The Onset of the Fast Phase in the Optokinetic Response of the Crab, Carcinus

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Abstract
1. The onset of the fast phase in the optokinetic response is a part of a centrally determined programme. 2. An eyecup moving in a slow phase toward the mid line governs the onset of the fast phase for both eyes and leads by 40-60 msec. 3. Repeated reflex withdrawal of a governing eyecup suppresses the onset of the fast phase; similar stimulation of a governed eyecup has no effect on fast phases. 4. After shocks applied near the governing eye the onset of the fast phase occurs earlier in the slow phase. Mechanical stimulation has the same effect. 5. If the eyecup receives a shock whenever it moves too near the mid line in a slow forward phase, it soon changes the position of onset of the fast phase so that the shocks are avoided. The tonic motor frequency to muscle 21 at which the fast phase normally occurs is reduced. The effect is lasting. 6. This plastic change is adaptive but is not learning; the earlier onset occurs irrespective of how or when the shocks are applied. The reinforcement need not supply any possibility of association. 7. In a behaviour pattern which depends on a centrally determined programme of motor impulses for each stimulus situation the element of plasticity is one further component of the central pattern, so arranged that it is adaptive.