Arousal Inverted

Arousal output is an inverted U-shaped function of arousal input (Figure 3). That is, the effect of arousal input on mental performance and such physiological indicators as electrodermal activity (EDA) initially rises. With further arousal input from trait and state sources, it asymptotes, then decreases as arousal input increases. 

Arousal input and arousal output the invert-U function. 

Acute exposure to caffeine raises arousal levels, subject to the inverted-U principle. 

Given the mixed results in the literature, it is difficult to know just how caffeine does affect memory. To some extent, the differential effects may depend on the memory assessment method (recall or recognition) and the time frame (immediate or delayed). Gender differences may also cloud the picture, as discussed above. Even when these differences are taken into account, however, unexplained discrepancies remain. One partial explanation may be that the differential effects of caffeine are a function of the subject s memory load. For example, Anderson65 found that caffeine enhanced low load memory tasks but was detrimental in high load tasks. This could be due to the increased arousal induced by the high load task, which, in the presence of caffeine, could produce overarousal. The drop in arousal output as the subject crossed the peak of the inverted U-shaped function could cause the memory deficits observed in some studies. 

Results showed that both novelty and white noise improved recall performance under placebo and decreased it under caffeine.6 It appears that the additional arousal generated by novelty and white noise served to push caffeine subjects over the top of the inverted-U curve and hence decrease their recall performance. Thus, it may be that caffeine does improve memory performance under conditions that otherwise produce low arousal. However, any condition causing overarousal, whether or not it is a part of the memory task itself, can yield performance decrements. 

The results of these two studies and others provide further support for the theoretical inverted U-shaped arousal function. In the first case,93 high doses of caffeine pushed subjects over the top of the curve and hence impaired vigilance performance. As time passed and the subject became more accustomed to the experimental situation, arousal returned to more optimal levels and performance improved. In the second study,94 caffeine combined with moderately high intensity situational stimulation (white noise) to drive arousal up beyond the point of transmarginal inhibition and, again, decrease performance. 

There does, however, appear to be a ceiling on the acute dosage of caffeine that will enhance reaction time. At relatively low doses given prior to simple tasks or highly practiced complex tasks, the drug does enhance RT.41 104-117-143-144 However, these results may not apply to more complex tasks that have not been extensively practiced. For example, Lieberman79 found that 64 mg of caffeine decreased RT on a simple visual task in which the subject had to identify an object. However, the same dose of caffeine had no effect on RT when the subject had to choose objects in a more complex task. In fact, caffeine has been found to have detrimental effects on reaction times in some complex tasks.51 104 145 Again, there appears to be an inverted-U relationship between overall arousal — induced by the combination of caffeine and other arousal factors — and performance on reaction time tasks. 

Most findings for emotional functioning are also consistent with this theory. Mood states become more positive until arousal, including the caffeine component, crosses threshold, then deteriorate with further arousal increments. Happiness similarly increases, then decreases as arousal rises. Data on such arousal-relevant personality dimensions as extraversion and impulsivity clearly support the inverted-U hypothesis. What this suggests, then, is that many of the effects of caffeine are mediated by its effect on arousal and its interaction with other arousal agents. Such a finding... 

As we noted earlier, caffeine affects not only arousal, but also other behavioral influences such as attentional focus. However, it now appears that these other effects may be secondary to the impact of the drug on arousal and that a multi-factorial model incorporating the inverted-U function may best describe that relationship. Most results in the literature to date are supportive of the biobehavioral model proposed here. However, research on the psychological effects of acute and habitual caffeine... 

Parrott AC and Hindmarch I (1975). Arousal and performance - the ubiquitous inverted U relationship. Comparison of changes in response latency and arousal level in normal subjects induced by CNS stimulants, sedatives and tranquillizers. IRCS Medical Science, Clinical Pharmacology and Therapeutics, Psychiatry and Clinical Psychology, 3, 176. 

Fig. 6. The Yerkes-Dodson principle based on the inverted U-shaped function describing the relationship between arousal and efficiency. A similar relationship holds for the relationship between DA activity (e.g. within the mesocortical DA system) and performance efficiency. Note that the optimal level of performance may also be a function of the nature of the task in the classical formulation, easy tasks being best performed at higher levels of arousal than hard or difficult tasks.

However, a dispute about folate fortification arouse in the last years as the decreasing trend of colon cancer inverted in some countries with mandatory folate fortification since its implementation (Mason et al. 2007). Therefore, many countries in the EU refuse mandatory fortification and favour the consumption of foods endogenously high in folates or increasing folate content in foods, generally. 

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