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Neurons and Their Neurons and Their ConnectionsConnections

Janusz A. Starzyk

Based on book Cognition, Brain and Consciousness ed. Bernard J. Baars

Cognitive ArchitecturesCognitive Architectures

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IntroductionIntroduction

Neurons did not change much for millions of years

The brain can be viewed as a hyper complex surface of neurons.

Sensory and motor cortex are viewed as processing hierarchies of neurons.

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IntroductionIntroduction

A single neuron may have thousands of inputs (dendrites) and one or more outputs (axons).

Neurons grow extending their axons and connecting to other neurons in the interconnected structure

A bipolar neuron

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Neurons’ GrowthNeurons’ Growth

This growth can be observed in the lab and under stimuli the network can learn a control function

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Real and idealized neuronsReal and idealized neurons

Neurons have been idealized into the classical integrate and fire neuron (right).

In this neuron inputs from dendrites are accumulated and if total voltage value exceeds -50 mV it triggers fast traveling action potential in the cell’s axon.

Neuron sends its signal by firing spikes from the cell body to terminals synapses.

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Excitation and InhibitionExcitation and Inhibition

Classical neurons are connected by excitatory and inhibitory synapses.

There are many classes of neurons, neurochemicals, and mechanisms for information processing

Many factors determine neuron activity – the sleep-waking cycle, availability of chemicals, and more.

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Excitation and Inhibition (cont.)Excitation and Inhibition (cont.)

Transmission of signals through axons is assisted by wrapping the axons in Myelinating Schwann cells.

The cells improve the conduction velocity of signals. At the breaks known as the nodes of Ranvier, the action

potentials are regenerated.

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A SynapseA Synapse A spike in the presynaptic

cell triggers release of neurotransmitter that diffuses across the synaptic gap and changes potential of postsynaptic cell.

Efficiency of signal transmission corresponds to synaptic weigh in network of neurons

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Working AssumptionsWorking Assumptions Neurons adds graded voltage inputs until total membrane voltage

exceeds -50 mV and then fires. Connections are either excitatory or inhibitory and its strengths is

represented by the connection weight. The weight can be normalized between -1 and 1.

Artificial neural networks that use simple neuron models can be used for pattern recognition or unknown function approximation.

Neurons can form one-way or bidirectional pathways to transfer information from one part of the brain to other.

Cortex is a massive 6-layer array of neurons. Arrays of neurons are called maps.

Stable collections of neurons form Hebbian cell assemblies

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A simple reflex circuitA simple reflex circuit An example of a spinal

(knee-jerk) reflex.

Sensory neurons pick up the tap and transmit it to the spinal cord.

An interneuron links the sensory impulses to motor neurons bypassing higher level

brain function and making the leg jump

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A simple reflex circuitA simple reflex circuit While reflex circuits can be triggered by

outside stimuli, they are integrated into voluntary, goal driven activities.

Many times this is unconscious and almost automatic. Voluntary goal driven brain mechanisms,

are associated with cortex. Sophisticated subcortical activity is also

engaged in planning and executing actions.

Spinal centers communicate with higher centers while carrying sensorimotor reflexes and return feedback signals to brain.

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There are several types of receptors, however, they are all similar in structure and function.Sensory nerves have parallel pathways sending sensory information to thalamus and sending back feedback

information 90% of neurons go

backwards towards the source

Most sensory and motor pathways split and cross over the midline of the body

Different types of receptorsDifferent types of receptors

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Similarities between sensor pathwaysSimilarities between sensor pathways

This image shows the similarities between the different sensory streams.

arm vs. leg,

high frequency vs. low frequency, and

foveal vs. peripheral vision.

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Sensory InteractionsSensory Interactions

Sensory regions interact with thalamic nuclei (RTN)Notice similarities between cortical input and output layers in all these senses

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Lateral inhibition is used to differentiate between neighboring cells

This gives better resolution at various levels of sensory perception In retina it helps to spot a tiny point At higher level it helps to differentiate e.g. between ‘astronomy’ and

‘astrology’

Lateral InteractionsLateral Interactions

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Visual demonstration of lateral inhibition

Notice that lateral inhibition applies to adjacent black squares, color perception, and even perception of direction

Lateral InteractionsLateral Interactions

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Mapping of the brainMapping of the brain

Visual quadrants map to cortical quadrants

Mapping is observed for various senses

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Neuron organizationNeuron organization

Neurons organize into layers. The figure below shows a single layer of pyramid neurons at 200 micrometers.

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Visual MapsVisual Maps

Neuron connections form various pathways In V1 the upper pathway is sensitive to location ‘where The lower pathway is sensitive to color, shape contrast and object

identity ‘what’

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Layers have 2-way connectionsLayers have 2-way connections

Neuronal layers have both feed-forward and feedback connections between layers/arrays.

Lower levels tend to be sensitive to simpler stimuli, while higher levels respond to more complex stimuli.

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Sensory and motor hierarchiesSensory and motor hierarchies Sensory and motor

systems appear to be arranged in hierarchies with information flowing between each level of the sensory and motor hierarchies.

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Ambiguous stimuliAmbiguous stimuli

Ambiguous stimuli pose choices for interpretation. It all depends on how the image is perceived and what ever preconceived notions you may have.

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Hebbian LearningHebbian Learning

“Neurons the fire together, wire together”

Long term potentiation (LTP) and long term depression (LTD)

The figure depicts Hebbian learning in cell assemblies.

At t1 input is encoded into connection weights.

Memory is retained at times t2 & t3.

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A Three Layer NetworkA Three Layer Network

Hidden layer makes the network more flexible

Backpropagation is used to adjust network weights to match the input to a desired output.

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A pattern recognition networkA pattern recognition network

An example of an auto-associative network that matches its output with its input.

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A self-organizing networkA self-organizing network Self-organizing

networks appear often in biological organisms.

A self-organizing network can be used for face recognition.

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Neural DarwinismNeural Darwinism

Gerald Edelman proposed that brain is a massive selectionist organ where neurons develop and make connections following Darwinian principle of selection of the fittest.

In biological evolution, species adapt by reproduction, mutation that leads to diverse forms, and selection.

A similar process occurs in the immune system, where millions of immune cells adapt to invading toxins.

Thus selectionism leads to flexible adaptation.

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Symbolic ProcessingSymbolic Processing Neural nets can handle

both distributed numerical

values as well as

symbolic expressions. The figure shows proposed by

McClelland and Rogers merge between symbolic features and their associations expressed by connections of a neural network

Brain uses adaptation and representation to learn the world.

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Coordinating Neural NetsCoordinating Neural Nets Neurons’ activation is coordinated by

large-scale rhythms to signify their activities.

Epileptic seizures are also caused by slow, intense, regular waves that lead to a loss of consciousness

Thus there must be a balance between integration and differentiation.

A high density of gamma rhythms has been related to conscious visual perception and understanding of spoken words.

Alpha rhythms are associated with an absence of focused attentional tasks.

Theta rhythms coordinate hippocampal region and the frontal cortex during retrieval of memories.

And delta rhythms signal deep sleep, are believed to group fast neuronal activities to consolidate learned events.

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Coordinating Neural NetsCoordinating Neural Nets

This figure illustrates hypothesis how brain rhythms coordinate large number of neuron cells’ firing.

Neurons that fire in synch with the dominant rhythm are strengthened by feedback from many other neurons, while those that fire out of synch are weakened.

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The basic question in cognitive neuroscience is how the nerve cells work together to perform cognitive functions like perception, memory and action.

Models of neurons were developed and used to build functional processing networks.

Artificial neural networks and biologically inspired networks are useful to study cognitive processing.

Sensory and motor systems are complex hierarchies of neurons organized in two or three dimensional arrays.

In vision, touch and motor control arrays of neurons are topographically arranged as maps of the spatial surroundings.

Hierarchies are bidirectional pathways, that allow signals to travel up, down and laterally.

A major function of downwards pathway is to resolve sensory ambiguities.

Lateral inhibition is used to emphasize differences between inputs.

SummarySummary