EXPERIMENTAL

NEUROLOGY

117,313-324

(19%)

Long-Latency Auditory-Evoked Potentials: Role of Polysensory Association Cortex in the Cat LINDAW.

DICKERSON*ANDJENNIFERS.BUCHWALD

Department of Physiology, Brain Research Institute, and Mental Retardation Research Center, UCLA SchooE of Medicine, Los Angeles, California

The objective of this study has been to define the role of polysensory association cortex in the generation of “wave NA” and of “wave C,” long-latency auditoryevoked potentials recorded from the vertex of conscious cats as, respectively, a marked negative potential of latency 30-48 msec followed by a broad positive wave of latency 50-75 msec. Wave C may represent the feline analogue of the longer latency human auditory-evoked potential wave P2, insofar as both waveforms are very large amplitude, long duration positivities characterized by long recovery cycles. Based on previous studies of wave C and the generators of other middle-latency evoked potentials, we hypothesized that both wave N, and wave C might reflect, at least in part, the cortical culmination of a nonlemniscal line auditory association system arising in reticulothalamic projections to intralaminar and associated ventral thalamic regions. Relays from these thalamic areas are known to project to polysensory association cortex, including pericruciate gyrus, anterolateral gyrus, and medial suprasylvian gyrus. Therefore we implemented a series of lesion experiments to characterize the role of each of these cortical areas in the production of wave N, and wave C. Our results indicate that all three polysensory association areas contribute significantly to both waves N, and C, although the largest effects followed ablation of the pericruciate area alone. Thus, the generator substrates of waves N, and C appear to involve a long-recovery cycle system which functionally incorporates activation of association cortex. 0 1992 Academic Press, he.

INTRODUCTION Disruptions of complex auditory processing may be evaluated and partially diagnosed in human clinical practice through the use of click-evoked auditory responses extracted from the EEG by means of computer-based signal averaging techniques. These auditory-evoked responses are detectable within the first millisecond following the stimulus and extend through time for more than 500 msec. Such field poten* To whom reprint Georgetown University, D.C. 20007.

requests 3900

may be sent at Dept. Pharmacology, Reservoir Road N. W., Washington,

tials represent the spatially and temporally distinguishable responses of several brain regions and brain systems. Identification of the brain region or system which produces a particular evoked-response component is therefore of substantial clinical importance, but is also of fundamental physiological importance in the understanding of hierarchical interactions between the various brainstem and forebrain pathways conveying auditory information. In this regard, the cat has proved to be an extremely useful experimental model for studies of auditory information processing and sensoryevoked potentials (1, 2). In response to repeated click stimuli, the sequence of auditory-evoked potentials which can be recorded from the vertex of the awake cat begins with the auditory brain stem responses (ABRs). The ABRs originate in the auditory nuclei of the brain stem within the first 10 msec after the stimulus (1). Following these short-latency components are the middle-latency responses, defined in the cat as occurring within lo-100 msec after the acoustic click, and specifically including these positive potentials: wave 7 (lo-12 msec latency), wave A (17-25 msec), and wave C (50-75 msec) (3,7). In a similar latency range, a pronounced negativity, wave NA, occurs within 30-48 msec following the stimulus. In temporal sequence, wave C follows wave NA. Lesion data and recordings of local units and field potentials indicate that wave 7 is generated by primary auditory cortex (3,16). Wave A has been attributed to activation of cholinergic projections from the pedunculopontine tegmental (PPT) nucleus of the ponto-mesencephalic reticular formation to muscarinic receptors within the thalamic intralaminar nuclei, centralis lateralis (CL) and centromedian (CM), ventralis lateralis (VL), and ventralis anterior (VA) (6, 10, 11, 32). Wave C has been shown to be of telencephalic origin in that it is abolished by complete bilateral hemispherectomy (3). Wave C is not, however, affected by ablation of primary auditory cortex bilaterally, or by frontal lobectomy anterior to the pericruciate cortex (3,ll). The foregoing results for wave C appear to be true for wave NA as well, suggesting that wave N, and wave C may be functionally linked. In a continuation of experiments to determine the generator origins of vertex-recorded auditory re-

313

0014-4&366/92 All

$5.00

Copyright 0 1992 by Academic Press, Inc. rights of reproduction in any form reserved.

314

DICKERSON

AND BUCHWALD

sponses, the objective of this study was to investigate the source or sources of waves N, and C. Therefore, several lines of evidence from other laboratories were considered in the formulation of our hypotheses regarding the possible origins of these middle-latency components (4). In either awake or chloralose-anesthetized cats, several areas of polysensory association cortex produce responses to auditory stimuli, as demonstrated by studies of local field potentials (28, 29) and of single-unit responses (14). These areas include portions of the pericruciate gyrus (PCA), anterior lateral gyrus (ALA), and the medial suprasylvian gyrus (MSA). Single-unit studies of polysensory association cortex, especially PCA, ALA, and MSA, showed that the auditory response latencies generally fell in the 16-50-msec range, and that onset response patterns were common (13). Thus, the longer unit response latencies were similar to the latency range of wave C. Auditory input to the cortical association areas appears to originate from several thalamic nuclei, particularly the rostra1 intralaminar nuclei and adjacent regions of the ventral lateral nucleus. These projections were demonstrated recently by electrophysiological recordings of locally generated auditory responses in PCA, ALA, and MSA combined with retrograde transport of fluorescent dye markers injected at these cortical recording sites (25). While a number of subcortical areas received labeled dye, very dense marker accumulation was notably present in the CL, CM, VL, and VA. These data confirm and extend the results of previous physiological findings (26). Moreover, single-unit studies suggest strong similarities between the auditory response characteristics found in medial and intralaminar thalamus, particularly CM/Pf and MD, and those of the cortical association areas (12-14, 21). Because of the foregoing data, and because waves NA and C occur immediately after wave A, we hypothesized that they might evolve from projections of intralaminar thalamus and associated regions (the probable source of wave A, see 10,32) to the cortical polysensory association areas PCA, ALA, and MSA. Also, the prolonged duration of wave C suggested the possibility of dual or multiple generators. Thus, the major objective of this study was to investigate possible relations between polysensory association cortex and waves C and NA by a series of lesions directed at specific regions of association cortex. METHODS Surgery and Postoperative

Care

All experimental animals were obtained from the UCLA Vivarium and were housed therein. All procedures were carried out in accordance with NIH stan-

dards and all protocols were approved by the UCLA Animal Use Committee. Prior to anesthesia, each cat was observed to be generally healthy and alert as judged by normal exploratory behavior in the laboratory and orientation to acoustic stimuli. Chlorpromazine was given at l-2 mg/kg 2 h before surgery. Approximately 15-30 min before surgery, atropine sulfate (0.44 mg/kg) was given subcutaneously. General anesthesia was produced with intravenous injection of pentobarbital “titrated” to produce the desired anesthetic level as judged by absence of reflex responses (approximately 30 mg/kg). The respiration level was monitored carefully. Stainless-steel screws were implanted in the skull just off midline at stereotaxic level A7 (15), the “vertex”, to function as electrodes for monitoring evoked potentials. Three linked screws for the reference electrodes were inserted in the bone overlying the frontal sinus at about A23. The leads from the EEG and reference electrodes terminated in a plastic socket (Amphenol Co.) embedded in the methyl methacrylate-based dental cement head cap mounting. Routine application of topical antibiotic was made throughout the postoperative period. A minimum of 1 week postsurgical recuperation period was scheduled after the implant surgery before experimental recording sessions commenced. After control recordings (see below), bilateral ablations of areas of dorsal and frontal association cortex were undertaken, specifically PCA, ALA, and MSA. At the time of surgery, 2.0 mg of dexamethasone SC was administered to minimize brain edema and a similar dose was given for 4 days thereafter, half in the morning, half at night. Postsurgical care included frequent examinations of the animal’s neurologic signs and general physical status. Beginning on the third postoperative day, each cat was given 50 mg chloramphenicol twice per day for 8 days and a topical anesthetic-antibiotic ointment (Lanabiotic, Combe, Inc., White Plains, NY) was routinely applied to incision sites during the recovery period. Once the series of experiments had been completed for a given animal, the animal was deeply anesthetized with pentobarbital (40 mg/kg) and perfused with 10% buffered formalin. The surface area of the brain lesion was examined postmortem, and the brain was photographed. Subsequently, 80-pm frozen brain sections were stained with cresyl violet, and the stained tissue slides were inspected to determine the extent and depth of the ablations. Reconstruction of the lesions onto appropriate brain atlas sections was then carried out to localize the areas from which cortical tissue had been surgically excised. Stimulation and Recording Procedures During recording sessions, the cat was restrained comfortably in a canvas bag within a sound isolation

CORTICAL

LESION

EFFECTS

chamber (Industrial Acoustics) with the head at a fixed location. The cats were quiet but awake as indicated by direct behavioral monitoring and the presence of desynchronized EEG activity throughout all experimental sessions. Acoustic stimuli were delivered “free field” through a Shure 533 SA microphone driven as a speaker which was centered in front of the cat’s nose, 15 cm from each external auditory meatus. Square waves were produced using a Grass S88 stimulator and fed through an attenuator switch with lo-dI3 increments. The square wave pulses were used to drive the speaker to produce O.lmsec rarefaction clicks, the peak-to-peak amplitude of which was adjusted to 70 dB SPL (re 20 u Pa). Monopolar surface recordings of evoked potentials from the vertex location were referenced to the linked set of three widely dispersed screws embedded over the frontal sinuses. This reference does not pick up EMG activity as compared to an extracephalic neck reference, and in general shows no activity when referenced to the neck (11). Vertex macroelectrode activity was amplified 50,000X through a Grass P511A amplifier, with 3-3000Hz filter settings and a 60-Hz notch filter. The signals were processed “on-line” using a DEC PDP 11123 laboratory computer. Prior to lesion procedures, each cat underwent a series of 10 baseline recording sessions over a period of 3 weeks, in order to establish the latency, amplitude, and configuration of the middle-latency evoked responses for each animal. Moderately loud clicks of standard intensity were presented slowly at a steady rate of O.l/sec, 0.2/set, 0.5/set, and also in a series of faster rates, l.O/sec, 2.0/set, 5.0/set, and lO.O/ set for each animal. The evoked potential recordings were done under conditions designed to minimize behavioral habituation by limiting the number of times a set of stimuli were presented, by varying the order of presentation of different stimulus rates within any given session, and by limiting the duration of recording sessions to less than 2 h. Acquisition of post-ablation data began approximately 1 week after lesion surgery, and proceeded for 10 sessions at regularly spaced intervals, ending at 5 weeks after the surgery. Stimulating and recording procedures were identical both pre- and postablation surgery. Statistical

Methods

and Data Analysis

Evoked-potential recordings of the first 100 msec following the stimulus were represented by 666 data points (150-psec sampling interval); successive three-point averages were then made to produce a data set of 222 data points on which the actual statistics were based. Thus, 2.22 data points represented 1 msec of the realtime evoked response. Within each day’s experimental session a total of 20 “blocks” of data were collected. A single data block con-

ON

CAT

315

LLRs

sisted of the summed and averaged evoked responses to 100 presentations of the acoustic click stimulus presented at a steady rate. Different stimulation rates (as noted previously) were tested and there were two different blocks for any given rate during each recording session. The order of presentation of each rate was varied from one session to another. Thus, for a given stimulus rate, at 100 trials/block, 2 blocks per session, and 10 recording sessions prior to the lesion surgery, the preoperative “grand average” for each cat consisted of calculations made on a point-by-point basis, of the waveforms produced in response to 2000 single presentations of the stimulus at each rate. The postoperative grand averages were similarly compiled. For the parametric analysis of the response behaviors of waves NA and C to changes in stimulus rate, a computer program was devised to select the maximum deflection point for each wave within specified poststimulus latency ranges. Those latency windows were set at 30-48 msec for the large negative wave NA and at 50-70 msec for the positive-going wave C. The rate response behaviors of waves NA and C were fitted to a quadratic equation of the general form: amplitude

= intercept

+ b, *rate + b,* (rate)2

where b, and b, are constants used to fit the curve to the data. Linear regressions with terms for both rate and (rate)” were fit to describe the changes in the amplitude of a particular evoked potential. Goodness-of-fit F statistics for the overall model were used to determine whether these regressions fit the data when tested for significance at the P d 0.05 level. Tests of whether the coefficients were equal to zero for linear and quadratic terms (i.e., rate and rate2, respectively) at the 0.05 significance level were used to answer questions regarding: (1) presence of an effect of different rates of stimulation, and (2) rate-response patterns. P values of less than 0.05 were used to accept the alternative hypotheses that linear and/or quadratic effects were present (i.e., nonzero). The effects of the cortical ablation procedures on the amplitudes of wave N, and wave C (at a given stimulus rate) were analyzed by comparing the prelesion grand average, on a point-by-point basis using paired t tests (8). Thus, the averaged evoked response to all presentations at O.l/sec of the click stimulus prior to the ablation surgery were compared to the averaged responses obtained postsurgically for that same stimulation rate. This same type of pre- and postablation comparison was performed separately for each stimulus rate. The amplitudes of wave N, and wave C were tested (within the appropriate poststimulus latency ranges) for the presence of a significant effect of the ablation surgery. The significance level “Y” is a function of the pointby-point paired t test P values, such that

316

DICKERSON

lsoj

160

120 80

AND

BUCHWALD

for each of 16 cats. Acoustic clicks were presented at different stimulation rates: O.l/sec, 0.2/set, 0.5/set, 1.01 set, B.Olsec, 5.0/set, and lO.O/sec. The general response pattern for all of the cats in this study was that the amplitudes of waves NA and C decrease in response to increasingly fast rates of stimulation. A typical example of this effect is shown in Fig. 1A. Table 1 summarizes these results, giving the P values for tests of coefficients equal to zero for linear and quadratic terms (i.e., rate and rate2, respectively). Goodness-of-fit statistics showed that these regressions fit the data well when tested for significance at the P = 0.05 level, for nearly all of the cats. Those animals whose data failed to fit the quadratic model were those subjects for which the data set was incomplete for stimulus rates of 2.0/set or faster. Typical patterns of the sequential decrease in wave C amplitudes in response to increas-

A

1 B

TABLE

0

10

20

30

40

50

60

70

80

Summary of Probability Values for Quadratic Model: P Values for Tests of Coefficients Equal to Zero for Linear and Quadratic Terms*

100

90

TIME LEGEND:

RATE

---

0.1

2

___-------

Wave

0.2 5

-------

0.5 10

--

Wave

N,,

C

,

FIG. 1. (A) Prelesion. Plots of the grand averages of evoked potential responses to different rates of stimulus presentation, ranging from 0.1-10 clicks/set. Time base in msec, amplitude in pV. Preoperative data shown for cat GA. (B) Postlesion. Change in rate response behavior of evoked potential grand averages following PCA cortex ablation. Data shown for cat GA.

Cat Group

- P).

The value Y < 0.022 was taken as the limit for statistical significance equivalent to a P value of 0.05. To avoid artifacts of multiple t test comparisons, our interpretation of “significant effect” required that five or more successive points (each group of five representing 2.25 msec) show significant differences (at P < 0.05). With respect to evoked potential methods, this is a very conservative limit in terms of the probability with which successive data points would randomly reach significance (8). Some of these data have appeared previously in abstract form (5).

RN DP” HR

Rate

Rate2

0.008 0.007 0.02 0.0001

0.02 0.02 0.04 0.003

0.0009 0.07 0.01 0.04

0.006 0.29 0.04 0.19

0.01 0.004 0.03 0.005 0.02

0.03 0.01 0.05 0.02 0.05

0.005 0.02 0.28 0.002 0.0005

0.03 0.04 0.60 0.01 0.0016

0.007 0.06 0.01 0.02

0.02 0.15 0.04 0.05

0.0005 0.02 0.24 0.001

0.0012 0.06 0.49 0.003

0.05 0.03 0.0007

0.10 0.07 0.004

0.0006 0.05 0.0002

0.003 0.08 0.003

III

NK JN” NE” FI Group

Rate2

II

XA AE AP” SC KK Group

Rate I

ME ZE NB” GA Group

Y = -log(l

1

IV

RESULTS

Normative

Data: Parametric

Study

The initial series of experiments quantified the normal amplitudes and latencies of wave NA and of wave C

0 Data includes rates * Data includes rates *P values are from alternative hypothesis, in favor of the alternative.)

through 5.0/set through 2.0/set tests with null not = 0. (Low

but not faster. but not faster. hypothesis: coefficients = 0; P values reject null hypothesis

CORTICAL

LESION

EFFECTS

ON

CAT

317

LLRs

140-

SOO-

-++ t _ t

+ -+b

150-

5s-

FI

+

ME

t

t t

t

0 ,, 0

,

,

,

,

,

,

,

,

S

12

50 , ( ( (

, 10

0

12

+ , ( ,

(

S

of wave C in response AI3, and HR.

HR

to different

ingly rapid rates of stimulation are shown for four cats in Fig. 2. This clearly illustrates the basis for the quadratic modeling of the rate-response behavior in that there is a single inflection point where the rate of decrease in wave C amplitude levels off. A similar trend is observed for the data set of the negative potential, wave NA, as illustrated in Fig. 3. The latencies to the peak negative deflection of wave NA occurred within 30-42 msec for all the cats across all stimulation rates. For any particular cat, the latency of wave NA appeared to be very consistent. The poststimulus latencies to the peak of wave C fell in the range 50-70 msec for all rates of stimulation across cats. The ABRs and wave 7 (12-15 msec latency) showed no consistent change at any of these stimulation rates. Wave A (approximately 25-msec latency) clearly decreased in amplitude at rates of 5-lO/sec but these reductions were less dramatic than those exhibited by waves NA and C. At slower rates of 0.2-2/set, wave A appeared to be enhanced (compared to its response to 0.1 Hz) due to the relative decrease in the deep negative trough of wave NA, on the leading slope of which wave A occurs. Effects of Association

Cortex Ablution

t ( 10

AE

FIG. 2. Plots of the peak amplitudes click/set). Data shown for cats FI, ME,

( (

Surgeries

After baseline recording sessions were completed, bilateral cortical ablations were performed. Those ani-

rates

of stimulus

presentation

(ranging

from

0.1 click/set

to 10.0

mals for which postsurgical data was obtained are listed in Table 2, which summarizes the locations of the cortex ablations as confirmed by the results of brain histology. (No postsurgical data was obtained from animals of Group IV, Table 1, due to surgical mortality.) On the whole, the surgeries removed the targeted cortices. Where the area of ablated tissue was smaller than intended, it is likely that fringe areas surrounding the site of a well-defined lesion were also damaged sufficiently to impair normal physiological function. There were a few instances of inadvertent damage consisting of small nicks in adjoining regions, e.g., primary auditory (AI) cortex, (Table 2), or in somatosensory (SI) cortex. Group I: Bilateral

PCA Lesions

The lesion objective for Group 1 cats, PCA cortex was defined to include the anterior sigmoid gyrus in area 6 and the posterior sigmoid gyrus, including a part of area 4 next to the cruciate sulcus. The pericruciate cortex was completely ablated bilaterally in three cats and largely removed in the fourth, except for the anterior sigmoid gyrus on one side (Table 2). The depth of aspiration included all cortical tissue and extended into the underlying white matter in the targeted area. In cat ZE an additional area, region SI of somatosensory cortex, was damaged on both sides. Typical histological results are illustrated in Fig. 4.

318

DICKERSON

AND BUCHWALD

O-

-2O+

+

+

+

+

-

FI

-2oo-

+

+

+

ME

-llO-

++ t

a400 s

- t+ I,

0

,

,

,

,

,

,

,

,

5

12

(

-loo+~,

,

0

10

,

,

1 2

,

,

,

,

,

6

(

10

k d

o-

go-

+

+

t

+

+

t -2m-

AE

+ ++

1

t -400

+

1 t zt ,

0

HR

-160-

,

,

,

,

1 2

,

6

,

,

,

,

, 10

-260+,,

,

0 1

2

,

,

,

5

,

,

,

,

,

10

Rate/Second FIG. 3. Plots of the maximum amplitudes of the negative potential wave N* in response to different rates of stimulus presentation (ranging from 0.1 click/set to 10.0 clickisec). Data shown for cats FI, ME, AE, and HR.

Subsequent to bilateral PCA lesions in Group 1 cats, the middle-latency evoked-potential responses showed statistically significant amplitude changes. Typical changes in evoked-potential responses to stimulation at 0.1 click/set before and after lesion surgery and the plots of the significance levels of these changes are shown in Fig. 4. The significance level Y (as shown in Fig. 4) is a function of the point-by-point paired t test P values, such that Y = -log(l

- P),

where the value Y < 0.022 was taken as the limit for statistical significance equivalent to a P value of 0.05 (8). To summarize these statistical data, in all Group I cats both wave N, and wave C amplitudes are significantly and substantially reduced following bilateral lesion of PCA (Table 3). Considering the postlesion responses for wave N, and wave C in more detail, the response amplitudes in all four Group 1 cats are greatly reduced at all stimulation rates. The general pattern of decreased response amplitudes with increasingly rapid stimulation is preserved (Fig. lB), which suggests the possibility of contributions from areas outside the lesion sites to the activity remaining in these markedly attenuated waveforms. Cortex lesions which attenuated wave NA often rendered the 20-25 msec wave A clearer and larger in am-

plitude (Fig. 4) even though the filter settings of this study were not designed for optimal recordings of wave A. The most conservative interpretation of the apparent increases in amplitude within the latency range of wave A is that these changes are due to removal of part of the competing signal of wave N,, which overlaps the latency range of wave A. In summary, bilateral lesions of PCA cortex in all four Group I cats produced substantial and statistically significant reductions in the amplitudes of wave NA and wave C. Wave A in contrast either showed no apparent change, or, occasionally, was somewhat more clearly defined postoperatively. Grand averages of the pre- and postlesion recordings for all cats in this group are shown in Fig. 5. Group II: Bilateral

Lesions of PCA and ALA and MSA

The objective for Group II cats was to remove bilaterally all three areas of polysensory association cortex from which auditory evoked responses have been obtained: PCA, ALA, and both the anterior and the posterior regions of the medial suprasylvian gyrus (A-MSA and P-MSA). Typical histological results are illustrated in Fig. 6. The cortex and underlying white matter of both the anterior sigmoid gyrus and the posterior sygmoid gyrus of PCA were successfully excised in this group of animals (Table 2). Most of the anterior regions of MSA and

CORTICAL

LESION

EFFECTS

TABLE

319

ON CAT LLRs

2

Summary of Association Cortex Ablations PCA

ALA

Anterior sigmoid gYms (area 6)”

Posterior sigmoid W-S (area 4)

AI

MSA

Anterior area (areas 1, 53, 18, 19)

Anterior area (area 5)

Midectosylvian gYms (areas 50,22)

Posterior area (area 7)

L

R

L

R

L

R

L

R

L

R

L

R

X X X X

X X 0 X

X X X X

X X X X

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

X X X 12 1 2

X X X ! ;,

X X X X X

X X X X X

s k 1 k X

X X L K X

X X X s s

X X X 2 ;r

X 0 1 0’ 0

L 2, 1 6 0

0 0 0 0 0

0 0 I 6 1 2

NK JN NE

0 0 0

0

0

0

3

31 34 1

X X 3

k

;r

1 a 2 i X

0 0 0

1 tl 1

0

f 1 i 0

0

FI

x X X X

Cat Group I ME ZE NB GA Group II XA AE AP SC KK Group III

0 0 0

0 x 1z

0 + 12

;, X X

ii

Note. L, left, R, right; X, ablation of designated area; 0, no damage; i, ‘; ablated. o See Reinoso-Suarez, 1961.

ALA were removed successfully. However, in only two of the animals (cats XA and AP) was there any significant removal of posterior MSA. The most consistent postablation electrophysiological finding in the five cats of group II was a large and statistically significant reduction in peak amplitude of wave N,, as shown in Fig. 6. These reductions of wave NA extended well into the leading negative edge of wave NA, even into the latency domain of wave A and wave 7 in four cats. In four of the five cats there was also a well-defined and statistically significant reduction in amplitude of wave C postoperatively. There was also a lengthening of wave C peak latency in cats AP and KK (Fig. 6). Only in cat XA was this reduction in wave C amplitude not significant. Variability in peak latencies across recording sessions probably contributed to the lack of significant differences between the pre- and postlesion amplitude data in this animal. In summary, the findings for group II indicate that all five cats exhibited substantial and significant postoperative reduction in wave NA. Four of the five cats also

showed statistically significant reductions in wave C. Grand averages of the pre- and postlesion recordings for all cats in this group are shown in Fig. 7. Group III:

Bilateral

Lesions of ALA and MSA

In the four animals of Group III, the goal was to remove cortical association areas ALA and the anterior and posterior regions of MSA. Most or all of ALA was excised bilaterally in all but one cat (cat JN). Bilaterally-complete ablation of anterior MSA (area 5) was achieved in all four animals. In two cats (NK and JN), most of the anterior region of posterior MSA (area 7) was removed bilaterally. In the other two cats (NE and FI), the posterior MSA was mostly spared due to difficulties in completely visualizing the target area. In summary, the lesions were largely successful in removing areas ALA and A-MSA, but removed only anterior portions of P-MSA in two of the cats, sparing P-MSA in the other two (Table 2). As was true for lesions in the other two groups, the areas of ablation always removed the

320

DICKERSON

AND

BUCHWALD

1 :, P L :

xl-10.” -40

: -70 E -100 -120. -1‘0 .l,O~, 0

A20

A24

A22

A27

7

FIG. 4. Plots of prelesion and postlesion evoked-potential comer, along with the corresponding plot of levels of significance. ing white matter. Data shown for cat ME.

A24

recordings Histology

cortical gray matter and extended well into the underlying white matter. Within the latency range of wave NA, statistically significant reductions in absolute amplitude were seen for all cats (Table 3). Similarly, all cats in this group showed a reduction in wave C. Cats NE and NK showed wave C amplitude reductions, although these reductions were not statistically significant. Finally, cats JN and FI exhibited decreases in the amplitudes of wave C which were both large and statistically significant. In summary, all four cats demonstrated significant reductions in wave N,; and two cats showed significant reductions in wave C. All four cats in group III had abla-

at a stimulus shows cortical

A27

rate of 0.1 click/set are shown in the upper right lesions (areas of stippling) extending into underly-

tion of cortex in areas ALA and anterior MSA. Grand averages of the pre- and postlesion recordings for the cats in this group are shown in Fig. 8. DISCUSSION

Our initial studies characterized the parametric responses of waves NA and C. The middle-latency auditory-evoked potentials NA and C decrease in amplitude at moderately rapid rates (e.g. 0.5/set) of stimulus presentation. This finding contrasts with previous studies of the auditory brainstem responses and of the primary auditory cortex response, wave 7, in that these shorter-

CORTICAL

TABLE Significant Postoperative

Group

II: PCA

+ ALA

XA AE AP SC KK Group

in Peak Amplitudes Wave

C

I: PCA ME ZE NB GA

Group

EFFECTS

3

Reductions

Wave NA

LESION

III: ALA NK JN NE FI

* * * *

* * * *

* * * * *

* * * *

+ MSA

+ MSA * * * *

* *

* P < 0.05.

latency potentials can follow rapid rates of stimulus presentation with little or no decrement in peak amplitudes (1,3). The rate response behaviors of wave NA and wave C can be described by a quadratic equation with terms for rate and rate2, indicating that the decrease in wave amplitude observed with increasing stimulation rates is not strictly linear, but has exponential properties as well. In order to characterize the role of polysensory association cortex in the production of wave NA and wave C, the effects of removing different cortical polysensory association areas were studied. This series of cortical ablation experiments produced large and significant decreases in both wave NA and wave C. All 13 cats in this study exhibited a statistically significant reduction in the negative deflection of wave NA, and 10 of these cats showed statistically significant reductions in wave C. Taken together, our results indicate that the generator systems of waves NA and C depend on contributions from activity in all three areas of polysensory association cortex, but possibly to a somewhat greater degree on PCA, because ablation of PCA alone is sufficient to produce the most consistent and marked reductions. The effects of these cortex ablations cannot be attributed to nonspecific postsurgical impairment of the animals’ health, neurologic or otherwise. In no case was evoked-potential data collected for statistical analysis if the animal exhibited lethargy or other impairment. By the end of the first week following the ablation surgery,

ON

CAT

321

LLRs

the animals appeared essentially normal in general health and behavior when allowed to engage in exploratory activity in the laboratory. There were no apparent signs of neurologic impairment with the exception of a deficient “placing reflex” in animals with lesions of PCA. All of these animals exhibited normal amplitude ABRs and wave 7 following surgery, indicating that the primary auditory pathway was functioning normally throughout its course from brainstem levels up to and including primary auditory cortex. To the extent that the ABRs and wave 7 can also be used to assess neurological health or dysfunction of particular brain regions (e.g., 17, 18,20,31), the intact evoked potentials arising in the primary auditory pathway support the behavioral observations that the animals were in good neurological condition. Not only were the ABRs and wave 7 of normal amplitude, latency and general configuration, but the middle latency component, wave A, was likewise undiminished by the ablation surgeries. A final point regarding the functional viability of other forebrain systems relates to a parallel study of event-related endogenous potentials. This study, which utilized the same animals reported herein, showed no significant decrease in the long-latency cognitive “cat-P3” potential following these association cortex ablations (9). Thus, the normal amplitudes and configurations of these evoked potentials arising in widely separated regions of the brain indicate a generally normally functioning brain. Lastly, the ablation effects on waves NA and C cannot be attributed to changes in volume conduction properties nor to nonspecific mass-action consequences of large cortical lesions. Prior studies indicated that large bilateral lesions of primary auditory cortex or removal of frontal cortex anterior to pericruciate cortex had no effect on wave C or wave NA (3,11). Furthermore, in the present study, the degree of wave NA and wave C peak amplitude reduction did not show any obvious relation 225 i 175 j

FIG. 5. Grand postlesion recording all cases.

averages of Group sbssions. Stimulus

I cats across presentation

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Plots of prelesion FIG. 6. along with the corresponding AP.

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and postlesion evoked-potential plot of levels of significance.

recordings at a stimulus rate of 0.1 click/set shown in the upper right Histology shows cortical lesions extending into white matter. Data shown

to the amount per se of cortical tissue removed, but related rather to the location of the lesions. For example, lesions restricted to the pericruciate area were as effective as more extensive lesions which included other cortical association areas. Finally, waves NA and C were recorded from the same vertex electrode as were the unaffected shorter latency evoked potentials and the likewise undiminished long latency P3 (9).

corner, for cat

The present data showing that vertex recorded auditory potentials NA and C depend at least in part upon polysensory cortex are strengthened by recent studies of locally generated field potentials. For example, at optimal sites in PCA, ALA, and MSA for recording local auditory responses, retrograde tracers were injected (25). The resultant fluorescence histology showed that heavy projections from intralaminar thalamus and ad-

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FIG. 7. post-lesion all cases.

Grand averages of Group recording sessions. Stimulus

II cats across presentation,

the 10 pre0.1 click/set

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jacent regions, previously shown to be associated with auditory responses (ll), terminate in each of the polysensory association cortex areas studied in the present experiment. There is, in addition, a long history of studies in these polysensory cortical association areas of both local evoked potentials and single-unit responses to auditory, somatosensory, visual, and nociceptive stimuli, as has been discussed extensively elsewhere (e.g., 13,14,22,24,27-29). These studies provide strong support for the concept of a central association system projecting to polysensory association cortex, a system which is clearly separate from the primary, “lemniscalline” auditory pathway projecting to the auditory cortex. One question left unresolved by this study is the extent to which other regions besides these polysensoryassociation areas (PCA, ALA, MSA) might contribute to waves N, and C. In most cases the ablations resulted in substantial diminution of the amplitudes of waves NA and C, but these waves generally were not completely eliminated. Perhaps we would have been successful in completely abolishing waves NA and C if we had been able to eliminate all components of association cortex. An alternative interpretation is that there are other telencephalic regions (3) which contribute to waves NA and C. Based on anatomical and physiological evidence, contributing areas might include additional targets of intralaminar thalamus projections such as the basal ganglia, claustrum, insular cortex, or the anterior cingulate cortex (19, 26,30). In summary, we conclude that all three polysensory association areas (PCA, ALA, MSA) contribute substantially to the generator systems of both waves NA and C. However, the most dramatic reductions in amplitude followed bilateral ablation of the pericruciate area alone. The relative importance of each area to waves NA and C apparently reflects the redundancy and overlapping of function inherent in association cortex. Most importantly, a relatively specific relationship be-

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tween this association-cortical system and waves NA and C is indicated by the absence of lesion effects on either the shorter latency potentials (ABRs, wave 7, wave A), or the longer latency cognitive potential (cat P3) (9). When considered in light of previous work, the present study suggests functional linkages between the middle-latency component, cat wave A, and the longer latency components, cat waves N, and C. In this view, the ascending choline@ system, originating in the pedunculopontine nucleus of the mesencephalic reticular formation (lo), receives auditory input from the area of the lateral lemniscus and projects this information forward to muscarinic receptors (6) of thalamic neurons in centromedian, centrolateral, parafasicular, ventral anterior, and ventrolateral nuclei (32), with resultant postsynaptic generation of cat wave A. Ascending projections from these thalamic neurons, in turn, could relay the auditory signals presynaptically to polysensory association cortex, resulting in cat wave NA. Subsequent postsynaptic activation of cell clusters in the various association cortices could then result in the large amplitude, long duration positivity, the cat wave C. While only one of several possible hypotheses, this speculation will require additional data and quantitative analyses of the relevant pre- and postsynaptic loci for validation. Our studies of long-latency auditory-evoked responses in the cat take on additional significance as they suggest parallels with human-evoked potentials. Clearly, the development of animal models has been of great value in other studies of the ABRs, middle-latency responses, and P3 generator systems (1,2). With regard to the present data, the feline wave C shows characteristics that are very akin to those of the human P2. Both wave C and the human P2 are long-latency, long-duration positive potentials easily recorded from the scalp, both are markedly larger than any of the preceding auditory-evoked potential components, and both waves show a prolonged recovery cycle to repetitive click stim225 175 125 75$ P q ,$

25. -25. -75. -125.

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FIG. 8. postlesion all cases.

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uli. Thus, it is possible that the human P2 may involve contributions from association cortex similar to those for wave C in cats.

16.

ACKNOWLEDGMENTS The authors express appreciation to Drs. S. Purdue and D. Guthrie for statistical consultation and to Dr. J. Schwafel for figure preparation. This research was funded by USPHS Grants HD 05958 and NS 25400.

“* 18.

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