Direction-dependent corticofugal modulation of frequency-tuning curves of inferior collicular neurons in the big brown bat, Eptesicus fuscus

This study examined if corticofugal modulation of subcortical frequency-tuning curves varied with sound direction. Both excitatory and inhibitory frequency tuning curves of inferior collicular neurons of the big brown bat, Eptesicus fuscus were plotted before and during electrical stimulation in the auditory cortex at two sound directions (contra-40 degrees and ipsi-40 degrees). Most collicular neurons had broader excitatory frequency-tuning curves at contra-40 degrees but had broader inhibitory frequency-tuning curves at ipsi-40 degrees. Cortical electrical stimulation changed the excitatory minimum thresholds of most collicular neurons at a greater degree at ipsi-40 degrees than at contra-40 degrees. However, cortical electrical stimulation produced a greater increase in the sharpness of excitatory frequency-tuning curves of most corticofugally inhibited collicular neurons at contra-40 degrees but produced a greater decrease in the sharpness of excitatory frequency-tuning curves of most corticofugally facilitated collicular neurons at ipsi-40 degrees. Cortical electrical stimulation also produced a greater change in the sharpness of inhibitory frequency-tuning curves of most corticofugally inhibited collicular neurons at contra-40 degrees than at ipsi-40 degrees. Possible mechanisms for this direction-dependent corticofugal modulation of frequency-tuning curves of collicular neurons are discussed.     

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.     

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     

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。     

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.     

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.     

Echo SPL influences the ranging performance of the big brown bat,Eptesicus fuscus

Four bats of the species Eptesicus fuscus were trained in a two-alternative forced-choice procedure to discriminate between two phantom targets that differed in range. The rewarded stimulus was located at a distance of 52.7 cm, while the other unrewarded stimulus was further away. Only one target was presented at a time.In the first experiment we measured the range discrimination performance at an echo SPL of –28 dB relative to the bat's sonar transmission. A 75% correct performance level was arbitrarily defined as threshold and was obtained at a delay difference of 80 s, corresponding to a range difference of 13.8 mm.In the second experiment the delay difference was fixed at 150 s and the echo SPL varied between –8 and –48 dB relative to sonar emissions. The performance of the bats depended on the relative echo SPL. At –28 dB the bats showed the best performance. It deteriorated at an increase of the relative echo SPL to –18 dB and –8 dB. The performance also deteriorated when the relative echo SPL was reduced to –38 dB and –48 dB. Only at low relative echo SPLs did the bats partially compensate for the reduction in echo SPL and increased the SPL of their emitted signals by a few dB.Our results support the hypothesis that neurons exhibiting paradoxical latency shift may be involved in encoding target range. This hypothesis predicts a decrease in performance at high echo SPLs as we found it in our experiments. The observed reduction in performance at very low echo SPLs may be due to a decrease in S/N ratio.     

Retinofugal projections of the big brown bat, Eptesicus fuscus and the neotropical fruit bat, Artibeus jamaicensis

Retinal connections were studied in Eptesicus fuscus and Artibeus jamaicensis using anterograde axonal degeneration and autoradiographic techniques following unilateral enucleations and uniocular injections of radioactive amino acids. Although each retina projected bilaterally to the brainstem, the number of silver grains in the emulsion of autoradiographs indicated that nearly all fibers in the optic nerve entered the contralateral optic tract. Ipsilaterally, a major portion of the projection ended in the suprachiasmatic nucleus; caudal to the suprachiasmatic nucleus, the amount of label was so small that individual silver grains were counted to determine the location and quantity of label in other ipsilateral nuclei. In both species the retinal projection terminated bilaterally in the suprachiasmatic, dorsal lateral geniculate, ventral lateral geniculate, and pretectal olivary nuclei and contralaterally in the posterior pretectal nucleus, superficial gray layers of the superior colliculus, and nuclei of the accessory optic system. In Eptesicus the projection to the nucleus of the optic tract ended contralaterally, and in Artibeus it ended in this nucleus bilaterally. The results of this study revealed a basic theme in the optic projection of the two ecologically different microchiropterans. The results differed, however, in that the projection was larger and visually related nuclei were better developed in Artibeus. Such variations are presumed to relate to eye size and the relative use of vision by the two chiropterans.     

Detection of jitter in intertarget spacing by the big brown bat Eptesicus fuscus

We trained bats to detect intertarget jitter, i.e., relative motion between two virtual (electronically synthesized) targets. Both targets were themselves moving with respect to nearby objects (e.g., the microphone and speaker used to create the virtual targets) so that the only reliable cue available to the bats was variation in intertarget spacing. Given a target at 80 cm and another at 95, 110 or 125 cm, the threshold for intertarget jitter (ITJ) of the two bats tested was <10 μs, corresponding to <1.7 mm of range. When, for one bat, we increased the range instability of the targets by adding varying amounts of random range shift to the target complex (while preserving the correct intertarget spacing), ITJ threshold worsened. When we presented three targets, one of which was jittering, the bat's threshold improved to 0.9 μs (equivalent to 0.16 mm). If no second target was presented, i.e., if the task was to detect jitter added to a single moving target, then bats' jitter threshold was very high (>200 μs). Eptesicus fuscus appears to be very good at detecting changes in intertarget spacing, which might prove valuable for detecting targets moving relative to the background or for constructing a spatial image of a complex environment. Accepted: 7 April 1997     

Binaural and frequency representation in the primary auditory cortex of the big brown bat, Eptesicus fuscus

This study examines the binaural and frequency representation in the primary auditory cortex (AC) of the big brown bat, Eptesicus fuscus, by using an ear-phone stimulation system. All 306 cortical neurons studied were excited by contralateral sound stimulation but they were either excited, inhibited or not affected by ipsilateral sound stimulation. These cortical neurons were columnarly organized according to their binaural and frequency-tuning properties. The excitation-excitation columns which occupy about 15% of the AC are mainly aggregated within an oval-shaped area of the central AC. The excitation-inhibition neurons and binaural neurons with mixed properties are distributed in the remaining 85% of the surrounding primary AC. Although the best frequency (BF) of these neurons shows a tendency to decrease from high to low along the anteroposterior axis of the primary AC, systematic variation in BF is not always consistent across the entire mapping area. In particular, BFs of cortical neurons isolated in the anterior AC vary quite unsystematically such that neurons with similar BFs are aggregated in isolated patches. Isofrequency and binaural columns are segregated into bands that intersect each other. Accepted: 13 August 1997     

Echo delay versus spectral cues for temporal hyperacuity in the big brown bat,Eptesicus fuscus

Big brown bats can discriminate between echoes that alternate in delay (jitter) by as little as 10–15 ns and echoes that are stationary in delay. This delay hyperacuity seems so extreme that it has been rejected in favor of an explanation in terms of artifacts in echoes, most likely spectral in nature, that presumably are correlated with delay. Using different combinations of digital, analog, and cable delays, we dissociated the overall delay of jittering echoes from the size of the analog component of delay, which alone is presumed to determine the strength of the apparatus artifact. The bats' performance remains invariant with respect to the overall delay of the jittering echoes, not with respect to the amount of analog delay. This result is not consistent with the possible use of delay-related artifacts produced by the analog delay devices. Moreover, both electronic and acoustic measurements disclose no spectral cues or impedance-mismatch reflections in delayed signals, just time-delays. The absence of artifacts from the apparatus and the failure of overlap and interference from reverberation to account for the 10-ns result means that closing the gap between the level of temporal accuracy plausibly explained from physiology and the level observed in behavior may require a better understanding of the physiology.Abbreviations FM frequency-modulated - XCR cross-correlation function     

Terrestrial locomotor behaviors of the big brown bat (Vespertilionidae: Eptesicus fuscus)

Mammal Research - Although living bats (Mammalia: Chiroptera) demonstrate a wide variety of terrestrial behaviors and abilities, most research on terrestrial locomotor behaviors of bats has focused...     

在背景干扰条件下大棕蝠对静止及移动目标的探测

采用“双选”的心理物理学方法 ,研究了训练后的大棕蝠 (Eptesicusfuscus)在背景干扰的条件下探测半圆形目标的能力。半圆形目标系以静止、旋转、摆动或不同组合的旋转与摆动呈现于蝙蝠。在测试室 ,蝙蝠-目标间距从 3 0cm开始 ,依次递增 10cm直至 110cm为止。当蝙蝠 -目标间距小于 80cm时 ,目标回声的强度总是大于背景干扰声。由旋转目标反射的回声强度则依正弦波调制。结果发现 :蝙蝠对目标的正确探测率随蝙蝠 -目标间距的增加而降低 ;在每一特定间距 ,对移动目标的正确探测率均高于对静止目标的正确探测率     

Preference of a revolving target to a stationary one by the big brown bat, Eptesicus fuscus

Utilizing a three-ramp platform, we studied the detection of a revolving and a stationary target in the presence of background clutter by trained Eptesicus fuscus. During the test, the mean amplitude of echo from either target was always larger than that of the background echoes at the bat-to-target distance of 30, 70 and 100 cm. The amplitude of the echo reflected back from a revolving target was modulated between a maximum and a minimum value. An electric motor was used to revolve a target. The frequency contents of the motor noise were mostly below 1 kHz. While the total percent response of approaching either target is always more than 90% at every bat-to-target distance tested, the bats approach a revolving target more frequently than a stationary one. Echolocation pulses emitted by the bats during the test were recorded and analyzed. The bats shortened their pulse durations and interpulse intervals and lowered the frequency contents as they entered into the crawling phase from the searching phase. Potential interference of background echoes and ambient noise with the performance of the bats is discussed. The preference of a revolving target to a stationary one by the bats is perhaps due to the fact that a revolving target has a higher releasing value than a stationary one does.     

Frequency tuning, latencies, and responses to frequency-modulated sweeps in the inferior colliculus of the echolocating bat, Eptesicus fuscus

Neurons in the inferior colliculus (IC) of the awake big brown bat, Eptesicus fuscus, were examined for joint frequency and latency response properties which could register the timing of the bat's frequency-modulated (FM) biosonar echoes. Best frequencies (BFs) range from 10 kHz to 100 kHz with 50% tuning widths mostly from 1 kHz to 8 kHz. Neurons respond with one discharge per 2-ms tone burst or FM stimulus at a characteristic latency in the range of 3–45 ms, with latency variability (SD) of 50 μs to 4–6 ms or more. BF distribution is related to biosonar signal structure. As observed previously, on a linear frequency scale BFs appear biased to lower frequencies, with 20–40 kHz overrepresented. However, on a hyperbolic frequency (linear period) scale BFs appear more uniformly distributed, with little overrepresentation. The cumulative proportion of BFs in FM₁ and FM₂ bands reconstructs a scaled version of the spectrogram of FM broadcasts. Correcting FM latencies for absolute BF latencies and BF time-in-sweep reveals a subset of IC cells which respond dynamically to the timing of their BFs in FM sweeps. Behaviorally, Eptesicus perceives echo delay and phase with microsecond or even submicrosecond accuracy and resolution, but even with use of phase-locked FM and tone-burst stimuli the cell-by-cell precision of IC time-frequency registration seems inadequate by itself to account for the temporal acuity exhibited by the bat. Accepted: 21 June 1997