Article Figures & Data
Figures
- Fig. 1.
Cochlear and neural delays. A, Neural delays. A schematic representation of the coincidence detection circuit in the barn owl is shown. Axons from the ipsilateral NM (Ipsi NM) enter the NL on the dorsal surface; those from the contralateral NM (Contra NM) enter the NL on the ventral surface. The segments of these axons within the NL serve as delay lines. Binaural coincidence detectors (numbered1–5) fire maximally when inputs from the two sides arrive simultaneously. A coincidence occurs when the sum of the acoustic delay and neural delay on one side equals that on the other side. Neuron 1 fires maximally when the sound reaches the contralateral ear first (contra-leading ITD) because a longer path delays the neural signal from the ipsilateral ear. If the sound source moves toward the ipsilateral side (ipsi-leading ITD), a coincidence occurs in neurons 2–5 with shorter axonal paths. This array of delays forms a map of the ITD in the dorsoventral dimension of the NL. B, Cochlear delays computed from frequency-dependent latencies of primary auditory fibers of barn owls (Köppl, 1997; in conjunction with the cochlear model of Carney and Yin, 1988). The cochlear delay changes as a function of BF (indicated for each plot) and is the basis for the stereausis theory (Shamma et al., 1989). Note that the change in delay is smaller for higher frequencies.
- Fig. 2.
Frequency-tuning curves. Threshold (A) and isointensity (B) tuning curves have similar CFs and BFs. ABI, Average binaural intensity (equaling sound levels at two ears divided by 2).
- Fig. 3.
Relationships between preferred frequency and tuning width. A, W50 plotted against BFs.B, Q10 plotted against CFs. Frequency tuning sharpens as CF increases (increasing Q10).
- Fig. 4.
Binaural frequency tuning determined by different methods. Frequencies derived from the period of broadband ITD curves are plotted against BFs (A) and the center frequency (B) obtained for binaural stimuli. The center frequency is a better approximation of the frequency derived from the period of the ITD curve than is BF.
- Fig. 5.
Distribution of frequency mismatches and the preferred ITD. A, Most neurons showed a left–right frequency mismatch of ≤200 Hz (n = 21).B, Prediction of how the best ITD must change as a function of frequency mismatch and BF using frequency-dependent latencies of primary auditory fibers for the owl (Köppl, 1997). BF was changed in steps of 0.5 kHz and is indicated for the lower (3 kHz) and upper (8.5 kHz) limits. C, Differences in the ipsilateral and contralateral center frequencies of 31 NL neurons are plotted against their best ITDs (filled circles). There is no correlation between frequency mismatch and ITD tuning. Error bars indicate SDs. The open circles show the interaural frequency mismatches necessary for encoding ITDs by cochlear delays for each neuron [based on the Bonham-Lewis model adapted for the owl (Köppl, 1997; Bonham and Lewis, 1999]. D, The correlation between predicted and observed ITDs showed a correlation of r = 0.38 (t(29) = 2.23; p < 0.05) (solid line). Note that most of the positive correlation is attributable to three neurons. Predicted and observed ITDs also covaried with BF. Removing the effect of BF resulted in a nonsignificant correlation between predicted and observed ITDs (see Results). The dashed line shows a perfect fit between predicted and observed ITDs.
- Fig. 6.
Topographic distribution of ITD tuning in the NL. Data from the present work are indicated by open circlesand a regression line (solid line), indicating the change in ITD tuning per micron of depth. For comparison, a regression line (dashed line) from a previous study of magnocellular axons is shown (Carr and Konishi, 1990). Negative ITD tuning shifts indicate changes toward ipsilateral side leading. Differences in depth were calculated by subtracting the smaller depth from the larger depth.
