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V. Pigeons compute distance and direction independently

Suppose that arrows represent conflicting vectors dictating where to search for a target. A weighted average of the entire vectors lies on the straight line joining the endpoints of the arrows. Independent averaging of distance and direction separately leads to a point on the arc joining the two endpoints, where the length of the averaged vector stays the same.

In the above training situation, the end of arrow represents a target's position. When the black bar is turned in the crucial test, its dictate conflicts 90 degrees with all other landmark. Results of experiments like this, favor the thesis of independent averaging in pigeons (Cheng, 1994) and honeybees (Cheng, 1998).

Independent averaging suggests separate modular systems, one devoted to analyzing distance from a landmark, and another devoted to analyzing direction from a landmark. Separate systems make functional sense when the cues relied on by each system are different, and the tasks can be done separately without compromising the overall goal. Modular functional architecture has the ‘evolutionary advantage' in that each system can be ‘separately tuned' to do its task better and better without compromising the performance of another system.

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