Okumura (1990) assumed wave-like dispersal of new lexical features from Izuhara, Tsushima's politico-cultural center.
[1] Tsushima's general resemblance to mainland Kyūshū dialects is obvious, but exactly how it has evolved remains an open question.
Tsushima is often classified as a Hichiku (northwestern Kyūkyū) dialect, but some Hōnichi (eastern Kyūshū)-like elements are sometimes observed.
However, Okumura (1990) argued that the Tsutsu accent system of Tsushima could hardly have a Chikuzen-like ancestor and was more close to the Buzen subgroup of the Hōnichi dialect.
[3] However, they were unable to test the hypothesis by conducting field work because the access to the island was restricted by the army for its strategic importance.