Neuroscience 154 (2008) 65–76

THE VOLLEY THEORY AND THE SPHERICAL CELL PUZZLE

P. X. JORISa* AND P. H. bSMITH proved to dovetail very well with the physiological parcel- aLaboratory of Auditory Neurophysiology, K.U.Leuven, Campus GHBlation of response categories based on responses to short O&N2, Herestraat 49 bus 1021, B-3000 Leuven, Belgium tone bursts Kiang( et al., 1965a; Pfeiffer,). 1966Other bDepartment of Anatomy, University of Wisconsin–Madison, 1300 Uni-studies lent further credence to Osen’s scheme in terms of versity Avenue, Madison, WI, USA projections patterns Warr,( 1982) and intrinsic electrical properties Oertel,( 1999). Osen’s insightful observations have thus served as an organizational principle which Abstract—Temporal coding in the auditory nerve is strikingly transformed in the cochlear nucleus. In contrast to fibersenabled in the remarkable progress in the understanding of the auditory nerve, some neurons in the cochlear nucleusthis nucleus in the 1970s and 1980s. can show “picket fence” phase-locking to low-frequency The study of the CN highlights one of the most inter- pure tones: they fire a precisely timed action potentialesting at features of the auditory system: its morphological every cycle of the stimulus. Such synchronization enhance-and physiological specializations to process temporal in- ment and entrainment is particularly prominent in neuronsformation in the acoustic waveform. We focus here on with the spherical and globular morphology, described temporalby processing and two neuron types, called the Osen [Osen KK (1969) Cytoarchitecture of the cochlear nuclei spherical and globular cells by Osen, and point out an in the cat. J Comp Neurol 136:453– 483]. These neurons re- ceive large axosomatic terminals from the auditory nerve—unsolved puzzle. the end bulbs and modified end bulbs of Held—and project to binaural comparator nuclei in the superior olivary complex. THE VOLLEY THEORY The most popular model to account for picket fence phase- locking is monaural coincidence detection. This mechanismOne hundred years ago, Lord RayleighStrutt, 1907( ) is plausible for globular neurons, which receive a large showednum- a relationship between the perceptual localization ber of inputs. We draw attention to the existence of enhancedof sound and the interaural phase of tones at the two ears. phase-locking and entrainment in spherical neurons, whichEven earlier,Thompson (1877) had described the sensi- receive too few end-bulb inputs from the auditory nerve to tivity of humans to ongoing interaural phase differences for make a coincidence detection of end-bulb firings a plausible mechanism of synchronization enhancement. © 2008 IBRO.low-frequency tones. These observations established un- Published by Elsevier Ltd. All rights reserved. equivocally that temporal information at the two ears is accessed by the CNS and that it is used in spatial percep- Key words: temporal coding, binaural, synchronization, am-tion. These early pioneers thereby provided strong support plitude modulation, cochlear nucleus, jitter. for the “telephone” theory, as opposed to the “resonance” theory: theories which can be traced back to Helmholtz Contents and Rutherford and which today are referred to as tempo- The volley theory 65 ral and place coding. Single unit phase-locking and Besides the binaural psychophysical observations, its enhancement 66 Relationship to morphological cell types in CN there68 was also physiological evidence for the telephone Mechanisms of synchronization enhancement 7 0theory (seeDavis, 1984for an interesting personal histor- The puzzle of enhanced synchronization in SBCs ical72 account). Early recordings of gross evoked potentials Acknowledgments 73 showed responses that phase-locked to the stimulus References 73 waveform up to several WeverkHz ( and Bray, 1930a). Investigators puzzled over this for two reasons. First, it was Biological taxonomy is always fraught with splitting knownvs. from single cell recording in other systems that lumping difficulties. Kirsten Osen’s morphological parcel-neurons display refractory behavior and are limited in their firing rates to a few hundred spikes per second (microelec- lation of the cochlear nucleus Osen,(CN) 1969( ) was a trode recordings from single auditory neurons only became landmark achievement because it hit exactly the right level available much later: Galambos and Davis, 1943; Tasaki, along the splitter-lumper dimension. Her parcellation 1954). How could auditory neurons have temporal infor- *Corresponding author. Tel: ϩ32-16-34-57-41; fax: ϩ32-16-34-59-93. mation above frequencies corresponding to “normal” firing E-mail address: [email protected] (P. X. Joris). Abbreviations: AN, auditory nerve; AVCN, anteroventral cochlear nu- rates? Second, how could neurons be phase-locked and at cleus; CF, characteristic frequency; CN, cochlear nucleus; GBC, glob- the same time carry intensity information in their discharge ular bushy cell; ITD, interaural time difference; LSO, lateral superior rate? The latter was seen as a requirement based on olive; MNTB, medial nucleus of the trapezoid body; MSO, medial observations in other sensory systems (Adrian, 1928). The superior olive; SBC, spherical bushy cell; SOC, superior olivary com- plex; TB, trapezoid body; VCN, ventral cochlear nucleus; VNLL, ven- volley theory (Wever and Bray, 1930b) solved these diffi- tral nucleus of the lateral lemniscus; VS, vector strength. culties and argued that the resonance and telephone the- 0306-4522/08$32.00ϩ0.00 © 2008 IBRO. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.neuroscience.2008.03.002

65 66 P. X. Joris and P. H. Smith / Neuroscience 154 (2008) 65–76 ories were not mutually exclusive. Wever and Bray rea- 1975; Rhode and Smith, 1986; Rhode and Kettner, 1987; soned that single fibers can be synchronized to the stim- Joris et al., 1994a). Both components, high VS and en- ulus waveform even if they do not fire at every stimulus trainment, likely contribute to the strongly phase-locked cycle, and that the combined output of a group of fibers can gross potentials measured in the CNS (Boudreau, 1965). carry the temporal waveform in a volley of spikes. In their The combination of enhanced phase-locking and entrain- words, “The situation is something like beating a tattoo with ment means that these neurons can be described as “vol- the two hands working alternately, and establishing a total ley-detectors”: they seem to collect phase-locked spikes frequency double that of either hand” (Wever and Bray, from a group of AN inputs to produce a precise pulse train 1930b). Here we illustrate that the temporal code which is at the stimulus frequency (Fig. 2D). distributed over different fibers at the level of the auditory Fig. 1 illustrates phase-locking for an AN fiber and a nerve (AN) is transformed at successive synaptic levels to VCN neuron, tuned to the same frequency of 670 Hz (A). a more robust code at the single cell level. Fig. 1B and C shows dot rasters in response to 50 short tones at this frequency. There is a vertical alignment of SINGLE UNIT PHASE-LOCKING AND dots in both cases, but clearly this alignment is better in C. The red dots indicate spike times which are identical, ITS ENHANCEMENT within a 50 ␮s window, across responses to at least two Computer-aided AN recordings (Kiang et al., 1965b; Rose stimulus presentations (of the 50 shown): for the black dots et al., 1967; Johnson, 1980) systematically demonstrated there is no matching spike time in any of the other spike phase-locking at frequencies far higher than maximal firing trains. The VCN neuron (C) tends to fire a spike on every rates sustained by AN fibers, which are ϳ300 Hz. As cycle over a narrow phase-range of the sinusoidal stimulus hypothesized by Wever and Bray (1930b), AN fibers skip waveform, resulting in a preponderance of spike times that cycles, even at very low frequencies, and the upper limit of are coincident across stimulus repetitions. The AN neuron phase-locking is thus not imposed by refractoriness (even is more stochastic in its firing, often skipping one, two, or though this statement is still encountered, e.g. Shepherd, more cycles, and the spikes are less well aligned across 1994). The most popular metric used to quantify phase- repetitions. The cycle histograms (Fig. 1D, E) show the locking is the vector strength (VS, Goldberg and Brown, instantaneous discharge rate directly as a function of stim- 1969). Spikes randomly distributed with respect to phase ulus phase. A flat distribution would indicate an absence of result in a VS near 0, while spikes occurring at a fixed phase-locking at the frequency of the histogram. There is phase yield values near 1 (Fig. 1). With this measure, phase-locking in both fibers, but there is more dispersion in phase-locking in the AN shows a low-pass characteristic the AN response. The same distributions are also shown in with an upper limit of ϳ4–5 kHz in the cat (Johnson, 1980; polar format, from which an averaged vector can be cal- Joris et al., 1994a) and somewhat lower in rodents (Palmer culated. The magnitude of this vector, normalized for the and Russell, 1986; Paolini et al., 2001; Taberner and Liber- overall discharge rate, is the VS and is much higher for the man, 2005). The exact limiting step(s) at the level of the VCN fiber (0.93) than for the AN fiber (0.64). Despite a cochlea are not known, but a number of candidate pro- stimulus frequency that is high relative to “routine” neuro- cesses such as hair cell membrane capacitance have nal firing rates, this VCN neuron also showed entrainment. been proposed (Palmer and Russell, 1986; Weiss and This is illustrated by the dominance of inter-spike intervals Rose, 1988). equal to the stimulus period (Fig. 1G), while the AN fiber Phase-locking changes in quality in the ascending au- shows a multimodal distribution (Fig. 1F) indicating fre- ditory system. Generally, there is a decrease in the upper quent skipping of stimulus cycles. frequency limit at successive synaptic levels. More surpris- So far, we have only discussed phase-locking to the ingly, central auditory neurons often show an enhance- fine-structure of pure tones i.e. to the fluctuations of instan- ment of phase-locking relative to the AN. There are two taneous pressure in these waveforms. Sounds also have a aspects to this phenomenon. First, discharges are re- temporal envelope, which is perceptually important, and stricted to a narrower range of phase angles, reflected in auditory neurons phase-lock to these envelopes (reviewed higher VS values (Fig. 1). For example, when studied at by Joris et al., 2004). Again such phase-locking can be their characteristic frequency (frequency of lowest thresh- enhanced in CN neurons relative to the AN, and there are old, CF), some neurons of the ventral cochlear nucleus phenomenological parallels between the enhancement of (VCN) show higher VS values than AN fibers. This hap- phase-locking to fine-structure and to envelopes. For ex- pens for frequencies below approximately 1 kHz (Joris et ample, the enhancement of envelope phase-locking does al., 1994a). Equally striking is the observation that neurons not extend to modulation frequencies as high as in the AN. may lack enhanced phase-locking at their CF but may Fig. 3 illustrates enhanced phase-locking to both fine- instead show enhancement for frequencies in their low- structure and envelope for a neuron recorded in the supe- frequency tail (see below) (Rhode and Smith, 1986; Joris rior olivary complex (SOC), which contains output fibers et al., 1994b; Rhode, 2008). from the VCN and their postsynaptic targets, organized A second aspect of enhanced phase-locking, which into several different nuclei. Fig. 3A shows the frequency has received less attention, is entrainment. In response to threshold tuning curve. The other two panels in the left short tone bursts, some neurons can discharge a spike at column show VS (Fig. 3B) and average firing rate (Fig. 3C) every cycle up to frequencies of ϳ700 Hz (Godfrey et al., for pure tones, presented at many different sound levels P. X. Joris and P. H. Smith / Neuroscience 154 (2008) 65–76 67

80 A 60 SBC 40 AN

20 THRESHOLD (dB SPL) 0 100 1000

B D 0.25 AUDITORY NERVE FIBER 1000 50

0

25 F

1000 100

0 0 0102030 0 0.5 1 0510

C SPHERICAL BUSHY CELL E 0.25 G 6000 50 INSTANTANEOUS RATE (spikes/sec) RATE INSTANTANEOUS 0

25 # INTERVALS/BIN

REPETITION # 1000 100

0 0 0102030 0 0.5 1 0510 POST STIMULUS TIME (ms) PHASE (cycle) INTERSPIKE INTERVAL (ms)

Fig. 1. Phase-locking and its enhancement. (A) Frequency threshold tuning curves for a fiber recorded in the AN (with lowest thresholds) and one in the TB. The TB fiber is one of two labeled high-sync SBC in Smith et al. (1993). The solid circle indicates CF and threshold for the AN fiber and the stimulus level and frequency for the responses in B, D, F. The empty circle indicates CF and threshold for the SBC and the stimulus level and frequency for its responses in the lower row of panels. (B, C) Dot rasters of responses to 50 repetitions to short (25 ms) tones at the CF (670 Hz). Red dots indicate coincidences across the 50 spike trains within a 50 ␮s window, i.e. if a pair of spikes differs 50 ␮s or less in spike time, both spike occurrences are colored red. Conversely, a black mark means that there is no other spike within 50 ␮s in any of the other spike trains. (D, E) Period or cycle histograms of responses to 100 repetitions as in B, C. The data are also shown in polar form. The magnitude (VS) is 0.64 for the AN fiber and 0.93 for the SBC (average phase is 0.64 and 0.84 cycles, respectively). The total number of spikes in these histograms is 667 (AN fiber) and 1775 (SBC). The period histograms are on the same scale, but not the polar plots, where the outer circle indicates instantaneous rates of 1000 and 6000 spikes/s. (F, G) Inter-spike intervals of the response over the same 10–25 ms window. The dots under the abscissa indicate integer multiples of the stimulus period. To remove onset effects, the period (D, E) and inter-spike interval (F, G) histograms are for a 10–25 ms response window; the number of bins is always 100 and the scales of the axes are identical for AN and SBC responses. The average spike rate was 233 spikes/s (AN) and 474 spikes/s (SBC). Spontaneous rate was 90 spikes/s (AN) and 175 spikes/s (SBC). The maximal entrainment index (Joris et al., 1994a) was 0.34 (AN) and 0.91 (SBC).

(dB SPL is indicated with color code on the ordinate of which has a high VS of 0.95. Moreover the neuron also panel A). Although this neuron does not phase-lock to pure entrains almost perfectly to low-frequency stimuli, if the tones at its CF (2460 Hz, indicated with a vertical line), it stimulus level is sufficiently above threshold. Indeed, at 80 shows enhanced synchronization for frequencies in the dB the firing rate (Fig. 3C) is close to equality (dashed line low-frequency “tail” of the tuning curve (Fig. 3B). An ex- in C) with the stimulus frequency up to about 500 Hz, and ample of a period histogram to a tone in the low frequency this entrainment is present for the entire (1 s) duration of tail (at 400 Hz) is shown in the top panels (histogram “X”), the stimulus. Thus a 300 Hz tone evokes 300 spikes/s, a 68 P. X. Joris and P. H. Smith / Neuroscience 154 (2008) 65–76

temporal representation than the more stochastic repre- sentation at the level of the AN. For binaural tasks that AN CN involve a comparison of the timing of events at the two ears, the high-sync representation seems advantageous. A An ideal observer analyses shows indeed better binaural discrimination thresholds when based on phase-locked VCN fibers projecting to binaural nuclei, than when based on AN responses (Louage et al., 2006). For other percep- B tual attributes that may be based on temporal coding (e.g. pitch, Cariani and Delgutte, 1996a) it is less clear whether the high-sync representation of VCN neurons is also an C enhanced (in the sense of superior) representation. Al- though it has been little studied, CN fibers with high-sync high-frequency tone responses seem to have smaller dynamic ranges than AN fibers (Joris et al., 1994a, Fig. 1), so that they are probably not good encoders of sound intensity, at least not in terms D of their average rates. The conflicting demands of the encoding of frequency, time, and intensity were already low-frequency tone discussed in the first half of the previous century (Davis, high-frequency AM tone 1984). At present, the dominant view is that such multiple demands are the raison d’être for the morphological and Fig. 2. Factors that potentially affect the number of coincident spikes. physiological diversity in the CN (Kiang et al., 1973; Irvine, Depicted are three AN fibers converging on a CN neuron that acts as 1986; Rhode and Smith, 1986; Cant and Benson, 2003; a coincidence detector. The arrangement in A and B, where these afferents also innervate a single inner hair cell, is hypothetical. For Young and Oertel, 2004). ease of presentation, action potentials (red if coincident) are depicted on axons as if these were timelines. (A) Intrinsic coincidences reflect a structural feature (e.g. collaterals from the same fiber, innervation of RELATIONSHIP TO MORPHOLOGICAL CELL the same hair cell) rather than coincidences arising from coupling to TYPES IN CN the stimulus. (B) In the absence of intrinsic coincidences, chance coincidences are still possible. (C) In response to a high-frequency Before discussing the relationship of high-sync responses tone, there is no coupling of spikes to the fine-structure of the stimulus to the CN cell types described by Osen, we focus on two of and the probability of coincidences in the afferents is small. (D) In the cell types described by her which are of particular response to low-frequency tones or amplitude-modulated (AM) tones which cause coupling of the spikes to the stimulus, a higher number of interest to us: the spherical and globular cells (Osen, coincidences is obtained, leading to a high rate of output spikes that 1969). Our focus has two reasons. As mentioned, the use are strongly phase-locked to the stimulus. of temporal information by the CNS is clearly established physiologically and psychophysically for binaural hearing. 400 Hz tone evokes 400 spikes/s, etc. In stark contrast, the The basic circuits involved in the time comparisons are response to CF tones is quite low. The other panels (D–G) known, and spherical and globular neurons are an essen- show responses to amplitude modulated tones, which are tial component of these circuits. Although there are still discussed below. several open questions and controversial issues on even Enhanced phase-locking to fine-structure is not only the most basic features of these circuits (Joris and Yin, present for pure tones, but also for non-periodic stimuli. 2007), there is no disagreement that temporal comparison This is somewhat less straightforward to quantify than of the sound waveforms at the two ears requires phase- phase-locking to pure tones or other periodic stimuli. A locking. For no other human perceptual ability is this es- coincidence or autocorrelation analysis (Joris, 2003; Joris tablished as well. A second reason to focus on spherical et al., 2006) shows that, compared with AN responses and globular cells is an intriguing puzzle regarding the (Louage et al., 2004), many VCN neurons show temporally mechanisms underlying synchronization enhancement, as “enhanced” properties not only to tones but also to broad- will become clear below. band noise, again both in terms of entrainment and re- These neurons have a restricted distribution in the duced jitter (Louage et al., 2005). Henceforth, we will use anteroventral cochlear nucleus (AVCN), in the spherical the term “high-sync” as a general term to indicate en- cell area and globular cell area. The correspondence of hanced phase-locking to fine-structure. Osen’s scheme to the other main CN parcellation scheme One important issue is whether enhanced phase-lock- (Brawer et al., 1974) has been discussed elsewhere (Cant ing is functionally relevant. “Enhanced” implies something and Morest, 1984; Irvine, 1986). We will use the nomen- positive, but high-sync responses actually represent the clature of Cant and Morest (1984), which fuses the two stimulus waveform in a rather distorted way. For example, schemes: spherical bushy cells (SBCs) and globular bushy the waveform of low-frequency sine waves is more faith- cells (GBCs). fully tracked by the probability of discharge of AN fibers As summarized by Cant (1991), there is an SBC path- than by high-sync fibers. One can question whether a way and a GBC pathway, which provide direct excitatory highly regular pulse train at the stimulus period is a better and indirect inhibitory input, respectively, to binaural nuclei P. X. Joris and P. H. Smith / Neuroscience 154 (2008) 65–76 69

A CF 80 X 1000 X 60 0 YZ 1000 0102030 0 40 00.51 TIME (ms) YZ 20 1000 1000 0 0

THRESHOLD (dB SPL) 00.51 00.51 INST. RATE (spikes/sec) RATE INST. 0 PHASE (cycle) 100 1000 10000 PURE TONES AMPLITUDE MODULATION B X D F f = 1.5 kHz Y f = 2.4 kHz Z 1 1 c 1 c

0.5 0.5 0.5 VECTOR STRENGTH VECTOR 0 0 0 100 1000 10000 10 100 1000 10 100 1000 CEG 500 500 500 Y X Z

FIRING RATE (spikes/s) FIRING RATE 0 0 0 100 1000 10000 10 100 1000 10 100 1000 FREQUENCY (Hz) MODULATION FREQUENCY f (Hz) m

Fig. 3. Example of enhanced phase-locking to pure tones and to the envelope of amplitude-modulated tones. (A) Threshold tuning curve (2 runs). (B, C) Responses to pure tones,1sinduration, over a range of frequencies and SPLs. (D–G) Responses to amplitude-modulated tones, 600 ms in duration, over a range of frequencies and SPLs. The bottom row shows average firing rates (C, E, G). The middle row (B, D, F) shows strength of phase-locking. The cycle histograms for three data points, indicated with the arrows in B–G, are shown in top panels X, Y, Z. The stimulus frequencies and levels are also indicated in A. The dashed line in the bottom panels indicates equality between response rate in spikes/s and stimulus frequency (in Hz). The abscissa scale is the same for all main panels. The small panels (top right, X–Z) show period histograms for a pure tone of 400 Hz (60 ϭ ϭ dB) and for amplitude modulation with envelope frequency fm 410 Hz and carrier fc 1.5 kHz (Y) or 2.4 kHz (X) at 50 dB. The topmost histogram shows the post-stimulus time histogram to a 25 ms, 60 dB tone at the CF of 2.4 kHz. There is uncertainty regarding the identity of this neuron. Spikes were monopolar and were recorded ipsilateral to the driving ear at a depth of 3.6 mm in the TB/SOC witha4M⍀ tungsten electrode (Thomas Recording, Giessen, Germany) which has the ability to record from axons. The location was just dorsolateral of the MSO at a position agreeing with the tonotopic map of this nucleus (Guinan et al., 1972). Although the physiological responses are monaural and “GBC-like” in several respects, the response to short tone bursts had an onset latency of 4.7 ms and notch duration of 2 ms, which are rather high for a 2.4 kHz GBC neuron; the responses to envelope modulation differ from a previous sample of GBC neurons (Joris and Yin, 1998); and the recording was remarkably stable for many hours. Possibly the responses were derived from an SOC neuron receiving GBC input (e.g. from the lateral nucleus of the TB, Cant and Hyson, 1992). Whether GBC or SOC neuron, the reasoning regarding the mechanisms of the increase in firing rate for pure tones and amplitude modulation remains the same, but possibly the responses reflect two stages of coincidence detection (one operating in the CN on AN fibers, one operating in the SOC on GBC fibers). in the SOC. Most of the information available on these and Ryugo, 1989; Spirou et al., 1990; Ryugo and Sento, pathways is based on degeneration studies (e.g. Warr, 1991; Smith et al., 1991, 1993). 1966, 1969, 1982; Osen, 1970) or studies using axonal In the cat the SBC population in the rostral-most AVCN transport of neural tracers either injected grossly (e.g. has CFs that span the low and medium frequency range. Tolbert et al., 1982; Shneiderman and Henkel, 1985; Cant Their cell bodies are larger than their counterparts in the and Casseday, 1986) or into single cells or axons (Sento caudal region of this nucleus, where the CFs span the 70 P. X. Joris and P. H. Smith / Neuroscience 154 (2008) 65–76 entire frequency range (Osen, 1969). The consensus from synchronization in the AVCN and MNTB, though not to the the studies mentioned above is that large SBCs send same degree as in the cat. Enhancement and entrainment projections to the appropriate frequency regions of the appear to be present in the rat LSO as well (Caspary and medial superior olive (MSO) bilaterally and the lateral su- Finlayson, 1991) and was observed in a non-bushy neuron perior olive (LSO) ipsilaterally where they form excitatory in gerbil (Feng et al., 1994; Ostapoff et al., 1994). En- terminations on the dendrites of principal cells. Other SOC hanced synchronization was not seen in SBCs of the projections from this population include both ipsilateral and guinea pig (Winter and Palmer, 1990). In birds, the phe- contralateral periolivary nuclei and perhaps the contralat- nomenon has been described in the chick (Fukui et al., eral ventral nucleus of the lateral lemniscus (VNLL) as 2006), but in the owl there is only minor enhancement in well. Projections of the small spherical cells in more caudal nucleus magnocellularis relative to the AN (Koppl, 1997). It areas of the AVCN are less well documented but it appears should be pointed out that even in the cat there are several that they have the ipsilateral LSO as their main target and extensive studies that show little difference between the may not project across the midline. highest VS values found in the VCN vs. the AN (Bourk, The more oval-shaped cell bodies of the GBCs with 1976; Blackburn and Sachs, 1989), while the difference is their somewhat less dense, more highly spread out den- rather striking in axonal recordings from SBC and GBC dritic arbors are located in the more caudal reaches of the fibers in the trapezoid body (TB) (Joris et al., 1994a,b; AVCN in and around the nerve root area (Harrison and Louage et al., 2005). Possible explanations for these dis- Warr, 1962; Osen, 1969; Rhode, 2008). Their primary crepancies have been offered elsewhere (Joris et al., termination sites appear to be on cells in brainstem nuclei 1994a): we return to this issue in the final section. that provide inhibitory inputs to other regions. These in- clude the large calyx of Held terminals onto glycinergic MECHANISMS OF SYNCHRONIZATION cells of the contralateral medial nucleus of the trapezoid ENHANCEMENT body (MNTB) that provide inhibitory inputs to LSO and MSO cells (Glendenning et al., 1985; Kuwabara et al., There have been a surprisingly large number of modeling 1991; Banks and Smith, 1992; Cant and Hyson, 1992; studies on the phenomenon of enhanced synchronization Smith et al., 1998; Chirila et al., 2007; Kopp-Scheinpflug et and entrainment. The most common model is one in which al., 2008), terminals onto neurons of the ipsilateral lateral coincidence of a number of excitatory inputs on neurons nucleus of the TB that provide inhibitory inputs to MSO with a short membrane time constant (Oertel, 1983)is cells (e.g. Cant and Hyson, 1992; Spirou and Berrebi, required before a postsynaptic spike is generated (Joris et 1997; Spirou et al., 1998); and a contralateral projection to al., 1994a; Rothman and Young, 1996). The degree of superior parolivary nucleus or dorsomedial periolivary nu- enhancement in such models is a complex interplay of the cleus that sends a GABAergic projection to the inferior number of inputs, their amplitudes, their temporal and colliculus (e.g. Kuwabara et al., 1991; Smith et al., 1991; spatial distribution, and several postsynaptic factors (Roth- Kulesza and Berrebi, 2000). Other projections to the VNLL man et al., 1993; Rothman and Young, 1996; Kuhlmann et and the ipsilateral LSO (in certain species) have been al., 2002; Reed et al., 2002; Rothman and Manis, 2003; Ito reported (Friauf and Ostwald, 1988; Kuwabara et al., 1991; and Akagi, 2005; Maki and Akagi, 2005; Xu-Friedman and Smith et al., 1991). Regehr, 2005a,b). Increasing the number of subthreshold Which neurons show enhanced phase-locking relative inputs increases the VS values (Rothman et al., 1993; to the AN? The phenomenon is surprisingly common, es- Joris et al., 1994a; Rothman and Young, 1996; Ito and pecially at “tail frequencies.” One major exception is the Akagi, 2005; Xu-Friedman and Regehr, 2005a,b). A coin- dorsal CN where phase-locking to fine-structure is very cidence scheme is plausible for enhanced synchronization limited (Goldberg and Brownell, 1973; Rhode and Smith, in GBCs because these cells have a large number of AN 1986). Perhaps one of the earliest high-sync examples in inputs (Spirou et al., 2005) which are presumably sub- the literature is in a study of the MSO, in which the re- threshold (Smith and Rhode, 1987; Paolini et al., 1997; sponse to a 375 Hz tone shows a unimodal interval histo- Rhode, 2008), as well as limited temporal summation due gram (Moushegian et al., 1967). Scattered examples are to their fast membrane time constant (Wu and Oertel, present in many CN recordings from the cat, and show that 1984; Manis and Marx, 1991). Similar ideas apply to other high-sync responses can be generated by several of the cell types in the CN which have enhanced temporal prop- main cell types described by Osen (1969): SBCs, GBCs, erties relative to the AN and which receive many small octopus cells, and multipolar cells (Lavine, 1971; Godfrey subthreshold AN inputs (Winter and Palmer, 1995; Jiang et et al., 1975; Rhode and Smith, 1986; Rhode and Kettner, al., 1996; Cai et al., 1997, 2000; Levy and Kipke, 1998; 1987; Carney, 1990; Joris et al., 1994a,b; Rhode, 2008), Oertel et al., 2000; Kalluri and Delgutte, 2003a,b). It has as well as by neurons of the MNTB, MSO, and LSO (Yin been particularly challenging to model the combination of and Chan, 1990; Finlayson and Caspary, 1991; Joris and enhanced synchronization and other physiologically ob- Yin, 1995; Smith et al., 1998; Tollin and Yin, 2005). The served properties (entrainment, spontaneous rate, shape phenomenon has also been encountered in the CN and of poststimulus time histograms at higher frequencies, MNTB of macaque monkeys (Joris and van der Heijden, etc.) while at the same time maintaining realistic physio- 2004). It seems less prominent in rodents: an extensive logical and anatomical parameters. While coincidence de- study in the rat (Paolini et al., 2001) found enhanced tection is a common ingredient in most GBC models, its P. X. Joris and P. H. Smith / Neuroscience 154 (2008) 65–76 71 exact biological implementation is not entirely clear yet frequency tone, chance occurrences of coincidences (Rothman and Young, 1996; Spirou et al., 2005). across inputs elicit output spikes in the GBC neuron (Fig. The presence of high-sync responses in GBCs is func- 2C). An increase in sound level generates an increased tionally important in several ways. GBCs provide, via the firing rate in the AN fibers, and thus also an increased firing MNTB, highly phase-locked inhibition to the LSO (Smith et rate in the GBC because the number of chance coinci- al., 1991, 1998), and low-frequency LSO neurons are in- dences increases. A second way to increase the correla- deed sensitive to interaural time differences (ITDs) of low- tion among the inputs is to lower the tone frequency to the frequency sounds (Finlayson and Caspary, 1991; Joris range of phase-locking (Fig. 2D). Indeed, lowering the tone and Yin, 1995; Tollin and Yin, 2005). Second, this inhibition to frequencies in the “tail” of the tuning curve that generate is also supplied to the MSO (Spangler et al., 1985; Adams phase-locking, causes an increase in firing rate of GBC and Mugnaini, 1990; Smith, 1995; Smith et al., 1998), and neurons accompanied by exquisite phase-locking and en- there is evidence that it influences ITD-processing in this trainment (Joris et al., 1994b). This behavior can be mod- nucleus (Grothe and Sanes, 1994; Brand et al., 2002). The eled with a coincidence operation (Rothman and Young, MNTB is predominantly a high-frequency nucleus (Guinan 1996). Note that lowering the stimulus frequency causes et al., 1972; Tsuchitani, 1977). Because GBCs also phase- these increased firing rates in the GBC neurons even lock to the stimulus envelope (Rhode and Greenberg, though their AN inputs, which are presumably tuned to the 1994; Joris and Yin, 1998), they provide, again via the same CF, show decreased firing rates under these circum- MTNB, inhibition which is phase-locked to envelopes, so stances. A third way to increase the correlation among the that the LSO is sensitive to ITDs of the envelope of high- inputs is amplitude modulation of a tone (Fig. 2D), even frequency sounds (Joris and Yin, 1995; Batra et al., 1997). when the spectral components of the stimulus are outside Sound envelopes carry important information not only re- the range of phase-locking to fine-structure. Within certain garding sound position in space but also regarding sound limits (Joris and Yin, 1992) amplitude modulation causes identity: study of responses to stimulus envelopes is thus temporal alignment of spikes across AN fibers, and should interesting not only from a binaural but even more from a therefore increase the number of coincidences driving the monaural viewpoint. Furthermore, such responses also postsynaptic GBC neuron. bear on the proposed mechanisms of coincidence detec- Fig. 3 shows responses of a neuron recorded ventrally tion. Before turning to enhanced synchronization in SBCs, in the SOC with a CF of 2.4 kHz (see legend regarding the we pause to examine similarities in enhanced phase-lock- identity of this neuron). The response rate to pure tones at ing to fine-structure and envelope. its CF (2.4 kHz, C) is moderate, from a spontaneous rate of GBCs with CFs just above the range of phase-locking four spikes/s to a maximum of 125 spikes/s to short tone are an interesting test case of coincidence detection (Joris bursts and even lower to the 1 s tones shown in Fig. 3C (35 et al., 1994b). In principle, coincidences among the spike spikes/s). As already discussed, lowering the frequency of trains of inputs to CN neurons, at the millisecond time a pure tone causes a dramatic increase in firing rate, with scale of interest here, can be affected by 1) intrinsic cor- high synchronization and entrainment. The panels of the relations in firing between AN fibers, e.g. of fibers inner- middle and right column show very similar behavior to vating the same inner hair cell, 2) phase-locking to the amplitude modulation. For example, when a 2.4 kHz car- fine-structure of the sound, 3) phase-locking to the enve- rier tone is modulated at 410 Hz, its spectral components lope of the sound or the local cochlear vibration pattern. As are in a range (see circle and bar “Z” in panel A) where far as known, source 1 (Fig. 2A) is not present and is not pure-tone phase-locking and high firing rates are absent further considered here, though admittedly absence of for tones presented individually. The amplitude modulation evidence here does not constitute evidence of absence causes a tremendous increase in firing rate with high syn- (Johnson and Kiang, 1976; Kiang, 1990; Young and Sa- chronization (F) and entrainment (G). Similar behavior is chs, 2008). Sources 2 and 3 are stimulus-induced (Fig. 2D) observed for a carrier frequency of 1.5 kHz (middle col- and can thus be manipulated by the experimenter. Source umn). This is in sharp contrast to the responses of AN 2 is not present for pure tones above the range of pure- fibers, which show little change in firing rate with variations tone phase-locking, i.e. a few kHz in mammals (Fig. 2C). in modulation frequency or modulation depth (Joris and Source 3 is not present for unmodulated pure tones or at Yin, 1992). The similarity in firing rates of the responses to high rates of amplitude modulation, if we only consider the pure tones and to amplitude modulation, over the common ongoing part of the response and ignore effects of onset range of frequencies (100–1000 Hz), is particularly and offset of the stimulus. striking. For GBC neurons with a CF above a few kHz, pure These responses illustrate how the temporal spike pat- tones above the phase-locking limit generate AN input terns in the AN can be transformed at the earliest stages of spike trains that are random relative to each other. In fact, the central auditory system into a rate code. Stimuli with such tones are the only stimulus for which spike trains different spectrum but the same periodicity can give rise to evoked in the AN inputs to the GBC neuron are mutually firing rates equaling this periodicity. It will be interesting to random. If the GBC neuron is a coincidence detector, its examine how invariant such responses are for spectrum, firing rate should increase by increasing the correlation using stimuli that are more amenable to parametric manip- among the inputs to the neuron. A first way to achieve this ulation (Cariani and Delgutte, 1996a,b; Wiegrebe and Win- is to increase the stimulus level. In response to a high- ter, 2001). 72 P. X. Joris and P. H. Smith / Neuroscience 154 (2008) 65–76

THE PUZZLE OF ENHANCED the TB. Field potentials are also absent when using high- SYNCHRONIZATION IN SBCs impedance micropipettes in the AVCN, and such record- ings indeed reveal high-sync GBC neurons (Rhode, 2008). SBCs are the most numerous projection neurons of the CN On the other hand, there is a clear bias toward recording (Osen, 1970) and provide the excitatory input to MSO from thicker axons in the TB and the axons of SBCs are neurons. Sensitivity to ITDs in MSO neurons is the premier thinner in diameter than those of GBCs. Possibly a com- example of temporal sensitivity in the mammalian CNS, bination of these factors explains why high-sync phase- and it is therefore important to understand the temporal locking has been much less reported in units from the behavior of SBC neurons. While monaural coincidence AVCN than from its output tract. Taken together, the avail- detection may be an adequate model for enhanced syn- able data show that both kinds of bushy cells can show chronization and entrainment in globular bushy neurons enhanced synchronization but the prevalence of this be- and other CN neurons with many AN inputs, it is problem- havior is unclear. It is interesting to note that at higher CFs atic for SBCs. First, we review the evidence that SBCs there is a clear difference between SBCs and GBCs: only show enhanced phase-locking. the latter show enhanced synchronization when stimulated Out of seven intra-axonally labeled low-frequency TB in their low-frequency tail (Joris et al., 1994b). Diversity in fibers with high-sync behavior, two appeared to be axons phase-locking behavior in SBCs, and the respective roles of SBCs; the other five were axons of GBCs (Joris et al., of pre- and postsynaptic factors (Sento and Ryugo, 1989; 1994a). The physiology for one labeled spherical bushy Cant, 1991; Cao et al., 2007), are interesting topics in neuron is shown in Fig. 1, the anatomy and physiology of themselves. the other labeled neuron is shown in Fig. 5 of Smith et al. Neurons in the spherical cell area have only between (1993). The cell body was not recovered in those two one and four inputs. This number is derived from morpho- cases but the morphological class was inferred based on logical observations after bulk tracer injections in the AN the main projection targets (MSO for SBCs; MNTB for (Ryugo and Sento, 1991); from quantitative comparison of GBCs). While the morphological classification of these two AN end bulbs and SBC counts (Melcher, 1993); and from neurons is thus indirect, the observation that the direct EM observations (Nicol and Walmsley, 2002). At least for projection from the CN to MSO is purely derived from the cat, four inputs seem already on the high side and the SBCs (Cant and Casseday, 1986) leaves little room for modal number of end-bulb inputs is probably two (Ryugo doubt. Similarly, the presence of a labeled calyx of Held in and Sento, 1991). the MNTB in the other five high-sync neurons shows that In summary, the available evidence suggests that these axons were from GBCs. Thus, even though the SBCs with a small number of large inputs show the high- number of labeled high-sync neurons is very small, the sync phenomenon. Can enhanced synchronization and data strongly suggest that both spherical and globular entrainment be obtained with few, suprathreshold inputs? bushy neurons display this behavior. In a dynamic clamp study of bushy cells in slices of the There are other, more indirect, indications that both mouse AVCN (Xu-Friedman and Regehr, 2005a,b), the groups of neurons show enhanced synchronization. Re- effect was studied of the number of simulated active inputs cordings in the TB of the cat show a paucity of responses as well as the shape of their temporal input distribution with VS values in the range of the AN (Joris et al., 1994a; (Gaussian or alpha) on the jitter and reliability of the Louage et al., 2005), suggesting that the bulk of low- postsynaptic response. Again, jitter became smaller with frequency inputs to the binaural nuclei is synchronized an increasing number of inputs. Interestingly, for alpha- more strongly than the AN. Also, some of the high-sync TB distributed inputs jitter reduction was achieved with even a fibers have high spontaneous rates. For CFs in the phase- small number of suprathreshold inputs. This occurs locking range, low and high spontaneous rates are rather through a “first-come-only-served” kind of mechanism: the strongly associated with GBCs and SBCs, respectively first arriving suprathreshold input triggers the postsynaptic (Spirou et al., 1990; Joris et al., 1994a), again suggesting neuron. Similar reductions in jitter are present in the sim- that both GBCs and SBCs contribute high-sync responses. ulations of Rothman and colleagues (Rothman and Young, However, there are also reasons to doubt that these TB 1996; Rothman and Manis, 2003) but require a large num- recordings give the full picture. Extensive AVCN record- ber of inputs (Ն10), which is not realistic for SBCs. Exam- ings by Rose and colleagues did not reveal high-sync ination of real AN spike trains shows that patterns as neurons (there is brief mention of the phenomenon in Rose observed in the SBC of Fig. 1 cannot be obtained from a et al., 1974, but no supporting data are shown). Likewise, combination of few suprathreshold inputs. This is illus- several population studies of the VCN reported no or few trated by a simple simulation (Louage, van der Heijden, high-sync neurons (Bourk, 1976; Blackburn and Sachs, Joris, unpublished observations) on actual AN responses. 1989; Winter and Palmer, 1990). Possible reasons for this Two or three spike trains from a single AN fiber were discrepancy between studies of the AVCN and studies of combined into a single new spike train (mimicking conver- its output tract have been given earlier (Joris et al., 1994a). gence on a SBC neuron), and a sliding window was then The most straightforward explanation is recording bias: applied to impose refractoriness. When this is repeated for strongly phase-locked field potentials in the AVCN hamper random combinations of spike trains from the same AN spike isolation with traditional metal electrodes, while this fiber (e.g. combinations of three spike trains drawn from is not a problem for axonal recordings with micropipettes in the 50 spike trains shown in Fig. 1B), the resulting simu- P. X. Joris and P. H. Smith / Neuroscience 154 (2008) 65–76 73 lated SBC output shows no or at best a modest increase in Brand A, Behrend O, Marquardt T, McAlpine D, Grothe B (2002) VS. In fact, it seems unlikely that spike trains as in Fig. 1C Precise inhibition is essential for microsecond interaural time dif- can be obtained by any combination of two or three spike ference coding. Nature 417:543–547. trains from Fig. 1B: there simply are not enough input Brawer JR, Morest DK, Kane EC (1974) The neuronal architecture of the cochlear nucleus of the cat. 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(Accepted 5 March 2008) (Available online 8 March 2008)