Neural map of interaural phase difference in the owl's brainstem.

Advances in Experimental Medicine and Biology

Ashida

Wagner

et al. 2016

Axons from the nucleus magnocellularis (NM) and their targets in nucleus laminaris (NL) form the circuit responsible for encoding interaural time difference (ITD). In barn owls, NL receives bilateral inputs from NM, such that axons from the ipsilateral NM enter NL dorsally, while contralateral axons enter from the ventral side. These afferents act as delay lines to create maps of ITD in NL. Since delay-line inputs are characterized by a precise latency to auditory stimulation, but the postsynaptic coincidence detectors respond to ongoing phase difference, we asked whether the latencies of a local group of axons were identical, or varied by multiples of the inverse of the frequency they respond to, i.e., to multiples of 2π phase. Intracellular recordings from NM axons were used to measure delay-line latencies in NL. Systematic shifts in conduction delay within NL accounted for the maps of ITD, but recorded latencies of individual inputs at nearby locations could vary by 2π or 4π. Therefore microsecond precision is achieved through sensitivity to phase delays, rather than absolute latencies. We propose that the auditory system "coarsely" matches ipsilateral and contralateral latencies using physical delay lines, so that inputs arrive at NL at about the same time, and then "finely" matches latency modulo 2π to achieve microsecond ITD precision.

Section: Introductionsupporting

confidence: 60%

The Role of Conduction Delay in Creating Sensitivity to Interaural Time Differences

Advances in Experimental Medicine and Biology

Ashida

Wagner

et al. 2016

“…Interaural time difference is mapped in the dorso-ventral direction in the barn owl nucleus laminaris, and not in the mediolateral direction (Fig. 3, [62,63,111]). The organization of the low best frequency region of barn owl nucleus laminaris is closer to the basal land bird pattern, except that the neurons are not neatly arranged in a cellular monolayer, with their dendrites polarized in the dorso-ventral dimension.…”

Section: Phase-locking Below 2 Khzmentioning

confidence: 99%

“…The possible absence of systematic delay lines in the low-frequency circuit is not in conflict with the presence of ITD sensitivity, where binaural inputs are all that is required. Interdigitating nucleus magnocellularis axons are only necessary to form a map of ITD, as shown for the 4-7.5 kHz region of barn owl nucleus laminaris [20,111].…”

Section: Phase-locking Below 2 Khzmentioning

confidence: 99%

“…Collaterals from the ipsi-and contralateral nucleus magnocellularis, entering the nucleus laminaris from its dorsal and ventral border, respectively, interdigitate across its thickness and form multiple delay lines along each isofrequency band [19,20]. Recordings in the barn owl's nucleus laminaris have shown that this innervation pattern generates multiple maps of the same ITD, each covering 150-200 ls [19,20,87,111].…”

Section: Delay Line-coincidence Detection Circuits In Birdsmentioning

confidence: 99%

“…Despite the differences in organization of nucleus laminaris in owls and chickens, delay lines and coincidence detectors underlie ITD sensitivity in both species [29,48,87,111]. Very similar principles apply to the coincidence detector neurons of the mammalian superior olive [36,128], but there is only anatomical evidence for delay lines in mammals, and the existence of a map of interaural time differences is still in question.…”

Section: Coincidence Detectors In Birds and Mammalsmentioning

confidence: 99%

See 2 more Smart Citations

Coding interaural time differences at low best frequencies in the barn owl

Journal of Physiology-Paris

Köppl

2004

In birds and mammals, precisely timed spikes encode the timing of acoustic stimuli, and interaural acoustic disparities propagate to binaural processing centers. The Jeffress model proposes that these projections act as delay lines to innervate an array of coincidence detectors, every element of which has a different relative delay between its ipsilateral and contralateral excitatory inputs. Thus, interaural time difference (ITD) is encoded into the position of the coincidence detector whose delay lines best cancel out the acoustic ITD. Neurons of the avian nucleus laminaris and mammalian MSO phase-lock to both monaural and binaural stimuli but respond maximally when phase-locked spikes from each side arrive simultaneously, i.e. when the difference in the conduction delays compensates for the ITD. McAlpine et al. [Nat. Neurosci. 4 (2001) 396] identified an apparent difference between avian and mammalian ITD coding. In the barn owl, the maximum firing rate appears to encode ITD. This may not be the case for the guinea pig, where the steepest region of the function relating discharge rate to interaural time delay (ITD) is close to midline for all neurons, irrespective of best frequency (BF). These data suggest that low BF ITD sensitivity in the guinea pig is mediated by detection of a change in slope of the ITD function, and not by maximum rate. We review coding of low best frequency ITDs in barn owls and mammals and discuss whether there may be differences in the code used to signal ITD in mammals and birds.

Development of the time coding pathways in the auditory brainstem of the barn owl

Boudreau

1996

J. Comp. Neurol.

The barn owl's head grows after hatching, causing interaural distances to more than double in the first 3 weeks posthatch. These changes expose the bird to a constantly increasing range of interaural time cues. We have used Golgi and ultrastructural techniques to analyze the development of the connections and cell types of the nucleus magnocellularis (NM) and the nucleus laminaris (NL) with reference to the growth of the head. The time coding circuit is formed but immature at the time of hatching. In the month posthatch, the auditory nerve projection to the NM matures, and appears adult-like by posthatch day (P)21. NM neurons show a late growth of permanent dendrites starting at P6. Over the first month, these dendrites change in length and number, depending upon rostrocaudal position, to establish the adult pattern in which high best frequency neurons have few or no dendrites. These changes are not complete by P21, when NM neurons still have more dendrites than in the adult owl. The neurons of NL have many short dendrites before hatching. Their number is greatly reduced by P6, and then does not change during later development. Like NM neurons, NL neurons and dendrites grow in the first month posthatch, and at P21, NL dendrites are longer than those in the adult owl. Thus, the auditory brainstem circuits grow in the first month after hatching, but are not yet mature at the time the head reaches its adult size.