Contact local medical college/university departments of cellular biology, neurology, neuroscience, neurosurgery, or psychiatry. Many scientists/physicians are happy to address lay audiences about their field of research and will respond favorably to an inquiry. The more precise a target schedule you have, the better; calendars are very tight.
Contact local hospitals and medical centers. Staff neurologists, psychiatrists, and physical and occupational therapists may be happy to speak to your group.
Check for BAW Partners located near your organization. Other partners may have readily available speakers but limited public programming opportunities. Consider joint programs as a means to support both your efforts. The Campaign Clearinghouse can provide you with detailed contact information.