Biosonar spatial resolution along the distance axis: revisiting the clutter interference zone

Many echolocating bats forage close to vegetation - a chaotic arrangement of prey and foliage where multiple targets are positioned behind one another. Bats excel at determining distance: they measure the delay between the outgoing call and the returning echo. In their auditory cortex, delay-sensitive neurons form a topographic map, suggesting that bats can resolve echoes of multiple targets along the distance axis - a skill crucial for the forage-amongst-foliage scenario. We tested this hypothesis combining an auditory virtual reality with formal psychophysics: we simulated a prey item embedded in two foliage elements, one in front of and one behind the prey. The simulated spacing between 'prey' (target) and 'foliage' (maskers) was defined by the inter-masker delay (IMD). We trained Phyllostomus discolor bats to detect the target in the presence of the maskers, systematically varying both loudness and spacing of the maskers. We show that target detection is impaired when maskers are closely spaced (IMD<1 ms), but remarkably improves when the spacing is increased: the release from masking is approximately 5 dB for intermediate IMDs (1-3 ms) and increases to over 15 dB for large IMDs (≥9 ms). These results are comparable to those from earlier work on the clutter interference zone of bats (Simmons et al., 1988). They suggest that prey would enjoy considerable acoustic protection from closely spaced foliage, but also that the range resolution of bats would let them 'peek into gaps'. Our study puts target ranging into a meaningful context and highlights the limitations of computational topographic maps.