Eur J Appl Physiol (1992) 65:241-245
Applied Physiology Journal of
and Qccupational Physiology © Springer-Verlag 1992
The influence of moving auditory fields on postural sway behaviour in man R. W. Soames and S. A. Raper Anatomy and Human BiologyGroup, BiomedicalSciences Division, King's CollegeLondon, Strand, LondonWC2R 2LS, England Accepted April 21, 1992
Summary. Postural sway behaviour was assessed, using a standard biomechanical measuring platform, in 30 young subjects (15 men, 15 women) during 60 s of erect standing in various combinations of visual input and moving auditory fields. The sway parameters investigated were mean lateral, antero-posterior, radius and velocity of sway, the area within the sway profile and the length of the sway path. The findings support the view that moving auditory fields have a destabilising influence on postural sway behaviour, and suggest that under the appropriate conditions postural sway can be "driven" by the auditory environment. Key words: Postural sway - Moving auditory fields Vision
Introduction Postural stabilisation is a dynamic process involving multisensory inputs and results from the continuous incorporation of motor commands formulated by the central nervous system (Mirka and Brookhart 1981). The ability to stand erect is regulated predominantly by visual, proprioceptive and vestibular afferent information, which is itself organised in a dominantly hierarchical manner (Paulus et al. 1984). Postural imbalance can therefore be considered to be a function of the mismatch between these various inputs. Once developed, however, postural control remains "plastic" and can be modified with practice (Fearing 1924) and prolonged optical reversal of vision (Gonshor and Melvill Jones 1980). During erect standing "automatic postural adjustment mechanisms" provide the majority of sway compensation which, according to Forssberg and Nashner (1982), are mediated primarily by active, support surface inputs. Although the vestibular system provides the essential gravitational reference, the subsequent relative
Correspondence to: R. W. Soames
weighting between the proprioceptive, vestibular and visual inputs appears to be context-dependent. Although it is widely accepted that a constant interplay between visual, vestibular and proprioceptive information provides the basis for the maintenance and control of posture, there is evidence to suggest that the auditory environment and auditory feedback can also influence sway behaviour. Era and Heikkinen (1985) reported that individuals exposed to noise at work have greater magnitudes of postural sway than do those without such exposure. Njiokiktjien (1973) had previously reported that when loaded with an auditory task individuals showed a decrease in postural sway. However, this latter finding probably reflects nothing more than the influence of mental activity on sway behaviour (Skaggs et al. 1932). The influence of auditory fields on postural sway has recently been demonstrated by Raper and Soames (1991), who found that its effect is generally to increase sway. They concluded that auditory stimulation always has a destabilising effect on posture, but with visual feedback this influence is offset. There appears, however, to be no direct interaction between the visual and auditory systems. Harris (1972) had earlier shown that exposure to an asymmetric intermittent 1000-Hz tone, at intensities between 65 and 105 dB, resulted in poor performance in a balancing task. In contrast, Bensel et al. (1968) observed a suppression of lateral sway during presentation of a similar tone at 70 dB. Nevertheless Harris (1972) proposed from his results that the acoustic energy had an effect on vestibular receptors, adding that the intermittency rate may be the most important variable for stimulating the vestibular system. It has been shown, in the guinea pig at least, that slow time varying pressure changes in the external auditory meatus are correlated with a variety of eye movements, including nystagmus, and that these responses are eliminated following eighth nerve section but retained after destruction of the cochlear by loud sounds (Parker et al. 1968). Furthermore, increases and decreases in action potential pulse rate have been observed
242 in some single vestibular ganglion nerve cells, again in the guinea pig (Parker and von Gierke 1971), in response to pressure changes in the external auditory meatus, suggesting that there can be vestibular activation by intense infra- and audiofrequency sound. Obviously if such findings were applicable to man they could form the basis for understanding orientation and equilibrium difficulties encountered not only in aerospace operations but in many high sound intensity industrial environments. The object of the present study was to determine whether moving, as opposed to stationary, auditory fields also have a detrimental effect on postural sway behaviour. In addition it was of some interest to determine whether postural sway could be "driven" by acoustic stimulation, as appears to be the case under certain conditions of visual feedback (Lee and Aronson 1974; Bles et al. 1977).
Methods Thirty subjects (15 men and 15 women) aged between 19 and 30 years agreed to participate in the study in which postural sway behaviour was recorded in 12 combinations of auditory and visual field conditions. The criteria for inclusion in the study were that subjects had no history of auditory or vestibular disease, and that their hearing was within normal limits. The latter was determined using standard audiometric techniques. Postural sway behaviour was recorded using a biomechanical measuring platform linked on-line to a computer; full details are given in Raper and Soames (1991). The following sway parameters were calculated using commercially available software: mean lateral and antero-posterior sway (mm), area within the sway path (ram2), mean radius of sway (mm), length of the sway path (mm) and mean velocity of sway (ram.s-i). As well as recording postural sway under quiet conditions (i.e. no auditory field), two dynamic auditory field conditions were created by "moving" the sound between speakers, either from side-to-side (SS) or from front-to-back (FB), with a frequency of 0.1 Hz. For each of these conditions both a pure tone (250 Hz) and general background conversation were employed, each being presented at an intensity of 65 dB. Furthermore, each auditory field condition was presented with and without visual feedback (i.e. eyes open and eyes closed). The speakers were placed at head height approximately 0.5 m from the subject. The presentation of the various combinations of auditory and visual fields was randomised in order to minimise any carry-over effects from one test to the next. Each subject assumed a comfortable position on the platform, there being no restriction on foot placement except that it was within the confines of the platform. Once comfortable, an outline of the feet was made so that foot position remained constant for each subject throughout the experiment. Between each of the 12 test sessions, each of which lasted 60 s, subjects were allowed to relax.
Results In the following all significant differences are at the 1% level unless otherwise stated. The sway behaviour observed under the equivalent sets of quiet conditions showed there to be no significant difference in any of the parameters investigated. Consequently a combined mean and standard deviation were determined and used in all subsequent analyses.
Vision The absence of visual feedback significantly increased the magnitude of all sway parameters for all auditory field conditions (Table 1). No significant differences were observed in any of the parameters studied between the three types of auditory field when standing with visual feedback (eyes open) or without (eyes closed).
Sex Generally there was no difference in sway behaviour between men and women (Table 2), the only difference being in lateral sway and the radius of sway with pure tones. In each case the women showed significantly greater magnitudes than did the men. Except for lateral sway in women, there was no difference in sway for either sex between the three auditory field conditions. The women showed a significantly increased lateral sway for both the pure tone and voices auditory fields compared with the silence condition; the men showed a significantly increased area of sway for both the pure tone and voices auditory fields compared with the silence condition.
Auditory fieM The influence of moving sounds on sway behaviour was partly dependent on whether the sound source was a pure tone or whether it was background conversation. With pure tones, significant differences were observed in lateral, antero-posterior and the area of sway, while with background conversation the differences were in lateral sway and the radius and area of sway. The mean and standard derivation of the sway magnitudes for each of these parameters are given in Table 3. For each of the above parameters, except antero-posterior sway with pure tones, the magnitude was always significantly greater with an auditory field than when standing in silence; both the pure tone and voices conditions elicited similar sway magnitudes. When the moving auditory field was background conversation significant differences were observed in lateral sway and the radius and area of sway between sounds presented in the SS and FB directions, with the FB direction giving the greater magnitudes (Table 3). With pure tones as the sound source the SS presentation produced greater lateral sway than did FB. However, for antero-posterior sway the FB presentation elicited greater sway than did SS. There was no difference between the SS and FB presentations for the area of sway. From the three-way analyses of variance conducted on the data there was no interaction between vision, the direction of the sound source or the m e n / w o m e n groups, except for the area of sway with the pure tone auditory fields. In this case a significant ( P < 0.01) interaction between all three variables was observed.
243 Table 1. Means (SD) of the various sway parameters during 60 s of erect standing with and without visual feedback
Without*
Visual feedback Auditory field
With Tone
Voices
Silence
Tone
Voices
Silence
Sway parameter Xm (mm) Ym (mm) Radius (mm) Length(ram) Area (ram 2) Velocity (mm.s -1)
2.24 (0.92) 3.77 (1.58) 4.58 (1.68) 516.7 (133.9) 75.9 (39.9) 8.82 (2.13)
2.17 (0.79) 3.53 (1.26) 4.38 (1.61) 507.5 (112.2) 72.3 (40.6) 8.55 (1.93)
2.09 (0.60) 3.28 (0.95) 4.13 (1.21) 496.7 (102.3) 63.0 (23.3) 8.59 (1.80)
2.72 (1.01) 4.81 (1.67) 5.86 (1.66) 656.0 (154.4) 119.1 (63.3) 11.11 (2.53)
2.70 (0.97) 4.69 (1.44) 5.99 (2.19) 652.8 (146.1) 116.6 (65.0) 10.88 (2.57)
2.61 (0.90) 4.61 (1.27) 5.55 (1.41) 658.1 (140.2) 99.0 (46.5) 11.03 (2.77)
Xm, Lateral sway; Ym, antero-posterior sway * All without values are significantly greater (P
Applied Physiology Journal of
and Qccupational Physiology © Springer-Verlag 1992
The influence of moving auditory fields on postural sway behaviour in man R. W. Soames and S. A. Raper Anatomy and Human BiologyGroup, BiomedicalSciences Division, King's CollegeLondon, Strand, LondonWC2R 2LS, England Accepted April 21, 1992
Summary. Postural sway behaviour was assessed, using a standard biomechanical measuring platform, in 30 young subjects (15 men, 15 women) during 60 s of erect standing in various combinations of visual input and moving auditory fields. The sway parameters investigated were mean lateral, antero-posterior, radius and velocity of sway, the area within the sway profile and the length of the sway path. The findings support the view that moving auditory fields have a destabilising influence on postural sway behaviour, and suggest that under the appropriate conditions postural sway can be "driven" by the auditory environment. Key words: Postural sway - Moving auditory fields Vision
Introduction Postural stabilisation is a dynamic process involving multisensory inputs and results from the continuous incorporation of motor commands formulated by the central nervous system (Mirka and Brookhart 1981). The ability to stand erect is regulated predominantly by visual, proprioceptive and vestibular afferent information, which is itself organised in a dominantly hierarchical manner (Paulus et al. 1984). Postural imbalance can therefore be considered to be a function of the mismatch between these various inputs. Once developed, however, postural control remains "plastic" and can be modified with practice (Fearing 1924) and prolonged optical reversal of vision (Gonshor and Melvill Jones 1980). During erect standing "automatic postural adjustment mechanisms" provide the majority of sway compensation which, according to Forssberg and Nashner (1982), are mediated primarily by active, support surface inputs. Although the vestibular system provides the essential gravitational reference, the subsequent relative
Correspondence to: R. W. Soames
weighting between the proprioceptive, vestibular and visual inputs appears to be context-dependent. Although it is widely accepted that a constant interplay between visual, vestibular and proprioceptive information provides the basis for the maintenance and control of posture, there is evidence to suggest that the auditory environment and auditory feedback can also influence sway behaviour. Era and Heikkinen (1985) reported that individuals exposed to noise at work have greater magnitudes of postural sway than do those without such exposure. Njiokiktjien (1973) had previously reported that when loaded with an auditory task individuals showed a decrease in postural sway. However, this latter finding probably reflects nothing more than the influence of mental activity on sway behaviour (Skaggs et al. 1932). The influence of auditory fields on postural sway has recently been demonstrated by Raper and Soames (1991), who found that its effect is generally to increase sway. They concluded that auditory stimulation always has a destabilising effect on posture, but with visual feedback this influence is offset. There appears, however, to be no direct interaction between the visual and auditory systems. Harris (1972) had earlier shown that exposure to an asymmetric intermittent 1000-Hz tone, at intensities between 65 and 105 dB, resulted in poor performance in a balancing task. In contrast, Bensel et al. (1968) observed a suppression of lateral sway during presentation of a similar tone at 70 dB. Nevertheless Harris (1972) proposed from his results that the acoustic energy had an effect on vestibular receptors, adding that the intermittency rate may be the most important variable for stimulating the vestibular system. It has been shown, in the guinea pig at least, that slow time varying pressure changes in the external auditory meatus are correlated with a variety of eye movements, including nystagmus, and that these responses are eliminated following eighth nerve section but retained after destruction of the cochlear by loud sounds (Parker et al. 1968). Furthermore, increases and decreases in action potential pulse rate have been observed
242 in some single vestibular ganglion nerve cells, again in the guinea pig (Parker and von Gierke 1971), in response to pressure changes in the external auditory meatus, suggesting that there can be vestibular activation by intense infra- and audiofrequency sound. Obviously if such findings were applicable to man they could form the basis for understanding orientation and equilibrium difficulties encountered not only in aerospace operations but in many high sound intensity industrial environments. The object of the present study was to determine whether moving, as opposed to stationary, auditory fields also have a detrimental effect on postural sway behaviour. In addition it was of some interest to determine whether postural sway could be "driven" by acoustic stimulation, as appears to be the case under certain conditions of visual feedback (Lee and Aronson 1974; Bles et al. 1977).
Methods Thirty subjects (15 men and 15 women) aged between 19 and 30 years agreed to participate in the study in which postural sway behaviour was recorded in 12 combinations of auditory and visual field conditions. The criteria for inclusion in the study were that subjects had no history of auditory or vestibular disease, and that their hearing was within normal limits. The latter was determined using standard audiometric techniques. Postural sway behaviour was recorded using a biomechanical measuring platform linked on-line to a computer; full details are given in Raper and Soames (1991). The following sway parameters were calculated using commercially available software: mean lateral and antero-posterior sway (mm), area within the sway path (ram2), mean radius of sway (mm), length of the sway path (mm) and mean velocity of sway (ram.s-i). As well as recording postural sway under quiet conditions (i.e. no auditory field), two dynamic auditory field conditions were created by "moving" the sound between speakers, either from side-to-side (SS) or from front-to-back (FB), with a frequency of 0.1 Hz. For each of these conditions both a pure tone (250 Hz) and general background conversation were employed, each being presented at an intensity of 65 dB. Furthermore, each auditory field condition was presented with and without visual feedback (i.e. eyes open and eyes closed). The speakers were placed at head height approximately 0.5 m from the subject. The presentation of the various combinations of auditory and visual fields was randomised in order to minimise any carry-over effects from one test to the next. Each subject assumed a comfortable position on the platform, there being no restriction on foot placement except that it was within the confines of the platform. Once comfortable, an outline of the feet was made so that foot position remained constant for each subject throughout the experiment. Between each of the 12 test sessions, each of which lasted 60 s, subjects were allowed to relax.
Results In the following all significant differences are at the 1% level unless otherwise stated. The sway behaviour observed under the equivalent sets of quiet conditions showed there to be no significant difference in any of the parameters investigated. Consequently a combined mean and standard deviation were determined and used in all subsequent analyses.
Vision The absence of visual feedback significantly increased the magnitude of all sway parameters for all auditory field conditions (Table 1). No significant differences were observed in any of the parameters studied between the three types of auditory field when standing with visual feedback (eyes open) or without (eyes closed).
Sex Generally there was no difference in sway behaviour between men and women (Table 2), the only difference being in lateral sway and the radius of sway with pure tones. In each case the women showed significantly greater magnitudes than did the men. Except for lateral sway in women, there was no difference in sway for either sex between the three auditory field conditions. The women showed a significantly increased lateral sway for both the pure tone and voices auditory fields compared with the silence condition; the men showed a significantly increased area of sway for both the pure tone and voices auditory fields compared with the silence condition.
Auditory fieM The influence of moving sounds on sway behaviour was partly dependent on whether the sound source was a pure tone or whether it was background conversation. With pure tones, significant differences were observed in lateral, antero-posterior and the area of sway, while with background conversation the differences were in lateral sway and the radius and area of sway. The mean and standard derivation of the sway magnitudes for each of these parameters are given in Table 3. For each of the above parameters, except antero-posterior sway with pure tones, the magnitude was always significantly greater with an auditory field than when standing in silence; both the pure tone and voices conditions elicited similar sway magnitudes. When the moving auditory field was background conversation significant differences were observed in lateral sway and the radius and area of sway between sounds presented in the SS and FB directions, with the FB direction giving the greater magnitudes (Table 3). With pure tones as the sound source the SS presentation produced greater lateral sway than did FB. However, for antero-posterior sway the FB presentation elicited greater sway than did SS. There was no difference between the SS and FB presentations for the area of sway. From the three-way analyses of variance conducted on the data there was no interaction between vision, the direction of the sound source or the m e n / w o m e n groups, except for the area of sway with the pure tone auditory fields. In this case a significant ( P < 0.01) interaction between all three variables was observed.
243 Table 1. Means (SD) of the various sway parameters during 60 s of erect standing with and without visual feedback
Without*
Visual feedback Auditory field
With Tone
Voices
Silence
Tone
Voices
Silence
Sway parameter Xm (mm) Ym (mm) Radius (mm) Length(ram) Area (ram 2) Velocity (mm.s -1)
2.24 (0.92) 3.77 (1.58) 4.58 (1.68) 516.7 (133.9) 75.9 (39.9) 8.82 (2.13)
2.17 (0.79) 3.53 (1.26) 4.38 (1.61) 507.5 (112.2) 72.3 (40.6) 8.55 (1.93)
2.09 (0.60) 3.28 (0.95) 4.13 (1.21) 496.7 (102.3) 63.0 (23.3) 8.59 (1.80)
2.72 (1.01) 4.81 (1.67) 5.86 (1.66) 656.0 (154.4) 119.1 (63.3) 11.11 (2.53)
2.70 (0.97) 4.69 (1.44) 5.99 (2.19) 652.8 (146.1) 116.6 (65.0) 10.88 (2.57)
2.61 (0.90) 4.61 (1.27) 5.55 (1.41) 658.1 (140.2) 99.0 (46.5) 11.03 (2.77)
Xm, Lateral sway; Ym, antero-posterior sway * All without values are significantly greater (P
