Medical devices and methods are designed to bias stretch receptors in the stomach wall of a patient to treat obesity. Biasing of the stretch receptors by pre-stretching induces an early sensation of satiety, causing the patient to consume less food. Biasing of the stretch receptors can be achieved by the placement of bulking devices within the wall of the stomach, e.g., in the mucosa, submucosa or muscle layer. The bulking devices may be expandable and, in some embodiments, may take the form of a hydrogel prosthesis that expands following implantation in a wall of the stomach.