At the beginning of the experiment, subjects were asked to fixate a ‘+’ sign centrally presented on the screen. By pressing an arrow key, subjects indicated the perceived direction of the coherently moving dots. Two training blocks of 32 trials with slow and moderately fast moving dots (0.8 and 12.5 deg/s) were performed to accustom subjects to the experiment. For the experiment, subjects were divided into two groups. Nine subjects (5 females, aged 26–35 years, mean 28.8 ± 2.8 years) did the vertical task and had to differentiate whether the dots moved coherently upward or downward.The experiment was performed in two different positions, i.e. sitting in an upright position and lying sideways on the left side (Fig. 1A–B).
The second group of nine subjects (6 females, aged 18–48 years, mean 25.00 ± 9.01 years) did the horizontal task and had to differentiate whether the dots moved coherently to the right or left. The two different positions were the same as in the vertical task (Fig. 1C–D).
For each speed, the motion coherence threshold was established by using a single-interval, two-option, forced-choice procedure.Patients had to indicate the direction of the coherent motion.The signal-to-noise ratio was one to one at the beginning. A correct answer led to a decrease of the signal-to-noise ratio by 1 dB, whereas an incorrect answer increased the signal-to-noise ratio by 3 dB. For each speed, 128 trials were conducted. Subsequently,the proportion of correct responses was calculated. The level at which subjects are expected to judge 75% correctly is defined as the coherence level. To find the coherence level for each speed and direction, a probit analysis was conducted.