Independence of Echo-Threshold and Echo-Delay in the Barn Owl
(A) Overlapping lead (direct) and lag (simulated reflection) sounds. The temporal overlap defines periods of time during which both sounds were superposed, flanked by periods when the lead or lag sources were present alone. (B) Stimuli presented in the standard precedence effect, paradigm. The lead (gray) and lag (black) sounds were of equal length (30 ms) and onset-delay was 1.5, 3, 6, 12, or 24 ms. (C) Stimuli in which lag-alone segments were experimentally lengthened or shortened while maintaining a constant duration lead-alone segment (constant lead/lag delay). Lead and lag sounds were of unequal lengths. (D) Stimuli in which lead-alone segments were experimentally lengthened or shortened while maintaining a constant duration lag-alone segment (converse of C). When the lead-alone segment was 24 ms, the length of the lag-alone segment was shortened to 12 ms only in our physiological experiments (indicated by asterisks). (E) Single-source sounds among which paired-source stimuli were randomly interspersed in our behavioral experiments. Their durations were roved from 6–54 ms to invalidate duration as a possible cue. (F) Placement of sound sources in our physiological experiments. The plot represents the frontal hemisphere of the owl's auditory space . Positive azimuths and elevations correspond to loci to the right and above an owl, respectively. A cell's SRF is shown in pseudo-color along with a scale bar indicating the average spike number over 4 repetitions. The source in the optimal location within the cell's SRF is referred to as the target. A second source placed at a location diametrically opposed across the owl's center of gaze from the target is referred to as the masker. In the experiments, the target or masker could lead, allowing us to examine a cell's response to simulated direct sounds and echoes.