Empirical Explorations of SYNCHEM.

During the past several years, a substantial body of experience has accumulated in the use of SYNCHEM, a large-scale program which is able to discover synthesis routes for relatively complex organic structures without on-line guidance on the part of its chemist user. These results indicate that the approach to computer-directed organic synthesis route discovery embodied in the program has been valid and reasonable, and that SYNCHEM is likely to be fruitful from the point of view of its intended users as well as for our research objectives in artificial intelligence. The experiments have revealed a number of insufficiencies in the program as well. Most of these are rectified in SYNCHEM2, a revised version of the program which includes, among other improvements, a more highly developed synthesis search algorithm and the routine consideration of stereochemistry.

Vitamin D deficiency, behavioral atypicality, anxiety and depression in children with chromosome 22q11.2 deletion syndrome.

Chromosome 22q11.2 deletion syndrome (22q11.2DS) is a complex developmental disorder with serious medical, cognitive and emotional symptoms across the lifespan. This genetic deletion also imparts a lifetime risk for developing schizophrenia that is 25-30 times that of the general population. The origin of this risk is multifactorial and may include dysregulation of the stress response and immunological systems in relation to brain development. Vitamin D is involved in brain development and neuroprotection, gene transcription, immunological regulation and influences neuronal signal transduction. Low levels of vitamin D are associated with schizophrenia, depression and anxiety in the general population. Yet, little is known about how vitamin D levels in children with 22q11.2DS could mediate risk of psychosis in adulthood. Blood plasma levels of vitamin D were measured in children aged 7-16 years with (n=11) and without (n=16) 22q11.2DS in relation to parent reports of children's anxiety and atypicality. Anxiety and atypicality in childhood are risk indicators for the development of schizophrenia in those with 22q11.2DS and the general population. Children with 22q11.2DS had lower vitamin D levels, as well as elevated anxiety and atypicality compared with typical peers. Higher levels of anxiety, depression and internalizing problems but not atypicality were associated with lower levels of vitamin D. Vitamin D insufficiency may relate to higher levels of anxiety and depression, in turn contributing to the elevated risk of psychosis in this population. Further study is required to determine casual linkages between anxiety, stress, mood and vitamin D in children with 22q11.2DS.

A summary of resource theories from a behavioral perspective.

This paper presents a concise review of the development of limited capacity metaphors for explaining performance limits. Capacity views have been mainly inspired by limited capacity computers of the sixties and seventies, which are now replaced by parallel distributed processors with a virtually unlimited capacity. This is among the reasons that the view is no longer popular, while, in addition, the predictions of the notion of a simple unidimensional pool of resources have failed to be confirmed. The alternative assumption of multiple resources runs the risk of becoming too loose a set of faculties without internal coherence. There is a similar problem, with Fleishman's mental abilities approach which is based on factor-analysis. Yet, this approach has had some success as a taxonomy for complex tasks and deserves, therefore, more close scrutiny. At present, the theoretical emphasis is on connectionist notions of confusion and interference, which are more in line with stage-like views on information processing However, so far, they suffer from unsufficiently detailed constraints to predict performance in concrete dual tasks. This may be the reason that, as yet, they have not led to much new experimental research.

Discrete vs. continuous processing: the fate of an incompletely processed perceptual dimension.

A relevant issue in the debate on continuous vs. discrete processing of information is whether incompletely processed information does or does not affect a subsequent reaction. Two stimuli (SL, SR) were presented on eye level, subtending a visual angle of 45 degrees. SL was always inspected first, followed by a saccade to SR and finally by a same/different response. The fixation time of SL (TL), the saccadic time (TM) and the time from fixating SR to the response (TR) were separately measured. SL and SR consisted of two-dimensional stimuli (size and letter shape) constructed in such a way that encoding size took longer for one group and encoding shape took longer for another group of 10 subjects. All subjects were tested in three conditions: Shape was relevant in one, size in the second, and both dimensions were relevant in the third condition. TL was less when encoding the relevant dimension was fast. When both dimensions were relevant, TL was about as long as when only the slow dimension was relevant, suggesting parallel and interference-free processing during TL. When only the slow dimension was relevant, TR (same) was much longer when the fast dimension differed. When the fast dimension was relevant, TR (same) was slightly longer when the slow dimension differed, which can be handled by either model. The experiment was repeated with three well-practiced and less variable subjects who carried out sufficient trials to measure TR as a function of TL. The results of this study were in line with the discrete model: A different slow and irrelevant dimension did not affect the same response regardless of the duration of TL. Interestingly, subjects were capable of retrieving the slow dimension, suggesting a code which can be used for retrieval but which does not affect the same/different response.

Resources and dual-task performance; resource allocation versus task integration.

This study aims at contributing to the explanation of dual-task performance in terms of either resource allocation or of task interference and integration. Twenty-four subjects carried out, single and in combination, a motor interval production task and a perceptual target detection task on the basis of combined memory and display search. The demands of the target detection task were varied by increasing or decreasing the presentation rate of successive search displays. Furthermore, the presentation rate was either constant or variable. The dual-task condition had a negative effect on interval production, the extent of which was unaffected by either rate or variability of display presentation. This means that there was no evidence for synchronizing interval production with display presentation, so that the major opportunity for task integration did not substantiate. It is suggested that the two tasks use different resource pools in addition to a common mechanism, the limited capacity of which causes a general interference in dual-task conditions.

Additive factors analysis of reaction time with alphanumericals and line orientations as stimuli.

Most additive factors method (AFM) analyses of choice reaction time (CRT) have used alphanumerics whereas tests of single process models have often used line length or line orientation. The suggestion is raised that commonly observed additive effects of variables on CRT might not apply to stimuli of the latter category. This would mean a severe limitation of the AFM in that the stage structure of choice reactions would be stimulus specific. The issue is addressed in two experiments. The first showed additive effects of stimulus quality and stimulus-response compatiblility for both alphanumerics and line orientations as stimuli. The second showed that for both stimulus categories the effect of stimulus quality was fully reflected in visual fixation time. Together the results argue against a single central process and favour a stage model of CRT.

Perceptual processing and speed-accuracy trade-off.

Three experiments are described on speed-accuracy trade-off during a visual fixation. In a typical trial subjects started by fixating a fixed position at their left side at which a degraded digit (SL) was presented. At about the same time an intact digit (SR) was briefly presented at a fixed right side position. SL and SR were separated by a binocular visual angle of 45 degrees. The task was to verbally report both digits, which implied that the eyes should be rapidly shifted from SL to SR, so as not to miss SR. Under these conditions the distribution of the fixation latencies of SL appeared to consist of two parts, suggesting the occurrence of either anticipations or reactions. This argues against a strategy of optional stopping of information accrual during the fixation of SL and is in line with a strategy of either fully neglecting or fully encoding SL. However, this interpretation meets the difficulty that, at trials where SL is reacted to, subjects showed a shorter fixation latency than in a control condition, in which they had ample time to view both SL and SR. The results of the second and third study supported the view that, when under time stress, subjects can end a fixation as soon as a preliminary code of SL is obtained. This code (a) is insensitive to the complexity of SL, (b) cannot be elaborated during the saccade to SR, and (c) can be used as a starting point for further analysis, as soon as the eyes reach SR.

The Eriksen flanker effect revisited.

Four studies are reported on the potential role of perceptual interference in a standard Eriksen flanker task. In the first study, incongruent flanker letters showed the usual effect on choice reaction time (CRT) to the target letter but had no effect on the visual fixation time (VFT) needed to distinguish target and flankers. In the second experiment, the effect of incongruent flankers was studied in the context of a same-different response in regard to the target letter and a subsequently presented single letter. The effect of incongruent flankers vanished at an interstimulus interval of 200 ms. In Experiment 3, the same-different task was used in the paradigm of the functional visual field with a target-flankers combination as stimulus on the left (SL) and a single letter as stimulus on the right side (SR) of the visual field. Flankers did neither affect VFT nor the same-different CRT suggesting that target selection may proceed during the saccade from SL to SR. In Experiment 4 effects were studied of flanker-to-target and target-to-single-letter similarity. Flanker-to-target similarity did neither affect VFT nor same-different CRT but target-to-single-letter similarity prolonged same-different CRT. Together, the results suggest parallel perceptual processing of target and flankers, followed by competition of responses to the target and to the incongruent flankers. In line with earlier research, processes of response selection and response competition appear not to be tied to VFT but to proceed in parallel with the saccade from SL to SR.

Stimulus control of visual fixation duration in a single saccade paradigm.

Subjects performed a task in which they successively inspected two stimuli presented at an equal distance of the left (SL) and the right (SR) of the visual meridian and subtending a 45 degrees visual angle. This was followed by a joint response on the basis of the status of both stimuli. The instruction was to fixate the position of SL at the start of a trial, followed by a single saccade to SR. In earlier studies on this paradigm it was suggested that the fixation time of SL (TL) may serve as a modern version of the (d)-reaction in which the time for achieving perceptual identification is measured uncontaminated by decision. This suggestion has the implicit assumption that, at least in this single saccade paradigm, the saccade from SL to SR is triggered when perceptual identification has been completed. There is the potential alternative that the duration of TL is programmed in advance, depending on the general processing demands of the stimuli in a block of trials. These two options were tested in two experiments. In the first, presentation of SL was delayed with a fixed or with a variable interval (200-400 ms) during a block of trials. In the second, the stimulus quality of SL was varied between and within blocks of trials. The results of either manipulation argue against preprogramming TL, and they are consistent with the hypothesis that the saccade is triggered upon completion of perceptual processing. Thus, TL in the single saccade paradigm appears a serious candidate for the (d)-reaction.

Signal processing during and across saccades.

Visual perceptual processing has been found to occur exclusively during fixations of the eye (Sanders and Houtmans, 1985; Sanders and Rath, 1991). Does fixation time also reflect postperceptual processes such as target classification and response selection as well, or can these processes continue during a saccade? In a series of experiments on this question two signals were presented at an angular distance of 100 degrees. At the start of a trial subjects fixated the left signal (S1), which was always a letter that did or did not belong to a predefined target set of variable size. Then they moved to the right signal (S2) which indicated the mapping of target and no target responses onto the response keys. Subjects were capable of starting the saccade upon identification of S1, so that target classification of S1 might occur either during the saccade or while fixating S2. Although the duration of the saccade was long enough to complete target classification, the effect of set size was still partially reflected in the fixation duration of S2. This could either be due to incomplete processing during the saccade or to interference while integrating S1 and S2. The results of two additional conditions were in support of the latter hypothesis, since they showed similar effects while no saccade was involved. Subsequent experiments confirmed the conclusion that target classification can continue during a saccade. The final experiment showed that response selection can continue during a saccade as well: The effect of S-R compatibility was reduced in case of a small saccade (8 degrees) and disappeared altogether when a large saccade was carried out (100 degrees).

Overlapping stage models and reaction time additivity: effects of the activation equation.

Many experimental factors have been found to affect mean reaction time (RT) additively in factorial experiments. What sorts of RT models are compatible with this fact? Sternberg (1969) showed that serial, discrete-stage models are consistent with additivity, and as a result additivity has sometimes been regarded as evidence in favor of such models. However, McClelland (1979) showed that an alternative "cascade" model, which violates crucial assumptions of discrete-stage models, also predicts RT additivity in many cases. This article examines various modified versions of the cascade model, and shows that mean RT additivity arises from many overlapping stage models, including some with thresholds, information quantization, and nonlinear activation transformations. This suggests that other aspects of the data besides mean RT additivity should be examined to distinguish between serial and overlapping stage models.

Changing trends in treatment of substance abuse: a look at some of the factors involved in third-party providers and treatment planning.

In order to better focus our care in providing patients with treatment that matched their needs, an inpatient study was done on patient discharges for 1990 involving 267 patients. Our study points out the influence of the changing drug scene in this country. In approximately 40% of the patients admitted, the drug of choice is other than alcohol. The importance of family and employer involvement in effective treatment stands out. Fewer patients are being admitted but those admitted are sicker and require more treatment. Chemical dependency programs in Louisiana and the United States are experiencing significant declines in the utilization of their service and many are closing. The patient census has been impaired primarily by the expense of or lack of insurance coverage by third-party payors. For patients with insurance coverage the criteria for inpatient treatment by third-party reviewers are so stringent that staff assessment recommendations are frequently over-ruled. Substance abuse is a chronic relapsing illness, yet many insurance companies limit coverage both in time and money. In no other field of medicine do patients leave treatment prematurely as they do in substance abuse.