Length discrimination thresholds (in degrees of visual angle + SE) are shown for the three groups. A lower threshold indicates better performance. The DD group showed inferior ability to perform this task, differing significantly from the CA [high IQ] group, and (non-significantly) from the CA low IQ group. Neither age nor reading performance proved good predictors of performance, whilst nonverbal intelligence appears more important. (** denotes significantly different from DD group, p < .01).