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B Webb

Publications and source records attributed to B Webb.

At least 55 records · Page 3Linked to original sources
PubMed1989

Some additional predictions and further tests of the Marr-Ullman model of motion perception.

The Marr-Ullman model for motion detection in the human visual system functions by means of the dual input of polarity-specific edge detectors and luminance change detectors. Moulden and Begg (1986) found a polarity-specific motion aftereffect which they claimed provided support for this dual input model. The logic of their experiment is examined, and it is shown that several additional predictions arise from the Marr-Ullman model, which were not supported by Moulden and Begg's study. A more powerful experiment was carried out and these additional predictions were disconfirmed, although the polarity-specific effect did emerge. A consideration of alternative explanations of this effect led to a second experiment in which an attempt was made to discover the actual determinants of the effect. This revealed that polarity-specific units are unlikely to play any part in the phenomenon. It was concluded, in the light of this and other evidence, that one of a class of alternative models is more likely to be the actual mechanism for motion perception. However, careful consideration of the Marr-Ullman model indicated that it may be untestable in principle if various differentially weighted levels of neural integration are envisaged.

Attention↗
PubMed1981

Particulate acetylcholinesterase in amniotic fluid and its implications for neural tube defect screening.

Acetylcholinesterase activity in mid-trimester amniotic fluid has been determined by a quantitative spectrophotometric method and a qualitative electrophoretic technique in a limited (76 samples) retrospective study. Differential centrifugation studies on amniotic fluid from normal pregnancies demonstrated the presence of particle-associated acetylcholinesterase sedimenting only at relatively high centrifugal forces. It is postulated that this particle-associated activity was a contributory factor to an observed false-positive incidence of 6.6% when quantitative acetylcholinesterase measurement was used to discriminate between normal pregnancies and those associated with neural tube defects. Discrimination on the basis of electrophoretic analysis of acetylcholinesterase resulted in no misclassification of pregnancies. Particulate acetylcholinesterase would not be expected to influence the interpretation of the electrophoretic pattern due to its size-related exclusion from gel pores. The implication of these observations in the accurate prenatal diagnosis of neural tube defects is discussed.

Acetylcholinesterase↗
PubMed1979

Orienting responses and locus of control.

Few studies have attempted to relate the locus of control (Rotter, 1966) variable to physiological activity. This paper describes the heart-rate and skin conductance responses of 40 subjects exposed to a series of auditory stimuli, in relation to their locus of control scores. It was found that those scoring relatively 'externally' on the locus of control scale gave less decelerative and more accelerative heart-rate responses and tended to give larger skin conductance responses when there was a change in stimulus characteristics, than those scoring 'internally'. These results may be interpreted to support Lazarus' (1966) suggestions that control over a situation lessens the threat perceived in that situation. They also indicate that internal subjects tend to seek information to increase their control.

Adult↗