Psychophysiological aspects of caffeine consumption

The effects of a caffeine placebo and suggestion on blood pressure, heart rate, well-being and cognitive performance

We studied the effect of suggestion and different instructions in a balanced placebo design. One hundred and fifty-nine subjects were randomized into a 2*4 factorial design. All subjects except a control group received a caffeine placebo. Subjects were randomized to a condition which was identical to an earlier study, or received an information about scientifically proven effects of caffeine (factor 1). The second factor varied instructions: subjects were either made to expect coffee, no coffee or were in a double blind condition and were told either coffee or placebo would be applied. Dependent measures were blood pressure, heart rate, well-being and a cognitive task. There was one main effect of the instruction factor (P=0.03) on diastolic blood pressure, with the group 'told caffeine' reporting significantly smaller decrease in diastolic blood pressure than controls and subjects in the double blind condition. There were no other main effects on both the instruction or suggestion factor, and no interactions. Contrary to the literature, instruction effects were very small. This was apparently due to the fact that placebo-caffeine in the dose used in this study--one cup of strong 'coffee'--did not produce expectancy effects strong enough to affect the parameters measured. It is concluded that the placebo-caffeine research paradigm is not suitable for researching instruction effects in Germany, and that reported effects should be reproduced with tighter controls.

Effects of caffeine and gender on physiology and performance: further tests of a biobehavioral model

The present study extended testing of our biobehavioral model describing the effects of arousal and caffeine to include an examination of gender differences and their interactions with habitual and acute caffeine ingestion. Males and females selected as high or low habitual caffeine users were randomly assigned to receive either caffeine or a placebo and exposed to novel and repetitive recall tasks and to simple auditory stimuli. Electrodermal activity and recall task performance were recorded. The four major factors examined, including habitual caffeine use, acute ingestion, gender, and stimulus novelty, affected behavior, physiology, or both. Results showed that habitual caffeine usage systematically affected tonic arousal (skin conductance level) and improved recall task performance. Acute caffeine ingestion increased phasic arousal (skin conductance response amplitude) and reduced habituation rates. Gender interacted with other factors to significantly affect both tonic and phasic arousal, and females performed better than males on the recall tasks. These results were partially supportive of the theoretical model, and further work is needed to examine the interactions of acute and chronic caffeine intake with gender and novelty.

Bilateral electrodermal activity: reliability, laterality and individual differences

A first aim of the present study was to estimate the short- and long-term stability of individual response patterns in bilateral electrodermal activity (EDA). A second aim was to examine the relationship of individual brain laterality to both peak amplitude and peak time based electrodermal asymmetry. Additionally, subjects were presented with verbal and spatial tasks to estimate the balance of left/right asymmetry in cerebral activation at time of testing. Finally, the influence of smoking, coffee consumption and subjective circadian phase (morningness/eveningness, subjects' rise time) on bilateral asymmetry and EDA lability was examined. Results indicated moderate to high short-term reliabilities of EDA laterality coefficients, but insufficient long-term stability. Handedness and conjugate lateral eye movements (CLEMs) were not related to asymmetry of EDA, but a significant interaction between CLEM tendency and smoking/nonsmoking on laterality of both EDA parameters was observed. Amount of coffee consumption was also significantly related to electrodermal asymmetry. Analysis of performance data demonstrated that intraindividual shifts of EDA laterality from one recording session to a following one were associated with corresponding shifts in accuracy of verbal/spatial performance. Degree of subjects' electrodermal lability differentiated significantly between speed and accuracy of performance in both verbal and spatial tasks, and was substantially related to subjects' rise time.

Effects of habitual caffeine use and acute ingestion: testing a biobehavioral model

A recently proposed model of the biobehavioral effects of caffeine suggests that acute ingestion impacts physiology and behavior differentially depending on the level of habitual usage of the drug. Acute ingestion and habitual usage are particularly expected to affect arousal and attentional processes. Subjects in the present study were preselected for high and low habitual caffeine use, given caffeine or a placebo, exposed to white noise or no white noise, and asked to perform on several tasks. Included were an arousal/habituation task (pure tones), reaction time, paired associates, anagrams, and vigilance. Electrodermal activity and performance were recorded. As predicted, virtually all effects were on the arousal/habituation and attentional (vigilance) tasks. Both acute ingestion and habitual use increased tonic EDA, and chronic use also reduced phasic responding, especially in the presence of a strong habituating stimulus. Both acute and habitual use also liberalized the vigilance response criterion, in that subjects risked more false alarms in order to attain more hits. In addition, habitual use increased sensitivity and reduced accuracy, and acute ingestion increased vigilance response time in the presence of white noise. Overall, the model was partially supported by these early results, though considerable further research is needed.

Caffeine-induced anxiety and increase of kynurenine concentration in plasma of healthy subjects: a pilot study

The concentration of kynurenine, a neuroactive tryptophan metabolite, in blood plasma after pharmacologically induced anxiety was studied. Anxiety was provoked in 15 healthy volunteers by an anxiogenic dose of caffeine. Kynurenine concentration was markedly increased at the peak of anxiety and returned to normal after anxiety had abated. Possible causes responsible for this effect are discussed. There is a correlation between kynurenine concentration in blood plasma and indices of state and trait anxiety (Spielberger-Khanin scale) and anxiety (Hamilton scale) at baseline. The correlation disappears at the peak of anxiety. It is suggested that kynurenine is involved in caffeine-induced anxiety in humans. The absence of correlation at the peak of caffeine-induced anxiety is discussed.