Apparatus (18, 18′) is provided, which includes one or more fundic sensors (74), configured to be applied to a fundus of a subject (10), and to generate a fundic signal. The apparatus further includes a control unit (90), which is configured to make a tentative determination of ingestion by the subject (10), determine fundic impedance responsively to the fundic signal, and calculate a duration of a period during which the fundic impedance rose during the tentatively-determined ingestion. Upon finding that the duration is less than a fundic rise duration threshold value, the control unit (90) determines that the tentative determination of ingestion is a false positive. Other embodiments are also described.