Rapid adaptation of the vestibulo-ocular reflex and induced self-motion perception

When a rigid object moves toward the eye, it is usually perceived as being rigid. However, in the case of motion away from the eye, the motion and structure of the object are perceived nonveridically, with the percept tending to reflect the nonrigid transformations that are present in the retinal image. This difference in response to motion to and from the observer was quantified in an experiment using wire-frame computer-generated boxes which moved toward and away from the eye. Two theoretical systems are developed by which uniform three-dimensional velocity can be recovered from an expansion pattern of nonuniform velocity vectors. It is proposed that the human visual system uses two similar systems for processing motion in depth. The mechanism used for motion away from the eye produces perceptual errors because it is not suited to objects with a depth component.     

The definability of objective becoming in Minkowski spacetime

In his recent article On Relativity Theory and Openness of the Future (1991), Howard Stein proves not only that one can define an objective becoming relation in Minkowski spacetime, but that there is only one possible definition available if one accepts certain natural assumptions about what it is for becoming to occur and for it to be objective. Stein uses the definition supplied by his proof to refute an argument due to Rietdijk (1966, 1976), Putnam (1967) and Maxwell (1985, 1988) that Minkowski spacetime leaves no room for objective becoming whatsoever. However, Stein's proof does not seem to go far enough. By considering only what events have become from the standpoint of any given event, Stein's uniqueness proof fails from the outset to allow for a more general kind of becoming whereby it is understood to occur from the standpoint of events on the particular worldlines followed by observers. This suggests that there may, after all, be more than one way to define objective becoming in Minkowski spacetime once each observer's worldline is allowed to figure in the definition. This suspicion is further aroused by two recent proposals for objective, worldline-dependent becoming due to Peacock (1992) and Muller (1992) who advocate ways of defining becoming that are not equivalent to the definition Stein's uniqueness proof delivers. Nevertheless, we show that Stein's uniqueness proofcan be extended in a natural way to cover this more general kind of becoming, provided one does not enrich standard Minkowski spacetime by privileging certain sets of worldlines over others in an unwarranted manner. Thus we aim to reinforce Stein's point that standard Minkowski spacetime does make room for objective becoming, but in essentially only one way, despite arguments and proposals to the contrary.     

Latency of sequential eye movements: implications for reading

The best available estimates indicate that the average minimum latency of saccadic eye movements (175-200 msec) approaches the mean duration of fixations in reading (200-250 msec). This fact presents a problem for models of reading which assume that an eye movement is initiated only after substantial information is processed on a fixation. Three experiments are reported that support earlier estimates of saccadic latency; the experiments were conducted under conditions in which the length of measured latencies could not reflect a motoric refractory period, spatial uncertainty, or temporal uncertainty.     

A developmental study of attribute encoding in free recall

In two studies, subjects from second-grade, sixth-grade, and college were presented lists of unrelated words for single-trial free recall. Embedded in the list were critical items that were either semantically or phonemically related or else were completely unrelated. The sets of related items were either massed or distributed in the longer list. For second-graders, recall of Phonemic words was better than recall of Semantic words while the reverse was true for sixth-graders. Recall of Semantic distributed words by second-graders and of Phonemic distributed words by sixth-graders did not differ from recall of unrelated words. College students recalled Phonemic and Semantic words equally well and all related words better than unrelated words. Developmental trends were seen in the salience of particular attributes and in the utilization of low salient attributes.     

Detection and identification thresholds for consonant-vowel syllables

Detection and identification thresholds were obtained for 6- and 10-year-old normal children and normal adults using five-formant synthesized consonant-vowel ([baHdflrfea]) stimuli. Sixyear-old children were found to have poorer detection than adults, just as they do for pure tones. For the identification task, the slopes of the performance-intensity functions were more shallow for 6-year-old children than for 10-year-olds and adults. Consequently, compared with 10-yearolds and adults, 6-year-old listeners require a greater increase in stimulus intensity above detection threshold to identify these stimuli at a high performance level. The influence of acoustic characteristics of the stimuli on all listeners is also discussed.     

Book reviews

Blanchard, R. J. & Blanchard, D. C. (Eds.). (1986). Advances in the study of aggression, Volume 2. Orlando, Florida: Academic Press. Pp. 228. ISBN 0-12-037702-5. $60.00 (hardback).

Dobbing, J. (Ed.). (1987). Sweetness. Berlin: Springer-Verlag. Pp. 282. ISBN 3-540-17045-6. £42.

Ellen, P. & Thinus-Blanc, C. (Eds.). (1987). Cognitive processes and spatial orientation in animal and man. Dordrecht: Martinus Nijhoff. Vol. 1. Experimental animal psychology and erhology. Pp. 331. ISBN 90-247-3447-9. £60.95. Vol. 2. Neurophysiology and developmental aspects. Pp. 321. ISBN 90-247-3448-7. £60.95