A novel method for measuring saccade velocity has been developed in our laboratory. When a moving flicker radiant is displayed in front of the eye, a row of radiants is "painted" onto the eye. Thus, the eye simultaneously perceives several radiants. This phenomenon is known as visual persistence, and occurs because of the limited time resolution of the eye. In the same way, when a tiny flicker radiant is displayed at a fixed location during a saccade (in a dark room), a row of radiants is perceived. As the spatial interval between the perceived neighbouring radiants is relative to the speed of the eye, the saccade velocity can be calculated from this interval (Ando, 1994, Neuro-Ophthalmology Japan, 11: 318-325).

The vacuum fluorescent display (VFD) is widely used in Japan in digital clocks and graphic equalisers as a display device that is able to emit a bright green light that flickers at high frequency. In this study, a radiant was used that incorporated a VFD (32 x 32 dots matrix with 0.3 mm dot pitch) capable of emitting flickering signals, on or off simultaneously. The flicker frequency could be adjusted from 100 Hz to 990 Hz and the light duration was fixed at 0.1 ms. The configuration pattern of the radiant was either an open circle or a solid rectangle (9.6 mm width x 9.6 mm height), which was produced on a Windows PC then loaded onto the VFD through the serial port. The VFD and fixation targets (which consisted of red LED's) were located on a arch-shaped arm (300 mm radius) attached to a rotator (400 mm radius), to measure every possible saccade direction.

Guided by visual and audible cues (LED's and beep sounds), subjects looked alternately at two fixation targets (0.5 Hz) using only one eye. The flicker pattern was then displayed on the saccade trajectory. To the subject, the image appeared as either a row of circles or rectangles. When the frequency of the radiant is low, a gap between the circles or rectangles should be perceived. At high frequency, no gap should be perceived. Measurements were taken using gradual increments from low to high frequency and the subject was asked to report the presence or absence of gaps. The saccade velocity was calculated using the radiant frequency that closed the gap between the perceived radiants (where the subject responded "no gap" for the first time).

This method for measuring saccade velocity has the following advantages:

1. the apparatus is simple and inexpensive;
2. any direction of saccade can be measured with the same accuracy;
3. results can be obtained without instrument calibration;
4. the methodology is non-invasive and safe.

The advantages for using the matrix VFD, over a row of optical fibres as previously reported, is that the radiants are more distinguishable since it emits a certain 2D pattern, and that it is easier to be equipped.