This was a study of the effects of prolonged head rotation on the apparent frontal parallel plane, and on kinesthetic jugment of the "straight ahead" position of the head. Both meaures were obtained before and after rotation periods of 30 seconds, 2 minutes, and 4 minutes, during which the head was deviated 20 degrees left, and no visual stimulation was allowed. The apparent frontal parallel plane judgments, pre and post adaptation, were made with the subject's face congruent with the frontal plane of his body. The results indicate an adaptation of head position of under correction or adaptation towards the direction of deviation; the apparent frontal plane was rotated toward the subject on the side toward which his head had been rotated. A significant visual after-effect was detected in the 30 second period, and this increased significantly over the 2 and 4 minute periods. These are the shortest adapting periods yet demonstrated, to our knowledge.
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