(This article appeared in Volume 25, Number 3, 2008, pp. 469–474).
The abscissa in Fig. 1 (p. 471) was labeled in units of cd/m2. The correct units are trolands (Td), and a new Fig. 1, so labeled, is shown below.
On p. 473, column 1, the third sentence in the second paragraph under the heading Effects of stimulus duration should read as follows:
“The mean Tc-estimates for their two observers were 33.5 ms for SPP and 49 ms for PPP, compared with 26.3 (SPP) versus 38.4 (PPP) for our controls.”