Late responses as aids to diagnosis in peripheral neuropathy.

+1-175at > 15-. These figures are not comparable to average variability ranges of +/-26-566" at <30`j found in numerous studies that have specifically endeavoured to perform the very best accuracy and reproducibility with automated and manual perimetry (static or kinetic) using skilled examiners and normal subjects.7'" Even under these optimal conditions variability increases markedly past 30-and is in addition up to 50% worse in selected and experienced patients with various ocular pathologies.8 II Non-selected patients will obviously have even greater variability. Our greatest concern is with the potential artifact of compensatory eccentric fixation as caused by changes in head position during perimeter testing, thus incorrectly indicating field increases in particular areas, that is, the visually impaired hemifield in hemianopic patients. Patients are sufficiently intelligent to realise that they have been trying for days to enlarge their impaired hemifield area and that they are not trying to increase their normal visual field. This selective attention combined with relatively high interstimulus interval and unstablished head movements for any given trial are potentially independent and can easily cause the patient to unknowingly make different compensations that, as pointed out in our paper, may be undetected by the examiner. Furthermore, such an artiface would not be indicated by an abnormal location of the blind spot as mapped by the perimeter because during those test trials the patient would have no need to make such compensations, that is, he would have no reason to extend his blind spot. Similarly, it does not make sense that any of the above compensatory mechanisms would be used by either the patient or the therapist to reduce the patient's visual field. Compensatory mechanisms by the very definition, selectively favour alternative physiological mechanisms that are associated with desired change. Therefore we must still maintain that a bite-plate, as it is used in almost all of modern visual psychophysics, coupled with an eye monitor system, would eliminate the potential confounded variable of uncontrolled head movements and eccentric fixation. Zihl and von Cramon believe that their exact training methods must be replicated and that such "an investigation depends critically upon the degree of correspondence with the [their] original study". This statement, however, is problematical. We would agree with Zihl and von Cramon's comments that our failure to find significant visual field increases with training could be explained in terms of differences in patient groups, if they had used methods that were not potentially contaminated by artifacts. Until this is done, no conclusions about correlative brain mechanisms can be made. This argument reminds us of the story about the horse named Hans that was reported by its owner to be able to count. As the story goes, people from many miles around came to watch "Clever Hans" correctly stomp his hoof on the ground in response to any verbally presented number.This was startling because other horses had previously not be able to count it seemed clear that this was a very special horse. The implication was that with proper training, perhaps, other horses might also be trained to count. Fortunately, someone eventually found that Hans' owner had unknowingly confounded results by slightly nodding his head just at the predicted moment that the animal was to stop stomping the ground. The fact of the matter was that Hans could not do the trick unless he