Possible Event-Related Potential Correlates of Voluntary Attention and Reflexive Attention in the Emei Music Frog

Attention, referring to selective processing of task-related information, is central to cognition. It has been proposed that voluntary attention (driven by current goals or tasks and under top-down control) and reflexive attention (driven by stimulus salience and under bottom-up control) struggle to control the focus of attention with interaction in a push-pull fashion for everyday perception in higher vertebrates. However, how auditory attention engages in auditory perception in lower vertebrates remains unclear. In this study, each component of auditory event-related potentials (ERP) related to attention was measured for the telencephalon, diencephalon and mesencephalon in the Emei music frog (Nidirana daunchina), during the broadcasting of acoustic stimuli invoking voluntary attention (using binary playback paradigm with silence replacement) and reflexive attention (using equiprobably random playback paradigm), respectively. Results showed that (1) when the sequence of acoustic stimuli could be predicted, the amplitudes of stimulus preceding negativity (SPN) evoked by silence replacement in the forebrain were significantly greater than that in the mesencephalon, suggesting voluntary attention may engage in auditory perception in this species because of the correlation between the SPN component and top-down control such as expectation and/or prediction; (2) alternately, when the sequence of acoustic stimuli could not be predicted, the N1 amplitudes evoked in the mesencephalon were significantly greater than those in other brain areas, implying that reflexive attention may be involved in auditory signal processing because the N1 components relate to selective attention; and (3) both SPN and N1 components could be evoked by the predicted stimuli, suggesting auditory perception of the music frogs might invoke the two kind of attention resources simultaneously. The present results show that human-like ERP components related to voluntary attention and reflexive attention exist in the lower vertebrates also.

Neural activities in music frogs reveal call variations and phylogenetic relationships within the genus Nidirana

The characteristics of acoustic signals co-evolve with preferences of the auditory sensory system. However, how the brain perceives call variations and whether it can reveal phylogenetic relationships among signalers remains poorly understood. Here, we recorded the neural signals from the Emei music frogs (Nidirana daunchina) in response to broadcasted calls of five different species of the same genus. We found that responses in terms of the different amplitudes of various event-related potential (ERP) components were correlated with diversification trends in acoustic signals, as well as phylogenetic relationships between N. daunchina and heterospecific callers. Specifically, P2 decreased gradually along the ordinal decline of similarities in acoustic characteristics of calls compared with those from conspecifics. Moreover, P3a amplitudes showed increasing trends in correspondence with callers' genetic distances from the subject species. These observations collectively support the view that neural activities in music frogs can reflect call variations and phylogenetic relationships within the genus Nidirana.

Hierarchical auditory perception for species discrimination and individual recognition in the music frog

The ability to discriminate species and recognize individuals is crucial for reproductive success and/or survival in most animals. However, the temporal order and neural localization of these decision-making processes has remained unclear. In this study, event-related potentials (ERPs) were measured in the telencephalon, diencephalon, and mesencephalon of the music frog Nidirana daunchina. These ERPs were elicited by calls from 1 group of heterospecifics (recorded from a sympatric anuran species) and 2 groups of conspecifics that differed in their fundamental frequencies. In terms of the polarity and position within the ERP waveform, auditory ERPs generally consist of 4 main components that link to selective attention (N1), stimulus evaluation (P2), identification (N2), and classification (P3). These occur around 100, 200, 250, and 300 ms after stimulus onset, respectively. Our results show that the N1 amplitudes differed significantly between the heterospecific and conspecific calls, but not between the 2 groups of conspecific calls that differed in fundamental frequency. On the other hand, the N2 amplitudes were significantly different between the 2 groups of conspecific calls, suggesting that the music frogs discriminated the species first, followed by individual identification, since N1 and N2 relate to selective attention and stimuli identification, respectively. Moreover, the P2 amplitudes evoked in females were significantly greater than those in males, indicating the existence of sexual dimorphism in auditory discrimination. In addition, both the N1 amplitudes in the left diencephalon and the P2 amplitudes in the left telencephalon were greater than in other brain areas, suggesting left hemispheric dominance in auditory perception. Taken together, our results support the hypothesis that species discrimination and identification of individual characteristics are accomplished sequentially, and that auditory perception exhibits differences between sexes and in spatial dominance.

Low-frequency electroencephalogram oscillations govern left-eye lateralization during anti-predatory responses in the music frog

Visual lateralization is widespread for prey and anti-predation in numerous taxa. However, it is still unknown how the brain governs this asymmetry. In this study, we conducted behavioral and electrophysiological experiments to evaluate anti-predatory behaviors and dynamic brain activities in Emei music frogs (Nidirana daunchina), to explore the potential eye bias for anti-predation and the underlying neural mechanisms. To do this, predator stimuli (a model snake head and a leaf as a control) were moved around the subjects in clockwise and anti-clockwise directions at steady velocity. We counted the number of anti-predatory responses and measured electroencephalogram (EEG) power spectra for each band and brain area (telencephalon, diencephalon and mesencephalon). Our results showed that (1) no significant eye preferences could be found for the control (leaf); however, the laterality index was significantly lower than zero when the predator stimulus was moved anti-clockwise, suggesting that left-eye advantage exists in this species for anti-predation; (2) compared with no stimulus in the visual field, the power spectra of delta and alpha bands were significantly greater when the predator stimulus was moved into the left visual field anti-clockwise; and, (3) generally, the power spectra of each band in the right-hemisphere for the left visual field were higher than those in the left counterpart. These results support that the left eye mediates the monitoring of a predator in music frogs and lower-frequency EEG oscillations govern this visual lateralization.

Sex differences in vocalization are reflected by event-related potential components in the music frog

Sex differences in vocalization have been commonly found in vocal animals. It remains unclear, however, how animals perceive and discriminate these differences. The amplitudes and latencies of event-related potentials (ERP) components can reflect the auditory processing efficiency and time course. We investigated the neural mechanisms of auditory processing in the Emei music frog (Nidirana daunchina) using an Oddball paradigm with ERP. We recorded and analyzed eletroencephalogram (EEG) signals from the forebrain and midbrain when the subjects listened to white noise (WN) and conspecific sex-specific vocalizations. We found that (1) both amplitudes and latencies of some ERP components evoked by conspecific calls were significantly higher than those by WN, suggesting the music frogs can discriminate conspecific vocalizations from background noise; (2) both amplitudes and latencies of most ERP components evoked by female calls were significantly higher or longer than those by male calls, implying that the ERP components can reflect sex differences in vocalization; and (3) there were significant differences in ERP amplitudes between male and female subjects, suggesting a sexual dimorphism in auditory perception. Together, the present results indicate that the music frog could discriminate conspecific calls from noise, male's calls from female's ones, and sexual dimorphism of auditory perception existed in this species.

Dynamics of electroencephalogram oscillations underlie right-eye preferences in predatory behavior of the music frog

Visual lateralization is a typical characteristic of many vertebrates; however, its underlying dynamic neural mechanism is unclear. In this study, predatory responses and dynamic brain activities were evaluated in the Emei music frog (Nidirana daunchina) to assess the potential eye preferences and their underlying dynamic neural mechanism, using behavioral and electrophysiological experiments, respectively. To do this, when the prey stimulus (live cricket and leaf as control) was moved around the frogs in both clockwise and anticlockwise directions at constant velocity, the number of predatory responses were counted and electroencephalogram (EEG) absolute power spectra for each band were measured for the telencephalon, diencephalon and mesencephalon. The results showed that: (1) no significant differences in the number of predatory responses could be found for the control (leaf), but the number of predatory responses for the right visual field (RVF) was significantly greater than that for the left visual field (LVF) when the live cricket was moved into the RVF clockwise; (2) compared with no stimulus in the visual field and stimulus in the LVF, the power spectra of each EEG band were greater when the prey stimulus was moved into the RVF clockwise; and (3) the power spectra of the theta, alpha and beta bands in the left diencephalon were significantly greater than those of the right counterpart for the clockwise direction, but similar significant differences presented for the delta, theta and alpha bands in the anticlockwise direction. Together, the results suggested that right-eye preferences for predatory behaviors exist in music frogs, and that the dynamics of EEG oscillations might underlie this right eye/left hemisphere advantage.

Preference of spectral features in auditory processing for advertisement calls in the music frogs

BACKGROUND

Animal vocal signals encode very important information for communication during which the importance of temporal and spectral characteristics of vocalizations is always asymmetrical and species-specific. However, it is still unknown how auditory system represents this asymmetrical and species-specific patterns. In this study, auditory event related potential (ERP) changes were evaluated in the Emei music frog (Babina daunchina) to assess the differences in eliciting neural responses of both temporal and spectral features for the telencephalon, diencephalon and mesencephalon respectively. To do this, an acoustic playback experiment using an oddball paradigm design was conducted, in which an original advertisement call (OC), its spectral feature preserved version (SC) and temporal feature preserved version (TC) were used as deviant stimuli with synthesized white noise as standard stimulus.

RESULTS

The present results show that 1) compared with TC, more similar ERP components were evoked by OC and SC; and 2) the P3a amplitudes in the forebrain evoked by OC were significantly higher in males than in females.

CONCLUSIONS

Together, the results provide evidence for suggesting neural processing for conspecific vocalization may prefer to the spectral features in the music frog, prompting speculation that the spectral features may play more important roles in auditory object perception or vocal communication in this species. In addition, the neural processing for auditory perception is sexually dimorphic.

Auditory perception exhibits sexual dimorphism and left telencephalic dominance in Xenopus laevis

Sex differences in both vocalization and auditory processing have been commonly found in vocal animals, although the underlying neural mechanisms associated with sexual dimorphism of auditory processing are not well understood. In this study we investigated whether auditory perception exhibits sexual dimorphism in Xenopus laevis. To do this we measured event-related potentials (ERPs) evoked by white noise (WN) and conspecific calls in the telencephalon, diencephalon and mesencephalon respectively. Results showed that (1) the N1 amplitudes evoked in the right telencephalon and right diencephalon of males by WN are significantly different from those evoked in females; (2) in males the N1 amplitudes evoked by conspecific calls are significantly different from those evoked by WN; (3) in females the N1 amplitude for the left mesencephalon was significantly lower than for other brain areas, while the P2 and P3 amplitudes for the right mesencephalon were the smallest; in contrast these amplitudes for the left mesencephalon were the smallest in males. These results suggest auditory perception is sexually dimorphic. Moreover, the amplitude of each ERP component (N1, P2 and P3) for the left telencephalon was the largest in females and/or males, suggesting that left telencephalic dominance exists for auditory perception in Xenopus.

Summary: Investigation of auditory neural mechanisms in the South African clawed frog (Xenopus laevis) indicates that auditory perception exhibits sexual dimorphism and left telencephalic advantage.

The right thalamus may play an important role in anesthesia-awakening regulation in frogs

Background

Previous studies have shown that the mammalian thalamus is a key structure for anesthesia-induced unconsciousness and anesthesia-awakening regulation. However, both the dynamic characteristics and probable lateralization of thalamic functioning during anesthesia-awakening regulation are not fully understood, and little is known of the evolutionary basis of the role of the thalamus in anesthesia-awakening regulation.

Methods

An amphibian species, the South African clawed frog (Xenopus laevis) was used in the present study. The frogs were immersed in triciane methanesulfonate (MS-222) for general anesthesia. Electroencephalogram (EEG) signals were recorded continuously from both sides of the telencephalon, diencephalon (thalamus) and mesencephalon during the pre-anesthesia stage, administration stage, recovery stage and post-anesthesia stage. EEG data was analyzed including calculation of approximate entropy (ApEn) and permutation entropy (PE).

Results

Both ApEn and PE values differed significantly between anesthesia stages, with the highest values occurring during the awakening period and the lowest values during the anesthesia period. There was a significant correlation between the stage durations and ApEn or PE values during anesthesia-awakening cycle primarily for the right diencephalon (right thalamus). ApEn and PE values for females were significantly higher than those for males.

Discussion

ApEn and PE measurements are suitable for estimating depth of anesthesia and complexity of amphibian brain activity. The right thalamus appears physiologically positioned to play an important role in anesthesia-awakening regulation in frogs indicating an early evolutionary origin of the role of the thalamus in arousal and consciousness in land vertebrates. Sex differences exist in the neural regulation of general anesthesia in frogs.

Auditory neural networks involved in attention modulation prefer biologically significant sounds and exhibit sexual dimorphism in anurans

Allocating attention to biologically relevant stimuli in a complex environment is critically important for survival and reproductive success. In humans, attention modulation is regulated by the frontal cortex, and is often reflected by changes in specific components of the event-related potential (ERP). Although brain networks for attention modulation have been widely studied in primates and avian species, little is known about attention modulation in amphibians. The present study aimed to investigate the attention modulation networks in an anuran species, the Emei music frog (Babina daunchina). Male music frogs produce advertisement calls from within underground nest burrows that modify the acoustic features of the calls, and both males and females prefer calls produced from inside burrows. We broadcast call stimuli to male and female music frogs while simultaneously recording electroencephalographic (EEG) signals from the telencephalon and mesencephalon. Granger causal connectivity analysis was used to elucidate functional brain networks within the time window of ERP components. The results show that calls produced from inside nests which are highly sexually attractive result in the strongest brain connections; both ascending and descending connections involving the left telencephalon were stronger in males while those in females were stronger with the right telencephalon. Our findings indicate that the frog brain allocates neural attention resources to highly attractive sounds within the window of early components of ERP, and that such processing is sexually dimorphic, presumably reflecting the different reproductive strategies of males and females.

The First Call Note Plays a Crucial Role in Frog Vocal Communication

Vocal Communication plays a crucial role in survival and reproductive success in most amphibian species. Although amphibian communication sounds are often complex consisting of many temporal features, we know little about the biological significance of each temporal component. The present study examined the biological significance of notes of the male advertisement calls of the Emei music frog (Babina daunchina) using the optimized electroencephalogram (EEG) paradigm of mismatch negativity (MMN). Music frog calls generally contain four to six notes separated approximately by 150 millisecond intervals. A standard stimulus (white noise) and five deviant stimuli (five notes from one advertisement call) were played back to each subject while simultaneously recording multi-channel EEG signals. The results showed that the MMN amplitude for the first call note was significantly larger than for that of the others. Moreover, the MMN amplitudes evoked from the left forebrain and midbrain were typically larger than those from the right counterpart. These results are consistent with the ideas that the first call note conveys more information than the others for auditory recognition and that there is left-hemisphere dominance for processing information derived from conspecific calls in frogs.

Resting-state brain networks revealed by granger causal connectivity in frogs

Resting-state networks (RSNs) refer to the spontaneous brain activity generated under resting conditions, which maintain the dynamic connectivity of functional brain networks for automatic perception or higher order cognitive functions. Here, Granger causal connectivity analysis (GCCA) was used to explore brain RSNs in the music frog (Babina daunchina) during different behavioral activity phases. The results reveal that a causal network in the frog brain can be identified during the resting state which reflects both brain lateralization and sexual dimorphism. Specifically (1) ascending causal connections from the left mesencephalon to both sides of the telencephalon are significantly higher than those from the right mesencephalon, while the right telencephalon gives rise to the strongest efferent projections among all brain regions; (2) causal connections from the left mesencephalon in females are significantly higher than those in males and (3) these connections are similar during both the high and low behavioral activity phases in this species although almost all electroencephalograph (EEG) spectral bands showed higher power in the high activity phase for all nodes. The functional features of this network match important characteristics of auditory perception in this species. Thus we propose that this causal network maintains auditory perception during the resting state for unexpected auditory inputs as resting-state networks do in other species. These results are also consistent with the idea that females are more sensitive to auditory stimuli than males during the reproductive season. In addition, these results imply that even when not behaviorally active, the frogs remain vigilant for detecting external stimuli.