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Psychological correlates of walking speed in the visually impaired W. D. ALAN BEGGS
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Blind Mobility Research Unit , University of Nottingham , Nottingham, NG7 2RD, UK Published online: 30 May 2007.
To cite this article: W. D. ALAN BEGGS (1991) Psychological correlates of walking speed in the visually impaired, Ergonomics, 34:1, 91-102, DOI: 10.1080/00140139108967291 To link to this article: http://dx.doi.org/10.1080/00140139108967291
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ERGONOMICS,
1991, VOL. 34, NO. 1,91-102
Psychological correlates of walking speed in the visually impaired Blind Mobility Research Unit, University of Nottingham, Nottingham NG7 ZRD, UK
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Keywords: Blind mobility; Locomotion; Evaluation.
Recent attempts to extend mobility evaluation techniques to include the partidly sighted as well as the totally blind have demanded the development of new measures. One of these, the percentage of preferred walking speed (PPWS)reflects the slower walking speed of visually impaired pedestrians. This reduction in speed may be due either to the impoverished visual information available for the control of Locomotion, or it may be due to a strategic response to the stress associated with travel. Using a clientderived mood checklist, this latter hypothesis was confirmed. In contrast, visual status, as measured by both acuity and field loss, w2s unrelated to PPWS. This mobility index is an important addition to existing measures, which are concerned with safety, efficiency and visual function.
1. Introduction The development of indices of travel proficiency in visually impaired pedestrians is an essential first step in evaluative studies of various kinds of intervention. Usually, these interventions take the form of sensory substitution devices, of which the long cane is a familiar example; electronic mobility aids, such as the Sonic Pathfinder (Heyes 1984) also come under this heading. More recently, the evaluation of training methods has posed the same demand. However, finding suitable dependent variables by which the impact of intervention of various kinds may be assessed has been an enduring problem for researchers in this field. To date, a number of avenues have been explored. One such approach concerns the stress experienced by visually impaired travellers; stress is one aspect of travel where intervention might be expected to have an effect. The earliest attempts to use physiological measures of stress of visually impaired travel, such as heart-rate (Wycherly and Nicklin 1970;.Peake and Leonard 1971) as an evaluative roo1 were not successful, mainly because heart rate in totally blind pedestrians is very high (at about 160 bpm), and seems to vary very little with moment-temoment changes in stress, or with the provision of aids. Accordingly, a set of behavioural indices of travel were developed by Armstrong (1 975), based on his analysis of three important aspects of mobility, namely safety, efficiency and stress. His safety indices included counts of the client walking into the road or making bodily contact with obstacles, and several long cane events, such as contacts with the shorelines (the kerb and hedge). EfKciency was measured by the proportion of time spent in productive travel, that is, a measure of how much time not spent immobile or walking in erroneous directions. In practice, these indices can be derived from the analysis of a video of a visually impaired pedestrian travelling along a standard route. Stress, while a very important psychological component of visually impaired travel, was not measured by Armstrong. 0014-0139/91 $3.00 0 199 1 Taylor & Francis Ltd.
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These measures were primarily designed to be used with totally blind longcane users, who represent quite a small proportion of the population of visually impaired travellers. The need to extend evaluative techniques to include partially sighted travellers, some of whom may use only a guide or symbol cane, has been a stimulus for the development of newer evaluative indices which are applicable to such a population. Behaviourally, partially sighted pedestrians operate at a level of competence which means that earlier indices suffer from floor or ceiling effects-they seldom, if ever, contact obstacles, or become confused enough to need to stop during a journey. Nevertheless, their performance can seem qualitatively very different from that of a sighted traveller, and they, too, complain about travel stress. Three indices of performance during travel by partially sighted pedestrians have recently been developed at the Blind Mobility Research Unit at Nottingham. These indices were needed to evaluate the impact of innovatory training practices, although they may, of course, be used to evaluate other interventions. The first of these is the Percentage of Preferred Walking Speed, or PPWS (Clark-Carter et al. 1 986a); recently, the indices of Preview and Detection of Obstacles (Dodds and Davis 1989) have been added to the list. PPWS picks up the most obvious behavioural difference between fully sighted and visually impaired travel, that is, walking speed; in contrast, the latter measures capture something of the visually impaired traveller's ability to see objects in his or her path. This paper will focus on the PPWS measure, and attempt to identify some of the reasons why visually impaired travellers walk slower than they would prefer. In practice, PPWS is based on a comparison between the time taken to cover the route unaided, and the time taken to cover it using the experimenter as a guide. However, unlike the normal sighted guide technique, where the sighted person leads the visually impaired traveller in respect of both walking speed and direction, when measuring PPWS, the subject is asked to regard the experimenter as a perfect mobility aid, and to walk at the speed he or she would prefer. Clark-Carter et a1.k analysis of the underlying mechanism for differences in walking speed drew heavlly on a physiological model advanced by Cotes and Meade (1960) and Walters et al. (1978). These workers showed how energy expenditure is related to walking speed, and that at one's preferred walking speed, energy expenditure is at an optimal level. Above or below this speed, more energy is needed to cover a given distance. Clark-Carter et al. showed that PPWS was related to route complexity, and varied between from about 40% to 85% for long-cane users; guide-dog users achieved slightly over 100%, suggesting perhaps that they were being pulled along by the dog. Thus, the measure seems to have a sufficient range to avoid the floor and ceiling effects which limited the usefulness of the earlier evaluative indices. They were also able to show that the range of an ultrasonic aid, the Sonic Pathfinder (Heyes 1984) affected PPWS, with an increase in range leading to an increase in PPWS (Clark-Carter ef al. 1986b). It has also been suggested that the walking speed of a visually impaired traveller may be related to the amount of stress experienced (e.g. Shingledecker 1978), or the level of the traveller's confidence, as Clark-Carter et al. (1986a) proposed. Although they tested the hypothesis that psychological factors are implicated in these changes in walking speed, Clark-Carter et al. ( I 986a) were unfortunately unable to confirm this. Using SACL, the Stress and Arousal Check List (Mackay et al. 1978) to measure psychological stress and arousal, they found
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no relationship between stress or arousal and PPWS, and were forced to conclude that if stress were related to PPWS, it was likely to be physiological, rat her than psychological. They offered no evidence to support their quite reasonable assumption that confidence levels might be implicated in the walking speed of visually impaired travellers. The question of what underlying factors might account for slower wallung speed in visually impaired pedestrians thus remains unanswered. Two possibilities present themselves. An information-processing analysis suggests that a reduction in walking speed may be related to reduced visual information pickup by visually impaired pedestrians. This hypothesis is easy to test, since it predicts that walking speed will be a function of visual status. Both foveal and peripheral visual information may be involved; although the role of peripheral visual information in locomotion is thought to be more important than central information (Paillard 1980), adequate visual acuity may also be important in real-world travel, as it helps a person to determine his or her location during a journey. An alternative hypothesis is that walking speed is strategically adjusted to help reduce feelings of stress and lack of confidence aroused by the loss of visual status. Thus, walkmg more slowly can be seen as a coping response. Unfortunately, the role of psychological factors in visually impaired travel is as yet not well understood, and work in this area is only now beginning to be undertaken. The seminal, and relatively recent work of Welsh (1 980) described how mobility training is often made more difficult by the presence of a number of personal or social factors. The former category includes emotional components such as fear and anxiety; low motivation and dependency; and the impact of sight loss on a person's self-concept. In addition, social factors such as the role of individual members of the person's family and the attitude of society as a whole towards sight Ioss may need to be taken into account. It is certainly not immediately clear the extent to which each of these might be related to walking speed, although the personal factors would seem to be the more likely candidates. Welsh's account seems to have been intuitively drawn from experienced practitioners' reports of the difficulties clients face. The data are therefore second-hand, and the analysis atheoretical. Clearly, it would be more valid to have first-hand reports of visually impaired travellers' experiences while making a journey; it would also be valuable to have some theoretical underpinning for these findings. Recently, Beggs (1990) asked visually impaired clients about their feelings while travelling. A.questionnaire was then developed, using words which clients identified as being descriptive of their mobility-related feelings. A factor analysis of data from this questionnaire, given at the end of a short but fairly complex route, revealed five factors; these were called Self Efficacy, Vigilance, Disorientation, Cognitive Effort, and Role Acceptance, and accounted for 64-1% of the common variance. The details of this factor analysis are shown in table 1. Self Efficacy, the first and largest factor, seems to encapsulate the three concept's central to self efficacy theory (Bandura 1977, 1982); that is, perceived environmental demand, perceived competence, and any associated emotional distress. Self-efficacy theory is useful in a number of contexts where people are required to perform under some sort of duress, and gives a coherent account of motivation, confidence and cognitive anxiety. These are some of the personal
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Table 1. The results of the factor analysis of the mobility-related feeldings data [adapted from Beggs (199O)J. Factor 1. Self efficacy (20.1%) (1) Sense of competency or mastery Feeling that things are not under control (0.75)
Feeling incapable (0.54) (2) Perceived environmental threat
Feeling unsafe (0.79) Feeling vulnerable (0.67) Feeling at risk (0.63) (3) Resulting emotional distress Not feeling relaxed (0.69) Feeling anxious (0.57) Not feeling selfcoddent (0.5 I) Feeling confused (0.50) Downloaded by [] at 23:13 01 February 2015
Factor 2. Vigilance (1 7.096)
Being cautious (0.80) Concentrating (0.74) Being careful (0.72) Being on your guard (0.7 1) Being observant (0.63) Factor 3. Role acceptance (1 1.6%) Feeling of standing out (0.85) Feeling conspicuous (0.8 1)
Feeling self-conscious (0.54) Factor 4. Disorientation (7.9%) Feeling disorientated (0.83) Not feeling sure or your surroundings (0.50) Factor 5. Cognitive effort (7.4%) Being alert (0.70)
Being wary (0.58)
factors which Welsh (1980) identified as having important effects on mobility performance; recently, Dodds (1 989) discussed the rote of perceived self-emcacy in motivation to undertake mobility training. The Vigilance factor seems to be in part related to the dangerous nature of visually-impaired mobility. However, it may be that unproductive thinking, such as worry and catastrophising (Beck 1976) are also involved, leading to the client being overly cautious; this speculation, however, awaits confirmation. The third factor, Role Acceptance, seems to be related to a failure to-acceptone's changed role as a visually impaired traveller, and was named appropriately. It may be, however, that it also reveals a maladaptive cognitive process, such as attending to images of oneself as an unskilled visually impaired traveller. Potentially, this is open to remediation in a way which adjustment to sight loss is not. Two smaller factors, Disorientation and Cognitive Effort, describe feelings associated with a sense of being lost, and the need to be as alert as possible to pick up the cues needed for travel. Although it would seem reasonable that mobility-related feelings of this sort
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might in some way be correlated with performance, unfortunately, and rather surprisingly, Clark-Carter et al. (1986a) failed to find a relationship between PPWS and either stress or arousal measured by SACL (Mackay et al. 1978). Although SACL works in a number of situations, and with numerous populations, it may be that the reason for its failure lies in the fact that it is inappropriate to use SACL with a visually impaired population. Partly, this may be because SACL is designed to be self-administered; with a visually impaired population, it has to be administered orally, with responses being recorded by a third party. However, the results of the research reported above suggest another, more likely reason. The precise words used in mood checklists of this type are crucially important; they must accurately define a semantic space which is assumed to be shared by the members of the population in question. It is argued that for visually impaired travellers, the words used in SACL were inappropriate, even though they are similar to those used in other mood checklists. For example, the second factor in the CSAI-2, the Competitive State Anxiety Inventory (Martens et a/. 1984) contains items related to the somatic components of anxiety often felt by competitive athletes-racing heart, clammy hands and so on. It also includes the words 'nervous', 'jittery* and 'tense', relating to the subjective experience of increased autonomic arousal. These words also appear in the Arousal scale of SACL, probably because Arousal is, in part, the cognitive appraisal of somatic anxiety. Clearly, both SACL and CSA1-2 are measuring much the same thing. Equally clearly, SACL does not tap the feelings that visually impaired travellers experience while on a journey. None of the words which cluster on the Self Efficacy factor, even those which are associated with emotional distress, appear on the Stress scale of SACL. One of the words which appears on the SelfEfficacy factor, 'feeling relaxed' is an item on the Arousal scale of SACL, while 'feeling alert*,which occurs on the Cognitive Arousal factor, also appears on the Arousal scale of SACL. It would therefore seem likely that the failure of ClarkCarter et of. to relate stress and arousal to walking speed may simply have been due to the choice of an inappropriate instrument. The relationship of psychological factors to PPWS thus remains an unanswered question. Fortunately, at the time when the mobility-related feelings data were being collected, PPWS measurements were also made. In this paper, the relationship of the psychological items to PPWS is reported. In addition, the visual status of the clients was available from their records, and thus it was possible to determine the relative contribution of both psychological and visual factors to walking speed, as well as their relationships to each other. 2. Method 2.4. Subjects A total of seventy-one clients were used in the study referred to earlier (Beggs 1990). They were all residents at a vocational rehabilitation course, and were selected for inclusion in the study if the following conditions applied:
(1) They had at least minimal outdoor mobility skills. (2) They spoke English as a native tongue. (3) They had any degree of visual impairment.
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For the purpose of quantification in the present study, those who had total loss of vision, perception of light, or 'hand movements' or 'counts fingers' measurements of acuity were eliminated from the analysis reported below; in addition, as it is not possible to derive a legitimate field status score for totally blind or untestable clients, they were therefore also eliminated. The result was a sample of fifty-five clients with some degree of partial sight. They varied from clients who had acuities as low as 0.5160 to those with acuities as high as 619; and with what were described in their records as mild to severe peripheral and central field losses. Partially sighted people form a very heterogeneous group, often presenting with some combination of loss of acuity and field. For example, a person suffering from macular degeneration may have extremely poor acuity and some central field loss, but an intact peripheral field. His or her ability to travel will be much less impaired than a person with the progressive condition retinitis pigmentosa. Such a person may have virtually normal acuity in reasonable lighting, but suffer from a very severely contracted field of less than five degrees; in addition, he or she may be unable to see in extreme levels of illumination. These conditions lead to particular difficulties; for example, the former may be unable to read road signs, but the latter will be unable to avoid obstacles, particularly other pedestrians. Both, therefore, will be handicapped in different ways. The subjects for this experiment were thus a wide, but not atypical sample of the sort of clients most Mobility Officers will meet.
3. Procedure
3.1 . Mobility-relatedfeelings Each of the clients was asked to make a short journey of about 100 metres in a Table 2. The mobility-related feelings items as they appear on the five-point scales on the questionnaire.
Anxious Observant Unwary Confused Safe Do not stand out Incapable Relaxed Alert Cautious Disorientated At risk Unselfconscious Things under control On your guard Not concentrating Careful Sure of your surroundings Vulnerable Conspicuous Self-confident
Not anxious Not observant Wary
Not confused Not safe Stand out Capable Not relaxed Not alert Not cautious Not disorientated Not at risk Selfconscious Things not under control Not on your guard Concentrating Not careful Not sure of your surroundings Not vulnerable Not conspicuous Not selfconfident
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very quiet suburban street, which had a number of trees, lampposts, driveway openings, and so on to negotiate. At the end of the journey, they were presented with the questiomaire containing the mobility-related feelings words in the form of five-point Likert-type scales. The questionnaire is shown in table 2. Presentation was oral. The clients were first asked to imagine how it would be possible to measure subjective feelings such as temperature on a hot--cold scale, with only five points-very hot or cold, rather hot or cold, or neither hot nor cold. This they were easily able to do, and responded verbatry to the questionnaire in a similar way. Their responses were recorded by the experimenter.
3.2. Walking speed Calculation of PPWS requires the measurement of both actual and preferred walking speed. The first was recorded as subjects negotiated the route unaided. Some totally blind clients took up to three minutes to complete this journey; most of the clients included in this sample completed it in between one minute and one minute and thirty seconds, although a few took up to two minutes and thirty seconds. After the questionnaire was administered, the PPWS technique was first verbally explained to the subject; then experimenter and subject walked back to the start of the route to practise using the PPWS technique. All subjects were able to understand, and respond to the simple instructions. A second pass was then made, in the original direction of travel, to ensure that any initial lack of confidence and uncertainty was overcome; preferred walking speed was recorded on this run. Times to walk the route in this way were about one minute, which is approximately normal sighted walking speed. 3.3. Visual status From in-house records, recent acuity and field loss measurements were available. Acuity measurements were transformed into angular measurements, to enable them to be more easily quantified for analysis; for example, a client with 3/60 vision could be described as having 0.05 vision. The acuity measurements in clients' better eye were used in the subsequent analysis. Field losses were also quantified from the original Friedman Analyser charts. In the past, a number of methods of quantifying field losses have been used, usually relying on measuring losses within hemifields or sectors of the chart (e.g. Newcornbe 1969). Using conventional perimetry, Ross (1983) developed a scoring system that considered the losses within each quadrant of a central, 5 degree area, those in the four quadrants of an intermediate area extending from 5 degrees out to 25 degrees from the centre, and those falling in the quadrants of an outer area, extending from 25 degrees to the periphery. She argued that, functionally, loss of information from these areas may have different consequences; this is almost certainly true for mobility (e.g. Marron and Bailey
1982). Data from the Friedman Analyser does not extend as far into the periphery as that from an arc perimeter, but it is possible to use a similar scoring technique. The Friedman Analyser charts record individual responses to groups of up to three stimuli presented simultaneously at various positions on a 26-5 degree circular array around a central fixation point. On the completed chart, it is possible to see those unsuccessful responses falling within a central 6 degree '
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circle, as well as those falling outside this area. In addition, it is also possible to see those that fail in each of the four quadrants of these areas of the chart. Those stimuli which were not detected at threshold were counted in each of the eight areas of the charts. These eight scores were then combined to give six estimates of field loss in each measured eye for each client. These were upper and lower hemifield scores in central and peripheral areas, and the total scores in each of these areas. Score from the better eye were used inthe analysis. 4. Results First, a multiple linear regression, using only acuity and measures of overall peripheral loss was run, to test the hypothesis that visual factors might be related to PPWS.The multiple correlation coefficient was -0.32, accounting for 10.50% of the variance; this just failed to reach significance (p= 0.056). Almost all of this multiple relationship was due to the correlation between peripheral loss and PPWS (r= -0.31 7); acuity was unrelated to PPWS (r=O-002). Clearly, visual factors alone are poorly related to PPWS; however, it was possible that they might operate indirectly on walking speed via their relationship to the mobility reIated feelings, which themselves might be related to PPWS.Inspection of the correlation matrix which included aLl the psychological factors, acuity measurements, and all the available measures of peripheral function (upper and lower hemifields in the central 6 degree circle, and in the area between 6 and 26.5 degrees, and total scores within the central region and the outer area) showed that the correlations between all the visual and psychological factors were very low, and none were statistically signihcant. They varied between r=O.OO and r-0.40, making it unlikely that visual factors played any significant role in determining walking speed. As a final verification of this, the twenty4ne scores on the questionnaire, the better-eye acuity data, and the six estimates of peripheral function for the better eye were all entered into a stepwise multiple linear regression with the PPWS scores as the criterion variable. This analysis showed that five of the psychological variables together accounted for 63.3% of the variance of PPWS; adding further items failed to make a statistically significant improvement to this ,model. As would be expected from the above results, visud factors entered the regression very late; for example, total peripheral loss was the seventeenth item, acuity the twenty-sixth. The proportion of the variance accounted for by the five items are shown in table 2.
Table 3. The cumulative proportion of the variable of PPWS accounted for by the five statistically significant predicator variables. Mobility-related feeling Disorientated On your guard Confused Things out of control Incapable
Cumulative percentage of variance accounted for 37.03
50.3 1 58-25
60.38 63.30
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5. Discussion Two hypotheses, information processing and coping, were suggested to account for the differences in walking speed of visually impaired pedestrians. The failure to find a clear relationship between visual status and walking speed suggests that the choice of walking speed is a strategic decision, rather than one which is forced upon a partidy sighted pedestrian by the information-processing demands of the task. Four out of five of the mobility-related feelings related to PPWS are drawn from the Self Efficacy factor, suggesting that PPWS is instead related to the distress-lack of confidence and cognitive anxiety-associated with the client's perceived level of competence to tackle the mobility task. There now seems little doubt that confidence and cognitive anxiety are negatively correlated; in sports, for example, the Competitive State Anxiety Inventory (CSA 1-2) (Martens et al. 1984) measures both somatic and cognitive anxiety, as well as a self-confidence factor. These latter seem to 'represent opposite ends of a cognitive evaluation continuum, self-confidence being viewed as the absence of cognitive anxiety, or conversely, cognitive anxiety being the lack of selfconfidence' (p. 17). The other mobility-related feeling, 'being on your guard' may be associated with wony. Clark-Carter et al. (1 986a) were therefore right in their suggestion that PPWS is in part related to these sorts of psychologica1 factors. This research has underlined the importance of using a valid instrument to measure such processes. Rather surprisingly, the data reported here show that visual factors are poorly related, either directly or indirectly via their effect on mobility-related feelings, to the walking speed of visually impaired pedestrians. Earlier Marron and Bailey ( 1982) showed that although mobility performance was unrelated to visual acuity, contrast sensitivity and field loss were joint predictors of success on their task. One reason for the discrepancy between this research and that of Marron and Bailey may be that their performance measure (success in avoiding obstacles) was very different from walking speed, and direct comparison between these studies may not be possible. The Detection of Obstacles index (Dodds and Davis 1989) may be more appropriate in Marron and Bailey's context, and is similarly related to visual status. PPWS, however, remains an important measure of normal mobility performance. On the other hand, it may be that contrast sensitivity is a more appropriate measure of visual status than acuity, especially in the context of mobility, and may be related to PPWS.Contrast sensitivity testing is a more comprehensive method of determining the ability of the eye to resolve detail, which is a function not only of stimulus size (or spatial frequency), but also of figurelground contrast. The familiar Snellen chart, using letters to test acuity, sets contrast at its maximum, and varies stimulus size; in contrast sensitivity testing, both contrast and spatial frequency are varied. The peak of the inverted Ucuwe relating log contrast sensitivity and spatial frequency (the contrast sensitivity function) can be taken as a measure of optimal visual function, although the precise shape of the curve may also be important. This peak bears no simple relationship to acuity measurements. It may therefore be that had the results of contrast sensitivity tests been available, they might have been related to either feelings or PPWS.This possibility needs to be tested. It is important to note that 36.7% of the variance of PPWS in this group remains to be accounted for. While visual factors appear to be only weakly '
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related to wallung speed, it may be possible that the physiological stress, or somatic anxiety associated with travel is implicated in walking speed differences, as Clark-Carter ef al. (1986a) suggested. However, psychological and physiological responses to stress almost always covary (e.g. Deffenbacher 1977), and it is therefore dficult to see how much additional variance might therefore be accounted for by physiological arousal. Of course, other physiological differences between clients, such as physical fitness, leg length and so on may be involved. It seems more likely that some additional psychological factors not measured in this experiment may be important. Since walking speed seems to be a strategic choice, in part determined by the client's need to maintain anxiety at an acceptable level, it is reasonable to assume that other factors may influence this choice. Some clients are extremely restrained in their mobility performance, others can seem positively reckless. These differences in impulsivity were not measured in this experiment, but may be important enough to account for some of the remaining variance of PPWS. Alternatively, some of the other factors identified by Welsh (1 980) may be involved; the more likely contenders for this role are dependency and the self-concept. Finally, it remains possible that other feelings, not described by Beggs (1 990),are implicated. These might be related to a more general response to loss of sight; depression, for example, is common in visually impaired people. Other possibilities include a sense of helplessness (Seligman 197 3 , an external locus of control (Rotter 1966), or low self-esteem (Coopersmith 1967). These speculations remain untested at present. , The most immediate role for PPWS will be to enable research to investigate the impact of different approaches to training. Training style differences (e.g. Warren el at. f 982) may particularly affect the relatively intangible psychological factors which underlie this index of mobility performance. Pilot work on style differences in mobility training (Beggs 1987) has suggested that a style focusing on responsibility sharing and co-operative gaolsetting has its effects at this level; PPWS offers a convenient way to measure' these effects behaviourally. The PPWS measure may also be important for the evaluation of secondary travel aids which add little additional usable sensory information to that provided by the primary aid, the long cane. Kay (1 980) suggested that the benefit of an ultrasonic aid was apparent largely in terms of improved confidence; earlier behavioural measures of mobility performance fail to pick this up. It is easy to dismiss such claims as untestable, simply because these psychological differences are unobservable behaviourally. It is now possible to test Kay's claim in a satisfactory way, by using PPWS;indeed, in showing that the range of the Sonic Pathfinder affects PPWS,Clark-Carter et al. (1 986b) may have already partially confirmed Kay's belief that effective secondary aids have their effect at a psychological level. This work has clarified the nature of PPWS as an evaluation tool. PPWS seems to be a mobility index which is related to the way the client responds to the stress of travel, unlike the Preview and Detection of Obstacles-assessment indices (Dodds and Davis 1989). These latter indices were developed specifically to evaluate the extent to which partially sighted people use their residual vision during travel, and are thus useful, but indirect measures of their mobility performance. Taken together, however, these new indices enable a much broader view to be taken of mobility performance, and relate it to both visual and
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psychological factors. Unlike the visual performance indices, PPWS measures are appropriate for totally blind clients, too; values of PPWS vary from 100%for the best partially sighted travellers to around 40% for inexperienced totally blind travellers. Although they were not included in this experiment, it would not be surprising to find that these very slow walking speeds reflect the more extreme reactions to sight loss commonly encountered in totally blind clients. The PPWS measure also complements existing be havioural measures of mobility performance (Armstrong 197 9, and extends the Nottingham evaluative methodology to take account of behavioural responses to the stress of travel. It is an important new tool which, together with the existing techniques, will make it possible to tease out the effects of different kinds of interventions which have their effects at various levels. . Acknowledgement The Blind Mobility Research Unit is funded by the Department of HeaIth and Social Security, and is directed by Professor C. I. Howarth. The author acknowledges the contribution of the members of the Unit to this work, and the generous co-operation of staff and residents at the RNIB Employment Rehabilitation Centre, Torquay.
References A R M ~ O N J. G ,D. A. 1975, Evaluation of man-machine systems in the mobility of the
visually handicapped, in R. M. Pickett and T. J. Triggs (eds), Human Fatiors in Health Care (Lexington Books, Lexington, MA). A. 1977, Self-eficacy: Toward a unifying theory of behavioural change, BANDURA, Psychological Review, 84, 1 9 1-2 15. BANDURA, A. 1982, Selfefficacy mechanism in human agency, American Psychologist, 37, 1 22-1 47. BECK,A. T. 1976, Depression: Clinical, Experimental and TheoreticalAspects (Harper and Row, New York). BEGGS, W. D. A. 1987, Mobility training today, I: Differences in approach, British Journal of Visual Impairment, V , 1 3- 16. . BEGGS,W. D. A. 1990, The emotional correlates of visually impaired travel, Journal of Visual Impairment and Blindness (in the press). CLARK-CARTER, D. D., HEYES,A. D. and HOWARTH,C. I. 1986a, The efficiency and walking speed of visually impaired travellers, Ergonomics, 29, 779-789. CLARK-CARTER, D. D.,HEYES,A. D. and H O W A C. R I. ~ ,1986b, The effect of non-visual preview upon the walking speed of visually impaired people. Ergonomics, 29, 1575-1 58 1. COOPERSMITH, S. 1967, The Antecedents of SelfEsieem (Little, Brown, Boston). COTES, J. E. and MEADE,F. 1960, The energy expenditure and mechanical energy demand in walking, Ergonomics, 3, 97- 119. . DEFFENBACHER, J. L. 1977, Relationship of worry and emotionality to performance on the Miller Analogies Test, Journal of Educational Psychology, 69, 1 9 1- 195. DODDS,A. G.D. 1989, Motivation reconsidered: the role of selfefficacy in rehabilitation, British Journal of Visual Impairment, MI, 11-1 5. DODDS,A. G. D. and DAW, D. P. 1989, Assessment and training of low vision for mobility, Journal of Visual Impairment and Blindness, 83, 439-446. HEYES,A. D. 1984, Sonic Pathfinder: A programmable guidance aid for the blind, Wireless World, 90, 26-29, 62. KAY, L. 1980, The Sonicguide, long cane and dog guide: Their compatibility, Journal of Visual Impairment and Blindness, 74, 277-279.
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MACKAY, C. J., COX, T., BURROWS, G. C. and LAZURINI,A. J. 1978, An inventory for the
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measurement of self-reported stress and arousal, British Journal of Social and Clinical Psychology, 17, 283-284. MARRON,J. A. and BAUEY, I. L. 1982, Visual factors and orientationmobility performance, American Journal of Optometry and Physiological Optics, 59,
4 13426. MARTENS,R.,BURTON,D. V w , R. S., BUMP,L. A. and S M ~D., E. 1984, Competitive State Anxiety Inventory 2 (Unpublished, University of Illinois). NEWCOMBE, F. 1969, Missile Wounds of the Brain: A Study of Psychological Deficits (Oxford University Press, Oxford). PAILLARD, J. 1980, The multichanneling of visual cues and the organisation of a visually guided response, in G. E. Stelmach and J. Requin (eds), Tutorials in Motor Behavior (North-Holland, Amsterdam). PEAKE,P. and LEONARD, J. A. 197 1, The use of heart rate as an index of stress in blind pedestrians, Ergonomics, 14, 189-204. Ross, J. E. 1983, The functional aspects of visual disorder, PhD thesis, University of Oxford. RO'ITER, J. B. 1966, Generalised expectancies for internal versus external control of reinforcement, Psychological Monographs, 80, 1 , 609. SEUGMAN, M. E. P. 19 75, Helplessness: On, Depression, Development and Death (Freeman, San Francisco). SHINGEDECKER, C. A. 1978, he effects of anticipation on performance and processing load in blind mobility, Ergonornia, 21, 355-371. WARREN, A., FLEGG, D.and LAW,C. 1982, Poise-Project On Instructor Style Egectiveness (Industrial Training Research Unit Ltd., Cambridge). WALTERS,R. L.,HISUP, H.J., PERRY, J. and ANTON-, D. 1978, Energetics: Application to the study and management of locomotor disabilities, Orthopedic Clinics of North America, 9, 35 1-377. WELSH, R. L. 1980, Psychosocial dimensions, in R. L. Welsh and B. B. Blasch (eds), Foundations of Orientation and Mobility (American Foundation for the Blind, New
York). WYCHERLEY, R. and NICKUN, B. 1970, The heart rate of blind and sighted pedestrians on a town route, Ergonomics, 13, 18 1- 1 92.
Revised manuscript received 27 April 1990. Manuscript accepted 9 May 1990.
Ergonomics Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/terg20
Psychological correlates of walking speed in the visually impaired W. D. ALAN BEGGS
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Blind Mobility Research Unit , University of Nottingham , Nottingham, NG7 2RD, UK Published online: 30 May 2007.
To cite this article: W. D. ALAN BEGGS (1991) Psychological correlates of walking speed in the visually impaired, Ergonomics, 34:1, 91-102, DOI: 10.1080/00140139108967291 To link to this article: http://dx.doi.org/10.1080/00140139108967291
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ERGONOMICS,
1991, VOL. 34, NO. 1,91-102
Psychological correlates of walking speed in the visually impaired Blind Mobility Research Unit, University of Nottingham, Nottingham NG7 ZRD, UK
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Keywords: Blind mobility; Locomotion; Evaluation.
Recent attempts to extend mobility evaluation techniques to include the partidly sighted as well as the totally blind have demanded the development of new measures. One of these, the percentage of preferred walking speed (PPWS)reflects the slower walking speed of visually impaired pedestrians. This reduction in speed may be due either to the impoverished visual information available for the control of Locomotion, or it may be due to a strategic response to the stress associated with travel. Using a clientderived mood checklist, this latter hypothesis was confirmed. In contrast, visual status, as measured by both acuity and field loss, w2s unrelated to PPWS. This mobility index is an important addition to existing measures, which are concerned with safety, efficiency and visual function.
1. Introduction The development of indices of travel proficiency in visually impaired pedestrians is an essential first step in evaluative studies of various kinds of intervention. Usually, these interventions take the form of sensory substitution devices, of which the long cane is a familiar example; electronic mobility aids, such as the Sonic Pathfinder (Heyes 1984) also come under this heading. More recently, the evaluation of training methods has posed the same demand. However, finding suitable dependent variables by which the impact of intervention of various kinds may be assessed has been an enduring problem for researchers in this field. To date, a number of avenues have been explored. One such approach concerns the stress experienced by visually impaired travellers; stress is one aspect of travel where intervention might be expected to have an effect. The earliest attempts to use physiological measures of stress of visually impaired travel, such as heart-rate (Wycherly and Nicklin 1970;.Peake and Leonard 1971) as an evaluative roo1 were not successful, mainly because heart rate in totally blind pedestrians is very high (at about 160 bpm), and seems to vary very little with moment-temoment changes in stress, or with the provision of aids. Accordingly, a set of behavioural indices of travel were developed by Armstrong (1 975), based on his analysis of three important aspects of mobility, namely safety, efficiency and stress. His safety indices included counts of the client walking into the road or making bodily contact with obstacles, and several long cane events, such as contacts with the shorelines (the kerb and hedge). EfKciency was measured by the proportion of time spent in productive travel, that is, a measure of how much time not spent immobile or walking in erroneous directions. In practice, these indices can be derived from the analysis of a video of a visually impaired pedestrian travelling along a standard route. Stress, while a very important psychological component of visually impaired travel, was not measured by Armstrong. 0014-0139/91 $3.00 0 199 1 Taylor & Francis Ltd.
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These measures were primarily designed to be used with totally blind longcane users, who represent quite a small proportion of the population of visually impaired travellers. The need to extend evaluative techniques to include partially sighted travellers, some of whom may use only a guide or symbol cane, has been a stimulus for the development of newer evaluative indices which are applicable to such a population. Behaviourally, partially sighted pedestrians operate at a level of competence which means that earlier indices suffer from floor or ceiling effects-they seldom, if ever, contact obstacles, or become confused enough to need to stop during a journey. Nevertheless, their performance can seem qualitatively very different from that of a sighted traveller, and they, too, complain about travel stress. Three indices of performance during travel by partially sighted pedestrians have recently been developed at the Blind Mobility Research Unit at Nottingham. These indices were needed to evaluate the impact of innovatory training practices, although they may, of course, be used to evaluate other interventions. The first of these is the Percentage of Preferred Walking Speed, or PPWS (Clark-Carter et al. 1 986a); recently, the indices of Preview and Detection of Obstacles (Dodds and Davis 1989) have been added to the list. PPWS picks up the most obvious behavioural difference between fully sighted and visually impaired travel, that is, walking speed; in contrast, the latter measures capture something of the visually impaired traveller's ability to see objects in his or her path. This paper will focus on the PPWS measure, and attempt to identify some of the reasons why visually impaired travellers walk slower than they would prefer. In practice, PPWS is based on a comparison between the time taken to cover the route unaided, and the time taken to cover it using the experimenter as a guide. However, unlike the normal sighted guide technique, where the sighted person leads the visually impaired traveller in respect of both walking speed and direction, when measuring PPWS, the subject is asked to regard the experimenter as a perfect mobility aid, and to walk at the speed he or she would prefer. Clark-Carter et a1.k analysis of the underlying mechanism for differences in walking speed drew heavlly on a physiological model advanced by Cotes and Meade (1960) and Walters et al. (1978). These workers showed how energy expenditure is related to walking speed, and that at one's preferred walking speed, energy expenditure is at an optimal level. Above or below this speed, more energy is needed to cover a given distance. Clark-Carter et al. showed that PPWS was related to route complexity, and varied between from about 40% to 85% for long-cane users; guide-dog users achieved slightly over 100%, suggesting perhaps that they were being pulled along by the dog. Thus, the measure seems to have a sufficient range to avoid the floor and ceiling effects which limited the usefulness of the earlier evaluative indices. They were also able to show that the range of an ultrasonic aid, the Sonic Pathfinder (Heyes 1984) affected PPWS, with an increase in range leading to an increase in PPWS (Clark-Carter ef al. 1986b). It has also been suggested that the walking speed of a visually impaired traveller may be related to the amount of stress experienced (e.g. Shingledecker 1978), or the level of the traveller's confidence, as Clark-Carter et al. (1986a) proposed. Although they tested the hypothesis that psychological factors are implicated in these changes in walking speed, Clark-Carter et al. ( I 986a) were unfortunately unable to confirm this. Using SACL, the Stress and Arousal Check List (Mackay et al. 1978) to measure psychological stress and arousal, they found
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no relationship between stress or arousal and PPWS, and were forced to conclude that if stress were related to PPWS, it was likely to be physiological, rat her than psychological. They offered no evidence to support their quite reasonable assumption that confidence levels might be implicated in the walking speed of visually impaired travellers. The question of what underlying factors might account for slower wallung speed in visually impaired pedestrians thus remains unanswered. Two possibilities present themselves. An information-processing analysis suggests that a reduction in walking speed may be related to reduced visual information pickup by visually impaired pedestrians. This hypothesis is easy to test, since it predicts that walking speed will be a function of visual status. Both foveal and peripheral visual information may be involved; although the role of peripheral visual information in locomotion is thought to be more important than central information (Paillard 1980), adequate visual acuity may also be important in real-world travel, as it helps a person to determine his or her location during a journey. An alternative hypothesis is that walking speed is strategically adjusted to help reduce feelings of stress and lack of confidence aroused by the loss of visual status. Thus, walkmg more slowly can be seen as a coping response. Unfortunately, the role of psychological factors in visually impaired travel is as yet not well understood, and work in this area is only now beginning to be undertaken. The seminal, and relatively recent work of Welsh (1 980) described how mobility training is often made more difficult by the presence of a number of personal or social factors. The former category includes emotional components such as fear and anxiety; low motivation and dependency; and the impact of sight loss on a person's self-concept. In addition, social factors such as the role of individual members of the person's family and the attitude of society as a whole towards sight Ioss may need to be taken into account. It is certainly not immediately clear the extent to which each of these might be related to walking speed, although the personal factors would seem to be the more likely candidates. Welsh's account seems to have been intuitively drawn from experienced practitioners' reports of the difficulties clients face. The data are therefore second-hand, and the analysis atheoretical. Clearly, it would be more valid to have first-hand reports of visually impaired travellers' experiences while making a journey; it would also be valuable to have some theoretical underpinning for these findings. Recently, Beggs (1990) asked visually impaired clients about their feelings while travelling. A.questionnaire was then developed, using words which clients identified as being descriptive of their mobility-related feelings. A factor analysis of data from this questionnaire, given at the end of a short but fairly complex route, revealed five factors; these were called Self Efficacy, Vigilance, Disorientation, Cognitive Effort, and Role Acceptance, and accounted for 64-1% of the common variance. The details of this factor analysis are shown in table 1. Self Efficacy, the first and largest factor, seems to encapsulate the three concept's central to self efficacy theory (Bandura 1977, 1982); that is, perceived environmental demand, perceived competence, and any associated emotional distress. Self-efficacy theory is useful in a number of contexts where people are required to perform under some sort of duress, and gives a coherent account of motivation, confidence and cognitive anxiety. These are some of the personal
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Table 1. The results of the factor analysis of the mobility-related feeldings data [adapted from Beggs (199O)J. Factor 1. Self efficacy (20.1%) (1) Sense of competency or mastery Feeling that things are not under control (0.75)
Feeling incapable (0.54) (2) Perceived environmental threat
Feeling unsafe (0.79) Feeling vulnerable (0.67) Feeling at risk (0.63) (3) Resulting emotional distress Not feeling relaxed (0.69) Feeling anxious (0.57) Not feeling selfcoddent (0.5 I) Feeling confused (0.50) Downloaded by [] at 23:13 01 February 2015
Factor 2. Vigilance (1 7.096)
Being cautious (0.80) Concentrating (0.74) Being careful (0.72) Being on your guard (0.7 1) Being observant (0.63) Factor 3. Role acceptance (1 1.6%) Feeling of standing out (0.85) Feeling conspicuous (0.8 1)
Feeling self-conscious (0.54) Factor 4. Disorientation (7.9%) Feeling disorientated (0.83) Not feeling sure or your surroundings (0.50) Factor 5. Cognitive effort (7.4%) Being alert (0.70)
Being wary (0.58)
factors which Welsh (1980) identified as having important effects on mobility performance; recently, Dodds (1 989) discussed the rote of perceived self-emcacy in motivation to undertake mobility training. The Vigilance factor seems to be in part related to the dangerous nature of visually-impaired mobility. However, it may be that unproductive thinking, such as worry and catastrophising (Beck 1976) are also involved, leading to the client being overly cautious; this speculation, however, awaits confirmation. The third factor, Role Acceptance, seems to be related to a failure to-acceptone's changed role as a visually impaired traveller, and was named appropriately. It may be, however, that it also reveals a maladaptive cognitive process, such as attending to images of oneself as an unskilled visually impaired traveller. Potentially, this is open to remediation in a way which adjustment to sight loss is not. Two smaller factors, Disorientation and Cognitive Effort, describe feelings associated with a sense of being lost, and the need to be as alert as possible to pick up the cues needed for travel. Although it would seem reasonable that mobility-related feelings of this sort
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might in some way be correlated with performance, unfortunately, and rather surprisingly, Clark-Carter et al. (1986a) failed to find a relationship between PPWS and either stress or arousal measured by SACL (Mackay et al. 1978). Although SACL works in a number of situations, and with numerous populations, it may be that the reason for its failure lies in the fact that it is inappropriate to use SACL with a visually impaired population. Partly, this may be because SACL is designed to be self-administered; with a visually impaired population, it has to be administered orally, with responses being recorded by a third party. However, the results of the research reported above suggest another, more likely reason. The precise words used in mood checklists of this type are crucially important; they must accurately define a semantic space which is assumed to be shared by the members of the population in question. It is argued that for visually impaired travellers, the words used in SACL were inappropriate, even though they are similar to those used in other mood checklists. For example, the second factor in the CSAI-2, the Competitive State Anxiety Inventory (Martens et a/. 1984) contains items related to the somatic components of anxiety often felt by competitive athletes-racing heart, clammy hands and so on. It also includes the words 'nervous', 'jittery* and 'tense', relating to the subjective experience of increased autonomic arousal. These words also appear in the Arousal scale of SACL, probably because Arousal is, in part, the cognitive appraisal of somatic anxiety. Clearly, both SACL and CSA1-2 are measuring much the same thing. Equally clearly, SACL does not tap the feelings that visually impaired travellers experience while on a journey. None of the words which cluster on the Self Efficacy factor, even those which are associated with emotional distress, appear on the Stress scale of SACL. One of the words which appears on the SelfEfficacy factor, 'feeling relaxed' is an item on the Arousal scale of SACL, while 'feeling alert*,which occurs on the Cognitive Arousal factor, also appears on the Arousal scale of SACL. It would therefore seem likely that the failure of ClarkCarter et of. to relate stress and arousal to walking speed may simply have been due to the choice of an inappropriate instrument. The relationship of psychological factors to PPWS thus remains an unanswered question. Fortunately, at the time when the mobility-related feelings data were being collected, PPWS measurements were also made. In this paper, the relationship of the psychological items to PPWS is reported. In addition, the visual status of the clients was available from their records, and thus it was possible to determine the relative contribution of both psychological and visual factors to walking speed, as well as their relationships to each other. 2. Method 2.4. Subjects A total of seventy-one clients were used in the study referred to earlier (Beggs 1990). They were all residents at a vocational rehabilitation course, and were selected for inclusion in the study if the following conditions applied:
(1) They had at least minimal outdoor mobility skills. (2) They spoke English as a native tongue. (3) They had any degree of visual impairment.
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For the purpose of quantification in the present study, those who had total loss of vision, perception of light, or 'hand movements' or 'counts fingers' measurements of acuity were eliminated from the analysis reported below; in addition, as it is not possible to derive a legitimate field status score for totally blind or untestable clients, they were therefore also eliminated. The result was a sample of fifty-five clients with some degree of partial sight. They varied from clients who had acuities as low as 0.5160 to those with acuities as high as 619; and with what were described in their records as mild to severe peripheral and central field losses. Partially sighted people form a very heterogeneous group, often presenting with some combination of loss of acuity and field. For example, a person suffering from macular degeneration may have extremely poor acuity and some central field loss, but an intact peripheral field. His or her ability to travel will be much less impaired than a person with the progressive condition retinitis pigmentosa. Such a person may have virtually normal acuity in reasonable lighting, but suffer from a very severely contracted field of less than five degrees; in addition, he or she may be unable to see in extreme levels of illumination. These conditions lead to particular difficulties; for example, the former may be unable to read road signs, but the latter will be unable to avoid obstacles, particularly other pedestrians. Both, therefore, will be handicapped in different ways. The subjects for this experiment were thus a wide, but not atypical sample of the sort of clients most Mobility Officers will meet.
3. Procedure
3.1 . Mobility-relatedfeelings Each of the clients was asked to make a short journey of about 100 metres in a Table 2. The mobility-related feelings items as they appear on the five-point scales on the questionnaire.
Anxious Observant Unwary Confused Safe Do not stand out Incapable Relaxed Alert Cautious Disorientated At risk Unselfconscious Things under control On your guard Not concentrating Careful Sure of your surroundings Vulnerable Conspicuous Self-confident
Not anxious Not observant Wary
Not confused Not safe Stand out Capable Not relaxed Not alert Not cautious Not disorientated Not at risk Selfconscious Things not under control Not on your guard Concentrating Not careful Not sure of your surroundings Not vulnerable Not conspicuous Not selfconfident
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very quiet suburban street, which had a number of trees, lampposts, driveway openings, and so on to negotiate. At the end of the journey, they were presented with the questiomaire containing the mobility-related feelings words in the form of five-point Likert-type scales. The questionnaire is shown in table 2. Presentation was oral. The clients were first asked to imagine how it would be possible to measure subjective feelings such as temperature on a hot--cold scale, with only five points-very hot or cold, rather hot or cold, or neither hot nor cold. This they were easily able to do, and responded verbatry to the questionnaire in a similar way. Their responses were recorded by the experimenter.
3.2. Walking speed Calculation of PPWS requires the measurement of both actual and preferred walking speed. The first was recorded as subjects negotiated the route unaided. Some totally blind clients took up to three minutes to complete this journey; most of the clients included in this sample completed it in between one minute and one minute and thirty seconds, although a few took up to two minutes and thirty seconds. After the questionnaire was administered, the PPWS technique was first verbally explained to the subject; then experimenter and subject walked back to the start of the route to practise using the PPWS technique. All subjects were able to understand, and respond to the simple instructions. A second pass was then made, in the original direction of travel, to ensure that any initial lack of confidence and uncertainty was overcome; preferred walking speed was recorded on this run. Times to walk the route in this way were about one minute, which is approximately normal sighted walking speed. 3.3. Visual status From in-house records, recent acuity and field loss measurements were available. Acuity measurements were transformed into angular measurements, to enable them to be more easily quantified for analysis; for example, a client with 3/60 vision could be described as having 0.05 vision. The acuity measurements in clients' better eye were used in the subsequent analysis. Field losses were also quantified from the original Friedman Analyser charts. In the past, a number of methods of quantifying field losses have been used, usually relying on measuring losses within hemifields or sectors of the chart (e.g. Newcornbe 1969). Using conventional perimetry, Ross (1983) developed a scoring system that considered the losses within each quadrant of a central, 5 degree area, those in the four quadrants of an intermediate area extending from 5 degrees out to 25 degrees from the centre, and those falling in the quadrants of an outer area, extending from 25 degrees to the periphery. She argued that, functionally, loss of information from these areas may have different consequences; this is almost certainly true for mobility (e.g. Marron and Bailey
1982). Data from the Friedman Analyser does not extend as far into the periphery as that from an arc perimeter, but it is possible to use a similar scoring technique. The Friedman Analyser charts record individual responses to groups of up to three stimuli presented simultaneously at various positions on a 26-5 degree circular array around a central fixation point. On the completed chart, it is possible to see those unsuccessful responses falling within a central 6 degree '
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circle, as well as those falling outside this area. In addition, it is also possible to see those that fail in each of the four quadrants of these areas of the chart. Those stimuli which were not detected at threshold were counted in each of the eight areas of the charts. These eight scores were then combined to give six estimates of field loss in each measured eye for each client. These were upper and lower hemifield scores in central and peripheral areas, and the total scores in each of these areas. Score from the better eye were used inthe analysis. 4. Results First, a multiple linear regression, using only acuity and measures of overall peripheral loss was run, to test the hypothesis that visual factors might be related to PPWS.The multiple correlation coefficient was -0.32, accounting for 10.50% of the variance; this just failed to reach significance (p= 0.056). Almost all of this multiple relationship was due to the correlation between peripheral loss and PPWS (r= -0.31 7); acuity was unrelated to PPWS (r=O-002). Clearly, visual factors alone are poorly related to PPWS; however, it was possible that they might operate indirectly on walking speed via their relationship to the mobility reIated feelings, which themselves might be related to PPWS.Inspection of the correlation matrix which included aLl the psychological factors, acuity measurements, and all the available measures of peripheral function (upper and lower hemifields in the central 6 degree circle, and in the area between 6 and 26.5 degrees, and total scores within the central region and the outer area) showed that the correlations between all the visual and psychological factors were very low, and none were statistically signihcant. They varied between r=O.OO and r-0.40, making it unlikely that visual factors played any significant role in determining walking speed. As a final verification of this, the twenty4ne scores on the questionnaire, the better-eye acuity data, and the six estimates of peripheral function for the better eye were all entered into a stepwise multiple linear regression with the PPWS scores as the criterion variable. This analysis showed that five of the psychological variables together accounted for 63.3% of the variance of PPWS; adding further items failed to make a statistically significant improvement to this ,model. As would be expected from the above results, visud factors entered the regression very late; for example, total peripheral loss was the seventeenth item, acuity the twenty-sixth. The proportion of the variance accounted for by the five items are shown in table 2.
Table 3. The cumulative proportion of the variable of PPWS accounted for by the five statistically significant predicator variables. Mobility-related feeling Disorientated On your guard Confused Things out of control Incapable
Cumulative percentage of variance accounted for 37.03
50.3 1 58-25
60.38 63.30
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5. Discussion Two hypotheses, information processing and coping, were suggested to account for the differences in walking speed of visually impaired pedestrians. The failure to find a clear relationship between visual status and walking speed suggests that the choice of walking speed is a strategic decision, rather than one which is forced upon a partidy sighted pedestrian by the information-processing demands of the task. Four out of five of the mobility-related feelings related to PPWS are drawn from the Self Efficacy factor, suggesting that PPWS is instead related to the distress-lack of confidence and cognitive anxiety-associated with the client's perceived level of competence to tackle the mobility task. There now seems little doubt that confidence and cognitive anxiety are negatively correlated; in sports, for example, the Competitive State Anxiety Inventory (CSA 1-2) (Martens et al. 1984) measures both somatic and cognitive anxiety, as well as a self-confidence factor. These latter seem to 'represent opposite ends of a cognitive evaluation continuum, self-confidence being viewed as the absence of cognitive anxiety, or conversely, cognitive anxiety being the lack of selfconfidence' (p. 17). The other mobility-related feeling, 'being on your guard' may be associated with wony. Clark-Carter et al. (1 986a) were therefore right in their suggestion that PPWS is in part related to these sorts of psychologica1 factors. This research has underlined the importance of using a valid instrument to measure such processes. Rather surprisingly, the data reported here show that visual factors are poorly related, either directly or indirectly via their effect on mobility-related feelings, to the walking speed of visually impaired pedestrians. Earlier Marron and Bailey ( 1982) showed that although mobility performance was unrelated to visual acuity, contrast sensitivity and field loss were joint predictors of success on their task. One reason for the discrepancy between this research and that of Marron and Bailey may be that their performance measure (success in avoiding obstacles) was very different from walking speed, and direct comparison between these studies may not be possible. The Detection of Obstacles index (Dodds and Davis 1989) may be more appropriate in Marron and Bailey's context, and is similarly related to visual status. PPWS, however, remains an important measure of normal mobility performance. On the other hand, it may be that contrast sensitivity is a more appropriate measure of visual status than acuity, especially in the context of mobility, and may be related to PPWS.Contrast sensitivity testing is a more comprehensive method of determining the ability of the eye to resolve detail, which is a function not only of stimulus size (or spatial frequency), but also of figurelground contrast. The familiar Snellen chart, using letters to test acuity, sets contrast at its maximum, and varies stimulus size; in contrast sensitivity testing, both contrast and spatial frequency are varied. The peak of the inverted Ucuwe relating log contrast sensitivity and spatial frequency (the contrast sensitivity function) can be taken as a measure of optimal visual function, although the precise shape of the curve may also be important. This peak bears no simple relationship to acuity measurements. It may therefore be that had the results of contrast sensitivity tests been available, they might have been related to either feelings or PPWS.This possibility needs to be tested. It is important to note that 36.7% of the variance of PPWS in this group remains to be accounted for. While visual factors appear to be only weakly '
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related to wallung speed, it may be possible that the physiological stress, or somatic anxiety associated with travel is implicated in walking speed differences, as Clark-Carter ef al. (1986a) suggested. However, psychological and physiological responses to stress almost always covary (e.g. Deffenbacher 1977), and it is therefore dficult to see how much additional variance might therefore be accounted for by physiological arousal. Of course, other physiological differences between clients, such as physical fitness, leg length and so on may be involved. It seems more likely that some additional psychological factors not measured in this experiment may be important. Since walking speed seems to be a strategic choice, in part determined by the client's need to maintain anxiety at an acceptable level, it is reasonable to assume that other factors may influence this choice. Some clients are extremely restrained in their mobility performance, others can seem positively reckless. These differences in impulsivity were not measured in this experiment, but may be important enough to account for some of the remaining variance of PPWS. Alternatively, some of the other factors identified by Welsh (1 980) may be involved; the more likely contenders for this role are dependency and the self-concept. Finally, it remains possible that other feelings, not described by Beggs (1 990),are implicated. These might be related to a more general response to loss of sight; depression, for example, is common in visually impaired people. Other possibilities include a sense of helplessness (Seligman 197 3 , an external locus of control (Rotter 1966), or low self-esteem (Coopersmith 1967). These speculations remain untested at present. , The most immediate role for PPWS will be to enable research to investigate the impact of different approaches to training. Training style differences (e.g. Warren el at. f 982) may particularly affect the relatively intangible psychological factors which underlie this index of mobility performance. Pilot work on style differences in mobility training (Beggs 1987) has suggested that a style focusing on responsibility sharing and co-operative gaolsetting has its effects at this level; PPWS offers a convenient way to measure' these effects behaviourally. The PPWS measure may also be important for the evaluation of secondary travel aids which add little additional usable sensory information to that provided by the primary aid, the long cane. Kay (1 980) suggested that the benefit of an ultrasonic aid was apparent largely in terms of improved confidence; earlier behavioural measures of mobility performance fail to pick this up. It is easy to dismiss such claims as untestable, simply because these psychological differences are unobservable behaviourally. It is now possible to test Kay's claim in a satisfactory way, by using PPWS;indeed, in showing that the range of the Sonic Pathfinder affects PPWS,Clark-Carter et al. (1 986b) may have already partially confirmed Kay's belief that effective secondary aids have their effect at a psychological level. This work has clarified the nature of PPWS as an evaluation tool. PPWS seems to be a mobility index which is related to the way the client responds to the stress of travel, unlike the Preview and Detection of Obstacles-assessment indices (Dodds and Davis 1989). These latter indices were developed specifically to evaluate the extent to which partially sighted people use their residual vision during travel, and are thus useful, but indirect measures of their mobility performance. Taken together, however, these new indices enable a much broader view to be taken of mobility performance, and relate it to both visual and
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psychological factors. Unlike the visual performance indices, PPWS measures are appropriate for totally blind clients, too; values of PPWS vary from 100%for the best partially sighted travellers to around 40% for inexperienced totally blind travellers. Although they were not included in this experiment, it would not be surprising to find that these very slow walking speeds reflect the more extreme reactions to sight loss commonly encountered in totally blind clients. The PPWS measure also complements existing be havioural measures of mobility performance (Armstrong 197 9, and extends the Nottingham evaluative methodology to take account of behavioural responses to the stress of travel. It is an important new tool which, together with the existing techniques, will make it possible to tease out the effects of different kinds of interventions which have their effects at various levels. . Acknowledgement The Blind Mobility Research Unit is funded by the Department of HeaIth and Social Security, and is directed by Professor C. I. Howarth. The author acknowledges the contribution of the members of the Unit to this work, and the generous co-operation of staff and residents at the RNIB Employment Rehabilitation Centre, Torquay.
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Revised manuscript received 27 April 1990. Manuscript accepted 9 May 1990.
