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Seeking Secrets of the Sonar of Dolphins
by DouglasMoreman

This inventor has a partial theory, about the sonar of dolphins, that guides his designing of software for imaging sonar. The chin, and perhaps other parts of the face, of a bottlenose dolphin contain, in this theory, "echotriggers," sensors that are exquisitely and especially sensitive to echoes of the consistently-shaped "clicks" that dolphins of that species emit. The echotriggers, responding to features of the clicks, send brief signals directly to "modeling neurons" in the brain, by-passing the cochlea entirely. An hypothesized "sonic retina" in the brain has at least one thin layer L of modeling neurons, like a layer in the retina of an eye. For each modeling neuron N in L, there is imagined to be, in front of the dolphin, a thin conical region D of rays, directions from the head, and an interval [R1,R2] of ranges such that N is fired by a click that arrives from some echoing point in a direction in D and at a distance from the dolphin that is in [R1,R2]. There is, for neuron N, a set S of echotriggers such that the firing of N results from the nearly enough simultaneous firings of a sufficient subset of S. When N fires, this somehow indicates to some part of the brain that an object exists roughly in the direction of the axis of the cone D. Range-resolution can be augmented by cues and neuronal layers not discussed herein. The hypothesized modeling neurons of the biosonar might be analogous to ganglion neurons in the retina of an eye.
Numbers of neurons in an eye give an indication of possible numbers of neurons involved in sonic imaging. The more than 90 million photoreceptors of a human retina communicate with more than a million retinal ganglion neurons and these communicate into the brain. The tip of a human finger can contain more than 1000 touch-sensors; so, it seems plausible that there are 1000 or more echotriggers in the chin of a dolphin.