Brain regions controlling courtship behavior in the bluehead wrasse

Bluehead wrasses (Thalassoma bifasciatum) follow a socially controlled mechanism of sex determination. A socially dominant initial-phase (IP) female is able to transform into a new terminal-phase (TP) male if the resident TP male is no longer present. TP males display an elaborate array of courtship behaviors, including both color changes and motor behaviors. Little is known concerning the neural circuits that control male-typical courtship behaviors. This study used glutamate iontophoresis to identify regions that may be involved in courtship. Stimulation of the following brain regions elicited diverse types of color change responses, many of which appear similar to courtship color changes: the ventral telencephalon (supracommissural nucleus of the ventral telencephalon [Vs], lateral nucleus of the ventral telencephalon [Vl], ventral nucleus of the ventral telencephalon [Vv], and dorsal nucleus of the ventral telencephalon [Vd]), parts of the preoptic area (NPOmg and NPOpc), entopeduncular nucleus, habenular nucleus, and pretectal nuclei (PSi and PSm). Stimulation of two regions in the posterior thalamus (central posterior thalamic [CP] and dorsal posterior thalamic [DP]) caused movements of the pectoral fins that are similar to courtship fluttering and vibrations. Furthermore, these responses were elicited in female IP fish, indicating that circuits for sexual behaviors typical of TP males exist in females. Immunohistochemistry results revealed regions that are more active in fish that are not courting: interpeduncular nucleus, red nucleus, and ventrolateral thalamus (VL). Taken together, we propose that the telencephalic-habenular-interpeduncular pathway plays an important role in controlling and regulating courtship behaviors in TP males; in this model, in response to telencephalic input, the habenular nucleus inhibits the interpeduncular nucleus, thereby dis-inhibiting forebrain regions and promoting the expression of courtship behaviors.

Neural basis of acoustic species recognition in a cryptic species complex

Sexual traits that promote species recognition are important drivers of reproductive isolation, especially among closely related species. Identifying neural processes that shape species differences in recognition is crucial for understanding the causal mechanisms of reproductive isolation. Temporal patterns are salient features of sexual signals that are widely used in species recognition by several taxa, including anurans. Recent advances in our understanding of temporal processing by the anuran auditory system provide an opportunity to investigate the neural basis of species-specific recognition. The anuran inferior colliculus consists of neurons that are selective for temporal features of calls. Of potential relevance are auditory neurons known as interval-counting neurons (ICNs) that are often selective for the pulse rate of conspecific advertisement calls. Here, we tested the hypothesis that ICNs mediate acoustic species recognition by exploiting the known differences in temporal selectivity in two cryptic species of gray treefrog (Hyla chrysoscelis and Hyla versicolor). We examined the extent to which the threshold number of pulses required to elicit behavioral responses from females and neural responses from ICNs was similar within each species but potentially different between the two species. In support of our hypothesis, we found that a species difference in behavioral pulse number thresholds closely matched the species difference in neural pulse number thresholds. However, this relationship held only for ICNs that exhibited band-pass tuning for conspecific pulse rates. Together, these findings suggest that differences in temporal processing of a subset of ICNs provide a mechanistic explanation for reproductive isolation between two cryptic treefrog species.

Latency for facultative expression of male-typical courtship behaviour by female bluehead wrasses depends on social rank: the 'priming/gating' hypothesis

Although socially controlled sex transformation in fishes is well established, the underlying mechanisms are not well understood. Particularly enigmatic is behavioural transformation, in which fish can rapidly switch from exhibiting female to male-typical courtship behaviours following removal of 'supermales'. Bluehead wrasses are a model system for investigating environmental control of sex determination, particularly the social control of sex transformation. Here, we show that the onset of this behavioural transformation was delayed in females that occupied low-ranking positions in the female dominance hierarchy. We also establish that expression of male-typical courtship behaviours in competent initial-phase (IP) females is facultative and gated by the presence of terminal-phase (TP) males. Dominant females displayed reliable TP male-typical courtship behaviours within approximately 2 days of the removal of a TP male; immediately following reintroduction of the TP male, however, females reverted back to female-typical behaviours. These results demonstrate a remarkable plasticity of sexual behaviour and support a 'priming/gating' hypothesis for the control of behavioural transformation in bluehead wrasses.

The numerical abilities of anurans and their neural correlates: insights from neuroethological studies of acoustic communication

Acoustic communication is important in the reproductive behaviour of anurans. The acoustic repertoire of most species consists of several call types, but some anurans gradually increase the complexity of their calls during aggressive interactions between males and when approached by females. In these interactions, males may closely match the number of calls or notes in a sequence that a neighbour produces, thereby revealing their numerical abilities. Anurans are also able to discern the number of sequential properly timed pulses (notes). The temporal intervals between successive pulses provide information about species identity and call type. A neural correlate of this numerical ability is evident in the responses of 'interval-counting' neurons, which show 'tuning' for intermediate to fast pulse rates and respond only after at least a threshold number of pulses have occurred with the correct timing. A single interpulse interval that is two to three times the optimal value can reset this interval-counting process. Whole-cell recordings from midbrain neurons, in vivo, have revealed that complex interplay between activity-dependent excitation and inhibition contributes to this counting process. Single pulses primarily elicit inhibition. As additional pulses are presented with optimal intervals, cells become progressively depolarized and spike after a threshold number of intervals have occurred.This article is part of a discussion meeting issue 'The origins of numerical abilities'.

Species specificity of temporal processing in the auditory midbrain of gray treefrogs: long-interval neurons

In recently diverged gray treefrogs (Hyla chrysoscelis and H. versicolor), advertisement calls that differ primarily in pulse shape and pulse rate act as an important premating isolation mechanism. Temporally selective neurons in the anuran inferior colliculus may contribute to selective behavioral responses to these calls. Here we present in vivo extracellular and whole-cell recordings from long-interval-selective neurons (LINs) made during presentation of pulses that varied in shape and rate. Whole-cell recordings revealed that interplay between excitation and inhibition shapes long-interval selectivity. LINs in H. versicolor showed greater selectivity for slow-rise pulses, consistent with the slow-rise pulse characteristics of their calls. The steepness of pulse-rate tuning functions, but not the distributions of best pulse rates, differed between the species in a manner that depended on whether pulses had slow or fast-rise shape. When tested with stimuli representing the temporal structure of the advertisement calls of H. chrysoscelis or H. versicolor, approximately 27 % of LINs in H. versicolor responded exclusively to the latter stimulus type. The LINs of H. chrysoscelis were less selective. Encounter calls, which are produced at similar pulse rates in both species (≈5 pulses/s), are likely to be effective stimuli for the LINs of both species.

Counting on dis-inhibition: a circuit motif for interval counting and selectivity in the anuran auditory system

Information can be encoded in the temporal patterning of spikes. How the brain reads these patterns is of general importance and represents one of the greatest challenges in neuroscience. We addressed this issue in relation to temporal pattern recognition in the anuran auditory system. Many species of anurans perform mating decisions based on the temporal structure of advertisement calls. One important temporal feature is the number of sound pulses that occur with a species-specific interpulse interval. Neurons representing this pulse count have been recorded in the anuran inferior colliculus, but the mechanisms underlying their temporal selectivity are incompletely understood. Here, we construct a parsimonious model that can explain the key dynamical features of these cells with biologically plausible elements. We demonstrate that interval counting arises naturally when combining interval-selective inhibition with pulse-per-pulse excitation having both fast- and slow-conductance synapses. Interval-dependent inhibition is modeled here by a simple architecture based on known physiology of afferent nuclei. Finally, we consider simple implementations of previously proposed mechanistic explanations for these counting neurons and show that they do not account for all experimental observations. Our results demonstrate that tens of millisecond-range temporal selectivities can arise from simple connectivity motifs of inhibitory neurons, without recourse to internal clocks, spike-frequency adaptation, or appreciable short-term plasticity.

Time computations in anuran auditory systems

Temporal computations are important in the acoustic communication of anurans. In many cases, calls between closely related species are nearly identical spectrally but differ markedly in temporal structure. Depending on the species, calls can differ in pulse duration, shape and/or rate (i.e., amplitude modulation), direction and rate of frequency modulation, and overall call duration. Also, behavioral studies have shown that anurans are able to discriminate between calls that differ in temporal structure. In the peripheral auditory system, temporal information is coded primarily in the spatiotemporal patterns of activity of auditory-nerve fibers. However, major transformations in the representation of temporal information occur in the central auditory system. In this review I summarize recent advances in understanding how temporal information is represented in the anuran midbrain, with particular emphasis on mechanisms that underlie selectivity for pulse duration and pulse rate (i.e., intervals between onsets of successive pulses). Two types of neurons have been identified that show selectivity for pulse rate: long-interval cells respond well to slow pulse rates but fail to spike or respond phasically to fast pulse rates; conversely, interval-counting neurons respond to intermediate or fast pulse rates, but only after a threshold number of pulses, presented at optimal intervals, have occurred. Duration-selectivity is manifest as short-pass, band-pass or long-pass tuning. Whole-cell patch recordings, in vivo, suggest that excitation and inhibition are integrated in diverse ways to generate temporal selectivity. In many cases, activity-related enhancement or depression of excitatory or inhibitory processes appear to contribute to selective responses.

Roles of syntax information in directing song development in white-crowned sparrows (Zonotrichia leucophrys)

Syntactical cues play an important role in song learning in songbirds. White-crowned sparrows (Zonotrichia leucophrys), whose song typically consists of four to five different phrases, fail to construct normal songs if exposed to all phrase types presented singly (Plamondon, Goller, & Rose, 2008; Soha & Marler 2001b). The specific role of acquired syntax information in guiding ontogenetic trajectories of syntax, however, and the respective contributions of instructive and selective processes to syntax ontogeny remain unknown. We tutored white-crowned sparrows with syntax information ranging from acoustic isolation to full song. Manipulation of tutor syntax influenced developmental trajectories of syntax assembly, suggesting that instructive processes contribute to syntax ontogeny. Early in development, birds tutored with full song or phrase pairs preferentially produced phrase pairings matching tutor syntax. Birds tutored with single phrases showed decreased diversity of pairwise syntactical combinations immediately after tutoring compared with other tutor groups, further illustrating the role of instructive processes. Overproduction of song material was also observed, suggesting that selective forces play a role in syntax development as well. Finally, consistent with the notion that innate influences guide syntax ontogeny, birds from all groups exhibited many similarities in trajectories of syntax assembly.

Mechanisms of long-interval selectivity in midbrain auditory neurons: roles of excitation, inhibition, and plasticity

Stereotyped intervals between successive sound pulses characterize the acoustic signals of anurans and other organisms and provide critical information to receivers. One class of midbrain neuron responds selectively when pulses are repeated at slow rates (long intervals). To examine the mechanisms that underlie long-interval selectivity, we made whole cell recordings, in vivo, from neurons in the anuran inferior colliculus (anuran IC). In most cases, long-pass interval selectivity appeared to arise from interplay between excitation and inhibition; in approximately 25% of these cases, the delayed inhibition to a pulse overlapped with the excitation to the following pulse at fast pulse repetition rates (PRRs), resulting in a phasic "onset" response. In the remaining cases, inhibition appeared to precede excitation. These neurons did not respond to fast PRRs apparently because delayed excitation to a pulse overlapped with the inhibition to the following pulse. These results suggest that the relative timing of inhibition and excitation govern differences in the response properties of these two cell types. Loading cells with cesium increased their responses to fast AM rates, supporting a role for inhibition in long-interval selectivity. Three cells showed little or no evidence of inhibition and exhibited strong depression of excitation. These findings are discussed in the context of current models for long-pass interval selectivity.

Encoding and processing biologically relevant temporal information in electrosensory systems

Wave-type weakly electric fish are specialists in time-domain processing: behaviors in these animals are often tightly correlated with the temporal structure of electrosensory signals. Behavioral responses in these fish can be dependent on differences in the temporal structure of electrosensory signals alone. This feature has facilitated the study of temporal codes and processing in central nervous system circuits of these animals. The temporal encoding and mechanisms used to transform temporal codes in the brain have been identified and characterized in several species, including South American gymnotid species and in the African mormyrid genus Gymnarchus. These distantly related groups use similar strategies for neural computations of information on the order of microseconds, milliseconds, and seconds. Here, we describe a suite of mechanisms for behaviorally relevant computations of temporal information that have been elucidated in these systems. These results show the critical role that behavioral experiments continue to have in the study of the neural control of behavior and its evolution.

Pulse rise time but not duty cycle affects the temporal selectivity of neurons in the anuran midbrain that prefer slow AM rates

Recovery-type auditory neurons in the anuran inferior colliculus (IC) respond with band-pass or low-pass selectivity for sinusoidal AM. These cells respond to each modulation cycle at slow AM rates and respond only at the onset of fast AM or pulse repetition rate (PRR) stimuli, failing to recover from the effects of early pulses. This selectivity is not altered by changes in pulse duty cycle. The recovery process is governed therefore by the interpulse interval and not the dimension of the gap between sound pulses. Most of these neurons preferred fast rise times, which is characteristic of the sound pulses in the calls of Hyla regilla and Rana pipiens, the two species selected for this study.

Insights into neural mechanisms and evolution of behaviour from electric fish

Both behaviour and its neural control can be studied at two levels. At the proximate level, we aim to identify the neural circuits that control behaviour and to understand how information is represented and processed in these circuits. Ultimately, however, we are faced with questions of why particular neural solutions have arisen, and what factors govern the ways in which neural circuits are modified during the evolution of new behaviours. Only by integrating these levels of analysis can we fully understand the neural control of behaviour. Recent studies of electrosensory systems show how this synthesis can benefit from the use of tractable systems and comparative studies.

Species-typical songs in white-crowned sparrows tutored with only phrase pairs

Modern theories of learned vocal behaviours, such as human speech and singing in songbirds, posit that acoustic communication signals are reproduced from memory, using auditory feedback. The nature of these memories, however, is unclear. Here we propose and test a model for how complex song structure can emerge from sparse sequence information acquired during tutoring. In this conceptual model, a population of combination-sensitive (phrase-pair) detectors is shaped by early exposure to song and serves as the minimal representation of the template necessary for generating complete song. As predicted by the model, birds that were tutored with only pairs of normally adjacent song phrases were able to assemble full songs in which phrases were placed in the correct order; birds that were tutored with reverse-ordered phrase pairs sang songs with reversed phrase order. Birds that were tutored with all song phrases, but presented singly, failed to produce normal, full songs. These findings provide the first evidence for a minimal requirement of sequence information in the acoustic model that can give rise to correct song structure.

Structure and Function of Neurons in the Complex of the Nucleus electrosensorius of Sternopygus and Eigenmannia: Diencephalic Substrates for the Evolution of the Jamming Avoidance Response

The ability to discriminate the sign of the difference in frequency (DF) between two wavelike signals is integral to the jamming avoidance response (JAR) of weakly electric gymnotiform fish such as Eigenmannia. 'Whole-cell' intracellular recordings from neurons in the nucleus electrosensorius (nE) of Sternopygus, a gymnotiform that lacks the JAR, revealed that this nucleus receives information from both the ampullary and the tuberous electrosensory systems. Most tuberous units responded to DF stimuli, and many of these cells were DF sign-sensitive, i.e., they responded differently for one sign of DF. Although the distribution of ampullary units was somewhat restricted, sign-sensitive units were found in all areas of the nE in Sternopygus. Whole-cell recordings made in the nE of Eigenmannia revealed that, as in Sternopygus, sign-sensitive cells were not restricted to areas associated with control of the JAR. The diverse neurophysiology and connectivity of the nucleus electrosensorius suggests that, besides its role in the JAR of some members of the order, this nucleus likely serves as an interface between sensory input and neural circuits controlling other behaviors and endocrinological states in other gymnotiforms as well. The discovery of sign sensitivity in the nE of Sternopygus indicates that this property is not uniquely associated with the presence of a JAR; rather, the ability to discriminate the sign of DF may be relevant to many other behavioral contexts in gymnotiforms. Existing evidence indicates that the JAR evolved more than once in this group; the presence of sign sensitivity in ancestral gymnotiforms may have made this parallelism more likely.

Roles for short-term synaptic plasticity in behavior

Short-term synaptic plasticity is phylogenetically widespread in ascending sensory systems of vertebrate brains. Such plasticity is found at all levels of sensory processing, including in sensory cortices. The functional roles of this apparently ubiquitous short-term synaptic plasticity, however, are not well understood. Data obtained in midbrain electrosensory neurons of Eigenmannia suggest that this plasticity has at least two roles in sensory processing; enhancing low-pass temporal filtering and generating phase shifts used in processing moving sensory images. Short-term synaptic plasticity may serve similar roles in other sensory modalities, including vision.

Interval-integration underlies amplitude modulation band-suppression selectivity in the anuran midbrain

We examined the mechanisms that underlie 'band-suppression' amplitude modulation selectivity in the auditory midbrain of anurans. Band-suppression neurons respond well to low (5-10 Hz) and high (>70 Hz) rates of sinusoidal amplitude modulation, but poorly, if at all, to intermediate rates. The effectiveness of slow rates of sinusoidal amplitude modulation is due to the long duration of individual 'pulses'; short-duration pulses (<10 ms) failed to elicit spikes when presented at 5-10 pulses s(-1). Each unit responded only after a threshold number of pulses (median=3, range=2-5) were delivered at an optimal rate. The salient stimulus feature was the number of consecutive interpulse intervals that were within a cell-specific tolerance. This interval-integrating process could be reset by a single long interval, even if preceded by a suprathreshold number of intervals. These findings indicate that band-suppression units are a subset of interval-integrating neurons. Band-suppression neurons differed from band-pass interval-integrating cells in having lower interval-number thresholds and broader interval tolerance. We suggest that these properties increase the probability of a postsynaptic spike, given a particular temporal pattern of afferent action potentials in response to long-duration pulses, i.e., predispose them to respond to slow rates of amplitude modulation. Modeling evidence is provided that supports this conclusion.

Voltage-gated Na+ channels enhance the temporal filtering properties of electrosensory neurons in the torus

Regenerative processes enhance postsynaptic potential (PSP) amplitude and behaviorally relevant temporal filtering in more than one-third of electrosensory neurons in the torus semicircularis of Eigenmannia. Data from in vivo current-clamp intracellular recordings indicate that these "regenerative PSPs" can be divided in two groups based on their half-amplitude durations: constant duration (CD) and variable duration (VD) PSPs. CD PSPs have half-amplitude durations of between 20 and 60 ms that do not vary in relation to stimulus periodicity. In contrast, the half-amplitude durations of VD PSPs vary in relation to stimulus periodicity and range from approximately 10 to 500 ms. Injection of 0.1 nA sinusoidal current through the recording electrode demonstrated that CD PSPs and not VD PSPs can be elicited by voltage fluctuations alone. In addition, CD PSPs were blocked by intracellular application of either QX-314 or QX-222, whereas VD PSPs were not. These in vivo data suggest, therefore, that CD PSPs are mediated by voltage-dependent Na+ conductances.

Auditory midbrain neurons that count

Many acoustic communication signals, including human speech and music, consist of a precise temporal arrangement of discrete elements, but it is unclear whether this precise temporal patterning is required to activate the sensory neurons that mediate signal recognition. In a variety of systems, neurons respond selectively when two or more sound elements are presented in a particular temporal order and the precise relative timing of these elements is particularly important for 'delay-tuned' neurons, including 'tracking' types, in bats. Here we show that one class of auditory neurons in the midbrain of anurans (frogs and toads) responds only to a series of specific interpulse intervals (IPIs); in the most selective cases, a single interval that is slightly longer or shorter than the requisite interval can reset this interval-counting process.

Short-term synaptic plasticity as a temporal filter

Synaptic efficacy can increase (synaptic facilitation) or decrease (synaptic depression) markedly within milliseconds after the onset of specific temporal patterns of activity. Recent evidence suggests that short-term synaptic depression contributes to low-pass temporal filtering, and can account for a well-known paradox - many low-pass neurons respond vigorously to transients and the onsets of high temporal-frequency stimuli. The use of depression for low-pass filtering, however, is itself a paradox; depression induced by ongoing high-temporal frequency stimuli could preclude desired responses to low-temporal frequency information. This problem can be circumvented, however, by activation of short-term synaptic facilitation that maintains responses to low-temporal frequency information. Such short-term plasticity might also contribute to spatio-temporal processing.

Integration and recovery processes contribute to the temporal selectivity of neurons in the midbrain of the northern leopard frog, Rana pipiens

This study examined the mechanisms underlying amplitude modulation selectivity in the anuran auditory midbrain. Single units were recorded extracellularly in the torus semicircularis of the northern leopard frog, Rana pipiens. Two physiologically distinct classes of neurons were identified, based on their response latencies and their selectivities to pulse repetition rates. Cells in one group had short response latencies (median = 31 ms) and responded best to pulse repetition rates below 40 Hz. Tuning to low amplitude modulation rates was largely determined by recovery processes and phasic response properties. Cells in the second group had much longer latencies (median=81 ms) and were generally selective for pulse repetition rates greater than 40-50 Hz. Sensitivity to higher amplitude modulation rates resulted from integration processes; these units only responded when a threshold number of pulses were presented at a minimum pulse density (amplitude modulation rate). At amplitude modulation rates above their best rate, their responses decreased, apparently due to inadequate recovery time between pulses.

Short-term synaptic plasticity contributes to the temporal filtering of electrosensory information

Short-term synaptic depression and facilitation often are elicited by different temporal patterns of activity. Short-term plasticity may contribute, therefore, to temporal filtering by impeding synaptic transmission for some temporal patterns of activity and facilitating transmission for other patterns. We examined this hypothesis by investigating whether short-term plasticity contributes to the temporal filtering properties of midbrain electrosensory neurons. Postsynaptic potentials were recorded in response to sensory stimuli and to direct stimulation of afferents, in vivo. Stimulating afferents with pairs of pulses at a rate of 20 pairs/sec ["tetanus (20 Hz)"] induced PSP depression. This PSP depression was similar to that observed for electrosensory stimuli of the same temporal frequency. Analysis of PSPs elicited by a pair of pulses that preceded versus followed the tetanus revealed that PSP depression was caused by synaptic depression, not by a loss of facilitation. Behavioral studies indicate that fish can detect slow changes in signal amplitude (slow AM) in backgrounds of fast fluctuations. Correspondingly, midbrain neurons respond well to slow AM even in the presence of fast AM. In many neurons, facilitation enhanced responses to trains (8-10 pulses; 100 Hz) that represented activity patterns elicited by slow AM, despite induction of synaptic depression by a tetanus (20 Hz). The interplay between synaptic depression and facilitation, therefore, can act as a filter of temporal information. Some neurons that showed little facilitation nonetheless responded to low temporal-frequency information after induction of depression by fast information; this likely results from the convergence of inputs with different temporal filtering properties.

Frequency-dependent PSP depression contributes to low-pass temporal filtering in Eigenmannia

This study examined the contribution of frequency-dependent short-term depression of PSP amplitude to low-pass temporal filtering in the weakly electric fish Eigenmannia. Behavioral and neurophysiological methods were used. Decelerations of the electric organ discharge frequency were measured in response to continuous and discontinuous electrosensory stimuli. Decelerations were strongest (median = 4.7 Hz; range, 3.5-5.9 Hz) at continuous beat rates of approximately 5 Hz and weakest (median = 0.4 Hz; range, 0.0-0.8 Hz) at beat rates of 30 Hz. Gating 20 or 30 Hz stimuli at a rate of 5 Hz, however, elicited decelerations that were sixfold greater than that of continuous stimuli at these beat rates (median = 2.6 Hz; range, 2.0-4.7 Hz for 30 Hz). These results support the hypothesis that short-term processes enhance low-pass filtering by reducing responses to fast beat rates. This hypothesis was tested by recording intracellularly the responses of 33 midbrain neurons to continuous and discontinuous stimuli. Results indicate that short-term depression of PSP amplitude primarily accounts for the steady-state low-pass filtering of these neurons beyond that contributed by their passive and active membrane properties. Previous results demonstrate that passive properties can contribute up to 7 dB of low-pass filtering; PSP depression can add up to an additional 12.5 dB (median = 4.5). PSP depression increased in magnitude with stimulus frequency and showed a prominent short-term component (t(1) = 66 msec at 30 Hz). Initial PSP amplitude recovered fully after a gap of 150 msec for most neurons. Remarkably, recovery of PSP amplitude could be produced by inserting a brief low-temporal frequency component in the stimulus.

Female choice and plasticity of male calling behaviour in the Pacific treefrog

Male Pacific treefrogs, Hyla regilla, use advertisement and encounter calls to regulate intermale spacing within breeding choruses. When either type of call produced by a neighbour is detected above a particular amplitude, a resident frog responds aggressively by producing encounter calls. These 'aggressive thresholds' differ for the two call types and are plastic: males rapidly resume advertisement calling (accommodate) following repeated presentation of these calls above their aggressive threshold. We tested the hypothesis that this plasticity is the result of female preference for the advertisement call over the encounter call. Female choice was tested in a two-speaker phonotaxis assay with alternating presentation of the two call types. Of the 12 females that met our criteria for demonstrating a phonotaxic response, each approached the speaker playing the advertisement call rather than the encounter call. This strong female preference for the advertisement call supports our hypothesis, and suggests that the plasticity in male calling behaviour allows males to maximize the time spent in producing advertisement calls to attract a female by rapidly adjusting their aggressive behaviour to changes in male spacing within choruses. Copyright 1999 The Association for the Study of Animal Behaviour.

Resolving competing theories for control of the jamming avoidance response: the role of amplitude modulations in electric organ discharge decelerations

The algorithm for the control of the jamming avoidance response (JAR) of Eigenmannia has been the subject of debate for over two decades. Two competing theories have been proposed to explain how fish determine the correct direction to shift their pacemaker frequency during jamming. One theory emphasizes the role of time-asymmetric beat envelopes, while the other emphasizes the role of amplitude- and phase-difference computations that arise from the differences in spatial geometry of the electric fields of neighboring fish. In repeating earlier experiments, we found that the decision to raise or lower the pacemaker frequency reliably above or below its resting level depends on the latter process, and that frequency deceleration responses to amplitude modulation appear to be sufficient to explain previous experimental results on which the former theory is based. Specifically, fish of the genus Eigenmannia show differential deceleration responses to asymmetric beat envelopes. The deceleration responses do not require phase modulation and show a sensitivity for amplitude modulation depth and selectivity for amplitude modulation rate comparable with that of JARs that are elicited when amplitude- and phase-difference information is available.

Mechanisms for generating temporal filters in the electrosensory system

Temporal patterns of sensory information are important cues in behaviors ranging from spatial analyses to communication. Neural representations of the temporal structure of sensory signals include fluctuations in the discharge rate of neurons over time (peripheral nervous system) and the differential level of activity in neurons tuned to particular temporal features (temporal filters in the central nervous system). This paper presents our current understanding of the mechanisms responsible for the transformations between these representations in electric fish of the genus Eigenmannia. The roles of passive and active membrane properties of neurons, and frequency-dependent gain-control mechanisms are discussed.

Long-term temporal integration in the anuran auditory system

Analysis of the temporal structure of acoustic signals is important for the communication and survival of a variety of animals including humans. Recognition and discrimination of particular temporal patterns in sounds may involve integration of auditory information presented over hundreds of milliseconds or seconds. Here we show neural evidence for long-term integration in the anuran auditory system. The responses of one class of auditory neurons in the torus semicircularis (auditory midbrain) of frogs reflect the integration of information, gathered over approximately 45-150 ms, from a series of stimulus pulses, not stimulus energy. This integration process is fundamental to the selective responses of these neurons for particular call types.

Passive and active membrane properties contribute to the temporal filtering properties of midbrain neurons in vivo

This study examined the contributions of passive and active membrane properties to the temporal selectivities of electrosensory neurons in vivo. The intracellular responses to time-varying (2-30 Hz) electrosensory stimulation and current injection of 27 neurons in the midbrain of the weakly electric fish Eigenmannia were recorded. Each neuron was filled with biocytin to reveal its anatomy. Neurons were divided into two biophysically distinct groups based on their frequency-dependent responses to sinusoidal current injection over the range 2-30 Hz. Fourteen neurons showed low-pass filtering, with a maximum decline in the amplitude of voltage responses of >2.6 dB (X = 4.30 dB, s = 1.10 dB) to sinusoidal current injection. These neurons also showed low-pass filtering of electrosensory information but with larger maximum declines in postsynaptic potential amplitude (X = 9.53 dB, s = 3.34 dB; n = 10). These neurons had broad dendritic arbors and relatively spiny dendrites. Five neurons showed all-pass filtering, having maximum decline in the amplitude of voltage responses of <2.0 dB (X = 1.16 dB, s = 0.61 dB). For electrosensory stimuli, however, these neurons showed low-, band-, or high-pass filtering. These neurons had small dendritic arbors and few or no spines. Voltage-dependent "active" conductances were revealed in eight neurons by using several levels of current clamp. In four of these neurons, the duration of the voltage-dependent conductances decreased in concert with the period of the electrosensory stimulus, whereas in the other four neurons the duration of the voltage-dependent conductances was relatively short (<30 msec) and nearly constant across sensory stimulation frequencies. These conductances enhanced the temporal filtering properties of neurons.

Temporal filtering properties of ampullary electrosensory neurons in the torus semicircularis of Eigenmannia: evolutionary and computational implications

Weakly electric fish have parallel electrosensory systems, the phylogenetically older ampullary system and the novel tuberous system. The tuberous system is an adaptation related to the evolution of active electrolocation. To examine the evolutionary relationship of the ampullary and tuberous systems, the temporal filtering properties of ampullary neurons in the dorsal torus semicircularis of Eigenmannia were studied. 'Whole-cell' recordings were made in vivo using patch-type pipettes. The responses of 19 neurons to sinusoidal electric signals (< 40 Hz) were recorded and the anatomy of these neurons demonstrated by injection of biocytin. All eight low-pass ampullary neurons had broad, relatively smooth post-synaptic potentials (psps) that at low frequencies nicely reflected the sinusoidal stimuli. These neurons had somata of 10-14 microns diameter and thick, spiny dendrites. Eight high-pass neurons were recorded, representing three physiological classes. The first class (3 neurons) had psps that roughly followed the sinusoidal time course of the stimulus; the psp morphology was similar to low-pass neurons. The second class had many small, fast, individual psps; their rate of occurrence varied with the stimulus. Finally, four neurons showed psps that were of constant width across stimulus frequencies. All three classes of high-pass neurons had small somata (8-10 microns diameter) with thin dendrites and either few or no spines. Some of these neurons had large varicosities on the dendrites. Three neurons had band-pass filtering properties: neurons that showed strong band-pass properties were morphologically similar to low-pass neurons. Comparisons of the temporal filtering, shapes of post-synaptic potentials, and anatomy of ampullary and tuberous neurons in the torus suggest that the circuitry for tuberous processing in the torus may have evolved as an elaboration or duplication of the ampullary system. The mechanisms underlying the low-pass filtering characteristics of tuberous neurons therefore appear to have predated the evolution of the tuberous system and to have served as a pre-adaptation for the evolution of the jamming avoidance response. In addition, these data support the hypothesis that spine density influences the temporal filtering properties of neurons.

New techniques for making whole-cell recordings from CNS neurons in vivo

Patch-type pipettes increasingly are being used to obtain intracellular 'whole-cell' recordings from neurons. Here we describe our methods for making whole-cell recordings in vivo from midbrain neurons in an electric fish. Novel elements in the procedure are: A device for micropositioning the pipette when near a cell, use of a 'Picospritzer' for cleaning the pipette tip and cell surface, and an electroporetic method for perforating the patch following seal formulation. In addition, we show that extracellular and intracellular recordings can be made from the same neuron. Stable intracellular recordings can be made from neurons at least as small as 10 microns.

Hierarchical sensory guidance of mauthner-mediated escape responses in goldfish (Carassius auratus) and cichlids (Haplochromis burtoni)

Acoustically-evoked escape behaviors were compared between goldfish (Carassius auratus), a hearing specialist, and the cichlid Haplochromis burtoni, a hearing nonspecialist. Fish were startled with compressive and rarefying, stimuli presented alone or together, and with compressive pulses preceded by a visual cue or after exposure to cobalt, an inhibitor of lateral line-innervated neuromast hair cells. These acoustic startle stimuli can evoke Mauthner neuron firing and are similar to but weaker than those produced by a largemouth bass (Micropterus salmoides) feeding on guppies. When sound stimuli were presented alone, both species avoided the direction of either the compressive or rarefying stimulus. If a light preceded and was contralateral to the compressive sound pulse, goldfish continued to avoid the sound source; cichlids avoided the visual cue and turned toward the sound. Goldfish performance improved significantly when the visual cue was in the same direction as the sound source. Goldfish performance also improved significantly after exposure to 0.1 mmol l-1 cobalt solution for 24 hours before testing, but cichlids would not startle after cobalt acclimation. A compressive pulse presented to one side of a fish simultaneously with a rarefying pulse on the other side causes the entire fish to accelerate with the water current. This strongly and directly accelerates the ear but tends to reduce both the pressure changes transduced by the swimbladder and activation of the mechanosensory lateral line. In this test, goldfish reliably avoided the compressive pulse. Cichlids, however, randomly avoided either speaker polarity but significantly avoided the speaker which had a faster onset. With more closely matched speakers, cichlids also preferentially avoided the compressive stimulus. Thus, the primitive sensory condition for auditory activation and guidance of Mauthner-neuron-initiated escape responses may have evolved to detect the initially compressive sounds associated with ram-type predators.

Temporal filtering properties of midbrain neurons in an electric fish: implications for the function of dendritic spines

Electrosensory neurons in the torus semicircularis (midbrain) of the weakly electric fish Eigenmannia vary considerably in their dendritic structure and responses to modulations of the amplitude of electric organ discharges. We investigated possible relations between these properties by recording intracellularly and labeling individual neurons while modulating stimulus amplitude over rates of approximately 2-20 Hz. Morphologically distinct cell types generally differed in their responses to these stimuli. The amplitude envelope of the stimulus was nicely reflected in fluctuations of the membrane potential of heavily spined neurons. The amplitude of these stimulus-related depolarizations decreased markedly as the stimulus modulation rate was increased. For aspiny or sparsely spined neurons, however, the amplitude of stimulus-related depolarizations either increased or remained constant over this range of modulation rates. In these cells, the amplitude envelope of the stimulus was not well represented in the membrane potential. Instead, fast EPSPs were observed that varied in number over time in accordance with the amplitude envelope of the stimulus. Aspiny neurons in the tectum also coded the amplitude envelope of stimuli with poor fidelity. The amplitude of stimulus-related depolarizations, however, decreased as the rate of modulation of stimulus amplitude was increased, consistent with the notion that tectal neurons receive afferent input from the spiny toral neurons. Spiny neurons appear, therefore, to act as low-pass filters of temporal information in sensory signals. Aspiny cells, however, code high temporal frequencies. These data support the hypothesis that dendritic spines contribute to the low-pass filtering of inputs to neurons.

Evidence for the role of dendritic spines in the temporal filtering properties of neurons: the decoding problem and beyond

The question of relations between structure and function acutely applies to the search for the functional role of dendritic spines. While dendritic spines are a prominent and widespread structural feature of neurons in the central nervous system, their function is poorly understood. Because the conducting core of a spine stem can be of extremely small dimensions, a large axial resistance to current flow and "low-pass" filtering of inputs have been hypothesized. Here we show that neurons in the dorsal torus semicircularis of the electric fish Eigenmannia show real-time fluctuations in their transmembrane potential that reflect modulations in the amplitude of a high-frequency sinusoidal carrier signal. In 18 neurons recorded intracellularly and labeled with Lucifer yellow, the decrease in the magnitude of these potentials with increasing rate of amplitude modulation (i.e., low-pass temporal filtering) was positively correlated (r = 0.79, P < 0.001, over a range of one to two octaves in modulation rate) with the mean dendritic spine density (range, 0-0.38 spine per micron of dendritic length) of the cell. The acquisition of synaptic input through dendritic spines may be a general mechanism for achieving the temporal filtering that underlies real-time signal processing in the central nervous system.

Differential distribution of ampullary and tuberous processing in the torus semicircularis of Eigenmannia

Gymnotiform electric fish sense low- and high-frequency electric signals with ampullary and tuberous electroreceptors, respectively. We employed intracellular recording and labeling methods to investigate ampullary and tuberous information processing in laminae 1-5 of the dorsal torus semicircularis of Eigenmannia. Ampullary afferents arborized extensively in laminae 1-3 and, in some cases, lamina 7. Unlike tuberous afferents to the torus, ampullary afferents had numerous varicosities along their finest-diameter branches. Neurons that were primarily ampullary were found in lamina 3. Neurons primarily excited by tuberous stimuli were found in lamina 5 and, more rarely, in lamina 4. Cells that had dendrites in lamina 1-3 and 5 could be recruited by both ampullary and tuberous stimuli. These bimodal cells were found in lamina 4. During courtship, Eigenmannia produces interruptions of its electric organ discharges. These interruptions stimulate ampullary and tuberous receptors. The integration of ampullary and tuberous information may be important in the processing of these communication signals.

Abstract art and emotion: expressive form and the sense of wholeness

This paper limits itself to abstract art the better to concentrate on the relation between the emotionally expressive power of esthetic form--apart from its narrative content--and emotional responsiveness. The emotionally expressive power of art--not to be confused with the artist's own emotions--has to do with the way sensuous esthetic forms highlight the rhythmic changes of tension and release inherent in ordinary perceptual experience. Tension and release are useful terms in thinking about how the perception of an esthetic structure is transformed (transduced) into feelingful psychological meanings, and contributes to the subjective sense of wholeness. The sense of wholeness may be illusory and/or authentic depending on the mix of elements in the individual's responsiveness. It comes about through a unified organization of tension and release, as embodied in expressive forms such as rhythm and rhyme, resonating with tension and release evoked in the observer's associations to such psychological issues as separation and reunion. Having stirred the viewer's emotional responsiveness, the art work provides a reliable "container" for the objectification of latent emotions.

Anatomical and functional organization of the prepacemaker nucleus in gymnotiform electric fish: the accommodation of two behaviors in one nucleus

The diencephalic prepacemaker nucleus (PPn) of gymnotiform electric fish projects to the medullary pacemaker nucleus and modulates its regular firing frequency. Each firing of the pacemaker, in turn, drives an electric organ discharge (EOD). Two types of PPn neurons were retrogradely labeled from the pacemaker with HRP in Eigenmannia and Apteronotus. In both species, smaller ovoidal cells were found in the dorsomedial part of the PPn (PPn-G), and larger multipolar cells were found in the ventrolateral part of the PPn (PPn-C). This morphological distinction between the two subnuclei in the PPn was paralleled by a functional dichotomy. Microiontophoresis of L-glutamate in the PPn-G of both species elicited slow and gradual accelerations of EOD frequency characterized by a time constant on the order of seconds. The elicited frequency modulations were similar to those observed during the jamming avoidance response and during courtship. Glutamate stimulation of the PPn-C, in contrast, produced fast and abrupt frequency modulations characterized by a time constant on the order of milliseconds. These abrupt modulations resembled "chirps" observed during courtship and aggression. Similar behavior was produced by intracellular current injection into a PPn-C neuron of Apteronotus, and intracellular labeling of this neuron with Lucifer Yellow revealed a multipolar PPn-C neuron similar to those retrogradely labeled with HRP.

'Recognition units' at the top of a neuronal hierarchy? Prepacemaker neurons in Eigenmannia code the sign of frequency differences unambiguously

The electric fish, Eigenmannia, is able to discriminate the sign of the frequency difference, Df, between a neighbor's electric organ discharges (EODs) and its own. The fish lowers its EOD frequency for positive Dfs and raises its frequency for negative Dfs to minimize jamming of its electrolocation ability by a neighbor's EODs of similar frequency. This jamming avoidance response (JAR) is controlled by a group of 'sign-selective' neurons in the prepacemaker nucleus (PPN) that is located at the boundary of the midbrain and the diencephalon (Fig. 1). Extracellular recordings from a total of 35 neurons revealed a great similarity between behavioral and neuronal response properties: 1. All neurons fired vigorously for negative Dfs and were almost silent for positive Dfs, regardless of the orientation of the jamming stimulus, and thus discriminated the sign of Df unambiguously (Fig. 2). 2. In accordance with behavioral observations, individual neurons failed to discriminate the sign of Df when the jamming stimulus had the same field geometry as the signal mimicking the animal's own EOD (Fig. 3). 3. Df magnitudes which evoke strongest JARs, usually 4 to 8 Hz, also induced most vigorous responses in sign-selective neurons (Fig. 5). 4. Behavioral and neuronal thresholds for the detection of small jamming signals were similar. Threshold for sign selectivity was reached when the amplitude ratio of the jamming signal to the EOD mimic, measured near the head surface, was 0.001. This value corresponds to a maximal temporal disparity (a necessary cue for performing a correct JAR) of 1 to 2 microseconds for signals received by the two sides of the body in a transverse jamming field (Fig. 7). 5. The effects of two jamming fields, offered orthogonally to each other, may interact nonlinearly at the behavioral as well as at the neuronal level. A positive Df presented in one field may suppress behavioral and neuronal responses to modulations of the sign of Df in the other field (Fig. 8c).

The optic tectum of the gymnotiform electric fish, Eigenmannia: labeling of physiologically identified cells

A total of 47 tectal neurons of the weakly electric fish, Eigenmannia, were studied physiologically and labelled by intracellular injection of Lucifer Yellow. With the exception of two cell types, all cells could be classified in accordance with the Golgi studies of Sas and Maler. The dominant stimulus modality of neurons was correlated with their laminar location. Neurons of the stratum opticum only responded to visual stimuli, such as modulations of the light level or the motion of an object. They showed, however, no directional preferences for motion. Neurons of the stratum griseum centrale were predominantly driven by electrosensory stimuli, most often those associated with the movement of an object, and generally were very sensitive to the direction of motion. Integration of different sensory modalities was found in neurons with dendrites invading laminae with different sensory inputs. In addition, small axons of interneurons appear to relay information across laminae. Large multipolar neurons in the deep tectum responded to the motion of objects, often preferring a particular direction of motion. Some of these large multipolar neurons of the deep tectum also discriminated the sign of the frequency difference between a mimic of a neighbor's sinusoidal electric organ discharge and the animal's own signal. These neurons are potential candidates for the control of the jamming avoidance response. These neurons were morphologically indistinguishable from large multipolar neurons of the deep tectum that either responded to moving objects or to acoustical stimuli. Individual large cells of the deep tectum project to various targets (Fig. 1) and probably contribute to the control of different behavioral responses. This suggests that the nature of such responses would then depend upon the constitution of sets of neurons recruited by a given stimulus situation, and the role of individual tectal neurons would neither be particularly specific nor very significant.

A time-comparison circuit in the electric fish midbrain. I. Behavior and physiology

Behavioral experiments show that the weakly electric fish, Eigenmannia, detects differences in timing as small as 400 nsec between electric signals from different parts of its body surface. The neural basis of this remarkable temporal resolution was investigated by recording from elements of the phase-coding system, a chain of electrotonically connected neurons devoted to the processing of temporal information. Each element of this system fires a single action potential for every cycle of the electric signal (either the fish's own electric organ discharge or a sinusoidal signal of similar frequency). For phase-coding primary afferents and midbrain neurons, the temporal resolution was determined by measuring each unit's capacity to lock its spike to a particular phase of the stimulus cycle. The jitter of a neuron's response (measured as the standard deviation of the timing of the spikes with respect to the stimulus) decreases from the level of the primary afferent (mean = 30 microsec) to the midbrain torus (mean = 11 microsec); these results can be correlated with morphological measures of convergence. The temporal resolution of single neurons is still inferior to that displayed at the behavioral level.

Species specificity and temperature dependency of temporal processing by the auditory midbrain of two species of treefrogs

The mating (advertisement) calls of two sibling species of gray treefrogs, Hyla versicolor and Hyla chrysoscelis, are spectrally identical but differ in trill rate; being higher for H. chrysoscelis. Single-unit recordings were made from the torus semicircularis of both species to investigate the neural mechanisms by which this species-specific temporal feature is analyzed. Using sinusoidally amplitude-modulated (AM) white noise as a stimulus, the temporal selectivity of these midbrain auditory neurons could be described by five response categories: 'AM nonselective' (34%); 'AM high-pass' (7%); 'AM low-pass' (6%); 'AM band-suppression' (12%); 'AM tuned' (40%). The distributions of temporal tuning values (i.e., modulation rate at which each AM-tuned unit responds maximally) are broad; in both species, neurons were found which were tuned to modulation rates greater than those found in their advertisement calls. Nevertheless, the temporal tuning values for H. versicolor (median = 25 Hz) were significantly lower than those for H. chrysoscelis (median = 32.5 Hz). The temporal selectivities of AM band-suppression neurons were found to be temperature dependent. The modulation rate at which a response minimum was observed shifted to higher values as the temperature was elevated. These results extend our earlier findings of temperature-dependent temporal selectivity in the gray treefrog. The selectivity of band-suppression and AM-tuned neurons to various rates of amplitude modulation was largely, but not completely, independent of whether sinusoidal or natural forms of AM were used.

Sensitivity to amplitude modulated sounds in the anuran auditory nervous system

Auditory responses were recorded from single units in the eighth nerve and in the midbrain torus semicircularis of the leopard frog (Rana pipiens). Acoustic stimuli included sinusoidally amplitude-modulated (AM) tones and noise, as well as pure tones. Mean spike rates were measured at various rates of AM, and the degree to which a unit's spikes were restricted to a particular phase of the modulation cycle was described by a synchronization coefficient. The firing rate of eighth-nerve fibers was largely independent of the rate of AM over the modulation range 10 to 150 Hz. Further, the general shape of the spike rate vs. AM-rate function was invariant with either depth of modulation or sound-pressure level (SPL). Although virtually all eighth-nerve fibers exhibited significant synchronization to the envelope of AM, the shape of the synchronization function depended on the unit's best-excitatory frequency (BEF). Fibers with highest BEF's, presumed to innervate the basilar papilla, generally showed greater synchronization as the AM rate was increased (up to 100-150 Hz). Fibers tuned to the low-and midfrequency region, which innervate the amphibian papilla, exhibited low-pass synchronization characteristics. As the depth of modulation was reduced, the degree of synchronization of eighth-nerve fibers decreased. For a given depth of modulation an increase in sound level tended to decrease the degree of synchronization, but significant synchronization could still be observed at stimulus intensities at least 65 dB above threshold. On the basis of the spike rate vs. AM-rate functions, the temporal selectivity of single cells in the torus could be characterized by five response types: AM nonselective (spike rate was largely independent of the AM rate); AM high-pass (activity increased as the AM rate was increased); AM low-pass (response was greatest for slow AM rates and decreased at high rates); AM band-suppression (these neurons responded well to low and high AM rates, but responded weakly to intermediate rates); and AM-tuned (spike rate was greatest over a narrow range of modulation rates). In these measurements the depth of modulation was held constant at 100%. The five response categories are not discrete, but rather reflect representative examples along a continuum with regard to temporal selectivity. The temporal selectivity exhibited by toral units in their firing rates was not evident in their AM-synchronization functions.(ABSTRACT TRUNCATED AT 400 WORDS)

The anuran superficial reticular nucleus: evidence of homology with nuclei of the lateral lemniscus

The superficial reticular nucleus (SR) of ranid frogs is part of a lateral cell column extending from the isthmus to the rostral tegmentum. The caudal part of this nucleus receives input from lower brainstem auditory nuclei and projects bilaterally to the torus semicircularis. On the basis of its position, connections, and sensitivity to acoustic stimuli, the caudal SR appears to be homologous to all or part of the mammalian nuclei of the lateral lemniscus.

A characteristic effect of hallucinogens on investigatory responding in rats

The disruption of the temporal distribution of investigatory responses by rats in a novel hole-board following lysergic acid diethylamide-25 (LSD), as described in a companion paper (Geyer and Light, 1979), was found to be a characteristic effect of a variety of hallucinogens. Similar effects were produced by indoleamine hallucinogens, such as LSD, N,N-dimethyltryptamine, and psilocin, and by phenylethylamine hallucinogens, such as mescaline or 2,5-dimethoxy-4-methylamphetamine (DOM). Congeners of DOM that are inactive in humans had no significant effects. Furthermore, of a variety of other psychoactive drugs tested, only apomorphine produced an effect similar to that of the hallucinogens. These results suggest that a simple behavioral measure of exploration in a hole-board may provide a useful animal model with which to examine the common effects of hallucinogens.

Effects of maternal-fetal monitoring on pregnancy outcome in a high risk pregnant population

A program was designed to determine the effects of monitoring all patients admitted to labor and delivery at the District of Columbia General Hospital during 1976. This paper reports the findings and discusses the value of electronic surveillance on fetal and perinatal survival.