multiple brain systems generating the rat auditory evoked...

240 Brain Research, 602 11993) 240-251) g) 1993 Elsevier Science Publishers B.V. All rights reserved 0006-8993/93/$06.00

BRES 18483

Multiple brain systems generating the rat auditory evoked potential. I. Characterization of the auditory cortex response

Gregory V. Simpson a,b and Robert T. Knight b

" Unit'ersity of California, Berkeley, CA 94702 (USA) and b Department of Neurology, Unicersity of California, Dat'is, Veterans Medical Center, Martinez, CA 94553 (USA)

(Accepted 25 August 1992)

Key words: Auditory evoked potential; Auditory cortex response; Multiple auditory system; Rat

The objectives of this study were to characterize the auditory cortex response in the rat and to examine its contributions to the auditory evoked potentials (AEPs) recorded from the dorsal and lateral skull. This was accomplished by simultaneously recording AEPs from the cortical surface and from skull screw electrodes in anesthetized animals. The initial positive-negative response (P17-N32) was largely restricted to the cortical region corresponding to area 41. More detailed examination of the AEP mapping revealed multiple subcomponents (P9, PI4, P17, P19) underlying the initial positivity, with differing topographies. Stimulus-response properties further dissociated the multiple positive subcomponents. Reversible local neurochemical suppression confirmed the auditory cortical origin of these AEPs. The auditory cortex-generated AEPs were refractory to barbiturate anesthesia which eliminated all dorsal skull AEPs, indicating that primary auditory cortical AEPs do not make a significant contribution to the dorsal skull-recorded ('vertex') AEPs. The findings raise issues regarding multiple parallel auditory processing systems and their associated AEPs.

l N T R O D U C T I O N

Auditory evoked potentials (AEPs) are used in a broad range of neuropharmacological and neurophysi- ological studies of sensory and cognitive processes in animals and humans. Substantial evidence for auditory cortical sources of the obligatory exogenous AEP components in humans has been obtained from studies of scalp topography (e.g. refs. 8, 34, 35), lesion effects (e.g. ref. 15), intracranial recordings (e.g. refs. 6, 22), dipole source localization (e.g. refs. 25, 26) and magne- toencephalography (e.g. ref. 9). Although these studies indicate that human exogenous AEPs receive major contributions from the superior temporal plane and lateral temporal lobe, contributions from sources outside the sensory cortices, such as the hippocampus, cingulate and association cortices, have not been ruled out .

The obligatory AEP components recorded in non- human primates ~ have been shown to have major contributions from auditory cortex, although possible

smaller amplitude contributions from other brain areas have not been eliminated. Studies in other animals have reported source activity from both auditory cortex 5'~2 and subcortical sources l~. Buchwald and col-

leagues have concluded that different components of the AEP in the cat are generated by each of these systems 5'1°'~1. Simpson and Knight 28"29 have suggested

that at least two parallel auditory systems are involved in generating the rat AEP. Recently, it has been pro- posed that the AEP in the guinea pig also originates from more than one generator system 17.

In humans it can be difficult to differentiate the contributions of auditory cortex to surface AEPs from those of other candidate sources such as frontal lobe, hippocampus and subcortical sources, due to the su- perposition of the potential fields. Delineation of contributions from sources other than auditory cortical areas is important in the functional description of the generator systems underlying the AEP. Definition of AEP sources in animals can also guide generator localization studies in humans.

Correspondence: G.V. Simpson, Department of Neurology, Albert Einstein College of Medicine, R.F. Kennedy Center, Room 915, 1410 Pelham Parkway So., Bronx, NY 10461, USA. Fax: (1) (212) 824-3058.

The anatomy of the rat auditory system is well suited for dissociating the contributions of auditory cortex and other brain structures to the AEP. Rat auditory cortex forms a nearly planar sheet on the ventral lateral brain surface which is parallel to the lateral skull and orthogonal to the dorsal skull surface 18'19'23. This geometry suggests that the AEP

fields produced by auditory cortex should be largely circumscribed to the lateral skull and contribute little to no amplitude to the AEPs commonly recorded on the dorsal skull, particularly at the vertex 28. One of the first steps in understanding the generator systems underlying rat AEPs is to examine this hypothesis.

Mapping of the initial positive-negative (P1-N1) AEP on the lateral cortical surface in anesthetized rats reveals a maximum amplitude over auditory cortex 2'21"28. However, the contribution of this activity to

lateral and dorsal skull AEP recordings has not been determined, except for the preliminary results of Simp- son and Knight 28. Recordings at the surface and depths of auditory cortex have revealed inversions of the P1 and N1 components 3'2, supporting the idea that auditory cortex is the principle generator of the P1 and N1 recorded over auditory cortex. The present study em- ploys epicortical mapping and reversible local neurochemical suppression to further examine this hypothesis. The P1 appears to have at least two subcomponents 3, although previous reports have not made this distinction when mapping the P1 over the lateral cortex 2'21"2s. The present study determines whether dif-

ferent P1 subcomponents correspond to different thalamic projection fields in auditory cortex. The above issues are addressed in the present study through epicortical mapping of the AEP over the lateral cortical surface with simultaneous sampling of the distribution over the skull in the anesthetized rat and by reversible local neurochemical suppression of the auditory cortex response. The second paper in this series 3° investigates the contributions of auditory cortex to the AEP in the alert unrestrained rat, and addresses how multiple generator systems underlie the rat AEP.

MATERIALS AND METHODS

Animals Adult male Fischer 344 rats approximately 12-months old were

used. Eight hemispheres (6 rats) were mapped for auditory cortical responses and 10 locations in 6 additional hemispheres (6 rats) were examined for the effects of y-aminobutyric acid (GABA) on the auditory evoked responses.

Surgical procedures All animals were given atropine to control secretions and then

anesthetized with pentobarbital (50 mg/kg) . They were placed in a stereotaxic frame and held steady with an incisor bar and hollow ear

241

Ref FCx Vx PL ACx Gnd

6 ' 6 1 l l l ~ 5 ~ l l l I i i

Bone Flap Fig. 1. Recording sites. Dorsal view (top) illustrates locations of the skull screw electrodes. The lateral view (bottom) depicts the location of the craniotomy used for epicortical recordings and mapping. Posterior to anterior stereotactic coordinates are indicated in mm

with the ]nteraural Line as zero.

bars through wh~h auditory stimuli were delivered. The skull was exposed along the midline and laterally following resection of the temporalis muscle on each side. The locations of the electrode sites and the craniotomy are depicted in Fig. 1. Stainless steel screw electrodes were implanted in the skull over frontal cortex (FCx), 2 mm anterior to bregma and 1 mm lateral to the central suture; at the vertex (Vx), 5.5 mm anterior to the interaural line and 1 mm lateral to the central suture; at the posterolateral dorsal skull (PL), 4.5 mm anterior to the interaural line and 5 mm lateral to the central suture at the lateral edge of the dorsal skull, immediately dorsal to auditory cortex; and in the lateral skull over auditory cortex (ACx), 4.5 mm anterior to the interaural line and 4.5 mm ventral to the dorsal skull on one side. The reference electrode was located over the frontal sinus and the ground screw was inserted over the cerebellum.

A cranial flap was made exposing the dorso-lateral surface of the brain from the interaural line to 12.5 mm anterior and ventrally to the rhinal fissure on the side opposite the screw electrodes. Care was taken not to damage the dura. The dura was kept moist by continual application of warm Ringer 's solution (37°C). Cortical response mapping was conducted with epidural recordings. Following the comple- tion of the cortical mapping studies on the exposed hemisphere, the other hemisphere was similarly exposed and studied.

Animals were maintained in a deep state of anesthesia with additional injections of pentobarbital and their EEG and reflexes were monitored at all times. The body temperature was monitored with an anal probe and maintained at 37°C with a water heating pad. Ringer 's solution, saline washes and the G A B A solution were maintained at body temperature (37°C) in a water bath.

Recording procedures Recordings were made from FCx, Vx, PL and ACx skull screw

electrodes and from silver-silver chloride ball electrodes 0.5 mm in diameter for recordings from the cortex (epidural and direct). All leads were referenced to the frontal sinus and the EEG was amplified by Grass P511K amplifiers with bandpass filter settings of 10 Hz -3 kHz. The amplified EEG was fed to a 4-channel Nicolet signal averager for averaging the evoked potentials, to a signal analyzer for continual on-line monitoring of the power spectrum of the EEG and to an oscilloscope and polygraph for monitoring the ongoing EEG.

242

All analog data were stored on FM tape and on-line averages of the evoked potentials were stored on floppy disks. Post-stimulus epochs with excess movement or muscle activity were automatically rejected by the computer.

Experimental procedures

Auditory stimuli Clicks (0.1 ms duration) were delivered either monaurally or

binaurally through the hollow ear bars at 50 dB above the animal's threshold as determined by BAEP audiometry, in the presence of 25 dB of broadband noise (1 Hz-20 kHz) to mask bone conduction.

Brainstem auditor)' et,oked potential (BAEP) audiometry BAEP thresholds were determined binaurally and for each ear in

order to control for click stimulus threshold differences when com- paring the effects of ipsilateral and contralateral ear stimulation on the cortical evoked response. BAEP thresholds were obtained using clicks at 10/s, sum of 1,024, recording bandpass of 100 Hz-3 kHz. BAEP threshold was defined as the click intensity at which a replicable wave IV could no longer be recorded. Both ascending and descending 2 dB steps were used and the mean intensity level from these two approaches was then used as the threshold (see ref. 27 for details).

Cortical mapping of the auditory response Epidural-evoked potentials (sums of 32) to binaural click stimuli

delivered at 0.5/s were recorded using ball electrodes 0.5 mm in diameter. Recordings were made from 4 electrodes at a time spaced 1 mm apart in a dorso-ventral line and this array was then moved in 1 mm steps across the cortical surface. Evoked potentials were recorded twice at each site. Order of mapping steps, anterior to posterior, dorsal to ventral cortex, were counter balanced across animals. All recording sites were designated by their stereotaxic coordinates. At the end of the experiment the brains were measured. The brain sizes were sufficiently similar across animals that the means of the brain dimensions were used in constructing the anatomical representation of the response map (Fig. 6). The anterior-posterior length was 16.0 mm (S.D. = 0.31), and the ventral- dorsal height was 7.0 mm (S.D. = 0.20), measured from the rhinal fissure to the dorsal surface at a point 5 mm anterior to the interaural line. Amplitudes at each recording site were converted to percentage of maximum response amplitude for that hemisphere. Linear interpolation of percent amplitude values between spatially adjacent sites was used to derive the contour lines indicating the percent amplitude boundaries. These boundaries demarcate regions containing response amplitudes greater than or equal to the boundary line. For example, the 75% boundary line encompasses the cortical region where amplitudes are from 75% to 100% of maximum.

Ear of stimulation and rate of stimulation effects After finding the site of maximal evoked response, evoked poten-

tials were recorded from that epidural location, in 5 animals, to click stimuli delivered at 0.5/s to the right ear, left ear and binaurally. In 4 animals, evoked potentials were recorded from the maximal response region to binaural clicks at 1.0/s, 5 /s and 10/s stimulation rates.

GABA effects The cortical surface was rapidly mapped to identify the region

yielding the largest responses, this area was then used for subsequent recordings and applications of saline and GABA solutions. The dura was carefully opened over this region and the immediately surround- ing area to allow application of solutions to the cortical surface while recording. In order to determine that the effects of GABA were due to direct local cortical actions, recordings were made at the site of topical GABA and saline applications (the 'treated' site) in the region of maximal responses and simultaneously at another ('non- treated') site 1.5 mm away. The non-treated site was selected on the

basis of having evoked potential amplitudes which were at least 7551 of those recorded at the treated site. The location of the non-treated site varied across animals, but was typically ventro-rostral to the treated recording site.

Evoked potentials were recorded to binaural click stimuli delivered at 0.5/s. Saline and GABA solutions were kept at body temperature and applied to the cortical surface at the "treated' recording site by means of a Hamilton micro-syringe in a stereotaxic holder. The protocol for saline (control) applications consisted of two recordings prior to application of saline ('pre-saline'), followed by application of 1 /~1 of saline to the cortical surface at the recording site, which was allowed to diffuse for 5 s and was then washed with Ringer's solution (body temperature) for 10 s. Evoked potential averages (sums of 16) were recorded immediately following the wash (0-30 s) and at 45-75 s, 90-120 s and 150-180 s post-wash. The same protocol was followed for the GABA applications, substituting 1 p.l of GABA solution (100 /xg/p.I in a saline solution) for the saline. In 4 animals this procedure was repeated at another location within the maximal response region (typically about 1 mm away from the first site), resulting in 10 locations tested in 6 hemispheres.

Ana[yse3 Peak amplitudes of the components were measured relative to

the pre-stimulus baseline. Peak latencies were measured from stimulus onset with 0.5 ms subtracted for the air conduction time from the headphone through the hollow ear bar to the ear. The effects of stimulation rate were analyzed by subjecting the amplitude and latency measures to a repeated measures ANOVA. The effects of stimulating the contralateral vs. ipsilateral ears were analyzed with t-tests (two tailed).

RESULTS

A L P components

T h e w a v e f o r m o f t he e p i d u r a l A L P r e c o r d e d ove r

a u d i t o r y c o r t e x in t h e b a r b i t u r a t e a n e s t h e t i z e d an ima l

is s h o w n in Fig. 2. T h e A L P c o m p r i s e d a pos i t ive p e a k

o c c u r r i n g at 9 ms ( P 9 : 8 . 6 ms, 0.1 S .E .M. ) , a l a rge r

a m p l i t u d e pos i t ive p e a k at 17 ms ( P 1 7 : 1 7 . 4 ms, 0.53

S .E .M. ) , a n d a l a rge b r o a d n e g a t i v e t r o u g h at 32 ms

( N 3 2 : 3 2 . 4 ms, 1.3 S .E .M. ) , w h i c h was m o r e va r i ab l e in

l a tency , a m p l i t u d e a n d d u r a t i o n . R e c o r d i n g s wi th 120

ms e p o c h s r e v e a l e d no f u r t h e r c o m p o n e n t s fo l lowing

t h e N32. T h e s e f i n d i n g s a re c o n s i s t e n t w i th t h o s e re-

p o r t e d by B o r b e l e y a n d Hal l 4.

Skull distribution

T h e l a rge a m p l i t u d e (up to 150 /~V) A E P s r e c o r d e d

ove r au d i t o ry co r t ex ( P 9 - P 1 7 a n d N32) w e r e t ight ly

r e s t r i c t e d to a few s q u a r e m i l l i m e t e r s o f l a t e ra l cor t ica l

s u r f a c e a n d e x h i b i t e d a s t e e p p o t e n t i a l g r a d i e n t . S imul -

t a n e o u s r e c o r d i n g s f r o m the skull s c r ew e l e c t r o d e lo-

c a t e d ove r a u d i t o r y c o r t e x ( A C x ) r e v e a l e d t h e s a m e

P 9 - P 1 7 a n d N32 ( see Fig. 2), w h e r e a s , only a smal l

a m p l i t u d e P17 ( less t h a n 10% o f t h a t r e c o r d e d at A C x )

c o u l d be r e c o r d e d at t he d o r s o l a t e r a l skull j u n c t u r e

a b o u t 5 ram. away ( P L site) , a n d no c o m p o n e n t s w e r e

r e c o r d e d , fo l lowing t h e b r a i n s t e m r e s p o n s e s , f r o m d o r -

sal s i tes at t h e v e r t e x (Vx s i te) a n d ove r f r o n t a l co r t ex

( F C x site) . Fig. 2 s h o w s t h e c o n f i n e m e n t o f t he P 9 - P 1 7

FCx

Vx

PL

ACx

ACx epidurol

P17

+

1 I

50 rnsec f 1 [ S 1 0 3 0

Fig. 2. Example of epidural AEP recorded from auditory cortex (ACx epidural) and from the skull screw electrodes FCx, Vx, PL and ACx (see Fig. 1). AEPs in this and all other figures were recorded to clicks at 0.5/s, sum of 32, bandpass of 10 Hz to 3 kHz, unless indicated otherwise. In this and all other figures of AEPs, the time of st imulus arrival at the animal 's ear (stimulus onset plus air conduction time through the hollow ear bars) is marked by the arrow with a n ' S ' underneath. Note: calibration bar is 60 /zV for the epidural ACx recording, 30/xV for the ACx skull screw electrode and 20 IzV

for the other skull sites.

complex and N32 to the lateral skull. The restriction of the small amplitude P9 to the lateral surface is further illustrated in Fig. 3 where the P9 was isolated by using a higher stimulation rate (10/s). Fig. 3 shows the BAEP, which is a broadly distributed far-field response, recorded at all electrode sites on dorsal as well as lateral surfaces. However, the P9 was not found at dorsal sites (Vx, FCx) including the dorsolateral border site (PL), but was recorded only over auditory cortex (ACx site).

Cortical surface distribution The distribution of the AEP was sampled at 1-1.5

mm steps over the cortical mantle from the interaural line to 11-12 mm anterior to it and from the rhinal fissure to 6 mm dorsal to it. An example of the spatial distribution of the AEP is given in Fig. 4, which illustrates the AEPs recorded at sites over the cortical surface in rat R3. Fig. 5 illustrates the locations and overlap of the P17 distributions for the 8 hemispheres which were mapped. Each contour line encompasses the region having P17 amplitudes from 75% to 100% of maximum for a given hemisphere, and the locus of the maximal response for each hemisphere is indicated by an asterisk. A map of the grand mean (across hemispheres) distribution of the P17 is given in Fig. 6 with boundary lines demarcating the regions of 75-

243

100%, 40-100% and 20-100% response amplitudes. The asterisks denoting the maximal response loci for the 8 hemispheres are retained in the figure. The locations of the regions containing response amplitudes of 40-100% of maximum for the P9 and N32, were essentially the same as the 40-100% region for the P17. Due to the smaller or more variable amplitudes of the P9 and N32, particularly those below 40% of maximum amplitude, quantitative comparisons of the distributions for the P9, P17 and N32 were not made. The potential gradients over the cortical surface were similar for all three components (P9, P17, N32), exhibiting a steep spatial decrement from 75% to 20% of maximal response amplitude. Responses typically dropped to 10% of maximum or less within a millimeter of the 40% region and decreased most precipitously in the dorsal direction (Fig. 6).

The region of 40-100% response amplitudes is co- extensive with Krieg's 18J9 anatomically derived map of area 41 as shown in Fig. 6. Patterson's 23 core area corresponds to the projection area of the ventral division of the medial geniculate body as determined with the HRP tracing technique. This correlates well with Krieg's ]8']9 area 41, or auditory koniocortex, based on cyto- and myelo-architectonic grounds. It should be noted that no stereotaxic coordinates, or other mapping coordinates for the graphic depictions of these areas were used by Krieg or Patterson. Consequently, the overlay of their maps with the AEP amplitude map is an estimate based upon anatomical landmarks. Multiple PI subcomponents (P14, P17, P19). Examina- tion of the latencies of the positive peaks recorded from posterior to anterior regions indicates three different positivities with overlapping distributions, a P14,

Fox

A C x

vx ~

P 9

I t t 5 10 msec - s

Fig. 3. Skull distribution of AEP recorded from skull screw electrodes at FCx, Vx, PL and ACx, to clicks at 10 /s , sum of 512,

bandpass 10 Hz to 3 kHz. Calibration bar = 1.5 pN.

244

a P17 and a P19, with the P17 being 5-10 times the amplitude of the other two components. The maximal response (P17) tends to mask the small P14 and P19 peaks except in the regions having low P17 amplitudes. In Fig. 4 for example, small amplitude responses can be seen in the lower left and lower right corners having slightly earlier and later latencies, respectively, than the peak latency of the maximal response.

In 4 hemispheres, a small P14 response (20-30% of maximum) was maintained in the more extreme anterior region from 8 to 10 mm anterior to the interaural line and from 1 to 2 mm dorsal to the rhinal fissure (Fig. 6). This region is located in the most anterior end of the projection area of the medial division of the medial geniculate and outside of the ventral division projection area 2°'23. This corresponds to area 40 and

borders on the posterior side of area 2A ~s'~'~. There is not an abrupt shift in amplitude between the low amplitude (20% of maximum) region adjacent to the maximal response area and this more anterior P14 region (20-30% of maximum P17 amplitude). This is consistent with two components (P14 and P17) having different but overlapping surface distributions. The small positivity (10-20% of maximum) recorded in the ventral section of the posterior region of the PI7 distribution had a longer latency (P19) than the PI7 recorded in the maximal response region. The location of this P19 region (2-3 mm anterior and 2-3 mm dorsal) corresponds to the border zone between Krieg's areas 41 and 36.

To further dissociate these AEP components (P14, PI7, P19), simultaneous recordings were made in the

P~7

H32

. t J \ , . . . . .

1 I 9 6

@ x / ~ 4 ~ ' - / - '~ ' -~

.t ' : k , .... 4-. .... " .

- I

- S

- 4

- 3

- 2

! ; - - ! I I ~ i

7 6 $ 4 3 2 !

Fig. 4. Distribution of the AEP over the lateral cortical surface. The lower portion shows a magnification of the lateral cortical surface indicated in the upper left inset, with its corresponding stereotactic coordinates indicated in mm from the interaural line in the poster ior-anter ior direction and from the rhinal fissure in the ventral-dorsal direction. The upper right insert shows the maximum AEP (marked by an asterisk in the lower portion) and indicates the main AEP components , P9, P17, N32. The lower portion shows the AEPs recorded at multiple locations over the lateral cortical surface in rat R3. The recording location for each AEP graphically corresponds to the beginning of each AEP. The time scale for all waveforms is 60 ms which includes a pre-st imulus time of 5 ms and 55 ms post-stimulus. Stimulus onset is indicated by the hash mark and

the length of the hash mark is equal to 40/~V (positivity up).

10 5 0 I I I I I I I I I I I

- - 7

0

* Max. Response g ~ l ~ ' ~ (~ 75-1007o of max. [ ' - 7

Fig. 5. Map of the cortical surface distribution of the P17 for each of the 8 mapped hemispheres. The upper portion shows a magnification of the cortical region indicated in the lower right inset, with the stereotactic coordinates indicated in millimeters from the interaural line in the posterior-anterior direction and from the rhinal fissure in the ventral-dorsal direction. The asterisks indicate the locations of the maximal response for each of the eight hemispheres mapped (6 animals). Each of the overlapping contour lines demarcates the boundary of the region in which P17 amplitudes were 75-100% of

the maximum amplitude for that hemisphere.

10 5 0 I I I I I I I I I I I

- - 7

/

0 75-100% of max. 0 40-100% of max. ','." 20-100% of max. {..~ Anfer ior resp. :']'. Area 41 (Krieg)

Fig. 6. Map of the cortical surface distribution of the P17. The upper portion shows a magnification of the cortical region indicated in the lower right inset, with the stereotactic coordinates indicated in millimeters from the interaural line in the posterior-anterior direction and from the rhinal fissure in the ventral-dorsal direction. The asterisks indicate the locations of the maximal response for each of the 8 hemispheres mapped (6 animals). The thin solid line indicates the boundary of the region in which P17 amplitudes were 75-100% of the maximum amplitude. The thick solid line indicates the boundary of the 40-100% of maximum response region and the dashed line indicates the 20-100% of maximum response region. The dashed-dotted line indicates the anterior region where 4 hemispheres showed relatively preserved responses (20-30% of maxi-

mum). The dotted line indicates the location of Krieg's area 41.

245

anterior response region (P14; area 40), the maximal P17 location (area 41) and in the posterior response region (P19; border of areas 41-36). Examples (from rat R4) of the AEPs recorded simultaneously from these three sites are given in Fig. 7. The P9 was small at the anterior and posterior areas, but had the same latency at all three locations. The latency of the small and variable N32 could not be consistently measured at the anterior and posterior areas.

The posterior responses included small and variable P9 and N32 components. The positivity following P9 was broad and frequently revealed two peaks, an earlier shoulder at 16.8 ms (P17) and a later peak which occurred about 2 ms later in all animals (P19). The latency of this posterior peak, P19 (19.0 ms, S.D. = 1.89), was significantly longer (P < 0.001) than the P17 (16.8 ms, S.D. = 1.59). The AEPs in the anterior region consisted primarily of a positivity peaking at 14.2 ms (P14) which occasionally had a pronounced shoulder on its falling phase at about 17 ms (P17). In the 4 animals which had sufficiently large responses for measurement in this anterior region, the P14 (mean = 14.2 ms, S.D. = 1.32) was significantly shorter in latency (P < 0.05) than the P17 (mean = 16.8, S.D. = 1.59).

Evidence for auditory cortex origins Steep potential gradient. The steep potential gradient over the auditory cortex surface, and the lack of any significant volume conduction of the P9, P17 and N32 to the dorsal surface, support the hypothesis that auditory cortex is the source of these components. To better establish the intracortical origin of these components the effects of topically applied GABA on the AEP were determined. Elimination by GABA. Epicortical application of GABA (100/~g in 1 ~1 of saline) reduced the P17 and N32 by 79.3% and 81.5%, respectively, in 30 s, while the saline control caused a reduction of only 9.6% and 15.2%, respectively (see Table I and Figs. 8 and 9). Simultane- ous recordings 1.5 mm away showed little or no reduction in any of the components during these applications of GABA.

In some cases, the P9 was reduced, while the components following it were eliminated, as in the example in Fig. 8. The GABA effects always followed a particu- lar pattern. The N32 and P17 were reduced or eliminated and the P9 was reduced only if the N32 and P17 were eliminated.

Partial reduction of the P17 revealed an unchanged rising phase in the AEP up to 14 ms. Similar effects can be seen in the recovery phase from elimination of

Z 0 F -

J Q.. (3L

I'-'-

0 Q.

{/3 r-', Z 0 (_) h i I l l

the P17, i l lustrated in Fig. 8. The recovery phase seen

in these cases often had a shoulder a round 14 ms.

Properties of the auditory cortex response Ear of stimulation. The ear of s t imulat ion (ipsilateral

vs. contrala teral to the recording hemisphere) had large

affects on the P17 and N32 recorded in the maximal

response region of auditory cortex (see Table II and

Fig. 10). St imulat ion of the ear ipsilateral to the

recording hemisphere produced P17 and N32 ampli-

tudes of only 35.3% and 41.3%, respectively, of those

produced by contrala teral s t imulat ion. The latencies of

the P17 and N32 were significantly longer ( P < 0.01) to

ipsilateral as compared to contra la tera l s t imulat ion (see

Table II).

The response to ipsilateral ear s t imulat ion consisted

of a small positivity which peaked at 19.2 ms (P19), but

in some cases there was a b roaden ing or a positive

shoulder at about 17 ms (P17). This waveform was

larger in ampli tude, but similar in morphology and

latency to the P19 recorded in the small response

region of posterior cortex which receives the heaviest

callosal input from area 41 of the opposite hemisphere 7.

The P9 componen t latency was the same for ipsilateral,

contrala teral or b inaura l s t imulation.

P17 and N32 latencies were comparable to b inaura l

and contralateral s t imulation, and al though the ampli-

tudes tended to be larger to b inaura l s t imulat ion these

effects were not significant. P17 and N32 ampli tudes

- - -M "p"

P19

Post - - ~ - ~ . ~ . . , - ~ P14

Max ~ " ~ ~ '~f

i t i -- I - - F S 10 30 5 0 m s e c

Fig. 7. Example of AEPs (l animal) recorded simultaneously from three different regions, to illustrate the subcomponents P14, PI7 and P19. The 3 recording locations are shown in the lateral brain view on the left. The 40-100% of maximum P17 region is indicated, to serve as a landmark, by the encircled area on the lateral brain surface to the left. AEPs were recorded from the posterior region labeled "P' (Post), from the extreme anterior response region labeled 'A' (Ant) and from the maximal response location labelled 'M' (Max). Note: calibration bar = 50 ~V for the Max AEP: 25 #V for the Post and

Ant AEPs.

were larger ( P < 0.01) and latencies shorter ( P < 0.05)

to b inaura l or contrala teral stimuli relative to ipsilat-

eral stimuli (see Table II). P17 ampl i tude to b inaura l

s t imulat ion was smaller (21.9% less; P < 0.05) than the

GABA SALINE

P r e

3 0 75 1 2 0 ~ "

1 8 0

t -

I I I 1 t

5 10 3 0 5 0 m s e c $ 10

246

! !

50 50

Fig. 8. Reversible suppression of the AEP with epicortical application of GABA (left) vs. saline control (right). The top trace, on the left and right, is the pre-treatment recorded AEP (Pre). The successively lower traces show the AEP at 30, 75, 120 and 180 s following the epicortical application of either GABA or saline at the recording site. AEPs were recorded at the maximal response site (example from one animal).

Calibration bar = 50/.LV.

TABLE I

Effects of epicortical GABA application on the P17 and N32 components recorded directly from auditory cortex

Mean ampli tudes (n = 10) of the P17 and N32 following epicortical application of saline or G A B A (followed by saline wash) are given as percentages of their pre-application ampli tudes with S.E.M., at post-application times of 30, 75, 120 and 180 s.

Time P17 N32

post-application Saline GABA Saline GABA (s)

30 90.4 20.7 ** 84.8 18.5 ** (3.6) (4.1) (4.6) (3.3)

75 104.7 31.4 ** 108.3 33.5 ** (5.5) (5.9) (7.1) (5.5)

120 104.5 49.7 106.5 52.7 (4.8) (6.2) (8.1) (8.5)

180 105.2 78.3 105.9 80.8 (3.2) (6.0) (6.1) (6.3)

** P < 0.01.

sum of the P17 amplitudes to ipsilateral and contralateral stimulation. The same comparison for the N32 component showed that it tended to be slightly smaller to binaural stimulation (7.5%), but the effect did not reach significance. S t imula t ion rate. Increased stimulation rates produced increased latencies and decreased amplitudes for the P17 and N32 components recorded at the maximal response region of auditory cortex and revealed an intermediary P14 component (see Fig. 11 and Table III). In some animals a P14 component could be seen

"ID 100

O.

E 75 .o.. \ o,o\., -- /o

, r P17 (Sal ine) • ( ) "6 2s "1 N32 Saline [] P17 (GABA) • N32 (GABA)

0 ( p R ~" ) . . . . . .

0 30 60 90 120 150 180

T ime P o s t - D r u g (sec.) Fig. 9. Reversible effects of epicortical application of G A B A vs. saline control on the P17 and N32 recorded from the maximal response region of auditory cortex. Post-application mean amplitudes (n = 10) of the P17 (open symbols) and N32 (filled symbols) are plotted as percentages of their respective pre-application ampli tudes (Pre), for 30, 75, 120 and 180 s following epicortical application at the recording site of either saline (circles) or G A B A (squares). S.E.M.s are indicated by error bars and asterisks indicate pos t -GABA amplitudes which are significantly smaller (P < 0.01) than post-saline

amplitudes.

247

TABLE II

Ear of stimulation effects on the AEP components P17, N32 and P9 recorded from the maximal response region of auditory cortex

Mean latencies (ms) and amplitudes (/zV) and S.E.M. are given for AEPs recorded from the maximal response region of auditory cortex to clicks delivered to the ear ipsilateral to the recording site, contralateral to it and binaurally. The sums of the amplitudes to ipsilateral and contralateral stimulation are given (I and C) for comparison with binaural stimulation.

lpsilateral Contralateral Binaural I and C

Lat. Amp. Lat. Amp. Lat. Amp. Amp.

P17 19.2 45.9 16.6 131.9 16.6 134.4 177.8 (1.1) (7.5) (0.8) (11.8) (0.7) (6.0) (15.7)

N32 38.5 27.0 32.1 63.8 32.5 69.5 90.8 (2.7) (5.3) (2.5) (10.2) (2.6) (8.0) (15.4)

P9 8.4 - 8.4 - 8.6 - - (0.2) - (0.2) - (0.2) - -

as a shoulder on the rising phase of the P17 even at low stimulation rates. However, in all animals tested, increasing the stimulation rate to 10/s increased the latency of the P17 sufficiently to reveal a small P14 component as illustrated in Fig. 11.

Increasing the stimulation rate from 1/s to 5 /s and to 10/s, did not change the latency of the P9 (Table III). Increasing the stimulation rate from 1/s to 5 /s significantly increased the latencies of the P17 and N32 (P < 0.05 for both). Stimulation at 10/s significantly increased the latencies of the P17 and N32 when compared to 1/s stimulation (P < 0.05 for both), and when compared to 5/s stimulation for the P17 (P < 0.05), but the N32 latency increase was not significant..

Stimulation rates of 5 /s decreased the amplitudes of the P17 and N32 amplitudes to 31.4% and 36.8%, respectively, of their amplitudes at 1/s stimulation. Amplitudes were further reduced at 10/s stimulation

P17

CONTRA -

/

/

IPSI ~ +

I' I I I I S 10 30 50 rnsec

Fig. 10. Effects of ear of stimulation on epicortical AEPs (example from 1 animal). AEPs were recorded from the maximal response region of auditory cortex, to clicks delivered at 0.5/s to both ears (BIN), to the ear contralateral to the recording site (CONTRA) and to the ear ipsilateral to the recording site (IPSI). Calibration bar =

75/zV.

248

P17

1/sec P 9

P14 5/see

1 O/see +

t I I I S 10 3 0 5 0 m s o c

Fig. 11. Effects of stimulation rate on the epicortical AEP (example from 1 animal). AEPs were recorded from the maximal response region of auditory cortex to binaural clicks delivered at l / s , 5 / s and 10/s. Note: calibration bar is 60 p,V for I / s ; 30 ~V for 5 / s ; 15 ~zV

for 10/s.

to 16.1% and 15.2%, respectively, of their amplitudes at 1 / s stimulation, and to 50.9% and 47.5%, respectively, of their amplitudes at the 5 / s stimulation rate. All of these decreases in amplitude were significant (all P < 0.01; see Table III).

DISCUSSION

Auditory cortex response The major components of the initial auditory cortex

response recorded in this study consisted of a double

TABLE III

Stimulation rate effects on the AEP components, P17 and N32, recorded from the maximal response region of auditory cortex

Mean latency (ms) and amplitude (p.V) values are given for binaural click stimulation rates of l / s , 5 / s and 10/s. Ampli tudes are also expressed as the percentage of the amplitude for that component at the 1 / s (Amp. % of l / s ) or 5 / s (Amp. % 5 / s ) stimulation rates.

Stimu- PI 7 lation Lat. rate

N32

Amp. Amp. Amp. Lat. Amp. Amp. Amp. % o f %0]" % o f % o f 1/s 5/s l /s 5/s

1 / s 16.4 117.0 - 25.0 96.6 (0.7) (15.1) (3.4) (10.6) - -

5 / s 18.1 38.0 31.4 - 29.6 32.5 36.8 - (0.8) (10.3) (5.6) (3.8) (6.4) (10.7) -

10 / s 21.1 20.0 16.1 50.9 35.3 14.4 15.2 47.5 (1.2) (6.2) (3.1) (4.9) (4.5) (2.7) (2.7) (8.4)

peaked P1 (P9 and P17) and an N1 (N32) similar to that found in the rat by Borbely and Hall 4 and in other species IA6'22'32. Later evoked potential components

were eliminated by the deep level of barbituate anesthesia employed, consistent with previous studies in a variety of animals including rat 4, cat 32 and monkey ).

The mapping, stimulus-response properties and neurochemical manipulations of this investigation revealed that the rat P1 component is more complex than previ- ously noted.

Multiple P1 subcomponents Cortical surface mapping of the major components,

P1 (P17) and N1 (N32), was consistent with previous maps 2'28, being largely restricted to the surface of anatomically defined area 41 Is J°, or primary auditory

cortex (PAC) defined as the ventral MG (vMG) projection area of auditory cortex 23. However, more detailed

examination of the P1 component revealed multiple subcomponents with differing distributions. In a preliminary report of recordings from three hemispheres, LeMessuier and Woolsey2~ found a similar distribution for the P1, but observed that in one hemisphere there was a second P1 response region located more anteri- orly. In the present study about half the animals had a positivity in the region corresponding to the border of areas 40 and 2a, and to a secondary cortical projection region of MG reported by LeDoux et al 2°. This positivity (P14) occurred significantly earlier than the major positivity over area 41 (P17), and appears to be a functionally distinct component.

Detailed mapping of the P1 over the cortical surface indicated that the major positivity recorded from area 41 (P17) appears to be a positive envelope containing 4 subcomponents. These consist of a small P9, a small P14, a large P17 and a small P19. The large P17 tended to obscure the smaller P14 and P19 subcomponents when recorded from area 41. However, the P14 and P19 could often be detected in recordings over area 41 as shoulders on the rising and falling phases of the P17.

Epicortical distributions further dissociated these P1 subcomponents and linked them with discrete thalamic and callosal projection fields in auditory cortex. Recordings from area 41, which receives thalamic afferents from both ventral (v) and medial (m) divisions of the MG, and callosal inputs from area 41, showed all 4 subcomponents P9, P14, P19 and a large P17. In contrast, the P17 was reduced while the P14 was relatively spared in the border region of areas 40 and 2a, which receives only mMG afferents. While the P17 was greatly diminished, the P19 was relatively spared in the border region of areas 41 and 36 which receives the heaviest callosal inputs from the opposite area 417, as

well as dorsal MG afferents, but not vMG or mMG afferents 23. Stimulus-response properties and ear of

delivery effects further dissociated these subcomponents. Increased stimulation rates differentially reduced the P17 recorded from area 41, revealing a P14 component. Ipsilateral ear stimulation also decreased the P17, more clearly revealing the P19 in area 41.

These findings suggest that the P1 component recorded from PAC (area 41) consists of 4 subcomponents (P9, P14, P17 and P19) associated with the following anatomical substrates. The early latency of the P9 (7 ms in the unanesthetized rat) and its stability over increasing stimulation rates, argues that it repre- sents initial pyramidal cell responses to vMG afferents. Recordings with a higher bandpass and high stimulation rate (10/s) revealed a small negativity preceding the P9 (see Fig. 3) which may reflect the incoming thalamocortical volley and initial stellate cell activation 31'33. The P14 may be generated by cortical re-

sponses to mMG input and /or intracortical synaptic activation in PAC. The P17 is probably generated by vMG thalamocortical and intracortical poly-synaptic activity in PAC which is specific to koniocortex. Corti- cal responses to callosal inputs probably contribute to the P19, however, it may also reflect dorsal MG activation of belt cortices (areas 36 and 20).

The P14 and P19 components recorded from anatomically distinct auditory cortical areas adjacent to PAC (area 41) appear to index activity in secondary auditory fields. This is consistent with a single unit study of the characteristic frequencies (CFs) of neurons in rat auditory cortex 24. A tonotopically organized (rostral-caudal) primary auditory area was found corresponding with anatomically defined PAC and with the distribution of the P17 in the current study. They also report recordings in adjacent cortical areas in which the CFs were clearly discontinuous with the neighboring isofrequency contours in PAC, but did not have a clear tonotopic organization.

Auditory cortex response properties Increasing rates of stimulation (from 1/s to 10/s)

resulted in decreased amplitude and increased latencies of the P17 and N32, while the latency of the P9 remained stable. The P17 and N32 amplitudes to ipsilateral stimulation are about 35-42% of those to contralateral stimulation. Binaural stimulation results in about a 25% reduction in P17 and N32 amplitudes relative to the sum of ipsilateral and contralateral stimulation. This is consistent with single unit evidence for a greater proportion of binaural suppression units, particularly at sound levels well above threshold 24.

249

Confirmation of auditory cortex origin of P1 and N1 The present study has shown that the surface distri-

bution of the P9, P17, N32 and the other P1 subcomponents are highly localized to a small region of the lateral cortical and skull surfaces. The steep potential gradients over the cortical surface are indicative of a local source for these components. Reversible focal elimination of the P17 (and P14, P19) and N32 at the site of topical GABA application supports local generation within auditory cortex. These components were eliminated only at the site of GABA application and not at neighboring recording sites, ruling out a global effect of GABA and further supporting a local source. The reduction in P9 amplitude by GABA, together with the circumscribed P9 surface distribution, supports the hypothesis that P9 is also generated within auditory cortex. The incomplete elimination of the P9 by epicortical GABA application may be due to insuffi- cient diffusion of GABA into the cortex.

Contribution of auditory cortex to dorsal skull-recorded AEPs

Recording the AEP in the barbiturate anesthetized animal provided an opportunity to define the surface distribution of the auditory cortex-generated P1 in relative isolation from contributions by other brain areas. The P1 distribution consisted of a small area of maximal potential over auditory cortex with a steep gradient over the lateral cortical surface; a very small potential at the dorso-lateral skull juncture and an occasional, barely discernable potential over the dorsal skull midline opposite in polarity to the response over the lateral surface. This fits well with the surface distribution that would be predicted, on the basis of anatomical geometry and biophysical considerations, for a dipolar source oriented orthogonal to the auditory cortical surface 28. Substantiation of this prediction makes it reasonable to propose that the same surface distribution of auditory cortex-generated AEPs can be expected in the unanesthetized rat. This would indicate that the large amplitude AEPs commonly recorded on the dorsal skull in unanesthetized rats, particularly at the vertex, are not generated by primary auditory cortex. This suggests that effects of behavioral and phar- macological manipulations on the rat dorsal AEP, may not indicate changes in primary auditory cortex activity.

The above findings raise three issues. The results from this study strongly suggest that there is no significant direct contribution from auditory sensory cortex to the AEPs recorded from the dorsal skull in the unanesthetized rat. The brain generators of the dorsal AEPs remain to be determined. Likely candidates, based upon anatomical geometry and location, are hippocam-

250

pal and cingulate cortices, the colliculi and possibly thalamus. Second, the absence of dorsal skull AEPs in the deeply barbiturate anesthetized rat raises the ques- tion as to whether the generators of these AEPs are entirely dependent upon intact extralemniscal function (which is disrupted with barbituate anesthesia). It is also possible that the absence of dorsal skull AEPs is due to a dependence upon normal auditory cortex function, since initial auditory cortical activity may be aberrant under barbiturate anesthesia (the P1-N1 waveform is delayed and prolonged4). These issues are addressed further in the next study 3°. Third, the present findings, in conjunction with related AEP recordings in the unanesthetized rat t4, indicate that there are multiple brain systems underlying the generation of AEPs in the rat.

Acknowledgments. Supported by the Veterans Administration Re- search Service and NINDS Grant NS21135. This work was submitted in partial fulfillment of a Ph.D. in Neuropsychology at the University of California, Berkeley. Special thanks are given to Drs. Curtis Hardyck, A. Leiman, D, Scabini and S. Brailowsky for helpful advice on this project, to R. Galambos and P. Stanton for valuable com- ments, and to J. Foxe and B. Curran for technical assistance.

R E F E R E N C E S

1 Arezzo, J.C., Pickoff, A. and Vaughan Jr., H.G., The sources and intracranial distribution of auditory evoked potentials in the alert monkey, Brain Research, 90 (1975) 57-73.

2 Barth, D.S. and Di, S., Three dimensional analysis of auditory evoked potentials in rat neocortex, J. Neurophysiol., 64 (1990) 1527-1536.

3 Borbely, A.A., Changes in click-evoked responses as a function of depth in auditory cortex of the rat, Brain Res., 21 (1970) 217-247.

4 Borbely, A.A. and Hall, R.D., Effects of pentobarbitone and chlorpromazine on acoustically evoked potentials in the rat, Neu- ropharmacology, 9 (1970) 575-586.

5 Buchwald, J., Hirman, C., Norman, R., Huang, C. and Brown, K., Middle and long latency auditory evoked responses recorded from the vertex of normal and chronically lesioned cats, Brain Res., 205 (1981) 91-109.

6 Celesia, G.G. and Puletti, F., Auditory cortical areas of man, Neurology, 19 (1969) 211-220.

7 Cipolloni, P.B. and Peters, A., The bilaminar and banded distribution of the callosal terminals in posterior neocortex of the rat, Brain Res., 176 (1979) 33-47.

8 Giard, M.-H., Perrin, F., Pernier, J. and Bouchet, P., Brain generators implicated in the processing of auditory stimulus de- viance: a topographic event-related potential study, Psychophysi- ology, 27 (1990) 627-640.

9 Hari, R., The neuromagnetic method in the study of the human auditory cortex. In F. Grandori, M. Hoke and G.-L. Romani (Eds.), Adeances in Audiology, Vol. 6, Raven Press, New York, 1989.

10 Harrison, J.B., Woolf, N.J. and Buchwald, J.S., Cholinergic neurons of the feline pontomesencephalon. I. Essential role in 'Wave A' generation, Brain Res., 520 (1990) 43-54.

11 Hinman, C.L and Buchwald, J.S., Depth evoked potential and single unit correlates of vertex midlatency auditory evoked responses, Brain Res., 264 (1983) 57-67.

12 Kaga, K., Hink, R.F., Shinoda, Y. and Suzuki, J., Evidence for a primary cortical origin of a middle latency auditory evoked potential in cats, Electroenceph. Clin. Neurophysiol., 50 (1980) 254-266.

13 Kelly, J.B. and Sally, S.L., Organization of auditory cortex in the

albino rat: Binaural response properties, Z Neurophysiol., 59 (1988) 1756-1769.

14 Knight, T., Brailowsky, S., Scabini, D. and Simpson, G.V., Sur- face auditory evoked potentials in the unrestrained rat: component definition, Electroenceph. Clin. Neurophysiol., 61 (1985) 430- 439.

15 Knight, R.T., Donatella, S., Woods, D.L., Clayworth, C. The effects of lesions of superior temporal gyrus and inferior parietal lobe on temporal and vertex components of the human AEP. Electroenceph. Clin. Neurophysiol., 70 (1988) 499-509.

16 Kraus, N., Smith, D.I. and Grossman, J., Cortical mapping of the auditory middle latency response in the unanesthetized guinea pig, Electroenceph. Clin. Neurophysiol., 62 (1985) 219-226.

17 Kraus, N., Smith, D.I. and McGee, T., Midline and temporal lobe MLRs in the guinea pig originate from different generator systems: a conceptual framework for new and existing data, Elec- troenceph. Clin. Neurophysiol., 70 (1988)540-558.

18 Krieg, W.J.S., Connections of the cerebral cortex. I. The albino rat. A. Topography of the cortical areas, 3". Comp. Neurol., 84 (1946) 221-275.

19 Krieg, W.J.S., Connections of the cerebral cortex. I. The albino rat. B. Structure of the cortical areas, 3. Comp. Neurol., 84 (1946) 227-323.

20 LeDoux, J.E., Sakaguchi, A. and Reis, D.J., Subcortical efferent projections of the medial geniculate nucleus mediate emotional responses conditioned to acoustic stimuli, J. Neurosci., 4 (1984) 683-698.

21 LeMessurier, D. and Woolsey, C., Auditory and visual areas of the cerebral cortex of the rat, Fed. Proc., 7 (1948) 70.

22 Liegeois-Chauvel, C., Musolino, A. and Chauvel, P., Localization of the primary auditory area in man, Brain, 114 (1991) 139-153.

23 Patterson, H., An anterograde degeneration and retrograde ax- onal transport study of the cortical projections of the rat medial geniculate body, Ph.D. Thesis, 1976.

24 Sally, S.L. and Kelly, J.B., Organization of auditory cortex in the albino rat: sound frequency, ./. Neurophysiol., 59 (1988) 1627-1638.

25 Scherg, M. and Von Cramon, D., Two bilateral sources of the late AEP as identified by a spatial-temporal dipole model, Elec- troenceph. Clin. Neurophysiol., 62 (1985)32-44.

26 Scherg, M. and Von Cramon, D., Evoked dipole source potentials of the human auditory cortex, Electroenceph. Clin. NeurophysioL, 65 (1986) 344-360.

27 Simpson, G.V., Knight, R., Brailowsky, S., Scabini, D. and Pros- pero-Garcia, O., Altered peripheral and brainstem auditory function in aged rats, Brain Res., 348 (1985) 28-35.

28 Simpson, G.V. and Knight, R.T., Rat auditory cortex: Primary evoked responses and their contributions to skull recorded evoked potentials, Soc. Neurosci. Abstr., 12 (1986) 1279.

29 Simpson, G.V. and Knight, R.T., Contributions of auditory cortex to auditory evoked potentials in the rat, Soc. Neurosci. Abstr., 13 (1987) 332.

30 Simpson, G.V. and Knight, R.T., Multiple brain systems generating the rat auditory evoked potential. II. Dissociation of the auditory cortex mediated generator system, Brain Res., 602 (1993) 251-263.

31 Steinschneider, M., Tenke, C., Schroeder, C., Javitt, D., Simpson, G.V., Arezzo, J.C. and Vaughan Jr., H.G., Cellular generators of the cortical auditory evoked potential initial component, Elec- troenceph. Clin. Neurophysiol., 84 (1992) 196-200.

32 Teas, D.E. and Kiang, N.Y., Evoked responses from the auditory cortex, Exp. Neurol., 10 (1964) 91-119.

33 Tenke, C.E., Simpson, G.V., Schroeder, C.E., Arezzo, J.C. and Vaughan Jr., H.G., Contributions of multiple neural elements to the click-evoked response of monkey auditory cortex, Soc. Neu- rosci. Abstr., 33 (1987) 331.

34 Vaughan Jr., H.G., and Ritter, W., The sources of auditory evoked responses recorded from the human scalp, Electroenceph. Clin. Neurophysiol., 28 (1970) 360-367.

35 Wood, C. and Wolpaw, J., Scalp distribution of human auditory evoked potentials. II. Evidence for overlapping sources and in- volvement of auditory cortex, Electroenceph. Clin. Neurophysiol., 54 (1982) 25-38.

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