Distributed memory and the representation of general and specific information.

Describes a distributed model of information processing and memory and applies it to the representation of general and specific information. The model consists of a large number of simple processing elements that send excitatory and inhibitory signals to each other via modifiable connections. Information processing is thought of as the process whereby patterns of activation are formed over the units in the model through their excitatory and inhibitory interactions. The memory trace of a processing event is the change or increment to the strengths of the interconnections that results from the processing event. The traces of separate events are superimposed on each other in the values of the connection strengths that result from the entire set of traces stored in the memory. The model is applied to a number of findings related to the question of whether abstract representations or an enumeration of specific experiences are stored in memory. The model simulates the results of a number of experiments that have been taken as evidence for the enumeration of specific experiences. At the same time, it shows how the functional equivalent of abstract representations—prototypes, logogens, and even rules—can emerge from the superposition of traces of specific experiences, when the conditions are right for this to happen. (50 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The effects of density and test conditions on measured compression and shear strength of cancellous bone from the lumbar vertebrae of ewes

An animal model (the ewe) was used to study mechanical parameters of cancellous bone specimens. Compression and shear tests were conducted on ewe vertebral trabecular bone (L1-L5) from old ewes (mean age: 9 years) under two different conditions: first, at room temperature in air ("standard" test conditions); and secondly, in a physiological saline bath regulated at 37 degrees C. The parameters obtained under "standard" test conditions with a uniaxial compression test were the mean value of the maximum strength (sigma max = 22.3 (7.06) MPa), Young's modulus (E = 1510 (784) MPa), the strain at maximum strength (epsilon sigma max = 3.21 (0.8) percent) and the energy absorbed during the test (W = 0.3 (0.12) MJ.m-3). No significant change was found when the test was carried out in a saline bath at 37 degrees C (p < 0.0005). An original shear test was performed to evaluate the shear strength which was found to vary from 7.5 (4.7) to 14.6 (8.53) MPa under "standard" test conditions depending on the method of calculation. Testing of the specimens in a 37 degrees C physiological saline bath induced a decrease in the shear strength from 32.5 percent (p < 0.0005) to 37.3 percent (p < 0.0001) of those measured under "standard" test conditions. The non-destructive measurement of the Bone Mineral Density (BMD) accounted for up to 73.3 percent of the maximum compressive strength sigma max and 61.5 percent of the maximum shear strength tau max determined in saline solution at 37 degrees C. These results showed that other parameters influencing the mechanical properties of trabecular bone and its structure appeared to be essential.     

Mechanical properties of ewe vertebral cancellous bone compared with histomorphometry and high-resolution computed tomography parameters

This paper is the result of discussions between scientists working in widely separated areas, united by an interest in the hippocampus. The discussions focused on the possible role of GABA in the development and maturation of the hippocampus and in neurodegeneration in Alzheimer's disease (AD). GABA neurons are among the first to differentiate in the hippocampus and the properties of GABA neurotransmission in the developing hippocampus are distinct from those in the adult. GABAergic transmission may play a role in the clustering and maturation of GABA receptors, as well as of receptors for other neurotransmitters. The development and maturation of synaptic connections involves changes in the organization of the cytoskeleton, and mechanical force generation is probably required to establish appropriate points of contact. This generation of force may require coupling of specific receptors to the cytoskeleton through specialized proteins. In AD, much of the developmental process is progressively unraveled in the hippocampus, as afferent fibers, most notably from entorhinal excitatory neurons and from basal forebrain cholinergic cells, degenerate. This denervation undoubtedly has consequences for receptor systems, dendritic morphology and the underlying cytoskeleton. GABA neurons remain in the AD hippocampus, and may actually contribute to abnormal firing and degeneration of remaining pyramidal neurons. This attempt to bring together data from different areas of research has allowed the development of a scheme which identifies significant specific gaps in our knowledge, which could be readily filled by focused experimental work.     

An interactive activation model of context effects in letter perception: I. An account of basic findings.

Describes a model in which perception results from excitatory and inhibitory interactions of detectors for visual features, letters, and words. A visual input excites detectors for visual features in the display and for letters consistent with the active features. Letter detectors in turn excite detectors for consistent words. It is suggested that active word detectors mutually inhibit each other and send feedback to the letter level, strengthening activation and hence perceptibility of their constituent letters. Computer simulation of the model exhibits the perceptual advantage for letters in words over unrelated contexts and is considered consistent with basic facts about word advantage. Most important, the model produces facilitation for letters in pronounceable pseudowords as well as words. Pseudowords activate detectors for words that are consistent with most active letters, and feedback from the activated words strengthens activations of the letters in the pseudoword. The model thus accounts for apparently rule-governed performance without any actual rules. (50 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)