Vigilance (psychology)

In modern psychology, vigilance, also termed sustained concentration, is defined as the ability to maintain concentrated attention over prolonged periods of time. During this time, the person attempts to detect the appearance of a particular target stimulus. The individual watches for a signal stimulus that may occur at an unknown time. The study of vigilance has expanded since the 1940s mainly due to the increased interaction of people with machines for applications involving monitoring and detection of rare events and weak signals. Such applications include air traffic control, inspection and quality control, automated navigation, military and border surveillance, and lifeguarding. In modern psychology, vigilance, also termed sustained concentration, is defined as the ability to maintain concentrated attention over prolonged periods of time. During this time, the person attempts to detect the appearance of a particular target stimulus. The individual watches for a signal stimulus that may occur at an unknown time. The study of vigilance has expanded since the 1940s mainly due to the increased interaction of people with machines for applications involving monitoring and detection of rare events and weak signals. Such applications include air traffic control, inspection and quality control, automated navigation, military and border surveillance, and lifeguarding. The systematic study of vigilance was initiated by Norman Mackworth during World War II. Mackworth authored 'The breakdown of vigilance during prolonged visual search' in 1948 and this paper is the seminal publication on vigilance. Mackworth's 1948 study investigated the tendency of radar and sonar operators to miss rare irregular event detections near the end of their watch. Mackworth simulated rare irregular events on a radar display by having the test participants watch an unmarked clock face over a 2-hour period. A single clock hand moved in small equal increments around the clock face, with the exception of occasional larger jumps. This device became known as the Mackworth Clock. Participants were tasked to report when they detected the larger jumps. Mackworth's results indicated a decline in signal detection over time, known as a vigilance decrement. The participants' event detection declined between 10 and 15 percent in the first 30 minutes and then continued to decline more gradually for the remaining 90 minutes. Mackworth's method became known as the 'Clock Test' and this method has been employed in subsequent investigations. Vigilance decrement is defined as 'deterioration in the ability to remain vigilant for critical signals with time, as indicated by a decline in the rate of the correct detection of signals'. Vigilance decrement is most commonly associated with monitoring to detect a weak target signal. Detection performance loss is less likely to occur in cases where the target signal exhibits a high saliency. For example, a radar operator would be unlikely to miss a rare target at the end of a watch if it were a large bright flashing signal, but might miss a small dim signal. Under most conditions, vigilance decrement becomes significant within the first 15 minutes of attention, but a decline in detection performance can occur more quickly if the task demand conditions are high. This occurs in both experienced and novice task performers. Vigilance had traditionally been associated with low cognitive demand and vigilance decrement with a decline in arousal pursuant to the low cognitive demand, but later studies indicated that vigilance is hard work, requiring the allocation of significant cognitive resources, and inducing significant levels of stress. Green and Swets formulated the Signal Detection Theory, or SDT, in 1966 to characterize detection task performance sensitivity while accounting for both the observer's perceptual ability and willingness to respond. SDT assumes an active observer making perceptual judgments as conditions of uncertainty vary. A decision maker can vary their response bias, characterized by c, to allow more or less correct detections, but at the respective cost of more or less false alarms. This is termed a criterion shift. The degree to which the observer tolerates false alarms to achieve a higher rate of detection is termed the bias. Bias represents a strategy to minimize the consequences of missed targets and false alarms. As an example, the lookout during a bank robbery must set a threshold for how 'cop-like' an approaching individual or vehicle may be. Failing to detect the 'cop' in a timely fashion may result in jail time, but a false alarm will result in a lost opportunity to steal money. In order to produce a bias-free measure, d' is calculated by measuring the distance between the means of the signal and non-signals (noise) and scaling by the standard deviation of the noise. Mathematically, this can be accomplished by subtracting the z-score of the hit rate from the z-score of the false alarm rate. Application of SDT to the study of vigilance indicates that in most, but not all cases, vigilance decrement is not the result of a reduction in sensitivity over time. In most cases a reduction of detections is accompanied by a commensurate reduction in false alarms, such that d' is relatively unchanged. Mental workload, or cognitive load, based on task differences can significantly affect the degree of vigilance decrement. In 1977, Parasuraman and Davies investigated the effect of two task difference variables on d', and proposed the existence of a vigilance taxonomy based on discrimination type and event rate. Parasuraman and Davies employed discrimination tasks which were either successive or simultaneous, and presented both at high and low event rates. Successive discrimination tasks where critical information must be retained in working memory generate a greater mental workload than simultaneous comparison tasks. Their results indicate the type of discrimination and the rate at which discriminable events occur interact to affect sustained attention. Successive discrimination tasks indicate a greater degree of vigilance decrement than simultaneous discriminations, such as comparisons, but only when event rates are relatively high. For detection tasks, empirical evidence suggests that an event rate at or above 24 events per minute significantly reduces sensitivity. Further investigation has indicated that when the discrimination task is difficult, a decrement can occur when the mental workload is low, as with simultaneous comparisons, at both high and low event rates. The effect of event rate on monitoring task performance can be affected by the addition of non-target salient objects at varying frequencies. Clock test research conducted in the late 1950s and 1960s indicates that an increase in event rate for rare irregular low salience signals reduced the vigilance decrement. When non-target 'artificial' signals similar to target signals were introduced, the vigilance decrement was also reduced. When the 'artificial' signal differed significantly from the target signal, no performance improvement was measured. Other dimensions beyond event rate and discrimination task difficulty affect the performance of vigilance tasks and are factors in the Vigilance Taxonomy. These include but are not limited to: sensory modality, or combinations of sensory modalities; source complexity; signal duration; signal intensity; multiple signal sources; discrete versus continuous events; intermittent versus continuous attention requirement; observer skill level; and stimulation value. Initial Vigilance Taxonomy studies relied on assumptions regarding the mental workload associated with discrimination tasks, rather than a direct quantification of that workload. Successive discriminations, for example, were assumed to impose a greater workload than simultaneous discriminations. Beginning in the late 1990s, neuroimaging techniques such as positron emission tomography (PET), functional magnetic resonance imaging (fMRI) and Transcranial Doppler sonography (TCD) have been employed to independently assess brain activation and mental workload during vigilance experiments. These neuroimaging techniques estimate brain activation by measuring the blood flow (fMRI and TCD) or glucose metabolism (PET) associated with specific brain regions. Research employing these techniques has linked increases in mental workload and allocation of attentional resources with increased activity in the prefrontal cortex. Studies employing PET, fMRI and TCD indicate a decline in activity in the prefrontal cortex correlates with vigilance decrement. Neuroimaging studies also indicate that the control of vigilance may reside in the right cerebral hemisphere in a variety of brain regions.