Perceived length across the physiological blind spot.

Objects falling across the physiological blind spot appear "complete" despite the absence of photoreceptors. Completion of objects may occur across the blind spot because (1) the blind spot is filled in with the background (the associative explanation); (2) the opposite sides of the blind spot may be contiguously represented in the cortex (i.e. the blind spot is simply sewn up-the retinotopic explanation); or (3) the blind spot may be sewn up, with compensatory expansion occurring around the blind spot (the compensation explanation). These theories would predict no size distortions regardless of object size; constant size distortions regardless of object size; and distortions that depend on the size of the object, respectively. To evaluate these explanations, we measured size distortions at the blind spot. We measured length distortions at the blind spot using a criterion-free two-alternative forced-choice method with feedback. Observers compared the lengths of test bars presented across the blind spot with lengths of reference bars presented at the corresponding location in the fellow eye. Test bar lengths ranged from 7-14 deg. Reference bar lengths were in the range of +/- 3 deg of test bar length. From the observers' responses the perceived length of each bar at the blind spot was estimated. Estimates of the precision of length discrimination at the blind spot were also obtained. Our results were consistent with the associative explanation. In all seven observers, length distortions at the blind spot were smaller than 1 deg (< 20% of the vertical height of the blind spot) for all bar lengths tested. For bars that were presented across the blind spot, the precision with which observers could discriminate length was comparable to that of normal periphery (Weber fraction approximately 20%). Both the veridicality and precision of perceived length are preserved around the blind spot.