Parallel thalamocortical pathways for echolocation and passive sound localization in a gleaning bat, Antrozous pallidus

Evolution and Ecology of Silent Flight in Owls and Other Flying Vertebrates

We raise and explore possible answers to three questions about the evolution and ecology of silent flight of owls: (1) do owls fly silently for stealth, or is it to reduce self-masking? Current evidence slightly favors the self-masking hypothesis, but this question remains unsettled. (2) Two of the derived wing features that apparently evolved to suppress flight sound are the vane fringes and dorsal velvet of owl wing feathers. Do these two features suppress aerodynamic noise (sounds generated by airflow), or do they instead reduce structural noise, such as frictional sounds of feathers rubbing during flight? The aerodynamic noise hypothesis lacks empirical support. Several lines of evidence instead support the hypothesis that the velvet and fringe reduce frictional sound, including: the anatomical location of the fringe and velvet, which is best developed in wing and tail regions prone to rubbing, rather than in areas exposed to airflow; the acoustic signature of rubbing, which is broadband and includes ultrasound, is present in the flight of other birds but not owls; and the apparent relationship between the velvet and friction barbules found on the remiges of other birds. (3) Have other animals also evolved silent flight? Wing features in nightbirds (nocturnal members of Caprimulgiformes) suggest that they may have independently evolved to fly in relative silence, as have more than one diurnal hawk (Accipitriformes). We hypothesize that bird flight is noisy because wing feathers are intrinsically predisposed to rub and make frictional noise. This hypothesis suggests a new perspective: rather than regarding owls as silent, perhaps it is bird flight that is loud. This implies that bats may be an overlooked model for silent flight. Owl flight may not be the best (and certainly, not the only) model for "bio-inspiration" of silent flight.

Binaural Response Properties and Sensitivity to Interaural Difference of Neurons in the Auditory Cortex of the Big Brown Bat, Eptesicus fuscus

This study examines binaural response properties and sensitivity to interaural level difference of single neurons in the primary auditory cortex (AC) of the big brown bat, Eptesicus fuscus under earphone stimulation conditions. Contralateral sound stimulation always evoked response from all 306 AC neurons recorded but ipsilateral sound stimulation either excited, inhibited or did not affect their responses. High best frequency (BF) neurons typically had high minimum threshold (MT) and low BF neurons had low MT. However, both BF and MT did not correlate with their recording depth. The BF of these AC neurons progressively changed from high to low along the anteromedial-posterolateral axis of the AC. Their number of impulses and response latency varied with sound level and inter-aural level differences (ILD). Their number of impulses typically increased either monotonically or non-monotonically to a maximum and the latency shortened to a minimum at a specific sound level. Among 205 AC neurons studied at varied ILD, 178 (87%) and 127 (62%) neurons discharged maximally and responded with the shortest response latency at a specific ILD, respectively. Neurons sequentially isolated within an orthogonal electrode puncture shared similar BF, MT, binaurality and ILD curves. However, the response latency of these AC neurons progressively shortened with recording depth. Species-specific difference among this bat, the mustached bat and the pallid bat is discussed in terms of frequency and binaurality representation in the AC.

Accurate sound localization behavior in a gleaning bat, Antrozous pallidus

Acute auditory processing in bats is typically associated with echolocation. A subset of bats, called gleaners, listens to prey-generated noise to hunt surface-dwelling prey. Gleaners depend less on echolocation to hunt and, therefore, accurate localization of prey-generated noise is necessary for foraging success. Here we studied azimuth sound localization behavior in the pallid bat, a gleaning bat in which spatial encoding has been studied extensively. We tested pallid bats on a relatively difficult open loop task (single sound, duration ≤ 200 ms). The bats were trained to face the midline when stimulus was presented, and this was confirmed with video analysis. Bats localized broadband noise (5-30 kHz) from 1 out of 11 speakers spaced evenly across the horizontal plane of the frontal sound field. Approach to the correct speaker was rewarded. Pallid bats show accurate localization near the midline with mean errors between 3-6°. Remarkably, the accuracy does not decline significantly at peripheral locations with bats averaging  <~7° error upto 72° off midline. Manipulation of stimulus bandwidth shows that higher frequencies (20-30 kHz) are necessary for accurate localization. Comparative studies of gleaning bats will reveal convergent adaptations across auditory systems for non-echolocation-based behaviors in bats.

Tuning for rate and duration of frequency-modulated sweeps in the mammalian inferior colliculus

Responses of auditory duration-tuned neurons (DTNs) are selective for stimulus duration. We used single-unit extracellular recording to investigate how the inferior colliculus (IC) encodes frequency-modulated (FM) sweeps in the big brown bat. It was unclear whether the responses of so-called “FM DTNs” encode signal duration, like classic pure-tone DTNs, or the FM sweep rate. Most FM cells had spiking responses selective for downward FM sweeps. We presented cells with linear FM sweeps whose center frequency (CEF) was set to the best excitatory frequency and whose bandwidth (BW) maximized the spike count. With these baseline parameters, we stimulated cells with linear FM sweeps randomly varied in duration to measure the range of excitatory FM durations and/or sweep rates. To separate FM rate and FM duration tuning, we doubled (and halved) the BW of the baseline FM stimulus while keeping the CEF constant and then recollected each cell’s FM duration tuning curve. If the cell was tuned to FM duration, then the best duration (or range of excitatory durations) should remain constant despite changes in signal BW; however, if the cell was tuned to the FM rate, then the best duration should covary with the same FM rate at each BW. A Bayesian model comparison revealed that the majority of neurons were tuned to the FM sweep rate, although a few cells showed tuning for FM duration. We conclude that the dominant parameter for temporal tuning of FM neurons in the IC is FM sweep rate and not FM duration.

NEW & NOTEWORTHY Reports of inferior colliculus neurons with response selectivity to the duration of frequency-modulated (FM) stimuli exist, yet it remains unclear whether such cells are tuned to the FM duration or the FM sweep rate. To disambiguate these hypotheses, we presented neurons with variable-duration FM signals that were systematically manipulated in bandwidth. A Bayesian model comparison revealed that most temporally selective midbrain cells were tuned to the FM sweep rate and not the FM duration.

Adaptations for Substrate Gleaning in Bats: The Pallid Bat as a Case Study

Substrate gleaning is a foraging strategy in which bats use a mixture of echolocation, prey-generated sounds, and vision to localize and hunt surface-dwelling prey. Many substrate-gleaning species depend primarily on prey-generated noise to hunt. Use of echolocation is limited to general orientation and obstacle avoidance. This foraging strategy involves a different set of selective pressures on morphology, behavior, and auditory system organization of bats compared to the use of echolocation for both hunting and navigation. Gleaning likely evolved to hunt in cluttered environments and/or as a counterstrategy to reduce detection by eared prey. Gleaning bats simultaneously receive streams of echoes from obstacles and prey-generated noise, and have to segregate these acoustic streams to attend to one or both. Not only do these bats have to be exquisitely sensitive to the soft, low frequency sounds produced by walking/rustling prey, they also have to precisely localize these sounds. Gleaners typically use low intensity echolocation calls. Such stealth echolocation requires a nervous system that is attuned to low intensity sound processing. In addition, landing on the ground to hunt may bring gleaners in close proximity to venomous prey. In fact, at least 2 gleaning bat species are known to hunt highly venomous scorpions. While a number of studies have addressed adaptations for echolocation in bats that hunt in the air, very little is known about the morphological, behavioral, and neural specializations for gleaning in bats. This review highlights the novel insights gleaning bats provide into bat evolution, particularly auditory pathway organization and ion channel structure/function relationships. Gleaning bats are found in multiple families, suggesting convergent evolution of specializations for gleaning as a foraging strategy. However, most of this review is based on recent work on a single species – the pallid bat (Antrozous palli dus) – symptomatic of the fact that more comparative work is needed to identify the mechanisms that facilitate gleaning behavior.

Topography of sound level representation in the FM sweep selective region of the pallid bat auditory cortex

Sound level processing is a fundamental function of the auditory system. To determine how the cortex represents sound level, it is important to quantify how changes in level alter the spatiotemporal structure of cortical ensemble activity. This is particularly true for echolocating bats that have control over, and often rapidly adjust, call level to actively change echo level. To understand how cortical activity may change with sound level, here we mapped response rate and latency changes with sound level in the auditory cortex of the pallid bat. The pallid bat uses a 60-30 kHz downward frequency modulated (FM) sweep for echolocation. Neurons tuned to frequencies between 30 and 70 kHz in the auditory cortex are selective for the properties of FM sweeps used in echolocation forming the FM sweep selective region (FMSR). The FMSR is strongly selective for sound level between 30 and 50 dB SPL. Here we mapped the topography of level selectivity in the FMSR using downward FM sweeps and show that neurons with more monotonic rate level functions are located in caudomedial regions of the FMSR overlapping with high frequency (50-60 kHz) neurons. Non-monotonic neurons dominate the FMSR, and are distributed across the entire region, but there is no evidence for amplitopy. We also examined how first spike latency of FMSR neurons change with sound level. The majority of FMSR neurons exhibit paradoxical latency shift wherein the latency increases with sound level. Moreover, neurons with paradoxical latency shifts are more strongly level selective and are tuned to lower sound level than neurons in which latencies decrease with level. These data indicate a clustered arrangement of neurons according to monotonicity, with no strong evidence for finer scale topography, in the FMSR. The latency analysis suggests mechanisms for strong level selectivity that is based on relative timing of excitatory and inhibitory inputs. Taken together, these data suggest how the spatiotemporal spread of cortical activity may represent sound level.

Environmental acoustic cues guide the biosonar attention of a highly specialised echolocator

Sensory systems experience a trade-off between maximizing the detail and amount of sampled information. This trade-off is particularly pronounced in sensory systems that are highly specialised for a single task and thus experience limitations in other tasks. We hypothesised that combining sensory input from multiple streams of information may resolve this trade-off and improve detection and sensing reliability. Specifically, we predicted that perceptive limitations experienced by animals reliant on specialised active echolocation can be compensated for by the phylogenetically older and less specialised process of passive hearing. We tested this hypothesis in greater horseshoe bats, which possess morphological and neural specialisations allowing them to identify fluttering prey in dense vegetation using echolocation only. At the same time, their echolocation system is both spatially and temporally severely limited. Here, we show that greater horseshoe bats employ passive hearing to initially detect and localise prey-generated and other environmental sounds, and then raise vocalisation level and concentrate the scanning movements of their sonar beam on the sound source for further investigation with echolocation. These specialised echolocators thus supplement echo-acoustic information with environmental acoustic cues, enlarging perceived space beyond their biosonar range. Contrary to our predictions, we did not find consistent preferences for prey-related acoustic stimuli, indicating the use of passive acoustic cues also for detection of non-prey objects. Our findings suggest that even specialised echolocators exploit a wide range of environmental information, and that phylogenetically older sensory systems can support the evolution of sensory specialisations by compensating for their limitations.

Congruent representation of visual and acoustic space in the superior colliculus of the echolocating bat Phyllostomus discolor

The midbrain superior colliculus (SC) commonly features a retinotopic representation of visual space in its superficial layers, which is congruent with maps formed by multisensory neurons and motor neurons in its deep layers. Information flow between layers is suggested to enable the SC to mediate goal-directed orienting movements. While most mammals strongly rely on vision for orienting, some species such as echolocating bats have developed alternative strategies, which raises the question how sensory maps are organized in these animals. We probed the visual system of the echolocating bat Phyllostomus discolor and found that binocular high acuity vision is frontally oriented and thus aligned with the biosonar system, whereas monocular visual fields cover a large area of peripheral space. For the first time in echolocating bats, we could show that in contrast with other mammals, visual processing is restricted to the superficial layers of the SC. The topographic representation of visual space, however, followed the general mammalian pattern. In addition, we found a clear topographic representation of sound azimuth in the deeper collicular layers, which was congruent with the superficial visual space map and with a previously documented map of orienting movements. Especially for bats navigating at high speed in densely structured environments, it is vitally important to transfer and coordinate spatial information between sensors and motor systems. Here, we demonstrate first evidence for the existence of congruent maps of sensory space in the bat SC that might serve to generate a unified representation of the environment to guide motor actions.

Functional segregation of monaural and binaural selectivity in the pallid bat auditory cortex

Different fields of the auditory cortex can be distinguished by the extent and level tolerance of spatial selectivity. The mechanisms underlying the range of spatial tuning properties observed across cortical fields are unclear. Here, this issue was addressed in the pallid bat because its auditory cortex contains two segregated regions of response selectivity that serve two different behaviors: echolocation for obstacle avoidance and localization of prey-generated noise. This provides the unique opportunity to examine mechanisms of spatial properties in two functionally distinct regions. Previous studies have shown that spatial selectivity of neurons in the region selective for noise (noise-selective region, NSR) is level tolerant and shaped by interaural level difference (ILD) selectivity. In contrast, spatial selectivity of neurons in the echolocation region ('FM sweep-selective region' or FMSR) is strongly level dependent with many neurons responding to multiple distinct spatial locations for louder sounds. To determine the mechanisms underlying such level dependence, frequency, azimuth, rate-level responses and ILD selectivity were measured from the same FMSR neurons. The majority (∼75%) of FMSR neurons were monaural (ILD insensitive). Azimuth tuning curves expanded or split into multiple peaks with increasing sound level in a manner that was predicted by the rate-level response of neurons. These data suggest that azimuth selectivity of FMSR neurons depends more on monaural ear directionality and rate-level responses. The pallid bat cortex utilizes segregated monaural and binaural regions to process echoes and prey-generated noise. Together the pallid bat FMSR/NSR data provide mechanistic explanations for a broad range of spatial tuning properties seen across species.

Mechanisms of Sound Localization in Two Functionally Distinct Regions of the Auditory Cortex

The auditory cortex is necessary for sound localization. The mechanisms that shape bicoordinate spatial representation in the auditory cortex remain unclear. Here, we addressed this issue by quantifying spatial receptive fields (SRFs) in two functionally distinct cortical regions in the pallid bat. The pallid bat uses echolocation for obstacle avoidance and listens to prey-generated noise to localize prey. Its cortex contains two segregated regions of response selectivity that serve echolocation and localization of prey-generated noise. The main aim of this study was to compare 2D SRFs between neurons in the noise-selective region (NSR) and the echolocation region [frequency-modulated sweep-selective region (FMSR)]. The data reveal the following major differences between these two regions: (1) compared with NSR neurons, SRF properties of FMSR neurons were more strongly dependent on sound level; (2) as a population, NSR neurons represent a broad region of contralateral space, while FMSR selectivity was focused near the midline at sound levels near threshold and expanded considerably with increasing sound levels; and (3) the SRF size and centroid elevation were correlated with the characteristic frequency in the NSR, but not the FMSR. These data suggest different mechanisms of sound localization for two different behaviors. Previously, we reported that azimuth is represented by predictable changes in the extent of activated cortex. The present data indicate how elevation constrains this activity pattern. These data suggest a novel model for bicoordinate spatial representation that is based on the extent of activated cortex resulting from the overlap of binaural and tonotopic maps.

SIGNIFICANCE STATEMENT

Unlike the visual and somatosensory systems, spatial information is not directly represented at the sensory receptor epithelium in the auditory system. Spatial locations are computed by integrating neural binaural properties and frequency-dependent pinna filtering, providing a useful model to study how neural properties and peripheral structures are adapted for sensory encoding. Although auditory cortex is necessary for sound localization, our understanding of how the cortex represents space remains rudimentary. Here we show that two functionally distinct regions of the pallid bat auditory cortex represent 2D space using different mechanisms. In addition, we suggest a novel hypothesis on how the nature of overlap between systematic maps of binaural and frequency selectivity leads to representation of both azimuth and elevation.

New perspectives on trophic guilds of arthropodivorous bats in North and Central America

Trophic guilds are useful concepts for advancing our knowledge of trophic structure of communities, dynamics of species interactions, redundancy in ecosystem services, resilience to disturbances, response to climate change, conservation strategies, etc. For insectivorous bats, current literature suggests 8 trophic-related guilds. These include 3 guilds based on the openness of foraging areas, 3 based on the style of feeding, and 2 recently proposed subguilds among gleaners. Some gleaners are “passive,” using densely cluttered vegetation in which echolocation is ineffective, and others are “actively” gleaning, using echolocation to procure prey. None of these guilds is based on the actual diets of bats. We analyzed 33 reports of diet composition representing 51 species of arthropod-feeding bats inhabiting North and Central America. We wanted to determine if the classical guild structure was concordant with the actual diets of bats and to compare guild structure in the Nearctic with that in the Neotropics. Discriminant function and principle component analyses generated 5 groups of genera based on the proportion of various arthropod taxa (mainly orders) in their diets. These groups were very different from classical guilds and showed almost no overlap among bat genera between the 2 continental regions. A similar analysis based on prey flying ability and hardness of their exoskeletons suggested 4 guilds that were more consistent with classical guild concepts, had higher rates of unambiguous guild assignment, and also showed major continental differences. Our results suggest a new arrangement of 4 guilds for arthropod-feeding bats in North and Central America that are based primarily on 2 features of their prey. New molecular techniques should allow us to build on this arrangement by significantly improving the taxonomic level of prey identification.

Los gremios tróficos son conceptos útiles para la mejora de nuestros conocimientos sobre la estructura trófica de las comunidades, la dinámica de las interacciones entre especies, la redundancia en los servicios de los ecosistemas, la capacidad de resistencia a las perturbaciones, la respuesta al cambio climático, las estrategias de conservación, etc. Para los murciélagos insectívoros la literatura actual sugiere ocho gremios tróficos. Estos incluyen tres gremios basados en la apertura de las zonas de alimentación, tres con base en el estilo de alimentación, es decir, la búsqueda aérea, caza de arrastre sobre superficies de agua, y recolección de presas en superficies, además de dos sub-gremios propuestos recientemente para los de hábitos recolectores. Algunos gremios son “pasivos”, los cuales forrajean en espacios excesivamente saturados de elementos de vegetación y hacen un uso de ecolocación es ineficaz, y otros recolectores “activos” los cuales utilizan la ecolocación para adquirir presas. Ninguno de estos gremios se basa en las dietas reales de murciélagos. Se analizaron 33 artículos sobre composición de la dieta, los cuales representan 51 especies de murciélagos de alimentación de artrópodos que habitan en Norte y Centro América. El objetivo del presente trabajo fue determinar si la estructura de los gremios clásicos era concordante con las dietas reales de los murciélagos, y comparar la estructura de los gremios entre las regiones Neártica y Neotrópical. En análisis de función discriminante y componentes principales se generaron 5 grupos de géneros con base a la proporción de los diferentes taxones de artrópodos (principalmente órdenes) contenidos en sus dietas. Estos grupos fueron muy diferentes de los gremios clásicos y mostraron casi ningún solapamiento entre los géneros de murciélagos de las dos regiones continentales. Un análisis similar con base en la capacidad de vuelo y la dureza de los exoesqueletos de las presas, ha apuntado a la conformación de cuatro gremios que estaban más en consonancia con los conceptos clásicos de gremio, tuvieron tasas más altas de la asignación inequívoca, y también mostró grandes diferencias continentales. Nuestros resultados sugieren un nuevo arreglo de cuatro gremios de murciélagos de alimentación de artrópodos en Norte y Centro América que se basan principalmente en dos características de su presa. Las nuevas técnicas moleculares deben permitir que construyamos sobre este acomodo, mejorando significativamente el nivel taxonómico de identificación presa.

Parvalbumin and calbindin expression in parallel thalamocortical pathways in a gleaning bat, Antrozous pallidus

The pallid bat (Antrozous pallidus) listens to prey-generated noise to localize and hunt terrestrial prey while reserving echolocation to avoid obstacles. The thalamocortical connections in the pallid bat are organized as parallel pathways that may serve echolocation and prey localization behaviors. Thalamic inputs to the cortical echolocation call- and noise-selective regions originate primarily in the suprageniculate nucleus (SG) and ventral division of medial geniculate body (MGBv), respectively. Here we examined the distribution of parvalbumin (PV) and calbindin (CB) expression in cortical regions and thalamic nuclei of these pathways. Electrophysiology was used to identify cortical regions selective for echolocation calls and noise. Immunohistochemistry was used to stain for PV and CB in the auditory cortex and MGB. A higher percentage (relative to Nissl-stained cells) of PV(+) cells compared with CB(+) cells was found in both echolocation call- and noise-selective regions. This was due to differences in cortical layers V-VI, but not layers I-IV. In the MGB, CB(+) cells were present across all divisions of the MGB, with a higher percentage in the MGBv than the SG. Perhaps the most surprising result was the virtual absence of PV staining in the MGBv. PV staining was present only in the SG. Even in the SG, the staining was mostly diffuse in the neuropil. These data support the notion that calcium binding proteins are differentially distributed in different processing streams. Our comparative data, however, do not support a general mammalian pattern of PV/CB staining that distinguishes lemniscal and nonlemniscal pathways.

Development of echolocation calls and neural selectivity for echolocation calls in the pallid bat

Studies of birdsongs and neural selectivity for songs have provided important insights into principles of concurrent behavioral and auditory system development. Relatively little is known about mammalian auditory system development in terms of vocalizations or other behaviorally relevant sounds. This review suggests echolocating bats are suitable mammalian model systems to understand development of auditory behaviors. The simplicity of echolocation calls with known behavioral relevance and strong neural selectivity provides a platform to address how natural experience shapes cortical receptive field (RF) mechanisms. We summarize recent studies in the pallid bat that followed development of echolocation calls and cortical processing of such calls. We also discuss similar studies in the mustached bat for comparison. These studies suggest: (1) there are different developmental sensitive periods for different acoustic features of the same vocalization. The underlying basis is the capacity for some components of the RF to be modified independent of others. Some RF computations and maps involved in call processing are present even before the cochlea is mature and well before use of echolocation in flight. Others develop over a much longer time course. (2) Normal experience is required not just for refinement, but also for maintenance, of response properties that develop in an experience independent manner. (3) Experience utilizes millisecond range changes in timing of inhibitory and excitatory RF components as substrates to shape vocalization selectivity. We suggest that bat species and call diversity provide a unique opportunity to address developmental constraints in the evolution of neural mechanisms of vocalization processing.

Mechanisms underlying azimuth selectivity in the auditory cortex of the pallid bat

This study focused on mechanisms underlying azimuth selectivity in the primary auditory cortex (A1) of pallid bats. The pallid bat listens to prey-generated noise (5-35 kHz) to localize and hunt terrestrial prey. The region of A1 tuned between 5 and 35 kHz consists of two clusters of neurons distinguished by interaural intensity difference (IID) selectivity: binaurally inhibited (EI) and peaked. The first aim of this study was to use sequential dichotic/free-field stimulation to test the hypothesis that IID is the primary cue underlying azimuth selectivity in neurons tuned in the prey-generated noise frequency band. IID selectivity and ear directionality at the neuron's characteristic frequency (CF) were used to predict azimuth selectivity functions. The predicted azimuth selectivity was compared with the actual azimuth selectivity from the same neurons. Prediction accuracy was similarly high for EI neurons and peaked neurons with low CF, whereas predictions were increasingly inaccurate with increasing CF among the peaked neurons. The second aim of this study was to compare azimuth selectivity obtained with noise and CF tones to determine the extent to which stimulus bandwidth influences azimuth selectivity in neurons with different binaural properties. The azimuth selectivity functions were similar for the two stimuli in the majority of EI neurons. A greater percentage of peaked neurons showed differences in their azimuth selectivity for noise and tones. This included neurons with multiple peaks when tested with tones and a single peak when tested with noise. Taken together, data from the two aims suggest that azimuth tuning of EI neurons is primarily dictated by IID sensitivity at CF. Peaked neurons, particularly those with high CF, may integrate IID sensitivity across frequency to generate azimuth selectivity for broadband sound. The data are consistent with those found in cat and ferret A1 in that binaurally facilitated neurons depend to a greater extent (compared to EI neurons) on spectral integration of binaural properties to generate azimuth selectivity for broadband stimuli.

Systematic representation of sound locations in the primary auditory cortex

The primary auditory cortex (A1) is involved in sound localization. A consistent observation in A1 is a clustered representation of binaural properties, but how spatial tuning varies within binaural clusters is unknown. Here, this issue was addressed in A1 of the pallid bat, a species that relies on passive hearing (as opposed to echolocation) to localize prey. Evidence is presented for systematic representations of sound azimuth within two binaural clusters in the pallid bat A1: the binaural inhibition (EI) and peaked (P) binaural interaction clusters. The representation is not a "point-to-point" space map as seen in the superior colliculus, but is in the form of a systematic increase in the area of activated cortex as azimuth changes from ipsilateral to contralateral locations. The underlying substrate in the EI cluster is a systematic representation of the medial boundary of azimuth receptive fields. The P cluster is activated mostly for sounds near the midline, providing a spatial acoustic fovea. Activity in the P cluster falls off systematically as the sound is moved to more lateral locations. Sensitivity to interaural intensity differences predicts azimuth tuning in the vast majority of neurons. Azimuth receptive field properties are relatively stable across intensity over a moderate range (20-40 dB above threshold) of intensities. This suggests that the maps will be similar across the intensities tested. These results challenge the current view that no systematic representation of azimuth is present in A1 and show that such representations are present locally within individual binaural clusters.

Comparison of auditory responses in the medial geniculate and pontine gray of the big brown bat, Eptesicus fuscus

The inferior colliculus has been well studied for its role of transmitting information from the brainstem to the thalamocortical system. However, it is also the source of a major pathway to the cerebellum, via the pontine gray (PG). We compared auditory responses from single neurons in the medial geniculate body (MGB) and PG of the awake big brown bat. MGB neurons were selective for a variety of stimulus types whereas PG neurons only responded to pure tones or simple FM sweeps. Best frequencies (BF) in MGB ranged from 8 kHz to > 80 kHz. BFs of PG neurons were all above 20 kHz with a high proportion above 60 kHz. The mean response latency was 19 ms for MGB neurons and 11 ms for PG neurons. MGB and PG contained neurons with a variety of discharge patterns but the most striking difference was the proportion of neurons with responses that lasted longer than the stimulus duration (MGB 13%, PG 58%). Both nuclei contained duration-sensitive neurons; the majority of those in MGB were band pass whereas in the PG they were long pass. Over half of the neurons in both nuclei were binaural. Differences between these nuclei are consistent with the idea that the thalamocortical pathway performs integration over time for cognitive analysis, thereby increasing selectivity and lengthening latency, while the colliculo-pontine pathway, which is more concerned with sensory-motor control, provides rapid input and a lasting trace of an auditory event.

Development of parallel auditory thalamocortical pathways for two different behaviors

Auditory thalamocortical connections are organized as parallel pathways that originate in different divisions of the medial geniculate body (MGB). These pathways may be involved in different functions. Surprisingly little is known about the development of these connections. Here we review studies of the organization and development of auditory thalamocortical pathways in the pallid bat. The pallid bat depends primarily on passive hearing of prey-generated noise for localizing prey, while reserving echolocation for general orientation and obstacle avoidance. In the inferior colliculus (IC) and the auditory cortex, physiological studies show that noise and echolocation calls are processed in segregated regions. Injection of retrograde tracers in physiologically characterized cortical sites show that the ventral division of the MGB (MGBv) projects to the cortical region selective for noise. The cortical region selective for echolocation calls receives input from the suprageniculate (SG) nucleus in the dorsal MGB, but not from the MGBv. Taken together, these studies reveal parallel IC-MGB-cortex pathways involved in echolocation and passive listening. There is overlap of thalamocortical pathways during development. At 2-weeks postnatal, when the bat begins to exhibit adult-like hearing thresholds, the SG projects to both noise- and echolocation call-selective regions. The MGBv, as in adults, projects only to the noise-selective region. The connections become adult-like only after 2-months postnatal. These data suggest that parallel auditory thalamocortical pathways may segregate in an experience-dependent fashion, a hypothesis that remains to be tested in any species.

Experience-dependent development of vocalization selectivity in the auditory cortex

Vocalization-selective neurons are present in the auditory systems of several vertebrate groups. Vocalization selectivity is influenced by developmental experience, but the underlying mechanisms are only beginning to be understood. Evidence is presented in this review for the hypothesis that plasticity of timing and strength of inhibition is a mechanism for plasticity of vocalization selectivity. The pallid bat echolocates using downward frequency modulated (FM) sweeps. Nearly 70% of neurons with tuning in the echolocation frequency range in its auditory cortex respond selectively to the direction and rate of change of frequencies present in the echolocation call. During development, FM rate selectivity matures early, while direction selectivity emerges later. Based on the time course of development it was hypothesized that FM direction, but not rate, selectivity is experience-dependent. This hypothesis was tested by altering echolocation experience during development. The results show that normal echolocation experience is required for both refinement and maintenance of direction selectivity. Interestingly, experience is required for the maintenance of rate selectivity, but not for initial development. Across all ages and experimental groups, the timing relationship between inhibitory and excitatory inputs explains sweep selectivity. These experiments suggest that inhibitory plasticity is a substrate for experience-dependent changes in vocalization selectivity.

Multiple mechanisms shape selectivity for FM sweep rate and direction in the pallid bat inferior colliculus and auditory cortex

The inferior colliculus and auditory cortex of the pallid bat contain a large percentage of neurons that are highly selective for the direction and rate of the downward frequency modulated (FM) sweep of the bat's echolocation pulse. Approximately 25% of neurons tuned to the echolocation pulse respond exclusively to downward FM sweeps. This review focuses on the finding that this selectivity is generated by multiple mechanisms that may act alone or in concert. In the inferior colliculus, selectivity for sweep rate is shaped by at least three mechanisms: shortpass or bandpass tuning for signal duration, delayed high-frequency inhibition that prevents responses to slow sweep rates, and asymmetrical facilitation that occurs only when two tones are presented at appropriate delays. When acting alone, the three mechanisms can produce essentially identical rate selectivity. Direction selectivity can be produced by two mechanisms: an early low-frequency inhibition that prevents responses to upward sweeps, and the same asymmetrical two-tone inhibition that shapes rate tuning. All mechanisms except duration tuning are also present in the auditory cortex. Discussion centers on whether these mechanisms are redundant or complementary.

GABA shapes a systematic map of binaural sensitivity in the auditory cortex

A consistent organizational feature of auditory cortex is a clustered representation of binaural properties. Here we address two questions. What is the intrinsic organization of binaural clusters and to what extent does intracortical processing contribute to binaural representation. We address these issues in the auditory cortex of the pallid bat. The pallid bat listens to prey-generated noise transients to localize and hunt terrestrial prey. As in other species studied, binaural clusters are present in the auditory cortex of the pallid bat. One cluster contains neurons that require binaural stimulation to be maximally excited, and are commonly termed predominantly binaural (PB) neurons. These neurons do not respond to monaural stimulation of either ear but show a peaked sensitivity to interaural intensity differences (IID) centered near 0 dB IID. We show that the peak IID varies systematically within this cluster. The peak IID is also correlated with the best frequency (BF) of neurons within this cluster. In addition, the IID selectivity of PB neurons is shaped by intracortical GABAergic input. Iontophoresis of GABA(A) receptor antagonists on PB neurons converts a majority of them to binaurally inhibited (EI) neurons that respond best to sounds favoring the contralateral ear. These data indicate that the cortex does not simply inherit binaural properties from lower levels but instead sharpens them locally through intracortical inhibition. The IID selectivity of the PB cluster indicates that the pallid bat cortex contains an increased representation of the frontal space that may underlie increased localization accuracy in this region.

Development of auditory thalamocortical connections in the pallid bat, Antrozous pallidus

Auditory thalamocortical connections are organized as parallel pathways originating in various nuclei of the medial geniculate body (MGB). The development of these pathways has not been studied. Therefore it remains unclear whether thalamocortical connections segregate before the onset of hearing or whether refinement of exuberant thalamocortical connections occurs following hearing onset. We studied this issue in the pallid bat. In adult pallid bats, parallel thalamocortical pathways represent two different sounds used in two different behaviors. The suprageniculate (SG) nucleus of the dorsal division of the MGB (MGBd) projects to a high-frequency cortical region selective for the echolocation calls, but not to a low-frequency cortical region sensitive to noise transients used in the localization of prey. Conversely, the ventral division (MGBv) projects to the low-frequency, but not the high-frequency, cortical region. Here we studied the development of these parallel pathways. Based on retrograde tracer injections in electrophysiologically characterized cortical loci, we show that there is an asymmetrical overlap in projection patterns from postnatal (P) day 15-60. The low-frequency region receives extensive input from both the SG and the MGBv. In contrast, the high-frequency region receives the great majority of its input from the SG, as in adults, whereas projections from the MGBv appear to make only a minor contribution, if any. By P150, these pathways are segregated and adult-like. These data suggest that these anatomically segregated pathways arise through postnatal refinement of initially overlapping connections.

The auditory cortex of the bat Phyllostomus discolor: Localization and organization of basic response properties

BACKGROUND

The mammalian auditory cortex can be subdivided into various fields characterized by neurophysiological and neuroarchitectural properties and by connections with different nuclei of the thalamus. Besides the primary auditory cortex, echolocating bats have cortical fields for the processing of temporal and spectral features of the echolocation pulses. This paper reports on location, neuroarchitecture and basic functional organization of the auditory cortex of the microchiropteran bat Phyllostomus discolor (family: Phyllostomidae).

RESULTS

The auditory cortical area of P. discolor is located at parieto-temporal portions of the neocortex. It covers a rostro-caudal range of about 4800 mum and a medio-lateral distance of about 7000 mum on the flattened cortical surface. The auditory cortices of ten adult P. discolor were electrophysiologically mapped in detail. Responses of 849 units (single neurons and neuronal clusters up to three neurons) to pure tone stimulation were recorded extracellularly. Cortical units were characterized and classified depending on their response properties such as best frequency, auditory threshold, first spike latency, response duration, width and shape of the frequency response area and binaural interactions. Based on neurophysiological and neuroanatomical criteria, the auditory cortex of P. discolor could be subdivided into anterior and posterior ventral fields and anterior and posterior dorsal fields. The representation of response properties within the different auditory cortical fields was analyzed in detail. The two ventral fields were distinguished by their tonotopic organization with opposing frequency gradients. The dorsal cortical fields were not tonotopically organized but contained neurons that were responsive to high frequencies only.

CONCLUSION

The auditory cortex of P. discolor resembles the auditory cortex of other phyllostomid bats in size and basic functional organization. The tonotopically organized posterior ventral field might represent the primary auditory cortex and the tonotopically organized anterior ventral field seems to be similar to the anterior auditory field of other mammals. As most energy of the echolocation pulse of P. discolor is contained in the high-frequency range, the non-tonotopically organized high-frequency dorsal region seems to be particularly important for echolocation.

When brains expand: mind and the evolution of cortex

OBJECTIVE

To critically examine the relationship between evolutionary and developmental influences on human neocortex and the properties of the conscious mind it creates.

METHODS

Using PubMed searches and the bibliographies of several monographs, we selected 50 key works, which offer empirical support for a novel understanding of the organization of the neocortex.

RESULTS

The cognitive gulf between humans and our closest primate relatives has usually been taken as evidence that our brains evolved crucial new mechanisms somehow conferring advanced capacities, particularly in association areas of the neocortex. In this overview of neocortical development and comparative brain morphometry, we propose an alternative view: that an increase in neocortical size, alone, could account for novel and powerful cognitive capabilities. Other than humans' very large brain in relation to the body weight, the morphometric relations between neocortex and all other brain regions show remarkably consistent exponential ratios across the range of primate species, including humans. For an increase in neocortical size to produce new abilities, the developmental mechanisms of neocortex would need to be able to generate an interarchy of functionally diverse cortical domains in the absence of explicit specification, and in this respect, the mammalian neocortex is unique: its relationship to the rest of the nervous system is unusually plastic, allowing great changes in cortical organization to occur in relatively short periods of evolution. The fact that even advanced abilities like self-recognition have arisen in species from different mammalian orders suggests that expansion of the neocortex quite naturally generates new levels of cognitive sophistication. Our cognitive and behavioural sophistication may, therefore, be attributable to these intrinsic mechanisms' ability to generate complex interarchies when the neocortex reaches a sufficient size.

CONCLUSION

Our analysis offers a parsimonious explanation for key properties of the human mind based on evolutionary influences and developmental processes. This view is perhaps surprising in its simplicity, but offers a fresh perspective on the evolutionary basis of mental complexity.

Development of functional organization of the pallid bat auditory cortex

The primary auditory cortex is characterized by a tonotopic map and a clustered organization of binaural properties. The factors involved in the development of overlain representation of these two properties are unclear. We addressed this issue in the auditory cortex of the pallid bat. The adult pallid bat cortex contains a systematic relationship between best frequency (BF) and binaural properties. Most neurons with BF<30 kHz are binaurally inhibited (EO/I), while most neurons with BF>30 kHz are monaural (EO). As in other species, binaural properties are clustered. The EO/I cluster contains a systematic map of interaural intensity difference (IID) sensitivity. We asked if these properties are present at the time the bat acquires its full audible range (postnatal day [P] 15). Tonotopy, relationship between BF and binaural properties, and the map of IID sensitivity are adult-like at P15. However, binaural facilitation is only observed in pups older than P25. Frequency selectivity shows a BF-dependent sharpening during development. Thus, overlain representation of binaural properties and tonotopy in the pallid bat cortex is remarkably adult-like at an age when the full audible range is first present, suggesting an experience-independent development of overlapping feature maps.