All stimulus files generated movement along a single centrally located horizontal axis. There were two basic stimulus patterns: periodic single-frequency sinusoids and a nonperiodic sum of sinusoids. The periodic sinusoids used were 0.2, 0.4, 0.6, and 0.8 Hz. The nonperiodic sum of sinusoids contained the nonharmonically related frequencies of 0.12, 0.35, 0.65, 0.8, and 1.0 Hz. Stimulus file characteristics are shown in
Table 1 .
The stimulus files were designed to drive the target (a 0.28° × 0.28° plus sign [+]) through approximately 10° of visual angle. Peak velocities were computed by applying a first central difference algorithm to the stimulus position files:
\[(vel_{i}{=}\ \frac{(pos_{i{+}1}{-}pos_{i{-}1})}{2{\Delta}t})\]
These increase as a function of frequency in the periodic files. The peak velocity of the nonperiodic sum of sinusoids file fell between those of the 0.6- the 0.8-Hz periodic files. All peak velocities were kept below 30°/s to promote smooth eye tracking.
22 For each stimulus frequency, there were two stimulus files, one starting to the left and one starting to the right. The directional starts were presented randomly so that the subject could not predict initial target direction. The order of the stimulus frequency presentation was also randomized. The target starting and stopping location was always at 0° of visual angle. Eye position was monitored to ensure a 0° starting position at the beginning of each data collection. For the scotoma trials, this resulted in immediate target blanking. The target would reappear only when outside the scotoma boundaries as a result of target movement or subject eye movement. Each stimulus file was 32 seconds long. During the first 30 seconds, there was a moving target. For the last 2 seconds, there was a stationary target positioned at 0°. A 15-second section of each stimulus file is presented in
Figure 1 .
Calibration files were presented to the subjects before each stimulus file to compute the gain and offset of the limbus (feedback) system. During this process, subjects were asked to fixate a target presented successively in five discrete display locations (0°, ±5°, and ±10°). A first-order linear regression curve fit through the collected data provided gain and offset calibration values for that specific trial. In the artificial scotoma trials, each calibration was used to determine the preselected boundaries of the scotoma in the subsequent stimulus file. The subjects were instructed on the importance of remaining still, especially between the calibration- and stimulus-file target presentations. Calibration and stimulus files were presented alternately throughout the experiment.
Data were collected in four 1-hour sessions. Each session contained 20 stimulus files, separated into four 5-file trials. Each trial used a single simulated scotoma size. The scotomas had horizontal half-widths of 0°, 1°, 2°, and 3°. Each subject completed the four sessions within a period of 2 weeks. Subjects were allowed to practice on the no-scotoma files only before data collection.
All subjects were told that a horizontally moving visual target would be presented on the display and that they were to “track that target as accurately as possible.” Before the simulated scotoma trials, subjects were educated on the effects of scotoma. They were told what to expect visually, but were not instructed on compensatory tracking strategies. They were not encouraged to develop peripheral retinal loci, nor were they told to show a visual field preference. Preceding the simulated scotoma sessions, subjects were given an additional instruction to “keep the target on as much as possible.” The second instruction was included to avoid any interpretational biases with respect to the first instruction. Pilot experiments showed that without this second instruction, two interpretations existed. Using the first interpretation, the subject would continue to refoveate the target, causing it to disappear. Accurate tracking would cease until the target reappeared. Using the second interpretation, the subject would eccentrically view the target without active tracking. The combination of the two instructions was found to provide the most clarity.
During the simulated scotoma sessions, the subjects were asked to constrain their vertical tracking to the display region containing the stimulus. The vertical location of the stimulus was reinforced before each stimulus pattern with the nonblanking calibration pattern. The vertical-tracking limitations were imposed in an effort to provide cleaner horizontal eye position data from the limbus system. This became important when the target blanked during the simulated scotoma condition. The ISCAN system was used to monitor vertical eye movements for postcollection analysis.