As we have previously reported, 2.0 mA of anodal tDCS over right inferior frontal cortex significantly increased performance with training of a difficult hidden object detection task. In addition, we found that higher tDCS current was associated with increased measures of alerting attention. Furthermore, the proportion of hits was strongly and significantly correlated with attention scores for participants receiving 2.0 mA tDCS. This suggests that tDCS enhancement of object detection performance is in part related to enhancement of alerting measures of attention.
One possibility is that enhancement of alerting attention by tDCS led directly to an increased ability to detect objects in the immediate and delayed tests. A heightened state of alertness during testing would be expected to enhance participants’ ability to identify threatening objects hidden within the complex scenes that were used in this task. In this study we targeted the right inferior frontal cortex, a prominent node of the fronto-parietal attention network thought to support alerting attention as measured by the ANT. Enhancement of alerting attention means that, (1) after 2.0 mA tDCS, participants were quicker to respond to the cue stimulus within the 100 ms interval between cue and target onsets, resulting in a faster response to target stimuli, when compared with 0.1 mA controls, or (2) participants receiving 2.0 mA tDCS were able to achieve a heightened state of alertness overall, with greater priming of target perception and/or response. Enhancement of object detection could be explained to some degree by either or both of these interpretations of these results. If participants are faster to achieve a state of cue-induced alertness, then they might have more time to search for threat stimuli in the test images. This is unlikely to have led to the large increases in learning present in this study, however, as participants were given 2 seconds to view the images, and differences in alerting reaction time scores were on the order of 20 ms, or 1/100 of the stimulus duration in the object detection task. Alternatively, a continuously heightened state of alertness could speed processing of stimuli within the complex virtual environment in this task, leading to the ability to process a greater number of objects in the image and/or more time to consider the response to the image before the end of the response window. Another possible explanation for these results is modulation of perception. Perhaps tDCS enhanced perception for both the object detection task and the ANT, leading to greater performance on both measures. If this were the case, however, one might expect that orienting and executive attention would differ between tDCS groups. Further research is needed to disentangle these possible cognitive effects of tDCS over right inferior frontal cortex.
The frontal-parietal attention network assessed by the alerting measure of the ANT has been proposed by Coull et al.
 to be associated with vigilance in continuous performance tasks and has been specifically implicated in sustained attention during object selection
. TDCS may have prolonged sustained attention in this study, leading both to greater hit rate and alerting reaction time scores on the ANT task. Given that the immediate test and ANT were performed after approximately 1.5 hours of experimentation, it is possible that the differences here can be explained in part by differences in the fatigue between the groups receiving 0.1 mA and 2.0 mA tDCS. However, we did not find that the self-reported measure of fatigue assessed by our mood/state questionnaire was related to enhancement of alerting or that it differed between tDCS groups. Future studies might benefit from examining the relationship between tDCS of the right inferior frontal cortex and sustained attention more directly, using other attention tasks aside from the ANT.
Interestingly, no correlation was found between the alerting attention measure of the ANT and false alarm rate or d’. Our previous studies
[11, 12] have shown that tDCS significantly reduces false alarm rate and increases d’. It is likely that the results of these previous studies can be explained by tDCS effects on multiple cognitive domains, each of which may account for effects on different performance measures for this task. d’ is calculated by taking the difference between normalized hit rate and normalized false alarm rate. When considered here, this suggests that the nonsignificant relationship between alerting and d’ might be explained by a nonsignificant relationship between alerting and false alarm rate specifically. Perhaps tDCS enhancement of learning and memory during training led to a greater understanding of object identities and general rules of object locations, increasing participants’ d’ scores and decreasing false alarms, while enhancement of alerting led to greater achievement and maintenance of visual search performance during the object detection task, but did not improve the false alarm rate. Another possibility is that effects of tDCS on false alarm rate result from increased risk aversion. Fecteau et al.
 show that anodal tDCS near the region targeted in this study decreases risky behavior in the Balloon Analog Risk Task (BART), despite incentive for risky behavior. In this context, more risk-averse participants might be more cautious in responding to ambiguous stimuli. This would lead to a lower proportion of object-present responses in trails where no object was detected by the participant.
The ANT has previously been used to examine the relationship between attention and various psychiatric and neurological disorders, including borderline personality disorder
, attention-deficit hyperactivity disorder
, and depression
. Deficits in alerting as measured by the ANT have been found for elderly individuals relative to a younger population
, and alerting scores have been found to vary by subtype of ADHD
. Perhaps enhancement of alerting through modulation of the fronto-parietal alerting network with tDCS might reduce deficits found in these populations. Research into the application of tDCS to these clinical issues could lead to new treatments and interventions without the necessity of pharmacological therapy.
While this study demonstrates compelling evidence of tDCS effects on basic measures of attention, there were several limitations which should be mentioned. We did not collect baseline alerting scores in this study and it is possible that differences demonstrated between participants receiving 0.1 and 2.0 mA tDCS were due to pre-existing differences in alerting. This is unlikely, however, as there was no correlation between alerting scores and measures of performance on the object detection test performed before training began, and there was no difference in object detection at baseline. Also, the design of this experiment was chosen to maximize the effects on object detection performance by placing the anodal electrode over right inferior frontal cortex, but future studies examining the effects of tDCS on measures of attention obtained using the ANT might benefit from using multiple electrode configurations, targeting multiple nodes implicated in the different attention networks proposed by Posner and Peterson