This paper presents a review on vigilance enhancement using computerized means and discuss es their contradictory findings. It highlights the key differences between research findings and argues that variations in experimental protocol could be a significant contributing factor to the controversial results. In this paper, we found that, computerized means of enhancement are reliable and have significant effects on reducing vigilance decrement. Th e paper discuss es the challenges toward the enhancement techniques and suggest s several alternative strategies to reduce vigilance decrement. Furthermore, this review provides evidence to use computerized means of enhancement on vigilance studies, regardle ss of their practical challenges. 1 Introduction Sustained attention or vigilance is a higher brain cognitive state which allows us to accurately detect and respond to external stimuli over long periods of time. Humans differ in their ability to handle vig ilance and cognitive workload. We may suffer a drop in cognitive efficiency due to the excessive working memory demand and switching between multiple tasks. On the other hand, it is quite intriguing to note that vigilance decrement caused by monotonous sti muli may also lead to a drop in cogn itive efficiency . Monotonically repetitive or quasi -static types of stimuli can lead subjects to withdraw attentional effort and disengagement from the monitoring task leading to a drop in cognitive performance and incre ased error. Research shows that target detection performance decreases by 15 percent in 30 minutes of a monotonous task . But in modern human computer interface environment, systems should be optimized to help humans make the “correct” decision at the right time. Examples of jobs that are influenced by vigilance decrement include driving for long distances such as lorry drivers, military surveillance, industrial control, air traffic control and power plant operation. Vigilance decrement can lead to fatal con sequences. Driver fatigue is a concerning problem, causing many car fatalities every year. Over 800 fatalities and 30,000 injuries from car crashes were due to fatigue driving in the United States in 2009. In China, 1768 fatalities were attributed to fatig ue driving in 20 07. Previous research has reported that fatigue was responsible for 20 –30% of the total road fatalities [1, 2] . In the following section, we present the state of the art of enhancement techniques using computerized machine. 2 State of the arts The following sub -sections discuss in details the findings obtained by different studies and highlight the key differences between them taking in consideration several factors that affect vigilance. 2.1 Video Game Nowadays, Action Video Game (AVG) has become a routine activity among children and adolescents. Green and Bavelier defined AVG by those that have “fast motion and required vigilant monitoring of visual periphery and simultaneous tracking of multiple targets” [3]. Researchers have found positive effects of AVG on basic mental processing such as perception, s ustained attention, memory , cognitive skills and decision -making [3-10]. These studies showed significant improvement in the cognitive abilities of gamers compared to no -gamers as reported by detection rate of targe ts/accuracy, response time and false alarm. Trick et al., [11] found that AVG improved the ability of children and adults to keep track of a set of moving ob jects that were visually identical to other moving objects in the visual field. Anguera [12], demonstrated in an experiment conducted on 179 subjects, that cognitive abilities including sustained attention and preservation of multitasking of the subjects who received computer game training has significantly improved. Another study by Green [5] found that AVG improved performance at locating targets presented in rapid rate and at tracking s everal moving items in the presence of distractors . Several studies have reported that, training, getting knowledge of results (KR), and expertise level of the gamer made an important contribution to sustained attention [12-19]. Kluger et al. reported that, receiving feedback on the performance not only has an informational function, but also has motivational properties that have an important influence on learning [14]. Complete feedback provides player with information about signal characteristics in the task, particularly Hit and False Alarm (FA) rates . When players alerted to signals in this way, they tend to maintain sustained attention an d the proportion of correct detections will be high while the number of FA committed remains low. Others demonstrate d that KR provided during a vigilance task improved performance over time and transferred to a task similar to the training task [15-17]. In facts, getting KR improve d the overall performance during training as well as during subsequent test session in which feedback had been withdrawn [20-23]. Analysis of previo us studies reveal ed that action video game players (AVGPs) show ed significant advantages over non -video game players (NAVGPs) as it relates to vigilance performance (i.e. proportion of correct detections, response time, false alarms, etc.). Additionally, workload analysis showed that AVGPs rate the task lower than NVGPs in terms of total or global perceived workload [24]. Interestingly , AVG studies on sustained attention up to date has not found a systematic gender differences [25-27]. For instance, [26] did not find any difference in vigilance performance of male and female children aged between 3 and 7 years, and [25] found no gender differences in audi tory and visual vigilance tasks in both participant s, child and adult. Another study found that, a 10-h training in AVG was enough to compensate for baseline gender differences in spatial attention, and to reduce the gap in mental rotation skills [28]. Whether the initial difference was innate or a product of lesser exposure to this kind of activities in women is a matter of debate [7]. Althou gh, AVG has demonstrated its potential in enhancing cognitive abilities, w e believe that the use of computer games as a means to enhance vigilance might not be very practical and doomed to failure as it may take the attention off the current task at hand. 2.2 Transcranial Direct Current Stimulation Transcranial direct current stimulation (tDCS) is a neuromodulatory technique that applies a small current in the range of 1 to 2 mA to the scalp and is considered safe for periods up to 30 min [29]. The tDCS causes changes in neuronal excitability through membrane polarization and the changes in synaptic strength [30, 31] . The direction of the cortical effects mainly depends on the polarity and the waveform of the applied current. For example, a nodal stimulation typically depolarizes the resting membrane potential, bringing neurons closer to their firing threshold, while cathodal stimulation decreases neuronal excitability [32]. Changes in cortical excitability lasted over one hour and have been demonstrated after few minutes of stimu lation [33]. Interestingly, there has been an increasing amount of research in the use of tDCS on cognition and clinical studies [34- 36]. Recent studies have examined the effects of tDCS on vigilance, attention and at alleviating problem at workplace [34, 35, 37 -43]. However, the effects of tDCS depends on the period of stimulation as well as the electrode placement on the brain areas. For example, [37] applied 1mA to the dorsolateral PFC (in the F3 and F4 position) for 10 minutes reported vigilance enhanc ement in a simulated air traffic controller task. The tDCS increased target detection performance and operator discriminability as well as increased cerebr al blood flow velocity and oxygenation when compared with a sham condition. Similarly, [44] applied 2 mA to the left dorsolateral PFC (in F3 position) for 30 minutes and used the right upper arm as reference while people doing Mackworth Clock Test and Psychomotor Vigilance Task. The result s showed imp rovements on vigilance, to the same or greater extent than caffeine. [45] studied the effects of tDCS on sustained attention task, reported an increment in the mind wandering but no effects were found on accuracy or reaction time. These findings indicate that tDCS may be well -suited to enhance performance degradation and showed that the frontal lobe play a major role in mind -wandering behavior. Study in [46] applied tDCS to the parietal lobes of healthy adults during visual vigilant task with numerical characters, showed enhancement of numerical proficiency. Another studies s timulated the parietal lobes during training of a vigilance numerosity task showed improvements on discernable effects up to weeks [39-41]. The technique has reported positive impact on improving cognitive function of people with cognitive impairments by reducing symptoms of attention deficits [35, 42, 43] and alleviate vigilance problems at work [38]. However, not all studies showed positive effects of tDCS on sustained attention. Study in [47] applied 2 mA to parietal cortex for 30 minutes and found that tDCS negatively affected performance on the final block of a reaction time task. Other studies employing a go/no -go task and found no effects of tDCS on performance when increased demand was placed on inhibitory control and set shifting abilities [48-50] for review refer to [51]. The studies reported that, genetic factors modulate effects of tDCS on cognitive performance. Therefore, genetic var iability should be considered in the design and analysis of future tDCS studies. Additionally, it seems that tDCS effects are most apparent at higher levels of task difficulty but not simple target detection [52]. While tDCS appears to be quite versatile and non -invasive, there risk of triggering epileptic seizures and the effects of long -term measurement are not yet known. It is still doubtful whether tDCS will ever be practically useful enhancement method. 2.3 Music Stimulation Music plays a major role in self -regulation of emotion and cognitive abilities by modulating arousal and mood [53, 54] . Studies have shown that music is able to improve attention and concentration, which resulted in better performance at work [55, 56] . Wokoun, attempted to improve vigilance performance using background music in industria l settings [56]. The author reported vigilance improvement through music listening by creating and administrating a music program designed to increase tempo at peak f atigue times. Similarly, Davies, [57] showed that background music increased the percentage of target detection in a visual vigilance task, particularly, when conditions were more difficult which suggested a specific modulation of the alertness state caused by listening to music. Scheufele, [58]introduced background music into a job training and found background music can help trainees focus and com plete d the job assignments more quickly . Corhan and Gounard [59] investigated the effects of different types of music on vi sual vigilance such as "rock" and "easy -listening'' music, and found that vigilance performance is best when background stimulation is discontinuous and contains elements of unc ertainty as the case of rock music. Since Corhan and Gounard [59] reported vigilance enhancement und er rock music , the enhancement may be attributable to the subjects' familiarity with the type of music rather than to its properties of discontinuity, rigor, and uncertainty. Indeed, Wolf and Weiner [60] have suggested that, familiari ty probably plays a l arge role in performance. Fontaine et al. [61, 62] and[63] reported th at, listening to familiar music significantly increased arousal and detection rate on a vigilance task, as well as reduced vigilance decrement. They also reported the important of psychological characteristics of noise in determining level of vigilance per formance. Konz and McDougal's [64] suggested that familiarity increase arousal and improve vigilance performance regardless of the type of music play ed. Similar study in visual neglect also found that preferred music enhanced patient’s performance on a perceptual report test by improving patients’ attention and vigilance [65]. Patients showed enhance visual awareness when completing the task with preferred music as compared to n on-preferred music and silence, suggested that music decrease visual neglect by i ncreasing attention resource. Recent studies [66-69] reported that music preference had a differential effect on miss rates where those who liked the music less tended to have increased false detection rate while those who at least moderately like the music improve their detecting rate . They suspect that non -preferred music was distracting and participants’ unawareness of the distraction led to poorer performance . Other researchers investigated the effects of specifi c type of volume and music on drivers’ vigilance. They found loud volumes affect simple vigilance, reaction time and movements whereas hard rock music affect tasks involving concentration and attention [70-72]. Musi c tempo may also influence the heartbeats of listeners and accelerate performance. According to recent survey, fast- tempo background music increase d performance compared with slow music while performing vigilance task [73]. Moris, reported that, playing music increased the speed and accuracy of task performance by surgeons, for review [74]. It showed that music reduce d heart rate, blood pressure, and muscle effort of surgeons while at the same time increasing the accuracy of surgical tasks. This might be due to that Mozart’s music was able to activate neuronal cortical circuit s related to attentive and cognitive functions [75, 76] . In fact, d ifferent types of music (preferred, fast tempo, classical, etc.) could improve performance in areas beyond vigilance tasks. Music enhance concentration, cognitive function and maintain alertness and vigil resulted in improving detection rate and reduce false alarms [77-79]. Although, music has found to improve performance during vigilance tasks it degrades performance in others [79]. Furnham and Strbac [80] found in a cognitive test, par ticipants performed be st in silence, background mu sic was second best for performance, and bac kground noise was lowest results. Compared to silence, both background music an d background noise negative ly affect work performance . Another finding revealed that playing background music with lyrics are likely to have significant negative effects on the concentration and attention of worker [81]. Future studies of music on vigilance should take in consideration the impacts of valence, tempo, familiarity and personal preference during the design and analysis. 2.4 Auditor y Binaural Beats Binaural beat is an auditory illusion perceived by the brain when two slightly different frequencies of sound played separately into each ear. For example, when you play a 250 Hz tone in the left ear and a 260 Hz tone in the right, it yiel ds a beat with a frequency of 10 Hz. The brain then uses a process known as frequency following response (FFR) to follow along at the new frequency (10 Hz). Low frequency binaural beats are associated with mental relaxation and high frequency beats with al ertness and attentional concentration [82]. A number of reports have suggested that listening to binaural beats can reduce vigilance decrements [83-87]. For example, Lane reported that, exposure to binaural beats for 30 min has led to reducing vigilance decrement [83]. The study tested participant performance dur ing a 1 -back vigilance test while listening to either delta 1.5 Hz, theta 4 Hz or beta 16 Hz range binaural beats. Participants showed improvement in target detection, and decreased in false alarms, task -related confusion, and fatigue while listening to be ta binaural beat. Frederick, [84] investigated the effects of beta binau ral beat at 18.5 Hz on vigilance task with short period of 5 min. The study found an increased in the magnitude of beta electroencephalography ( EEG ) rhythm by 21% . The increase of EEG rhythm was in the range of 13 Hz to 21 Hz with highest increase in ampl itude located at 18.5 Hz. Beauchene [88], investigated three different binaural beats at 5 Hz, 10 H z and 15 Hz on vigilance task of delayed match -to-sample for a period of 5 min. The result reported that, the 15 Hz binaural beat increased the response accuracy as well as improved connectivity across brain regions. Within the gamma band, the largest EEG steady state responses occurred with a binaural beat of 40Hz and primarily activated the frontal and parietal lobes [89]. Jirakittayakorn, [90] investigated high frequency gamma binaural beats at 40 Hz on vigilance performance while doing word list recall task for 30 min. The study found that, listening to 40 Hz binau ral beat for 20 min enhanced working memory function and improved mood state. Reedijk, [85] have recently shown that binaural auditory beats affect people control and monitor their visual attention. Particip ants listened to binaural beats at 10 Hz and 40 Hz while performing an attentional blink (AB) task, which assesses the efficiency of allocating attent ion over time. The AB reduced by the binaural beats in some participants, which suggests that beats have a specific impact on how people allocate their attention over time. Unfortunately, Goodinet.al., failed to meet the same conclusion as Lane using di fferent protocol [91]. In particular, Goodin assessed theta at 7 Hz and beta at 16 Hz in sh ort time of 13 min presented in 2 min tone . The author reported no significant differences in cortical frequency power during the period of binaural beats stimulation compared to using a white noise signal. The discrepancies may be due to the use of short -time stimulation and the choice of carrier tone may influence upon the efficacy of beat stimulation. Robust effect of binaural beat on vigilance could be obtain with the use of lower carrier tone as well as beat frequencies. Lorenza, [92] suggest that listening to high -frequency binaural beat of 40 Hz bias the individual attentional processin g towards a reduced spotlight of attention. The inconsistency in the results of the previous studies might be due to several variables including time duration of the experiment, individual differences, carrier frequency, specific frequency to fluctuate the cortical activities, cognitive test/vigilance task, attention to stimuli, sensory modality, spatial limitation as the case in EEG, age, circadian rhythm, and hearing ability. These factors should be taken in considerations when conduction the experiment a s well as during the analysis framework. 2.5 Haptic S timulation The haptic s timulation comprises two types of feedback: tactile and kinesthetic. The tactile feedback addresses the tactile perception from th e skin, such as vibrations whereas; kinesthetic feedback is based on our own muscular effort. Haptic stimulation has been established as a technique to enhance attention and its technologies deployed in cars revealed significant improvements on driving performance. The h aptic stimulation delivered at variou s areas of the car, such as the steering wheel, the seat, or the pedal, see review [93].Theses type of stimulations used as an alarm to warn the drivers of dangers (like goi ng out of the main road, crossing the traffic light, et..), so that they can pay more attention. Several studies have investigated the effects of haptic stimulation on sustained attention. Study in [94] found that 15 Hz haptic stimulation increased human sensorimotor rhythm band power as well as enhanced short -term attention to vigilance task. The results show ed that, the stimulation improved the attention level to stimuli, reaction speed and accuracy. Another study showed that, haptic s timulation enhanced vigilance level of pilots during stimulation flying task [95]. The study also demonstrated that, tactile feedback provide a higher detection rate and shorter responses times to unexpected events th an visual feedback. However, haptic stimulation is affected by age and sensitive to gender difference [96]. It is well known that, human haptic performance decreases as a function of age but the neural mechanism underlying these changes still not yet elucidated. Future studies on vigilance should take in consideration several factors when developing e xperimental protocol, design and analysis including age, gender, time -on-task and skin sensitivity, which could be affected by skin diseases. 2.6 Visual Modulation Visual modulation technique is recently proposed to reduce vigilance decrement in video survei llance paradigm [97]. The technique was based on integrating primary vigilance task with a visual noise in the form of artificial simulated rain. The effects of integrating the challenging events (simulated rain) into the primary task demonstrated significant enhancement on vigilance performance and reduced the reaction time to stimuli. It seems that, this type of enhancement increased the workload into a certain level, which in turns increased engagement of attentional resource and resulted in improving the vigilance level. However, the effects after stimulation on performance still not yet explored. More s tudies with large sample size are needed to further support the effectiveness of this technique on reducing vigilance decrement. 3 CONCLUSION We conclude that great progress has been made in exploring several computerized mean s of enhancements on vigilanc e. While findings of most enhancement techniques up to date are either solitary or contradictory, recent computerized enhancement techniques exert a positive effect on vigilance. Besides, the effects of enhancement seems to be influenced by several factor s such as time-on-task, and overall experimental protocol and do not last so long and the underlying neural mechanisms still yet not fully elucidated. Understanding how and where the enhancement percepts are generated and which cortical network are highly affected will help in optimization of the enhancement techniques to modulate overall cognition and mood states. Further studies with more accurate reporting of experimental protocols, will help to clarify the most promising effects.