Bilateral matching of frequency tuning in neural cross-correlators of the owl

Sound localization requires comparison between the inputs to the left and right ears. One important aspect of this comparison is the differences in arrival time to each side, also called interaural time difference (ITD). A prevalent model of ITD detection, consisting of delay lines and coincidence-detector neurons, was proposed by Jeffress (J Comp Physiol Psychol 41:35–39, 1948). As an extension of the Jeffress model, the process of detecting and encoding ITD has been compared to an effective cross-correlation between the input signals to the two ears. Because the cochlea performs a spectrotemporal decomposition of the input signal, this cross-correlation takes place over narrow frequency bands. Since the cochlear tonotopy is arranged in series, sounds of different frequencies will trigger neural activity with different temporal delays. Thus, the matching of the frequency tuning of the left and right inputs to the cross-correlator units becomes a ‘timing’ issue. These properties of auditory transduction gave theoretical support to an alternative model of ITD-detection based on a bilateral mismatch in frequency tuning, called the ‘stereausis’ model. Here we first review the current literature on the owl’s nucleus laminaris, the equivalent to the medial superior olive of mammals, which is the site where ITD is detected. Subsequently, we use reverse correlation analysis and stimulation with uncorrelated sounds to extract the effective monaural inputs to the cross-correlator neurons. We show that when the left and right inputs to the cross-correlators are defined in this manner, the computation performed by coincidence-detector neurons satisfies conditions of cross-correlation theory. We also show that the spectra of left and right inputs are matched, which is consistent with predictions made by the classic model put forth by Jeffress. This article is part of a special issue on Neuronal Dynamics of Sensory Coding.     

Sensitivity to spectral interaural intensity difference cues in space-specific neurons of the barn owl

Barn owls use interaural intensity differences to localize sounds in the vertical plane. At a given elevation the magnitude of the interaural intensity difference cue varies with frequency, creating an interaural intensity difference spectrum of cues which is characteristic of that direction. To test whether space-specific cells are sensitive to spectral interaural intensity difference cues, pure-tone interaural intensity difference tuning curves were taken at multiple different frequencies for single neurons in the external nucleus of the inferior colliculus. For a given neuron, the interaural intensity differences eliciting the maximum response (the best interaural intensity differences) changed with the frequency of the stimulus by an average maximal difference of 9.4±6.2 dB. The resulting spectral patterns of these neurally preferred interaural intensity differences exhibited a high degree of similarity to the acoustic interaural intensity difference spectra characteristic of restricted regions in space. Compared to stimuli whose interaural intensity difference spectra matched the preferred spectra, stimuli with inverted spectra elicited a smaller response, showing that space-specific neurons are sensitive to the shape of the spectrum. The underlying mechanism is an inhibition for frequency-specific interaural intensity differences which differ from the preferred spectral pattern. Collectively, these data show that space-specific neurons are sensitive to spectral interaural intensity difference cues and support the idea that behaving barn owls use such cues to precisely localize sounds.Abbreviations ABI average binaural intensity - HRTF head-related transfer function - ICx external nucleus of the inferior colliculus - IID interaural intensity difference - ITD interaural time difference - OT optic tectum - RMS root mean square - VLVp nucleus ventralis lemnisci laterale, pars posterior     

Sensitivity to interaural time difference and representation of azimuth in central nucleus of inferior colliculus in the barn owl

Standard electrophysiology and virtual auditory stimuli were used to investigate the influence of interaural time difference on the azimuthal tuning of neurons in the core and the lateral shell of the central nucleus of the inferior colliculus of the barn owl. The responses of the neurons to virtual azimuthal stimuli depended in a periodic way on azimuth. Fixation of the interaural time difference, while leaving all other spatial cues unchanged, caused a loss of periodicity and a broadening of azimuthal tuning. This effect was studied in more detail in neurons of the core. The azimuthal range tested and the frequency selectivity of the neurons were additional parameters influencing the changes induced by fixating the interaural time difference. The addition of an interaural time difference to the virtual stimuli resulted in a shift of the tuning curves that correlated with the interaural time difference added. In this condition, tuning strength did not change. These results suggest that interaural time difference is an important determinant of azimuthal tuning in all neurons of the core and lateral shell of the central nucleus of the inferior colliculus, and is the only determinant in many of the neurons from the core.     

A temporal window for lateralization of interaural time difference by barn owls

Summary Lateralization of interaural time difference by barn owls (Tyto alba) was studied in a dichotic masking experiment. Sound bursts consisted of two parts: binaurally time-shifted noise, termed the probe, was inserted between masking noise. The owls indicated that they detected and lateralized the time-shift in the probe by a head turn in the direction predicted from sign of the time-shift.The general characteristics of head turns in response to this stimulus was similar to the head turns elicited by free-field stimulation or to head turns in response to presentation of the probe alone.The owls could easily lateralize stimuli containing long probes. The number of correct turns decreased as probe duration decreased, demonstrating that the masking noise interfered with the owls' ability to lateralize the probe. The minimal probe duration that the animals could lateralize (minimal duration) became shorter as burst duration decreased. Minimal durations ranged from 1 ms to 15 ms for the two subjects and burst durations from 10 to 100 ms.These findings suggested that owls possess a temporal window. A fitting procedure proposed by Moore et al. (1988) was used to determine the shape of the temporal window. The fitting procedure showed that the shape of the owls' binaural temporal window could be described by the same algorithms as the human monaural temporal window. Thus, the temporal window is composed of a short time constant that determines the central part of the window, and a longer time constant that determines the shape at the skirts of the window.Abbreviations ERD equivalent rectangular duration - ILD interaural level difference - ITD interaural time difference - RSE relative signal energy - SNR signal-to-noise ratio     

On the ability of neurons in the barn owl's inferior colliculus to sense brief appearances of interaural time difference

Summary This paper investigates the ability of neurons in the barn owl's (Tyto alba) inferior colliculus to sense brief appearances of interaural time difference (ITD), the main cue for azimuthal sound localization in this species. In the experiments, ITD-tuning was measured during presentation of a mask-probe-mask sequence. The probe consisted of a noise having a constant ITD, while the mask consisted of binaurally uncorrelated noise. Collicular neurons discriminated between the probe and masking noise by showing rapid changes from untuned to tuned and back to untuned responses.The curve describing the relation between probe duration and the degree of ITD-tuning resembled a leaky-integration process with a time constant of about 2 ms. Many neurons were ITD-tuned when probe duration was below 1 ms. These extremely short effective probe durations are interpreted as evidence for neuronal convergence within the pathway computing ITD. The minimal probe duration necessary for ITD-tuning was independent of the bandwidth of the neurons' frequency tuning and also of the best frequency of a neuron. Many narrowly tuned neurons having different best frequencies converge to form a broad-band neuron. To yield the short effective probe durations the convergence must occur in strong temporal synchronism.Abbreviations ICc central nucleus of the inferior colliculus; - ICx external nucleus of the inferior colliculus; - ITD interaural time difference - LP Likelihood parameter     

巴基斯坦旁遮普中部仓鸮食性的季节性变化

在巴基斯坦对仓鸮食性的季节变化进行了研究。通过分析连续3年在6个地区搜集的2 360个仓鸮回吐食物团,发现其食物主要是小型哺乳动物(95.6 %)。其中,家(Suncus murinus)有最高的比例,达65.6%(冬季最多78%,夏季最少27%)。就生物量而言,小型哺乳动物占仓鸮食物总生物量的99% 。     

Pupillary dilation response as an indicator of auditory discrimination in the barn owl

The pupil of an awake, untrained, head-restrained barn owl was found to dilate in response to sounds with a latency of about 25 ms. The magnitude of the dilation scaled with signal-to-noise ratio. The dilation response habituated when a sound was repeated, but recovered when stimulus frequency or location was changed. The magnitude of the recovered response was related to the degree to which habituating and novel stimuli differed and was therefore exploited to measure frequency and spatial discrimination. Frequency discrimination was examined by habituating the response to a reference tone at 3 kHz or 6 kHz and determining the minimum change in frequency required to induce recovery. We observed frequency discrimination of 125 Hz at 3 kHz and 250 Hz at 6 kHz – values comparable to those reported by others using an operant task. Spatial discrimination was assessed by habituating the response to a stimulus from one location and determining the minimum horizontal speaker separation required for recovery. This yielded the first measure of the minimum audible angle in the barn owl: 3° for broadband noise and 4.5° for narrowband noise. The acoustically evoked pupillary dilation is thus a promising indicator of auditory discrimination requiring neither training nor aversive stimuli. Accepted: 28 February 2000     

Mechanisms of experience-dependent plasticity in the auditory localization pathway of the barn owl

Sound localization is a computational process that requires the central nervous system to measure various auditory cues and then associate particular cue values with appropriate locations in space. Behavioral experiments show that barn owls learn to associate values of cues with locations in space based on experience. The capacity for experience-driven changes in sound localization behavior is particularly great during a sensitive period that lasts until the approach of adulthood. Neurophysiological techniques have been used to determine underlying sites of plasticity in the auditory space-processing pathway. The external nucleus of the inferior colliculus (ICX), where a map of auditory space is synthesized, is a major site of plasticity. Experience during the sensitive period can cause large-scale, adaptive changes in the tuning of ICX neurons for sound localization cues. Large-scale physiological changes are accompanied by anatomical remodeling of afferent axons to the ICX. Changes in the tuning of ICX neurons for cue values involve two stages: (1) the instructed acquisition of neuronal responses to novel cue values and (2) the elimination of responses to inappropriate cue values. Newly acquired neuronal responses depend differentially on NMDA receptor currents for their expression. A model is presented that can account for this adaptive plasticity in terms of plausible cellular mechanisms. Accepted: 17 April 1999     

Schedule discrimination in a mixed schedule: implications for models of the variable-ratio, variable-interval rate difference

In Experiment 1, each of three humans knowledgeable about operant schedules used mouse clicks to respond to a "work key" presented on a monitor. On a random half of the presentations, work-key responses that completed a variable ratio (VR) 12 produced a tone. After five tones, the work key was replaced by two report keys. Pressing the right or left report key, respectively, added or subtracted yen50 from a counter and produced the work key. On the other half of the presentations, a variable interval (VI) associated with the work key was defined so its interreinforcer intervals approximated the time it took to complete the variable ratio. After five tone-producing completions of this schedule, the report keys were presented. Left or right report-key presses, respectively, added or subtracted yen50 from the counter. Subjects achieved high yen totals. In Experiment 2, the procedure was changed by requiring an interresponse time after completion of the variable interval that approximated the duration of the reinforced interresponse time on the variable ratio. Prior to beginning, subjects were shown how a sequence of response bouts and pauses could be used to predict schedule type. Subjects again achieved high levels of accuracy. These results show humans can discriminate ratio from interval schedules even when those schedules provide the same rate of reinforcement and reinforced interresponse times.     

The influence of metabolic state on the level of phosphorylation of the light-harvesting chlorophyll-protein complex in chloroplasts isolated from maize mesophyll

The activity of the protein kinase that phosphorylates the light-harvesting chlorophyll-protein of Photosystem II (LHCP) has been investigated in intact chloroplasts isolated from maize mesophyll cells. Measurements of ³²P incorporation into LHCP, ATP concentration, $\text{ATP}\text{ADP}$ ratio, ΔpH, chlorophyll fluorescence and oxygen evolution were made in the presence of different metabolic substrates. Without added substrate a high level of LHCP phosphorylation was observed which was suppressed by addition of oxaloacetate or phosphoglycerate but stimulated by pyruvate. Whereas no correlation was observed between LHCP phosphorylation and adenylate status, a clear effect of redox state on protein kinase activity was observed. A correlation between a highly reduced electron-transfer chain (produced under conditions which favour cyclic electron flow) and the maximum level of protein phosphorylation was observed. The regulation of kinase activity and its dependence on electron transfer and carbon assimilation are discussed.     

Roles for the VCP co-factors Npl4 and Ufd1 in neuronal function in Drosophila melanogaster

The VCP-Ufd1-Npl4 complex regulates proteasomal processing within cells by delivering ubiquitinated proteins to the proteasome for degradation. Mutations in VCP are associated with two neurodegenerative diseases, amyotrophic lateral sclerosis (ALS) and inclusion body myopathy with Paget's disease of the bone and frontotemporal dementia (IBMPFD), and extensive study has revealed crucial functions of VCP within neurons. By contrast, little is known about the functions of Npl4 or Ufd1 in vivo. Using neuronal-specific knockdown of Npl4 or Ufd1 in Drosophila melanogaster, we infer that Npl4 contributes to microtubule organization within developing motor neurons. Moreover, Npl4 RNAi flies present with neurodegenerative phenotypes including progressive locomotor deficits, reduced lifespan and increased accumulation of TAR DNA-binding protein-43 homolog (TBPH). Knockdown, but not overexpression, of TBPH also exacerbates Npl4 RNAi-associated adult-onset neurodegenerative phenotypes. In contrast, we find that neuronal knockdown of Ufd1 has little effect on neuromuscular junction (NMJ) organization, TBPH accumulation or adult behaviour. These findings suggest the differing neuronal functions of Npl4 and Ufd1 in vivo.     

Electrostatic interactions in the denatured state and in the transition state for protein folding: effects of denatured state interactions on the analysis of transition state structure

The development of electrostatic interactions during the folding of the N-terminal domain of the ribosomal protein L9 (NTL9) is investigated by pH-dependent rate equilibrium free energy relationships. We show that Asp8, among six acidic residues, is involved in non-native, electrostatic interactions with K12 in the transition state for folding as well as in the denatured state. The perturbed native state pK(a) of D8 (pK(a) = 3.0) appears to be maintained through non-native interactions in both the transition state and the denatured state. Mutational effects on the stability of the transition state for protein (un)folding are often analyzed in respect to change in ground states. Thus, the interpretation of transition state analysis critically depends on an understanding of mutational effects on both the native and denatured state. Increasing evidence for structurally biased denatured states under physiological conditions raises concerns about possible denatured state effects on folding studies. We show that the structural interpretation of transition state analysis can be altered dramatically by denatured state effects.