Objective      To investigate the behavioral characteristics of eyeblink responses during the extinction of delay and trace eyeblink conditionings in guinea pigs.        Methods      A total of 27 male guinea pigs (12-16 weeks-old

450-600 g) were randomly assigned into 2 groups

delay eyeblink conditioning group (n=13)

and trace eyeblink conditioning group (n=14). Pure tone conditioned stimulus (CS) of 2 kHz was paired with corneal oxygen-puff unconditioned stimulus (US) to acquire delay and trace conditioned eyeblink responses (CR). The guinea pigs received CS alone during the extinction. A high-resolution spring-return potentiometer was used to monitor the activities of left orbicularis oculi muscles. Self-developed software written in the Matlab environment was used to quantitatively analyze the behavioral data.        Results      Compared with the trace-conditioned guinea pigs

the delay-conditioned guinea pigs acquired faster CRs (P0.05) and higher peak amplitude (P0.05). Nevertheless

there was no significant difference in the CR extinction rate between the 2 groups (P0.05). In addition

more obvious spontaneous recovery of the extinguished CRs was observed in the delay-conditioned guinea pigs (P0.05) as compared to the trace-conditioned guinea pigs.        Conclusion      The behavioral characteristics of delay CR extinction are different from those of trace CR extinction.