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Biomedical subjects

M R Dawson

Publications and source records attributed to M R Dawson.

14 recordsLinked to original sources
PubMed1997

Classification and staging of dementia of the Alzheimer type: a comparison between neural networks and linear discriminant analysis.

OBJECTIVE: To examine the utility of artificial neural networks (ANNs) for differentiating patients with Alzheimer disease from healthy control subjects and for staging the degree of dementia. DESIGN: Comparison of the classification abilities of ANNs with the statistical technique of linear discriminant analysis (LDA) using the results of 11 neuropsychological tests as predictors. PARTICIPANTS: Ninety-two patients with a diagnosis of probable Alzheimer disease (referred from a geriatric clinic) and 43 elderly control subjects (independently solicited). The patients met National Institute of Neurological and Communicative Disorders and Stroke-Alzheimer's Disease and Related Disorders Association criteria for probable dementia, with clinical ratings of dementia severity derived from the Cambridge Examination for Mental Disorders of the Elderly (CAMDEX). MAIN OUTCOME MEASURES: Classifications between and within groups were determined by using LDA and ANNs, and more detailed comparisons of the 2 methods were performed by using chi2 analyses and unweighted and weighted kappa statistics. RESULTS: Linear discriminant analysis correctly identified 71.9% of cases. Artificial neural networks, trained to classify the subjects using the same data, correctly classified 91.1% of the cases. Subsidiary analyses showed that although both techniques effectively discriminated between the control subjects and patients with dementia, the ANNs were more powerful in discriminating severity levels within the dementia population. The analyses for goodness of fit revealed that the ANN classification produced a better fit to the actual data. A comparison of the weighted proportion of agreement between the criterion and predictor variables also showed that the ANNs clearly outperformed LDA in classification accuracy for the full data set and patients-only data set. CONCLUSION: The results demonstrate the utility of ANNs for group classification of patients with Alzheimer disease and elderly controls and for staging dementia severity using neuropsychological data.

Aged↗
PubMed1994

Primitive fossil rodent from Inner Mongolia and its implications for mammalian phylogeny.

The evolutionary origin of rodents is obscured by the group's sudden and highly transformed first appearance in the fossil record in the latest Paleocene. We report here the discovery of nearly complete dental remains of an extraordinary new primitive rodent from strata of transitional Paleocene-Eocene age in Inner Mongolia, China. The strikingly conservative morphological features of this taxon, Tribosphenomys minutus, gen. et sp. nov., substantially modify previous ideas about the ancestral rodent morphotype, which in turn has important implications for understanding the origin of rodents and their relationship to other eutherian mammals. This new fossil, in conjunction with recent morphological and molecular evidence confirming rodent monophyly, indicates the need for a reassessment of phylogenetic affinities among gliriform eutherians. Our results indicate a sister-group position of the new taxon to other rodents, and support the alliance of lagomorphs (rabbits) and rodents (cohort Glires). They also suggest that paraphyly of an extinct assemblage, the 'Eurymylidae', and reveal an unexpectedly complex pattern of character evolution near the ancestry of Rodentia.

Animals↗
PubMed1994

Training redundant artificial neural networks: imposing biology on technology.

One biological principle that is often overlooked in the design of artificial neural networks (ANNs) is redundancy. Redundancy is the replication of processes within the brain. This paper examines the effects of redundancy on learning in ANNs when given either a function-approximation task or a pattern-classification task. The function-approximation task simulated a robotic arm reaching toward an object in two-dimensional space, and the pattern-classification task was detecting parity. Results indicated that redundant ANNs learned the pattern-classification problem much faster, and converge on a solution 100% of the time, whereas standard ANNs sometimes failed to learn the problem. Furthermore, when overall network error is considered, redundant ANNs were significantly more accurate than standard ANNs in performing the function-approximation task. These results are discussed in terms of the relevance of redundancy to the performance of ANNs in general, and the relevance of redundancy in biological systems in particular.

Algorithms↗
PubMed1994

Artificial neural networks that use single-photon emission tomography to identify patients with probable Alzheimer's disease.

Single-photon emission tomographic (SPET) images using technetium-99m labelled hexamethyl-propylene amine oxime were obtained from 97 patients diagnosed as having Alzheimer's disease, as well as from a comparison group of 64 normal subjects. Multiple linear regression was used to predict subject type (Alzheimer's vs comparison) using scintillation counts from 14 different brain regions as predictors. These results were disappointing: the regression equation accounted for only 33.5% of the variance between subjects. However, the same data were also used to train parallel distributed processing (PDP) networks of different sizes to classify subjects. In general, the PDP networks accounted for substantially more (up to 95%) of the variance in the data, and in many instances were able to distinguish perfectly between the two subjects. These results suggest two conclusions. First, SPET images do provide sufficient information to distinguish patients with Alzheimer's disease from a normal comparison group. Second, to access this diagnostic information, it appears that one must take advantage of the ability of PDP networks to detect higher-order nonlinear relationships among the predictor variables.

Aged↗
PubMed1994

Polarity matching in the Ternus configuration.

Two experiments were conducted to examine the effect of changes in the sign of element contrast on perceptions of the Ternus apparent motion display. In the first experiment, the contrast polarity of all three elements in the display were alternated from the first frame of view to the second. At short durations, this increased perceptions (relative to a control condition) of simultaneity in the display, decreased perceptions of element motion, and did not significantly affect perceptions of group motion. At long durations, this manipulation did not affect performance. In a second experiment, patterns of element polarity were manipulated to favour perceptions of either element motion or of group motion relative to a control condition in which all elements had identical contrast polarity. At a long duration, this manipulation affected perceptions of the configuration; this manipulation did not affect the appearance of the display at a short duration. Together, these results are inconsistent with the predictions of Grossberg and Rudd's [Psychological Review, 99, 78-121 (1992)] motion oriented contrast filter. However, they are consistent with a model of motion correspondence processing that includes a polarity matching constraint.

Contrast Sensitivity↗
PubMed1994

Simultaneity in the Ternus configuration: psychophysical data and a computer model.

The Ternus configuration is an apparent motion display which is typically described as being bistable; subjects usually describe seeing either element motion or group motion, depending upon temporal properties of the display. The results of an experiment are reported in which subjects are also permitted to report seeing four stationary display elements (simultaneity). It was found that simultaneity was produced when both frame durations and interstimulus intervals (ISIs) were brief. A weaker than expected effect of stimulus onset asynchrony (SOA) was revealed because this third type of judgment was obtained. Furthermore, statistical analyses indicated that SOA was not by itself the best predictor of judgment type. The interaction between duration and ISI was also an important predictor. This suggested that a complete account of the Ternus configuration requires two mechanisms: a visible persistence mechanism, governed by an SOA law, and a motion correspondence mechanism, governed by an ISI law. These two mechanisms were added to Dawson's (1991) [Psychological Review, 98, 569-603] autoassociative network for motion correspondence processing. The resulting model could generate each of the three interpretations of the Ternus configuration at appropriate combinations of frame duration and ISI.

Association↗
PubMed1991

The how and why of what went where in apparent motion: modeling solutions to the motion correspondence problem.

A model that is capable of maintaining the identities of individuated elements as they move is described. It solves a particular problem of underdetermination, the motion correspondence problem, by simultaneously applying 3 constraints: the nearest neighbor principle, the relative velocity principle, and the element integrity principle. The model generates the same correspondence solutions as does the human visual system for a variety of displays, and many of its properties are consistent with what is known about the physiological mechanisms underlying human motion perception. The model can also be viewed as a proposal of how the identities of attentional tags are maintained by visual cognition, and thus it can be differentiated from a system that serves merely to detect movement.

Attention↗
PubMed1989

The consistency of element transformations affects the visibility but not the direction of illusory motion.

Two experiments tested whether the consistency of element transformations affected perceptions of long-range apparent motion. Vertical lines were used to generate apparent motion against one of two different backgrounds, control and depth. Consistency was manipulated by changing the size of the vertical lines. In some displays, the size of the vertical lines remained constant during movement. In other displays, the size of the vertical lines changed during movement. Consistent movement occurred when the size manipulation was in agreement with the type of background used. In Experiment I, points of subject equality for the quality of motion relative to a standard display were measured. These PSEs indicated that consistent movement (e.g., line sizes held constant for control background displays) was more visible than inconsistent movement (e.g., line sizes constant for depth background displays). In Experiment II, a motion competition display was used to measure thresholds for perceived direction of motion. The depth background was used to make motion in one direction more consistent than motion in the opposite direction. However, no significant differences were noted between thresholds obtained in this condition and those obtained in a control condition. Thus the consistency of element transformations affected the quality of motion, but did not affect the perceived direction of motion. These results are consistent with Ullman's (The Interpretation of Visual Motion, MIT Press, 1979) two-component theory of apparent motion.

Distance Perception↗
PubMed1989

Apparent motion and element connectedness.

Two experiments attempted to determine the effect of the topological property of connectedness on the perception of apparent motion. In Experiment I, direction detection points at subjective equality (PSEs) were measured in a motion competition paradigm. Results indicated that apparent motion was not more likely to be seen between topologically identical elements (e.g., two connected figures) than between elements similar in appearance, but which differed with respect to connectedness. This failed to replicate one finding of Chen (1985). Experiment II tested the possibility that connectedness might affect the quality or visibility of apparent motion. Displacement PSEs for the visibility of motion were measured. Apparent motion between identical figures was significantly more visible than apparent motion between figures similar in appearance, but of different connectedness. These results are discussed in terms of a two-stage model of the long-range motion system.

Humans↗
PubMed1987

Moving contexts do affect the perceived direction of apparent motion in motion competition displays.

Three experiments were performed to test Ullman's [The Interpretation of Visual Motion. MIT Press, Cambridge, Mass. (1979)] independence hypothesis, used in the minimal mapping theory of motion correspondence. Subjects were required to detect the direction of motion (left vs right) of an element in a motion competition display. In control conditions, threshold interelement distances were obtained for this task in the absence of any moving context. In experimental conditions, context elements that moved either to the left or to the right were added to the display. This resulted in changes in thresholds (relative to the control condition) that indicated that a context moving in one direction increased the probability of seeing the competition element move in the same direction. The magnitude of the context effect was shown to be related to the proximity of the context to the competition display, as well as to the number of elements in the context. These results are in conflict with Ullman's independence hypothesis. An alternative model of the motion correspondence process, which uses information about interdependencies between element movements, is briefly discussed.

Humans↗
PubMed1987

Spatio-temporal parameters and the three-dimensionality of apparent motion: evidence for two types of processing.

The minimum ISI required for perceiving apparent motion in depth was measured as a function of the 2D separation of stimuli and the physical separation of stimuli in depth. It was found that temporal thresholds increased as a function of the separation of stimuli in depth. This supports the results of previous research indicating that the perceived three-dimensionality of apparent motion in depth increases with ISI. In addition, the rate of threshold increase was significantly greater in displays with short 2D separations of stimuli than in displays with large 2D separations. This robust functional dissociation of thresholds indicates that the short-range system may be involved in the processing of apparent motion in depth in the former case.

Depth Perception↗
PubMed1979

Biopterin. VI. Purification and primary amino acid sequence of mammalian D-erythro-7,8-dihydroneopterin triphosphate synthetase.

An enzyme that catalyzes the conversion of 2-amino-6-(5'-triphosphoribosyl)amino-5- or 6-formamido-6-hydroxypyrimidine, but not of guanosine triphosphate, to quinonoid 6-(D-erythro-1'-2'-3'-trihydroxypropyl)dihydropterin triphosphate and formic acid has been purified to homogeneity from some mammalian brain and liver. The enzyme of a single strand is a basic protein of 9177 daltons consisting of 68 amino acid residues--except the enzyme from rat brain, which has one additional aspartic acid as residue 7. The enzyme possesses three free SH groups and, in its most active form, 1 mol of phosphate per mole of enzyme. Peptides isolated after hydrolysis with trypsin, chymotrypsin, or weak acid were separated by thin-layer chromatography and sequenced manually by Edman degradation. The complete sequence of the molecule was established as follows: (formula: see text)

Amino Acid Sequence↗