Organization of central auditory pathways in a teleost fish,Cyprinus carpio

Summary

• 1. Central auditory pathways were mapped in carp,Cyprinus carpio, by analyzing depth profiles of acoustic evoked field potentials (AEPs) and multiunit activity (MUA) in response to click stimuli. Central nuclei which respond to sound are not activated by lateral line stimulation, indicating that separate pathways are involved in processing these two sensory modalities in carp.

• 2. Two octaval nuclei within the carp medulla respond to sound, the descending octaval nucleus (DON) (Fig. 2) and the anterior octaval nucleus (AON) (Fig. 3). Single clicks produce localized MUA and negative AEPs with peak latencies of 6–8 ms in both nuclei.

• 3. In the midbrain, best AEP and MUA responses to auditory stimuli are found within the medial torus semicircularis (TS) (Fig. 4). A monophasic negative AEP, with a latency of 8–14 ms, is particularly prominent within the upper layers of the torus. Earliest unit responses begin at 7 ms.

• 4. Auditory activity within the carp diencephalon is encountered during electrode penetrations of the central posterior thalamic nucleus (CP) (Fig. 5), one of three nuclei comprising the dorsal thalamus. The thalamic field potential is a monophasic negative wave with a peak latency ranging from 18 to 23 ms. Auditory units within the thalamus have an initial response latency of 17–19 ms.

• 5. Two auditory areas were found within the carp telencephalon. The first, believed to receive direct input from the auditory thalamus, is within a central zone (Dc) of the caudal dorsal telencephalon (Fig. 6). In this region, click stimuli produce a biphasic field potential with an initial negative peak at a latency of 35–44 ms. Maximal field potentials and multiunit activity are present at depths of 600–900 μm below the telencephalic surface. Longer latency biphasic field potentials and multiunit activity were recorded from a second auditory area located 500 μm caudal to this Dc field, within the ipsilateral dorsomedial (Dm) telencephalon (Fig. 7). Acoustic field potentials and unit latencies ranged from 46 to 69 ms. Neuronal responses to lateral line stimulation were also recorded within this telencephalic region. Some spatial segregation of auditory and lateral line units was apparent; lateral line units were recorded from the surface of the telencephalon to depths of 200 μm and acoustic units were found at depths of 100–400 μm.

• 6. In response to paired clicks (Fig. 8), AEPs recorded from medullary and midbrain auditory nuclei show a depression at intervals of less than 20 ms, but a rapid recovery of AEP amplitude at longer stimulus intervals, with 80–100% recovery at intervals of 80–100 ms. In contrast, auditory nuclei within the diencephalon and the telencephalon are extremely refractory to paired stimuli, with only 40–50% recovery at stimulus intervals of 100 ms. A similar finding was observed in response to click trains (Fig. 9).