in many fish and amphibian species the otolith organs ( the saccule and utricle ) are important for the detection of sound , as well as serving a vestibular function ( lewis and narins , 1999 ) . throughout vertebrate evolution , new structures evolved for the detection of sound culminating in the cochlea ( manley et  al . , 2004 ) . nevertheless , an acoustic sensitivity of the otolith organs has been conserved in all classes of vertebrate , including primates ( young et  al . , 1977 ;
in humans , acoustic sensitivity of the otolith organs can be demonstrated by vestibular - dependent effects like nystagmus ( lackner and graybiel , 1974 ) or evoked electromyographic ( emg ) signals ( bickford et  al . , 1964 ) .
such emg responses can be measured either from muscles of the neck , e.g. the sternocleidomastoid muscle , reflecting the vestibular - collic reflex pathways ( the vestibular evoked myogenic potential or vemp : colebatch et  al . , 1994 ) or from extra - ocular eye muscles , reflecting activation of the vestibular ocular reflex pathways ( ocular vemp or ovemp : rosengren et  al . , 2005 ; todd et  al . , 2007 ) .
although the neck response is often now referred to as a cervical vemp ( or cvemp ) , in the rest of this text we use the original acronym vemp .
the use of vestibular evoked emg methods has enabled considerable advances in our knowledge of the sensitivity of the human otolith organs to acoustic stimulation . within the literature
there is , however , considerable confusion in the use of terms , especially with the introduction of the mini - shaker ( e.g. model 4810 , bruel & kjaer , denmark ) as a means of stimulation , along with the usual head - phones for the delivery of air - conducted ( ac ) sound , and the more conventional audiological vibrator ( e.g. model b71 , radioear corp .
the principal source of confusion is that the nature of the skull response changes as a function of stimulus frequency . at the higher frequencies typically employed in audiometry ,
the skull response is primarily a function of its reactive , i.e. elastic , properties , but for low - frequencies , less than about 8001000  hz , the skull response is characterised as whole - head quasi - rigid vibration in which there is zero phase between stimulus and response ( stenfelt et  al . , 2000 ; mcknight et  al . ,
this is further complicated by the existence of several skull resonances near 500  hz . in order to distinguish these response regimes we use here
the terms bc sound vs. low - frequency vibration , with the transition placed at around 200  hz , just below the skull resonances . in response to sound and vibration
the two otolith organs appear to have distinct tuning properties , with the saccule and utricle tuned to approximately 500  hz and 100  hz respectively ( todd and cody , 2000 ; todd et  al . , 2009 ) , likely a consequence of underlying biomechanical properties .
( 2011 , 2012 ) showed that stimulation with both head - phones and mini - shaker may produce distinct resonances at about 100  hz and 500  hz , suggesting that the two resonance peaks are specific to the different dynamic responses of the two end - organs .
sound and vibration modes of stimulation also have distinct threshold properties . for 500  hz ac sound activation , vestibular thresholds
are found at about 80  db above the auditory thresholds ( todd et  al . , 2008b ) ,
while using 100  hz vibration , vestibular thresholds may be as low as 15  db below the auditory threshold ( todd et  al . , 2008a ) .
having made some progress in establishing the natural frequencies and appropriate modes of stimulation of the otolith organs , these sensitivities may be used as a tool to investigate the central pathways , i.e. by stimulating at best frequency for the receptors one is maximally likely to excite higher order neurons .
several attempts have now been made to measure vestibular evoked potentials ( vseps ) of neurogenic origin . following a study by de waele et  al .
( 2001 ) , which showed the existence of short - latency potentials ( 815  ms ) in response to electrical stimulation , todd et  al .
these acoustically evoked short - latency vseps were confirmed as being vestibular as they were absent in avestibular patients but present in deaf subjects with intact vemps ( rosengren and colebatch , 2006 ) . a later study by todd et  al .
( 2008b ) used a source analysis to confirm that the short - latency vseps are dominated by the pathways underlying the vestibular - ocular reflex , but also suggested activity in frontal cortex .
( 2011 ) used an alternative source analysis method to suggest that a wider range of vestibular cortical areas contribute to the short - latency potentials activated by sound .
such studies complement animal work using linear or rotational whole body acceleration to evoke short - latency vestibular responses ( sohmer et  al . , 1999 ; jones et  al . , 2011 ) .
while there is agreement on the existence of short - latency vestibular evoked effects , and some progress made in elucidating the sub - cortical and cortical generators in humans , a question which has not been addressed is the contribution , if any , made by vestibular receptors to the late auditory evoked potentials ( laeps ) .
these are characterised by a series of potentials , which are usually measured at the vertex , between about 50 and 250  ms , i.e. the p1 , n1 and p2 ( although the p1 is sometimes considered as a pb wave following the mid - latency response ( mlr ) na , pa and nb waves ( picton , 2011 ) ) .
source analysis indicates that the primary generators are bilateral tangential and radial sources in superior temporal cortex , with additional generators in the frontal cortex ( naatanen and picton , 1987 ; scherg et  al . , 1989 ) .
the aim of the present study was to address the above question by looking for evidence of changes in 500  hz ac sound evoked laep when stimuli are presented at intensities above vestibular threshold and to carry out a source analysis .
fourteen healthy subjects were selected for this study ( mean age  =  28.3 ; sd  =  6.9 ; 5 females and 9 males ) .
prior to any testing , all participants gave written informed consent according to the declaration of helsinki .
the experimental stimuli employed for obtaining vestibular responses were ac 2-ms , 500-hz , single cycle tone pips .
ac stimuli were delivered by insert earphones ( 3a insert earphone , e - a - rtone gold , guymark uk limited ) . the maximum input voltage , which was set to 1  v pp , resulted in a maximum output on the amplifier equivalent to a peak spl of 135.9  db re 20  pa ( as measured by the llpk parameter with linear frequency weighting ) and an rms spl of 115.4  db re 20  pa ( measured by the lai parameter , with a - frequency weighting and impulse time weighting ) .
stimulus calibration was carried out using a gras iec711 coupler ( ra0045 ) and a pressure - field microphone ( model 4134 ) with a 2260 investigator ( brel and kjaer , naerum , denmark ) .
the stimuli were generated using customised software with a laboratory interface ( power 1401 , cambridge electronic design , cambridge , uk ) and a commercial or custom amplifier . in normally hearing subjects use of ac stimuli
will give rise to evoked potentials of cochlear origin , i.e. auditory evoked potentials ( aeps ) , and thus any vestibular evoked potentials ( vseps ) would be mixed in with aeps . for this reason after obtaining the subjects ' vemp thresholds we recorded eeg responses both below and above the vemp threshold in two separate sessions .
the vemp threshold is necessarily higher than the receptor threshold due to synaptic attenuation , but is convenient to use as it fairly easy to obtain and changes in the infra - ocular waveforms , i.e. presence and absence of ovemps , are clearly recognizable above and below this level .
however although this threshold does not guarantee that the eeg response is free from vestibular influence we expected that they would be predominantly cochlear in origin .
audiograms were obtained for both ears using an amplivox audiometer ( amplivox ltd , uk ) with telephonics tdh 49 earphones ( telephonics corp . ,
each subject satisfactorily achieved pure tone air conduction thresholds of 20  db hl at 125 , 250 , 500 , 1000 , 2000 , 4000 and 8000  hz bilaterally , according to british society of audiology ( bsa ) ( 2011 ) recommended procedures .
psychophysical auditory thresholds of the stimulus used for evoked response recording were determined using psylab ( v2.0 , hansen , 2006 ) using 3-alternative forced choice ( 3afc ) , one - up two - down adaptive method to track the 79.4% point on the psychometric function ( levitt , 1971 ) .
the signal , i.e. 2-ms , 500-hz , single - cycle tone - pip , was randomly presented to the subject in one of the three intervals and delivered unilaterally through insert earphones ( 3a insert earphone , e - a - rtone gold , guymark uk limited ) .
the initial signal level was set to 81  db llpk ; this was reduced by 4  db after two successive correct responses and increased by 4  db after an incorrect response .
after four reversals the measurement phase began and the step size was reduced to 1  db .
subjects were tested lying supine on a couch , with the backrest to approximately tilted 3045 from the horizontal , and lifted their heads against gravity to activate the sternocleidomastoid ( scm ) muscles .
surface emg was measured from the ipsilateral scm using self - adhesive ag / agcl electrodes .
active surface electrodes were placed over the middle of the scm muscle belly and were referred to electrodes placed on the medial clavicle .
emg was amplified , bandpass filtered ( 5 hz1 khz ) and sampled using a power 1401 interface ( ced ltd . ,
the emg was sampled at a rate of 5  khz , starting 10  ms before to 80  ms following stimulus onset , and averaged .
stimuli were delivered by insert earphones ( 3a e - a - rtone gold , guymark uk limited ) . up to 200 stimuli
vemp thresholds ( vt ) were determined for each subject by reducing the stimulus intensity in 5  db steps over successive trials and were defined as the smallest intensity at which a  vemp could be produced in at least two trials .
vseps were recorded with subjects comfortably seated with their gaze directed straight ahead to a screen displaying silent movies at a viewing distance ( about 70  cm ) .
this recording position was adopted in order to avoid significant eye movement and alpha - wave artifact .
ac pips were randomly presented between 600 and 1000  ms , up to a total of 400 stimuli per trial . evoked potentials ( eps )
were recorded in two test sessions ; sub- and supra - threshold intensities were used in a first test session , i.e.  6 , 0 ,  + 6 ,  + 12 and  + 18  db re vt , and only sub - threshold intensities were presented in a second part , i.e.  6 ,  12 ,  18 and  24  db re vt .
additional electrodes were also placed below each eye ( i.e. infra - ocular electrodes , io1 and io2 ) , at deep frontal ( f9 and f10 ) and at ear - lobe locations ( a1 and a2 ) .
electrode offset ( i.e. running average of the voltage measured between cms and each active electrode ) was maintained below 20  v .
recordings were made with a band - pass of between 0.16  hz and 1  khz .
artefact elimination , epoching and averaging of eps were carried out using the besa 5 software .
epochs were 350  ms in length , from 50  ms before to 300  ms following the stimulus onset .
after collection , eps were filtered at 1200  hz and referenced either to the ear - lobe electrodes or to an average reference using scan software ( v4.3 , neuroscan , usa ) .
amplitudes and latencies of mid and long - latency aeps were measured at response peaks .
besa software ( version 5.1 megis software gmbh , germany ) was used for dipole modelling .
the radial thickness of the head , scalp , bone and csf were 85 , 6 , 7 and 1  mm respectively with conductivities set to 0.33 , 0.33 , 0.0042 and 1.0 , respectively . prior to conducting the source analysis changes in the global field power with intensity
vemp thresholds ( vt ) were recorded in all healthy subjects , with a mean ( sd ) threshold of 108.7 ( 6.1 ) db peak sound pressure level ( pkspl ) and 109.3 ( 6.5 ) db pkspl for left and right air - conducted ( ac ) stimulation , respectively .
absolute auditory thresholds were 26.0 ( 5.3 ) db pkspl and 26.2 ( 4.1 ) db pkspl for left and right ac stimulation , respectively . combined together these are equivalent to 82.7 and 83.1  db sensation level ( sl ) , similar values to that found by todd et  al .
as noted in the methods section the vestibular receptor threshold is likely to be below this , possibly by as much as 10  db or more , i.e. at around 70  db sl ( todd et  al . , 2010 ) .
grand means for the major conditions are illustrated in figs .  1 and 2 .
sub - threshold conditions ( 12 ,  18 and  24  db re vt ) produced a typical auditory brainstem response ( abr ) and middle / long - latency auditory evoked response ( mlr / laer ) pattern consisting of the slow wave v followed by the na , pa , nb , pb / p50/p1 , n1 , p2 waves .
these were well illustrated in channel fcz ( fig .  2 , aeps indicated with grey labels ) .
in contrast , supra - threshold conditions ( + 6 ,  + 12 ,  + 18  db re vt ) produced responses with an altered morphology .
the altered morphology includes short - latency waves ( todd et  al . , 2008b ) .
these waves are clearest in the infra - ocular ( io ) , prefrontal ( fpz ) and inion ( iz ) leads ( fig .  2 , vseps indicated with black labels ) .
the io responses consist of a series of alternating waves , the earliest of which occurs at 10  ms and is referred to as an n10 ( todd et  al . , 2008b ) .
the polarity and latency of these waves correspond with ovemps measured using a differential montage ( todd et  al . , 2007 ) .
in the prefrontal lead ( fpz ) , an n15 ( todd et  al . ,
2008b ) with similar latency to the latency of na component of mlr is apparent . in the parietal lead ,
such positivity has conventionally been referred to as p10 ( todd et  al . , 2008b ) , although this tends to merge with the wave v of the abr after low - pass filtering at 200  hz . in the inion lead ( iz ) , a series of waves analogous to the io waves can be observed , the earliest occurring at about 10  ms .
in addition to the above , some additional later waves not previously described were also observed . in particular a prominent prefrontal negativity ( labelled  n *  in fig . 
2 ) with similar latency to the nb component of mlr was apparent , followed by a corresponding positivity ( labelled  p *  ) with a similar latency to the pb / p1 deflection of the mlr / laer , but with a more definitely positive character than for the sub - threshold condition .
this n*p * deflection could be observed clearly in the midline frontal electrodes , fpz and fcz .
a further change in morphology was observed in the later waves with enhancement of the n1 potential compared to the p2 . as the transition from nb / pb to n*/p
* is quite a subtle one in the grand mean we also illustrate in fig . 
3 the transition in the individual responses at leads io2 and fcz when passing through the vemp threshold . at  6  db re vt
there is little or no sign of the ovemp waveform in the individual traces , although there is a hint of a small ovemp in the grand mean . similarly at fcz
there is no consistent nb / pb deflection , but a hint in the grand mean . at  + 18  db re vt in contrast
both ovemp and n*/p * waveform are consistently present in the individual traces , with the peak  peak ovemp and n*/p * amplitudes being approximately the same . in order to investigate statistically the effects of stimulus intensity on the amplitude of the responses , we measured peak  peak values of the potentials at the latency of the pa  nb and nb  pb components of mlrs ( including the n*p * components : fig . 
4a , b ) , and the peak values of the n1 and p2 components of laers ( fig . 
4c , d ) , for stimuli presented at  24 ,  18 ,  12 ,  6 , 0 ,  + 6 ,  + 12 , and  + 18  db re vt .
as there was no significant difference in the  6  db condition recorded at two separate sessions , the average value of the two was used .
in order to quantify this we carried out a slope analysis by means of linear regression of both individual amplitudes and grand mean amplitudes for the sub- ( 24 ,  18 ,  12 and  6  db ) vs. supra - threshold ( 0 ,  + 6 ,  + 12 and  + 18  db ) conditions . a t - test ( n  =  14 ,   =  .05 , two - sided ) to compare the sub- vs. supra - threshold regression parameters for each wave yielded essentially the same result , which was that the mlr peak  peak amplitudes showed a significant increase in slope ( at the 5% level ) when passing through the vestibular threshold .
for the pa  nb the slope changed from 12 to 42  nv per db ( p  =  .042 ) and for the nb
 pb ( n*p * ) from  1039  nv per db ( p  =  .003 ) . for the n1 and p2 potentials there was no significant change in slope .
in addition to the slope analysis anovas were carried out separately on the amplitudes for the mlr and laer amplitudes with within - subjects factors of  wave  ( pa  nb and nb 
pb for the mlr and n1/n74 and p2 for the laer ) and  intensity 
( + 18 ,  + 12 ,  + 6 , 0 ,  6 ,  12 ,  18  db re vt ) . for the mlr epoch
the anova yielded a main effect of intensity ( f(7 , 91 )  =  15.6 , p  <  .001 ) and a significant linear contrast ( f(1 , 13 )  =  46.3 , p  < 
.001 ) , but also a significant quadratic contrast ( f(1 , 13 )  =  9.0 , p  <  .05 ) , consistent with the slope change detected above . for the laer amplitudes the anova also yielded a main effect of intensity ( f(7 , 91 )  =  12.5 , p  <  .001 ) and a linear contrast ( f(1 , 13 )  =  21.5 , p  <  .001 ) , but in this case a significant 5th order contrast ( f(1 , 13 )  =  6.1 , p  <  .05 ) indicating a more complex slope pattern .
changes in the latencies of pa , nb / n * , pb / p * , n1 and p2 with intensity are shown in fig .  5 .
all waves show a general trend of decreased latency with increase in intensity , with a shallowing slope .
anovas carried out separately for each of the waves in the range of  12 to  + 18  db confirmed that the nb / n * and pb / p * each showed significant main effects of intensity , ( f(5 , 65 )  =  6.7 , p  <  .01 ) and ( f(5 , 65 )  =  5.2 ,
there was a shift in latencies with increase in stimulus intensity , from 50.3 to 42.1  ms for the nb / n * and from 60.6 to 52.8  ms for the pb / p * , both by about 8  ms .
in contrast the mlr pa and the laer n1 and p2 latencies did not show any significant effects , indicating that the intensity function had reached its asymptote for these waves .
this patient had a history of vestibular dysfunction with absent vemps for right side ac stimulation .
comparison of the waveforms of ac evoked responses in selected eeg leads with the average response in a sample of normal subjects on right ear stimulation shows clear differences .
aeps are mostly intact in the patient whereby waves v , na , pa , n1 and p2 are well - formed . in contrast
there is absence of the ovemp waveform in io2 , the n15 and n * components in fpz , the n*p * deflection at fcz , the p10 component at pz and the inion response at iz . fig . 
7 illustrates the changes in the gfp with intensity , which mirror the changes in morphology noted above . at intensities below vt four lobes of the gfp
can be discerned which correspond to aeps na , pa , n1 and p2 . at the highest intensity , in contrast
the earliest three of these have been previously described and correspond primarily to activities associated with short - latency vseps , which are dominated by vemp sources ( todd et  al . ,
the lobe preceding the n1 activity is not present in the sub - threshold conditions and corresponds to the growth of the n * deflection described above . at sub - threshold intensities
there is no particular focus in the scalp map but with an increase in intensity the n * develops two midline foci , one at fpz and the other at fcz .
given the novelty of the n*p * deflection we focused the source analysis to the lobe of the gfp associated with this activity , i.e. between approximately 32 and 52  ms , the latencies of the minima either side of the 4th lobe .
the modelling strategy employed was to start with the simplest models and gradually build up complexity using a genetic algorithm in which the number of sources could be specified .
for the simplest models , i.e. 1 , 2 or 3 dipoles , a unique solution could be obtained which indicated that 90% of the variance could be explained by contributions from an ocular source and from a cortical midline source in a middle cingulate location .
the third source , which was located to a deep area , explained just an additional 3% of variance .
all three sources were contralateral to the midline . for more than 3 sources no unique solution could be obtained , but a number of additional sources were indicated , including a contralateral cortex source and a cerebellar source , as well as a deep source indicated in the unique solutions . since an ocular source was implicated we also investigated a set of models in which a single dipole was replaced by a symmetrical pair , with an additional 2 , 3 or 4 sources .
all three of these models located the ocular pair within the orbit , and again located a second source in the midline , with additional sources in contralateral , cerebellar and deep locations . for an ocular pair and three dipoles the residual variance
as the ocular sources are likely to be complex , including possibly a combination of vemp and eog signals , we also investigated a class of model in which the ocular pair of dipoles was replaced by a pair of regional sources .
as above the first dipole was always located to the midline in a cingulate region with the second to a contralateral region .
examination of the morphology of the midline current source ( sources 3 and 5 in fig . 
8b ) suggests that its activity is not confined to the fitting interval employed but continues to contribute significantly to the later components . in order to investigate this
further we also carried out a source analysis of the 5th lobe of the gfp associated with the n1 , i.e. for the interval 52102  ms , independently of the 4th lobe . a standard approach to modelling the n1 is to fit a pair of regional sources .
we applied this approach , as well the simple dipole source approach employed above . about 94% of the variance
could be explained with just two dipoles , one located in the contralateral temporal lobe and the other in a midline position . with three dipoles
the ipsilateral temporal lobe was also implicated , along with the midline and contralateral sources . if a pair of regional sources was applied , the resultant locations were medial of temporal lobe , indicative of the presence of an additional midline source .
thus , for a symmetrical pair of regional sources and a single dipole ( illustrated in fig . 
8c ) , the regional sources were located within the bilateral transverse temporal gyrus ( 53 ,  19 , 8) with the single source located to the mid - cingulate area , accounting for about 97% of variance .
consistent with the single dipole approach , the largest current is from the mid - line source , followed by the contralateral source , with a relatively small contribution from the ipsilateral source .
in the present study , we have provided evidence that for 500-hz ac - stimuli there are systematic changes that take place in laeps at intensities above the vemp threshold and have tentatively identified a new mid - latency potential which appears to have a significant vestibular contribution .
the changes that take place are of two kinds : changes in the slope of the amplitude functions with intensity of the ac stimulus and changes in the morphology and distribution of the potentials above and below vt .
there are a number of prior studies that indicate aeps tend to plateau and saturate with increasing intensity . for the pb component saturation
occurs at quite low intensity , from about 50 to 70  db normal hearing level ( nhl ) ( thornton et  al . , 1977 ; ozdamar and kraus , 1983 ) , while for the n1/p2 components saturation occurs at higher levels , i.e. at about 8090  db nhl ( picton et  al . , 1974 ; dierks et  al . , 1999 ;
our data also show evidence of early saturation of the pa  nb and nb  pb components in the form of a plateau up to the vestibular threshold , consistent with the earlier literature , but then this is followed by an increase in slope at around the vestibular threshold where the n*p * component is recruited . although the n1/p2 waves in our case do not show a significant change in slope they do exhibit a plateau or inflexion near the vestibular threshold ( about 80  db sl ) followed by a continued increase in amplitude .
prior studies of changes in the latency of aeps as a function of intensity are consistent in revealing a general tendency of the latency to reduce with intensity but with a decreasing rate of change ( slope ) as intensity increases , possibly to a point of saturation , i.e. where there is no further significant change . for
the na / pa waves a saturation may occur early at about 4050  db nhl ( maurizi et  al . , 1984 ) , but for the n1/p2 component , although the largest change in slope takes place up to 40  db nhl , the latency continues to decrease up to 90  db nhl , albeit at a slower rate ( picton , 2011 ) .
prior studies of the pb component , however , indicate a more complex non - monotonic latency - intensity function ( ozdamar and kraus , 1983 ) . in our results we detected a significant latency shift in the nb / n * and the pb / p * components in passing through the vestibular threshold , but not for the na or the n1 and p2 components .
this result supports an interpretation that the na , n1 and p2 components of the aep had reached near saturation but an additional process was occurring in the generators responsible for the n*p * deflection .
considering the changes in waveform morphology , the most dramatic change around vt occurs in the infra - ocular and inion leads with the appearance of ovemp and inion response waveforms .
however , in addition to these changes , there was also the development of the n*p * deflection .
the avestibular patient had absent ovemp , n15 , p10 , and inion responses and also lacked the n*p * deflection .
these two waves also showed a significant increase in the peak  peak slope in the amplitude function of intensity and a significant change in their latency in passing through the vestibular threshold . taken together with the latency changes , our evidence supports the view that the n*p * deflection is vestibular in origin .
for this reason we refer to these hereafter as vestibular n42/p52 waves , in order to distinguish them from their aep counterparts the nb / pb waves .
the pb or p50 when considered with the laers is referred to as a p1 , preceding the n1 and p2 .
thus our data supports the case that vestibular receptors do indeed contribute to the laers , which are believed to be cortical in origin , and thereby provides a new method of investigating vestibular - cortical projections in intact human subjects .
although source analysis can not establish definitively a causal locus or generator site it is a useful method for providing hypotheses for such sites .
our analysis implicated mid - cingulate activity , along with a source emanating from the eyes , as accounting for 90% of the variance associated with the n42/p52 , and , along with contralateral auditory cortex , accounting for 94% of the laep n1 .
the recruitment of cingulate cortex above vestibular threshold plausibly accounts for the changes in slope we observed in the amplitude - intensity functions of both n42 and n1 .
activity within the cingulate gyrus is consistent with the literature on vestibular cortical projections in which a  vestibular cingulate region  has been identified in primate studies ( guldin and grusser , 1998 ) . in humans
the vestibular cingulate area extends from anterior cingulate to middle and more posterior cingulate areas ( lopez and blanke , 2011 ) . in our case
the source was located in a mid - cingulate area ( ttc y - coordinate ranging from  5 to  20 ) corresponding approximately with the posterior aspect of ba24 ( anterior cingulate ) and anterior portions of ba 23 and 31 ( posterior cingulate ) .
a number of imaging studies using caloric and galvanic vestibular stimulation have observed activity in this area ; ( lobel et  al . , 1998 ; suzuki et  al . , 2001 ; fasold et  al . , 2002 ) .
recent attempts that have been made using fmri with acoustic stimulation to investigate saccular cortical projections have also observed cingulate activation .
( 2007 ) reported activation in both the anterior ( ba32 ) and posterior cingulate ( ba31 ) although this was not replicated by schlindwein et  al .
the anterior cingulate region is considered to have two distinct functional and anatomic divisions , i.e. dorsal / rostral vs. ventral / caudal ( lopez and blanke , 2011 ) . the dorsal cingulate is generally associated with motor function , including ocular - motor control , and is connected with frontal eye fields , pivc and other vestibular areas ( guldin and grusser , 1998 ) . the ventral portion is considered to be more limbic in function , and is connected to anterior insula and other subcortical structures .
of particular relevance to the present work was a study by smith et  al .
( 2012 ) who showed that there are strong cross - modal visual - vestibular interactions in an area they termed the cingulate sulcus visual area ( csv ) which is  closely involved in encoding egomotion  and  strongly responsive to coherent optic flow  .
( 2008 ) , which showed activity in both cingulate cortex and planum temporale to coherent visual motion .
the existence of a vestibular contribution to aeps , if confirmed , has at least two important consequences .
the first is that any auditory studies , which make use of high signal intensity need to take into account the possibility that results are contaminated with non - cochlear contributions .
for example , a large number of studies make use of the loudness dependence of aeps ( ldaep ) to make inferences about the activity of certain catecholamine transmitters , including serotonin and dopamine ( dierks et  al . , 1999 ) .
such methods have been applied to a number of psychiatric disorders , including depression , schizophrenia and obsessive - compulsive disorder .
it is possible that the ldaep results are influenced by activation of the vestibular system , especially as it appears that the highest intensity stimuli used in these studies , which may exceed 100  db spl , are the most critical ( gallinat et  al . , 2000 ) .
this is of particular importance because the central projections of the vestibular system include insular , cingulate and autonomic pathways , which employ transmitters of interest ( balaban and yates , 2004 ; lopez and blanke , 2011 ) .
a second potential consequence is that it is quite possible that acoustically activated non - cochlear ( vestibular ) projections may play a role in normal hearing , for example , in the vocal behaviour of primate and amniote vertebrate species ( todd and cody , 2000 ; todd and merker , 2004 ; todd , 2007 ) , as well as the vocal behaviour of anamniotes for which the otolith organs are established as auditory . in the case of humans
many music environments , such as at concerts , are of high - intensity and well above the threshold levels we have considered ( todd and cody , 2000 ) .
similarly , the singing voice is high - intensity and very likely produces self - activation of the vestibular apparatus ( todd , 1993 ) as was suggested as long ago as the 1930s by tait ( 1932 ) . given a probable role of cingulate cortex , such activation could contribute to the affective responses to sound or in listening to music ( todd , 2001 ) . as noted above
mccue and guinan ( 1994 ) found a rate threshold in cat vestibular afferents of 90 db spl for 50 ms tones , with a phase - locking threshold 10 db lower .
combined with the appropriate psychophysical correction for short tone bursts , e.g. meddis and lecluyse ( 2011 ) , this places the receptor rate threshold at near 70 db sl .
it is likely , therefore , that vestibular receptors contribute at intensities well below those experienced in the loud environments described in the previous paragraphs .
the presence at everyday intensities of a vestibular component in cortical potentials from the temporal lobe , hitherto considered purely cochlear in origin , raises the possibility that acoustic activation of the otolith organs could contribute directly to auditory discrimination , as well as to affective processes .
auditory interaction which allows vestibular inputs to improve temporal and spatial aspects of hearing ( emami and daneshi , 2012 ; brimijoin and akeroyd , 2012 ; probst and wist , 1990 ) , to contribute to speech perception and in metrical aspects of musical perception ( emami et  al . , 2012 ; phillips - silver and trainor , 2008 ) , and which indicates a general association between hearing loss and vestibular dysfunction ( akin et  al . , 2012 ; zuniga et  al . , 2012 ; kumar et  al .
, 2010 ; wang and young , 2007 ) . given the well - established cross - over from vestibular to auditory pathways at the level of the brain - stem , ( e.g. barker et  al . , 2012 ) , and thalamus , ( roucoux - hanus and boisacq - schepens , 1977 ; blum et  al . , 1979 ) , vestibular effects at the level of temporal cortex should be expected , especially as activation of superior temporal lobe is consistently indicated in vestibular imaging studies ( lopez et  al . , 2012 ) .
we have in this paper presented evidence to support a vestibular contribution to aeps , and have tentatively identified a possible new component , the n42/p52 , which appears to be vestibular in origin .
some of this evidence is quite subtle , but this probably explains why it has hitherto not been remarked upon .
auditory interaction for hearing research it is important that further work is conducted to substantiate these . among the most important of these future studies should be work with a larger sample of avestibular patients to investigate differences in aeps .
given also the limitations of the vemp threshold in identifying a receptor threshold new methods for establishing a n42/p52 threshold directly from aeps should be developed .
finally higher resolution imaging methods will be required to substantiate the brain areas suggested by the besa .
 acoustic sensitivity of the vestibular apparatus is well - established , but the contribution of vestibular receptors to the late auditory evoked potentials of cortical origin is unknown . evoked potentials from 500  hz tone pips were recorded using 70 channel eeg at several intensities below and above the vestibular acoustic threshold , as determined by vestibular evoked myogenic potentials ( vemps ) . in healthy subjects both auditory mid- and long - latency auditory evoked potentials ( aeps ) , consisting of na , pa , n1 and p2 waves , were observed in the sub - threshold conditions . however , in passing through the vestibular threshold , systematic changes were observed in the morphology of the potentials and in the intensity dependence of their amplitude and latency . 
 these changes were absent in a patient without functioning vestibular receptors . 
 in particular , for the healthy subjects there was a fronto - central negativity , which appeared at about 42  ms , referred to as an n42 , prior to the aep n1 . 
 source analysis of both the n42 and n1 indicated involvement of cingulate cortex , as well as bilateral superior temporal cortex . 
 our findings are best explained by vestibular receptors contributing to what were hitherto considered as purely auditory evoked potentials and in addition tentatively identify a new component that appears to be primarily of vestibular origin . 
Introduction
Methods
Results
Discussion
Concluding remarks