Visual persistence as measured by reaction time

Latency of reaction to onset of a visual display was subtracted from latency of reaction to offset. Persistence was defined as difference between the two latency values. Persistence was inversely related to stimulus duration and was comparable for monoptic presentation and for presentation of the first half of the stimulus duration to one eye and the second half to the other. A power function described the relation between persistence and stimulus duration. The possible effects of central intermittency on this type of reaction time measure are discussed.     

Psychophysiological reactions to understimulation and overstimulation

Catecholamine output, heart rate, and performance efficiency were examined in 28 Ss during two contrasting stimulus conditions, one of understimulation, and one of overstimulation. During understimulation the subject spent 3 hr performing a vigilance task, and during overstimulation he was exposed for the same time period to a complex sensorimotor test. Both understimulation and overstimulation produced a significant increase of adrenaline and noradrenaline release as compared with a control condition involving a medium amount of stimulation. Subjects who excreted relatively more adrenaline performed significantly better during understimulation, whereas subjects with relatively lower excretion rates of adrenaline tended to perform better under overstimulation. When performance efficiency was related to heart rate it was shown that high-heart rate subjects performed better during understimulation, while low-heart rate subjects performed better during overstimulation.     

Methylphenidate increases selectivity of visual scanning in children referred for hyperactivity

Visual scanning patterns were investigated in 32 children referred for symptoms of hyperactivity in a doubleblind crossover comparison of methylphenidate and placebo treatments. Total errors, response latency, and visual fixations were recorded as the child scanned computer-generated visual matching-to-sample problems. Results indicated that the number of fixations on the standard stimulus in the matching task was significantly larger in the methylphenidate state. Drug treatment also resulted in a significant increase in the number of systematic comparisons between the standard and the variants in the task. However, the increased selectivity of attention to the standard stimulus was not accompanied by a reduction of total errors. It was suggested that the stimulant drug may increase attentional selectivity even when such a shift fails to produce improvement in task performance.This research is based in part upon a master's thesis completed by the first author in the department of Psychology, University of Guelph. The authors are indebted to Dean H. Owen for providing the random polygons used in the experiment. The research was supported by grants from the Ontario Mental Health Foundation and the Medical Research Council of Canada to James M. Swanson.     

Development of strategies for recalling letter sequences

Kindergarten, second-grade, and fourth-grade children (6, 8, and 10 years of age, respectively) were asked to recall letter sequences on a task that required the use of: (a) a verbal strategy; (b) a positional strategy; or (c) either strategy. Additional trials, allowing either strategy, were interspersed among those requiring a verbal or positional strategy; strategy use was determined from performance on these interspersed trials. The results show that children at all grade levels attempted to use a task-appropriate strategy. Performance on the verbal task showed a more marked developmental increase than did performance on the positional task. A second experiment replicated the first with the interspersed trials removed. A third experiment was designed to determine whether letter names interfered with older children's efficient use of a positional strategy. Even with this source of interference removed, the developmental pattern of Experiments 1 and 2 was replicated. There was a Sex by Condition interaction in Experiments 2 and 3 which showed that males do better in the condition requiring a positional strategy, and females do better in the condition requiring a verbal strategy.     

Aphasia, dyslexia and the phonological coding of written words

A possible account of the reading difficulty of certain aphasic-dyslexic patients includes the notion that they are impaired in translating the written word into a phonological code via grapheme-phoneme conversion rules. This notion was tested in two experiments, both utilizing orthographically regular non-words (like dake) as stimuli. The first experiment provides an analysis of two patients' (largely successful) attempts to repeat non-words, and their (almost totally unsuccessful) attempts to read them. Second, in a lexical decision task (is this written letter-string a word or not?), the finding that normals are slowed by non-words homophonic with real words (like flore) was replicated using a modified technique. This effect, attributable to phonological coding, was not shown by the patients. At the same time, their ability to discriminate between words and non-words was essentially intact. Consideration was given to mechanisms which might underlie such patients' correct and erroneous readings of words and non-words.     

Cerebral lateralization, preferred cognitive mode, and reading ability in normal children

In a sample of 105 children (aged 6 to 12), a stylistic preference for a verbal approach to problem-solving correlated positively with reading ability (independently of age) and degree of right-ear advantage in the report of dichotic stop consonant-vowel syllables (but not with verbal intelligence or hand preference). Verbal strategic preference may: (1) foster the acquisition of reading skill, and (2) bias attention toward input from the right by generating differential left-hemisphere activation.     

OBJECTIVE AND SUBJECTIVE PERFORMANCE AS INFLUENCED BY DRUG-INDUCED VARIATIONS IN ACTIVATION LEVEL

Performance was measured by seven sensorimotor and perceptual tests given to 30 students in six successive trials, spread over five hours, before and after the intake of either 15 mg dexamphetamine, zoo mg Pentobarbitone, or a placebo. Subjective performance, level of aspiration, wakefulness, and mood were measured by a scaling method. A close agreement was found between the amount of objective and subjective improvement following dexamphetamine, whereas the objective impairment induced by Pentobarbitone was not reflected in the subjective measures. These results were examined in relation to the other indices of activation.     

The Individuality of Species: Some Reflections on the Debate

The thesis that species are individuals, and not classes as the traditional view had it, has been influential in the last 25 years. In this paper David Hull's arguments for the thesis are surveyed, as well as some counterarguments presented by Philip Kitcher. It is claimed that though species can be conceptualized as individuals, we are not compelled to view them in that way. The importance of the issue seems to have been somewhat exaggerated. However, it might happen that empirical findings concerning species selection would strengthen the case for the thesis, thereby rendering it straightforwardly empirical. The applicability of teleological explanation is suggested as a criterion for biological individuality. This revised version was published online in June 2006 with corrections to the Cover Date.