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The mean percentage of lever presses made on the lever associated with 2 pellets (as opposed to 1 pellet) of food by measurement phase (Phase 1 = baseline) for the (a) Control and (b) UHT group. Upper x-axis: 2 kHz = 2 pellet tone; lower x-axis: 4 kHz = 2 pellet tone. The means and SEM are derived from data summarised at the subject-level; the data for the ‘reference tones’ (i.e. 2 kHz and 4 kHz) pertain to non-reinforced trials only.
