Electrophysiologic Signs of Auditory Distraction in ... · J Am Acad Audiol 11: 36-45 (2000) Electrophysiologic Signs of Auditory Distraction in Elderly Listeners Amy L. Fisher* Murvin

J Am Acad Audiol 11 : 36-45 (2000)

Electrophysiologic Signs of Auditory Distraction in Elderly Listeners Amy L. Fisher* Murvin R. Hymel* Jerry L. Cranford* Albert R. DeChicchist

Abstract

Brain mapping was used to investigate the ability of young and elderly female listeners to attend to /ga/ syllabic events at one ear in the presence of speech babble competition at the opposite ear. An oddball stimulus presentation paradigm was used to record the N, and PZ components of the late auditory evoked potential (LAEP) from 19 scalp locations . With speech competition, elderly listeners exhibited significantly larger reductions in PZ amplitude than did young listeners. The competition produced no changes in N, amplitude in either group. These findings contrast with those of an earlier study in which age-related reductions in N, but not PZ amplitude were found when listeners attended to tones rather than speech stimuli in the presence of speech competition . These studies suggest that amplitude reductions in differ-ent LAEP components may provide electrophysiologic indices of age-related breakdowns in processing sounds in the presence of competition . Which LAEP components are affected may depend on experimental variables such as task difficulty or the nature of the stimuli (e .g ., speech vs nonspeech) .

Key Words: Auditory distraction, auditory event-related potentials, auditory evoked poten-tials, brain map, elderly listeners, N,, processing negativity, Pz P3oo, scalp topography, selective attention, speech competition

Abbreviations: ANOVA= analysis of variance, CID = Central Institute for the Deaf, EEG = electroencephalographic activity, GFP = global field power, LAEP = late auditory evoked potential, PTA = average of pure-tone threshold hearing levels at 500, 1000, and 2000 Hz, SL = sensation level, SRT = speech reception threshold, WRS = word recognition score

B

eginning in the early to mid 1970s, there has been a large and concerted research effort aimed at identifying electrophys-

iologic correlates of cognitive processing in the human brain. Much of this work has focused on attention. In 1973, Hillyard et al developed a spe-cial experimental paradigm that allowed the first controlled recording of the electrical corre-lates of selective attention in the human brain (Woods, 1990). This paradigm involved the pre-sentation of discriminably different oddball tonal sequences to the two ears on each test run. Hill-yard et al reported that when normal young lis-

*Department of Communication Sciences and Disorders, East Carolina University, Greenville, North Carolina ; tUniversity of Georgia, Athens, Georgia

Reprint requests : Jerry L . Cranford, Department of Communication Sciences and Disorders, East Carolina University, Greenville, NC 27858

teners selectively attended to tones at one ear, the Ni component of the late auditory evoked potential (LAEP) was significantly larger in amplitude than when they ignored the tones. Earlier research (e.g ., Hernandez-Peon et al, 1956) had suggested that such attention effects might be the result of a peripheral gating mech-anism that augmented the neurophysiologic response (i.e., the exogenous component) to infor-mation in the attended channel while attenu-ating information in the nonattended channel. The Hillyard et al (1973) study, along with sub-sequent studies (Picton and Hillyard, 1974 ; Schwent and Hillyard, 1975; Parasuraman, 1978 ; Hansen and Hillyard, 1980 ; Alho et al, 1987 ; Naatanen and Picton, 1987 ; Giard et al, 1988, 1991 ; Alho, 1992 ; Woods, 1995), has obtained evidence that attention-related increases in Ni amplitude, rather than reflect-ing a peripheral gating process, represent a sep-arate hypothetical endogenous neural process,

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referred to as the "processing negativity" (PN) (Naatdnen et al, 1978), which is invoked by the listener's selective attention to the target sound . The PN, as its name implies, is reflected by a slow negative wave that, depending on the nature of the experimental task, may overlap with NJ to produce an apparent enhancement of this early component of the LAEP

More recently, Martin and Cranford (1989) and Cranford and Martin (1991) reported find-ing the apparent opposite of an attention effect. Rather than measuring electrophysiologic changes that occur when participants selec-tively listen to specific sounds, Cranford and Martin used electrophysiologic responses to determine whether speech competition at one ear might interfere with the ability of young and elderly listeners to selectively attend to tonal stimuli at the opposite ear. Whereas both young and elderly female listeners showed significant decreases in NI-P2 peak-to-peak amplitudes in the presence of speech competition, the magni-tude of this effect was significantly greater for the elderly participants . This finding suggests the possibility of some form of decline in the ability of elderly persons to selectively attend to or process sounds at one ear when potentially dis-tracting or competing sounds are present at the opposite ear. In a recent replication and exten-sion of the Cranford and Martin (1991) study, Hymel et al (1998) found that although the amplitudes of both the NJ and P2 components of the LAEP were significantly reduced in the presence of the competing speech babble, an age-related effect only occurred with NY Thus, it appears that changes in the LAEP and, in par-ticular, the N1 component may reflect brain processes related to contralateral competition and selective attention. When listeners are selec-tively attending to tones, the N1 response increases in amplitude, and, conversely, when competing stimuli are present at the opposite ear, the amplitude of Ni is reduced.

The purpose of the present study was to further investigate the effects of contralateral speech babble competition on the different com-ponents of the LAEP More specifically, we were interested in determining if similar or different age-related amplitude effects would be found if participants discriminated speech sounds rather than tonal stimuli in the presence of competition. The present report describes new findings indi-cating that when listeners discriminate speech sounds in the presence of contralateral speech babble, it is the later P2 component, rather than NJ, that is reduced in amplitude.

Auditory Distraction in Elderly Listeners/Fisher et al

METHOD

Participants

Ten adult females were tested in each of two age groups : a "young" group (mean = 23 years, 3 months; range = 20 years, 10 months to 27 years, 9 months) and an "elderly" group (mean = 68 years, 1 month; range = 60 years, 3 months to 79 years, 1 month) . The young par-ticipants were volunteers from the university stu-dent body, whereas the elderly participants were recruited from church and recreational pro-grams within the community. Women were selected to preclude possible systematic gender differences in topographic location of LAEP wave components (Baumann et al, 1991). All partici-pants were right handed and had a negative history of neurologic disorders, head trauma and/or surgery, otologic disease (including otitis media), vertigo or persistent tinnitus, ototoxic drug use, speech and language disorders, and sig-nificant occupational and recreational noise exposure . All participants from the older group were ambulatory and in good health.

Apparatus and Procedures

Audiometric Testing

A battery of audiometric tests was admin-istered to the participants while they were seated in a sound-treated test booth, the mean results of which are shown in Table 1. For all partici-pants, pure-tone thresholds at 500, 1000, 2000, and 4000 Hz were better than 25 dB HL, whereas interaural differences in thresholds were less than 10 dB HL. Speech recognition thresholds (SRTs) were obtained using Central Institute for the Deaf (CID) W-1 word lists. Word recognition scores were obtained at 40 dB SL re the SRT using CID W-22 word lists and were better than 92 percent for all participants . Tympanometry screening revealed that all par-ticipants had normal middle ear function (ASHA, 1990). Finally, the Cortical Function Assess-ment test (Herst et al, 1990) was used as a screening tool to rule out depression and demen-tia in the elderly participants . The test was based on a standard pass-or-fail criteria, with 100 percent of the participants passing.

Electrophysiologic Testing

An oddball stimulus paradigm (Squires and Hecox, 1983) was used to evoke the LAER The

37

Journal of the American Academy of Audiology/Volume 11, Number 1, January 2000

Table 1 Mean Audiomet

Young

Mean

ric Test Results

Group

SD Mean

Elderly Group

SD

Pure-tone average (dB HL) Left ear 1 .6 1 .9 10 .9 5 .4 Right ear 1 .7 3 .3 12 .2 7 .5

Speech reception threshold (dB HL) Left ear 5 .0 6 .0 17 .5 11 .6 Right ear 5 .0 7 .1 16 .9 9.2

Word recognition score (%) Left ear 95.0 2 .8 98.0 3.0 Right ear 97.0 1 .9 96 .5 4.0

frequent stimulus consisted of/da/ syllabic events with an 80 percent probability of occurrence, while /ga/ syllables served as the rare event (20% probability) . These syllables, along with the calibration tone, were imported into the Neu-roScan sound edit program from the Dichotic Nonsense Syllables subtest of the Speech Recog-nition and Identification Materials Disc 1.1 pro-duced by the Auditory Research Laboratory at the Veterans Administration Medical Center in Long Beach, CA. Each syllable was 240 msec in duration. Three hundred presentations (240 fre-quent and 60 rare) were made during each test run with a 1.3 ± 0.1 sec interstimulus interval. Randomized presentation of the syllables was performed using a PC-based computer stimulus package (NeuroScan STIM). Competition was provided using the Auditec multitalker tape played on a professional tape deck (Marantz PMD-40). Test and competing stimuli were pre-sented at 55 dB SL re the PTA of the ear receiv-ing the respective signal . All stimuli were presented via insert receivers (Etymotic ER-3A) .

Electrophysiologic recordings were con-ducted in a sound-treated and electrically shielded test booth with the participants seated in a reclining chair. Participants were asked to maintain visual fixation on a target placed at eye level on the wall in front of them and to blink normally in order to minimize contamination by ocular movement . Two recording runs were obtained per ear per participant: one run with the syllables presented to one ear with no con-tralateral competition and one run with speech competition presented to the contralateral non-test ear. The presentation order of the initial test ear was counterbalanced across participants . Participants were instructed to ignore the speech competition and keep a mental count of the total number of /ga/ syllables they heard, and report this number to the experimenter at the end of

the test run. If any participant's count deviated by ± 15 or more from the actual number pre-sented (i .e ., 60), they were reinstructed and the test run was repeated a second time . In the absence of competition, no test runs had to be repeated . With competition, four subjects (all elderly) received a second test run. The test runs that had the more accurate count were used for subsequent data analyses . .

Neuroelectric activity was recorded from 19 electrodes (FP,, FP,, F7, F3, F, F4, F3, T3, C3, C, C4, T4, T5, P3, P, P4, T6, Ol, 02) fixed to the scalp with an appropriately sized commercial electrode cap (Electro-Cap International) according to the International 10-20 system and referenced to linked earlobes . Ocular movement was monitored by electrodes placed vertically above and below the left eye. Electrode impedances were maintained below 5000 ohms . Individual sweeps of time-locked electroencephalographic (EEG) activity extended from -100 to +1000 msec relative to stimulus onset. EEG activity was amplified 1000 times, analog filtered (1-70 Hz, 24 dB/octave slope), and digitized at an analog-to-digital rate of 500/sec with a PC-based NeuroScan system and SynAmps 16-bit amplifiers .

The digitized epochs were then sent to a microcomputer for offline averaging, digital fil-tering (1-40 Hz, 24 dB/octave slope), and con-struction of topographic brain maps . The 100-msec prestimulus recording obtained at each electrode site was used to establish a base-line to correct for the DC level of background EEG activity. Ocular movement artifacts were digitally removed from the epochs (Semlitsch et al, 1986). Epochs containing artifacts exceeding ±50 microvolts were rejected from averaging. Epochs were sorted by type and averaged into frequent and rare stimulus waveforms. For the frequent stimuli, the mean number of sweeps used in deriving averaged waveforms was 227

38


(SD = 24.4) for the elderly participants and 225 (SD = 21.0) for the young participants . For the rare stimuli, the waveforms were derived from a mean of 57 sweeps for both the elderly and young participants (with SDs of 7.7 and 4.4, respectively) . Finally, grand-averaged wave-forms were constructed by group/ear/condition for analysis .

To facilitate production of topographic maps, global field power (GFP) waveforms were con-structed for each grand-averaged waveform (Skrandies, 1989, 1990) . Based on these GFP waveforms, latencies of maximal activity across all electrode locations were selected for the Nl,

P2, and P~ � events . These latencies were then used as the temporal center for the respective maps . For N1 and P2, topographic maps were constructed with a 10-msec window centered on the GFP latency. As the P3oo is a broader event, topographic maps were constructed using a wider 50-msec window centered on the GFP latency. A 4-point linear interpolation algorithm was used for constructing all maps . Voltage scales for the different group maps were set to maximize the number of color gradients pre-sent, allowing more detailed visualization of the topographic location of scalp electrical activity.

RESULTS

T able 2 shows the mean numbers (along with standard deviations and ranges) of rare /go/ stimuli that the young and elderly partici-pants reported hearing during each of the four test runs . These data indicate that whereas the interparticipant variability in counting accu-racy by both groups was minimal during test runs without contralateral competition, the elderly participants exhibited increased vari-ability in their counts during runs with speech competition . Fmax homogeneity of variance tests (Steel et al, 1997) found that, in the absence of contralateral competition, the two participant groups exhibited similar variance levels in their counts at both the left (F [9, 91 = 1.62 ; p = .24)

and right ears (F [9, 91= 1.04; p = .48) . However, in the presence of contralateral competition, the elderly group's count of the /ga/ stimuli was sig-nificantly more variable than that of the young group at the left ear (F [9, 91= 5.23; p = .01) but not at the right ear (F [9, 91 = 2 .88; p = .06) .

Electrophysiologic Findings

Figure 1 displays the topographic maps of the Ni and P2 components of the LAEP as a func-tion of group (i .e ., young vs elderly), listening con-dition (quiet vs competition), and test ear (right vs left). An examination of the maps indicate that, for both Ni and P2, the areas of maximum activity appear to be symmetrically distributed around midline and centered between F and C.. For the elderly group, maximum P2 activity appears to be reduced in the presence of con-tralateral competition. With the Ni component, no systematic pattern of competition-related changes in maximum amplitude is evident. Fig-ure 2 shows grand-averaged Nl and P2 wave-forms recorded from Fz and Cz in the young and elderly participants . Inspection of the topo-graphic maps of Psoo (not shown) indicated that the region of maximum activity was centered at midline in the posterior parietal areas for both participant groups . In contrast to the findings of the Hymel et al (1998) study, the P3oo maps in the present experiment revealed no evidence of amplitude or topographic differences either between participant groups or in various test con-ditions (i .e ., left and right ear stimulation, with and without contralateral speech competition) .

To focus on possible topographic differences in the general region around

C' where maxi-

mum activity was typically observed, nine elec-trode locations centered around the vertex were selected for statistical analysis (i .e ., F3, F, F4, C3,

CP C4, P3, P, and P4) . Each participant's Nl and P2 amplitudes from the nine electrode locations were selected from averaged frequent stimulus waveforms. Ni amplitude was defined as the lowest negative voltage value (from baseline)

Table 2 Number of /ga/ Stimuli Participants Reported Hearing during each Test Run

Mean

Young Group

SD Range Mean

Elderly Group

SD Range

LE quiet 58.9 3.3 53-65 60 .7 5.3 49-69

LE competition 59.2 4 .5 53-70 58 .8 23.6 26-115 RE quiet 59.9 4 .5 52-67 57 .6 4 .3 49-62 RE competition 59.4 3 .6 55-65 57 .1 10 .3 37-67

39


Left Ear Young Group

Right Ear

Elderly Group

F

C,

LATENCY IN MILLISECONDS

(ANOVAs) were performed to investigate N1, P2, and P3o~ amplitude differences as a function of group, listening condition, test ear, and elec-trode location . The analyses were performed using SPSS computer software (SPSS, Inc ., Release 6.1.4) . Summaries of the ANOVA test findings for Nl and PZ are presented in Table 3 . As the sphericity assumption was violated for repeated measures, adjusted p values were deter-mined using Geisser-Greenhouse corrections . Relative treatment magnitude sizes (i .e ., the pro-portion of variance "explained" or "accounted"

Figure 2 Grand-averaged N, and Pz waveforms recorded in response to the frequent tones at scalp locations F and C from the young and elderly listeners . Waveforms were recorded in the left and right ears with and with-out speech babble present in the con-tralateral nontest ear.

for by the independent variable) are indexed by omega squared [w2] (Keren and Lewis, 1979 ; Keppel, 1991) . Experiments that produce omega-squared values of .01, .06, and .15 are considered to have small, medium, and large effect sizes, respectively (Cohen, 1977).

Findings with NZ

The only significant finding (p < .01) with the Ni component involved the electrode location variable . This result reflects the finding that, as

Table 3 Summary Table for the Four-Factor ANOVA Investigating N, and P2 Amplitude as a Function of Participant Group, Listening Condition, Test Ear, and Electrode Location

Source of

F

N,

Value

Pz

p

N,

Value

Pz N, Pz

Group 1, 18 4.57 0.28 05 60 01 00 Condition 1, 18 0.17 10.7 68 004* 00 24

Ear 1, 18 3.63 0.21 07 65 11 00 Location 8, 144 14.48 13 .1 < .001* <,001* 38 26 Group x condition 1, 18 0.00 11 .4 99 003* 00 25

Group x ear 1, 18 1 .27 0.49 27 49 01 00 Group x location 8, 144 3.19 15 .1 02 <.001 * 06 30

Condition x ear 1, 18 0.14 0.17 71 68 00 00 Condition x location 8, 144 1 .29 4.27 29 008* 01 14 Ear x location 8, 144 2.79 2.08 07 10 08 05

Group x condition x ear 1, 18 3 .87 0.38 07 54 12 00 Group x condition x location 8, 144 0 .67 1 .54 57 21 00 02 Group x ear x location 8, 144 1 .85 0.58 16 66 04 00

Condition x ear x location 8, 144 1 .04 0 .51 37 66 00 00 Four-way interaction 8, 144 0 .76 1 .50 49 23 00 02

*Considered significant at p < .01 .

41


shown in Figure 1, the magnitude of Nl ampli-tude varied considerably among the nine elec-trode locations examined in the present study.

Findings with P2

As seen in Table 3, statistically significant (p < .01) main effects were found for both lis-tening condition and electrode location . As evi-denced in the topographic maps, these main effects reflect the finding that P2 amplitude was reduced in the presence of competition and that the amplitudes exhibited considerable variabil-ity over the array of nine electrodes . Three sig-nificant (p < .01) two-way interactions were also found. The group by listening condition inter-action indicates that the elderly participants exhibited greater competition-related reduc-tions in P2 amplitude than did the young par-ticipants. This effect is depicted in Figure 3. The group by electrode location interaction reflects the finding, shown in Figure 1, that the older group exhibited maximum P2 amplitudes that were located more anteriorly on the scalp than were those of the young group. The lis-tening condition by electrode location interaction is due to the finding that, for both participant groups, the areas of maximum amplitude activ-ity were situated more posteriorly on the scalp in the competition condition than in the quiet condition.

Finally, Pearson product-moment correlation tests were performed to investigate whether the difficulty that individual elderly listeners had in discriminating /go/ stimuli in the presence of

4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 0.0

0 QUIET 0 COMPETITION

YOUNG ELDERLY

LISTENING CONDITION

Figure 3 Effects of contralateral speech competition on amplitude of P2 component of late auditory evoked poten-tial measured in groups of young and elderly listeners. Error bars represent ±1 standard error.

competition was related to the magnitude of the competition-related decreases in P2 amplitude. Nonsignificant correlations were found at both the left (r = -.49, p = .15) and the right ears (r = .62, p = .06) .

Findings with P3oo

Similar to NJ, the only significant fording with the P300 component of the LAEP was a main effect (F [8, 1441= 21.64, p < .001) involving elec-trode location. As with Ni and P2, this finding is the result of the considerable variability in amplitude found among the array of nine elec-trodes examined in the present study.

DISCUSSION

T he present project was a direct follow-up to the study by Hymel et al (1998) . Except for testing new groups of participants plus using speech as opposed to tones as test stimuli, the present investigation used identical procedures (including data analyses) to those reported in the earlier project. Hymel et al required partici-pants to count the occurrence of 2-kHz tone bursts randomly interspersed among more fre-quently occurring 1-kHz tones. On all test runs, including those in which speech competition was present in the nontest ear, the young and elderly participants exhibited minimal prob-lems counting the rare tones . All participants reported counts that were accurate within {-4 of the actual number of rare tones presented (60) . In contrast, the present project found behav-ioral evidence of impaired counting of the rare /go/ stimuli by the elderly participants when contralateral speech competition was present (see Table 2) . In the absence of speech competi-tion, the two groups maintained similar levels of accuracy in their counts . Thus, it appears that the counting of rare tones in the presence of contralateral speech competition may be an easier discrimination task for elderly listeners than counting rare /go/ stimuli.

Differences in electrophysiologic findings were also found between the two projects . Whereas Hymel et al (1998) found significant decreases in both Ni and P2 amplitude in the presence of contralateral speech babble, an age-related effect only occurred with the Ni com-ponent . In marked contrast, the present inves-tigation found no evidence of competition-related decreases in Ni amplitude with either participant group. The present elderly listeners did, however,

42


exhibit significantly larger competition-related decreases in Pz amplitude than did the young lis-teners . Thus, the findings of the two projects together suggest that, when performing a rela-tively easy tonal discrimination task in the pres-ence of contralateral competition, elderly listeners exhibit significantly larger amplitude decreases in the earlier occurring N1 component of the LAEP, whereas, with a more difficult speech discrimination task, they exhibit larger amplitude reductions with the later P2 compo-nent. However, since different groups of elderly persons were tested in the two experiments, it must be noted that these observed behavioral or electrophysiologic differences (or both) may have reflected the performance of different subgroups of the elderly population . In spite of this possi-ble problem, the two projects may provide impor-tant new clues to understanding the nature of the neural mechanisms underlying the con-tralateral competition effect .

As briefly described in the Introduction, there have been a large number of research reports (Schwent and Hillyard, 1975 ; Parasur-aman, 1978 ; Hansen and Hillyard, 1980; Giard et al, 1988) that have hypothesized the exis-tence of a separate endogenous neural process in the human brain related to selective auditory attention . This hypothetical attentional mech-anism, PN (Naatanen et al ., 1978), is reflected by a negative shift in baseline activity that begins in the approximate latency window of the

exogenous N1 response and lasts for several hundred msec . When listeners are selectively attending to specific stimuli, the PN summates

with the ongoing exogenous components of the LAEP to produce apparent voltage shifts in the amplitudes of individual components . In inves-tigating the PN, researchers have used a tech-nique of computing difference waves (referred to as Nd waves) that involve subtracting the wave-forms elicited when listeners ignore the sounds from waveforms recorded when they are attend-ing to the same sounds . Numerous investigations (see Woods, 1990, for an excellent review) have shown that the specific morphologic character-istics of the Nd wave, including amplitude, begin-ning and ending latency, etc ., varies with the specific nature of the discrimination task (e.g .,

type of stimuli, task difficulty, interstimulus interval, etc .) . There is also evidence (Hansen

and Hillyard, 1980 ; Naatanen,1982,1985) that the Nd wave may consist of separate early and late components that may reflect different func-tions . It has been reported (Naatanen, 1982 ;

Hansen and Hillyard,1980,1983 ; Hillyard et al, 1987 ; Alho et al, 1986 ; Woods, 1990) that the early and late Nd components exhibit different scalp topographies from each other and from the exogenous Ni component, which provides further evidence that these events are the prod-ucts of independent neural generators .

The finding that selective attention results in apparent increases in N, amplitude (e.g., Hill-yard et al, 1973), whereas contralateral compe-tition produces decreases in N, amplitude (e.g ., Hymel et al, 1998), suggests the possibility that the same underlying neural process may be involved in both phenomena . The attention and competition effects may reflect some form of bipolar brain response in which baseline activ-ity changes in one direction with selective atten-tion and changes in the opposite direction with competition or distraction . Two of the present

authors (Hymel et al, 1999) recently completed a new study designed to test this hypothesis of

a bipolar brain mechanism by directly compar-ing the effects of selective attention and con-tralateral competition in a group of 12 young

female listeners . The test paradigm involved having participants attend to and count rare tones (in an oddball task) in one ear while, in dif-ferent test runs, discriminably different odd-ball tonal sequences were either present or absent at the opposite ear. In dichotic runs, the tones in the two ears occurred in a temporally nonoverlapping fashion with an interstimulus

interval of 800 msec . Two types of difference waveforms were computed . To examine attention effects, the waveforms elicited during dichotic

runs when the participants ignored tones in a specific ear were subtracted from the waveforms elicited during other dichotic runs when par-ticipants had been attending the same tones in that ear. To measure competition effects, the waveforms recorded when listeners attended to tones during monotic test runs were subtracted from the waveforms obtained when participants attended to the same tones during dichotic test runs . If the attention and competition effects were the product of some form of bipolar neural

process, it would be expected that the morphol-ogy of the respective group difference waves

(i .e ., the amplitudes and latencies of any observed

peaks of activity) would be similar but opposite in polarity. Without going into specific details, several statistically significant differences were found in the morphology of the two difference waves, which suggests that distinctively different neural mechanisms may underlie the attention


and competition effects . Examination of the scalp distributions of the different peaks of the two group difference waves failed, however, to provide evidence for the existence of separate neural generators with different physical locations. Finally, this new study also obtained additional important information that con-traindicates the possibility that the competi-tion effect found in the authors' earlier research (Cranford and Martin, 1991 ; Hymel et al, 1998), as well as that found in the present project, could have been due solely to central masking. In all three projects, the contralateral speech babble (which consisted of the Auditec four-talker babble tape) temporally overlapped the occur-rence of the tones in the test ear. In the new study, in which a significant (p < .001) con-tralateral competition effect involving decreases in P2 amplitude was also found, the tones in the contralateral ear were separated temporally from the tones in the test ear by 800 msec .

The present project, therefore, provides additional experimental evidence that specific age-related electrophysiologic changes may be observed in the human brain when listeners are "distracted" by competing sounds . Although, as described in the preceding paragraph, the authors now have new unpublished evidence that the selective attention effect may reflect dif-ferent underlying neural processes than those associated with the competition effect, addi-tional studies will be needed to further define the specific nature of both phenomena. The Hymel et al (1998) investigation, in combination with the present project, also indicates that the under-lying neural processes involved in the competi-tion effect, like those for the attention effect, may be differentially sensitive to factors related to either task difficulty or the specific nature of the stimuli involved (e.g., speech vs nonspeech) . Specific task-related variables may influence whether the earlier or later components of the LAEP are more affected by these endogenous processes. If distinctively different underlying neural processes can be identified for both atten-tion and competition effects in the brain, and if future research can refine the means of record-ing these phenomena, this would open up a whole new area of clinical investigation. Im-paired selective attention and increased vul-nerability to distraction are common symptoms that underlie a multitude of brain pathologies ranging from dementia in aging to attention deficit disorders in children .

Acknowledgment. The authors would like to thank Dr. Andrew Stuart for providing critical reviews of ear-lier versions of the present manuscript.

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