Response to Westlund's commentary: 'Can conditioned reinforcers and variable-ratio schedules make food and fluid control redundant?' - Prescott, M; Brown, V; Flecknell, P; Garrod, K; Lemon, R; Parker, A; Ryder, K; Schultz, W; Scott, L; Watson, J; Whitfield, L (Journal of Neuroscience Methods, 2011)