Cross-cultural differences in colour vision: Acquired ‘colour-blindness’ in Africa

We report a study of the incidence of ‘colour-blindness’ in southern and central Africa, and we compare the African data with data from various European groups. There was a surprisingly high incidence of tritan errors (yellow–blue defect). The likelihood of making tritan errors increased with age, and was greater in rural areas than in towns. In Europe, no tritan errors were made by samples from the U.K., Eire or Spain, but some tritan errors were made by a sample from southern Greece. In contrast, most of a British sample of people over sixty-five years old makes tritan errors. Although tritan errors were the most frequent, they were often accompanied by protan and deutan errors. This mixed pattern of errors is consistent with the condition being acquired rather than congenital. Many languages of southern Africa categorise blues and greens with the same term. If the tritanopia we report has been endemic, it may have reduced the ‘perceptual pressure’ to split the blue-with-green categories into separate blue and green terms; a speculation consistent with Rivers, W. H. R. (1901. Introduction to A. C. Haddon (Ed.), Reports on the Cambridge Anthropological Expedition to the Torres Straits. Cambridge: Cambridge University Press) © 1998 Elsevier Science Ltd. All rights reserved..     

An analysis of the word-superiority effect

It is easier to decide which of two letters was presented tachistoscopically if the critical letter was in a word rather than in a scrambled word. We showed that this word-superiority effect holds just as strongly for pronounceable nonwords as for words, even when the critical letters are constant over all trials. This finding rules out word meaning and familiarity as variables accounting for the effect. In addition, it was found that the superiority of pronounceable stimuli holds for two-letter stimuli as well as four, and it is therefore concluded that the effect is not due to a memory limitation. An explanation of the effect in terms of the use of additional acoustic information is ruled out by showing that the effect was not diminished when the two possible words sounded exactly alike. An experiment using correctly and incorrectly spelled chemical formulas suggested that spelling regularities, regardless of pronounceability per se, account for the superiority effect. Finally, when decisions about two critical letters must be made on each trial, the correlation between being correct on one and on the other is higher for pronounceable stimuli under some conditions.     

Repeated vs varied distractors in immediate memory

Ss heard groups of eight spoken digits presented for immediate recall. Irrelevant distractor letters were interpolated between digits, using either the same letter interpolated eight times or eight different letters. The latter condition produced significantly more errors in recall. Error scores were also affected by rate of presentation of items and by whether letter-digit sequences started with a letter or a digit. It is suggested that selective immediate recall may bear some resemblance to aspects of visual search.     

The relative lability of mobile and stationary components in a visual-motor adaptation task

The results of previous studies of intermanual transfer of prism adaptation may have been biased because the arm was mobile during training, rather than for instance the head. It was hypothesized that adaptation will be more associated with a joint when it is mobile rather than stationary. Intermanual transfer when arm was mobile and head stationary was compared with that when head was mobile and arm stationary. Also measured was the adaptation persisting when the mobile part in training was stationary in testing. The results showed, against the original hypothesis, that adaptation tends to be associated with a stationary joint. This fits in with evidence that there is greater position uncertainty in a stationary joint and that this is associated with adaptability.     

Detection of temporal gaps within and between perceptual tonal groups

Two experiments were conducted to explore the hypothesis that tones widely separated in frequency are processed in separate communication channels. Listeners attempted to detect brief temporal gaps between items in a simple tonal sequence of two high tones followed by two low tones. Temporal resolution within both the high and low groups was near perfect; between-group detection scores were significantly lower. Results are interpreted as evidence for a processing time delay when shifts of focal attention occur between perceptual structures organized within the frequency domain.     

On the determination of redundancies in sociometric chains

The use of a matrix to represent a relationship between the members of a group is well known in sociometry. If this matrix is raised to a certain power, the elements appearing give the total number of connecting paths between each pair of members. In general, some of these paths will be redundant. Methods of finding the number of such redundant paths have been developed for three- and four-step chains by Luce and Perry (3) and Katz (2), respectively. We have derived formulas for the number of redundant paths of five and six steps; and in addition, an algorithm for determining the number of redundant paths of any given length.The research leading to this paper was supported by a grant from the Rockefeller Foundation.     

After-images from retina and brain

The evidence pointing to the retinal origin of after-images is considered. The reports of the occurrence of after-images from visual images of hallucinatory vividness are reviewed.

Experimental results are presented to indicate that a complementarily coloured afterimage may arise following the exposure of the temporarily blind retina to a coloured stimulus.

After-images, or after-effects, from vivid images are described in seventeen persons (mostly possessors of “number-forms”). They are found to move with the eyes and to show, in some persons, a degree of conformity with Emmert's Law which, while considerable, is less than that of after-images of real stimuli. In the case of one “eidetic” subject, the after-images from neither real nor imaged stimuli conformed with Emmert's Law. In some persons, after-images of images occur in complementary colours.

The retinal origin of after-images is affirmed, but that they can occur occasionally as a purely central phenomenon is acknowledged. The possible learned or inherent nature of after-images of central origin is discussed.