Author Response: Grating Visual Acuity in Infantile Nystagmus in the Absence of Image Motion

Matt J. Dunn, Tom H. Margrain, J. Margaret Woodhouse, Fergal A. Ennis, Christopher M. Harris, Jonathan T. Erichsen
2014 Investigative Ophthalmology and Visual Science  
We would like to thank Dell'Osso 1 for his critique of our work, as well as for highlighting the issue of visual acuity (VA) testing in the presence of infantile nystagmus (IN). It has long been assumed that VA could be improved by reducing the intensity of the nystagmus (i.e., the average velocity of the eye movements), and as cited by Dell'Osso, there are many such claims in the literature (discussed below). Over the last few decades, this intuitively appealing view has become entrenched as
more » ... ome entrenched as the theoretical basis for numerous therapeutic interventions. Indeed, Dell'Osso's critique of our study begins and ends by appealing to this notion, but we would remind him that intuition is no substitute for scientific rigor. STUDY DESIGN In our study, 2 we have demonstrated that there exists a fundamental underlying limitation in the VA of adults with IN, even in the absence of retinal image motion. By presenting grating stimuli using very brief flashes of light that were less than 1 ms in duration, we were able to virtually eliminate any motion blur induced by the eye movements themselves, thus unmasking the underlying VA. Subjects with IN and controls were tested under both constant and brief (tachistoscopic) lighting conditions. The brightness of the flash was adjusted so that control subjects showed no change in VA between the two conditions. However, contrary to the assumption that the eye movements of IN degrade VA, there was also no significant improvement whatsoever in subjects with IN when the effect of their eye movements (i.e., motion blur) was eliminated. Clearly, Dell'Osso has missed the point of our experiment. He erroneously states that we used a flash with a duration of 75 ms, whereas the duration was, in fact, only 0.76 ms. Such a brief presentation ensured that there was virtually no retinal smear caused by the nystagmus. His error is further compounded when he suggests that longer (i.e., 100 ms) presentations during foveating periods of the waveform might have worked better; a duration that long would produce substantial retinal smear and completely defeat the purpose of our experiment. Dell'Osso also expresses concern that the waveform characteristics of our participants might have affected the outcome. However, the fundamental premise of the experiment was to circumvent the eye movements altogether by eliminating image motion. Thus, our paradigm nullified the possible effect of waveform variations in order to provide a measure of the subjects' underlying spatial acuity threshold. For this reason, even if we were inclined to compute eXpanded Nystagmus Acuity Function (NAFX; an objective algorithm to characterize nystagmus waveforms), 3 it would have no meaning in this context. Moreover, the number of presentations available to each participant was unrestricted in order to overcome the possibility (as yet untested, despite Dell'Osso's claims) that detailed visual information cannot be gathered during nonfoveating portions of the waveform. The gratings used in our study were sufficiently large, so that the fovea was always pointing at the stimulus whenever the flash might have occurred. Dell'Osso also argues that poorer VA for vertical rather than horizontal gratings supports the notion that VA is limited by the horizontal nystagmus, and clearly dismisses the possibility of meridional amblyopia, as originally suggested
doi:10.1167/iovs.14-15070 pmid:25108007 fatcat:ii5ecmkkojhx3hcccleiotxiku