The purpose of our study was to investigate the effects of introducing visual and nonvisual conditions of the SOT in conjunction with an auditory cognitive task on balance and cognitive performance. Our results confirmed and extended the findings obtained by Broglio et al,5
which indicated that balance would be maintained at the expense of cognitive function with regard to both RT and errors. Similar to prior researchers,4,5,7,12
we found that young adults' postural stability increased during the fixed-fixed and fixed-sway conditions and remained unchanged during the remainder of the balance conditions. With respect to the cognitive task, we observed a concomitant increase in RT and number of errors with increasing difficulty of the balance task.
A physiologic explanation of our findings is that cerebral processing during dual-task conditions apparently modifies how the central nervous system controls postural stability. Under normal conditions, balance is controlled via integration of sensory information provided by the visual, vestibular, and somatosensory systems.5,13
Input based on limb positioning is transmitted to the basal ganglia. This signal is integrated with planned actions developed in the premotor cortex and supplementary motor cortex in the cerebellum. The descending pathway continues via alpha motor neurons, which innervate skeletal muscle, allowing for regulation of balance.5,14,15
Typically, the visual and somatosensory inputs provide the majority of information to maintain postural stability.5,16
Theoretically, our findings support the “posture-first” principle, which suggests that postural control is attentionally demanding, requiring increased allocation of attentional resources in accordance with the complexity of the postural task.17,18
Vuillerme and Nafati19
proposed 2 additional hypotheses to account for the maintenance of or increase in postural stability during the dual-task condition. The first suggests that increased attention during a reaction-timed cognitive task increases muscular stiffness and, subsequently, postural control.19
This hypothesis was supported by Hunter and Hoffman,4
who found decreased medial-lateral COP movement during a balance task in participants simultaneously performing a visual cognitive task.
The second hypothesis suggests that dual-task conditions facilitate control at the sensory-motor level.20
Although attentionally demanding, postural stability occurs primarily via automatic processes in everyday life, making a single-task condition involving balance alone somewhat unnatural. The authors19
of a related study instructed a sample of young participants to focus on reducing their sway, compared with a control group who received no instruction during a quiet-standing task. The experimental group, which allocated additional attention to reduce postural sway, had increased COP and center-of-gravity amplitudes and frequencies. Incorporating a secondary cognitive task into the dual-task method may better represent everyday and sport situations and force individuals to allocate attention to the secondary cognitive task, leaving postural stability to the aforementioned automatic processes. Simply stated, deliberately controlling posture is less efficient than controlling posture more automatically.20
In contrast to our results, decrements in balance during a cognitive task have been reported by Peterson,21
who observed compromised balance in gymnasts performing a cognitive task. Although an important finding, the author's use of a gross measure of balance (ie, walking on a balance beam) and cognitive task (serial sevens) did not allow subtle neurocognitive changes to be captured.5,21
These results are similar to those found in an older sample but opposite those found with a dual-task procedure in younger participants,7
who may possess greater ability to allocate attention. This ability may allow for muscle recruitment to maintain or improve postural stability with increased RT and error response rates during the dual-task protocol.
Our results are similar to those observed by Broglio et al.5
Participants' performance on an auditory executive-function task that assessed speed and accuracy revealed longer RTs under dual-task than single-task conditions. Notably, longer RTs and an increase in response errors were observed during dual-task conditions for trials that followed a category switch (consonants to vowels or odd numbers to even numbers) versus for trials in which the stimulus category did not change. Thus, the perturbation of balance produced specific effects on cognitive functioning. In addition, the increased complexity of the cognitive task demanded executive processing to inhibit responses to one stimulus set and to respond to a different, now relevant, stimulus set. The process-specific effects of balance disruption on cognitive performance may help to explain, at least in part, the conflicting results obtained by previous researchers who used cognitive tasks that did not depict subtle cognitive changes.
One limitation of this investigation was our study of a healthy sample to determine whether cognitive deficits existed in a nonconcussed state and to evaluate the dual-tasking model as a possible concussion-assessment test. Further research regarding this dual-task condition protocol will include a concussed sample for comparison. Other limitations were participant motivation and frustration during completion of the cognitive task. Although participant compliance and effort were considered good, extraneous variables such as these can only be controlled to a certain extent.
Our results are particularly important for researchers interested in assessing the effects of concussion on athletes' cognitive function. Currently, no single evaluative test can determine the effect of a concussion on cognitive function and help clinicians make return-to-play decisions. The relationships among self-reported symptoms, computerized neuropsychological testing, and postural stability are well documented in the concussion literature. When delivered separately, these tools have demonstrated sensitivities of 68%, 79%, and 62%, respectively; when delivered together, greater than 90% sensitivity was achieved.22
Although these results are encouraging, not all clinicians have access to these tests due to financial constraints and limited availability of the professional support needed to properly evaluate such tests.
The results of the present study suggest that measures of cognitive processes involved in performing complex computer-based tests while concurrently performing a balance task may provide a sensitive means of detecting subtle cognitive changes in a young, healthy sample. Although our findings show promise as an alternate tool for concussion assessment, continued research on a concussed sample is imperative before we implement this protocol in the management of concussion. Like any tool used for clinical decision making, each evaluative tool suggested to help in the management of sport-related concussion must meet the stringent criteria of the laboratory setting before being used in clinical practice.9,23,24
Our methods may be more academic and laboratory based, but the results provide meaningful contributions to aid in the development of a more clinically based tool. A novel tool that incorporates both a motor and a cognitive task to detect deficiencies associated with sport-related concussion may prove to be both time- and cost-effective for the clinician.