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The electrical activity of the antrum and proximal duodenum was recorded simultaneously with bar pressing for food in various feeding conditions. During restricted feeding (RF; food available from 1200 to 1600 hr; lights-on from 0800 to 2000 h), large-amplitude slow potentials of duodenal origin were observed 3-5 hr preceding food access. These potentials occurred in short trains at irregular intervals of approximately 1 min and appeared to be associated with the active phase of the migrating motility complex. At least 8-10 days on RF were required before these irregular contractions (ICs) reliably preceded food access. The distribution of ICs showed a circadian variation during food deprivation, and transients lasting 4-6 days were observed in response to 6- or 8-hr phase delays of food access. These observations suggest that ICs are modulated by a circadian mechanism. On the average, the increase in ICs occurred about 2 hr earlier than bar pressing in anticipation of food, and there was no specific relation between individual ICs and bar pressing. However, bar pressing frequently occurred 0.5 min or longer after an increase in electromyogram (EMG) amplitude. Unreinforced bar pressing was never observed during a quiescent EMG.

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