faces or interpret facial expressions

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Perception

Perception is a unified whole (binding problem)

  • Ability of the brain to construct uniform perceptions from a multitude of sensory impressions

  • In the visual system, structures are known that are activated more strongly by certain shapes, colors or movements in their receptive field than when other patterns are shown

  • The property of some neurons is the basis of the idea that the sensory information is broken down into such "basic components"

  • and then put back together again – question?

  • The binding problem raises the question of how these signals are linked to the overall impression Pathways for visual perception

  • Ventral stream

  • Dorsal stream

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Object constancy

  • Also called perceptual constancy

  • Tendency of animals and humans to see familiar objects as having standard shape, size, color, or location

  • Regardless of changes in the angle of perspective, distance or lighting

  • Impression rends to conform to the object as it is or is assumed to be, rather than to the actual stimulus

  • Responsible for the ability to identify objects under various conditions

  • Seem to be “taken into account” during a process of mental reconstitution of the known image

  • Reduced by limited experience with the object and by decreasing the number of environmental cues that aid in identification of the object

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View-invariant recognitio

  • Does not happen by simple analysis of the stimulus information

  • The perceptual system extracts structural information about the components of an object and the relationship between these components

  • key to successful recognition is that critical properties remain independent of viewpoint

  • Bicycle example

  • The properties might be features such as an elongated shape running along the long axis, combined with a shorter, stick-like shape coming off of one end

  • With two circular- shaped parts, we could recognize the object as a bicycle from just about any position

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View-dependent recognition

  • Posit that people have a cornucopia of specific representations in memory

  • Key idea is that the stored representation for recognizing a bicycle from the side is different from the one for recognizing a bicycle viewed from above

  • Our ability to recognize that two stimuli are depicting the same object is assumed to arise at a later stage of processing

  • They seem to place a heavy burden on perceptual memory

  • Each object requires multiple representations in memory, each associated with a different vantage point

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Shape encoding

  • recognition involve hierarchical representation in which each successive stage adds complexity

  • Simple features such as lines can be combined into …

  • Edges

  • Corners

  • Intersections

  • … which are grouped into parts and the parts grouped into objects (as processing continues up the hierarchy)

  • People recognize a pentagon because it contains five-line segments of equal length (joined together) to form five corners that define an enclosed region (figure 6.12)

  • The same five-line segments can define other objects such as a pyramid

  • Here there are only four points of intersection (not five) and the lines define a more complicated shape that implies – three dimensional

  • Investigate how to encode

  • Identify areas of the brain that are active when comparing contours that form a recognizable shape versus contours that are just jungled

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grand-mother cell coding

  • Assumption that the final percept of an object is coded by a single cell

  • Cells are constantly firing and refractory

  • Coding scheme of this nature would be highly susceptible to error

  • If a gnostic unit were to die, we would expect to experience a sudden loss for an object

  • Cannot adequately account for how it is possible to perceive novel objects

  • gnostic theory does not account for how the grandmother cell would have to adapt as grandmother changed over time

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Ensemble coding

  • Recognition is not due to one unit but to the collective activation of many units

  • Readily account for why we can recognize similarities between objects and may confuse one visually similar object with another

  • Both objects activate many of the same neurons

  • Losing some units might degrade our ability to recognize an object, but the remaining units might suffice

  • Account for our ability to recognize novel objects

  • Novel objects bear a similarity to familiar things

  • Our percept results from activating units that represent their features

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Encoding

  • Transform sensory data into a form of mental representation

  • In storage you keep encoded information in memory

  • In retrieval you pull out or use information stored in memory

  • Refers to how you transform a physical, sensory input into a representation that van be placed into memory

  • Encode our memories to store them

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Short term storage

  • Short term memory

  • Acoustic code is more important than a visual code

  • Semantics did not matter much for processing

  • Appears to be primarily acoustic

  • May be some secondary visual information than acoustic information

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Long term storage

  • Most information stored in long term memory primarily is encoded semantically

  • Acoustic information, semantic information, visual information – encoded in long term memory

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Decoding

  • Decoding techniques interrogate more of the information in the brain scan

  • Rather than asking which brain regions respond strongly to faces

  • They use both strong and war responses to identify subtler pattern of activity

  • These recordings are fed into a “pattern classifier” (computer algorithm that learns the patterns associated with each picture or concept)

  • Once a program has seen enough samples it can start to conclude what the person is looking at or thinking about

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Pattern recognition

  • Cognitive process that matches information from a stimulus with information retrieved from memory

  • Occurs when information from the environment is received and entered into short-term memory

  • Causing automatic activation of a specific content of long-term memory

  • Super important for humans and animals

  • Koala uses pattern recognition to find and consume eucalyptus leaves

  • Development of neural networks in the outer layer of the brain in humans has allowed for better processing of visual and auditory patterns

  • Six main theories of pattern recognition

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Template matching

  • Mist basic approach to human pattern recognition

  • Theory – assumes: every perceived object is stored as a “template” into long-term memory

  • Incoming information is compared to these templates to find an exact match

  • Example

  • A a a are all recognized as a but not b

  • This theory – can’t explain how new experiences can be understood without being compared to an internal memory template

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  • Prototype-matching

  • Compares incoming sensory input to one average prototype

  • Proposes – exposure to a series of related stimuli leads to the creation of a “typical” prototype based on their shared feature

  • Reduces number of stored templates by standardizing them into a single representation

  • Supports perceptual flexibility – allows for variability in the recognition of novel stimuli

  • Example

  • Suppose a child has never seen a lawn chair before, but given the characteristics of a regular chair, the child would still be able to recognize it as a chair

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  • Feature analysis

  • Try to explain how humans are able to recognize patterns in their environment

  • Proposes – nervous system sorts and filters incoming stimuli to allow the human or animal to make sense of the information

  • Proposes an increasing complexity in the relationship between detectors and the perceptual feature

  • When features repeat or occur in a meaningful sequence

  • We are able to identify these patterns because of our feature detection system

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  • Recognition-by-components

  • Proposes – humans recognize objects by breaking them down into their basic 3d geometric shapes called geons

  • Example

  • Break down something like a coffee cup

  • We have the hollow cylinder that holds the liquid and a curved handle off the side that allows to hold it

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