LETTERS TO THE EDITOR - Measurement of Kinaesthetic Sensitivity

SIR-Bairstow and Laszlo (DMCN, 28, 188-197), in commenting on our objections to their method of measuring kinaesthetic acuity, have failed to understand both the nature of these objections and the nature of the method which we proposed as an alternative. To take the latter point first, Bairstow and Laszlo apparently think that staircase estimation techniques (PEST) constitute a training procedure, and as such are unsuitable for testing acuity. In fact they are wrong; this is not a training procedure, because the subject receives no information about the success or failure of his response on any trial. In this regard the method does not differ from Bairstow and Laszlo’s own procedure.

completely ignore an essential point in our objections, namely that the method of constant stimuli requires a suitable range of stimulus values to bracket the threshold. It is not permissible to use a single set of stimulus values (ramp settings) for all subjects regardless of their acuity, nor can extrapolation upwards from subthreshold ramp settings-to which responding will be at or near chance level-be justified. The

Bairstow and Laszlo almost

essential point of the method is that it is only properly used when it gives a threshold value derived from settings appropriate to the individual subject, which the PEST method also does but with much less fuss and bother. Bairstow and Laszlo do not deal with this point; rather, they argue that %linical necessity’ obliged them to modify the procedure as they did. To this we can only reply that i f the modified procedure gives grossly inaccurate results then it is no compensation to argue that it is clinically convenient.

Bairstow and Laszlo say that the objections to their use of the method of constant stimuli can be avoided by taking the number of correct responses (to the predetermined range of ramp settings) as the measure of kinaesthetic acuity. This scoring system is an improvement in that it does not generate spurious threshold values by inappropriate extrapolation, but it represents a very coarse method of estimating acuity compared to staircase procedures. Thus i f subjects were guessing, 19 in every 20 (on average) would obtain a score between I1 and 23. It is not reassuring therefore to discover from the table of normative

A futher point needs to be made.

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0 * * data presented in Laszlo and Bairstow’(p. 119, Table 6.5) that the mean score for five-year-olds is 20 correct out of a possible 32, and that 23 correct represents the 85th percentile. It suggests that virtually all the subjects in this normative sample (the size of which is not given) were guessing. Evidently this scoring method is not suitable for children as young as five, and it is not much better for older children; even by 12 the mean score has risen only to 25/32. This difficulty stems from the same objection as that raised to the original procedure, that i f the ramp separations do not bracket the threshold, and the subject has to guess, one cannot extract from such data any meaningful estimate of acuity.

JOHN ELLIOTT KEVIN CONNOLLY

TONY DOYLE

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Department of Psychology, University of Sheffield, Sheffield SIO 2TN.

Reference 1 . Laszlo, J . I . , Bairstow, P. J . (1985) Perceptual-

Motor Behaviour. London: Holt, Rinehart & Winston.

Phillip Bairstow and Judith Laszlo have replied as follows:

SIR-Different methods can be used to estimate perceptual functioning. Doyle, Elliott and Connolly (DMCN, 28, 188-193) seem keen on staircase estimation techniques (PEST) for testing acuity. Nothing prevents them from using such a technique with our task, and we wish them success i f they should attempt to establish their own developmental norms. Their norms would be different from ours for reasons discussed previously (DMCN, 28, 194-197). We would like to stress, however, that correlating the two measures of sensitivity and then discussing which measure is better is an unskilful exercise; a low correlation, f o r example, does not shed light on the relative worth of the methods.

When using a PEST type design, subjects have in formation about the success of a response even when not

Three points are taken up here. ( I )

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in formed verbally. When the stimulus values ore sufficiently large, subjects know for certain that their response is correct; as the values are reduced, they become uncertain eventually. An ordered reduction in the difficulty of the task helps subjects to focus attention on features of the sensory input which can be used for making finer perceptual discriminations. A PEST procedure gives an estimate of acuity after such training. (2) There is nothing ‘grossly inaccurate’ about recording the number of correct responses on a set range of stimulus values, and there is no support f o r their claim. (3) The existence of subjects whose performance on the set values was no better than chance was reported clearly (DMCN, 23, 606-616, Table I), so Elliott and colleagues reveal nothing new.

PHILLIP J . BAIRSTOW JUDITH I. LASZLO

X-linked Mental Retardation and Infantile Spasms in Two Brothers

SIR-I wish to report mental retardation inherited in a sex-linked mode in three generations of a family, including two affected brothers with infantile spasms (Fig. 1) .

The affected members of the family are ail described as moderately or more severely mentally retarded. No common stigmata, CNS anomalies or other physical anomalies were found. Chromosomal analysis of both affected and unaffected family members showed no fragile X or other anomaly. Convulsions were reported during the first year of life in case 111-8, and generalized tonic-clonic convulsions occurred from I1 years of age in case 111-5. In the latter case encephalography showed dilated left cella media and cornu anterior. The birth of case 111-8 were described as protracted, but the births of the other affected members of the family were described as uncomplicated.

IV-3, both had infantile spasms. Case IV-5 was born normally in 1983 after

The affected brothers, cases IV-5 and