Biofeedback therapy improves bowel symptoms and anorectal function in patients with dyssynergic defecation, however its mechanism of action is not known. The investigators hypothesize that biofeedback therapy enhances gut-brain-gut communication by altering cortical processing of information and improving cortically mediated neuromuscular function of the gut. However, in order to better understand these mechanisms in patients, the investigators need to examine and establish normative data and compare findings with healthy subjects. The investigators specific aims are to examine and evaluate the following 40 normal subjects; (1) To evaluate the afferent cortical evoked potentials in response to the electrical stimulation of the anorectum and (2) To evaluate the corticofugal tracts (efferent) by recording the anal and rectal electromyographic responses following noninvasive lumbosacral and transcranial magnetic stimulation.

1. Cortical evoked potentials:A probe with 2 ring electrodes is placed in anus and rectum. A small amount of electrical current is passed. The cortical evoked responses to the anal and rectal electrical stimulation is measured from the scalp using a neurophysiology recorder.
2. Motor evoked potentials: A magnetic coil is placed on the scalp at the anorectal cortical site and discharged using magnetic energy. The anal and rectal motor evoked potentials are then measured using a probe with 2 ring electrodes.

Inclusion criteria:

• Right-handed adults between ages 18 years and 99 years, who are free of bowel disorders or other medical illnesses.

Exclusion Criteria:

• Potential subjects with comorbid illnesses; severe cardiac disease, chronic renal failure or previous gastrointestinal surgery except cholecystectomy and appendectomy.
• Neurologic diseases e.g.; head injury, epilepsy, multiple sclerosis, strokes, spinal cord injuries.
• People who have metal in their skull or under the skull, or have metal in the back or hips.
• People who have a cardiac pacemaker, implanted defibrillator or medication pump.
• Impaired cognizance (mini mental score of < 15) and/or legally blind.
• Pregnant or likely to conceive during the course of the study. Women with potential for pregnancy must be willing to use contraceptive measures during the study.
• Previous pelvic surgery, rectocele repair, bladder repair, radical hysterectomy.
• Rectal prolapse or anal fissure or anal surgery.

40 Healthy volunteers. 110 patients with chronic constipation and dyssynergic defecation and rectal hyposensitivity and rectal hypersensitivity