Irritable bowel syndrome (IBS) is the most common digestive disorder that affects more than 5% of population in Hong Kong. However, there is no effective treatment of IBS using Western Medicine. Acupuncture, a traditional therapeutic modality, has been used in China for thousands of years for various pain disorders. In addition to analgesia, acupuncture has also been shown to influence physiology of gastrointestinal tract. The investigators set out to evaluate the therapeutic value of acupuncture in IBS. The investigators will study its effects on rectal sensation and brain activity in patients with IBS.