(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.
![](https://static.cambridge.org/binary/version/id/urn:cambridge.org:id:binary-alt:20160712193823-03475-mediumThumb-S0952523808080747_fig1g.jpg?pub-status=live)
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.”