An important question in sensory neuroscience is what coding strategies and mechanisms are used by the brain to detect and discriminate among behaviorally relevant stimuli. There is evidence that sensory systems migrate from a distributed and redundant encoding strategy at the periphery to a more heterogeneous encoding in cortical structures. It has been hypothesized that heterogeneity is an efficient encoding strategy that minimizes the redundancy of the neural code and maximizes information throughput. Evidence of this mechanism has been documented in cortical structures. In this study, we examined whether heterogeneous encoding of complex sounds contributes to efficient encoding in the auditory midbrain by characterizing neural responses to behaviorally relevant vocalizations in the mouse inferior colliculus (IC). We independently manipulated the frequency, amplitude, duration, and harmonic structure of the vocalizations to create a suite of modified vocalizations. Based on measures of both spike rate and timing, we characterized the heterogeneity of neural responses to the natural vocalizations and their perturbed variants. Using information theoretic measures, we found that heterogeneous response properties of IC neurons contribute to efficient encoding of behaviorally relevant vocalizations.
Reinterpretation of research on the electric sense in aquatic organisms with ampullary organs results in the following conclusions. The detection limit of limnic vertebrates with ampullary organs is 1 microV cm(-1), and of marine fish is 20 nV cm(-1). Angular movements are essential for stimulation of the ampullary system in uniform d.c. fields. Angular movements in the geomagnetic field also generate induction voltages, which exceed the 20 nV cm(-1) limit in marine fish. As a result, marine electrosensitive fish are sensitive to motion in the geomagnetic field, whereas limnic fish are not. Angular swimming movements generate a.c. stimuli, which act like the noise in a stochastic resonance system, and result in a detection threshold in marine organisms as low as 1 nV cm(-1). Fish in the benthic space are exposed to stronger electric stimuli than fish in the pelagic space. Benthic fish scan the orientation plane for the maximum potential difference with their raster of electroreceptor organs, in order to locate bioelectric prey. This behaviour explains why the detection threshold does not depend on fish size. Pelagic marine fish are mainly exposed to electric fields caused by movements in the geomagnetic field. The straight orientation courses found in certain shark species might indicate that the electric sense functions as a simple bisensor system. Symmetrical stimulation of the sensory raster would provide an easy way to keep a straight course with respect to a far-field stimulus. The same neural mechanism would be effective in the location of a bioelectric prey generating a near-field stimulus. The response criteria in conditioning experiments and in experiments with spontaneous reactions are discussed.
A large range of aquatic vertebrates employs passive electroreception to detect the weak bioelectric fi elds that surround their prey. Bioelectric fi elds are dynamic in strength and frequency composition, but typically consist of a direct current (DC) and an alternating current (AC) component. We examined the biological relevance of these components for prey detection behaviour in the brown bullhead by means of a preference test. We gave each fi sh the choice between two small dipoles emitting a DC step or AC stimulus of variable strength, respectively. We used AC stimuli that were either representative for venti latory movements by prey (1 Hz sine wave) or optimal for the ampullary electroreceptor cells (10 Hz sine wave). In an attempt to present a more complex stimulus, we also used slightly modifi ed recordings of bioelectric prey fi elds, but this yielded no results. Brown bullheads prefer DC stimuli to 10 Hz sine waves if the stimulus intensity of either component is much larger. When the stimulus presentation consists of DC versus 1 Hz, most fi sh will choose randomly unless the stimulus intensities diff er greatly. Th en, they favour the component that had a higher amplitude during training. Our results suggest an intrinsic behavioural preference for very low frequency signals (<10 Hz) as well as plasticity in prey detection behaviour.
Heart rate deceleration (HRD) after exposure to novel stimuli is part of the orienting refl ex, and can be used as a tool to investigate the susceptibility of various organisms to sensory stimuli. HRD as response criterion was used in unrestrained catfi sh, Ameiurus ( Ictalurus ) nebulosus (Lesueur, 1819) to investigate its susceptibility to electrical stimuli. HRD in catfi sh occurs after stimulation with light, mechanical stimuli, and electrical stimuli. HRD shows habituation and correlates with stimulus strength. Th e response to sinusoidal electrical stimuli from 70 to 700 μV/cm p-p was determined in the range from 0.1 to 1000 Hz. Using HRD as response criterion we found that at 85 μV/cm catfi sh react to stimuli from 0.1 to 3 Hz. In the absence of stimuli, the heart rate develops an ultradian rhythm with periods of 7 to 15 min. About twice a day cardiac arrest of 1 min occurs. During anaesthesia oscillations with a period of about 1 min are recorded. Comparison of this study with others supports the notion that there exist at least two neural channels for processing electrical stimuli. One channel is involved in predation, namely processing the fast potential changes accompanying the passage of a bioelectric dipole; another is involved in processing uniform DC fi elds used for navigation.
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