Manipulating the Response Criterion in Visual Monitoring

Forty-eight subjects were required to detect long-duration brightness changes (signals) and ignore short-duration changes (nonsignals) occurring on an electroluminescent panel during a 60-min. monitoring session. Signal-to-nonsignal ratios (constant 1/9, changing 1/9 to 1/1, or constant 1/1) and signal detectability (0.3 or 0.6 sec. difference between signal and nonsignal duration) were combined factorially in a between-subject design. The changing signal-to-nonsignal ratio resulted in an intermediate level of signals correctly detected. The classical decrease in percent of signals detected over time occurred in the constant 1/9 ratio condition under both levels of signal detectability. Signal detection theory analyses were restricted to low detectable signals. A marked increase in β over the monitoring session occurred in the constant 1/9 ratio condition, whereas β remained low and relatively constant in the other signal-to-nonsignal ratio conditions. Both the limitations of a decision-theory interpretation and the implications of using changing signal-to-nonsignal ratios for manipulating the observer's effective response criterion were discussed.