In opposition to sense-data theory or indirect realism and subjective idealism (representationalism or what we perceive is determined by a psychological representation of the thing), it is James J. Gibson’s theory of perception according to which the structure of the world is directly available to perception (‘what you see is what you get’). Thus, on …
Author Archives: Brian Hopkins
Direct (monogenesis) and indirect (metagenesis and metamorphosis) development
Monogenesis is the direct development of an embryo, without metamorphosis, into an organism similar to the parent. Metagenesis, like the other form of indirect development, metamorphosis, consists of marked changes undergone in a series of successively produced individuals, extending from the one developed from the ovum to the final mature individual. Unlike metamorphosis, metagenesis involves …
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Direct corticomotoneuronal connections (or tracts)
The pyramidal fibers originating in the motor cortex and extending down the spinal cord where they synapse on alpha motoneurons. They connect to the extrafusal muscles of the hand and fingers with just one synapse, and thus are referred to as being ‘monosynaptic’. In the newborn macaque monkey, fibers of this tract have reached …
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Digraphs
Combinations of letters that represent a single phoneme (e.g., ‘gh’ in ‘laugh’). For the most part, these combinations are of consecutive letters, but sometimes they are split (e.g., the ‘o-e’ combination signifies a long ‘o’ sound, as in ‘hope’. See Diagraph, Phoneme
Diploid
From the Greek word meaning ‘double’, a cell or organism having two sets or copies (or homologs) of each somatic chromosome, usually one from the mother and one from the father, and thus twice the haploid number. Nearly all animal cells are diploid, except for the gametes, and some are polypoid (three or more copies …
Digitigrade locomotion
Upright locomotion with the toes at the end of the stance phase being in contact with the floor and the rest of the foot elevated; typical pattern involving heel strike to toes progression for bipedal locomotion in the human. Species who do not have heel strike, and thus a plantigrade locomotion include wolves, dogs, rats, …
Diffusion tensor imaging
A technique used to visualize elements with directional structure, such as bundles of myelinated axons, in the brain. The technique derives its name from the fact that a tensor is computed for each voxel, where a tensor is a mathematical array describing the orientation and magnitude of diffusion in all three spatial dimensions (i.e., the …
Diffusion tensor magnetic resonance imaging
As applied to neuroimaging, this magnetic resonance imaging (also known as diffusion tractography) uses information about the direction of water movement in white matter to determine the size and orientation of its constituent fiber tracts. The underlying principle stems for the fact that when water in unobstructed at body temperature, then the water molecules move …
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Diffusion anisotropy
An important consideration in brain imaging, it refers to the ability of water molecules to move more in certain directions than in others. When water is freely diffusing, it has no directionality (i.e., it is isotropic). In ordered structures such as the cerebral cortex, however, this tendency is most often dependent on, or preferentially restricted …
Differentiation (specific)
In developmental biology, the commitment or specialization of embryonic stem cells to becoming a specific cell type or tissue. See Competence (embryology, Differentiation (embryology), Differentiation (general), Stem cells