Differential age effects for case and hue mixing in visual word recognition.

The authors compare older adults' lexical-decision data with younger adults' data reported in P. Allen, A. F. Smith, et al. (2002). On the basis of their work, it was proposed that consistent-case wordswould be processed by the faster holistic (magnodominated) stream, but that mixed-case words would be processed by the slower analytic (interblob-dominated or blob-dominated) steams. Hue mixing was predicted to have no effect on consistent-case performance, but mixed-hue/mixed-case words were predicted to be recognized faster than monochrome/mixed-case words. Younger adults showed the predicted results, but older adults did not. These results suggest that holistic central processes are maintained, but that older adults exhibited an analytic decrement

Age differences in central (semantic) and peripheral processing: the importance of considering both response times and errors.

In this project we examined the effect of adult age on visual word recognition by using combined reaction time (RT) and accuracy methods based on the Hick-Hyman law. This was necessary because separate Brinley analyses of RT and errors resulted in contradicting results. We report the results of a lexical decision task experiment (with 96 younger adults and 97 older adults). We transformed the error data into entropy and then predicted RT by using entropy values separately for exposure duration (thought to influence peripheral processes) and word frequency (thought to influence central processes). For exposure duration, the entropy-RT functions indicate that older adults show higher intercepts and slopes than do younger adults, suggesting an encoding decrement for older adults. However, for word frequency, older adults show higher intercepts but not steeper slopes than younger adults. Older adults thus show a peripheral processing decrement but not a central processing decrement for lexical decision.

How low is too low? Use of cluster analysis to define low levels of mercaptopurine metabolites.

BACKGROUND: To group patients receiving treatment for acute lymphoblastic leukemia (ALL) according to their oral mercaptopurine (6-MP) metabolite levels and to establish cut-off points to screen for potential poor clinical outcome. PROCEDURE: Methodological study using 6-MP metabolite levels from 48 adolescent ALL patients enrolled in a multicenter adherence study. Cluster analysis was the primary analytical technique. We used two validation methods (a split-sampling and a simulation technique) for validating the results. RESULTS: Four clusters were retained in our initial analysis using our first group of patients (n = 27). Three clusters (labeled 1, 2, and 4) exhibited the expected negative correlation between the two metabolites, that is, "high" values of one were associated with "low" values of the other. One cluster (labeled 3) had "low" levels for both TGN and MMP. Five of the 27 adolescents had their 6-MP "held" during the study. Post-hoc examination of the results revealed that all five grouped in Cluster 3 during the time that their medications were stopped, but grouped in other clusters at other times. The median ANC was highest in Cluster 3, consistent with low therapeutic drug levels. Parameters were reproducible with both validation methods. Values below the respective 75th centile for both TGN and MMP in Cluster 3 for the complete sample (n = 48) are suggested as representing a potentially higher risk for relapse. CONCLUSIONS: This study provides an objective method for identifying patients at risk for treatment failure due to suboptimal 6-MP therapy; the clinical significance of this approach should be examined in future studies.

Stimulus contrast and word reading speed in Alzheimer's disease.

The oral word reading speed of patients with Alzheimer's disease (AD) and healthy young and older control participants was evaluated across a broad range of stimulus contrast levels in two experiments. The impact of stimulus repetition on reading speed also was examined. It was found that the older adult participants, and particularly the AD patients, were more sensitive to contrast reductions. Each subject group was able to read repeated words more rapidly than novel words but this repetition effect emerged only at lower stimulus contrast levels. It was concluded that AD patients have feature extraction speeds comparable to non-demented older adults but only when the stimuli are presented at a relatively high contrast. These findings suggest that the automatic encoding processes involved in word recognition remain intact in mildly demented AD patients given stimuli of sufficient strength.

Impact of stimulus integrity on age differences in letter matching.

Young and older adults were tested in both a letter-identification and a letter-matching task in which the integrity of the letter stimuli was manipulated through contrast reduction and low-pass spatial frequency filtering. The use of the contrast and filtering manipulations was an attempt to increase encoding difficulty in an effort to examine whether stimulus integrity impacts more than just the initial encoding of the letter pairs in a letter-matching task, namely the comparison process as indexed by fast-same and false-different effects. Of interest in terms of aging is whether a decline in information-processing performance often reported in the aging literature is related to the known encoding deficits of older adults. In the letter-identification task, both contrast reduction and filtering slowed letter-identification speed for both groups, with the effect being larger for the older adults. In the letter-matching task, decreased processing efficiency produced by the contrast-reduction and low-pass-filtering manipulations led to an overall increase in reaction time and errors, but it did not interact with the magnitude of the fast-same effect or false-different effects for either subject group. These findings suggest that the stimulus integrity manipulations only impact the encoding of the letter pairs in the matching task and not the comparison process. The results of the present study support a dual-process model of the matching task consisting of separate encoding and comparison processes. The finding of a larger fast-same effect for older adults suggests that the age effect is occurring at the comparison stage, but it is not impacted by the stimulus integrity manipulations. The findings are described within a generalized slowing framework.