The stimulus display is illustrated in Fig. 1. Four potential targets (consisting of figure 8 symbols) were displayed throughout each trial. Participants controlled the start of each trial by

pushing a button when they were ready with their gaze upon the central fixation point. After a variable fixation interval ABT-888 research buy (1000–1400 ms) the central fixation point turned into an arrow to indicate which of the four figure 8s would be the saccade target. At 25, 75, 150 or 250 ms after the onset of the arrow (the SOA), the figure 8 at the target location could change briefly (for 100 ms) into either E or 3, while the figure 8s at non-target locations could change into 5 or 2. Four trial types were used to allow the separate assessment of the effects of symbol-changes at target and at peripheral selleck screening library non-target locations: (1) ‘No-change’ trials, where all four figure 8s remained unchanged throughout the trial; (2) ‘Target’ trials, where only the figure 8 at the target location changed into E or 3, while the three figure 8s at the non-target locations remained unchanged; (3) ‘Distractor’ trials, where only the three figure 8s at the non-target locations changed into 5 and 2 and the figure 8 at the target location remained unchanged; and (4) ‘Target/Distractor’ trials where all four figure 8s changed at the SOA: at the target location into E or 3 and at the

non-target locations into 5 and 2. The task was presented in blocks of 52 trials and each block was presented in a different randomized order. Interspersed in each block were four No-change trials. The remaining 48 trials consisted of four trials of each trial type (Target, Target/Distractor or Distractor trials), at each of the four SOAs. On half of the trials in which Florfenicol the target symbols changed into discrimination symbols (i.e. Target and Target/Distractor trials), the figure 8 turned into E, on the other half into 3. Eye movements were recorded monocularly

using a video-based iView X Hi-Speed system (SMI, Berlin, Germany) at a sampling rate of 1250 Hz. This system uses a combination of corneal reflection and pupil tracking with a typical spatial accuracy of 0.25–0.5° and a tracking resolution of < 0.01°. Stimuli were displayed on a 21-inch CRT screen with a 100-Hz refresh rate on a display area of 400 × 300 mm, at a resolution of 800 × 600 pixels. The computer screen was positioned 600 mm in front of participants, who sat with head supported by the chin and forehead rest of the iView tracking column. As PD patients may have lower contrast sensitivity and a smaller ‘useful field of view’ than controls (Uc et al., 2005), high-contrast stimuli and small target amplitude were used to minimize any potential differences in perceptual ability between the groups.