Evidence for parallel processing in the frog's auditory thalamus

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Abstract
We have conducted anatomical and physiological experiments to investigate the functional organization of the dorsal thalamus in the northern leopard frog (Rana pipiens pipiens). Our studies provide evidence for parallel auditory processing at this level of the frog's brain. Acoustically evoked potentials were recorded from the posterior and central thalamic nuclei and several differences in sound‐evoked activity were noted between them: (1) the amplitude of acoustically evoked potentials (AEPs), in response to a standard search stimulus, was always greater in the central, as opposed to the posterior, nucleus; (2) the posterior, but not central, nucleus exhibited the phenomenon of nonlinear summation when 350‐Hz and 1,700‐Hz tones were presented simultaneously rather than individually; and (3) the central, but not posterior, nucleus showed selectivity for the repetition rate of pulsed sound signals. The posterior and central thalamic nuclei also possessed distinct innervation patterns as revealed by the HRP transport patterns arising from these structures. The central nucleus was reciprocally connected with the major auditory relay stations along the frog's central auditory pathway including the superior dive, nucleus of the lateral lemniscus, and the torus semicircularis. Major projections to the lateral thalamic nucleus, ventral hypothalamus, and the telencephalic striatal complex were also observed. The posterior nucleus, on the other hand, established reciprocal connections primarily with the medial reticular nucleus, ventral mid‐brain tegmentum, and structures constituting of the ventral thalamic nuclei, particularly the nucleus of Bellonci. Thus, time and frequency cues contained within the species mating call, and conveying information concerning species identity, appear to be processed independently within the frog's thalamus with separate neural channels for each.