The cursor, which had a circular shape with a diameter of 10 mm, was filled with the noise. The noise was randomly generated before each trial using the same procedure as the background noise. A gray ring frame 1.2 mm thick and with an intensity of 255/2 was added to the cursor. The drifting noise within the cursor was implemented in the following ways. First, a noise whose vertical size was larger than the cursor was generated, although the entire area of the noise was not presented as stimuli. Second, a part of this larger area of noise was cropped and pasted within the cursor, such that the vertical position of the cropped noise
ycut was determined depending on
x, the position of the cursor in the horizontal axis of the stimuli (
Figure 2). Specifically, the cropped area was determined by the following formula:
\begin{equation}
y_{cut} = A_{noise}\,exp\left(-\frac{(c_{x}-x)^{2}}{2(W/8)^{2}}\right),\quad
\end{equation}
where
Anoise is the number of pixels equivalent to the amplitude of noise drift [mm],
cx is the horizontal center of the screen (i.e., the horizontal center of the background noise), and
W is a pixel size equivalent to 35 mm (i.e., the size of the background noise). Note that the position of the cropped noise was changed only vertically and in coordinates relative to the cursor position, not in world coordinates (i.e., when
Anoise was zero, the noise was glued to the cursor). The maximum amplitude of noise drift
Anoise for each participant was chosen from one of the following nine levels: −7.00, −5.25, −3.50, −1.75, 0.00, 1.75, 3.50, 5.25, and 7.00 mm. When the amplitude of noise drift
Anoise was positive, the lower (i.e., positive side of the vertical axis) of the larger area of noise was cropped out; in this case the noise drift within the cursor was perceived to drift upward (i.e., toward the negative side of the vertical axis). To explain how the cropping changed the stimulus appearance, some snapshots of stimuli for the −7.0 mm and 7.00 mm drift conditions are shown in
Figure 2.