Environmental Pitch and Three Types of Pointing

Many studies have shown that large errors are made when setting a target (T) to visually perceived eye level (VPEL) in a pitched environmental surround. The error in judgement of VPEL is typically about 50% of the environmental pitch angle. An observer can, however, point to the level of the target (T) with much smaller errors (eg Stoper et al, 1992 Bulletin of the Psychonomic Society30 439, found a shift of pointing of only 4% of the environmental pitch). These small pointing errors are found when the observer reaches out with an unseen hand and touches the surface on which T is presented. We call this ‘type I pointing’. If longer distances (183 cm) are used the observer must walk (with closed eyes, as in ‘pin the tail on the donkey’) in order to touch the surface on which T is presented. We call this ‘type II pointing’; it results in much larger errors, approaching in angular magnitude the errors in judgement of VPEL. In the present experiments the observer indicated the level of T by touching a point on a unseen pole which was just to the right of the observer's eyes, and thus separated from T by the viewing distance [as in the ‘manual task’ used to judge apparent height by Stoper and Bautista (1992 Investigative Ophthalmology and Visual Science, Supplement33 962)]. We call this ‘type III pointing’. This method, for both long and short distances, produced large errors similar in magnitude to those of type II pointing. These results are explained by the assumptions that environmental pitch causes an error in the judgement of the apparent horizontal in the sagittal plane (sagittal apparent horizontal; SAH) and that SAH is used in pointing of types II and III, but not of type I.