Figure 1.

Experimental design. (A) Animals were implanted with EMG recording electrodes in the orbicularis oculi (O.O.) muscle of the upper eyelid and with stimulating electrodes on the supraorbital nerve. The latter was used for presentation of unconditioned stimulus (US). For classical eyeblink conditioning, we used a tone (20 ms, 2.4 kHz, 85 dB) as a conditioned stimulus (CS). The loudspeaker was located 30 cm in front of the animal's head. Animals were also implanted with stimulating (St.) electrodes at the Schaffer collaterals (coll.) and with a recording (Rec.) electrode in the CA1 area (see inset at the top). (B, C) Two photomicrographs illustrating the location of hippocampal recording (B) and stimulating (C) sites. Scale bars: 200 μm. DG, dentate gyrus, Sub., subiculum; D, L, M, V, dorsal, lateral, medial, and ventral. (D) Schematic representation of the conditioning paradigm, including CS and US stimuli, and the moment at which a paired pulse was presented to Schaffer collaterals (St. Hipp.). The two lower traces illustrate an EMG recording and an extracellular recording of hippocampal activity. Both traces were collected from the 9th conditioning session of a representative animal. Calibrations as indicated.