Eyespots in a reversible setting.

When eyespots were presented on a reversible figure, the total duration for which elements bearing the eyespots were seen as closer to the observer was found to be greater than the total for counterpart elements. It is speculated that the tendency to see eyespots as nearer than they really are is related to the manner in which they are responded to in nature.

Inferential methods for comparing a single case with a control sample: modified t-tests versus mycroft et al.'s (2002) modified anova.

Mycroft, Mitchell, and Kay (2002) have criticised existing inferential methods (e.g., Crawford & Howell, 1998) for comparing a single case with a control sample and propose that such comparisons be made using a modified ANOVA. It is argued that the assumptions made by Mycroft et al. are questionable and, even if they held, would not invalidate Crawford and Howell's method. Crawford and Howell's null hypothesis is that the patient is an observation from the control population whereas Mycroft et al.'s null hypothesis is that the control population and a notional population of patients have a common mean. Even if one accepts Mycroft et al.'s conceptualisation, their arguments only have force if (1) the variance of a notional population of patients was larger than that of the control population, and (2) patients with impaired performance were balanced exactly by patients whose performance had been enhanced relative to controls. Furthermore, the modified ANOVA would have the undesirable consequence of reducing statistical power unnecessarily and it requires users to provide some estimate of the variance of a hypothetical population.

Wanted: fully operational definitions of dissociations in single-case studies.

In contrast to the careful consideration given to the issue of what we can infer from dissociations in single-case studies, the more basic question of how we decide whether a dissociation is present has been relatively neglected. Proposals are made for fully operational definitions of a deficit, classical and strong dissociations, and double dissociations. In developing these definitions it was assumed that they should be based on the use of inferential rather than descriptive statistical methods. The scope of these definitions is limited to typical single-case studies in which patients are compared to control samples of a modest size. The operational definition of a classical dissociation incorporates a requirement that a patient's performance on Task X is significantly different from Task Y, in addition to the "standard" requirement that the patient has a deficit on Task X and is within normal limits on Task Y. We ran a simulation to estimate the Type I error rates when the criteria for dissociations are applied and found these to be low (Type I errors were defined as identifying an individual from the control population as having a dissociation). The inferential methods for testing whether the various criteria are met make use of t-distributions. These methods are contrasted with the widespread use of z to test for a deficit or a difference between tasks. In the latter approach the statistics of the control sample are treated as parameters; this is not appropriate when, as is normally the case, the control sample size is modest in size.