Temporal asynchrony interferes with vernier acuity

Two dots may be aligned vertically with a precision much higher than that expected from two-point resolution provided they are separated by a visual angle of 3-5 min of arc. This precision suffers when the two dots are not exposed synchronously. Neither onset nor offset asynchronies can be tolerated; exposure differences of the two components of the vernier task as low as 30 ms can lead to a reduction in performance when the total exposure is below 90 ms. This effect cannot be compensated for by synchronizing the onset of one stimulus component with the offset of the other, even when the two are of opposite contrast. The data suggest that vernier acuity may be subserved by a dynamical linking of cortical excitation generated by the synchronous arrival of signals within a range of locations in the cortex whose spatial separation is critical for optimal hyperacuity performance. The evidence presented in this paper must be taken into account when a physiological substrate for hyperacuity is considered.