STARTLE REFLEX FACILITATION AS A FUNCTION OF CLASSICAL EYEBLINK CONDITIONING IN HUMANS

I investigated whether classical eyeblink conditioning increases the a ctivating effect of the conditional stimulus (CS) on the relevant alph a motoneuron pool in the facial nucleus. The activating effect was mea sured by the reflex modification method, where startle reflexes, shari ng the facial nerve motor pathway with conditional and unconditional e yeblink reflexes, were elicited in the presence of reinforced and nonr einforced CSs (CS+ and CS-). To reduce influences of conditional arous al on startle, a weak airpuff to the eye was used as unconditional sti mulus and relatively short stimulus onset asynchronies (SOAs) of 300 a nd 600 ms were used in three startle tests presented while conditionin g was in progress. Two groups (N = 72) received 100% or 0% reinforceme nt (Groups 100 and 0) of a tone CS+. The CS- was not reinforced. Three startle tests, consisting of acoustic startle probes presented alone and after CS+ and CS- onset, were conducted early, middle, and late in conditioning. Results showed increased startle reflex facilitation af ter CS+ onset in the late startle test in Group 100, indicating that s tartle increased as a function of classical conditioning. Although ref lex facilitation at the 300-ms SOA in Group 100 is consistent with the hypothesis that CS presentations activated motoneurons in the facial nucleus, the present study cannot itself definitively support this hyp othesis. Reflex facilitation at the 600-ms SOA in Group 100 could be i nfluenced by conditional arousal.