Divided Attention

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December 2003

By Kim M. Chronister, MHS, OTR/L

The role that cognition plays in fall prevention programs has been overlooked.

Preventing falls among the elderly has received much attention. Single dimension fall prevention programs that focus on exercises designed to improve strength and flexibility have not been successful.1 Other, more multifaceted types of programs have been designed to improve the musculoskeletal system, examine effects of medication, evaluate physiologic and sensory systems, remove environmental hazards, and provide behavior modification education as a way to prevent falls. These types of programs also have not been very successful in preventing falls from occurring.2

Balance is the ability to remain upright when a person’s posture has been disturbed. The body is constantly regulating visual, proprioceptive, and vestibular input, making automatic adjustments through the spinal cord and brain stem. We are unable to remain completely still and therefore are in a constant state of postural sway. This sway is caused by an attempt to keep the center of gravity (COG) within the base of support (BOS). It is a natural response to the constant force of gravity and the minor internal disturbances caused by one’s own respirations and heartbeat.

Postural control following an external disturbance happens in two ways. The first is generating muscle torque of the trunk and limbs while the feet are kept in place. If this generation of muscle activity occurs at the ankle, it is called an ankle strategy, or if at the hip, a hip strategy. The other method involves making the BOS larger by taking a step. This is referred to as a stepping strategy.

AGING EFFECTS ON BALANCE

Kim M. Chronister, MHS, OTR/L

The effects of normal aging on the systems that control balance and posture are well known. The visual system provides detection and gives direct spatial orientation that is used for alignment and postural adjustment. Age-related changes in acuity and depth perception and a decrease in visual field may also negatively influence balance. Vestibular input is provided through a complex labyrinth lined with hair cells that supply information regarding motion of the head in space. Age diminishes the number of hair cells in the vestibular system, potentially making the information less reliable.

The proprioceptive system is also negatively impacted by age. There is an increased threshold of the joint, muscle, and cutaneous receptors, causing an increase in motor reaction time response. All of the sensory systems listed, coupled with a decrease in muscle strength and bone density, make for balance impairments and an increased risk for falls. It makes sense that programs designed to prevent falls focused on these areas but, again, these types of programs have not been effective. The prevalence of serious falls in the older adult population has stimulated the search for better understanding of the mechanisms of balance control and the impact of aging. It has recently been concluded that balance is a complex issue and cannot be isolated to just one causal factor.1 While it is an accepted fact that aging negatively impacts the biological systems that play a factor in balance, often overlooked is the cognitive system and its impact on balance as one ages.

BALANCE AND COGNITION

Traditionally, balance, posture, and even walking have been thought to be subconscious and highly automatic tasks. Due to this view, higher-level cognitive processes and their effects on maintaining balance and posture have been overlooked. This is surprising since this type of activity happens constantly in everyday life. A person walking across a busy street, engaging in an intellectual conversation when climbing stairs, or running through an airport while trying to remember their gate number, are all examples of engaging in a complex cognitive activity while remaining upright. It has been shown that, when requiring a person to engage in a dual task that includes a cognitive task while in a balance situation, the performance of one task may be chosen at the expense of the other.3-7 This type of trade-off is referred to as dual-task costs (DTCs).8,9

As a person ages, they progress from a system that automatizes postural control to correct balance perturbations almost instantaneously and therefore subconsciously, to one that, due to peripheral and central changes and disorders, becomes more conscious. There is also a balance strategy recovery continuum that correlates to attentional resources. Ankle strategies require less conscious attention than hip strategies, whereas stepping strategies require the most attention.

Interestingly, older adults rely most on stepping strategies to recover from balance perturbations.10 This places an older person at risk for falls as their method to remain upright is the most attentionally demanding. The dual-tasks cost will be a fall or a decreased accuracy in the cognitive task.

LIMITS OF ATTENTION

Are there limits to our abilities to attend to more than one task simultaneously? Capacity models of attention assume we have a certain amount of cognitive capacity to devote to various tasks at hand.11 The capacity theories propose that it is possible to do more than one task simultaneously as long as the “processing space” demands of these activities do not exceed the capacity limits of the system. If capacity limits are exceeded, interference would result and difficulty in performing one or more of these tasks would be experienced.

This type of paradigm is used to explain the balance/posture/cognition interplay. Posture is usually considered the primary task and a secondary task is anything that requires cognitive processing. It is presumed that if posture is an automatic response and does not require cognitive processing, then the reaction time of the secondary task would not be slowed under conditions when simultaneously performed with the postural task as compared to the control (no postural task) conditions. This is not the case. There is convincing evidence that postural control and balance do require attentional resources and are not automated tasks.4,6,7,12 Therefore, treatment of balance disorders must focus on automatizing the balance strategy in a wide variety of environments so it is not competing for attentional resources needed in complex environments and situations. Adding another task while standing results in increased sway, which is one predictor of falls.12 The more a person sways, the more likely they will need to engage in a balance strategy to remain upright. The more novel or difficult the task is, the higher the processing demands, which make balance and posture less automatic.13 Unfortunately, as one ages, it is harder to attend to two tasks simultaneously, as there is increased competition for the limited attentional resources. The elderly are using attentional resources just to remain upright.10 These resources are being increasingly taxed when a cognitive task is added.4,7,11

Postural control and balance require cognitive processing and are also affected by the complexity of the cognitive task.7,11 When people react to balance perturbations, concurrent cognitive demands adversely affect postural performance to a greater extent in older adults than young adults. This may be explained by the inability to switch attention from cognitive tasks to balance tasks when there are rapid and unexpected postural perturbations.13

Fall prevention and rehabilitation programs that are designed to prevent falls by improving balance need to include scenarios in which attention is drawn to another task while remaining upright. Of clinical importance is the fact that walking speed deteriorates when older adults engage simultaneously in a cognitive task.14 Walking speed is a predictor used in fall assessments. However, walking speed is timed without any distractions and all attention is on the task of walking. A more accurate predictor of falls would include speed while engaged in a cognitive task. Older adults have more trouble attending to both cognitive tasks and postural control, and this becomes overwhelming as the task complexity increases. As a person ages, more cognitive and attentional processing is needed to maintain balance during both standing and walking activities.4,8 With the interplay between balance and cognition, addressing cognition in balance restoration and fall prevention makes sense. Traditional fall prevention programs focus on the musculoskeletal system including flexibility, range of motion, and weight training. These traditional programs have not been very successful in preventing falls from occurring.2 Prevention programs that address higher-level cognitive processes while maintaining balance and posture appear to be nonexistent. Falls remediation programs need to incorporate dual tasks, not strictly balance activities in a static environment. Once the needed musculoskeletal components are improved in therapy, balance activities need to include cognitive tasks.

Older adults need to focus on their posture and balance, particularly in complex environments. Therapists need to design programs to automatize balance strategies not only in static environments, but also in dynamic situations. Once this has been mastered, balance needs to be further challenged by adding cognitive tasks.

Balance education and strategies also need to be tailored to a wide variety of circumstances. For example, it may be suggested that a person could ambulate in the home environment as long as there are minimal distractions. The television may need to be off until a person is safely seated and then turned on. Therapists may recommend that everyday tasks involving a strong cognitive component or a highly dynamic process, like grocery shopping, be done in a seated position for safety reasons. Remembering that balance and posture do compete for attentional resources and that these resources deplete with age may add to the success of fall prevention programs.

Kim M. Chronister, MHS, OTR/L, is an occupational therapist with Shands at Alachua General Hospital, Gainesville, Fla.

Editor’s Note: The references that accompany this article appear with the online version at www.rehabpub.com.

REFERENCES

van Haastregt JC, van Rossum E, Diederiks JP, Voorhoeve PM, de Witte LP, Crebolder HF. Preventing falls and mobility problems in community-dwelling elders. Geriatr Nurs. 2000;21:309-314.
Hill-Westmoreland EE, Soeken K, Spellbring AM. A meta-analysis of fall prevention programs for the elderly, how effective are they? Nurs Res. 2002;51:1-8.
Brauer SG, Woollacott M, Shumway-Cook A. The influence of a concurrent cognitive task on the compensatory stepping response to a perturbation in balance-impaired and healthy elders. Gait Posture. 2002;15:83-93.
Brown LA, Shumway-Cook A, Woollacott M. Attentional demands and postural recovery: the effects of aging. J Gerontol A Biol Sci Med Sci. 1999;54A:M165-171.
Kerr B, Condon SM, McDonald LA. Cognitive spatial processing and the regulation of posture. J Exp Psychol. 1985;11: 617-622.
Maki BE, Zecevic A, Bateni H, Kirshenbaum N, McIlroy WE. Cognitive demands of executing postural reactions: does aging impede attention switching? Neuroreport. 2001;12:3583-3589.
Shumway-Cook A, Woollacott M, Kerns KA, Baldwin M. The effects of two types of cognitive tasks on postural stability in older adults with and without a history of falls. J Gerontol A Biol Sci Med Sci. 1997;52A:M232-240.
Lindenberger U, Marsiske M, Baltes PB. Memorizing while walking: increase in dual-task costs from young adulthood to old age. Psychol Aging. 2000;15:417-436.
Li KZ, Lindenberger U, Freund AM, Baltes PB. Walking while memorizing: age-related differences in compensatory behavior. Psychol Sci. 2001;12:230-237.
Wojcik LA, Thelen DG, Schultz AB, Ashton-Miller JA, Alexander NB. Age and gender differences in single-step recovery from a forward fall. J Gerontol A Biol Sci Med Sci. 1999;54(1):M44-M50.
Kahneman D. Attention and Effort. Englewood Cliffs, NJ: Prentice Hall; 1997:20-29.
Melzer I, Benjuya N, Kaplanski J. Age-related changes of postural control: effect of cognitive tasks. Gerontology. 2001;47:189-194.
Baloh RW, Fife TD, Zwerling L, et al. Comparison of static and dynamic posturography in young and older normal people, J Am Geriatr Soc. 1994;42:402-412.

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December 2003