Temporally patterned pulse trains affect duration tuning characteristics of bat inferior collicular neurons

 This study examines the effect of temporally patterned pulse trains on duration tuning characteristics of inferior collicular neurons of the big brown bat, Eptesicus fuscus, under free-field stimulation conditions. Using a 50% difference between maximal and minimal responses as a criterion, the duration tuning characteristics of inferior collicular neurons determined with pulse trains of different pulse durations are described as band-pass, long-pass, short-pass, and all-pass. Each band-pass neuron discharged maximally to a specific pulse duration that was at least 50% larger than the neuron's responses to a long- and a short-duration pulse. In contrast, each long- or short-pass neuron discharged maximally to a range of long- or short-duration pulses that were at least 50% larger than the minimal responses. The number of impulses of an all-pass neuron never differed by more than 50%. When pulse trains were delivered at different pulse repetition rates, the number of short-pass and band-pass neurons progressively increased with increasing pulse repetition rates. The slope of the duration tuning curves also became sharper when determined with pulse trains at high pulse repetition rates. Possible mechanisms underlying these findings are discussed. Accepted: 25 August 1999     

Temporally patterned pulse trains affect directional sensitivity of inferior collicular neurons of the big brown bat, Eptesicus fuscus

The directional sensitivity of inferior collicular neurons of the big brown bat, Eptesicus fuscus, was studied under free field stimulation conditions with 3 temporally patterned trains of sound pulses which differed in pulse repetition rate and duration. The directional sensitivity curves of 92 neurons studied can be described as hemifield, directionally-selective, or non-directional according to the variation in the number of impulses with pulse train direction. When these neurons were stimulated with all 3 pulse trains, the directional sensitivity curves of 50 neurons was unchanged but that of the other 42 neurons changed from one type into another. When these pulse trains were delivered at high pulse repetition rate and short pulse duration, they significantly sharpened the directional sensitivity of two thirds of the neurons examined by reducing the angular range and increasing the slope of their impulse directional sensitivity curves. These pulse trains also sharpened the slope of the threshold directional sensitivity curves of 25 neurons studied. However, when directional sensitivity of collicular neurons was determined with pulse trains that differed only in pulse repetition rate or in pulse duration, significant sharpening of directional sensitivity was rarely observed in all experimental conditions tested. Possible mechanisms underlying these findings are discussed.     

Duration selectivity of bat inferior collicular neurons improves with increasing pulse repetition rate

Insectivorous big brown bats, Eptesicus fuscus, progressively increase the pulse repetition rate (PRR) throughout the course of hunting. While increasing PRR conceivably facilitates bats to extract information about the targets, it also inevitably affects sensitivity of their auditory neurons to pulse parameters. The present study examined the effect of increasing PRR on duration selectivity of this bat's inferior collicular (IC) neurons by comparing their impulse-duration functions determined at different PRRs. Impulse-duration functions plotted with the number of impulses in response to single pulses against pulse duration at different PRRs were described as short-pass, band-pass, long-pass, and all-pass. Short- or long-pass neurons discharged maximally to a range of short or long pulse durations. Band-pass neurons discharged maximally to one pulse duration. These three types of IC neurons were called duration tuned neurons. All-pass neurons were not duration tuned because they did not discharge maximally to any pulse duration. Increasing PRR improved duration selectivity of IC neurons by (1) increasing the number of duration tuned neurons; (2) decreasing the critical duration concomitant with increasing slope of the impulse-duration function; and (3) decreasing the 50% duration range of the impulse-duration function. This improved duration selectivity with PRR may potentially facilitate prey capture by bats.     

The effect of sound intensity on duration-tuning characteristics of bat inferior collicular neurons

Previous studies have shown that inferior collicular neurons of the big brown bat, Eptesicus fuscus, serve as short-, band-, long- and all-pass filters for sound durations. Neurons with band-, short- and long-pass filtering characteristics discharged maximally to a specific sound duration or a range of sound durations. In contrast, neurons with all-pass filtering characteristics do not have duration selectivity. To determine if duration-tuning characteristics of collicular neurons were tolerant to changes in sound intensity, we examined the duration-tuning characteristics of collicular neurons using a wide range of sound intensities. Duration-tuning characteristics examined included the type, bandwidth and slope of duration-tuning curves. Sound intensity delivered within 20 dB of minimum threshold did not affect duration-tuning characteristics of all collicular neurons studied. Sound intensities at still higher levels did not affect the tuning characteristics of two-thirds of collicular neurons but decreased the duration selectivity and changed the duration-tuning curves of the remaining one-third of neurons from one type to another. However, these two groups of duration-tuning collicular neurons were not separately organized inside the inferior colliculus. The biological relevance of these findings to bat echolocation is discussed.     

Bicuculline application affects discharge pattern and pulse-duration tuning characteristics of bat inferior collicular neurons

This study examines the contribution of GABAergic inhibition to the discharge pattern and pulse duration tuning characteristics of 101 bat inferior collicular neurons by means of bicuculline application to their recording sites. When stimulated with single pulses, 56 (55%) neurons discharged 1 or 2 impulses (phasic responders), 42 (42%) discharged 3–10 impulses (phasic bursters) and 3 (3%) discharged impulses throughout the stimulus duration (tonic responders). Bicuculline application increased the number of impulses and changed the discharge patterns of 66 neurons. Using 50% difference between maximal and minimal responses as a criterion, the duration tuning characteristics of these neurons can be described as band-pass (20, 20%), long-pass (17, 17%), short-pass (33, 32%), and all-pass (31, 31%). Each band-pass neuron discharged maximally to a specific duration (the best duration) which was at least 50% larger than the neuron's responses to a long-duration pulse and a short-duration pulse. In contrast, each long- or short-pass neuron discharged maximally to a range of long or short duration pulses. Bicuculline application changed the duration tuning characteristics of 65 neurons. Possible mechanisms underlying duration tuning characteristics and the behavioral relevance to bat echolocation are discussed. Accepted: 4 November 1998     

Encoding repetition rate and duration in the inferior colliculus of the big brown bat,Eptesicus fuscus

1. Encoding of temporal stimulus parameters by inferior collicular (IC) neurons of Eptesicus fuscus was studied by recording their responses to a wide range of repetition rates (RRs) and durations at several stimulus intensities under free field stimulus conditions. 2. The response properties of 424 IC neurons recorded were similar to those reported in previous studies of this species. 3. IC neurons were classified as low-pass, band-pass, and high-pass according to their preference for RRs and/or durations characteristic of, respectively, search, approach, or terminal phases of echolocation. These neurons selectively process stimuli characteristic of the various phases of hunting. 4. Best RRs and best durations were not correlated with either the BFs or recording depths This suggests that each isofrequency lamina is capable of processing RRs and durations of all hunting phases. 5. Responses of one half of IC neurons studied were correlated with the stimulus duty cycle. These neurons may preferentially process terminal phase information when the bat's pulse emission duty cycle increases. 6. While the stimulus RR affected the dynamic range and overall profile of the intensity rate function, only little effect was observed with different stimulus durations.     

Temporally patterned sound pulse trains affect intensity and frequency sensitivity of inferior collicular neurons of the big brown bat, Eptesicus fuscus

This study examined the effect of temporally patterned pulse trains on intensity and frequency sensitivity of inferior collicular neurons of the big brown bat, Eptesicus fuscus. Intensity sensitivity of inferior collicular neurons was expressed by the dynamic range and slope of rate-intensity functions. Inferior collicular neurons with non-monotonic rate-intensity functions have smaller dynamic ranges and larger slopes than neurons with monotonic or saturated rate-intensity functions. Intensity sensitivity of all inferior collicular neurons improved by increasing the number of non-monotonic rate-intensity functions when the pulse repetition rate of pulse trains increased from 10 to 30 pulses per second. Intensity sensitivity of 43% inferior collicular neurons further improved when the pulse repetition rate of pulse trains increased still from 30 to 90 pulses per second. Frequency sensitivity of inferior collicular neurons was expressed by the Q10, Q20, and Q30 values of threshold frequency tuning curves and bandwidths of isointensity frequency tuning curves. Threshold frequency tuning curves of all inferior collicular neurons were V-shape and mirror-images of their counterpart isointensity frequency tuning curves. The Q10, Q20, and Q30 values of threshold frequency tuning curves of all inferior collicular neurons progressively increased and bandwidths of isointensity frequency tuning curves decreased with increasing pulse repetition rate in temporally patterned pulse trains. Biological relevance of these findings to bat echolocation is discussed.     

后掩蔽效应对大棕蝠下丘神经元声反应的影响

以回声定位蝙蝠为模式动物,采用在体动物细胞外单位记录法,研究了后掩蔽效应对下丘神经元声反应的影响。结果显示,部分神经元(38％,12／31)对测试声刺激的反应明显受到掩蔽声的抑制,其后掩蔽效应强弱与掩蔽声和测试声的相对强度差(inter-stimulus level difference,SLD),以及测试声与掩蔽声之间的间隔时间(inter-stimulus onset asynchrony,SOA)有关：当掩蔽声强度升高或测试声强度降低时,后掩蔽效应增强;而SOA的缩短,亦可见后掩蔽效应增强。另外,相当数量的神经元(52%,16／31)对测试声刺激的反应并不受掩蔽声的影响,其中有的神经元只有在特定SLD和SOA时,才表现出后掩蔽效应。而少数下丘神经元(10％,3／31)在特定SLD和SOA时,掩蔽声对测试声反应有易化作用。上述结果表明,部分下丘神经元参与了声认知活动中的后掩蔽形成过程,推测下丘神经元在定型声反应特性中,对掩蔽声诱导的兴奋前抑制性输入与测试声诱导的兴奋性输入之间的时相性动态整合起关键作用。     

Sound duration and sound pattern affect the recovery cycles of inferior collicular neurons in leaf-nosed bat, Hipposideros armiger

The effects of sound duration and sound pattern on the recovery cycles of inferior collicular (IC) neurons in constant frequency-frequency modulation (CF-FM) bats were explored in this study. Five leaf-nosed bats, Hipposideros armiger (4 males, 1 female, 43-50 g body weight), were used as subjects. The extracellular responses of IC neurons to paired sound stimuli with different duration and patterns were recorded, and the recovery was counted as the ratio of the second response to the first response. Totally, 169 sound-sensitive IC neurons were recorded in the experiment. According to the interpulse interval (IPI) of paired sounds when neurons reached 50% recovery (50% IPI), the recovery cycles of these IC neurons were classified into 3 types: fast recovery (F, the 50% IPI was less than 15 ms), short recovery (S, the 50% IPI was between 15.1 and 30 ms) and long recovery (L, the 50% IPI was more than 30 ms). When paired CF stimuli with 2 ms duration was used, the ratio of F neurons was 32.3%, and it decreased to 18.1% and 18.2% respectively when 5 and 7 ms CF stimuli were used. The ratios of S and L neurons were 41.5%, 33.7%, 29.1% and 26.2%, 48.2%, 52.7% respectively when 2, 5 and 7 ms CF stimuli were used. The average 50% IPI determined after stimulation with paired 2 ms, 5 ms and 7 ms CF sounds were (30.2 ± 27.6), (39.9 ± 29.1) and (49.4 ± 34.7) ms, respectively, and the difference among them was significant (P< 0.01). When the stimuli of paired 2 ms CF sounds were shifted to paired 2 ms FM sounds, the proportion of F, S and L neurons changed from 32.3%, 41.5%, 26.2% to 47.7%, 24.6%, 27.7%, respectively, and the average 50% IPI decreased from (30.2 ± 27.6) to (23.9 ± 19.0) ms (P< 0.05, n = 65). When paired 5+2 ms CF-FM pulses were used instead of 7 ms CF sounds, the proportion of F, S and L neurons changed from 18.2%, 29.1%, 52.7% to 29.1%, 27.3%, 43.6%, respectively, and the average 50% IPI decreased from (49.4 ± 34.7) to (36.3 ± 29.4) ms (P< 0.05, n = 55). All these results suggest that the CF and FM components in echolocation signal of CF-FM bats play different roles during bats' hunting and preying on. The FM component of CF-FM signal presenting in the terminal phase can increase the number of F type neurons and decrease the recovery cycles of IC neurons for processing high repetition echo information, which ensures the bat to analyze the target range and surface texture more accurately.     

The influence of thermal conditions on rectal temperature,respiration rate and pulse rate of lactating Holstein-Friesian cows in the humid tropics

The effect of minimum, maximum and mean ambient air temperatures and the temperature-humidity index (THI) of the same and the previous day on morning (a.m.) and afternoon (p.m.) rectal temperatures (RT), respiration rates (RR) and pulse rates (PR) were studied in 17 Holstein-Friesian cows over the first 125 days in the 3rd and 4th lactations. Physiological responses showed a diurnal pattern, being lower in the mornings than the afternoons: 38.6 vs 39.0° C for RT, 52.2 vs 60.7 breaths/min for RR and 58.1 vs 64.1 beats/min for PR. Correlations between RT and RR (r = – 0.043 to –0.046) and RT and PR (r = –0.178 to –0.261) were low (P> 0.05). Correlations between RR and PR (r = 0.353 to 0.365) were moderate (P<0.05). Weather variables, especially ambient temperature of the previous day, were more important and influenced physiological responses to a greater extent than other thermal factors the same day. Generally, physiological responses were influenced to a greater extent by ambient temperature than THI. Weather variables explained variations in RT (5.1–59.6%), in RR (13.0–17.8%) and in PR (22.1–25.4%). Relationships between weather variables the previous day and physiological responses were contradictory, with minimum and maximum values showing a negative relationship in contrast with a positive relationship for mean values.     

Breaking the trade-off: rainforest bats maximize bandwidth and repetition rate of echolocation calls as they approach prey

Both mammals and birds experience a performance trade-off between producing vocalizations with high bandwidths and at high repetition rate. Echolocating bats drastically increase repetition rate from 2–20 calls s⁻¹ up to about 170 calls s⁻¹ prior to intercepting airborne prey in order to accurately track prey movement. In turn, bandwidth drops to about 10–30 kHz for the calls of this ‘final buzz’. We have now discovered that Southeast Asian rainforest bats (in the vespertilionid subfamilies Kerivoulinae and Murininae) are able to maintain high call bandwidths at very high repetition rates throughout approach to prey. Five species of Kerivoula and Phoniscus produced call bandwidths of between 78 and 170 kHz at repetition rates of 140–200 calls s⁻¹ and two of Murina at 80 calls s⁻¹. The ‘typical’ and distinct drop in call frequency was present in none of the seven species. This stands in striking contrast to our present view of echolocation during approach to prey in insectivorous bats, which was established largely based on European and American members of the same bat family, the Vespertilionidae. Buzz calls of Kerivoula pellucida had mean bandwidths of 170 kHz and attained maximum starting frequencies of 250 kHz which makes them the most broadband and most highly pitched tonal animal vocalization known to date. We suggest that the extreme vocal performance of the Kerivoulinae and Murininae evolved as an adaptation to echolocating and tracking arthropods in the dense rainforest understorey.     

GABAergic inhibition and the effect of sound direction on rate-intensity functions of inferior collicular neurons of the big brown Bat,Eptesicus fuscus

GABAergic inhibition shapes many auditory response properties of neurons in the inferior colliculus of the big brown bat, Eptesicus fuscus. This study examined the role of GABAergic inhibition on direction-dependent rate-intensity functions of bat inferior collicular neurons. When plotted at three sound directions (60 degrees contralateral, 0 degrees and 60 degrees ipsilateral relative to recording site), most collicular neurons had nonmonotonic and saturated rate-intensity functions at 60 degrees contralateral and 0 degrees but had monotonic rate-intensity functions at 60 degrees ipsilateral. The dynamic range of rate-intensity functions of majority (>90%) of collicular neurons significantly decreased as the sound direction changed from 60 degrees contralateral to 60 degrees ipsilateral. Bicuculline application increased or decreased the dynamic range of IC neurons in different degrees with sound direction and abolished direction-dependent intensity sensitivity of these IC neurons. Possible mechanisms for these observations are discussed.     

抑制性频谱整合对大棕蝠下丘神经元声强敏感性的影响

自由声场条件下 ,采用特定双声刺激方法研究了不同频率通道之间的非线性整合对下丘神经元声强敏感性的调制作用。实验在 1 2只麻醉与镇定的大棕蝠 (Eptesicusfuscus)上进行 ,双电极同步记录 2个配对神经元的声反应动作电位。主要结果如下 :1 )所获 1 1 0个 (5 5对 )配对神经元中 ,85 5 %表现为抑制性频谱整合作用 ,其余 1 4 5 %为易化性频谱整合 ;2 )阈上 1 0dB (SPL)放电率抑制百分比与神经元最佳频率 (BF)及记录深度呈负相关 ;3)抑制效率随声刺激强度升高而逐步下降 ;4 )当掩蔽声分别位于神经元兴奋性频率调谐曲线(FTC)内 (MSin) /外 (MSout)时 ,其抑制效率存在差异。前者的放电率抑制百分比及声反应动力学范围(DR)下降百分比均显著高于后者 ;5 )抑制性频谱整合导致 3类DR改变 :6 1 6 %为下降、 1 0 9%增加、另有2 7 5 %变化小于 1 0 %。本结果进一步支持如下设想 :下丘不同频率通道之间的抑制性频谱整合参与了对强度编码的主动神经调制活动     

A model linking immediate serial recall,the Hebb repetition effect and the learning of phonological word forms

We briefly review the considerable evidence for a common ordering mechanism underlying both immediate serial recall (ISR) tasks (e.g. digit span, non-word repetition) and the learning of phonological word forms. In addition, we discuss how recent work on the Hebb repetition effect is consistent with the idea that learning in this task is itself a laboratory analogue of the sequence-learning component of phonological word-form learning. In this light, we present a unifying modelling framework that seeks to account for ISR and Hebb repetition effects, while being extensible to word-form learning. Because word-form learning is performed in the service of later word recognition, our modelling framework also subsumes a mechanism for word recognition from continuous speech. Simulations of a computational implementation of the modelling framework are presented and are shown to be in accordance with data from the Hebb repetition paradigm.     

Sound-evoked oscillation and paradoxical latency shift in the inferior colliculus neurons of the big fruit-eating bat, <Emphasis Type=Italic>Artibeus jamaicensis</Emphasis>

Frequency tuning, temporal response pattern and latency properties of inferior colliculus neurons were investigated in the big fruit-eating bat, Artibeus jamaicensis. Neurons having best frequencies between 48–72 kHz and between 24–32 kHz are overrepresented. The inferior colliculus neurons had either phasic (consisting in only one response cycle at all stimulus intensities) or long-lasting oscillatory responses (consisting of multiple response cycles). Seventeen percent of neurons displayed paradoxical latency shift, i.e. their response latency increased with increasing sound level. Three types of paradoxical latency shift were found: (1) stable, that does not depend on sound duration, (2) duration-dependent, that grows with increasing sound duration, and (3) progressive, whose magnitude increases with increasing sound level. The temporal properties of paradoxical latency shift neurons compare well with those of neurons having long-lasting oscillatory responses, i.e. median inter-spike intervals and paradoxical latency shift below 6 ms are overrepresented. In addition, oscillatory and paradoxical latency shift neurons behave similarly when tested with tones of different durations. Temporal properties of oscillation and PLS found in the IC of fruit-eating bats are similar to those found in the IC of insectivorous bats using downward frequency-modulated echolocation calls.     

The effect of population structure on the rate of evolution

Ecological factors exert a range of effects on the dynamics of the evolutionary process. A particularly marked effect comes from population structure, which can affect the probability that new mutations reach fixation. Our interest is in population structures, such as those depicted by ‘star graphs’, that amplify the effects of selection by further increasing the fixation probability of advantageous mutants and decreasing the fixation probability of disadvantageous mutants. The fact that star graphs increase the fixation probability of beneficial mutations has lead to the conclusion that evolution proceeds more rapidly in star-structured populations, compared with mixed (unstructured) populations. Here, we show that the effects of population structure on the rate of evolution are more complex and subtle than previously recognized and draw attention to the importance of fixation time. By comparing population structures that amplify selection with other population structures, both analytically and numerically, we show that evolution can slow down substantially even in populations where selection is amplified.     

The effect of strychnine, bicuculline, and picrotoxin on X and Y cells in the cat retina

The effect of intravenous strychnine and the GABA antagonists picrotoxin and bicuculline upon the discharge pattern of center-surround-organized cat retinal ganglion cells of X and Y type were studied. Stimuli (mostly scotopic, and some photopic) were selected such that responses from both on and off-center cells were either due to the center, due to the surround, or clearly mixed. Pre-drug control responses were obtained, and their behavior following administration of the antagonists was observed for periods up to several hours. X-cell responses were affected in a consistent manner by strychnine while being unaffected by GABA antagonists. All observed changes following strychnine were consistent with a shift in center-surround balance of X cells in favor of the center. For Y-cell responses to flashing annuli following strychnine, there was either no shift or a relatively small shift in center-surround balance. Compared to X-cell responses to flashing lights, those of Y cells were very little affected by strychnine and in most cases were unaffected. It thus appears that glycine plays a similar role in receptive field organization of X cells as does GABA in Y cells (Kirby and Enroth-Cugell, 1976. J. Gen. Physiol. 68:465-484).     

The effect of multifunctionality on the rate of evolution in yeast

Multifunctional genes are expected to evolve at lower rates because mutations in such genes that improve one function might often have deleterious effects on other functions. Here we tested for an association between multifunctionality and evolutionary rates in genes of Saccharomyces cerevisiae, and we find a highly significant negative correlation between the number of biological processes in which a gene is involved in and its rate of evolution. However, the magnitude of this effect is small, and the results do not support the notion that multifunctionality limits a gene's rate of evolution.