Mating call selectivity in the thalamus and midbrain of the leopard frog (Rana p. pipiens): Single and multiunit analyses

Summary

• 1. A single and multiunit survey was made of the tuning properties of auditory neurons in the rostral midbrain and caudal thalamus of the northern leopard frog,Rana p. pipiens. Lesions of the recording sites revealed auditory neurons were located in the rostrolateral torus semicircularis, pretectal gray, pretectal central gray, optic tectum, and posterocentral and posterolateral thalamic nuclei.

• 2. Single unit analysis of 36 auditory neurons revealed that 10 responded only to specific tone combinations, or responded to single low frequency tones at high intensities (typically greater than 90 dB SPL) but their excitatory thresholds could be decreased as much as 40 dB with the addition of specific tones. All such neurons were similar in that they preferred combinations of low (<600 Hz) and high (>1,200 Hz) frequency tones. They were inhibited or not excited by intermediate frequency tones.

• 3. The majority of neurons responded to single tones and their excitatory thresholds were not decreased by multiple tone stimulation. They were typically broadly tuned, and in some cases displayed bimodal or trimodal excitatory tuning curves. Some could be inhibited by high intensity tones near the center of their excitatory frequency range. Others were totally unresponsive to these intermediate frequencies, resulting in two discontinuous excitatory tuning curves.

• 4. The multiunit survey at 26 recording sites examined responses to various single and multiple tone stimuli composed of 300, 750 and 1,750 Hz tones representing the low, mid and high frequency ranges of the three receptor populations. There was considerable variation in the relative excitatory capacities of these stimuli between recording sites. The trend which emerged was that (1) tone combinations were more effective than single tones, (2) combining tones often produced a non-linear summation of the responses to individual tones, (3) the 300+1,750 Hz stimulus (i.e., the low and high frequency combination) was always among the most excitatory stimuli, and (4) the addition of mid frequency tones often reduced the excitatory capacity of a stimulus.

• 5. A previous evoked potential study of the caudal thalamus (Mudry et al. 1977) reported a non-linear summation of the potential when frequencies within the response ranges of the amphibian (low and mid frequencies) and basilar papillae (high frequencies) were combined. Present single and multiunit analyses extend these results, and suggest that these regions of the anuran auditory system show a particular preference for tone combinations which approximate the spectral energy distribution of the species mating call.