"The most typical finding has been that the optimal interval is in the quarter-second range on either side of .5 seconds"
Exceptions to this rule:
KAMIN LJ.Traumatic avoidance learning: the effects of CS-US interval with a trace-conditioning procedure. J Comp Physiol Psychol. 1954 Feb;47(1):65-72. PMID 13130735
CHURCH RM, BRUSH FR, SOLOMON RL.Traumatic avoidance learning; the effects of CS-US interval with a delayed-conditioning procedure in a free-responding situation.J Comp Physiol Psychol. 1956 Jun;49(3):301-8. PMID 13332132
BRUSH FR, BRUSH ES, SOLOMON RL. Traumatic avoidance learning: the effects of CS-US interval with a delayed-conditioning procedure. J Comp Physiol Psychol. 1955 Aug;48(4):285-93. PMID 13252159