TERRAIN INFLUENCES THE ACCURATE JUDGMENT OF DISTANCE

Mathematically, three-dimensional space can be represented differently by the cartesian, polar, and other coordinate systems. However, in ph ysical sciences, the choice of representation system is restricted by the need to simplify a machine's computation while enhancing its effic iency(1). Does the brain, for the same reasons, 'select' the most cost -efficient way to represent the three-dimensional location of objects? As we frequently interact with objects on the common ground surface, it might be beneficial for the visual system to code an object's locat ion using a ground-surface-based reference frame(2). More precisely, t he brain could use a quasi-two-dimensional coordinate system (x(s), y( s)) with respect, to the ground surface (s), rather than a strictly th ree-dimensional coordinate system (x, y, z), thus reducing coding redu ndancy and simplifying computations(2-5). Here we provide support for this view by studying human psychophysical performance in perceiving a bsolute distance and in visually directed action tasks(6-11). For exam ple, when an object was seen on a continuous, homogeneous texture grou nd surface, the observer judged the distance to the object accurately. However, when similar surface information was unavailable, for exampl e, when the object was seen across a gap in the ground, or across dist inct texture regions, distance judgement was impaired.