CLICK-EVOKED POTENTIAL MAP FROM THE SUPERIOR OLIVARY NUCLEUS

Somatic Integration of Incoherent Dendritic Inputs in the Gerbil Medial Superior Olive

The medial superior olive (MSO) is a binaural nucleus that is specialized in detecting the relative arrival times of sounds at both ears. Excitatory inputs to its neurons originating from either ear are segregated to different dendrites. To study the integration of synaptic inputs both within and between dendrites, we made juxtacellular and whole-cell recordings from the MSO in anesthetized female gerbils, while presenting a "double zwuis" stimulus, in which each ear received its own set of tones, which were chosen in a way that all second-order distortion products (DP2s) could be uniquely identified. MSO neurons phase-locked to multiple tones within the multitone stimulus, and vector strength, a measure for spike phase-locking, generally depended linearly on the size of the average subthreshold response to a tone. Subthreshold responses to tones in one ear depended little on the presence of sound in the other ear, suggesting that inputs from different ears sum linearly without a substantial role for somatic inhibition. The "double zwuis" stimulus also evoked response components in the MSO neuron that were phase-locked to DP2s. Bidendritic subthreshold DP2s were quite rare compared with bidendritic suprathreshold DP2s. We observed that in a small subset of cells, the ability to trigger spikes differed substantially between both ears, which might be explained by a dendritic axonal origin. Some neurons that were driven monaurally by only one of the two ears nevertheless showed decent binaural tuning. We conclude that MSO neurons are remarkably good in finding binaural coincidences even among uncorrelated inputs.SIGNIFICANCE STATEMENT Neurons in the medial superior olive are essential for precisely localizing low-frequency sounds in the horizontal plane. From their soma, only two dendrites emerge, which are innervated by inputs originating from different ears. Using a new sound stimulus, we studied the integration of inputs both within and between these dendrites in unprecedented detail. We found evidence that inputs from different dendrites add linearly at the soma, but that small increases in somatic potentials could lead to large increases in the probability of generating a spike. This basic scheme allowed the MSO neurons to detect the relative arrival time of inputs at both dendrites remarkably efficient, although the relative size of these inputs could differ considerably.

Signatures of Somatic Inhibition and Dendritic Excitation in Auditory Brainstem Field Potentials

Extracellular voltage recordings (V_e ; field potentials) provide an accessible view of in vivo neural activity, but proper interpretation of field potentials is a long-standing challenge. Computational modeling can aid in identifying neural generators of field potentials. In the auditory brainstem of cats, spatial patterns of sound-evoked V_e can resemble, strikingly, V_e generated by current dipoles. Previously, we developed a biophysically-based model of a binaural brainstem nucleus, the medial superior olive (MSO), that accounts qualitatively for observed dipole-like V_e patterns in sustained responses to monaural tones with frequencies >∼1000 Hz (Goldwyn et al., 2014). We have observed, however, that V_e patterns in cats of both sexes appear more monopole-like for lower-frequency tones. Here, we enhance our theory to accurately reproduce dipole and non-dipole features of V_e responses to monaural tones with frequencies ranging from 600 to 1800 Hz. By applying our model to data, we estimate time courses of paired input currents to MSO neurons. We interpret these inputs as dendrite-targeting excitation and soma-targeting inhibition (the latter contributes non-dipole-like features to V_e responses). Aspects of inferred inputs are consistent with synaptic inputs to MSO neurons including the tendencies of inhibitory inputs to attenuate in response to high-frequency tones and to precede excitatory inputs. Importantly, our updated theory can be tested experimentally by blocking synaptic inputs. MSO neurons perform a critical role in sound localization and binaural hearing. By solving an inverse problem to uncover synaptic inputs from V_e patterns we provide a new perspective on MSO physiology.SIGNIFICANCE STATEMENT Extracellular voltages (field potentials) are a common measure of brain activity. Ideally, one could infer from these data the activity of neurons and synapses that generate field potentials, but this "inverse problem" is not easily solved. We study brainstem field potentials in the region of the medial superior olive (MSO); a critical center in the auditory pathway. These field potentials exhibit distinctive spatial and temporal patterns in response to pure tone sounds. We use mathematical modeling in combination with physiological and anatomical knowledge of MSO neurons to plausibly explain how dendrite-targeting excitation and soma-targeting inhibition generate these field potentials. Inferring putative synaptic currents from field potentials advances our ability to study neural processing of sound in the MSO.

A Test of the Stereausis Hypothesis for Sound Localization in Mammals

The relative arrival times of sounds at both ears constitute an important cue for localization of low-frequency sounds in the horizontal plane. The binaural neurons of the medial superior olive (MSO) act as coincidence detectors that fire when inputs from both ears arrive near simultaneously. Each principal neuron in the MSO is tuned to its own best interaural time difference (ITD), indicating the presence of an internal delay, a difference in the travel times from either ear to the MSO. According to the stereausis hypothesis, differences in wave propagation along the cochlea could provide the delays necessary for coincidence detection if the ipsilateral and contralateral inputs originated from different cochlear positions, with different frequency tuning. We therefore investigated the relation between interaural mismatches in frequency tuning and ITD tuning during in vivo loose-patch (juxtacellular) recordings from principal neurons of the MSO of anesthetized female gerbils. Cochlear delays can be bypassed by directly stimulating the auditory nerve; in agreement with the stereausis hypothesis, tuning for timing differences during bilateral electrical stimulation of the round windows differed markedly from ITD tuning in the same cells. Moreover, some neurons showed a frequency tuning mismatch that was sufficiently large to have a potential impact on ITD tuning. However, we did not find a correlation between frequency tuning mismatches and best ITDs. Our data thus suggest that axonal delays dominate ITD tuning.SIGNIFICANCE STATEMENT Neurons in the medial superior olive (MSO) play a unique role in sound localization because of their ability to compare the relative arrival time of low-frequency sounds at both ears. They fire maximally when the difference in sound arrival time exactly compensates for the internal delay: the difference in travel time from either ear to the MSO neuron. We tested whether differences in cochlear delay systematically contribute to the total travel time by comparing for individual MSO neurons the best difference in arrival times, as predicted from the frequency tuning for either ear, and the actual best difference. No systematic relation was observed, emphasizing the dominant contribution of axonal delays to the internal delay.

Neuronal coupling by endogenous electric fields: cable theory and applications to coincidence detector neurons in the auditory brain stem

The ongoing activity of neurons generates a spatially and time-varying field of extracellular voltage (Ve). This Ve field reflects population-level neural activity, but does it modulate neural dynamics and the function of neural circuits? We provide a cable theory framework to study how a bundle of model neurons generates Ve and how this Ve feeds back and influences membrane potential (Vm). We find that these "ephaptic interactions" are small but not negligible. The model neural population can generate Ve with millivolt-scale amplitude, and this Ve perturbs the Vm of "nearby" cables and effectively increases their electrotonic length. After using passive cable theory to systematically study ephaptic coupling, we explore a test case: the medial superior olive (MSO) in the auditory brain stem. The MSO is a possible locus of ephaptic interactions: sounds evoke large (millivolt scale)Vein vivo in this nucleus. The Ve response is thought to be generated by MSO neurons that perform a known neuronal computation with submillisecond temporal precision (coincidence detection to encode sound source location). Using a biophysically based model of MSO neurons, we find millivolt-scale ephaptic interactions consistent with the passive cable theory results. These subtle membrane potential perturbations induce changes in spike initiation threshold, spike time synchrony, and time difference sensitivity. These results suggest that ephaptic coupling may influence MSO function.

Predicting binaural responses from monaural responses in the gerbil medial superior olive

Accurate sound source localization of low-frequency sounds in the horizontal plane depends critically on the comparison of arrival times at both ears. A specialized brainstem circuit containing the principal neurons of the medial superior olive (MSO) is dedicated to this comparison. MSO neurons are innervated by segregated inputs from both ears. The coincident arrival of excitatory inputs from both ears is thought to trigger action potentials, with differences in internal delays creating a unique sensitivity to interaural time differences (ITDs) for each cell. How the inputs from both ears are integrated by the MSO neurons is still debated. Using juxtacellular recordings, we tested to what extent MSO neurons from anesthetized Mongolian gerbils function as simple cross-correlators of their bilateral inputs. From the measured subthreshold responses to monaural wideband stimuli we predicted the rate-ITD functions obtained from the same MSO neuron, which have a damped oscillatory shape. The rate of the oscillations and the position of the peaks and troughs were accurately predicted. The amplitude ratio between dominant and secondary peaks of the rate-ITD function, captured in the width of its envelope, was not always exactly reproduced. This minor imperfection pointed to the methodological limitation of using a linear representation of the monaural inputs, which disregards any temporal sharpening occurring in the cochlear nucleus. The successful prediction of the major aspects of rate-ITD curves supports a simple scheme in which the ITD sensitivity of MSO neurons is realized by the coincidence detection of excitatory monaural inputs.

A model of the medial superior olive explains spatiotemporal features of local field potentials

Local field potentials are important indicators of in vivo neural activity. Sustained, phase-locked, sound-evoked extracellular fields in the mammalian auditory brainstem, known as the auditory neurophonic, reflect the activity of neurons in the medial superior olive (MSO). We develop a biophysically based model of the neurophonic that accounts for features of in vivo extracellular recordings in the cat auditory brainstem. By making plausible idealizations regarding the spatial symmetry of MSO neurons and the temporal synchrony of their afferent inputs, we reduce the challenging problem of computing extracellular potentials in a 3D volume conductor to a one-dimensional problem. We find that postsynaptic currents in bipolar MSO neuron models generate extracellular voltage responses that strikingly resemble in vivo recordings. Simulations reproduce distinctive spatiotemporal features of the in vivo neurophonic response to monaural pure tones: large oscillations (hundreds of microvolts to millivolts), broad spatial reach (millimeter scale), and a dipole-like spatial profile. We also explain how somatic inhibition and the relative timing of bilateral excitation may shape the spatial profile of the neurophonic. We observe in simulations, and find supporting evidence in in vivo data, that coincident excitatory inputs on both dendrites lead to a drastically reduced spatial reach of the neurophonic. This outcome surprises because coincident inputs are thought to evoke maximal firing rates in MSO neurons, and it reconciles previously puzzling evoked potential results in humans and animals. The success of our model, which has no axon or spike-generating sodium currents, suggests that MSO spikes do not contribute appreciably to the neurophonic.

Directional hearing by linear summation of binaural inputs at the medial superior olive

Neurons in the medial superior olive (MSO) enable sound localization by their remarkable sensitivity to submillisecond interaural time differences (ITDs). Each MSO neuron has its own "best ITD" to which it responds optimally. A difference in physical path length of the excitatory inputs from both ears cannot fully account for the ITD tuning of MSO neurons. As a result, it is still debated how these inputs interact and whether the segregation of inputs to opposite dendrites, well-timed synaptic inhibition, or asymmetries in synaptic potentials or cellular morphology further optimize coincidence detection or ITD tuning. Using in vivo whole-cell and juxtacellular recordings, we show here that ITD tuning of MSO neurons is determined by the timing of their excitatory inputs. The inputs from both ears sum linearly, whereas spike probability depends nonlinearly on the size of synaptic inputs. This simple coincidence detection scheme thus makes accurate sound localization possible.

Oscillatory dipoles as a source of phase shifts in field potentials in the mammalian auditory brainstem

A popular model of binaural processing, proposed by Jeffress (1948), states that external interaural time delays (ITDs) are compensated by internal axonal delays allowing ITD to be spatially represented by a population of coincidence detectors in the medial superior olive (MSO). Isolating single-neuron responses in MSO is difficult because of the presence of a strong extracellular field potential known as the neurophonic, so that few studies have tested Jeffress's key prediction. Phase delays in the nucleus laminaris neurophonic in owls have been observed and are consistent with a Jeffress-like model. Here, we recorded neurophonic responses in cat MSO to monaural tones at locations along its dendritic axis. Fourier analysis of the neurophonic was used to extract amplitude and phase at the stimulus frequency. Amplitude, as a function of depth, showed two peaks separated by a dip. A half-cycle phase shift was observed at depths close to the dip, over a wide frequency range. Current source density analysis for contralateral (ipsilateral) stimulation shows a current source close to the neurophonic amplitude peak and a sink a few hundred micrometers ventromedially (dorsolaterally). These results are consistent with a dipole configuration: contralateral (ipsilateral) excitation causes a current sink at the ventromedial (dorsolateral) dendrites and a source at the soma and dorsolateral (ventromedial) dendrites. Incorporating these results in a dipole model explains the phase and amplitude patterns observed. We conclude that the half-cycle phase shift is consistent with a current dipole, making it difficult to derive measurements of axonal delays from the neurophonic.

Origin of the binaural interaction component in wave P4 of the short-latency auditory evoked potentials in the cat: evaluation of serial depth recordings from the brainstem

There is no general agreement on the origin of the binaural interaction (BI) component in auditory brainstem responses (ABRs). To study this issue the ABRs to monaural and binaural clicks with various interaural time differences (ITDs) were simultaneously recorded from the vertex and from a recording electrode aiming at the superior olive (SO) in cats. Electrode path was along the fibers of the lateral lemniscus (LL). Binaural difference potentials (BDPs), which were computed by subtracting the sum of the two monaural responses from the binaural response, were obtained at systematic depths and across a range of ITD values. It was observed that only a specific BDP deflection recorded at the level at which lemniscal fibers terminate in the nuclei of LL coincided in time with the most prominent BDP in the cat's vertex-recorded ABRs, the BDP in their wave P4. As ITD was increased, the latency shifts and amplitude decrements of the scalp-recorded far-field BDP wave exactly followed those recorded at this lemniscal near-field BDP locus. The data support our hypothesis that the BI component in wave P4 results from a binaural reduction in dischargings of axons ascending in the LL, with this reduction due to contralateral inhibition of the discharge activity of the inhibitory-excitatory units in the lateral nucleus of the SO. Furthermore, at the level of the SO, the BDP in the responses to contra-leading binaural clicks always had larger magnitudes than those evoked by ipsi-leading ones. This bilateral asymmetry is consistent with the view that the BDP in scalp-recorded ABRs is related to the function of sound lateralization.

A fiber tract model of auditory brain-stem responses

The hypothesis that auditory brain-stem responses (ABRs) are generated by action potentials in fiber tracts was tested by recording from frog sciatic nerves in a volume conductor. Far-field recording of the sciatic nerve action potentials confirmed the rule that the initial response of the electrode toward which the action potentials are traveling is positive with respect to the electrode from which they are retreating. This rule explained the initial polarity of response for transverse and longitudinal recordings in humans, and why right and left ear clicks evoke responses of opposite polarity for transverse recording, but the same polarity for longitudinal recording. This result was true for all of the waves evoked, up to 8 msec latency. A later myogenic potential known as the postauricular response did not invert with the ear stimulated. The possibility of a dendritic contribution to the ABR was discussed. It was concluded that the components of the ABR are more satisfactorily explained by action potentials than by somatic or dendritic synaptic activity.

Subcortical correlates of the auditory brainstem potentials in the monkey: referential responses

Referential responses correlated with the vertex auditory brain-stem potentials (ABSP) were recorded from different brainstem and diencephalic structures of large monkeys under barbiturate anesthesia. Polarity, latency, and amplitude of various response components were determined from structures located 2 mm apart along five different vertical trajectories aiming at the trapezoid body (TB), superior olivary complex (SOC), mesencephalic reticular formation (MRF), medial geniculate nucleus thalami (MG), and inferior colliculus (IC). Latency correlations, amplitude differences, voltage profiles, current-source-density-distribution, and current flows of the various response components were subsequently calculated. Subcortical referential responses were formed by seven initial fast positive components (I, II, A, B, C, D and E) and one late slow negative component (F) correlated to waves I to VII and SP3 of the vertex ABSP respectively. Positive components C and D' also correlated to SP1 and SN1 of ABSP respectively. Amplitude of component B at contralateral SOC and C, D' and F at contralateral MRF was significantly larger than those of other subcortical responses and vertex ABSP. A single component of the subcortical referential responses accompanied changes in voltage, CSD, and current flows of various brainstem structures. However, considering only major changes in these parameters, component B accompanied a voltage positive and a CSD positive-negative peak and an ascending current flow at the SOC; component C accompanied similar changes at MRF, and components D' and F accompanied voltage negative and CSD negative-positive-negative peaks and ascending and descending current flows at MRF. In contrast, no systematic changes in voltage, CSD and current flows accompanied components I, II, A, D and E.

Hippocampal potentials evoked by stimulation of olfactory basal forebrain and lateral septum in the rat

Electrophysiological characteristics of olfactory-hippocampal relations were examined because recent anatomical studies have described a substantial olfactory input to the hippocampus via the entorhinal cortex. Potentials evoked in the dorsal hippocampus of anesthetized rats by stimulation of the prepyriform cortex, pyriform cortex, diagonal band, lateral olfactory tract, anterior commissure, olfactory tubercle and anterior olfactory nucleus had similar characteristics, although latencies differed. For example, latencies were twice as long after stimulation of the obliquely oriented portion of the diagonal band than after stimulation of the prepyriform cortex. A relatively low-amplitude, initially negative wave was recorded in the subiculum, CA1 and CA2, and a relatively high-amplitude, initially positive wave was recorded in CA4 and the dentate gyrus. In CA3 negative potentials were observed at dorsal recording sites and positive potentials were recorded at more ventral sites. Peak latencies were usually two to four msec shorter for the negative than for the positive wave. Laminar distributions of responses evoked in the hippocampus by stimulation of the prepyriform cortex and diagonal band were evaluated by driving eight electrodes mounted on one carrier through the brain and were found to be strikingly similar. Maximal amplitudes of the negative wave were recorded at the level of stratum moleculare of CA1 and the subiculum, and peak amplitudes of the positive wave were associated with the hilus of the dentae gyrus. Transition from negative to positive waveforms occurred approximately at the hippocampal fissure. Although the negative and positive waves were usually elicited together, they also were separable in that only negative waves were recorded along some tracks and only positive waves along others. Also, various stimulation sites in the prepyriform cortex elicited stable high-amplitude positive waves accompanied by negative waves of varying amplitude. It is suggested that branches of the perforant path are involved in generation of the two waves and that activity in a number of olfactory structures may influence the hippocampus, probably via the perforant pathway. Thus, hippocampal potentials following prepyriform or diagnonal band stimulation were not abolished by transection of the fornix-fimbria. Dorsolateral septal stimulation evoked hippocampal responses with characteristics and distribution distinctly different from those evoked by stimulation of olfactory areas. The findings suggest that lateral septal stimulation may activate the hippocampus antidromically.

Superposition of binaural influences on single neuron activity in the medial superior olive elicited by electrical stimulation of the osseous spiral laminae

The single unit response of medial superior olive (accessory nucleus) neurons was investigated, when elicited by electrical stimulation of nervous processes in the osseous spiral laminae in both cochleae and also by antidromic stimulation from the inferior colliculi. The osseous spiral laminae were stimulated at the first, second or third turns with grounding of one of the unstimulated turns. Time of signal arrival differences (delta tau) at the two cochleae and also intensity differences (deltai) were varied. (2) It is concluded that (A) binaural interaction in the MSO is not based on totally excitatory or inhibitory influences from either ear, but on cycles of excitation and inhibition; (B) these influences approximately obey the laws of superposition, i.e., the influences add linearly to determine discharge. (3) It is demonstrated that: (a) electrical stimulation, unlike acoustical stimulation, elicits a regular neural response; (b) the neural response functions elicited by changes in delta tau are deltai exhibit maxima and minima and are not smooth functions; (c) fine latency changes in the neural response are correlated with changes in delta tau and deltai; (d) the stimulation of different turns of the osseous spiral laminae (producing different current flows) elicits a different neural response depending on the turn stimulated, an effect reflecting the anatomical tonotopic arrangement of the medial superior olive; (e) changes in the stimulus differences delta tau and deltai are correlated with the ratio of the spike count in a short period following the stimulus, to the spike count in a longer total period; (f) changes in the stimulus differences delta tau and deltai elicit a neural response which varies according to the turn stimulated.