Visual signal detection and the locus of foreperiod effects.

The effect of alertness on S's sensitivity to external signals was determined by varying the foreperiod in a visual signal detection task. Discriminability increased with increasing foreperiods up to an optimal foreperiod, beyond which it decreased slightly. This pattern of results parallels closely that of many foreperiod studies in which reaction time was the dependent variable. Because no speeded response was required in this signal detection task, it was concluded that alertness must be affecting an information processing stage prior to those associated with responding. Other evidence indicates that alertness does not affect sensory stages either. Alertness, by elimination, must be affecting the operation of a central stage, Posner's view of alertness which implicates the central stages is discussed.