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This work tackles the problem of whether the dissociation between two performances in a single-case study should be computed as the difference between the raw or between the standardized (e.g. z) scores. A wrong choice can lead to serious inflation of the probability of finding false dissociations and missing true dissociations. Two common misconceptions are that (i) standardized scores are a universally valid choice, or (ii) raw scores can be subtracted when the two performances concern the same "task/test", otherwise standardized scores are better. These and other rules are shown to fail in specific cases and a solution is proposed in terms of in-depth analysis of the meaning of each score. The scores that should be subtracted are those that better reflect "deficit severities" - the latent, unobservable degrees of damage to the cognitive systems that are being compared. Thus explicit theoretical modelling of the investigated cognitive function(s) - the "scenario" - is required. A flowchart is provided that guides such analysis, and shows how a given neuropsychological scenario leads to the selection of an appropriate statistical method for detecting dissociations, introducing the critical concept of "deficit equivalence criterion" - the definition of what exactly a non-dissociation should look like. One further, overlooked problem concerning standardized scores in general (as measures of effect size, of which neuropsychological dissociations are just one example) is that they cannot be meaningfully compared if they have different reliabilities. In conclusion, when studying dissociations, increases in false-positive and false-negative risks are likely to occur when no explicit neuropsychological theory is offered that justifies the definition of what are to be considered as equivalent deficit severities in both performances, and which would lead to appropriate selection of raw, standardized, or any other type of score. More generally, the choice of any measure in any research context needs explicit theoretical modelling, without which statistical risks cannot be controlled.

Twelve children with developmental dyslexia underwent a four-month treatment with tachistoscopic presentation of words, according to Bakker's methodology. One group received standard lateral presentation of words on a PC screen, while the other group received the same stimuli in random lateral position. The spatial distribution of visual attention was measured by means of the Form-Resolving Field (FRF; ), which was administered along with reading tests, before and after treatment. The FRF of children who received random presentation widened at -12.5 degrees on the left side, while the FRF in the group that received standard lateral presentation narrowed at that position. Both groups significantly improved in reading accuracy for both words and nonwords. Some hypotheses are proposed concerning the mechanisms responsible for the changes in the FRF and their correlation with improvements in word and nonword reading. The results of the present study are also compared with data suggesting a left "minineglect" in dyslexia.

Neglect of contralesional stimuli and perseverative behavior on ipsilesional stimuli may co-occur on a target cancellation task. Our aim was to investigate whether the presence vs. absence of contralesional targets can modify perseveration on ipsilesional targets. We studied four right brain-damaged patients with left neglect and perseverative behavior on screening cancellation tasks. We compared their cancellation performance in two conditions: (i) targets equally distributed on both sides of space and (ii) targets confined to the right side. One patient showed no significant difference in perseveration between these two conditions; in contrast, three patients perseverated significantly more in condition (i) than in condition (ii). These results suggest that, at least in some patients, information from the ‘neglected’ targets is not completely lost, but rather it affects behavior in the ipsilesional side. The traditional dichotomy ‘bad space–good space’ would not apply here.

Bisiach and his colleagues have developed a new version of Milner’s ‘‘landmark task’’ for the purpose of separating ‘‘perceptual’’ and ‘‘response’’ biases in neglect patients. Subjects are required to decide which is the longer (or the shorter) of the two portions of a pre-bisected horizontal line. The authors proposed two indices to measure perceptual and response bias respectively. However, these indices are not mathematically independent of one another. Moreover, they do not exploit all of the information available in the data, since they do not consider the effect of the different transection locations across trials. We now propose an alternative means of analyzing data from the revised Landmark task, which generates independent estimates of perceptual and response biases. The method and its theoretical foundation are summarized, and illustrative data obtained from brain damaged patients and control subjects are presented. ≈ 2002 Elsevier Science (USA)