Schaffer's Demon

Jonathan Schaffer (2010) has summoned a new sort of demon – which he calls the debasing demon – that apparently threatens all of our purported knowledge. We show that any debasing skeptical argument must attack the justification condition and can do so only if a plausible thesis about justification is false.     

Relativity of judgements about sound amplitude and the asymmetry of the same-different ROC

The problem of deciding whether two things are the same or different in magnitude can be solved by judging one magnitude relative to the other, or by making absolute judgements about the magnitude of each. The shape of the resulting receiver operating characteristic depends on which solution is adopted. In order to obtain empirical receiver operating characteristics, we therefore had subjects rate their confidence that two tone amplitudes were the same or different. Four subjects each made 500 ratings of three differences in amplitude. The asymmetry in the obtained characteristics indicated that subjects made relative rather than absolute judgements of the amplitudes, despite the fact that making absolute judgements would lead to better performance on the task.     

The Effect of Practice on the Control of Rapid Aiming Movements: Evidence for an Interdependency Between Programming and Feedback Processing

The purpose of this experiment was to investigate how the control of aiming movements performed as fast and as accurately as possible changes with practice. We examined: (1) the influence of visual feedback on the initial impulse and error correction phases of aiming movements during acquisition; and (2) the effect of removing visual feedback at different levels of practice. Results from the acquisition trials indicated that vision had a major impact on the organization of the initial impulse and error correction phases. Also, consistent with findings from research involving temporally constrained movements, the cost of removing vision was greater after extensive levels than after moderate levels of practice. Collectively, these results denote the importance of visual feedback to the learning of this particular class of aiming movements. Learning appears to be a dual process of improved programming of the initial impulse and increased efficiency of feedback processing. Practice not only acts on programming and feedback processes directly, but also indirectly through a reciprocal interplay between these two processes.     

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..