Figure 2. Trace eyeblink conditioning in rats was used to study the effect of disrupting dentate spike‐related hippocampal neural activity on learning.

The outline of the experiment is presented in A and an example of a typical conditioned eyeblink response in B. Examples of hippocampal stimulation via the vHC are shown in panels C–F. A, rats were trained in trace eyeblink conditioning using a 500 ms trace period. The conditioned stimulus (CS) was a 200 ms, 75 dB white noise auditory stimulus. The unconditioned stimulus (US) was a 100 ms electrical stimulation of the eyelid that elicited a robust blink of the eye. A total of 60 trials were presented during each daily session, with an intertrial interval (ITI) of 30–60 s. Conditioning was followed immediately by a 1 h rest period. During the rest period, animals were either left alone (normal control, NC) or the hippocampus stimulated via the ventral hippocampal commissure (vHC) contingent on dentate spikes (EXP). For animals in the yoked control group (YC) vHC stimulation was administered to a random brain state but during immobility. B, an example of electromyogram (EMG) from the stimulated eyelid during a trial in which the animal emitted a typical conditioned response. The pale grey box indicates the time period during which blinking was categorized as a conditioned response. C, an example of hilar local‐field potential during hippocampal stimulation (at 0 ms) via the vHC, immediately following a dentate spike (DS). D, same data as in C but with a higher time resolution. E, an example of hilar local‐field potential during hippocampal stimulation (at 0 ms) via the vHC, to a random brain state but during immobility. F, same data as in E but with a higher time resolution. Note the ∼10 ms delay in the hippocampal‐evoked response to vHC stimulation.