Laminar Distribution of Spectrotemporal Receptive Field Subtypes in the Primary Auditory Cortex of Echolocating Bats.

The primary auditory cortex (A1) is crucial for processing sound features and integrating multidimensional information. Evidence shows that cortical microcircuits in Layers 2/3 of A1 selectively integrate spectrotemporal information to create networks that respond to complex sounds in a species-specific manner, but many fundamental details about this neurocomputational process remain unknown. The bat auditory system is primarily adapted to analyze the echoes of their own calls for information about the identity and location of targets. Assuming bat A1 follows typical mammalian organization, we hypothesized that compared to Layer 4, neurons in Layers 2 and 3 would exhibit greater selectivity for complex features used in biosonar target discrimination, namely rapid amplitude modulations embedded in frequency-modulated sweeps. To test this hypothesis, we used microelectrode arrays to examine the laminar distribution of spectrotemporal receptive fields (STRFs) in A1 of adult Mexican free-tailed bats. Layer 4 neurons displayed downward frequency-modulated STRFs resembling biosonar pulses, while neurons in more superficial layers showed multipeaked STRFs resembling spectral echoes of preferred prey. We suggest that cortical microcircuits in bat A1 transform discrete spectral cues arriving in Layer 4 into broadband analyses of echo spectral envelope in Layers 2/3 that favor naturalistic sound features important for foraging bats.