Violating the main sequence: asymmetries in saccadic peak velocities for saccades into the temporal versus nasal hemifields.

Saccadic peak velocities during monocular and binocular presentation were tested. While the main sequence linear increase in peak velocities as a function of saccade amplitude is well documented, our results provide demonstrations of violations of the main sequence. Peak velocities during monocular presentation were considerably higher toward temporal than nasal stimuli. This nasal-temporal asymmetry (NTA) was not explained by amplitude differences and was most pronounced for the lowest amplitudes tested, decreasing with increased amplitude. Under binocular presentation, this NTA was much smaller. While the exact reasons for this difference in peak velocities between hemifields are unclear at present, we propose that anatomical NTAs result in stronger signals from the nasal, than temporal retina leading to higher peak velocities into the temporal visual hemifield. NTAs in peak velocity are consistent with NTAs in attentional choice and in attentional function, which might also be explained by anatomical NTA.