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Six separate participants volunteered for Experiment 1 (3 females) and Experiment 2 (4 females). All participants had normal vision. Written informed consent was obtained before the experiment. The experiments have been approved and permitted by the Ethics Commission, Institute for Psychology and Education, Ludwig-Maximilians-Universität München, Germany.
Experiments took place in a dark room. The participant sat in front of the monitor with viewing distance of 52 cm. The dominant eye was aligned with the center of the screen and monitored by a head supported eye Tracker (EyeLink 1000). After the calibration of the eye tracking system, the participant rested for about 20 minutes for dark adaptation. In Experiment 1, a neutral density plastic filter (LEE filter, reducing light 4 stops) was attached on the surface of the screen to reduce the luminance. The experiment consists of two sessions: the motion detection and the moving object localization (Figure 1).
Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0033651.g001 Schematic illustration of the stimuli used in the experiments. (a) Motion detection task. A dot appears on the left or the right side of the fixation point (FP) at a given eccentricity and revolves around the fixation for 100 ms. (b) Movement localization task. A dot moves from the left or the right towards the fixation point (motion-terminated condition) or moves away from fixation to the left or the right side (motion-initiated condition). (c). Spectral charts for the blue filter (blue curve) and the green filter (green curve). The scotopic luminance for the blue and the green was approximately 3.93 cd/m2 (used for the fixation and reference bars) calculated with the scotopic luminosity function (CIE 1951).
In the motion detection session, a trial started with a small dim fixation point (diameter: 0.17°; luminance: 0.28 cd/m2) and a warning tone (100 ms, 1000 Hz, 63 dB). The positions of the eye were monitored online. After random interval (300–500 ms), provided the position of eye had not deviated from the fixation point by more than 0.5 deg, a small dim dot (diameter: 0.17°; luminance: 0.028 cd/m2) appeared on the left or the right at a given eccentricity (7 levels, from 0.5° to 2.3° with steps of 0.3°) and revolved around the fixation at 5.0 radians/sec for 100 msec. The participant then had to indicate if he/she saw the rotating dot. Each eccentricity condition was repeated 24 times and counter-balanced on the left and the right sides and the direction of the motion. In addition, 14 catch trials (i.e. with no moving dot) were randomly mixed with the other trials. In the moving object localization session, a trial started with the presentation of the fixation point (diameter: 0.17°; luminance: 0.28 cd/m2) and the two vertical collinear reference lines (subtending: 0.08°×0.41°; luminance: 0.28 cd/m2) for 300–500 ms. The vertical positions of the reference lines were 1.2° above and below the fixation point. The horizontal position of the reference lines was varied from trial to trial (see further details below). When the eye was fixated on the fixation point (online, measured by eye tracker, and the deviance was less than 0.5°), another dim dot (diameter: 0.17°; luminance: 0.028 cd/m2) appeared. On half the trials, the dot started to move (at 5°/sec) from a position 8° to the left or the right of fixation towards the fixation point and vanished at the center (the motion-terminated condition, see Figure 1b). The participant had to indicate if the moving dot vanished to the right or to the left of the reference lines, which were positioned randomly between 0° and 1.8° with a step size of 0.3° away from the fixation point (on the same side as the movement). On the other half of the trials, the dim dot started to move from the center to the left or the right and vanished at the 8° position (the motion-initiated condition). The task was to indicate if the moving dot's first perceived position was to the left or the right of the reference lines. In this case the horizontal position of the reference lines was randomly chosen from 0.5° to 2.3° with steps of 0.3°. The range of the reference positions was chosen based on a pilot experiment. The motion-terminated condition and the motion-initiated condition were run in separated blocks, each with 28 trials. The order of the blocks was randomized. Each condition contained 7 levels of reference positions, which were randomly repeated 20 times and the left/right visual field presentations were counterbalanced. In Experiment 2, the stimuli and procedure were the same as in Experiment 1, excepting the following differences: A blue (Tokyo blue LEE filter, dominant wave length: 422 nm) or a green (Primary green LEE filter, dominant wave length: 501 nm) plastic filter was attached on the surface of the monitor in separated sessions. We used the cyan color for all stimuli on the screen to reduce the red spectrum. The spectral characteristics of the two filters for a cyan color on the screen are illustrated in Figure 1c. The intensities of the stimuli were adjusted for the two filters separately such that both scotopic luminances were approximately equal. The fixation point and reference lines were set to 3.92 cd/m2 (radiance shown in Figure 1c) and the moving/revolving dots were set to 0.95 cd/m2 (Measured by JETI spectrometer and calculated with the scotopic luminosity function, CIE, 1951). The motion detection task was run only in the blue filter condition since the detection of the green revolving dot was far above the threshold with the given luminance.
Participants' responses were first converted to proportions of visibility for the detection task, the motion-initiated and motion-terminated localization tasks. Psychometric curves were then fitted using a logistic function to each condition and points of subjective equality (PSEs) were estimated from the 50% point of corresponding psychometric function.
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