Abstract
Previous assessment of verticality by means of rod and rod and frame tests indicated that human subjects can be more (field dependent) or less (field independent) influenced by a frame placed around a tilted rod. In the present study we propose a new approach to these tests. The judgment of visual verticality (rod test) was evaluated in 50 young subjects (28 males, ranging in age from 20 to 27 years) by randomly projecting a luminous rod tilted between -18 and +18° (negative values indicating left tilts) onto a tangent screen. In the rod and frame test the rod was displayed within a luminous fixed frame tilted at +18 or -18°. Subjects were instructed to verbally indicate the rod's inclination direction (forced choice). Visual dependency was estimated by means of a Visual Index calculated from rod and rod and frame test values. Based on this index, volunteers were classified as field dependent, intermediate and field independent. A fourth category was created within the field-independent subjects for whom the amount of correct guesses in the rod and frame test exceeded that of the rod test, thus indicating improved performance when a surrounding frame was present. In conclusion, the combined use of subjective visual vertical and the rod and frame test provides a specific and reliable form of evaluation of verticality in healthy subjects and might be of use to probe changes in brain function after central or peripheral lesions.
Highlights
Under normal circumstances, we are able to use vision to precisely align objects with the gravitational vertical without any other reference frame
On the basis of rod and frame test (RFT), normal subjects were classified either as “field dependent” (FD), when the presence of the frame exerted a significant influence on the judgment of rod tilt, or as “field independent” (FI), when the presence of the frame did not interfere with the judgment of verticality
Whereas a large number of subjects ranged in intermediate index values, a small percentage of subjects showed high index values as a consequence of more correct guesses in subjective visual vertical (SVV) than in RFT, and were classified as FD
Summary
We are able to use vision to precisely align objects with the gravitational vertical without any other reference frame. The results of such studies suggest that altered SVV may be one of the factors underlying the difficulties that hemiplegic patients experience in keeping their balance after a recent stroke [3,4,7], which possibly is due to an abnormal internal representation of verticality [9]. In their original studies, Witkin and Asch [10,11] designed another test to assess the influence of surrounding oriented contours on the perception of verticality. A third category, known as “field intermediate” (FInt), was used for subjects for whom the presence of the frame exerted some influence
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