HYPERACUITY FOR SPATIAL LOCALIZATION OF CONTRAST-MODULATED PATTERNS

The acuity for localizing the position of a grating and other first or der patterns which are defined directly by the luminance distribution, is much higher than the resolution for such gratings, This well-descr ibed phenomenon usually is referred to as hyperacuity, and is regarded as a cortical function which is not limited by the optics and the sam pling properties of the eye. Second order patterns which can be define d by the distribution of local contrast gained some interest because t hey require more complex processing mechanisms than first order patter ns. We investigated how well gratings and bars which are exclusively d efined by the variation of the local contrast of static random dot pat terns can be localized in space. In this case localization acuity does not reach the precision which is known for first order patterns. Howe ver, the localization of contrast-modulated patterns can bie almost on e order of magnitude better than second order grating resolution, and therefore reaches into the hyperacuity range. In combination with find ings for motion-defined or stereo-defined patterns it is concluded tha t the brain mechanisms responsible for the localization of features in the visual scene have not only access to first order information whic h is available immediately from the retinal image, but in addition, to second order information which has to be extracted from the retinal i ntensity distribution by some sort of nonlinear processing.