Many simple cells of the visual cortex have long narrow receptive fields, which are strongly excited by lines oriented along their long axis. In the present psychophysical analysis, we assume that there are similar independent ‘subunits’ which contribute, by probability summation, to the detection of a line. If a line segment is shorter than the subunit length, then extending the line length will increase the sensitivity of all the subunits affected by the line, and a relatively large increase in visual sensitivity will occur, corresponding to this ‘physiological summation’ within subunits. However, for a line segment which is much longer than the subunit length, the main effect of extending line length is to stimulate more subunits, resulting in a relatively small increase in sensitivity owing to probability summation. Thus a study of sensitivity (reciprocal threshold) as a function of line length may be used to test the subunit model and to estimate the subunit length. Here we use a quantitative model to demonstrate that sensitivity/line length data may be well fitted, assuming independent subunits having a constant length of 8·6′—in good agreement with the length of Andrews' ‘ff’ units.