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Perceiving and Avoiding Collisions Across Species: How many
Mechanisms are Intrinsically Requisite?
Matthias S. Keil
[email protected]
Joan López Moliner [email protected]
Institute for Brain, Cognition and Behavior (IR3C)
Faculty for Psychology, Basic Psychology Department
Many different species (from insects to humans) show avoidance reactions with approaching objects on
a collision course. Collision avoidance is thought to involve different neuronal mechanisms than
computation of time-to-contact (ttc), which is deemed important for catching balls, or when birds of
prey are hunting. Several desirable properties made the TAU-function (defined by dividing an object's
angular size on the retina through its rate of expansion) the presently most popular model for ttc
perception (and corresponding neuronal computations): At each moment of the approach, it directly
computes an approximation of ttc, which is independent of object size.
Neuronal recordings from insects, however, suggested a completely different model for collision
avoidance, the so-called ETA-function. Its most distinctive property is that it reveals an activity peak
before collision, which is related to a threshold of angular size where avoidance reactions are triggered.
Moreover, in pigeons, the simultaneous occurrence of “ETA-function neurons” and “TAU-function
neurons” has been reported (apart from neurons which signal rate of expansion). But how could it be
that such different functions evolved in one and the same brain?
We provide a possible answer to this question with a new mathematical model, which produces TAUresponses and ETA-like responses as a consequence of different parameter values. Our model has noise
suppression properties that are superior to TAU & ETA-function, and makes correct predictions of size
and velocity effects. It consistently explains the results of a psychophysical experiment, where
subjects had to indicate ttc with respect to a reference, for different observation times of an
approaching object. We were also able to fit several (published) neuronal response curves from locusts
(and one bullfrog).
In conclusion, the new model indicates that collision avoidance and ttc perception are based on similar
neuronal circuits, thus providing insights into corresponding evolutionary mechanisms.