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 outgoing call and 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 about 5 dB for
intermediate IMDs (1-3 ms) and increases to over 15 dB for large IMDs (≥ 9
ms). These results are well comparable to earlier work on bats'
clutter interference zone (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.