Abstract
Echolocating animals adjust the transmit intensity and receive sensitivity of their sonar in order to regulate the sensation level of their echoes; this process is often termed automatic gain control. Gain control is considered not to be under the animal's cognitive control, but previous investigations studied animals ensonifying targets or hydrophone arrays at predictable distances. To test whether animals maintain gain control at a fixed level in uncertain conditions, we measured changes in signal intensity for a bottlenose dolphin (Tursiops truncatus) detecting a target at three target distances (2.5, 4 and 7 m) in two types of sessions: predictable and unpredictable. Predictable sessions presented the target at a constant distance; unpredictable sessions moved the target randomly between the three target positions. In the predictable sessions the dolphin demonstrated intensity distance compensation, increasing the emitted click intensity as the target distance increased. Additionally, as trials within sessions progressed, the animal adjusted its click intensity even from the first click in a click train, which is consistent with the animal expecting a target at a certain range. In the unpredictable sessions there was no significant difference of intensity with target distance until after the 7th click in a click train. Together, these results demonstrate that the bottlenose dolphin uses learning and expectation for sonar gain control.
Highlights
Bats and toothed whales are model organisms for the investigation of sensory processing
Our results indicate gain control is dependent on the animal’s expectation of target distance
Echolocation trials began with low source levels, increased rapidly in amplitude over approximately five clicks, reached a maximum source level, declined in amplitude for the remainder of each trial (Figure 2). Due to this pattern, comparing averaged source levels across conditions would mask changes in source levels that occurred through the click train
Summary
Bats and toothed whales are model organisms for the investigation of sensory processing. These two animal groups convergently evolved echolocation, an active sense relying on the integration of auditory, vocal and motor systems. A target at 100 m, the detection limit of the bottlenose dolphin [2], would return an echo more than 80 dB quieter than the outgoing signal [1]. Processing such a large range of echo intensities poses a challenge for the animal’s auditory system. Echolocators maintain a constant perceived echo level by changing both the transmit and receive sonar systems [3,4,5,6,7,8]
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