SPATIAL INTEGRATION IN POSITION ACUITY

To determine whether and how spatial integration takes place in positi on acuity, bisection and Vernier thresholds were measured in the fovea of four normal observers with spatially ''undersampled'' dark lines ( i.e. lines comprised of discrete samples). The size, contrast, and den sity of samples, and the separation of the lines were varied. For a gi ven sampling density, sample size (0.17-2.72 min) has negligible effec t on position threshold. For all sample sizes, position threshold decr eases as sampling density increases, indicating that spatial integrati on takes place. The form of spatial integration depends on line separa tion. At the optimal line separation (2 min for bisection and 0 min fo r Vernier), position threshold decreases as sampling density increases with a slope of about -0,8 on log axes, steeper than a slope of -0.5 as would be expected from statistical position averaging. This effect of sampling density can be completely explained by spatial contrast su mmation for visibility. At the 16 min line separation, position thresh old also decreases as sampling density increases but with a slope shal lower than -0.5. However, this effect of sampling density can not be e xplained by contrast summation. Position thresholds decrease even afte r discounting the effect of contrast summation on visibility, suggesti ng a genuine position averaging. These findings are independent of lin e orientation (horizontal or vertical), and hold for both random and u niform dot distributions, and for both bisection and Vernier. Thus, tw o separate mechanisms of position acuity are suggested. A spatial filt er mechanism operates at the optimal (or narrow) line separation where position threshold is critically dependent on stimulus visibility. A local sign mechanism operates at the relatively wider line separation where position acuity benefits from focal sign position averaging. For both mechanisms, spatial integration is not perfect.