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Brain Structure & Function
9 terms
Axon
Plain English
The long fiber extending from a neuron that carries electrical signals to other cells.
Technical Definition
The elongated projection of a neuron that conducts action potentials away from the cell body (soma) to synaptic terminals, enabling signal transmission over distances from micrometers to over a meter.
Cortical Region
Plain English
A defined area on the brain's outer surface responsible for specific tasks.
Technical Definition
A cytoarchitecturally or functionally delineated zone of the cerebral cortex, typically mapped using standardized atlases (e.g., Desikan-Killiany, Brodmann areas).
Functional Network
Plain English
A set of brain areas that activate together during a task.
Technical Definition
A spatially distributed set of brain regions exhibiting temporally correlated activity (measured via fMRI BOLD signal or electrophysiological coherence) during specific cognitive operations.
Gray Matter
Plain English
The outer layer of the brain containing neuron cell bodies.
Technical Definition
Regions of the central nervous system rich in neuronal somata, dendrites, and synapses; forms the cerebral cortex and subcortical nuclei.
Neurite
Plain English
A projection (axon or dendrite) extending from a neuron.
Technical Definition
Any process extending from a neuronal cell body, encompassing both axons and dendrites; in diffusion imaging, the term refers to the cylindrical compartment modeled by NODDI.
Node of Ranvier
Plain English
A gap in the myelin sheath where signals are regenerated.
Technical Definition
Periodic unmyelinated segments (~1 micrometer) along myelinated axons where voltage-gated Na+ channels cluster, enabling saltatory conduction by regenerating the action potential at each gap.
Oligodendrocyte
OL
Plain English
A brain cell that wraps axons in myelin insulation.
Technical Definition
A type of macroglial cell in the central nervous system that extends multiple processes, each forming a segment of myelin sheath around nearby axons; a single OL can myelinate up to 50 axonal internodes.
OPC
Oligodendrocyte Precursor Cell
Plain English
A stem-like cell that can become a new myelin-producing cell.
Technical Definition
A proliferative, NG2+/PDGFRα+ progenitor distributed throughout the CNS that differentiates into mature myelinating oligodendrocytes in response to developmental cues, neural activity, or injury-related signals.
White Matter
Plain English
The brain's internal wiring, made up of myelinated fiber bundles.
Technical Definition
Tissue of the CNS composed primarily of myelinated axonal tracts, glial cells, and extracellular matrix; it interconnects cortical and subcortical gray matter structures.
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Myelin & Conduction
6 terms
Conduction Velocity
Plain English
How fast an electrical signal travels along a nerve fiber.
Technical Definition
The speed (m/s) at which an action potential propagates along an axon, determined primarily by axon diameter and myelination status; ranges from ~0.5 m/s (unmyelinated C-fibers) to ~120 m/s (large myelinated fibers).
Demyelination
Plain English
The loss or thinning of the myelin insulation around nerve fibers.
Technical Definition
Degradation, thinning, or complete loss of the myelin sheath surrounding axons, caused by autoimmune attack, metabolic failure, aging-related oligodendrocyte dysfunction, or neuroinflammation.
Myelin
Plain English
A fatty insulating coating on nerve fibers that speeds up and stabilizes signal transmission.
Technical Definition
A lipid-rich (~70% lipid, 30% protein) multilayered membrane sheath produced by oligodendrocytes (CNS) or Schwann cells (PNS), enabling saltatory conduction and reducing capacitive current loss.
Myelin Plasticity
Plain English
The brain's ability to add or modify myelin insulation in response to learning and activity.
Technical Definition
Activity-dependent remodeling of myelin sheaths, including de novo myelination and modification of existing sheath thickness/length, mediated by OPC differentiation in response to neuronal firing patterns.
Saltatory Conduction
Plain English
The way electrical signals "jump" between gaps in myelin to travel faster.
Technical Definition
The mode of action potential propagation in myelinated axons whereby the depolarizing current jumps from one Node of Ranvier to the next, increasing conduction velocity by 10–100x compared to unmyelinated fibers.
Temporal Jitter
Plain English
Random variation in exactly when a signal arrives at its destination.
Technical Definition
Stochastic variation in action potential arrival times at a target synapse across repeated trials, measured as the standard deviation of spike latencies; increased jitter degrades phase-locking precision.
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Brain Networks & Synchrony
5 terms
Coherence
Plain English
Waves or signals moving in lockstep with each other.
Technical Definition
A frequency-domain measure (0–1) of the linear relationship between two signals at a given frequency; high coherence between brain regions indicates consistent phase relationships.
Decoherence
Plain English
Loss of synchronization between brain signals, disrupting communication.
Technical Definition
In the DDH context: the progressive loss of inter-regional oscillatory phase-locking caused by heterogeneous conduction delays from non-uniform demyelination across white matter pathways.
Hebbian Plasticity
Plain English
"Neurons that fire together wire together": synaptic learning.
Technical Definition
The principle that coincident pre- and post-synaptic activity strengthens synaptic connections (long-term potentiation), while decorrelated activity weakens them (long-term depression); operates at the synapse (vertex) level.
Oscillatory Synchrony
Plain English
Brain regions vibrating in coordinated rhythms to communicate.
Technical Definition
The temporal coordination of rhythmic neural activity (oscillations) across spatially separate brain regions, enabling information transfer through phase-locking in specific frequency bands.
Phase-Locking
Plain English
When two signals maintain a consistent timing relationship.
Technical Definition
A condition in which the phase difference between two oscillatory signals remains approximately constant over time, enabling reliable temporal coordination of neural populations.
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Imaging & Measurement
7 terms
Diffusion-Weighted MRI
dwMRI
Plain English
A brain scan that tracks water molecule movement to reveal fiber structure.
Technical Definition
An MRI technique that measures the diffusion of water molecules in tissue, providing contrast based on tissue microstructure; the directional dependence of diffusion reveals axonal fiber orientation and integrity.
Fractional Anisotropy
FA
Plain English
A score (0–1) measuring how organized nerve fibers are in a tract.
Technical Definition
A scalar DTI metric (0–1) quantifying the directional dependence of water diffusion within a voxel; FA=1 indicates perfectly anisotropic (single fiber direction), FA=0 indicates perfectly isotropic (random) diffusion.
ficv
Intracellular Volume Fraction
Plain English
The proportion of tissue occupied by nerve fibers; decreases with age.
Technical Definition
The fraction of the diffusion signal attributed to water molecules restricted within neurites (axons/dendrites), estimated by the NODDI model; age-related decline indicates neurite density loss.
fiso
Isotropic / Free Water Fraction
Plain English
The proportion of freely moving water in tissue; increases with tissue loss.
Technical Definition
The fraction of the diffusion signal from unrestricted isotropic water diffusion (CSF-like), estimated by NODDI; age-related increase indicates expanded extracellular space from cell/myelin loss.
Mean Diffusivity
MD
Plain English
A measure of how freely water moves in brain tissue; higher values suggest tissue damage.
Technical Definition
The average rate of water diffusion across all directions within a voxel (mm²/s), reflecting overall tissue density; elevated MD indicates increased extracellular water from tissue loss or edema.
NODDI
Neurite Orientation Dispersion and Density Imaging
Plain English
An advanced brain scan analysis that separates nerve fiber density from surrounding water.
Technical Definition
A multi-compartment biophysical model of diffusion MRI that decomposes the signal into intracellular (ficv), extracellular/isotropic (fiso), and orientation dispersion (odi) components.
odi
Orientation Dispersion Index
Plain English
How spread out nerve fibers are in different directions; largely unchanged with age.
Technical Definition
A NODDI parameter (0–1) quantifying the angular dispersion of neurite orientations within a voxel; odi=0 indicates perfectly parallel fibers, odi=1 indicates completely random orientations.
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White Matter Tracts
5 terms
Corpus Callosum
Plain English
The thick band of fibers connecting the left and right brain hemispheres.
Technical Definition
The largest white matter commissure (~200 million axons) connecting homologous cortical regions across hemispheres; subdivided into rostrum, genu, body, and splenium.
Corticospinal Tract
Plain English
The pathway carrying voluntary movement commands from the brain to the spinal cord.
Technical Definition
The principal descending motor pathway from primary motor cortex (precentral gyrus) through the internal capsule and brainstem to spinal motor neurons; mediates voluntary movement.
Fornix
Plain English
The main highway for memory signals between the hippocampus and other brain areas.
Technical Definition
A C-shaped white matter bundle forming the principal efferent pathway of the hippocampal formation, connecting it to the mammillary bodies, anterior thalamus, and septal nuclei; critical for episodic memory.
Internal Capsule
Plain English
A deep brain pathway carrying signals between the cortex and lower brain areas.
Technical Definition
A V-shaped white matter structure between the caudate/thalamus and lentiform nucleus, containing projection fibers connecting cortex to thalamus, brainstem, and spinal cord.
Uncinate Fasciculus
Plain English
A pathway connecting emotion and memory regions to the frontal lobe.
Technical Definition
A hook-shaped association fiber bundle connecting the anterior temporal lobe (amygdala, hippocampal formation) with the orbitofrontal cortex; involved in emotional regulation, memory, and language.
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Statistical Methods
9 terms
AIC
Akaike Information Criterion
Plain English
A score that balances model fit against complexity; lower is better.
Technical Definition
An information-theoretic model selection criterion defined as AIC = 2k − 2ln(L), where k = number of parameters and L = maximized likelihood; penalizes overfitting while rewarding goodness of fit.
Akaike Weight
ω
Plain English
The probability that a given model is the best description of the data.
Technical Definition
The normalized relative likelihood of model i: ωi = exp(−0.5·Δi) / Σj exp(−0.5·Δj), where Δi = AICi − AICmin; interpretable as the probability each model is the best approximating model.
Bootstrap
Plain English
A method of estimating statistical confidence by repeatedly resampling the data.
Technical Definition
A nonparametric resampling technique that generates an empirical sampling distribution by drawing B samples (with replacement) from the observed data; used here to construct 95% percentile confidence intervals.
Canonical Variate
Plain English
A weighted combination of variables that captures the strongest cross-domain pattern.
Technical Definition
A linear combination of original variables (imaging or cognitive) derived from CCA that maximizes the correlation with its paired variate from the other domain; structure correlations (canonical loadings) indicate each variable's contribution.
CCA
Canonical Correlation Analysis
Plain English
A technique for finding the strongest shared patterns between two datasets.
Technical Definition
A multivariate statistical method that identifies linear combinations of two variable sets (here: imaging metrics and cognitive scores) that maximize their mutual correlation, revealing latent modes of shared variance.
Confidence Interval
CI
Plain English
A range of values likely to contain the true result.
Technical Definition
An interval estimate expressing the uncertainty of a parameter estimate; a 95% CI means that if the study were repeated many times, 95% of computed intervals would contain the true population parameter.
FDR Correction
False Discovery Rate
Plain English
A method to reduce false positives when running many statistical tests at once.
Technical Definition
False Discovery Rate correction (Benjamini-Hochberg procedure): controls the expected proportion of false positives among rejected hypotheses when performing multiple comparisons, less conservative than Bonferroni but maintains statistical rigor.
Robust Linear Model
RLM
Plain English
A statistical method that is resistant to outlier data points.
Technical Definition
A regression technique using M-estimation (e.g., Huber psi-function with IRLS) that downweights influential observations, providing parameter estimates resistant to non-Gaussian errors without requiring normality assumptions.
Wilks' Lambda
Λ
Plain English
A test statistic for determining whether CCA results are statistically significant.
Technical Definition
A likelihood-ratio statistic for MANOVA/CCA significance testing, defined as the product of (1−R²i) across canonical correlations; smaller values indicate stronger multivariate associations.
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Key Concepts
3 terms
DDH
Decoherence via Demyelination Hypothesis
Plain English
The theory that cognitive aging is caused by uneven myelin loss disrupting brain network timing.
Technical Definition
The hypothesis that age-related cognitive decline is primarily driven by non-homogeneous degradation of myelin across white matter pathways, causing differential conduction delays that disrupt inter-regional oscillatory synchrony required for distributed cognitive processing.
Nonlinear Trajectory
Plain English
A decline pattern that accelerates over time rather than staying steady.
Technical Definition
An age-effect trajectory where the rate of change is not constant; in the DDH data, modeled as a quadratic polynomial (poly(age,2)) showing relative stability in midlife followed by accelerating decline after ~60 years.
Processing Speed
Plain English
How quickly the brain completes a mental task.
Technical Definition
The rate at which cognitive operations are executed, measured by reaction times on speeded tasks; reflects the efficiency of information transfer across brain networks and is highly sensitive to white matter integrity.