Binocular cues in the perception of distance of a point source of light

The ability to make egocentric distance estimates of a single point source of light, seen in darkness and without the cues of changing size and luminance, was investigated in sixteen observers. The attenuation required to maintain constant luminance, when the target was viewed from different distances, was shown to follow the inverse square law providing the angle subtended by the light was less than 20 s arc. Distance changes were also simulated by means of a split mirror which produced vergence cues, or by test lenses to provide accommodation cues. Over the range 0.5 to 9.2 m distance estimates were surprisingly accurate, although there was some overestimation of near and underestimation of far distances. Most observers made good judgements when only convergence cues were varied, whereas no observers made consistently good judgements when only accommodation cues were varied. The difficulties are discussed in terms of the accommodation-convergence link. When distance was simulated by changing convergence and accommodation cues, estimates were not as good as when real distance was changed. Since good estimates were made with brief target exposures, these judgements were not based on subsequent convergence or accommodation changes. It is suggested that the metric or reference against which the apparently absolute judgements were made was the efferent demand signal associated with a 'resting' position of convergence in darkness.     

Deconstructing the Tower of London: Alternative moves and conflict resolution as predictors of task performance

Despite widespread use the cognitive demands of the five-disc Tower of London (TOL) are unknown. Research suggests that conflict moves (those that are essential to the solution but do not place a disc in its final position) are a key aspect of performance. These were examined in three studies via a verification paradigm, in which normal participants were asked to decide whether a demonstrated move was correct. Experiment 1 showed that individual move latencies increase with the number of intermediate moves until the disc is placed in its goal position (resolution). Post hoc tests suggested that the number of alternative moves and moves to resolve a disc were independent predictors of performance. Experiment 2 successfully manipulated these factors in an experimental design. Experiment 3 showed that they remain determinants of performance as familiarity increased. Overall, errors on the task were significantly correlated with spatial memory. The implications of these findings for the use of the TOLin cognitive psychology and as an assessment tool are discussed.     

Book reviews

TOATES, F. Animal Behaviour: A Systems Approach. Chichester: John Wiley and Sons. 1980. Pp. 299. Hardback £17. Paper back £6.40 ISBN 0 471 27724.

DICKINSON, A. Contemporary Animal Learning Theory. Cambridge: University Press. 1980. Pp. 177. Hardback £12.50. Paperback £3.95. ISBN 0 521 23469 7.