2.1 Age-associated cognitive abilities relevant to driving performance in spatial-temporal context
Among various studies, cognitive predictors of driving performance including visual processing speed and attention, spatial abilities, motor speed, and executive function are most studied [5]. The control of movement is a complex interaction of cognitive and sensorimotor systems. In this section, the major changes that occur in the control and coordination of movement with respect to aging are reviewed.
2.1.1 Visual processing speed and attention
Even in normal aging there is a decline in various aspects of visual attention including selective attention, divided attention, sustained attention and switching attention. Driving in traffic requires the ability to attend to relevant information and to ignore irrelevant information in often complex visual scenes [6]. Therefore, the speed at which visual information is processed can be critical for negotiating challenging traffic situations [12,13,14,15]. It is commonly recognised that older adults are slower than younger adults in the speed of processing and that slowing of fundamental cognitive processes may have detrimental effects on more complex tasks [16]. Older adults showed a significantly decreased ability to divide attention when compared with young and middle-aged adults [17]. It would take an older driver 1.5 to 1.7 times longer on average than a younger driver to scan for information [18]. Apparently, for older drivers, visual attention ability is an important variable having a direct association with accident risk [13, 19].
2.1.2 Spatial abilities
Spatial abilities imply the generation, retention, retrieval, and transformation of visual-spatial information [20]. General spatial abilities, which peak during the second or third decades of life, decline steadily in later life. Older adults experience difficulty with spatial relations and with mental rotation tasks, for example, localizing objects in 3-D space and interpreting information displayed in the rear view and side mirrors of a car [18]. Previous studies have linked spatial cognition to older drivers’ driving behaviours [20,21,22]. A review of older drivers’ capabilities for safe driving [22] indicated that spatial cognition is frequently utilised in driving and related to the safe and efficient operation of a vehicle.
2.1.3 Psychomotor skill
Motor skills in driving, such as pedal control and steel wheel control are psychomotor skills developed as a result of constant practice, so individuals need little thought to perform them but require input from both the physical and the mental attributes, and coordinate in order to achieve a certain goal. Age-related motor skill declines can be found as balance and gait deficits, coordination deficits, movement slowing [23], and alterations in cognitive-motor processes [23]. Motor execution of the older adults may require more psychomotor ability and rely on cognitive control [24].
Poor movement skill in older adults results in reduced speed and accuracy [25]. Older drivers are slower to respond to situations [18], e.g., break reaction time, which affects ability to respond quickly to imminent danger; reduced control, which impacts safe driving in a timely fashion. Older adults take longer to initiate movement and carry through and are slower to accomplish eye movements to fixate on objects that are moving around the environment [18]. Older adults tend to adopt a different movement strategy when facing with a motor task that requires them to move steadily under temporal and/or spatial task constraints [12].
2.1.4 Executive function
Neuroimaging studies showed age-related changes in the prefrontal cortex which indicate the decline of executive function in normal aging [26]. Executive function is necessary for integrating information and making decision, which seems highly relevant to driving [12]. Executive function involves the control and coordination of cognitive operations [27]. Subtle executive changes in a more cognitively intact group (such as older people), show a stronger relationship with cognitively demanding driving tasks [28]. The extent to which visual perception and fine motor skills are well coordinated is critical in driving [5].
Executive function handles organizing actions in goal-oriented behaviors, specifically coordinating different centers of the brain to respond to environmental cues, planning the response and subsequently carrying it out, and dealing with situations that are hazardous or technically challenging [29,30,31]. Given that crashes in older adults involved often occur in complex traffic situations, such as intersections [32], it is reasonable to assume that difficulty occurs at the higher cognitive level of executive function [12] in older drivers.
2.2 The context of driving assessment in older adults
Since cognition and behaviour are often situational in context, laboratory findings, particularly concerning decision-making and related executive functions, may fail to predict behaviours in complex and dynamic tasks that people confront in their daily lives. Vehicle driving in real traffic involves human visual perception, cognition, and locomotion of the driver, vehicle movement, and the environment [33]. The driving environment can greatly influence a driver’s ability on the road. Age-related declines in the required component functions (vision, cognition, sensorimotor) for driving may often be “overloaded” in certain contextual situations [34]. Recent studies have emphasized an integrated approach to driver safety study which focuses on the overlapping and interacting area of the role of driver, vehicle and road environment [35]. To detect the driver state, various overt and covert measures, such as driving performance, visual attention, and traffic situation can be collected and interpreted in the driving context [8]. As movement does not take place by itself, gathering information on the context in which the movement occurred, such as the interactions of moving phenomena with their environment and with other individuals, can facilitate a deeper understanding of movement [36].
2.2.1 The driver-environment interaction
To drive safely, drivers need to perceive and interpret the relevant objects and elements in the current traffic situation so that they can consider these elements during planning and controlling their behaviour. Such elements may be other cars, the condition of the street or traffic signs [30]. For each of these elements, drivers not only must perceive them but also understand them according to their relevance to the driver’s goals. In addition, drivers must make assumptions about the future actions or states of these elements [30]. The concept of situation awareness has become very popular in psychology as well as in driving studies to describe these cognitive processes [37]. Situations require a driver to adapt his/her behaviour by changing speed or dangerous direction. Safer drivers should be able to anticipate these situations in order to take vehicle control actions to avoid a collision [38]. Endsley [39] stressed that the concept of situation awareness is best seen as encompassing perceptual and comprehension processes, but not decision-making and response execution processes.
2.2.2 The driver-vehicle interaction
Once in motion, the driver must continuously and actively make adjustments on the steering wheel and pedals to attain desired travel objectives, and to avoid dangerous situations such as driving off the road or losing control of the pedals [40,41,42]. Driving can be described as a control task in a changing environment “created by the driver’s motion with respect to a defined track with static and moving objects” [42]. The task includes requirements for route choice and following, coordination of manoeuvres in support of navigational objectives, and ongoing adjustments of steering and speed [42]. Control theory is predicated on the assumption that driver control actions are dependent on perceptual processes which select information compared to some standards. Drivers act to keep resulting discrepancies within acceptable limits in a negative feedback loop as the means of control in their goal-directed behaviour [42]. Target variables of control theory can be space and time margins and mental load specifically relating to control [43].
Smith [44] described the interacting phases in driving: first, to sense a situation and stimulus registered at the perceptual level; second, to recognize it and stimulus recognition at the cognitive level; then to decide how to respond at cognitive level; and lastly to execute the manoeuver at the motor level. Inputs from sensory and cognitive processes are important in determining what a driver choose to do and how the movements are organized and adjusted. Therefore, in driving assessments, environmental conditions, task requirements, and persons’ characteristics imposing spatial and temporal constraints must be sensed and evaluated to determine what have been done [45].
2.3 A conceptual driving behaviours and cognitive processes framework
Driving is a complex system in which the environment, driver and vehicle are influencing factors [46]. The system incorporates drivers’ visual patterns, the attribute of traffic environment and vehicle movement that are intertwined in space over time. The human-machine-environment framework (Fig. 1) links the cognitive variables and driving parameters, and various interactions associated with driving psychology and behaviours in a circular hierarchical structure.
The center of the conceptual framework (Fig. 1) abstracts the core of driving as driver-vehicle-environment interactions and where driving research should start from. The second circle (yellow) defines the cognitive variables with respect to the sub-sections of interactions. The driver’s perception, visual attention, spatial orientation and spatial visualization are the critical cognitive variables for driver-environment interaction; whereas psychomotor skill and executive function are the key cognitive abilities for driver-vehicle interaction. The situation awareness theory and driver control theory are used to support the notions of the two interaction phases. In driving, the two phases are instantaneously connected through the driver’s cognitive processes, which are demonstrated as the driver’s visual search and locomotion in the next circle of drivers’ behaviours. The third circle (blue) defines key parameters of driving behaviours and performance, and their linkages to the driver’s cognitive conditions. Eye fixation and duration, steel wheel and pedal controls are the variables of drivers’ main behaviours and performance.
The outer circle (grey) presents study variables of drivers’ behaviours and performance. They are conceptualised as the outcomes of psycho-spatial-temporal interactions on: what and how long drivers viewed their surroundings, how gazing behaviour associated with vehicle control (e.g., lane keeping and speed regulation). The links between grouped variables give indications on the underlying neuropsychological mechanisms responsible for driving behaviour and performance, such as the driver’s ability of visual-motor coordination determines the driver’s locomotion, and results in the vehicle lane position and the speed. The preliminary implication for older adults is that variability in the performance of older drivers may stem from age-related declines in cognitive functioning, and we must go beyond single risk factor studies of crashes to identify the factors most highly related and predictive of safe driving [47]. An important further implication of this framework is that research focus should be directed towards unpacking older drivers’ visual-motor coordination using a vision-in-action paradigm in order to determine the underlying cognitive mechanisms.
2.4 Unpacking visual-motor coordination in older drivers
The conceptual framework has emphasised that driving involves multiple tasks that require drivers to gather information and control the vehicle to achieve desired goals. In other words, drivers face coordination demands arising from the dependencies between sub-activities that constrain how tasks can be performed. Therefore, coordination problems should be managed by sub-activities that implement coordination effectively. A driver’s visual and motor behaviour must be coordinated, and how well it was coordinated determines the driving ability and indicates the level of the driver’s mobility and safety. When applying visual-motor coordination concept in a driving behaviour study, it is important to include key sub-activities in the data model. Therefore, driving assessment or training should always consider how overall goals can be subdivided into tasks so that the coordination can be improved.
Previous studies show that older drivers are less likely to be involved in crashes caused by fatigue, high speed, weather condition or alcohol, but are more likely to be involved in crashes involving: intersections; failure to yield the right of way; failure to identify hazards or to heed stop signs/traffic signals; turning and changing lanes [32, 48, 49]. Older drivers also tend to avoid driving in a busy traffic or at night, and especially at nights in wet conditions, however, older drivers are more likely to be involved in angled impact crashes [50]. It is more difficult for older drivers to gather and process information about the environment due to the gradual deterioration of sensory and cognitive processing capabilities [51]. According to Michon [52], there are three levels of hierarchical decisions which shape driving behaviours: strategic-level decisions such as route selections; tactical-level moment-to-moment decisions such as speed choice and lane changing; and operational-level second-to-second behaviour such as braking or steering. Older drivers are more capable to perform their strategic and tactical decisions than the operational-level ones, which involve quick vehicle handling to implement the decisions at the tactical-level. It is performed almost without conscious thoughts [53], thus requires effective executive functioning and coordination between actions. In general, visual-motor coordination can be construed as the extent to which visual perception and fine motor skills are well coordinated [54]. Older drivers have exhibited difficulty in adjusting their operational-level behaviours, consequently they would have shown problems in visual-motor coordination [55]. The investigation of cognitive mechanisms related to older drivers’ behaviours has been an area of high priority due to the increasing elderly population, the visual-motor coordination can be a sensitive but reliable measurement for the prediction of driving ability among older adults. On the other hand, older drivers must be aware of their abilities and limitations in visual-motor coordination so that they can compensate by employing defensive visual search and lane keeping behaviours to enhance their driving competencies.