Durations of stimuli displayed on video display terminals:: (n-1)/f plus persistence

Video display terminals (VDTs) have become the most popular apparatus for displaying stimuli in psychophysical experiments. Driven by modern desktop computers, they afford convenience and flexibility. There are dangers and limitations inherent in the technology, however. The problems include limited resolution, brightness, and contrast, as well as fixed wavelength contributions to color balance. In addition, VDT users may be susceptible to such symptoms as accommodative error (Jaschinski-Kruza, 1991; Schleifer, Sauter, Smith, & Knutson, 1990) and horizontal heterophoria (Dain, McCarthy, gr Chan-ling, 1988). Kennedy and Murray (1991), using a lexical decision task, found that flicker resulted in saccadic irregularities. At low refresh rates, subjects misjudge the extent of target displacements that take place during saccades,in the direction of a breakdown in space constancy (Macknik, Fisher, & Bridgeman, 1991). A serious and largely unrecognized danger is misinterpretation and limited control over stimulus duration at the millisecond time scale. All VDTs are illuminated by a spot moving rapidly across the screen in a raster pattern, covering the surface 60 to 120 times per second. Visual persistence makes screen illumination appear continuous, because the raster rates exceed the critical flicker fusion rate. Scanning begins with a single horizontal row of pixels illuminated successively across the screen. Just after the end of the scan, a horizontal synchronization pulse triggers another scan, slightly lower on the screen. At the end of the final scan, a Vertical synch pulse restarts the raster at the beginning. Early VDTs were strobed at 60 Hz, but as large screens with bright backgrounds were introduced, flicker in peripheral vision became noticeable. For this reason, faster strobe rates were introduced-66 Hz in Sony Trinitron tubes, then in several tubes at refresh rates of 75 Hz, 100 Hz, and 120 Hz. All of these rates are in use today. The motivation for faster rates was solely to avoid visible flicker; the industrial standard for VDTs states only that flicker should not be Visible for 90% of users.