Sound localization in anurans. II. Binaural interaction in superior olivary nucleus of the green tree frog (Hyla cinerea)

Abstract

In the evolution of the vertebrate auditory system, the superior olivary nucleus made its 1st appearance in anurans (frogs and toads). To gain more insight into the role played by this center, monaural and binaural response properties of 146 single neurons were studied in the superior olivary nucleus of the green tree frog H. cinerea. Of the neurons studied, 58% responded to monaural stimulation only; most of this population was excited by the contralateral ear. The remaining 42% of the neurons were sensitive to binaural stimulation. These binaural cells consisted of 3 different types: EE (n = 11), EI (n = 44) and complex (n = 6). Most of the cells were tuned to relatively narrow frequency bands with Q10 values ranging from 0.25-5. The distribution of best excitatory frequencies comprised 4 groups centered around 500, 1100, 1900 and 3000 Hz. The excitatory thresholds were scattered between 6 and 91 dB SPL [sound pressure level]. Two-thirds of the superior olivary neurons responded tonically throughout the duration of an excitatory tone burst; the rest of them responded to the transient features of a tone. Response latencies to tone bursts (1 ms rise-fall time) at 10 dB above threshold ranged from 5-38 ms. EE cells showed 3 types of binaural interaction patterns to tone bursts of equal intensities and times of arrival at the 2 ears: binaural summation, binaural facilitation and monaural dominance. Several EE cells were insensitive to interaural intensity differences (IID); their sensitivity to interaural time differences (ITD) was not examined. EI cells comprised the majority of the binaural population. Most (93%) of these cells were excited by contralateral stimulation and inhibited by ipsilateral stimulation. Almost all of the EI cells were sensitive to IID regardless of their best excitatory frequencies, but only the low-frequency-sensitive cells showed any sensitivity to relatively large ITD. The responses to IID and ITD were graded and maximal when the contralateral stimulus led or was more intense than the ipsilateral stimulus. The response properties of cells in the anuran''s superior olivary nucleus resemble those in the mammalian superior olive. The significance of this center in sound localization is discussed.