Colliculus Neurons Research Articles

Signalling routes and developmental regulation of group I metabotropic glutamate receptors in rat auditory midbrain neurons

Group I metabotropic glutamate receptors (mGluRs) are linked to intracellular Ca(2+) signalling and play important roles related to synaptic plasticity and development. In neurons from the central nucleus of the inferior colliculus (CIC), the activation of these receptors evokes large [Ca(2+) ](i) responses. By using optical imaging of the fluorescent Ca(2+) -sensitive dye Fura-2, we have explored which [Ca(2+) ](i) routes are triggered by group I mGluR activation in young CIC neurons and whether mGluR-induced [Ca(2+) ](i) responses are regulated during postnatal development. In addition, real-time quantitative RT-PCR was used to study the developmental expression of both group I mGluR subtypes, mGluR1 and mGluR5. Application of DHPG, a specific agonist of group I mGluRs, was used on CIC slices from young rats to elicit [Ca(2+) ](i) responses. A majority of responses consisted of an initial thapsigargin-sensitive Ca(2+) peak, related to store depletion, followed by a plateau phase, sensitive to the store-operated Ca(2+) entry blocker 2-APB. During postnatal development, from P6 to P17, DHPG-induced [Ca(2+) ](i) responses changed. The largest Ca(2+) responses were reached at P6, whereas lower peak and plateau responses were found after hearing onset, at P13-P14 and P17. qRT-PCR analysis also revealed important differences in the expression of both mGluR1 and mGluR5 subtypes during development, with the highest levels of both subtypes at P7 and a developmental decrease of both transcripts. Our results suggest both intra- and extracellular routes for [Ca(2+) ](i) increases linked to group I mGluRs in CIC neurons and a regulation of group I mGluR activity and expression during auditory development.

Changes of gamma-amino butyric acid and electrophysiology inferior colliculus after noise exposure in guinea pig

To explore the changes of inferior collicular (IC) neurons after noise exposure cochlea injury in guinea pig to elucidate the encoding mechanism of pure tones, observe the changes of IC gamma-amino butyric acid (GABA) after cochlear damage by acoustic trauma and understand the possible mechanism of symptoms such as noise-induced tinnitus, hyperacusis and loudness recruitment. The responses of IC neurons to pure tone stimuli were observed in guinea pig at Day 1 and Days 11-21 after cochlear damage induced by noise exposure. And the IC neurons of normal guinea pig were assigned as the controls. Reverse transcription-polymerase chain reaction (RT-PCR) was used to measure the concentrations of GABA(A) and GABA(B) receptors. (1) The types of frequency reaction area (FRA) in the experiment group were the same as those in the control group (V-shape 84.8%, W-shape 8.9%, N-shape 6.3%). But the percentages of types were markedly different at Day 1 (V-shape 63.9%, W-shape 18.1%, N-shape, 18.1%) and Days 11-21 (V-shape 84.2%, W-shape 12.3%, N-shape 3.5%) after noise exposure. (2) After noise exposure, there was a marked fault in characteristic frequency (CF) and depth function map corresponding to 4 kHz (noise frequency). The rake ratio of CF and depth linear function map in the experiment group was lower than that of the control group. The control group, Day 1 and Days 11-21 after noise exposure, the rake ratios were 6.6, 5.8, 5.2 respectively. (3) GABA(A)/GABA(B) receptors decreased markedly at Days 1, 11 and 21 post-exposure compared to normal controls. And the values increased gradually with the prolonged time after exposure. The above findings conformed to the changes of electrophysiology of IC. After acoustic trauma, the responses of IC neurons to pure tone stimuli change with the elongation of time. It may be explained by the changes of IC GABA receptors after noise exposure.

The adaptive value of increasing pulse repetition rate during hunting by echolocating bats

During hunting, bats of suborder Microchiropetra emit intense ultrasonic pulses and analyze the weak returning echoes with their highly developed auditory system to extract the information about insects or obstacles. These bats progressively shorten the duration, lower the frequency, decrease the intensity and increase the repetition rate of emitted pulses as they search, approach, and finally intercept insects or negotiate obstacles. This dynamic variation in multiple parameters of emitted pulses predicts that analysis of an echo parameter by the bat would be inevitably affected by other co-varying echo parameters. The progressive increase in the pulse repetition rate throughout the entire course of hunting would presumably enable the bat to extract maximal information from the increasing number of echoes about the rapid changes in the target or obstacle position for successful hunting. However, the increase in pulse repetition rate may make it difficult to produce intense short pulse at high repetition rate at the end of long-held breath. The increase in pulse repetition rate may also make it difficult to produce high frequency pulse due to the inability of the bat laryngeal muscles to reach its full extent of each contraction and relaxation cycle at a high repetition rate. In addition, the increase in pulse repetition rate increases the minimum threshold (i.e. decrease auditory sensitivity) and the response latency of auditory neurons. In spite of these seemingly physiological disadvantages in pulse emission and auditory sensitivity, these bats do progressively increase pulse repetition rate throughout a target approaching sequence. Then, what is the adaptive value of increasing pulse repetition rate during echolocation? What are the underlying mechanisms for obtaining maximal information about the target features during increasing pulse repetition rate? This article reviews the electrophysiological studies of the effect of pulse repetition rate on multiple-parametric selectivity of neurons in the central nucleus of the inferior colliculus of the big brown bat, Eptesicus fuscus using single repetitive sound pulses and temporally patterned trains of sound pulses. These studies show that increasing pulse repetition rate improves multiple-parametric selectivity of inferior collicular neurons. Conceivably, this improvement of multiple-parametric selectivity of collicular neurons with increasing pulse repetition rate may serve as the underlying mechanisms for obtaining maximal information about the prey features for successful hunting by bats.

Recovery cycles of inferior collicular neurons in the leaf-nosed bat, Hipposideros armiger

When stimulated with biologically relevant constant frequency–frequency modulation (CF–FM) sounds, the inferior collicular neurons of the CF–FM bat, Hipposideros armiger, either only discharged impulses to the onset (76%, single-on neurons) of the CF-FM sounds or to the onset of both CF and FM components of CF–FM sounds (24%, double-on neuron). Some neurons were single-on responders at low sound amplitude but become double-on responders at high sound amplitude. Single-on responders had longer latency and recovery cycle than double-on responders. While most neurons did not respond to the second sound when the paired CF-FM sounds overlapped, 3 single-on and 7 double-on neurons did such that they had “cyclic” recovery cycles with inter-pulse intervals. The different response latency and dynamic variation in the recovery cycle of these two types of neurons suggest they may serve as the neural basis underlying a bat's ability to perform echo ranging throughout different phases of hunting.

Properties of echo delay-tuning receptive fields in the inferior colliculus of the mustached bat

One role of the inferior colliculus (IC) in bats is to create neuronal delay-tuning, which is used for the estimation of target distance in the echolocating bat’s auditory system. In this study, we describe response properties of IC delay-tuned neurons of the mustached bat (Pteronotus parnellii) and compare it with those of delay-tuned neurons of the auditory cortex (AC). We also address the question if frequency content of the stimulus (pure-tone (PT) or frequency-modulated (FM) pairs stimulation) affects combination-sensitive interaction in the same neuron. Sharpness and sensitivity of delay-tuned neurons in the IC are similar to those described in the AC. However, in contrast to cortical responses, in collicular neurons the delay at which the neurons show the maximum response does not change with changes in echo level. This tolerance to changes in the echo level seems to be a property of collicular delay-tuned neurons, which is modified along the ascending auditory pathway. In the IC we found neurons that showed a facilitated delay-tuned response when stimulated with FM components and did not show any delay-tuning with PT stimulation. This result suggests that not only is echo delay-tuning generated in the IC but also its FM-specificity observed in the cortex could be created to some extent in the IC and then topographically organized at higher levels.

Plasticity in the acquisition of multisensory integration capabilities in superior colliculus

The multisensory integration capabilities of superior colliculus (SC) neurons are normally acquired during early postnatal life and adapted to the environment in which they will be used. Recent evidence shows that they can even be acquired in adulthood, and require neither consciousness nor any of the reinforcement contingencies generally associated with learning. This process is believed to be based on Hebbian mechanisms, whereby the temporal coupling of multiple sensory inputs initiates development of a means of integrating their information. This predicts that co-activation of those input channels is sufficient to induce multisensory integration capabilities regardless of the specific spatiotemporal properties of the initiating stimuli. However, one might expect that the stimuli to be integrated should be consonant with the functional role of the neurons involved. For the SC, this would involve stimuli that can be localized. Experience with a non-localizable cue in one modality (e.g., ambient sound) and a discrete stimulus in another (e.g., a light flash) should not be sufficient for this purpose. Indeed, experiments with cats reared in omnidirectional sound (effectively masking discrete auditory events) reveal that the simple co-activation of two sensory input channels is not sufficient for this purpose. The data suggest that experience with the kinds of cross-modal events that facilitate the role of the SC in detecting, locating, and orienting to localized external events is a guiding factor in this maturational process. Supported by NIH grants NS 036916 and EY016716.

Neural basis of feature-based contextual effects on visual search behavior

Searching for a visual object is known to be adaptable to context, and it is thought to result from the selection of neural representations distributed on a visual salience map, wherein stimulus-driven and goal-directed signals are combined. Here we investigated the neural basis of this adaptability by recording superior colliculus (SC) neurons while three female rhesus monkeys (Macaca mulatta) searched with saccadic eye movements for a target presented in an array of visual stimuli whose feature composition varied from trial to trial. We found that sensory-motor activity associated with distracters was enhanced or suppressed depending on the search array composition and that it corresponded to the monkey's search strategy, as assessed by the distribution of the occasional errant saccades. This feature-related modulation occurred independently from the saccade goal and facilitated the process of saccade target selection. We also observed feature-related enhancement in the activity associated with distracters that had been the search target during the previous session. Consistent with recurrent processing, both feature-related neuronal modulations occurred more than 60 ms after the onset of the visually evoked responses, and their near coincidence with the time of saccade target selection suggests that they are integral to this process. These results suggest that SC neuronal activity is shaped by the visual context as dictated by both stimulus-driven and goal-directed signals. Given the close proximity of the SC to the motor circuit, our findings suggest a direct link between perception and action and no need for distinct salience and motor maps.

Dynamic temporal signal processing in the inferior colliculus of echolocating bats

In nature, communication sounds among animal species including humans are typical complex sounds that occur in sequence and vary with time in several parameters including amplitude, frequency, duration as well as separation, and order of individual sounds. Among these multiple parameters, sound duration is a simple but important one that contributes to the distinct spectral and temporal attributes of individual biological sounds. Likewise, the separation of individual sounds is an important temporal attribute that determines an animal's ability in distinguishing individual sounds. Whereas duration selectivity of auditory neurons underlies an animal's ability in recognition of sound duration, the recovery cycle of auditory neurons determines a neuron's ability in responding to closely spaced sound pulses and therefore, it underlies the animal's ability in analyzing the order of individual sounds. Since the multiple parameters of naturally occurring communication sounds vary with time, the analysis of a specific sound parameter by an animal would be inevitably affected by other co-varying sound parameters. This is particularly obvious in insectivorous bats, which rely on analysis of returning echoes for prey capture when they systematically vary the multiple pulse parameters throughout a target approach sequence. In this review article, we present our studies of dynamic variation of duration selectivity and recovery cycle of neurons in the central nucleus of the inferior colliculus of the frequency-modulated bats to highlight the dynamic temporal signal processing of central auditory neurons. These studies use single pulses and three biologically relevant pulse-echo (P-E) pairs with varied duration, gap, and amplitude difference similar to that occurring during search, approach, and terminal phases of hunting by bats. These studies show that most collicular neurons respond maximally to a best tuned sound duration (BD). The sound duration to which these neurons are tuned correspond closely to the behaviorally relevant sounds occurring at different phases of hunting. The duration selectivity of these collicular neurons progressively increases with decrease in the duration of pulse and echo, P-E gap, and P-E amplitude difference. GABAergic inhibition plays an important role in shaping the duration selectivity of these collicular neurons. The duration selectivity of these neurons is systematically organized along the tonotopic axis of the inferior colliculus and is closely correlated with the graded spatial distribution of GABAA receptors. Duration-selective collicular neurons have a wide range of recovery cycle covering the P-E intervals occurring throughout the entire target approaching sequences. Collicular neurons with low best frequency and short BD recover rapidly when stimulated with P-E pairs with short duration and small P-E amplitude difference, whereas neurons with high best frequency and long BD recover rapidly when stimulated with P-E pairs with long duration and large P-E amplitude difference. This dynamic variation of echo duration selectivity and recovery cycle of collicular neurons may serve as the neural basis underlying successful hunting by bats. Conceivably, high best frequency neurons with long BD would be suitable for echo recognition during search and approach phases of hunting when the returning echoes are high in frequency, large in P-E amplitude difference, long in duration but low in repetition rate. Conversely, low best frequency neurons with shorter BD and sharper duration selectivity would be suitable for echo recognition during the terminal phase of hunting when the highly repetitive echoes are low in frequency, small in P-E amplitude difference, and short in duration. Furthermore, the tonotopically organized duration selectivity would make it possible to facilitate the recruitment of different groups of collicular neurons along the tonotopic axis for effective processing of the returning echoes throughout the entire course of hunting.

Forward Masking Induced by Off-Best Frequency Sound on Inferior Collicular Neurons of <I>Miniopterus magnater</I>

Forward Masking Induced by Off-Best Frequency Sound on Inferior Collicular Neurons of <I>Miniopterus magnater</I>

Differential susceptibility of astrocytic and neuronal function to 3‐chloropropanediol in the rat inferior colliculus

We have previously shown that systemic administration of S(+)3-chloropropanediol (3-CPD) produces a morphological loss of astrocytes in specific nuclei of the rodent brain that precedes loss of both neurones and endothelial tight junctions. Here, we have evaluated the differential susceptibility of neuronal and astrocytic function to 3-CPD, in order to see if this parallels the morphological selectivity. To do this, we have developed an in vivo method for monitoring astrocyte function over time by giving hourly 20-min bolus challenge exposures to ammonia via an implanted microdialysis probe and measuring the resulting transient increases in the extracellular glutamine : glutamate ratio. These challenge ammonia exposures evoked a stable response for at least 5 h when the probe was implanted in the rat inferior colliculus, but caused no behavioural response or morphological damage. Although 3-CPD produced a rapid and sustained abolition of the ammonia response within 2 h, the field potential response of inferior collicular neurones to sound fell significantly to 75.0 ± 3.9% pre-dose at up to 8 h but then fell markedly, reaching 20.5 ± 3.7% at 2 days. Blood flow in the inferior colliculus also showed only late changes, increasing substantially at 2 days. Astrocyte damage at the EM level was seen from 3 h, followed by loss of astrocytes from 18 h to a minimum of 7 ± 10% control at 3 days. The rapid abolition of the ammonia response suggests that in addition to selective astrocyte death, 3-CPD also produces an earlier impairment of astrocyte function that precedes loss of neuronal function. This initial functional selectivity of 3-CPD provides a potential investigative tool in neurochemical studies of astrocyte-neuronal interactions.

Response property of inferior collicular neurons inherited from peripheral origin in mouse

The mechanisms underlying spike time coding in auditory system are not well understood. Despite of several models proposed to describe the first spike latency (FSL), there is no comparison of their respective performance. Here, based on FSL data from the central nucleus of inferior colliculus (CIC) in mouse to tone stimuli with varying rise function, rise time, and amplitude, we examined the previous models by comparing the recorded FSL with derived the FSL, respectively. We found that the LIEFTS (leaky integration, event formation, temporal summation) threshold model produced better match with the recorded data than other models. In addition, the model suggested that the short time constants derived from the FSL data (<2ms) cannot be attributed to IC neurons themselves (normally longer than 10ms), but are similar to those for the inner hair cells (around 1.4ms). Our results suggested that LIEFTS threshold model is a better fit for FSL, and FSL properties in central neurons can be inherited along the central auditory pathway, likely through faithful relays from the peripheral origins.

Recovery Cycle of Inferior Collicular Neurons Determine Pulse Following Rate in CF-FM Bat*

The characteristics of recovery cycles in inferior collicular (IC) neurons of leaf-nosed bat (Hipposideros armiger) and effect of the recovery cycle on the following pulse repetition rate were studied using mimic CF-FM sound stimuli emitted by free flying bat. Recovery cycle of 93 IC neurons were obtained from IC of five bats with normal hearing. These neurons were classified into three types, i.e. long recovery (LR, 47.4%), moderate recovery (MR, 35.1%), and short recovery (SR, 17.5%), according to their inter pulse interval (IPI) (ms) of 50% recovery under two CF-FM sound stimulation condition. Each type of the neurons could also be categorized into different sub-types according to changes induced by IPI increasing such as single-IPI response area neurons, multi-IPI response area neurons, and monotonic-IPI response neurons. Mean IPIs of 50% recovery of LR, MR, and SR neurons were (64.0 ± 24.8), (19.6 ± 5.8), and (7.1 ± 2.4) ms, respectively (P 0.001). The calculated theoretically following pulse repetition rate (pulse per second, Hz) of LR, MR, and SR neurons by mean IPI of 50% recovery for each type were (18.2 ± 7.0), (55.4 ± 15.7), and (171.3 ± 102.9) Hz, respectively (P 0.001). These three types of IC neurons were well corresponding to their three hunting phases, search, approach, and catch phases. The sub-types of single-IPI response area neurons and multi-IPI response area neurons had hunting phase selectivity, and sub-type of monotonic-IPI response neurons had well sensitivity to IPI change, but their hunting phase selectivity was of a sort. These results demonstrated that recovery cycle of IC neurons determined the ability to follow pulse repetition rate and matched this bat's echolocation behavior.

Inferior colliculus responses to dual-site intralamina stimulation in the ventral cochlear nucleus

A major limitation of the present auditory brainstem implant (ABI) is its inability to access the tonotopic organization of the ventral cochlear nucleus (VCN). A previous study by our group indicated that stimulation of single sites within a given VCN frequency region did not always elicit frequency-specific responses within the central nucleus of the inferior colliculus (CIC) and in some cases did not elicit a response at all. For this study, we hypothesized that sequential stimulation (with a short interpulse delay of 320 μsec) of two VCN sites in similar frequency regions would enhance responsiveness in CIC neurons. Multiunit neural recordings in response to pure tones were obtained at 58 VCN and 164 CIC sites in anesthetized rats. Among the 58 VCN sites, 39 pairs of sites with similar characteristic frequencies were chosen for electrical stimulation. Each member of a VCN pair was electrically stimulated individually, followed by sequential stimulation of the pair, while recording CIC responses. On average, CIC sites were found to respond to dual-site VCN stimulation with significantly lower thresholds, wider dynamic ranges, a greater extent of activation with increasing current levels, and a higher degree of frequency specificity compared with single-site stimulation. Although these effects were positive for the most part, in some cases dual-site stimulation resulted in increased CIC thresholds and decreased dynamic ranges, extent of activation, and frequency specificity. The results suggest that multisite stimulation within VCN isofrequency laminae using penetrating electrodes could significantly improve ABI stimulation strategies and implant performance.

Enhanced c-Fos expression in superior colliculus, paraventricular thalamus and septum during learning of cue-reward association

Reward-mediated associative learning is important for recognizing the significance of environmental cues. Such learning involves convergence of multimodal sensory inputs with circuits involved in affective and memory processes. Dopamine-dependent plasticity in the striatum plays a pivotal role, but the wider circuits engaged in cue-reward association are poorly understood. To identify candidate structures that may be of particular interest for further detailed electrophysiological and functional analysis, we quantified c-Fos expression in a selection of brain structures. c-Fos is a well-known marker of cell activation with additional potential importance for synaptic plasticity. We compared c-Fos expression between animals exposed to 100 pairings of a novel conditioned stimulus with a subsequent reward, and control animals exposed to the same number of cues and rewards, but where the cues and rewards occurred at random with respect to each other. We found significant increases in c-Fos expression in the superior colliculus in the group exposed to cue-reward pairing. This is consistent with previous recordings in conscious animals, showing modulation of phasic visual responses of single collicular neurons depending on their association with reward. Further, the data also suggest the possibility that the thalamic paraventricular nucleus and septal nuclei may be selectively activated during cue-reward association learning. Little is known of the neurophysiological responses in these structures during such tasks, so the present results suggest they would be targets of interest for future single-neuron recording experiments, designed to confirm whether the neurons show learning-specific modulation.

Gaze position effects and position-dependent motor tuning from primate superior colliculus (SC) neurons during head-unrestrained visually guided movements

Gaze position effects and position-dependent motor tuning from primate superior colliculus (SC) neurons during head-unrestrained visually guided movements

Preceding Inhibition Silences Layer 6 Neurons in Auditory Cortex

A canonical feedforward circuit is proposed to underlie sensory cortical responses with balanced excitation and inhibition in layer 4 (L4). However, in another input layer, L6, sensory responses and the underlying synaptic circuits remain largely unclear. Here, cell-attached recordings in rat primary auditory cortex revealed that for the majority of L6 excitatory neurons, tonal stimuli did not drive spike responses, but suppressed spontaneous firings. Whole-cell recordings further revealed that the silencing resulted from tone-evoked strong inhibition arriving earlier than excitation. This pattern of inputs can be attributed to a parallel feedforward circuit with both excitatory and inhibitory inputs disynaptically relayed. In contrast, in the other neurons directly driven by thalamic input, stimuli evoked excitation preceding relatively weak inhibition, resulting in robust spike responses. Thus, the dichotomy of L6 response properties arises from two distinct patterns of excitatory-inhibitory interplay. The parallel circuit module generating preceding inhibition may provide a gating mechanism for conditional corticothalamic feedback.

A Duration Coding Mechanism Underlying Bat Echo Recognition

Microchiropterans emit ultrasonic signals and analyze the returning echoes to catch prey and avoid obstacles. During this active acoustic behavior, to recognize the echoes of self-emitted pulses is of primary importance for successful prey capture. This article reviews our previous studies of bat midbrain inferior collicular neurons in regarding to their multiple-parametric selectivity in duration, frequency and amplitude during the dynamic variation of pulse parameters throughout a target approaching sequence. These studies show that duration selectivity is a neuronal response property underlying an animal's recognition of sound duration which is an important feature contributing to the distinct spectral and temporal attributes of individual biological sounds. These studies also show that duration-selective collicular neurons have maximal echo frequency and amplitude selectivity when the duration of both pulse and echo matches the neuron's best duration. Based on these findings, we propose that the bat could conceivably utilize the maximal echo frequency and amplitude selectivity of these duration-selective collicular neurons to extract prey feature and size from the expected echoes for successful prey capture or avoidance of obstacles.

A Multi-Component Model of the Developing Retinocollicular Pathway Incorporating Axonal and Synaptic Growth

During development, neurons extend axons to different brain areas and produce stereotypical patterns of connections. The mechanisms underlying this process have been intensively studied in the visual system, where retinal neurons form retinotopic maps in the thalamus and superior colliculus. The mechanisms active in map formation include molecular guidance cues, trophic factor release, spontaneous neural activity, spike-timing dependent plasticity (STDP), synapse creation and retraction, and axon growth, branching and retraction. To investigate how these mechanisms interact, a multi-component model of the developing retinocollicular pathway was produced based on phenomenological approximations of each of these mechanisms. Core assumptions of the model were that the probabilities of axonal branching and synaptic growth are highest where the combined influences of chemoaffinity and trophic factor cues are highest, and that activity-dependent release of trophic factors acts to stabilize synapses. Based on these behaviors, model axons produced morphologically realistic growth patterns and projected to retinotopically correct locations in the colliculus. Findings of the model include that STDP, gradient detection by axonal growth cones and lateral connectivity among collicular neurons were not necessary for refinement, and that the instructive cues for axonal growth appear to be mediated first by molecular guidance and then by neural activity. Although complex, the model appears to be insensitive to variations in how the component developmental mechanisms are implemented. Activity, molecular guidance and the growth and retraction of axons and synapses are common features of neural development, and the findings of this study may have relevance beyond organization in the retinocollicular pathway.

Somatosensory cross-modal plasticity in the superior colliculus of visually deafferented rats

The effects of neonatal visual deafferentation on the final adult pattern of cortico-collicular connections from the rat primary somatosensory cortex barrel field were studied by injecting an anterograde tracer (BDA) into different locations of the barrel cortex. Collicular afferents originating in the barrel cortex normally end in the intermediate collicular strata (SGI and SAI). However, neonatal visual deafferentation caused an invasion of abundant somatosensory cortical afferents into the lateral portions of the superficial collicular strata (SGS and SO). Moreover, anterograde-labelled fibers in the intermediate strata were more densely packed in visually deafferented animals. In order to study the activity of the altered somatosensory cortico-collicular connection, the effects of two different types of whisker stimuli on c-fos expression in the SC were analyzed (apomorphine treatment and enriched environment exploration). In stimulated control animals, c-fos expression was clearly evident in neurons of the intermediate layers 2 h after whisker stimulation. Similar stimulation in adult animals that underwent neonatal visual deafferentation triggered higher levels of c-fos expression in the superficial collicular layers that were invaded by cortico-collicular axonal branches. In exploration experiments, increased levels of c-fos expression were also detected in lateral parts of the intermediate layers of visually deafferented animals. These results suggest that the ascending fibers of somatosensory cortical origin can recruit deafferented superficial collicular neurons that enabling them to participate in extravisual behavioural responses mediated by collicular circuits.

The auditory response properties of single-on and double-on responders in the inferior colliculus of the leaf-nosed bat, Hipposideros armiger

The present study examines the response properties of neurons in the central nucleus of the inferior colliculus (IC) of the CF–FM (constant frequency–frequency-modulated) bat, Hipposideros armiger using CF, FM and CF–FM sounds as stimuli. All 169 IC neurons recorded are tonotopically organized along the dorsoventral axis of the IC. Collicular neurons have V-shaped or upper-threshold frequency tuning curves. Those neurons tuned at the predominant second harmonic have extremely sharp frequency tuning curves and low minimum thresholds. Collicular neurons typically discharge impulses to both CF and FM sounds. However, when stimulated with CF–FM sounds, most (76%) neurons only discharge impulses to the onset of CF–FM sounds (single-on responders). The remaining neurons (24%) discharge impulses to both CF and FM components (double-on responders) of CF–FM sounds. The double-on responders have higher minimum threshold and longer latency to the FM component than to the CF component of CF–FM sounds. Our data show that the FM component of the CF–FM sounds contributes significantly in shaping the discharge pattern, latency and number of impulses of IC neurons. The present study suggests that using CF–FM sounds to study auditory response properties of the CF–FM bat may be essential for a better understanding of echo analysis by the CF–FM in the real world. Because the double-on responders have shorter response latency than single-on responders, we speculate that these two types of responders may be best suited for echo analysis during different phases of hunting.