THE CO-OWNER OF THE ULTRALIGHT VEHICLE WAS WATCHING THE FLT & OBSERVED THE ACCIDENT. HE SAID THE VEHICLE APPROACHED THE PASTURE & WAS DESCENDING FROM APRX 200 FT AS IF THE PLT WAS GOING TO LAND OR FLY BY. THE ULTRALIGHT PASSED OVER THE TAKEOFF/LANDING AREA AT ABOUT 30 TO 40 FT AGL & CONTINUED AT THE SAME ALT & GENERAL DIRECTION FOR APRX 200 YARDS, WHERE IT COLLIDED WITH TREE BRANCHES. SEVERAL OTHER WITNESSES CORROBORATED WHAT THE CO-OWNER HAD DESCRIBED. NO PREIMPACT DEFICIENCES OF THE ULTRALIGHT WERE FOUND. FOUR BOTTLES OF MEDICATION WERE FOUND WHICH INCLUDED ADAPIN, SINEQUAN, THORAZINE, & NARDIL. THE PLT'S DOCTOR REPORTED THAT HE (THE PLT) HAD BEEN TREATED FOR DEPRESSION FOR SEVERAL MONTHS & HAD PERIODS OF MENTAL INCAPACITATION. AUTOPSY & TOXICOLOGICAL EXAMS REVEALED NOTHING SIGNIFICANT TO THE ACCIDENT. THE PLT HAD NO FORMAL FLT INSTRUCTION TOWARD CERTIFICATION, BUT HAD RECEIVED 4 HRS GROUND SCHOOL & 40 TO 50 FLTS IN AN UNPOWER VEHICLE TOWED BEHIND AN AUTOMOBILE & A SUPERVISED SOLO FLT.