In our paper,
2 we pointed out that a correlation between VA and various aspects of the nystagmus waveform (e.g., foveation duration, intensity, etc.) appears to be based on intersubject comparisons. In rebuttal, Dell'Osso
1 cites an impressive list of 12 papers in support of intrasubject improvement in VA,
5–16 but in our view, his interpretation exaggerates any such support. Close inspection reveals that only one of these papers actually provides statistical evidence of such a change,
15 as measured using standard letter charts. Of the other studies, six contained three or fewer subjects.
6,8,9,11,13,16 In fact, four of these studies only used one subject: Dell'Osso himself.
6,9,11,16 Two of the papers cited were reviews, and thus contained no new data,
10,12 while the remaining three found statistically significant changes in nystagmus waveform characteristics, but failed to detect significant changes in VA.
5,7,14 In three of these studies,
6,9,11 VA was not even measured, but instead NAFX was used as an outcome measure, from which VA was predicted. NAFX is a computed number based on waveform shape and does not include any perceptual component. Thus, these studies only confirm a change in waveform, and any claim that this reflects improvement in VA is completely circular.
Despite the lack of clear evidence in the above studies, we are aware of a handful studies that have found a statistically significant change in VA in response to waveform modifications. These include the work of Hertle et al.,
17 who showed improvements in VA following head posture surgery, and McLean et al.,
18 who treated patients with memantine and gabapentin. We did not, and would not, suggest that VA cannot be improved at all in every subject with IN. On the basis of our results
2, 19,20 (and those of others
21,22 ), our conclusion was, and remains, that treatments that seek to slow the eye movements of adults with IN are likely to be fundamentally limited with regards to the improvements that can be expected in VA.