Selective effects of barbiturate and amphetamine on information processing and response execution

Aging, Reaction Time, and Stages of Information Processing

Using Sternberg's (1969) Additive Factors Method (AFM), previous investigations in search of the locus of age-related slowing in reactive capacity have found conflicting results possibly due to inconsistencies in research methodologies. This experiment was conducted to examine age differences in the performance of AFM intratask manipulations of a reaction time task using both fixed and variable foreperiod conditions with subject testing at both naive and practiced skill levels. Twenty male subjects, ten young and ten old, performed a visual four-choice RT task with intratask manipulations of stimulus-degradation, stimulus-response compatibility, and response-stimulus intervals (RSIs were fixed at 0, 2, and 5 sec and variable with random presentations at 0, 2, and 5 sec), once when subjects were naive and again when practiced. The results varied by level of practice and RSI, but clearly the older subjects had difficulty with the intratask manipulations. The older subjects took twice as long, on the average, to respond. Interactions of age by compatibility suggest that, according to the AFM, with age comes inordinately long delays in the response selection stage of information processing. Conclusions are made with caution since this research points to limitations and methodological confounds which serve to explain many of the equivocal findings in previous studies.

Histamine H1 receptor blockade predominantly impairs sensory processes in human sensorimotor performance

BACKGROUND AND PURPOSE

Centrally active antihistamines impair cognitive performance, particularly sensorimotor performance. The aim of the present study was to further elucidate the scarcely studied subprocesses involved in sensorimotor performance, which may be affected by H1 receptor blockade. Better knowledge about the cognitive deficits associated with histamine dysfunction can contribute to better treatment of clinical disorders in which histamine hypofunction may be a contributing factor, such as in schizophrenia.

EXPERIMENTAL APPROACH

Interactions of dexchlorpheniramine with specific task manipulations in a choice reaction time task were studied. Task demands were increased at the level of sensory subprocesses by decreasing stimulus quality, and at the level of motor subprocesses by increasing response complexity. A total of 18 healthy volunteers (9 female) aged between 18 and 45 years participated in a three-way, double-blind, crossover design. Treatments were single oral doses of 4 mg dexchlorpheniramine, 1 mg lorazepam and placebo. Behavioural effects were assessed by measuring reaction times and effects on brain activity by event-related potentials.

KEY RESULTS

Dexchlorpheniramine significantly slowed reaction times, but did not significantly interact with task manipulations. However, it did significantly interact with stimulus quality, as measured by event-related potentials. Lorazepam slowed reaction times and interacted with perceptual manipulations, as shown by effects on reaction times.

CONCLUSIONS AND IMPLICATIONS

The results confirm that the histamine system is involved in sensory information processing and show that H1 blockade does not affect motoric information processing. Histamine hypofunction in clinical disorders may cause impaired sensory processing, which may be a drug target.

Additive factors analysis of inhibitory processing in the stop-signal paradigm

This article reports an additive factors analysis of choice reaction and selective stop processes manipulated in a stop-signal paradigm. Three experiments were performed in which stimulus discriminability (SD) and stimulus-response compatibility (SRC) were manipulated in a factorial fashion. In each experiment, the effects of SD and SRC were assessed first for going and next for stopping. Two experiments yielded the anticipated additive relation between SD and SRC for going but stopping appeared to be insensitive to the SD manipulation. Increasing the SD demands in the third experiment by using a different display resulted in a significant (over-additive) interaction between SD and SRC for going and a non-significant (under-additive) interaction for stopping. The pattern of results that emerged from this set of experiments was interpreted to suggest that going and stopping are both similar and different. They are similar in that distinct stages can be identified in both going and stopping but they are also different, as selective stopping seems to be less sensitive to discrimination manipulations relative to going.

Influence of sleep deprivation and auditory intensity on reaction time and response force

Arousal and activation are two variables supposed to underlie change in response force. This study was undertaken to explain these roles, specifically, for strong auditory stimuli and sleep deficit. Loud auditory stimuli can evoke phasic overarousal whereas sleep deficit leads to general underarousal. Moreover, Van der Molen and Keuss (1979, 1981) showed that paradoxically long reaction times occurred with extremely strong auditory stimuli when the task was difficult, e.g., choice reaction or Simon paradigm. It was argued that this paradoxical behavior related to reaction time is due to active disconnecting of the coupling between arousal and activation to prevent false responses. If so, we predicted that for extremely loud stimuli and for difficult tasks, the lengthening of reaction time should be associated with reduction of response force. The effects of loudness and sleep deficit on response time and force were investigated in three different tasks: simple response, choice response, and Simon paradigm. According to our expectation, we found a detrimental effect of sleep deficit on reaction time and on response force. In contrast to Van der Molen and Keuss, we found no increase in reaction time for loud stimuli (up to 110 dB) even on the Simon task.

Effects of temazepam on memory and psychomotor performance: a dose-response study

In a randomised, three-period crossover study, psychomotor performance and memory were tested and mood assessed for 3 h after single doses of placebo (PL), 20 mg temazepam (T20) or 30 mg temazepam (T30) were given to six healthy females aged 21-23. A composite measure of psychomotor speed showed a dose-dependent slowing (Page's L trend test: p < 0.001; sign test PL vs T20 and T30 vs T20: p < 0.05). The number of errors in the performance of tasks was increased, significantly in the case of some measures. Explicit memory (Buschke Selective Reminding Task) showed significant impairment of long-term but not short-term memory (Page's L trend test: p < 0.05). The form of the dose-response curve was positively accelerating, with the difference in performance between T20 and T30 at least as great as that between PL and T20. Visual Analogue Scales showed a decrease in a factor representing functional integrity (Page's L trend test: p < 0.05;) sign test (PL vs T20: p < 0.05), but no changes in mood. These results show that 30 mg is a useful extension of the dose range of temazepam, being well tolerated and that it produces a substantially greater impairment of performance than 20 mg. Copyright 2001 John Wiley & Sons, Ltd.

Motor control and state regulation in children with ADHD: a cardiac response study

The goal of the current study was to investigate whether poor motor control in children with Attention-Deficit Hyperactivity Disorder (ADHD) was associated with a state regulation deficit. For this purpose, 28 ADHD and 22 healthy children carried out two Go No-Go tests: one with a fast stimulus presentation rate, and the other with a slow stimulus presentation rate. Groups were compared on RT performance and on specific cardiac measures, reflecting arousal, motor activation/inhibition, and effort allocation. No group difference in the arousal measure (mean heart rate) was found. Further, groups did not differ with respect to response inhibition: in both the fast and slow condition, ADHD children made comparable numbers of errors of commission to the control group, and the groups did not differ with respect to the heart rate deceleration after the onset of the No-Go signal, reflecting motor inhibition. Group differences were found with respect to motor activation and effort allocation in the condition with a slow presentation rate. In this condition: (1) ADHD children reacted more slowly to Go signals than control children, suggesting poor motor activation; (2) the heart rate deceleration before the onset of Go signals, which is believed to reflect motor preparation, was less pronounced in the ADHD children; (3) after Go signals, where a response was given, the cardiac shift from deceleration to acceleration, indicating response initiation, was delayed in ADHD children; and (4) ADHD children had greater heart rate variability (0.10 Hz component) than the control group, indicating that less effort was allocated. No group differences in motor activation and effort allocation were found in the condition with a fast presentation rate of stimuli. We conclude, therefore, that a slow presentation rate of stimuli brings the ADHD child in a non-optimal activation state.

A controlled daytime challenge of motor performance and vigilance in sleep bruxers

Many etiological factors have been suggested for sleep bruxism. Among these, elevated mental and physical alertness has been proposed to characterize sleep bruxers. The present study tests the hypothesis that, during the day-time, sleep bruxers are more vigilant and more prone to react to a motor command than are control subjects. Seven sleep bruxers, diagnosed polysomnographically according to validated research criteria, were matched for age and gender to seven control subjects. A simple reaction time task was selected to assess daytime vigilance and motor responsiveness. The following physiological measures were recorded: reaction time, error rate, electroencephalography, electrocardiography, electromyography, and video detection of body movements. Analysis of these variables showed no differences between groups. During the test, bruxers and controls showed a parallel decrease in EEG vigilance and heart rate over time. Frequency of orofacial and body movements was the same in both groups, and no clenching activity was observed during the experimental test. Subjects' visual analog scale ratings revealed that both controls and bruxers were more competitive after the test than before, and bruxers were slightly more anxious than controls before and after the test. Together, the results indicate that sleep bruxers are neither more vigilant nor more prone to react to a motor command during the daytime than are control subjects.

Dopamine and the mechanisms of cognition: Part I. A neural network model predicting dopamine effects on selective attention

BACKGROUND

Dopamine affects neural information processing, cognition, and behavior; however, the mechanisms through which these three levels of function are affected have remained unspecified. We present a parallel-distributed processing model of dopamine effects on neural ensembles that accounts for effects on human performance in a selective attention task.

METHODS

Task performance is stimulated using principles and mechanisms that capture salient aspects of information processing in neural ensembles. Dopamine effects are simulated as a change in gain of neural assemblies in the area of release.

RESULTS

The model leads to different predictions as a function of the hypothesized location of dopamine effects. Motor system effects are simulated as a change in gain over the response layer of the model. This induces speeding of reaction times but an impairment of accuracy. Cognitive attentional effects are simulated as a change in gain over the attention layer. This induces a speeding of reaction times and an improvement of accuracy, especially at very fast reaction times and when processing of the stimulus requires selective attention.

CONCLUSIONS

A computer simulation using widely accepted principles of processing in neural ensembles can account for reaction time distributions and time-accuracy curves in a selective attention task. The simulation can be used to generate predictions about the effects of dopamine agonists on performance. An empirical study evaluating these predictions is described in a companion paper.

Dopamine and the mechanisms of cognition: Part II. D-amphetamine effects in human subjects performing a selective attention task

BACKGROUND

A neural network computer model described in a companion paper predicted the effects of increased dopamine transmission on selective attention under two different hypotheses.

METHODS

To evaluate these predictions we conducted an empirical study in human subjects of D-amphetamine effects on performance of the Eriksen response competition task. Ten healthy volunteers were tested before and after placebo or D-amphetamine in a double-blind cross-over design.

RESULTS

D-amphetamine induced a speeding of reaction time overall and an improvement of accuracy at fast reaction times but only in the task condition requiring selective attention.

CONCLUSIONS

This pattern of results conforms to the prediction of the model under the hypothesis that D-amphetamine primarily affects dopamine transmission in cognitive rather than motor networks. This suggests that the principles embodied in parallel distributed processing models of task performance may be sufficient to predict and explain specific behavioral effects of some drug actions in the central nervous system.

Auditory stress effects on preparation and execution of graphical aiming: a test of the neuromotor noise concept

Effects of physical and mental stress, on the preparation and execution of a psychomotor task were studied to test the applicability of the neuromotor noise concept (Van Gemmert and Van Galen, 1997) as an explanation of stress effects. Central to this notion is that both physical stress and mental load raise neuromotor noise levels in the human information processing system. It is proposed that increased levels of neuromotor noise lead to decreased processing times during task preparation (activation effect), decreased or increased reaction times during task initiation, depending on task difficulty (impoverished signal-to-noise effect) and increased limb stiffness during task execution (biomechanical filtering effect). To test these predictions, an experiment was conducted in which two types of auditory stressors, physical stress and mental load, were manipulated across the stages of preparation, initiation, and execution of a graphical aiming task. The results confirmed the notion that the neuromotor noise concept is a tenable approach to explain the effects of stress on human performance.

Biological determinants of P300: the effects of a barbiturate on latency and amplitude

Polich & Kok (1995) (Biological Psychology, 41, 103-146) have recently argued that P300 is not only sensitive to specific 'cognitive' variables, but also to non-specific biological processes such as arousal. Fluctuations in arousal are said to be indexed by an inverse relationship between latency and amplitude. We tested this hypothesis with a drug that decreases arousal-the barbiturate secobarbital sodium. Twelve subjects performed a visual 80-20% oddball task at two levels of stimulus quality and after ingesting the drug (2.9 mg/kg body weight) or a placebo. Reaction time (RT) and P300 were collected simultaneously and the latter was analyzed on both a single trial and average basis. The RT results confirmed that secobarbital interacts with stimulus quality. Secobarbital slowed single trial P300 by about half as much as RT, and this slowing was additive with stimulus quality. Thus the two measures dissociated. Secobarbital did not influence P300 amplitude. Average P300 revealed the same pattern of results, although the size of the latency effects was somewhat attenuated. RT and P300 latency were more strongly correlated than P300 latency and amplitude. We propose that P300 latency reflected the slowing of stimulus evaluation produced by the depressant properties of the drug, and that fluctuations in arousal are not necessarily associated with a simple inverse relationship between P300 latency and amplitude.

Effects of body core temperature and brain dopamine activity on timing processes in humans

In a placebo-controlled study, the effects of experimentally induced increase in body core temperature and of the dopamine antagonist haloperidol on judgments of an apparent second, a speeded-tapping task, and temporal discrimination of intervals in the range of milliseconds and seconds were investigated in 40 healthy male subjects. A 0.7 degree C-increase in body core temperature due to 3-h exposure to an ambient temperature of 52 degrees C did not cause any statistically significant changes in timing tasks. Unlike heat exposure, 3 mg of haloperidol caused a pronounced impairment of performance on the temporal discrimination of intervals in the range of milliseconds and seconds (P < 0.01 and P < 0.001, respectively) as well as speeded tapping (P < 0.05). For temporal discrimination of intervals in the range of seconds, a significant interaction between ambient temperature and haloperidol could be established (P < 0.05) indicating that haloperidol caused a significant performance decrement only in subjects exposed to an ambient temperature of 28 degrees C but not in those exposed to 52 degrees C. The overall pattern of results suggests that temporal processing of intervals in the range of milliseconds can be considered a function of dopaminergic activity in the basal ganglia while temporal processing of longer intervals appears to be cognitively mediated. Furthermore, the hypothesis that timing processes in humans are modulated by changes in body core temperature could not be established.

Serotonin and human information processing: fluvoxamine can improve reaction time performance

Fluvoxamine is a specific serotonin reuptake inhibitor. Recent evidence suggests that this antidepressive drug shortens the reaction time (RT) of healthy volunteers. The first objective of the present study was to decipher whether this effect is due to an improvement in information processing per se or to the adoption of an error-prone strategy. The second objective was to locate the effect of fluvoxamine within the series of information processing stages by means of Sternberg's additive factor method. After administration of a single oral dose of fluvoxamine (100 mg) or a placebo (randomized double-blind, cross-over design), eight healthy volunteers performed a choice RT task in which stimulus intensity, stimulus-response compatibility and response repertoire were manipulated. Fluvoxamine shortened RT without decreasing the accuracy of the responses. This demonstrates that fluvoxamine improves information processing per se. The effect of fluvoxamine was additive on RT with the respective effects of stimulus intensity and stimulus-response compatibility. This result suggests that fluvoxamine spares the processing stages of stimulus preprocessing and response selection.

Effect of sleep deficit, knowledge of results, and stimulus quality on reaction time and response force

Some recent findings suggested that response force measured during reaction time experiments might reflect changes in activation. We performed an experiment in which the effect of sleep deprivation, knowledge of results, and stimulus quality on response force was studied in simple and choice reaction tasks. As expected, both simple and choice reaction times increased with sleep deficit. Further, simple and choice reactions were faster with knowledge of results and slowed down when stimulus quality was degraded. As sleep deprivation affects both arousal and activation, we expected a detrimental effect of sleep on force amplitude. On the other hand, knowledge of results was expected to increase force by its compensatory effect on arousal and activation. No effect of sleep deprivation on response force was found. Knowledge of results increased response force independently of sleep deprivation.

Effects of cold on human information processing: application of a reaction time paradigm

Only a very few studies on the effects of cold on human information processing appear to exist. Therefore, the present experiment was designed to study the effects of the experimentally induced lowering of body core temperature on information processing, while applying a reaction time paradigm. Thirty healthy male volunteers performed a stimulus evaluation-response selection reaction time task after exposure to ambient temperatures of either 28 or 5 degrees C. A 0.5 degree C-decrease in body core temperature resulted in a significant increase in both reaction and movement time indicating a general deteriorating effect of lowering of body core temperature on information processing. Mean reaction times were 538 ms and 549 ms for the control and the cold group, respectively (p < .05). The respective mean movement times were 298 ms and 269 ms (p < .001). Speed of stimulus evaluation was not sensitive to decreases in body core temperature. However, response complexity and body core temperature showed a significant interaction in their effect on movement time (p < .05), indicating that lowering of body core temperature is more likely to affect response-related stages of central information processing rather than stimulus evaluation. Furthermore, movement time appeared to be more sensitive to cold-induced effects on information processing as compared to reaction time. Additional correlational analyses suggest that the observed effects can be considered as independent of changes in skin temperature and experienced levels of thermal discomfort. Taken together, the results indicate that lowering of body core temperature differentially affects various stages of information processing.

Effects of closed-head injury on attentional processes: generality of Sternberg's additive factor method

The present study examined the generality of Shum, McFarland, Bain, and Humphreys' (1990) findings that closed-head injury (CHI) selectively impairs different processes of attention (operationalized in terms of stages of information processing) depending on the severity of, and the time since, injury. The procedure of Shum et al. was based on Sternberg's additive factor method (AFM), with the mode of information processing involved being a physical-directional matching of visual stimuli. The present study followed a similar procedure except that a name-matching task was used. This task was administered to 16 first-year psychology students and two groups of CHI patients (viz., severe short-term (SS) and severe long-term (SL)) with matched control groups. The results obtained replicated the study by Shum in that the SS group was found to be impaired on the identification and response-selection stages of information processing whereas the SL group was found to be impaired only on the response-selection stage. The present study confirms that the AFM is not limited to one specific mode of information process and strengthens the validity of the conclusions made by Shum et al. regarding the effects of CHI on attention.

Parsing attentional components during a simple reaction time task using sleep deprivation and amphetamine intervention

To examine the independent contributions of the attentional components of arousal and activation in performance, sleep deprivation was used as the attentional manipulation in a reaction time (RT) task. The subjects were 18 men who underwent 63 hr. of sleep deprivation during which time they periodically performed a simple auditory RT task with manipulations of temporal uncertainty and intensity. After 48 hr. sleep deprivation, subjects ingested either 20 mg d-amphetamine or placebo, then continued testing throughout Day 3. During sleep deprivation, performance was more impaired on trials associated with low temporal uncertainty (arousal) and high preparation (activation) than on trials associated with high temporal uncertainty and low preparation. Analysis indicated that sleep deprivation perturbed activation, leaving arousal relatively unimpaired and that amphetamine had a restorative effect on the sleep deprivation-impaired activation system. The stimulus of high intensity was disruptive on Day 1 but facilitative on Day 3, a result which was interpreted as an initial inhibition, then disinhibition of arousal. Results were interpreted to indicate that, in some instances, alterations in the less specific arousal and activation systems may underlie impairment or changes in the more specific information processing and motor output stages.

Perceptual and motor components of performance in three different reaction time experiments

Three reaction time experiments were conducted to examine the effects of time of day, stimulus intensity, stimulus modality, and constant and variable foreperiod on the perceptual and motor components of performance. These variables are all supposed to generate changes in arousal level. All the independent variables affected the perceptual component, while the motor component was significantly influenced only by foreperiod and modality. The results are discussed in relation to aspects of dependency/independency of the perceptual and motor components of human performance in reaction time experiments.

Attentional deficits in head-injured children: an information processing analysis

Attentional problems of closed head-injured (CHI) children were examined using an information processing (IP) approach. Based on Sternberg's (1969) additive factor method (AFM), the study examined attentional processes in terms of four stages and their corresponding task variables. A visual-spatial choice reaction-time task was undertaken with two groups of CHI children (severe and mild to moderate) and corresponding matched control groups. Results indicated that for this task both the CHI and the normal children exhibited a similar mode of linear, sequential information processing. It was found that the severe group was impaired not only in terms of slowed motor execution but also in terms of response selection. This group showed no impairment on the feature extraction, stimulus identification, and motor adjustment stages of processing. No evidence of impairment on any of the stages was found for the mild group. The utility of the AFM and implications of these findings are discussed with reference to CHI children and to neuropsychological assessment and rehabilitation.

Placebo controlled comparison of acute effects of ebastine and clemastine on performance and EEG

The effects of single oral doses of 10 and 20 mg ebastine were compared with placebo and 2 mg clemastine in a double-blind cross-over study in 16 healthy male volunteers. Clemastine produced the known pattern of changes, namely impairment of psychomotor performance, drowsiness, and a selective effect on cognitive processes. Earlier encoding in a perceptual stage was slowed whereas abstract classification processes were not affected. Electrophysiological measures of vigilance showed a general decrease in vigilance especially 2.5 and 4.5 h after dosing. In contrast at no time was any effect of ebastine different from that of the placebo. Ebastine 10 and 20 mg differed positively from clemastine in its effect on pursuit tracking, subjective rating of drowsiness and general discomfort. Ebastine 10 mg also differed positively from clemastine in the EEG features of vigilance. It is concluded that 10 and 20 mg ebastine were free from sedative adverse effects.

Reaction time and movement time measured in a key-press and a key-release condition

Reaction time (RT) and movement time (MT) are measured in two conditions, a key-press and a key-release condition. The latter makes a greater demand on perceptual feedback than the former. RT increases in the key-release condition and a fraction of MT denoted key-press time (KT) decreases in the key-press condition. It is argued that KT of a response button in the simple reaction-time experiment may serve as a proper measure of the motor component. The foreperiod prior to onset of the reacting stimulus affects RT, KT, and MT, suggesting that preparatory set or expectancy influences both the perceptual and motor components of simple human performance. The results indicate that the relation of RT and MT depends upon methodological conditions.

Effects of closed-head injury on attentional processes: an information-processing stage analysis

The present study, based on Sternberg's (1969) additive-factor method, examined attentional processes in terms of four information-processing stages (feature extraction, identification, response selection, and motor adjustment). Four task variables were used to operationally define the stages (signal quality, signal similarity, signal-response compatibility, and foreperiod uncertainty). In two studies, a visuo-spatial reaction-time task was undertaken by a group of university subjects (Experiment 1) and by three groups of closed-head-injured patients (severe short-term, severe long-term, and mild short-term) and their corresponding matched controls (Experiment 2). The results indicated that both patients and normals exhibited a similar mode of linear information processing. In addition, it was found that the severe short-term group was impaired on the response-selection stage and response selection stage; the severe long-term group was impaired only on the response-selection; and no evidence of impairment was found for the mild short-term patients. The implications of these findings with reference to the study of attentional processes in closed-head injured patients and to neuropsychological assessment and rehabilitation are discussed.

Stress and cognitive functioning in sport

This paper considers the relationship between stress and cognitive functioning and discusses some of the concepts and methods that may be adopted to examine the relationship between stress and sports performance. It considers only that literature that may be relevant to, or furthers, the understanding of sports performance under competitive stress. One of the most popular approaches has been to explain the relationship in terms of the unidimensional inverted-U hypothesis. The validity of this approach to explaining sports performances in specific situations is discussed and it is generally concluded that the inverted-U hypothesis is too vague and simplistic. The discussion considers multidimensional approaches and discusses the suitability and implications of some more recent and more complex models of stress and performance which are now available in the psychological literature. It is concluded that the relationship between stress and sports performance is an extremely complex one and involves the interaction between the nature of the stressor, the cognitive demands of the task being performed and the psychological characteristics of the individuals performing it.

Drugs and information processing in skilled performance

A concise review is presented of recent research on task aspects with respect to determining the effects of drugs on human information processing. It is concluded that progress in this area is hampered by lack of a theoretical basis to most behavioral tasks, preventing firm conclusions about the effects of drugs on either well-defined mental functions or on real-life performances. It is argued that the effects of drugs should only be tested in behaviorally well-researched tasks. Some proposals are discussed with an emphasis on perceptual-motor skills.

Dose effects of methaqualone on stimulus encoding in a memory scanning task

Three task variables, stimulus quality, memory set size and response type, were used in a Sternberg binary classification task to define stimulus encoding, short-term memory scanning, and response selection stages within a serial stage reaction process. Mean reaction times, and the slopes and intercepts of the function relating reaction time to memory set size, were used to test the hypothesis that performance deficits seen at two doses of methaqualone (2.9 mg/kg and 5.9 mg/kg), in the range formerly in common clinical use, were specific to the stimulus encoding stage of the reaction process. Mean reaction times were increased significantly by the methaqualone at both doses, but the effects of the two doses did not differ from one another. The intercept of reaction time as a function of set size showed significant main effects of methaqualone, stimulus quality, and response type, and a significant hyper-additive interaction of methaqualone with stimulus quality. At 2.9 mg/kg, the intercept was increased by methaqualone but only with degraded stimuli. At 5.9 mg/kg, the intercept was increased by methaqualone for both high and low quality stimuli. These results suggested a dose-dependent selective effect of methaqualone on the stimulus encoding stage of the reaction process.

Effects of d-methamphetamine on auditory and visual reaction times and detection thresholds in the baboon

Baboons were trained in both auditory and visual reaction time procedures to release a response lever in the presence of low-intensity stimuli. By varying the stimulus intensity from trial to trial, functions relating reaction time (elapsed time from stimulus onset to lever release) to stimulus intensity were established, and detection thresholds were measured. The effects of acute, IM injections of d-methamphetamine (0.001-1.0 mg/kg) were examined on these psychophysical performance baselines. Reaction times for acoustic stimuli generally were faster for higher drug doses, whereas reaction times for visual stimuli either lengthened or shortened, depending on both drug dose and individual differences among animals. Auditory thresholds were unaffected at all drug doses studied, whereas visual thresholds were generally elevated at doses of 0.1 mg/kg and above.

The effect of methylphenidate on information processing

Models of information processing currently popular in cognitive psychology divide the reaction process into a series of discrete separable stages. The distinction between one stage and another is verified by the additive factors method (AFM) as defined by Sternberg (1969). Task factors that do not interact with each other are inferred to affect different stages. The distinction between stimulus evaluation stages and response selection stages has been supported by brain event related potential (ERP) studies. The latency of the P300 component of the ERP is sensitive to changes in stimulus complexity but not to to changes in response complexity. The focus of this research is to determine the effects of stimulant drugs on stages of information processing using both reaction time (RT) and P300 latency within an AFM framework. Four doses of methylphenidate (MP) were used in a within-subjects design to examine the effects of MP on stimulus and response processing. We found that MP speeds RT, and that this effect does not interact with the effect of stimulus complexity on RT. MP dose interacts with response complexity, the dose for optimal speeding varying with the level of complexity. The latency of P300 is increased by stimulus complexity, and not by response complexity, nor is it affected by MP. These results show that the stimulant drug acts on processes involved in response selection, rather than in stimulus evaluation. Individual differences in drug response are dose dependent, but also point to an effect on response processing.

Information processing components of the auditory event related potential are reduced by cocaine

The effects of cocaine on a human electroencephalographic event related potential (ERP) were measured. Forty-eight subjects received one of three IV doses (0.2, 0.4, or 0.6 mg/kg) and placebo. Thirty-three subjects received one of three oral doses (2, 3, or 4 mg/kg). All IV and oral doses reduced amplitude of the auditory ERP P200 and P300 components during the oddball task. P200 latency decreased. N100 amplitude was reduced only after IV administration. The changes in ERPs occurred during the period of peak cardiovascular and subjective effects. The amplitude reduction in ERP components occurring before the P300 component is consistent with decrements in attention, specifically selective attention. The P300 amplitude reduction after cocaine suggests a disruption of stimulus evaluation resources. The findings are inconsistent with the notion that stimulants affect only response selection and execution. The degree to which stimulants alter cognitive processes prior to response selection may depend on the magnitude of the cardiovascular, subjective, and probably other noncognitive effects.

Visual evoked potential changes induced by methylphenidate in hyperactive children: dose/response effects

Visual event related potentials (ERP) were recorded from 21 hyperactive children aged 7-13 years under two attention conditions at 4 levels of methylphenidate dose (placebo, low, medium and high). ERP measures were very sensitive to age (under or over 10 years) and attention condition, but less sensitive to drug dose. There appeared to be two classes of drug dose effect on ERP amplitude, those that changed monotonically with dose and those from which dose interacted with attention non-monotonically. Drug effect on ERP amplitude may also depend on age so that opposite effects may occur in young and old children. No latency measures showed a dose effect. It appears that methylphenidate can speed reaction times without shortening ERP latency. This suggests the drug acts more on response-related processes than on stimulus evaluation.

EP components, visual processing stages, and the effect of a barbiturate

In a 2 x 2 x 2 factorial experiment, ten subjects carried out a visual choice reaction task. In addition to RT measurement, evoked potentials (EPs) were recorded from the central (Cz) and occipital (Oz) derivations. Independent variables were drug treatment (barbiturate versus placebo), visual stimulus intensity and visual stimulus degradation. The reaction time data showed that visual intensity and degradation had additive effects, which indicates that these variables affect independent stages, i.e. stimulus preprocessing and stimulus encoding. The effect of barbiturate on RT was additive with intensity but appeared to interact with degradation. This suggests a selective effect of that drug on the encoding state. EP components also showed selective effects of intensity, degradation and drug treatment. It was suggested that these components may be related to stages in the reaction process.