Fig. 1.

Experimental design. (A) Male and female rats were surgically implanted with electrodes to measure the electromyographic response (EMG) of the eyelid. Afterward, animals were injected with 1 dose of bromodeoxyuridine (BrdU), which labels 1 population of dividing cells. Females were cycling normally and injected during estrus. One week later, males and females were trained with trace eyeblink conditioning each day for 4 days. One day later, trained and untrained animals were killed to assess the number of new cells in the dentate gyrus of the hippocampus (12 days after the BrdU injection). (B) During trace conditioning, a CS of white noise was paired with a US (periorbital stimulation), which elicits an eyeblink response. The 2 conditioning stimuli were separated by a trace interval or temporal gap of 500 ms. As an animal learns that the 2 stimuli are associated, it blinks during the trace interval and in anticipation of the US. Eyeblinks were detected by an increase in the magnitude of the EMG. Those that occurred during the trace interval were considered CRs. The electrophysiological record shows an example of a CR that occurred just before the onset of the US.