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 occ ur across the blind spot because (1) the blind spot is filled in with the background (the associative explanation); (2) the opposite sides o f the blind spot may be contiguously represented in the cortex (i.e. t he 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 w ould predict no size distortions regardless of object size; constant s ize distortions regardless of object size; and distortions that depend on the size of the object, respectively. To evaluate these explanatio ns, 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 ba rs 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 pe rceived 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 sm aller than 1 deg (<20% of the vertical height of the blind spot) for a ll bar lengths tested. For bars that were presented across the blind s pot, the precision with which observers could discriminate length was comparable to that of normal periphery (Weber fraction approximate to 20%). Both the veridicality and precision of perceived length are pres erved around the blind spot.