Systematic variance thatcharacterizes an individual or a group of individuals. It can be accounted for in statisticalapproaches such as multiple level modeling. See Multilevel modeling (MLM), Person-centered approaches
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Permissive interaction
One of two types of proximate tissueinteractions (also referred to as secondary induction) duringembryogenesis. In permissiveinteraction, the inducing tissue provides signals to start differentiation in atissue that is already committed to respond. In contrast, instructive interaction refers to the inducing tissuetransmitting signals to cells to adhere to a specific developmental pathway. SeeCompetence (embryology), Differentiation (embryology), Embryogenesis, …
Persistent middle-ear effusion
An effusion that persists in the middle ear following an episode of acute otitis media. Can give rise to hearing loss or diminution, but is typically not painful. See Otitis media with effusion (OME)
Periventricular white matter
White matter connecting fiber systems on either side of the lateral ventricles. It is classified on the basis of which lobes in the brain it is located. Damage to periventricular white matter in preterm infants is typically evident in the collateral trigone n the floor of the ventricles. See Periventricular leucomalacia (PVL), Preterm infant, Ventricle
Periventricular hemorrhagic infarction (PHI)
A unilateral periventricular lesion of the brain where an schema of the brain is complicated by a hemorrhage. It is a neuropathological consequence of bleeding in the germinal matrix (a tissue with small, fragile blood vessels situated mainly in the periventricular regions of the head of the caudate nucleus in the thalamus), which is referred …
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Periventricular leucomalacia (PVL)
A bilateral lesion of the periventricular white matter thought to be due to ischema, although there is some debate about its causes. First described in 1869 by Rudolf Ludwig Karl Virchow (1821-1902), it is typified by damage to the cerebral white matter bordering on the lateral ventricles. Consists of areas of necrosis that often form cavitating cysts, …
Peristaltic
Worm-like movements in a tubular structureconsisting of alternating and orderly contractions and relaxations oflongitudinal and circular muscles leading to propulsion of substances in its centralcavity. Evident in first in esophagusand then the stomach and intestines of the digestive system in which there is arepeated process of contraction and relaxation as ingested food is movedthrough the …
Period effect
Aneffect on a mean value or association that changes at a particular time andaffecting all age groups and cohorts in a uniform manner. In longitudinal research, this effect has tobe distinguished from those relating to (chronological) age and cohort. See Cohort, Cohort effect, Longitudinal studies
Peripheral nervous system (PNS)
That part of the nervous system consisting of 12 pairs of cranial nerves (except for the optic nerve), 31 pairs of spinal nerves, their peripheral combinations, and the peripheral portions of the autonomic nervous system (consisting in turn of the sympathetic and parasympathetic nervous systems). The cranial nerves are connected directly to the brain, the spinal …
Perinatal focal damage
Localized damage to the brain of a fetus or infant, occurring during the period around birth (5 weeks before to 1 month after). Via longitudinal studies there are indications that the developmental consequences of such damage can be sometimes quite subtle ranging from early language acquisition problems to deficits concerning spatial analyses. See Fetus, Hypoxic-ischemic …