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Nine hierarchical and four nonhierarchical clustering algorithms were compared on their ability to resolve 200 multivariate normal mixtures. The effects of coverage, similarity measures, and cluster overlap were studied by including different levels of coverage for the hierarchical algorithms, Euclidean distances and Pearson correlation coefficients, and truncated multivariate normal mixtures in the analysis. The results confirmed the findings of previous Monte Carlo studies on clustering procedures in that accuracy was inversely related to coverage, and that algorithms using correlation as the similarity measure were significantly more accurate than those using Euclidean distances. No evidence was found for the assumption that the positive effects of the use of correlation coefficients are confined to unconstrained mixture models.
This study addresses three themes that recur in the research on student achievement: (a) developmental modeling ofintraindividual changes in achievement over time; (b) examination of the differences among subgroups within a classroom in the determinants of achievement; (c) description of the interactions among instructional variables in determining achievement differences. Eight classrooms were preselected on the basis of their widely differing slopes obtained in a regression analysis of pre- and posttest achievement scores. Mathematics achievement differences among sixth graders were analyzed in a four-wave design and explained by aptitude and instructional variables in a structural equation framework provided by LISREL. The results demonstrate the local nature of achievement models in that neither their measurement nor structural components proved generalizable across both groups of classrooms. Mention is also made, however, of technical problems and analytical ambiguities in the interpretation of these results.
Brain potentials during mental arithmetic: effects of extensive practice and problem difficulty
(1994)
Recent behavioral investigations indicate that the processes underlying mental arithmetic change systematically with practice from deliberate, conscious calculation to automatic, direct retrieval of answers from memory [Bourne, L.E.Jr. and Rickard, T.C., Mental calculation: The development of a cognitive skill, Paper presented at the Interamerican Congress of Psychology, San Jose, Costa Rica, 1991; Psychol. Rev., 95 (1988) 492-527]. Results reviewed by Moscovitch and Winocur [In: The handbook of aging and cognition, Erlbaum, Hillsdale, NJ, 1992, pp. 315-372] suggest that consciously controlled processes are more dependent on frontal lobe function than are automatic processes. It is appropriate, therefore to determine whether transitions in the locus of primary brain activity occur with practice on mental calculation. In this experiment, we examine the relationship between characteristics of event-related brain potentials (ERPs) and mental arithmetic. Single-digit mental multiplication problems varying in difficulty (problem size) were used, and subjects were trained on these problems for four sessions. Problem-size and practice effects were reliably found in behavioral measures (RT). The ERP was characterized by a pronounced late positivity after task presentation followed by a slow wave, and a negativity during response indication. These components responded differentially to the practice and problem-size manipulations. Practice mainly affected topography of the amplitude of positivity and offset latency of slow wave, and problem-size mainly offset latency of slow wave and pre-response negativity. Fronto-central positivity diminished from session to session, and the focus of positivity centered finally at centro-parietal regions. This finding suggests that frontal lobe processing is necessary as long as task performance is not automatized, while automatized arithmetic processing requires parietal brain activity only. The pre-response negativity observed in the first session and during more difficult tasks is assumed to reflect excitatory preparatory processes, which could be associated with activation of calculation strategies.
In panic disorder bodily sensations appear to play an important role as a trigger for anxiety. In our psychophysiological model of panic attacks we postulate the following vicious circle: individuals with panic attacks perceive even quite small increases in heart rate and interpret these changes as being catastrophic. This elicits anxiety and a further increase in heart rate. To evaluate this model we conducted a field study of 28 subjects with panic attacks and 20 healthy controls. A 24 hr ambulatory ECG was recorded and the subjects were instructed to report any cardiac perceptions during this period and to rate the anxiety elicited by these perceptions. The incidence of cardiac perceptions was about the same in both groups, but only subjects with panic attacks reported anxiety associated with such perceptions. Analysis of the ECGs revealed that in both groups heart rate accelerations preceded cardiac perceptions. Following cardiac perceptions, the healthy controls showed a heart rate deceleration, whereas the subjects with panic attacks had a further acceleration. This heart rate increase after cardiac perceptions was positively related to the level of anxiety elicited by the perceptions. These results provide clear evidence in support of the vicious circle model of panic attacks.
This article gives an overview of possibilities for the assessment offacial behavior. With regard to validity, results from a longitudinal study of 36 depressed patients and nine controls as weil as often schizophrenic patients and their relatives will be referred to. These results are used to illustrate the following principles which have to be taken into account when studying facial behavior: a) communication strongly facilitates facial expression, b) activation of facial behavior follows the "principle of least effort", and c) the principle of individual specificity applies to the association of nonverbal behavior and mood states. Making allowance for these principles has, among others, consequences a) for situations or conditions under which to asses behavior (specifically conditions of communication), b) for data analysis (e.g., dealing with frequent and rare events), and c) for empirical or experimental strategies (e.g., aggregation of single-case longitudinal comparisons). From the results on facial behavior during depression it can be concluded that the nonverbal reaction tendencies of endogenous and neurotic depressed patients differ. Moreover, the differential behavioral pattems observed cast doubt on the assumption of a homogeneity of affects in depression. Taking into account the conditions which govern it, facial behavior has proved to be a valid and, especially, a differential indicator for pathoIogic affective states and their changes. Given the fact that a psychiatric illness generally incorporates emotional problems it is more than surprising that little attention has been paid to the systematic study of emotional behavior. Some of the reasons for this will be clarified in the following.
According to more recent studies on memory development in young children, preschoolers and kindergarteners are able to demonstrate surprisingly good memory skills in natural as weH as in laboratory-type settings. This finding is not consistent with the results of a study by Istomina (1975), conducted in 1948, leading to the concJusion that (a) preschoolers do not use voluntary remembering, and (b) children generally recall better in play situations than in typical experimental settings. In this study, two experiments were conducted to replicate Istomina's research. In the first, it was shown that Istomina's findings were replicable when methodological problems in the procedure were ignored. Experiment 2 improved methodologically upon Istomina's experimental methods and did not produce results to support her concJusions. Four- and 6-year-olds showed voluntary memory in play activities as weH as in laboratory-type settings, and remembered equally weB in both contexts. The results did not support the assumption that memory performance in young children can be substantially facilitated by motivating contexts.
The present research examined whether children's awareness of phonological similarities between words with respect to rhyme and consonantal word onset is of the same importance for learning to read German as it was found to be for learning to read English. In two longitudinal studies differences in phonological sensitivity among children before learning to read (at age 6 to 7) were tested with versions of Bradley & Bryant's (1985) oddity detection task. Children's reading and spelling achievements were tested about one year later at the end of grade one, and again at around the age of 10. The main finding was a developmental change in the predictive relationship of rhyme and word-onset awareness. Rhyme awareness was only minimally predictive for reading and spelling achievement at the end of grade one, but gained substantially in predictive importance for reading and spelling achievement in grades three and four. No such predictive improvement was observed for word-onset awareness. It is proposed that rhyme awareness is initially of little importance, because in the first phase of learning to read German children rely heavily on indirect word recognition via grapheme--phoneme translation and blending. The gain in the predictive importance of rhyme awareness is explained by its helpful effect on the establishment of mental representations of written words. Such mental representations allow fast, direct word recognition and orthographically correct spellings. A wareness of larger phonological units is helpful for the efficient establishment of such representations, by allowing connections of recurring grapheme clusters in written words with phonology.
This article describes a 2-systems model that explains social behavior as a joint function of reflective and impulsive processes. In particular, it is assumed that social behavior is controlled by 2 interacting systems that follow different operating principles. The reflective system generates behavioral decisions that are based on knowledge about facts and values, whereas the impulsive system elicits behavior through associative links and motivational orientations. The proposed model describes how the 2 systems interact at various stages of processing, and how their outputs may determine behavior in a synergistic or antagonistic fashion. It extends previous models by integrating motivational components that allow more precise predictions of behavior. The implications of this reflective–impulsive model are applied to various phenomena from social psychology and beyond. Extending previous dual-process accounts, this model is not limited to specific domains of mental functioning and attempts to integrate cognitive, motivational, and behavioral mechanisms.