An Automatic Apparatus for the Continuous Method of Limits
Myron L. Braunstein · The American Journal of Psychology · 1962
To obtain threshold measurements under conditions in which a minimal amount of time is a consideration thresholds must be determined quicklythe automatic apparatus described here was devised. The procedure used is the continuous of limits. A variabIe stimulus is set to an intensity below threshold and is increased in intensity. S signals when he first perceives a difference. The stimulus-intensity continues to change, however, in the same direction for a brief interval, which is varied in duration from trial to trial in a pseudo-random manner. Then the direction of the change is reversed and S again signals when he no longer perceives a difference. After another brief pseudo-random interval, the direction of change is again reversed, and so on for as many series and reversals as desired. This differs from the standard of limits in that it permits the convergence of the stimulus-series about the threshold. It differs from Bekesy's method of adiustment'J in that it assures independence of successive determinations by interposing pseudo-random amounts of stimulus-change between successive series.1 The use of a converging series, in addition to permitting the rapid determination of the threshold, minimizes adaptation by keeping the stimulus close to the threshold. The apparatus consists of a motor driving a cam in which three grooves are cut, with different amounts of space between each two. A microswitch is activated each time a groove passes its position. It is thus activated in repeating cycles of three variable intervals. When S responds by pressing a button, indicating that he perceives or no longer perceives the stimulus or a diference, relays are so placed that the stimulus-intensity is reversed in direction of change the next time a groove hits the microswitch. As few as three variable delays thus interact with S's variability to produce a set of delays which appears random over long sequences of responses. As presently used, this device controls the motor on a variac, which regulates the voltage delivered to a projector-bulb. The bulb projects a spot onto an illuminated background. When S responds, indicating that he first sees the spot, the voltage continues to increase at a rate fixed for the particular motor-driven variac (4-v. per sec.), until the next time a groove passes the microswitch. The variac-motor is then reversed and the intensity of the stimulus decreases. The grooves on the cam are separated by spaces in the ratio of 3:4:5, and the cam is rotated at a speed of 10 rpm. This apparatus has been used on the centrifuge of the Navy Aviation Medical Acceleration Laboratory and reliable brightness-difference thresholds were determined in periods of 90 sec. or less while S was exposed to various levels of acceleration. Cornell Aeronautical Laboratory, Inc. MYRON L. BRAUNSTEIN