How we learn and refine motor skills and how we prevent performance breakdown in pressurised or demanding circumstances are among the most important questions within sport psychology and skill acquisition. The development of lightweight eye tracking technology has significantly advanced our understanding of what individuals fixate and attend to while performing sport skills. A growing body of evidence suggests that how gaze is controlled is critical for skills requiring precise cue selection, optimal timing and the ability to focus for long durations under extreme performance conditions . Such research suggests that in order to be successful at aiming to a target, the final fixation made by a performer must not only be located on the target, but must also be of a long enough duration to ensure accuracy. This particular gaze strategy, termed the quiet eye (QE; ), has been shown to be indicative of superior performance in a variety of sporting tasks (see Vine et al., 2012  for a recent review). Indeed, since its conception in 1996 over 70 articles have been published probing the role that the QE plays in underpinning skilled performance.
The Quiet eye:
The QE is defined for a given motor task as the final fixation or tracking gaze directed to a single location or object in the visuomotor work-space within 3degrees of visual angle (or less) for a minimum of 100 ms . The onset of the QE occurs before the critical phase of the motor task and the offset occurs when the final fixation deviates off the target for more than 100 ms. The QE has been shown to underpin successful performance, differentiating both expertise (inter-individual) and proficiency (intra-individual), with experts and successful sporting attempts characterised by longer QE durations .
Quiet eye training:
Perhaps of most interest to applied sport and performance psychologists is the body of evidence emerging which shows that the QE is a perceptual-cognitive skill that can be trained. The QE is not just a by-product of expertise, but appears to be an important mediator of skilful performance. QE training involves guiding decisions about where and when to fixate areas of interest within the visuomotor work-space whilst performing a skill. Using video modelling and verbal feedback, performers are guided to develop the same QE focus and visual control as expert performers. Recent studies examining the utility of QE training among a skilled populations have been undertaken in shotgun shooting , football penalty taking  and golf putting .
In Causer et al.’s (2011) study , 20 international level skeet shooters were assigned to either a QE trained or control group and tested before and after an 8-week intervention. The QE trained group were shown video feedback oftheir eye movements and taught a pre-shot routine aimed at lengthening their QE. After training, the QE trained group had a significantly earlier onset of QE on the clay, tracked it for longer and demonstrated significantly improved performance, while the control group revealed no significant changes in QE or performance.
Vine et al. (2011 ) revealed similar benefits for a brief (1 hour) QE training intervention for low handicap (mean = 2.4) golfers. Again, relative increases in QE and performance in a laboratory-based putting task were found, compared with a control group. The benefits also transferred to the competition environment with the QE trained golfers revealing a relative improvement in golf putting performance on the course; taking 1.92 fewer putts per round and holing 5% more putts from 6 to 10feet following the training.
QE training has also been shown to help novices to acquire targeting skills more quickly than when taught using ‘typical’ technical instructions. Vine and Wilson [9,10] examined the effect of QE training on the gaze control and performance of novice participants performing golf putts and basketball free throws, respectively. In both studies, two groups of novices, QE trained and control (technique focused instructions), performed 360 acquisition trials. Learning was assessed using retention tests, where no guidance or instructions were provided. In both studies, the QE trained group demonstrated better performance than their control group counterparts in these retention tests (although this advantage only approached significance in the 2010 golf putting study). The control groups also improved performance over the training period (and increased their QE durations significantly), however, QE training promoted a longer and more expert-like QE, resulting in a level of performance that was further along the learning curve.
Anxiety and the QE:
As top level sport is characterised by a demand to perform at peak levels in anxiety-provoking situations, research has also examined how anxiety causes a disruption to attentional control leading to degradation in task performance (see Wilson, 2012 ). As a result, several studies have revealed that the QE may be a useful index of optimal attentional control in targeting tasks, and sensitive to the influence of increased anxiety [12, 13]. The aforementioned studies (e.g. by Vine and Wilson [9, 10], Vine et al., 2011 ) have included experimental manipulations of anxiety, to determine any additional benefits of QE training upon performance in an anxiety provoking environment. The authors found that control groups displayed significantly shorter QE durations and performed significantly worse in pressure tests, compared with retention tests, whilst QE trained groups maintained effective QE durations and performance. Thus, the results indicate that QE training acted to protect performers from the adverse effects of anxiety upon attentional control and performance by maintaining effective QE durations.
On-going research is aiming to pinpoint the exact cognitive and neurological processes which may explain the benefits of a QE and QE training. Recent research by Vine et al., 2013  and Klostermann et al., 2013  used strict experimental manipulations to examine some of the key characteristics of the QE (i.e. location, duration and timing). Other work by Vine et al., in press  and Wood et al., 2012  has examined psychological explanations for the benefits of QE training such as increased perceptions of control and a more implicit form of learning.
QE and QE training interventions are receiving increasing attention from domains outside of sport in which motor skills must be learnt and then performed under pressure. Wilson and colleagues  have examined the QE of high and low motor coordination children, to examine whether the QE may explain differences in motor skill learning and performance. Vickers and Lewinski  examined the QE durations of experienced and novice police fire arms officers, and found that increased QE durations were associated with improved weapon accuracy. Finally, a body of work by Wilson, Vine and colleagues [20, 21] utilised eye tracking technology and QE theory to examine the gaze behaviours of laparoscopic surgeons. In a series of studies gaze training interventions were examined in terms of their benefits for skill acquisition, robustness under pressure (dual tasking) and the retention and transfer of surgical skills [22, 23].
The QE variable provides an interesting framework to examine perceptual cognitive expertise in motor skill acquisition and performance. The body of evidence to support QE theory is growing in quantity, is developing in quality and is expanding into new and interesting areas.
Dr Sam Vine is an Experimental Psychologist based in the Sport and health sciences department at the University of Exeter: