WHOLE-FIELD INTEGRATION, NOT DETAILED ANALYSIS, IS USED BY THE CRAB OPTOKINETIC SYSTEM TO SEPARATE ROTATION AND TRANSLATION IN OPTIC FLOW

For optimal visual control of compensatory eye movements during locomo tion it is necessary to distinguish the rotational and translational c omponents of the optic flow field. Optokinetic eye movements can reduc e the rotational component only, making the information contained in t he translational flow readily available to the animal. We investigated optokinetic eye rotation in the marble rock crab, Pachygrapsus marmor atus, during translational movement, either by displacing the animal o r its visual surroundings. Any eye movement in response to such stimul i is taken as an indication that the system is unable to separate the translational and the rotational components in the optic flow in a mat hematically perfect way. When the crabs are translated within a pseudo -natural environment, eye movements are negligible, especially during sideways translation. When, however, crabs were placed in a gangway be tween two elongated rectangular sidewalls carrying dotted patterns whi ch were translated back and forth, marked eye movements were elicited, depending on the translational velocity. To resolve this discrepancy, we tested several hypotheses about mechanisms using detailed analysis of the optic flow or whole-held integration. We found that the latter are sufficient to explain the efficient separation of translation and rotation of crabs in quasi-natural situations.