Temporal summation of receptive fields for line orientation in humans

The contrast threshold for line orientation was studied using two lines with the same orientation under three different experimental conditions (series): (1) the two lines were presented in the same part of the receptive field; (2) they were along the same straight line and separated by 14' visual angle; (3) they were parallel and displaced at 4' of visual angle. The lines were presented for the same duration with an interstimulus interval (ISI) varying between 0 and 300 ms. The threshold for line orientation when only the first or the second line was presented alone was also studied. In all series when the ISI was longer than 100 ms, the line orientation thresholds for two lines (T1,2) and for one line when presented alone (T1 or T2) were the same. At ISI varying from 50 to 100 ms the values for (T1,2) were similar to those expected with probability summation. In the third series at ISI from 0 to 10 ms, the same reciprocal inhibitory interactions prevailed as when the two lines were presented simultaneously. In the first series at the same ISI (0-10 ms), a complete temporal summation was also observed.