While the number of uninsured Massachusetts residents has fallen since 2006 and the number who have visited a doctor has increased, the state's shortage of physicians continues, with 'critical' levels in a few specialties. This year, neurosurgery joined internal medicine, urology and psychiatry as fields where there is a critical shortage, according to the Massachusetts Medical Society Physician Workforce Study released yesterday. Other fields in the slightly less drastic 'severe' rating include family medicine, dermatology and general surgery.