In the motor domain, dual-task manipulations are widely used for identifying those persons more at risk of falling in everyday-life context (Schaefer, 2014). In clinical practice, a phenomenon widely known for its simplicity of detection, clinical value and predictive power is the “Stops walking when talking” behavior, or SWWT. The presence of the SWWT failure is typically detected by asking questions (e.g., about medications) and determining whether the participant manages to complete both activities together or stops walking while responding (Figure 1A). Lundin-Olsson et al. (1997) showed that the majority of older people who were unable to continue walking while talking experienced a fall within 6 months from the test, while the risk of falling for seniors who were able to continue walking while talking was significantly lower (Lundin-Olsson et al., 1997). Since then, several studies have supported the effectiveness of the SWWT test in identifying older people at high risk of falls (Verghese et al., 2002; Beauchet et al., 2009; Ayers et al., 2014).

The drop in performance found in dual-task is not limited to predictions within the broad realm of motor performance but can also extend to cognitive aspects and detect impairments related to degenerative disorders. In fact, dual-task cost allows discriminating between healthy controls, mild cognitive impairment (MCI) patients, and Alzheimer's disease (AD) patients thanks to a good association between cognitive and molecular biomarkers, and a moderate prognostic value (Nielsen et al., 2018). A longitudinal study (Montero-Odasso et al., 2017) has shown a good prognostic value of a working memory dual-task on walking parameters in MCI patients. The larger the cost the higher the probability to develop severe cognitive disorders/dementia.

In short, research using dual-task suggests that dual-tasking can be a viable option for the sensitive and early detection of subclinical motor but also cognitive deficits in ecological contexts with good diagnostic and prognostic ability. Evaluating performance in dual-task conditions makes it possible to identify potential problems “sooner and better” compared to the canonical tests, which usually are not sensitive enough to detect subtle deficits. Consequently, it might be possible to reduce the emergence of such problems in the most complex situations of daily life, where they could lead to negative outcomes; indeed, for an aged person, a fall can have very serious consequences (Kannus et al., 2005). What about other potentially dangerous situations such as driving?

Driving is a complex task that can become particularly difficult for older people, who are exposed to specific risks due to motor, sensory and cognitive impairments. It is known that the need to simultaneously process two visual stimuli (i.e., a dual-task) induces a “shrinkage” of visual field, i.e., reduces the number of peripheral targets detected. A computer-based test widely used to verify this phenomenon is the “Useful Field of View” (Ball and Owsley, 1993; Sekuler et al., 2000; Edwards et al., 2006). In this test, a central visual identification task (primary task), is performed either in isolation or while paying attention to peripheral stimuli (secondary task), while ignoring distractors (Figure 1B). As in the motor field the SWWT phenomenon often anticipates a fall, the UFOV effectively predicts important everyday life outcomes, like car crash risk (Clay et al., 2005). Driving performance is significantly more impaired in those older people who present a particularly large, dual-task induced, reduction in their useful field of view (Rubin et al., 2007; Cross et al., 2009). The predictive power of UFOV might be due to its multidomain nature: the test requires not only visual processing but, when task complexity is increased, also high order attentional abilities, that are crucial when driving in traffic. This is in line with the fact that test batteries testing multiple cognitive domains, rather than vision alone, are more able to predict driving outcomes (Wood et al., 2008, 2013).

The UFOV is widely used by healthcare professionals, and the current commercial version (i.e., UFOV®. Visual Awareness Research Group, Punta Gorda, FL) requires only 10-15 minutes to be completed and can run on a personal computer (Wood and Owsley, 2014). Online versions might be developed in the future in order to reach a higher number of potentially at risk older drivers.

De Lucia et al. (2015) used a verbal dual-task to test whether closing-in behavior (i.e., the tendency to draw near or on the visual model/shape that is supposed to be copied) in Parkinson's patients is exacerbated by a second task taxing on verbal, non-spatial, domain. The tendency to deviate towards the model strongly correlated with executive dysfunction, and significantly increased when patients were engaged in dual-task compared with single-task conditions (De Lucia et al., 2015). Such findings suggest a relationship between reduction of attentional resources and the presence of closing-in.

Following a brain injury (caused for example by a stroke) patients may show a deficit called unilateral spatial neglect (Driver and Vuilleumier, 2001; Corbetta and Shulman, 2002; Bartolomeo et al., 2012; Vuilleumier, 2013), which is characterized by difficulties in processing a contralesional portion of the surrounding space (Bonato, 2012). This condition negatively impacts everyday life: right hemisphere damaged patients, for instance, can suffer injuries from hitting objects on their left. In clinical practice, the diagnosis of this disorder is carried out through specific paper tests that require, for example, to cross all the stimuli presented in a sheet. The majority of chronic patients present only mild, if any, deficits in these tests, despite experiencing difficulties in complex, everyday life situations. However, their deficits emerge again in computer-based tests (Rengachary et al., 2009). Building on the previously described clinical dual-tasking approaches we simulated, in a computerized, controlled and simplified context, a visuospatial environment requiring patients to pay attention to several aspects at the same time. Across different studies, we demonstrated that a secondary task (visual or auditory), performed during a main visuospatial task (i.e., naming whether a lateralized target appeared on the left or on the right of fixation), interferes with the perception of the contralesional space in chronic patients not showing neglect in classical tests (Bonato, 2015; Bonato et al., 2019). These seemingly unimpaired patients, while achieving good performance in the single-task conditions (i.e., execution of the visuospatial task only), lost this ability in the dual-task conditions, but only when the target was presented in the contralesional “weak” space. In striking contrast, targets presented on the non-compromised side were always perceived. In short, the left target was perceived by right hemisphere damaged patients only when they could focus on its appearance without having to pay attention to anything else. The fact that the impairment was very similar when the secondary task required to pay attention to a sound or a visual symbol provides important clues to the origin of this interesting phenomenon. One might even claim that the source of the distraction (whether a sound or an image) was not important as much as the presence of a distraction itself. In conclusion, it is possible to assume that, under the less demanding single-task condition, resources are employed to optimize an otherwise impaired performance. However, in the dual-task condition, resources were insufficient and the impairment couldn't be compensated.

The “implementation/operationalization” of the concept of “cognitive resources” in dual tasks has therefore once again proved to be a very sensitive approach in identifying subtle forms of cognitive deficits which represent a real diagnostic challenge for any clinician.