Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Plasticity of Neural Connections

Neurons and synapses define a person's individuality and personality. The neurons and synapses are established by genetics and ongoing lifetime of experiences.

The neural network is in a state of constant flux via the plasticity of synaptic connections.

Beginning with embryonic neural development and continuing through infancy, childhood, adolescence, and into adulthood, the neural network is sculpted by genetics and re-sculpted in the ever-changing environment.

Sensory signal patterns interact with this vast network of synaptic efficacies to form activated neural signal patterns constituting perceptions of consciousness.

Our brains are belief engines, evolved pattern recognition machines that connect the dots and create meaning out of the patterns that we think we see in nature. (Shermer; Believing Brain, 59)

Patternicity -- the tendency to find meaningful patterns in both meaningful and meaningless noise. (Shermer; Believing Brain, 60)


Research Study — Strengthening Synaptic Connections by NMDA for Long-Term Potentiation (LTP)

Research study — Cortical Microcircuits can perform Pattern Completion

Research study — Chromatin Remodeling Regulates Neural Coding

Research Study — Thalamocortical Signals Selectively Amplified via Recurrent Inputs

Research study — Cortical Inhibition Regulated by Sensory Experience


Plasticity of Synapses Mediates Memory

Biochemical changes in the synapses mediate memory in the relative near-term, whereas gene expression and protein changes in synapses and dendritic tree structures are consolidated over time for hippocampus-independent long-term memory.

The combinatorics of the brain's vast network of 1015 synapses provides a virtually infinite storeplace for memories.

The term "synaptic plasticity" was introduced by the Polish psychologist Jerry Konorski to describe the persistent, activity driven changes in synaptic efficacy that he assumed to be the basis of information storage in the brain.    (Konorski, 1948).   (Andersen; Hippocampus Book, 344)

The discovery of what has come to be known as long term potentiation (LTP) emerged from experiments that Per Andersen and Lomo, then a PhD student, were conducting at the University of Oslo during the mid-1960s on the phenomenon of frequency potentiation in excitatory hippocampal pathways. (Andersen; Hippocampus Book, 344)

The synaptic plasticity of long term potentiation and depression,    LTP and LTD, persist for hours if not longer. (Andersen; Hippocampus Book, 210)

The hippocampus appears to undergo dynamic modifications continually    in the form of dendritic extension and retraction    as well as synaptic formation and elimination. (Andersen; Hippocampus Book, 321)

The dentate gyrus is one of two brain regions (other is olfactory bulb) in which adult neurogenesis has been widely recognized. (Andersen; Hippocampus Book, 324)


Growth of new spines and changes in the structure of existing spines are possible substrates of synaptic plasticity in the hippocampus. (Andersen; Hippocampus Book, 136)

The diffusional resistance of spines could be very important for plasticity changes, such as those that occur with long term potentiation. (Arbib, Handbook of Brain Theory; Holmes; Dendritic Spines, 291)

Plasticity is a property of virtually all elements of the nervous system. (Osaka, Working Memory; Postle; Activated Long-Term Memory, 343)

CREB (cAMP-response element-binding protein) -- a type of protein that regulates the expression of genes, and fulfills a key role in neuronal plasticity. (Dudai; Memory from A to Z, 65)

The efficiency of neurotransmitter pulses in a synapse depends on the past history of pulses crossing the synapse. An unrelated metaphor - exercise can improve muscle efficiency; disuse can decrease it. Can think of synapses as weighted according to past experience. (Holland; Emergence, 85)

Adaptive changes in individual neurons in the nervous system take from seconds to hours. (Holland; Hidden Order, 9)

Experience-dependent brain plasticity typically declines after an early critical period during which circuits are established. Discordant vision through the two eyes during an early critical period results in the enduring loss of visual acuity (amblyopia) that reflects aberrant circuit remodeling within primary visual cortex (V1).

Behavioral plasticity can arise from plastic changes in the efficacy of synaptic transmission. These changes in synaptic function can be either short-term effects that rely on post-translational modification of existing synaptic proteins, or long-term changes that require changes in gene expression, new protein synthesis, and perhaps even growth of new synapses, or elimination of existing ones. (Purves; Neuroscience, 579)

Initial basis of long-lasting forms of synaptic plasticity in the mammalian CNS, such as LTP and LTD, entail post-translational changes that lead to altered distributions or density of postsynaptic AMPA receptors. (Purves; Neuroscience, 597)


Research study — Synapses Pruned by Microglia


Calcium Ions Promote Many Changes

Calcium ions can promote many changes, such as the activation of genes. (Greenfield; Centers of Mind, 45)

When certain genes are activated, the neuron may change the amounts and types of particular chemicals it contains and even undergo a modification in its overall appearance. It will have been adapted over a period of time to the sustained increase in input. It's response will have become weighted to respond in different ways to future signals. (Greenfield; Centers of Mind, 45)

After you gain experience for a while, during a period of rest, your brain sets in motion LTP, which is responsible for your adapting to or learning from the experience you have just had. (Greenfield; Centers of Mind, 45)


Consolidation of Memory and LTP Require Protein Synthesis

Long-term potentiation (LTP) at glutamatergic synapses is considered to underlie learning and memory and is associated with the enlargement of dendritic spines. Because the consolidation of memory and LTP require protein synthesis, it is important to clarify how protein synthesis affects spine enlargement.

Long-lasting forms of synaptic plasticity requires changes in gene expression. (Purves; Neuroscience, 597)

Brain imaging studies of musicians have shown they have more neuronal connections in the area of the brain that regulates the hand movements involved in musical performance. (Greenspan; First Idea, 7)

Learning is a lifelong process - early years are crucial - foundation for subsequent learning - extensive plasticity in early life - synapses do not stop changing. (LeDoux; Synaptic Self, 96)

Plasticity is a property of virtually all elements of the nervous system. (Osaka, Working Memory; Postle; Activated Long-Term Memory, 343)

Hebbian cell assembly -- spatiotemporal pattern of neuronal activity. (Calvin; Neil's Brain, 282)


Research study — Synaptic Remodeling, and Network Activity

Research study — Learning and Memory in Pyramidal Neuron Dendrites


The results of a research study support the hypothesis that plasticity changes in synapses during waketime lead to a net increase in synaptic strength in many brain circuits and that sleep is required for synaptic renormalization.


Research study — Sleep and Synaptic Homeostasis


Neurotrophins promote new synaptic connections

Neurotrophins - promote survival and growth of neurons. Neurotrophins released from postsynaptic cell, diffuse backward, taken up by presynaptic terminals; branch and sprout new synaptic connections. (LeDoux; Synaptic Self, 81)


Hebbian Plasticity

The LTP of Hebbian plasticity, which hypothesizes that ‘neurons that fire together wire together,’ begins the network formation prenatally and continues with lifelong changes that modify the efficacies of the brain’s 1015 synapses.

When an axon of cell A. is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased. (Hebb; Organization of Behavior, 62)

Hebb's rule -- concurrent activity in presynaptic and postsynaptic cells leads to a strengthening of the connections from the presynaptic to postsynaptic cell. (LeDoux; Synaptic Self, 80)

Hebb's rule - connections between neurons that are active together are strengthened. Neurons of the visual cortex forge their network of synapse connections; busy axons expand their bushy crowns, whereas idle neurons shrink away. Sensitive time for visual cortex limited to 'critical period', which differs from species to species. (Zeman; Consciousness, 203)

NMDA receptors appear to be particularly important in the mechanism in which activity promotes growth. NMDA receptors are able to detect a match between activity in the presynaptic and postsynaptic neuron. (LeDoux; Synaptic Self, 81)

Also, Eric Kandel and colleagues in a Columbia University have demonstrated that physiological plasticity is accompanied by axon branching and new synapse formation both during development and following learning. (LeDoux; Synaptic Self, 81)


Research Study — NMDA Receptor Ion Channel Crystal Structure


Visual cortex adapts to its surroundings. Difficult to specify the strength of every synapse of the visual system. A rough genetic sketch is sufficient. Visual system is provided a generous superfluity of potential interconnections; experience then selects the useful ones, 'fine tuning' the visual system. (Zeman; Consciousness, 204)

Experience may continue to 'sculpt' the visual cortex long after the period of maximum plasticity. Store information through synaptic change in the sensory regions in which it is processed. Plasticity and memory may share a common fundamental explanation in Hebb's rule. (Zeman; Consciousness, 205)

Hebbian plasticity. Synaptic plasticity. (LeDoux; Synaptic Self, 136-137)

Classical conditioning as Hebbian plasticity - (diagram) (LeDoux; Synaptic Self, 160)

Lateral nucleus of amygdala is a key site of plasticity during fear learning. (LeDoux; Synaptic Self, 124)

Glutamate receptors,  several types including:  AMPA receptor - regular synaptic transmission; NMDA receptor - synaptic plasticity (LeDoux; Synaptic Self, 144)

Edelman’s ‘Neural Darwinism’

Formation of Connections -- the outcome of neural process outgrowth to form and stabilize connections depends upon a complex set of cooperative and competitive mechanisms that are dynamic and also to some extent stochastic in their actions. (Edelman; Neural Darwinism, 115)

Theory of Neuronal Group Selection (TNGS) - (1) Developmental selection, (2) Experimental selection, (3) Reentry  (Edelman; Remembered Present, 43)


Research study — Plasticity of Dendritic Spines for Learning and Memory

Research study — Plasticity of Neural Connections — Recent Research

Research study — Plasticity of Synapses — We consider, in turn, each component of the synapse:   the presynaptic compartment,    the postsynaptic compartment,    and the synaptic cleft,    and discuss processes that undergo activity-dependent modifications    to alter synaptic efficacy.

Research study — Dendritic Spine Remodelling



Return to — Memory

Return to — Neural Network

Link to — Consciousness Subject Outline

Further discussion — Covington Theory of Consciousness