1,240 articles
Hearing
Hearing: the sense converting sound waves into electrical signals via the ear canal, eardrum, malleus-incus-stapes chain, and cochlear hair cells, with tonotopic coding carried by the auditory nerve to the brain; distinct from balance, the inner ear's other function.
Hearing aid
Hearing aid: wearable medical device amplifying sound for impaired hearing; air- and bone-conduction types, digital signal processing, FDA OTC category since October 2022, styles from BTE to CIC, and global use by only 17% of those who could benefit.
Hearing loss
Hearing loss (deafness): reduced hearing versus 20 dB thresholds, from mild to profound. Types sensorineural, conductive, mixed; 430 million need rehabilitation; first gene therapy (Otarmeni, OTOF) approved 2026.
Hearing range
Hearing range, the span of frequencies and intensities an ear detects; humans hear about 20 Hz to 20 kHz (adults often 15–17 kHz upper limit), best sensitivity 2000–5000 Hz, declining with age.
Hebbian theory
Hebbian theory, also called Hebbian learning or Hebb's rule, is the principle from Donald Hebb's 1949 book The Organization of Behavior that synapses strengthen when neurons repeatedly fire together.
Hemianopsia
Hemianopsia is the loss of one half of the vertical visual field, usually caused by stroke, brain tumor, or traumatic brain injury, occurring in homonymous or heteronymous forms depending on where the visual pathways are damaged.
Hemispatial neglect
Hemispatial neglect, also called unilateral spatial neglect or hemineglect, is a post-stroke attention disorder in which patients ignore the side of space opposite a brain lesion, usually the left after right hemisphere stroke.
Hemispherectomy
Hemispherectomy: neurosurgical removal or disconnection of one cerebral hemisphere to treat drug-resistant epilepsy. Includes anatomic hemispherectomy, functional hemispherectomy, and hemispherotomy; about 73% pooled seizure freedom, hydrocephalus the main complication.
Henry Molaison
Henry Gustav Molaison (H.M., 1926–2008), American epilepsy patient whose 1953 bilateral medial temporal lobe resection caused profound amnesia; decades of study by Brenda Milner revealed distinct memory systems and the hippocampus's role.
Hereditary benign intraepithelial dyskeratosis
Hereditary benign intraepithelial dyskeratosis, or HBID, is a rare inherited disorder causing white plaques on the conjunctiva and mouth, first described in the Haliwa-Saponi tribe of North Carolina in 1960.
Hereditary spastic paraplegia
Hereditary spastic paraplegia (HSP), or Strümpell-Lorrain syndrome, is a group of inherited disorders causing progressive leg spasticity and weakness from corticospinal tract degeneration; over 80 genes, SPG4 most common, pure form in about 90%.
Heritability of autism
Heritability of autism: the share of autism risk variance attributable to genetic factors, estimated at roughly 80% via twin, family, and SNP-based studies, with common variation explaining most risk.
Herpes simplex keratitis
Herpes simplex keratitis (HSK, herpetic keratitis) is HSV-1 corneal infection, a leading infectious cause of blindness. Dendritic epithelial disease heals with trifluridine or acyclovir; stromal disease needs steroids plus antivirals.
Herpes simplex virus stromal keratitis
Herpes simplex virus stromal keratitis (HSK, herpes stromal keratitis) is immune-mediated corneal stromal inflammation after HSV-1 infection, causing scarring and vision loss; treated with tapered topical corticosteroids plus oral antivirals, as established by the HEDS trials.
Heterosynaptic plasticity
Heterosynaptic plasticity: lasting synaptic strength changes at synapses not active during induction, also called modulatory input-dependent plasticity. It counteracts Hebbian runaway dynamics, stabilizes synaptic weights, and supports long-term memory maintenance.
Hexosaminidase (β-N-acetylhexosaminidase)
β-N-acetylhexosaminidase (Hex A αβ, Hex B ββ, Hex S αα; HEXA/HEXB) is a lysosomal GH20 enzyme; only Hex A with GM2 activator degrades GM2 ganglioside, and its deficiency causes Tay-Sachs and Sandhoff diseases.
High-altitude cerebral edema
High-altitude cerebral edema (HACE), the severe end-stage form of acute mountain sickness, is hypoxia-induced brain swelling with ataxia and altered mental status above roughly 4,000 m, treated by descent, oxygen, and dexamethasone.
Hilar mossy cell
Hilar mossy cell: glutamatergic principal neuron of the dentate gyrus hilus, marked by thorny excrescences, axons reaching up to 35,000 granule cells; key in pattern separation, vulnerable in epilepsy.
Hippocampal granule cell
Hippocampal granule cell, dentate gyrus principal neuron (dentate granule cell). Receives entorhinal input, fires ultra-sparsely, sends mossy fibers to CA3 as detonator synapses for pattern separation; generated by adult neurogenesis.
Hippocampal pyramidal neuron
Hippocampal pyramidal neuron (CA pyramidal cell): glutamatergic principal neuron of hippocampal fields CA3, CA2, CA1 and subiculum; receives ~30,000 CA3 synapses in CA1, fires at ~1.4 Hz, and shows marked within-type heterogeneity.
Hippocampus
Hippocampus: seahorse-shaped medial temporal lobe brain structure essential for forming new declarative memories and spatial navigation; subfields DG, CA3, CA2, CA1; defined by H.M. amnesia, place cells, and cognitive map theory.
Histaminergic neuron
Histaminergic neurons, the brain's only histamine-releasing neurons, sit in the tuberomammillary nucleus of the posterior hypothalamus, fire as wake-promoting pacemakers, project throughout the CNS, and signal via H1–H4 receptors.
Hoffmann's reflex
Hoffmann's reflex (Hoffmann sign, finger flexor reflex): bedside flick of the middle finger causing thumb and finger flexion, an upper-limb corticospinal tract sign; distinct from Paul Hoffmann's electrically evoked H-reflex.
Holoprosencephaly
Holoprosencephaly (HPE) is the most common human forebrain malformation, in which the cerebral hemispheres fail to separate. Alobar, semilobar, lobar, and MIH forms; SHH pathway disruption; about 1 in 10,000 live births.
Homer family
Homer family (Homer1, Homer2, Homer3; Vesl) proteins: postsynaptic density adaptor proteins binding mGluR1/5, IP3 receptors, and Shank via EVH1 domains; long forms multimerize, short Homer1a is an activity-induced dominant negative.
Homosynaptic plasticity
Homosynaptic plasticity: activity-dependent change (LTP or LTD) at synapses that were themselves active during induction, Hebbian, input-specific, NMDA receptor and calcium dependent, distinct from heterosynaptic plasticity at unstimulated synapses.
Horner's syndrome
Horner's syndrome (oculosympathetic paresis) is the triad of miosis, ptosis, and anhidrosis from disruption of the three-neuron sympathetic pathway to the eye; described by Horner in 1869, confirmed with apraclonidine or cocaine testing, and localized with hydroxyamphetamine.
Hue
Hue: the color attribute ordering sensations around a color circle, independent of lightness and saturation. Rooted in cone-opponent retinal processing, formalized from Newton's color circle to CIECAM02 hue angle.
Human brain
Human brain: the three-pound central organ of the human nervous system, with about 100 billion nerve cells, an 80-percent-of-volume cortex, a restrictive blood-brain barrier, and a threefold size increase over two million years of hominin evolution.
Human brain mapping
Human brain mapping: measuring and charting human brain structure and function with PET, fMRI, EEG, MEG, and related methods; also a Wiley neuroimaging journal founded in 1993.