Binocular vision revised: temporal vs. energy summation.

For binocular summation the upper limit of asynchrony in separate (dichoptic) stimulation for grating targets successively presented to the two eyes is 100 ms. The magnitude of binocular summation varies inversely with the temporal interval, peaking for simultaneous presentation, though similar energy is contained in the sum of two monocular inputs in either simultaneous or asynchronous stimulation. Thus, two types of binocular summation can be defined, based on its temporal properties: 1) energy summation, i.e., pooled intensities of stimuli simultaneously presented to the two eyes and 2) temporal summation, i.e., energy summation over time for monocular stimuli presented with a temporal delay. Here we directly compared temporal vs. energy summation for localized low-contrast 188 Gabor targets, recently shown to gain in both accuracy and RT for simultaneous binocular vs. monocular presentation (Yehezkel et al., VSS 2010). Subjects detected Gabor targets (6 cpd, contrast near monocular detection threshold) presented using dichoptic goggles for 60 ms to both eyes, either simultaneously (B), sequentially (RL and LR), or to one eye only (R and L). As expected, there was binocular summation (B vs. R and L) for accuracy and d-prime, but not for RT, consistent with longer binocular vs. monocular RT in a grouping task (Yehezkel et al., VSS 2006). Moreover, there was a disadvantage in RT to dichoptic simultaneous (B) vs. successive (RL or LR) stimulation. Accuracy and d-prime were also lowest for B (80%, 2.76) compared to RL (95%, 4.13) and LR (90%, 3.49). However, the lack of advantage to the twofold duration of simultaneous binocular presentation (BB vs. RL and BB vs. LR), both for RT and for accuracy or d-prime suggests saturation of temporal summation for stimulation of 120 ms. To conclude, since both successive and simultaneous binocular stimulation carry the same energy, these results suggest an advantage of temporal over energy summation.