Thirteen subjects with horizontal nystagmus volunteered for the study. First, the diagnosis of IN as reported by the subject or by their ophthalmologist was investigated by an optometrist using high-speed eye movement recording, ophthalmoscopy, slit-lamp examination, optical coherence tomography (OCT), and a detailed family history. Note that “idiopathic” IN is a diagnosis by exclusion and, as such, is reliant on the gamut of clinical tests performed. It remains possible that some patients labeled as idiopathic (including those in this study) may have undetected or subclinical forms of visual pathology. Two subjects with active noncongenital ocular pathology or noninfantile nystagmus were excluded: one on the basis of eye movement recordings (fusion maldevelopment nystagmus syndrome), and another for having Fuchs' endothelial dystrophy. Eleven subjects with IN remained to participate in the study (three female; 22–69 years [mean age, 48 years]). One of these (RC) showed clinical signs of albinism (iris transillumination, fundus hypopigmentation, and foveal hypoplasia), while another reported an undiagnosed childhood macular defect, appearing similar to hard drusen on fundoscopy, but not visible on OCT. A third subject (DT) had previously been diagnosed with achromatopsia by the ophthalmologist. Eleven normally sighted individuals with no history of ocular or neurologic disease were recruited via e-mail and word of mouth, from a deliberately similar age range (two female; 21–72 years [mean age, 53 years]). All control participants without an up-to-date sight test underwent a full eye examination on the day of the study. The investigation was carried out in accordance with the Declaration of Helsinki; informed consent was obtained from the subjects after explanation of the nature and possible consequences of the study. Ethical approval was granted by the Cardiff School of Optometry and Vision Sciences Research Ethics Audit Committee.
Binocular VA was measured by using a self-illuminated logMAR chart with distance spectacle correction at 3 m in a well-lit room. Subjects were given as long as they wished to view the chart, and encouraged to continue reading until at least four letters on a line were incorrectly identified.
Subjects were seated in a room whose walls had a mean luminance of approximately 60 cd/m
2, 2 m from a Sony GDM-F520 21-inch CRT monitor (Sony Electronics, Inc., San Diego, CA, USA). Maximum and minimum monitor luminance were measured as 100.90 cd/m
2 and 0.00 cd/m
2, respectively. Eye movements were recorded at 1000 Hz with an EyeLink 1000 (SR Research, Ottawa, ON, Canada). The chin and head were supported by a rest, and subjects were encouraged to adopt a comfortable position to view the screen, turning their head to use their null zone (angle of gaze at which nystagmus intensity is minimized
12) if preferred. Habitual spectacle correction (if any) was worn, and subjects viewed the screen binocularly. A SideWinder game pad (Microsoft, Redmond, WA, USA) was used to detect subject responses.
The experiment consisted of two phases: centrally presented gratings and peripherally presented gratings. The first task was intended to capture the time required to respond to visual information presented at the locus of fixation, whereas the latter task required a saccadic eye movement; thus, time to first fixation could be separated from time to discrimination.