MST野と運動視

Area MST and motion vision

　MST野は，方向選択性ニューロンの集まりであるV5/MT野から前方向性の投射を受ける領域として発見された運動情報処理専門の領域である。V5/MTに較べるとMSTでは個々のニューロンの受容野はずっと広く，網膜部位局在の配列が失われている。第一の特徴は視覚的追跡ニューロンで，眼球運動信号と背景像の動きの網膜信号を統合して視覚目標の動きをコードする。一方，自己の眼球運動で誘導される網膜像の動きに対する反応はMSTでは眼球運動信号で抑制される。MSTの奥行運動感受性ニューロンは，大きさ変化と両眼視差変化を組合せて三次元的運動方向をコードする。MSTで発見された回転感受性ニューロンは，運動方向の変化を手がかりとして三次元的な回転の方向をコードする。MSTdにはオプティックフローから自己運動の進行方向を読み取るニューロンがある。最近のf-MRIを使ったブレインマッピングにより，ヒトのMT野とMST野の違いが明らかになった。

　MST（medial superior temporal）area in macaques was defined anatomically by the forward corticocortical projection from neighboring V5/MT area. Most of MST neurons were directionally selective. Their visual receptive fields were larger than those of V5/MT neurons and were not retinotopically organized. One of the featuring cells in MST are the visual tracking（pursuit）neurons that receive retinal signals of moving background and the extraretinal signals of eye movement. It was concluded that visual tracking（VT）neurons encode the direction and velocity of the visual target of pursuit. Extraretinal signal of eye movement have inhibitory effect on the MST neurons activated by movement of external object to suppress their responses to self-induced movement of retinal image to be discriminated from real movement. The depth-movement-sensitive neurons in MST encode the direction of movement of object in three-dimensional space by combining the signals of size change and disparity change of the retinal image. Sakata et al（1985）discovered the rotation-sensitive neurons in MST. They respond selectively to the rotary movement of the visual stimulus in contrast to the linear movement. Many of the rotation-sensitive cells preferred rotation in depth, in horizontal sagittal and diagonal planes. They responded to the rotary movement of a single spot suggesting to compute rotary movement from the change of direction. Many of the visual neurons in MSTd responded to the optic flow of planar, circular and radial patterns. Many of them responded to the combination of circular and planar optic flow and were selective in FOE（focus of expansion）. These neurons may encode direction of heading of self motion. Recent f-MRI studies demonstrated activation of MT/MST area in human brain by moving visual stimulation. Some f-MRI studies demonstrated specific activation of MST by non-retinotopic radial and circular optic flow, as well as non visual pursuit eye movement.