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

Video display terminals (VDTs) have become the most popular apparatus for d isplaying stimuli in psychophysical experiments. Driven by modern desktop c omputers, they afford convenience and flexibility. There are dangers and li mitations inherent in the technology, however. The problems include limited resolution, brightness, and contrast, as well as fixed wavelength contribu tions to color balance. In addition, VDT users may be susceptible to such s ymptoms as accommodative error (Jaschinski-Kruza, 1991; Schleifer, Sauter, Smith, & Knutson, 1990) and horizontal heterophoria (Dain, McCarthy, gr Cha n-ling, 1988). Kennedy and Murray (1991), using a lexical decision task, fo und that flicker resulted in saccadic irregularities. At low refresh rates, subjects misjudge the extent of target displacements that take place durin g saccades,in the direction of a breakdown in space constancy (Macknik, Fis her, & Bridgeman, 1991). A serious and largely unrecognized danger is misin terpretation 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 ra ster pattern, covering the surface 60 to 120 times per second. Visual persi stence makes screen illumination appear continuous, because the raster rate s exceed the critical flicker fusion rate. Scanning begins with a single ho rizontal row of pixels illuminated successively across the screen. Just aft er 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 Vertic al synch pulse restarts the raster at the beginning. Early VDTs were strobed at 60 Hz, but as large screens with bright backgrou nds were introduced, flicker in peripheral vision became noticeable. For th is reason, faster strobe rates were introduced-66 Hz in Sony Trinitron tube s, 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 solel y to avoid visible flicker; the industrial standard for VDTs states only th at flicker should not be Visible for 90% of users.