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1Kresge Hearing Institute, The University of Michigan, Ann Arbor, Michigan 48109-0506; 2Department of Otolaryngology, Toho University, Tokyo 143-0015, Japan; and 3Department of Neuroscience, University of Connecticut Health Center, Farmington, Connecticut 06030-3405
Submitted 2 December 2003; accepted in final form 25 December 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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; Hoffman and Henson 1990; Malmierca et al. 1996; Shore et al. 1991; Syka et al. 1988a,b).
The floor of the fourth ventricle is a converging location for a portion of this descending system and thus a historical location for electrically activating it (Desmedt 1962; Galambos 1956; Wiederhold and Kiang 1970). For reviews of the early literature, see Klinke and Galley (1974), Wiederhold (1986), and Guinan (1996). Due to the compact nature of this fiber bundle, electrical stimulation presumably activates most, if not all, of the medial portion of the crossed olivocochlear (COCB) system that innervates the outer hair cells in the inner ear. Although individual efferent fibers may innervate restricted regions of the cochlea, COCB stimulation causes a fairly broad effect within the cochlea. Electrical activation of individual fibers going to a particular region of the cochlea has not been possible.
More recently, studies have shown that gross electrical activation of the IC produces an efferent effect within the cochlea. Rajan (1988, 1990) first showed a functional efferent connection from the IC to the cochlea by reporting that gross stimulation of the IC reduced the temporary threshold shift of the cochlear whole-nerve action potential (CAP) threshold. Dolan and Nuttall (1988) reported that gross electrical stimulation of the IC via a bipolar electrode reduced the amplitude of the CAP similar to stimulation of COCB at the floor of the fourth ventricle. The physical size of the IC is quite large in comparison to the bundle of COCB fibers at the floor of fourth ventricle, and this allows the placement of microelectrodes to electrically stimulate discrete regions (Mulders and Robertson 2000a; Popelar et al. 2002; Scates et al. 1999).
The architecture and connections of the IC suggest that the parts of the IC may play different roles in the auditory efferent system. The IC is divided into a central nucleus (ICC) surrounded by a dorsal (ICD) and lateral external cortex (ICX) (Faye-Lund 1985; Faye-Lund and Osen 1985; Morest and Oliver 1984). Normal activation of the olivocochlear system may involve the descending projections from the IC to the SOC (Caicedo and Herbert 1993; Mulders and Robertson 2002; Mulders et al. 2003; Schofield and Cant 1999; Thompson and Thompson 1993; Vetter et al. 1993). The ICC sends projections to the periolivary region of the SOC as well as to the dorsal nucleus of the lateral lemniscus and CN (Caicedo and Herbert 1993; Schofield 2001). The ICX also sends descending projections to the periolivary region of the SOC, the CN, and acousticomotor regions (Caicedo and Herbert 1993; Huffman and Henson 1990; Schofield 2001). Unlike the ICC, the ICX and ICD receive neocortical inputs (Huffman and Henson 1990). The ICX receives not only corticocollicular input from auditory and somatosensory neocortex, but it also receives multimodal inputs including inputs from the ICC, dorsal column nuclei, spinal cord, trigeminal nuclei, and superior colliculus (Caicedo and Herbert 1993; Huffman and Henson 1990).
To assess the functional differences of IC subdivisions on cochlear function, the present study used a microelectrode provided by the University of Michigan Center for Neural Communication Technology (Fig. 1) to physiologically characterize the best frequency of IC cells located at the recording electrode position. The same electrode was used to electrically stimulate the same region. The best-frequency region of the electrode position within the IC closely matched the tone burst frequency in which maximum reduction of the CAP was observed. This microstimulation caused varying effects on the CAP depending on which IC subdivision was stimulated.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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The animal's head was firmly fixed in a holder while the plastic tip of the speculum was sealed into the left ear canal. The speculum held a condenser microphone (1/2 in, B&K) to deliver the acoustic tones to evoke the CAP. The microphone was calibrated in a volume approximating the guinea pig external ear canal so that stimulus levels are expressed as dB SPL. CAP threshold to tones (1-ms rise/fall time, 12-ms duration, 10/s, 64 epochs) from 0.5 to 24 kHz were measured at the beginning, during, and at end of the each experiment. Animals were omitted from the data analysis if they showed >10 dB elevations in CAP threshold, compared with normal lab standards, during the experiment or showed elevated CAP thresholds at initial measurement.
The left bulla was exposed and opened to place 0.13-mm diam Teflon-insulated silver wire electrode on the round window. An Ag/AgCl ground electrode was placed in the soft tissue of the neck. The animal was rotated so that an extended right parietal craniotomy was performed to expose the occipital cortex and cerebellum. The occipital cortex was aspirated revealing the dorsal surface of the contralateral IC. The microelectrode (multichannel recording/stimulating electrode provided by The University of Michigan Center for Neural Communication Technology, Fig. 1) was inserted via a micromanipulator into the IC. The electrode has five fingers, and each finger has two recording sites (153 µm2) and a stimulating site (1,070 µm2) to pass current. Spacing of each finger (center to center) is 200 µm. Each stimulating site is positioned equally between the recording sites on each finger. Each recording and stimulating site is numbered as indicated in Fig. 1. The stimulating current was 120–220 µA consisting of bipolar pulses, 250 Hz for 250 ms, and 400 µs of duration for each pulse. Current pulses were passed between adjacent stimulating sites. The tone burst to evoke the CAP was 15 ms after the end of the electrical stimulation. The time between electrical stimulation offset and next electrical stimulation onset was set to 400 ms.
When the probe was inserted into the IC, a tuning curve was obtained from the 10 recording sites with tone bursts (100-ms duration and 400-ms interstimulation interval). The cellular activity of each recording site was audio/visually monitored while varying the frequency and intensity of the tone burst. Note that the cellular activity recorded at each site appeared to be predominately from one cell, but because of the area of the recording site, multiple cells may have contributed to the recording activity. For each frequency, the intensity was noted that produced an increase in spike activity. The tone burst to elicit a CAP was set at 10 dB above threshold [10-dB sensation level (SL)]. Two control responses (64 epochs each) were obtained and averaged. Two responses (64 epochs each) were then obtained with IC stimulation, averaged, and expressed as percent control CAP amplitude. This procedure was repeated for each adjacent pair of stimulating electrodes.
At the end of some experiments, a DC current (30 µA for 5 s or 100 µA for 10 s) was passed through adjacent pairs of stimulating sites to mark the position within the IC. These guinea pigs were given a cardiac perfusion with 4% paraformaldehyde in phosphate buffer. The brain was removed and placed in the preceding fixative overnight. After rinsing in phosphate buffer for 1 h, the brain was dehydrated in 100% ethyl alcohol (ETOH) and immersed in a 1:1 solution of JB-4 glycol methacrylate (Polysciences, Warrington, PA) infiltrating solution and 100% ETOH overnight. The brain was then immersed in 100% JB-4 infiltrating solution for 
24 h. Brains were embedded in plastic molds with JB-4 embedding solution and allowed to polymerize under UV light for 24 h. The resulting blocks were trimmed and 6-µm-thick sections cut on a Leitz 1516 microtome. Every other section through the brain were collected and baked onto a glass slide. Resulting slides were then stained with Paragon and cover slipped with Permount. Each slice was imaged with a light microscope, and the location of the recording and stimulation sites were confirmed.
The experimental protocol was approved by the Animal Care and Use Committee at the University of Michigan and conforms to the National Institutes of Health guidelines for the care and use of laboratory animals.
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RESULTS |
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10 to16 kHz. Two tuning curves from cells near recording sites 5 and 8 are shown in Fig. 3. The BFs at these sites were 13 and 16 kHz, respectively. Passing current through stimulating sites 3 and 4 produced a localized effect on the CAP with a maximum effect for an acoustic stimulus at 14 kHz. Each tone stimulus used to evoke the CAP was adjusted to be 10 dB above threshold. Increasing the current level increased the reduction in CAP amplitude. CAP amplitude was reduced by 20–80% for currents from 120 to 220µA. For each current level, the maximum reduction of the CAP amplitude was largest for the 14-kHz acoustic stimulus.
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4 to 10 kHz (see Fig. 4, top and bottom). Current stimulation through an adjacent stimulating site produced maximum reductions of the CAP amplitude for acoustic stimuli from 5 to 8 kHz (Fig. 4, middle). BFs from cells in the more medial location (Fig. 5, top and bottom) ranged from 6 to 11.5 kHz. From this more medial position, passing current through adjacent stimulating sites (Fig. 5, middle) affected CAP responses to acoustic stimuli ranging from 6 to 9.5 kHz. The increase in cellular BF with the more medial recording position is consistent with the tonotopic frequency map of the ICC.
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— ) at a given recording site within the IC and the frequency region within the cochlea maximally affected by current stimulation (
— ). There is a close match between BF and tonotopic location within the cochlea. Note that the frequency range of each measure is limited by the number of functioning recording and stimulating sites on the electrode.
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DISCUSSION |
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, 1978; Warr and Guinan 1979; White and Warr 1983) with projections to each cochlea. The efferent system is further divided according to location within the SOC and site of termination within the cochlea. The medial olivocochlear (MOC) system, located in the periolivary nuclei of the SOC (Vetter and Mugnaini 1992; Vetter et al. 1991; White and Warr 1983), innervates the outer hair cells and receives inputs from several locations including the IC (Mulders and Robertson 2000b, 2001; Robertson and Winter 1988; Thompson and Thompson 1991, 1993; Vetter et al. 1993). In addition, in guinea pigs, the MOC fibers give off collaterals to the cochlear nucleus (Benson and Potashner 1990; Winter et al. 1989). Excitatory effects of efferent activation on onset neurons of the cochlear nucleus have been reported (Mulders et al. 2003).
The present results are most likely due to the activation of the MOC not the middle ear muscles. Scates (Scates et al. 1999) found that the large reductions in DPOAE amplitude from stimulating the IC were unaffected by severing the middle ear muscles in one animal. Their reductions, 
25 dB, are large compared with the reductions in CAP amplitude equivalent to a 3- to 6-dB reduction in sound pressure reported here. Mulders and Robertson (2000a) investigated IC stimulation with and without paralysis in rats. Large reductions in DPOAE amplitude (5–12 dB) were reduced to 0–3 dB when the rats were paralyzed. The animals in this study were also paralyzed; this should inactivate the middle ear muscles. It is possible that the muscles were not completely paralyzed, but other evidence suggests that the middle ear muscles are not the reason for CAP reduction. Electrical stimulation within the ICD did not produce reduction of the CAP, and this suggests that current spread is not an issue (Mulders and Robertson 2000a). Moreover, the effects we observed were across a range of frequencies, including higher frequencies where middle ear muscle contractions have little effect. Contractions of the middle ear muscles primarily affect low frequency transmission but do not show the high-frequency-specific effects observed here (Borg et al. 1984). Similarly, stimulation of the ICX caused reductions of the CAP that extended to frequencies out of the range of middle ear muscle effects.
IC inputs to the MOC
For the purpose of this physiological study, we simplified the IC structure into three subnuclei. Such simplification allows gross statements about electrical stimulation of the subnuclei, and the effects within the cochlea. The present results showed that activating localized regions of the IC produced differential effects within the cochlea. As in previous studies (Mulders and Robertson 2000a; Scates et al. 1999), stimulation of the dorsal IC (e.g., dorsal cortex) was ineffective, and reductions in cochlear activity occurred only after stimulation of the central and lateral regions of the IC. The previously reported reductions in CAP amplitude (Mulders and Robertson 2000a) are also similar to those here.
Some of the previously reported effects of IC stimulation showed frequency preferences but not differences in tuning. For example, the Scates et al. (1999) investigation was limited to relatively low frequencies and reported greater effects at the lowest frequencies used. This may be related to potential middle ear effects or other masking related phenomenon. Mulders and Robertson (2000a) showed a maximal efferent effect for frequencies of 6–10 kHz.
It is possible that the electrical stimulation of the IC produces the effects within the cochlea by some other means than direct input to the MOC neuron. Because the IC is a major processing center for ascending and descending auditory processing, activation of the IC may influence any number of auditory structures. IC stimulation could activate ascending projections and direct cortical inputs to the cochlear nucleus that, in turn, may influence the olivocochlear system (Weedman and Ryugo 1996). We cannot rule out the potential cortical influence in this study, but the simplest pathway for IC activation of MOC neurons is a direct monosynaptic activation by IC inputs. Other projections from IC directly to the reticular formation are possible and could contribute to MOC activation.
Fine descending control of the MOC system
This report shows that the regions of ICC that provide descending input to the periolivary region have sharp tuning. These results are consistent with recent findings on the effects of IC stimulation on olivocochlear function (Mulders and Robertson 2000a; Mulders et al. 2003; Scates et al. 1999) that showed gross electrical stimulation of the IC produced broad efferent effects within the cochlea. However, the individual MOC fibers show sharper tuning characteristics similar to auditory nerve fibers (Brown 1989; Liberman and Brown 1986; Robertson and Gummer 1985). The present findings of sharp tuning in the ICC regions that activate MOC is consistent with an anatomical study (Malmierca et al. 1996) showing a tono-topically organized feedback projection from the ICC to the lower brain stem. Our results suggest that the IC efferent system may exercise a much finer level of influence than the "gross" effects previously described.
Several roles for the medial efferent system have been proposed including a simple attenuator, improving the signal-to-noise ratio, protection from acoustic trauma, and selective attention (Guinan 1996). In general, these proposed roles resulted from studies using electrical stimulation of the COCB at the floor of the fourth ventricle. Such stimulation likely activates most, if not all, of the medial efferent fibers innervating the outer hair cells. In the present study, stimulation of the ICC activates the MOC neurons synaptically rather than electrically. This more physiological stimulation reveals a fine control of the ICC over the MOC in the spectral domain.
The frequency-specific reductions observed with ICC stimulation may provide the basis for the phenomenon of an "attentional filter." Psychophysical experiments by Scharf and colleagues (Dai et al. 1991; Ebata and Scharf 1992; Scharf 1989; Scharf et al. 1987) suggest that the cochlear efferents may be involved in an attentional filter. They found reduced signal detection for unexpected frequencies distant from the signal frequency that was the focus of attention in the detection task, a phenomenon originally described by Green (1961). The frequency and level of the unexpected signal was, under different circumstances, clearly audible to the subject. The resolution of the present results cannot confirm that the medial efferent system is capable of accounting for the differences when stimuli are only 100 Hz apart as observed by Scharf and colleagues. However, Scharf et al. (1994, 1997) reported a broader "attention filter" in human subjects in which the olivocochlear bundle had been cut by vestibular neurotomy. Because the efferent transection was peripheral to the brain stem, the observed effects can be attributed to elimination of the efferents within the cochlea. These results provide strong evidence that the efferent system can selectively modulate transduction processes within the cochlea.
Coarse descending control from ICX
In contrast to ICC, the broad suppression of neural output associated with ICX stimulation may play a role in multimodal selective attention. The ICX is linked with other parts of the auditory system where prominent nonauditory inputs may modulate auditory activity (Malmierca et al. 2002). In this study, we show that ICX can affect cochlear activity via its presumed action on the MOC system. The effect of ICX stimulation reduces a broad of region of cochlear output, which if desired, would attenuate the auditory sensory input to the CNS in the presence of a nonauditory event. Previous studies suggested that selective attention exists in the auditory system. In the 1970s, Oatman and colleagues (Glenn and Oatman 1977; Oatman 1971, 1976; Oatman and Anderson 1977, 1980) showed that responses recorded from the round window of awake cats were reduced in a visual attention task. More recently, the role of selective attention and arousal state on cochlear emissions has been studied in humans (Ferberf-Viart et al. 1995; Froehlich et al. 1990, 1993a; Michie et al. 1996; Puel et al. 1988). Although Michie et al. (1996) did not find attention related differences in emissions, Froehlich et al. (1993b) found that the arousal state affected the amplitude of the transiently evoked otoacoustic emission (TEOAE). The TEOAE amplitude increases during sleep, and the efferent effect of contralateral noise is reduced during sleep. Thus the ICX control of the olivocochlear systems may differ substantially from that of the ICC.
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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This work was supported by National Institute of Deafness and Other Communication Disorders and Stroke Grants DC-04194, DC-0078, and DC-00189.
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FOOTNOTES |
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Address for reprint requests and other correspondence: D. F. Dolan, Kresge Hearing Institute, The University of Michgan, 1301 E. Ann St., Ann Arbor, MI 48109-0506 (E-mail: ddolan{at}umich.edu).
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) produced by current passed through stimulating sites 4 and 5 is within lateral external cortext (ICX). The 2nd lesion is located within the IC central nucleus (ICC) near the region of the rostral pole (RP). B, middle: blow-up of the lesion region in A.Top: a more posterior slice showing the lesion between sites 1 and 2 (see 
). Bottom: from a more anterior location (+36 microns) showing the lesion caused by passing current through sites 4 and 5 (see 
