Effects of Interaural Intensity Difference on the Processing of Interaural Time Difference in the Owl’s Nucleus Laminaris

Abstract

Interaural time and intensity differences (ITD and IID) are processed independently in the owl's auditory system. This paper examines whether this independence is established in nucleus laminaris (NL), the first site of ITD processing. A plot of discharge rate against time difference (ITD curve) is sinusoidal in NL. The ITDs that produce the peaks are called the most favorable ITDs, and those that produce the troughs are called the least favorable ITDs. IID had little effect on the discharge rates of laminaris neurons for the most and least favorable ITDs. The degree of peak-trough modulation changed slightly with variation in IID. In contrast, IID in tonal stimuli affected the temporal aspect of ITD curves depending on the difference between the stimulus frequency and the neuron's best frequency (BF). For frequencies below BF, IID caused large and systematic shifts in ITD toward the ear in which the sound was louder, whereas for frequencies above BF, IID caused small shifts in ITD toward the opposite ear. IID had little effect on ITD curves taken with BF or broadband noise. These results can be largely accounted for by the effects of frequency and intensity on the timing of impulses at the level of the cochlear nuclei. Thus, the processing of ITD by NL neurons is independent of IID for behaviorally relevant stimuli, because the timing of impulses is insensitive to sound level when the signal is broadband. Key words: owl; sound localization; nucleus laminaris; interaural time difference; interaural intensity difference; parallel pathwaysParallel processing of information is an important operation in many sensory systems (see, for example, owl's auditory system: Takahashi et al., 1984; monkey visual system: Maunsell et al., 1990;Merigan and Maunsell, 1993; fish electrosensory system: Heiligenberg, 1991). The barn owl's auditory brainstem offers a relatively simple system in which to analyze the relationship between parallel pathways. The owl uses interaural time and intensity differences (ITD and IID) for sound localization (Moiseff, 1989b). The independence of these cues is important for the species, because they code for different spatial coordinate axes, ITD mainly for azimuth and IID mainly for elevation. ITD and IID are processed in separate parallel pathways that start at the level of the cochlear nucleus. The avian auditory system has two anatomically and physiologically distinct cochlear nuclei, nucleus magnocellularis (NM) and nucleus angularis (NA). NM neurons lack dendrites and contain end bulbs of Held, whereas NA neurons have dendrites and bouton-type synapses (Parks, 1981;Jhaveri and Morest, 1982;Takahashi and Konishi, 1988a;Carr and Boudreau, 1993). NM neurons show phase locking, whereas NA neurons do not except for very low frequencies (Sachs and Sinnott, 1978;Sullivan and Konishi, 1984a;Konishi et al., 1985;Warchol and Dallos, 1990). The time-and intensity-processing pathways converge in the inferior colliculus, where neurons selective for combinations of ITD and IID are found. Partial inactivat...

Maps of interaural time difference in the chicken’s brainstem nucleus laminaris

“…Acceptable data according to these criteria usually fell within a range of 0.2-0.5 octaves around CF (median 0.33 octaves). The neurophonic amplitudes or, for single units, the spike rate, as a function of lTD were then fitted with a cosine function at the respective stimulus frequency (Viete et al 1997) to determine best IPD, defined as the peak closest to zero IPD. In cases where the minimum fell close to zero IPD and it was thus ambiguous which peak defined the best IPD, click responses and the CD (see next) were used to resolve the laterality.…”

Tonotopic Specialization of Auditory Coincidence Detection in Nucleus Laminaris of the Chick

“…Comparison with in vivo studies in the barn owl We reevaluated the ITD sensitivity of NL neurons of the barn owl on the frequency axis (1-8 kHz) by recalculating the time window and the percentage maximum slope (see definition in Results) from the reported ITD response curve (Carr and Konishi, 1990;Pena et al, 1996;Viete et al, 1997). The ITD sensitivity is critically dependent on the tonal period when the ITD response curve was normalized between the peak and the baseline activity (Fujita and Konishi, 1991).…”

Modeling coincidence detection in nucleus laminaris

“…As a result, NL, primarily in chick and barn owl, has already been the objective of many anatomical (Rubel and Parks 1975;Smith and Rubel 1979;Smith 1981;Rubel 1984, 1989a, b) and electrophysiological experiments (Carr and Konishi 1990;Warchol and Dallos 1990;Overholt et al 1992;Pena et al 1996;Reyes et al 1996;Viete et al 1997;Bruckner and Hyson 1998;Funabiki et al 1998;Yang et al 1999;Monsivais et al 2000;Pena et al 2001;Kuba et al 2002;Cook et al 2003). This wealth of information has also made it the target of several modeling efforts (Grün et al 1990;Agmon-Snir et al 1998;Dasika et al 1999;Simon et al 1999a, b;Dasika et al 2001;Simon et al 2001a, b;Cook et al 2003;Carr et al in press).…”