Long-latency optical responses from the dorsal inferior colliculus of Seba's fruit bat

We used a novel microendoscope system to record simultaneously optical activity (fluorescence of a calcium indicator dye) and electrical activity (multi-unit activity and local field potentials) from the dorsal inferior colliculus of the echolocating bat, Carollia perspicillata. Optically recorded calcium responses to wide-band noise and to frequency-modulated bursts were recorded at probe depths down to 1300 µm, with the majority of active sites encountered at more shallow depths down to 800 µm. Calcium activity exhibited long latencies, within the time span of 50-100 ms after stimulus onset, significantly longer than onset latencies of either multi-unit activity or local field potentials. Latencies and amplitude/latency trading of these electrical responses were consistent with those seen in standard electrophysiological recordings, confirming that the microendoscope was able to record both neural and optical activity successfully. Optically recorded calcium responses rose and decayed slowly and were correlated in time with long-latency negative deflections in local field potentials. These data suggest that calcium-evoked responses may reflect known, sustained inhibitory interactions in the inferior colliculus.