Intermittency in human manual tracking tasks

We confirm Craik's (1947) observation that the human manual1y tracking a visual target behaves like an intermittent servo-control1er. Such tracking responses are indicative of "sampled" negative-feedback control but could be the result of other, continuous, mechanisms. Tracking performance therefore was recorded in a task in which visual feedback of the position of the hand-held joystick could be eliminated. Depriving the subjects of visual feedback led to smoother tracking and greatly reduced the signal power of their responses between 0.5-1.8 Hz. Their responses remained intermittent when they used feedback of their own position but not of the target to track a remembered (virtual) target. Hence, intermittency in tracking behavior is not exclusively a signature of visual feedback control but also may be a sign of feedback to memorized waveforms. Craik's (1947) suggestion that the intermittency is due to a refractory period following each movement was also tested. The errors measured at the start of each intermittent response, during tracking of slow waveforms, showed evidence of a small error deadzone (measuring 0.7 cm on the VDU screen or 0.80 degrees at the eye). At higher target speeds, however, the mean size of starting errors increased, and the upper boundary of the distribution of starting error was close to that expected of a refractory delay of approximately 170 ms between responses. We consider a model of the control system that can fit these results by incorporating an error deadzone within a feedback control loop. We therefore propose that the initiation of intermittent tracking responses may be limited by a positional error deadzone and that evidence for a refractory period between successive corrective movements can be satisfied without evoking an explicit timing or sampling mechanism.     

Accuracy and precision of the PEAK Performance Technologies Motion Measurement System

This study evaluated the accuracy and precision of the PEAK Performance Technologies, lnc.'s motion analysis system for three-dimensional angle reconstruction. Pendular motion of a bar, on which 18 retroreflective markers were mounted, was videotaped at three different orientations (parallel, and rotated 30 degrees right and left) to a plane at which two standard video cameras were aimed. The videotaped motion was digitized off-line, and 32 angles between the 18 markers were calculated. intraclass correlation coefficients (ICCs) were calculated between trials within each pendulum orientation and across orientations to determine system precision, and between randomly selected trials and actual angles to determine accuracy. lCCs were in all cases greater than.99. Within-trial standard deviations ranged between 0.05-0.8 degrees for the different angles. Deviations from the actual angle averaged 0.0-0.8 degrees across all angles and orientations. The results indicate that accurate and reliable angular measurements can be made with this motion analysis system.     

Is the cerebellum a smith predictor?

The motor system may use internal predictive models of the motor apparatus to achieve better control than would be possible by negative feedback. Several theories have proposed that the cerebellum may form these predictive representations. In this article, we review these theories and try to unify them by reference to an engineering control model known as a Smith Predictor. We suggest that the cerebellum forms two types of internal model. One model is a forward predictive model of the motor apparatus (e.g., limb and muscle), providing a rapid prediction of the sensory consequences of each movement. The second model is of the time delays in the control loop (due to receptor and effector delays, axonal conductances, and cognitive processing delays). This model delays a copy of the rapid prediction so that it can be compared in temporal register with actual sensory feedback from the movement. The result of this comparison is used both to correct for errors in performance and as a training signal to learn the first model. We discuss evidence that the cerebellum could form both of these models and suggest that the cerebellum may hold at least two separate Smith Predictors. One, in the lateral cerebellum, would predict the movement outcome in visual, egocentric, or peripersonal coordinates. Another, in the intermediate cerebellum, would predict the consequences in motor coordinates. Generalization of the Smith Predictor theory is discussed in light of cerebellar involvement in nonmotor control systems, including autonomic functions and cognition.     

A logic of intentions and beliefs

Intentions are an important concept in Artificial Intelligence and Cognitive Science. We present a formal theory of intentions and beliefs based on Discourse Representation Theory that captures many of their important logical properties. Unlike possible worlds approaches, this theory does not assume that agents are perfect reasoners, and gives a realistic view of their internal architecture; unlike most representational approaches, it has anobjective semantics, and does not rely on anad hoc labeling of the internal states of agents. We describe a minimal logic for intentions and beliefs that is sound and complete relative to our semantics. We discuss several additional axioms, and the constraints on the models that validate them.     

The imp of the reverse: A phenomenology of the unconscious

The debate regarding the unitary versus multiple nature of the human person is examined with special attention to the division between conscious and unconscious. The roots of the unconscious are traced to the experience of alienation or feeling divided within. An existential analysis is provided in order to shed light on the nature of the unconscious. The experience of feeling divided within is explained as a metaphor that is taken literally. An existential-phenomenological explanation of the unconscious is provided which preserves the essential unity of the human person.     

A Review of MMPI and Rorschach interrelationships in adult samples

The MMPI and Rorschach are consistently ranked among the most widely used psychological assessment instruments across adolescent and adult clinical settings. Although there is an extensive research literature available on each instrument individually, relatively little research attention has been focused on the interrelationships between these measures. This article reviews the literature derived from 37 studies that have reported interrelationships between MMPI and Rorschach variables in adult populations. The results of these studies generally indicate limited or minimal relationships between the MMPI and Rorschach. A number of methodological issues, however, prevent drawing firm conclusions from the literature at this time. Directions for future research are discussed, including consideration of issues related to sources of alpha and beta error and the need for studies examining the incremental validity of combinations of Rorschach and MMPI data in prediction to relevant external criteria.     

On the comparison of TAT and free speech techniques in personality assessment

Schnurr, Rosenberg, and Oxman (1992) recently compared the free speech samples and Thematic Apperception Test (TAT) responses of 95 normal adults. They concluded that the two techniques are not interchangeable, and that the TAT, which proved superior in the prediction of individual differences, may be preferable to free speech instructions for eliciting data in content analytic studies. We disagree with both conclusions. Various forms of narrative speech samples may be highly correlated, so long as psychologically meaningful, well-validated, and higher order content categories are used. The use of first-order content categories is less likely to contribute to the study of personality.     

Perceived Health Barriers and Health Value in Seniors: Implications for Well-Being and Mortality1

There is growing evidence to suggest that the ways in which people think about their health problems may have significant implications for psychological and physical well-being. This possibility has the greatest consequences for those groups facing health problems on a daily basis, in particular, the elderly. This study examined the role of perceived health barriers and value for health in later life. A Perceived Health Barriers (absent, present) by Health Value (low, medium, high) 2.3 factorial design was analyzed for younger and older seniors on: number of life-threatening diseases, need for health care, and life satisfaction. In addition, a longitudinal design was used to examine the joint roles of perceived health barriers (PHBs) and health value (HV) on mortality. Findings generally indicated that individuals’ PHBs and HV are associated with health-related outcomes; however, the results differed for young and old elders. Among young elders, those with PHBs and those with high HV had the most negati e profile. That is, they had more diseases, higher health care needs, and lower life satisfaction. Among older elders, the findings were more complicated in that the relationship between PHB and the outcomes depended on HV. Overall, the patterns for old elders suggest that perceived health barriers are most relevant among those with medium value for health and least relevant among those with low value for health.