A C language technique for synchronizing millisecond timer software to the appearance of the stimulus on the IBM PC's video monitor is described. Tachistoscopic programs that use the technique can correct the mean 8.3-msec bias normally found in reported response latencies and reduce the associated error variance.Researchers considering the use of the IBM PC for tachistoscopic applications should know that the appearance of a stimulus on the video monitor is controlled by two asynchronous processes: the computer program and the video display circuitry. The program executes instructions that result in the storage of character data (or pixel images) in random access memory (RAM) that is dedicated to the video display. The display processing hardware recurrently reads video RAM and controls the intensity of the monitor's electron beam as it "draws" images. The result: the computer program-which also initiates the timing of response latencies-normally does not know the precise moment at which the display process causes the stimulus to appear on the screen.The decoupling of the program and video processes has a number of effects of interest to researchers who require precise control over brief stimulus displays and millisecond interval timing. As the vertical dimension of the stimulus increases, it becomes more likely that the stimulus will be drawn on the screen in parts. For example, the electron beam might be aimed at the middle of the screen when the program transfers the stimulus data to video RAM. The bottom portion of the stimulus would then appear first, followed by an interval during which the electron beam would be scanning the bottom and then the top of the screen (both areas presumably empty of stimulus parts). Finally, the top portion of the stimulus would appear. The interval between the appearance of first-and last-drawn portions would be 16.7 msec (assuming a monitor with a 6O-Hz refresh frequency).The situation for response latencies produces both biased results and increased error variance. If the program starts a millisecond timer following the execution of the commands to display the stimulus, the timer will have been running an average of 8.3 msec before the stimulus Correspondence may be addressed to Joseph G.
A machine language technique is described whereby the z.ao microprocessor of the Model III TRS-aO can be programmed to monitor position of the electron beam during CRT scanning. This technique provides the opportunity to synchronize the appearance of video displays with z.ao processing. The programmer can therefore be assured of crisp stimulus displays and precisely recorded reaction times. The computer's real-time clock operates on video circuitry as part of a routine that is initiated by a maskable interrupt. The real-time clock interrupt can be vectored from its normal use to a routine that signals the z.ao when the electron beam is at a known screen location. A machine language program and a TRSDOS BASIC program that demonstrate the technique are described.
The execution times of microcomputer high-level-language commands can be long enough to be of concern in experiments in which precise timing is a consideration. The problems in developing standard BASIC timing routines are addressed. A technique for using the Model III TRS-80-real-time clock to calibrate BASIC timinig routines is described, and representative execution times of selected commands are reported. It is concluded that high-level languages are too slow and that execution times are too variable for critical timing in experiments. On the other hand, machine language programs can provide the needed precision and control.Timing with millisecond precision is a major requirement for microcomputers used in research. The timing of stimulus exposures, interstimulus intervals, and response latencies can be accomplished by a software timer or by having the computer input signals from an external clock. Each technique has advantages and disadvantages. An external clock is reliable, but it is also an additional expense and must be interfaced with the computer. A software timer requires no additional equipment, but it can be sensitive to tampering. Moreover, timers written in high-level languages (e.g., BASIC or PASCAL) are too slow for millisecond precision. This has led some researchers to rely on external timers for critical timing. Regardless of the timer used, however, reliable event timing also depends on the execution time of program commands, whether they are written in a high-level language or in machine language.For purposes of this discussion, assume that an experiment consists of a series of discrete trials on which a stimulus is displayed, timed, and then masked. Also, a latency timer starts at stimulus onset and stops when the subject responds. Millisecond timing is required for stimulus exposure, interstimulus interval, and response latency: Either an external timer or a machine language software timer may be used for this.A detailed analysis of trial events shows that a reliable timer is not all that is needed to assure that program timing fits the intended specifications. With a software timer, the following operations must be performed: (1) display the stimulus, (2) start timing response latency, (3) time stimulus exposure, (4) display the mask, (5) monitor the subject's response, and (6) stop and read the latency timer. With an external timer, the operations are these: (1) read the clock for the start
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