First come, first served: Neuronal processing of multi-echo streams in the auditory cortex of echolocating bats

Echolocating bats emit acoustic pulses that get reflected off objects. The spatial information carried by the echoes enable bats to avoid obstacles in darkness. Usually, every pulse is followed by a cascade of echoes arising from multiple objects. By using echolocation sequences where a single pulse is followed by an echo cascade, we recently demonstrated that cortical neurons predominantly respond to the leading echo. Responses to lagging echoes from a cascade were suppressed suggesting that spatial information from the most immediate object are processed at the cortex level. In that study, the leading echo was typically the most intense, leaving it unclear whether the echo selectivity was due to echo order or echo level. Here we record from the auditory cortex of anaesthetized Carollia perspicillata, while stimulating the bats with echolocation sequences that contained echo cascades whose echo levels were either equally intense or where the leading echo was less intense than the lagging ones. Our results demonstrate that the echo level has only minor effects on neural processing and that the echo selectivity is mostly caused by the echo order. These results go in line with the neural time window of sensation hyothesis, proposed by Roverud and Grinnell. Whenever the bat hears a pulse, a neural time window opens, and any subsequent high-frequency signal within the spectral range of that pulse is by default classified as an echo, thereby closing the sensation window. This mechanism renders large parts of the cortex less responsive to distant objects, regardless of the echo intensity they produced.