The results of this study demonstrate links between several functional dopaminergic alleles and the propensity to both decline in performance and feel mentally fatigued. Specifically, our data suggest that the dopamine transporter, DAT1, as well as COMT, may have an impact on the rate at which the vigilance decrement occurs, and that the dopamine receptor DRD4 may be related to subjective declines in mental energy. This is one of the first demonstrations that these polymorphisms play such a role in attention in non-clinical populations.
For the two genes that were associated with TOT, alleles that typically confer risk of poorer cognitive performance (i.e. the Val/Val allele 
and the 10-repeat allele of the DAT1 VNTR 
) were protective against TOT declines in our sample, although selective studies on healthy populations have also found effects in the direction indicated in this work 
. The two alleles in question are thought to increase the availability of DA to post-synaptic neurons, thus promoting activity in striatal and prefrontal regions implicated in stable attentional performance. Thus, our results argue against the hypothesis that global variables such as mean reaction time and the slope variable of time-on-task are affected in the same way by dopaminergic activity
The direction of the dopamine effect in this study may be best explained by the tonic-phasic model of dopamine regulation. In a seminal set of papers, Grace and colleagues 
described this dissociation, and showed that dopamine exerts an influence on post-synaptic activity via two separate but related mechanisms. First, phasic DA release is triggered when bursts of action potentials are fired in response to environmental stimuli and reach DA axon terminals 
. This release leads to a large and rapid rise in intrasynaptic dopamine; however, this rise is also highly transient due to the immediate action of the DA transporter, which causes reuptake of DA into pre-synaptic terminals 
. In contrast, tonic DA activity refers to spontaneously occurring single spikes in DA neurons 
which determine the baseline levels of DA concentration in extracellular space 
In the long-term, the tonic and phasic DA systems are not independent. For instance, stimulants such as cocaine block reuptake of DA from the synaptic cleft 
, allowing increased leakage of DA into extracellular space. Abusers of these stimulants tend to have an increase in tonic DA activity, and a subsequent down-regulation of the post-synaptic DA system over time (for a comprehensive review, see 
). Furthermore, it has been demonstrated that individuals with relatively low dopaminergic tone also have larger phasic release of DA in response to an external stimulus (e.g. nicotine) 
. These results may have implications for cognitive functioning; for instance, a model of dopamine function in PFC has shown that tonic and phasic evels of DA interact to determine optimal performance on a task – phasic increases are most beneficial in PFC when basal (tonic) DA activity is low 
As mentioned previously, alleles associated with greater DA availability are usually found to be of benefit in a range of cognitive tasks. However, our data suggest that slope variables may be affected much more greatly by DA tone than phasic DA release, thus explaining the inferior performance of individuals with DA-promoting alleles. Differences between DAT1 and COMT genotype groups in this sample were found only for TOT variables, and not lapses, false alarms or reaction time variability, despite the fact that these variables are highly inter-correlated. We also found evidence that some subjects were able to ameliorate their level of TOT decline by withholding effort (possibly unconsciously), as mean first-minute reaction times were positively correlated with RRT slope. Nevertheless, DAT1 and COMT genotype groups did not differentiate performers on this variable either. We therefore speculate that individuals with chronically high DA availability may more quickly exhaust the benefits of greater phasic release when longer-term attentional effort is required, due to the long-term plastic changes in DA neurons in the striatum and PFC described above. These effects may be especially prominent when elicited by the PVT, which is a task with consistently high signal load and attentional demand.
The assertion that tonic, resting levels of DA may influence TOT changes is consistent with data from a recent study in which Lim et al. 
used fMRI to measure the baseline and task-related activity of a group of young subjects as they performed the Psychomotor Vigilance Task (PVT). The authors found that pre-task resting cerebral blood flow in the thalamus and middle frontal gyrus (MFG) predicted the extent of subsequent performance decline, with higher MFG activity at rest correlated with greater TOT decrements. Taken together with our current genetic data, we speculate that this higher MFG resting perfusion may be related to greater spontaneous firing rates which result from higher DA tone. Indeed, negative correlations have been found between striatal DAT density and regional cerebral blood flow in frontal cortex 
, providing in vivo
support for this claim. This view is also consistent with the notion that fMRI activity may represent an endophenotypic trait 
that mediates between genetic differences and behavioral phenotypes.
While both DAT1 and COMT were found to have an effect on PVT performance in this study, presumably via the modulation of levels of extracellular DA, we note that these molecules likely influence this phenotype via different sites in the brain. COMT acts to degrade DA primarily in PFC, with a ~40% difference in enzyme activity between the Val and Met alleles 
. Furthermore, COMT is significantly less active in the striatum than PFC, as DA persists in extracellular space for a much longer time in the PFC 
. In contrast, the DA transporter has a much larger effect on reuptake in the ventral striatum than in frontal cortex 
. Hence, DAT1 may exert its influence on attention indirectly, possibly by modulating levels of motivation during the task. Further research is needed to test the more fine-grained effects of these two polymorphisms on TOT decrements.
Fast vs. slow decay
By decomposing individual subject curves into two components using a set of exponential functions, we found that DAT1 and COMT polymorphisms were associated with differences in the slow, but not the fast decaying component. Effect sizes of genotype on the slow component were, in fact, slightly larger than for the simple linear fit. These data suggest that dopamine exerts its effects on longer-term TOT decay, which lends credence to the hypothesis that the direction of the effects observed in this experiment (i.e. COMTMet<COMTVal; DAT110-repeat absent<DAT110-repeat present) may only be observable when the dependent measure is a slow-evolving process.
As expected, performing the PVT for 20-minutes resulted not only in robust TOT decrements, but also significant declines in energy and mood. This is consistent with findings from previous studies which suggest that vigilance tests are a resource-demanding form of mental work 
. A factor analysis showed that energy and mood changes loaded onto separate scales, suggesting that different biological pathways may underlie these subjective changes. These factors correspond well with a two-system model of activation (energetic and tense arousal) proposed by Thayer 
, adding support for their validity. Moreover, in the current dataset, we found that objective TOT declines were correlated with changes in energy, but not mood, indicating that the former scale more closely reflects the phenomenology associated with mental workload.
Interestingly, subjective changes in energy were strongly associated with the DRD4 polymorphism; subjects with at least one copy of the 2-repeat allele tended to show a greater subjective change in energy over time. To our knowledge, this is the first association of the allele with this effect, although previous work has implicated DAT1, COMT and DRD2 in mental fatigue 
, and the role of dopamine in maintaining an energetic state has been extensively discussed 
. Experimentally, catecholamine depletion is known to decrease energy and vigor, and increase sleepiness, fatigue and sedation 
. Reproducing and explaining the effect of DRD4 on subjective mental fatigue represents a promising avenue for ameliorating this problem in real-world situations.
Finally, significant increases in anxiety, stress and depression represent a separate problem that is putatively caused by high mental workload. Our data suggested that changes in mood are not directly associated with TOT; nevertheless, they represent an undesirable side effect that may lead to other negative consequences. Further research is needed to characterize how these changes are instantiated in the brain, and how they might affect behavior and performance.
The current study has a small number of limitations. First, subjects were asked to abstain from caffeine, which may have caused some regular users to experience a withdrawal effect. However, data collected from other studies by the first author indicate that caffeine usage in Singapore students is low, and we thus suggest that any such effects were relatively minor. Second, and more importantly, we note that our sample size was moderate for a study of this nature, and that the effects observed were in the small to medium range, exposing us to the possibility of Type I error. Nevertheless, we count two points in our favor. First, we had strong a priori justification based on the previous clinical literature that the alleles under investigation are involved in sustained attention, and tested only a small number of these candidate genes. Second, more than one of the comparisons performed returned a significant result in the same direction, and as these form part of a single system, are unlikely when taken together to be spurious findings.
We note that the reliability of the TOT effect, as is typical with change scores, is only in the moderate range (.54), as are the intra-class correlations of the subjective measures used in this paradigm. This is not unexpected, as difference or slope scores by their nature have lower reliability than measures of central tendency 
. We believe that the relatively lower reliability of change scores speaks more to the method of their computation than against the trait-like nature of TOT vulnerability; however, we recommend that large sample sizes be used when studying this phenomenon in the future to ensure that results are valid and reproducible.
An increasing amount of evidence points to TOT vulnerability as being the result of the cortical attention system drawing on a brain-limiting resource that is determined by resting levels of neuronal activity. Our genetic data strengthen the case that one of the resources in question may be dopamine, and that these individual differences may be more highly related to tonic brain functioning than phasic task-related activity. We further demonstrate that TOT vulnerability is associated with changes in energy but not mood, and that DRD4 is robustly related to the feelings of energy depletion. The results of this study have implications for the emerging field of neuroergonomics 
, and add to our knowledge of the biological basis of mental “work” and its fatiguing effects.