Effects of Attention on Neuroelectric Correlates
of Auditory Stream Segregation
Joel S. Snyder1, Claude Alain1,2, and Terence W. Picton1,2
& A general assumption underlying auditory scene analysis is
that the initial grouping of acoustic elements is independent of
attention. The effects of attention on auditory stream segrega-
tion were investigated by recording event-related potentials
(ERPs) while participants either attended to sound stimuli and
indicated whether they heard one or two streams or watched
a muted movie. The stimuli were pure-tone ABA? patterns
that repeated for 10.8 sec with a stimulus onset asynchrony
between A and B tones of 100 msec in which the A tone was
fixed at 500 Hz, the B tone could be 500, 625, 750, or 1000 Hz,
and ? was a silence. In both listening conditions, an enhance-
ment of the auditory-evoked response (P1–N1–P2 and N1c) to
the B tone varied with ?f and correlated with perception of
streaming. The ERP from 150 to 250 msec after the beginning
of the repeating ABA? patterns became more positive during
the course of the trial and was diminished when participants
ignored the tones, consistent with behavioral studies indicat-
ing that streaming takes several seconds to build up. The N1c
enhancement and the buildup over time were larger at right
than left temporal electrodes, suggesting a right-hemisphere
dominance for stream segregation. Sources in Heschl’s gyrus
accounted for the ERP modulations related to ?f-based
segregation and buildup. These findings provide evidence for
two cortical mechanisms of streaming: automatic segregation of
sounds and attention-dependent buildup process that inte-
grates successive tones within streams over several seconds. &
Making sense of the acoustic environment requires
parsing sounds that originate from different physical ob-
jects and grouping together sounds that emanate from
the same object. These processes play a critical role in
a listener’s ability to identify and recognize complex
acoustic signals such as speech and music. The collec-
tion of internal processes that segregate and group
sounds to form representations of auditory objects is
called auditory scene analysis (Bregman, 1990). Without
auditory scene analysis, forming accurate representa-
tions of the external world would fail, especially in com-
plex situations wherein multiple objects produce similar
sounds. For example, a listener at a cocktail party must
process speech from one person while other speakers
are talking (Cherry, 1953). A similar situation arises when
a listener focuses on a single musical instrument in an
One popular paradigm for studying auditory scene
analysis presents low tones (A), high tones (B), and
silences (?) in a repeating ABA? pattern (see Figure 1).
When the difference in frequency between the A and B
tones is small and the repetition rate of the sequence is
slow, listeners hear a single stream of tones in a galloping
rhythm. When the frequency difference is large and the
repetition rate is fast, listeners report the sequence split-
ting into two streams of tones, each in a metronome-like
According to the ‘‘peripheral channeling hypothesis,’’
the most powerful cues for stream segregation are those
that lead to two or more nonoverlapping activations in
the cochlea (Hartmann & Johnson, 1991) such as pure-
tone frequency and ear of stimulation. This type of place-
based segregation is likely to be carried up through the
ascending auditory pathway to the tonotopic fields of
the auditory cortex (Kaas & Hackett, 2000). A recent
study (Fishman, Arezzo, & Steinschneider, 2004) sup-
ported this idea by presenting A and B tones that alter-
nated in frequency while recording multiunit activity
from macaque monkey primary auditory cortical neurons
that were maximally responsive to the A tones. As the
presentation rate and pitch separation of A and B tones
increased, the firing rate increased in response to the A
tones. Another study reported similar findings from the
primary auditory cortex of bats (Kanwal, Medvedev, &
Micheyl, 2003). Bee and Klump (2004) measured multi-
unit activity in the auditory forebrain (homologous to
the mammalian auditory cortex) of starlings that showed
correspondence with behavioral data from starlings and
Despite the power of the peripheral channeling hy-
pothesis, a number of cues besides those based on
1Baycrest Centre for Geriatric Care, Toronto,
D 2006 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 18:1, pp. 1–13
peripheral segregation can lead to streaming (for a re-
view, see Moore & Gockel, 2002), implying that centrally
computed features contribute to stream segregation.
Further supporting the existence of central mechanisms
for streaming is the finding that perception of streaming
does not occur immediately but takes several seconds to
build up (Anstis & Saida, 1985; Bregman, 1978). Addi-
tionally, the effect of a biasing sequence that increases
perception of streaming also lasts for several seconds
(Beauvois & Meddis, 1997), with a longer time constant
for musicians than nonmusicians (Beauvois & Meddis,
1997). Despite this slow buildup and decay for stream-
ing, transient events such as a brief silence in the ABA?
pattern or an attention shift can almost completely reset
the buildup process (Cusack, Deeks, Aikman, & Carlyon,
2004). The long time constants for buildup and decay
of streaming and the influence of musical experience
further suggest that critical aspects of streaming occur at
higher levels of the auditory system. In particular, the
long time constants would be consistent with neuro-
magnetic correlates of echoic memory in the auditory
cortex (Lu ¨, Williamson, & Kaufman, 1992), and compu-
tational modeling of streaming that use inhibitory time
constants typical of the auditory cortex (Kanwal et al.,
2003; McCabe & Denham, 1997). Thus, although it is
clear that low-level aspects of stream segregation oper-
ate at low-level stages of the auditory system, other as-
pects of streaming likely require computations in the
auditory cortex and other cortical structures.
Evidence of attentional effects on the buildup of
streaming has further implicated higher-level influences
on stream segregation (Carlyon, Cusack, Foxton, &
Robertson, 2001). When participants ignore the ABA?
pattern presented to one ear by listening to sounds
presented to the other ear and then switch their atten-
tion to the ABA? pattern, the buildup process of stream-
ing is diminished compared to when participants simply
attend to the ABA? patterns for the whole trial. The ap-
parent diminishment of streaming when ignoring the
ABA? pattern, however, might be in part due to the pro-
cess of switching attention rather than an actual effect of
ignoring the sounds (Cusack et al., 2004). Further casting
doubt on the influence of attention, ignored ABA?
patterns that would be perceived as streaming result in a
reduction in interference in a visual memory task, com-
pared to patterns that would not be perceived as stream-
ing (Macken, Tremblay, Houghton, Nicholls, & Jones,
2003). These behavioral studies thus lead to an uncertain
conclusion about whether, to what extent, and at what
level of processing listeners’ attention affects streaming.
Event-related potentials (ERPs) might help provide a
clearer answer to these issues by isolating neural events
that correspond to distinct aspects of streaming. Fur-
thermore, it is possible to record ERPs when participants
are actively listening as well as when they are ignoring
sounds, providing a simple means for evaluating the ef-
fects of attention on streaming.
Such an approach has been used in previous studies
to understand the perception of concurrently presented
auditory objects rather than simultaneously unfolding
auditory streams. As with stream segregation, percep-
tion of multiple concurrent sounds is promoted by in-
creased pitch separation. For example, if one shifts the
frequency of a partial in a multicomponent stimulus, all
Figure 1. Five cycles of
stimuli used during attend and
ignore conditions. Actual trials
were composed of 27 cycles.
Each bar represents a pure
tone in a galloping rhythm.
2 Journal of Cognitive NeuroscienceVolume 18, Number 1
the other frequencies of which derive from a single fun-
damental, this shifted component is heard as a separate
tone (Moore, Glasberg, & Peters, 1986). ERP research on
this perceptual phenomenon has identified a negative
peak at ?150 msec following presentation of the com-
plex sound called the ‘‘object-related negativity’’ (ORN).
The ORN amplitude varies in direct proportion with
perception of two simultaneous auditory objects (Alain,
Arnott, & Picton, 2001) and is not affected by selective
attention (Alain & Izenberg, 2003).
Earlier ERP research on sequential stream segregation
used randomly presented tones during selective atten-
tion (Alain & Woods, 1994; Alain, Achim, & Richer,
1993). For example, Alain and Woods (1994) presented
concurrent interleaved tone sequences of different fre-
quencies and showed behavioral and ERP evidence
that it was easier to selectively process particular
pitches when the other task-irrelevant tones were
grouped together. This suggested that perceptual
grouping overrides the effects of physical similarity
during selective attention and that auditory attention,
like visual attention, may be allocated to objects (Alain
& Arnott, 2000).
Studies by Winkler et al. (2003) and Sussman, Ritter,
and Vaughan (1999) used the mismatch negativity
(MMN) to study streaming. The MMN is a negative
ERP component that peaks ?150 msec following a
deviant stimulus in a sequence of homogeneous audi-
tory events (for a review, see Picton, Alain, Otten, Ritter,
& Achim, 2000). Studying streaming with the MMN
requires events that can only be perceived as deviants
if streaming has occurred. For example, Winkler et al.
presented simultaneous sequences, one in which the
intensity was constant except for occasional deviants and
one in which the intensity varied constantly. When the
two sequences overlapped in frequency, the constant
intensity variations in one sequence obscured the occa-
sional intensity deviants in the other sequence and no
MMN to the occasional deviants occurred. However,
when the two sequences were widely separated in fre-
quency, an MMN occurred to the occasional deviants,
suggesting that streaming had occurred prior to de-
tection of intensity deviants. Although the MMN indi-
cates that stream segregation has occurred, it reveals
little about the neural mechanisms underlying stream-
ing because it does not track ongoing processing of
The current study uses a more direct paradigm mea-
suring brain activity that tracks the ABA? pattern as
streaming occurs, in hopes of addressing some of these
limitations. Using 10.8-sec trials helped to determine
whether buildup of neural activity mirrors behavioral
buildup of streaming (Anstis & Saida, 1985; Bregman,
1978). To avoid motor contamination of the neural mea-
surements, participants were asked to indicate at the
end of the sequence whether they heard one or two
streams, enabling us to establish a relationship between
ongoing patterns of neural activity and whether stream-
ing had occurred. To identify segregation processes, we
compared activity for trials with different frequency
separations (?f) between A and B tones. To identify
buildup processes, on the other hand, we compared
activity at different 2-sec time bins within the 10.8-sec
trials. In one session, we collected ERP data while partic-
ipants made streaming judgments after the end of each
trial. In a separate session, we presented identical sound
patterns and asked the same participants to watch a
muted subtitled video of their choice and to ignore the
auditory stimuli. This manipulation was designed to
test whether stream segregation as indexed by ERPs
depends on focused attention, thereby allowing us to
clarify the stage at which attention affects streaming. The
use of muted subtitled movies is important because
the text dialogue effectively captures attention while
not interfering with auditory processing (Pettigrew
et al., 2004).
Based on place models of streaming (McCabe &
Denham, 1997; Beauvois & Meddis, 1996; Hartmann &
Johnson, 1991), we expected increases in activity as a
function of ?f corresponding to decreased overlap
between activations corresponding to the A and B tones.
We hypothesized that segregation processes would func-
tion independent of attention, whereas neural buildup
processes would be affected by attention as suggested
by behavioral studies (Cusack et al., 2004; Carlyon
et al., 2001).
Figure 2 shows the mean proportion of trials heard as
streaming for each of the ?f conditions. As expected,
the likelihood of reporting perception of two streams
increased with ?f. At 0 semitone (1 semitone = 1/12
octave), participants rarely reported hearing streaming,
whereas at 12 semitones, participants almost always
heard streaming by the end of the 10.8-sec trial. At
Figure 2. Group mean (±SE) proportion of trials heard as streaming
across participants (n = 10) for the four ?f levels.
Snyder, Alain, and Picton3
intermediate levels (4 and 7 semitones), participants
sometimes heard streaming and sometimes heard the
galloping pattern for the whole trial. There was a
significant main effect of ?f, F(3,27) = 88.02, p < .001,
with all adjacent levels of ?f differing from each other
( p < .05).
Neural Activity Reflecting
Figure 3A shows ERPs elicited by the onset of the se-
quence for attend and ignore conditions, collapsed
across ?f. The ERPs comprised P1 (?60 msec), N1
(?120 msec), and P2 (?160 msec) waves that were
maximal at frontocentral scalp regions (see Figure 3B).
There was also a clear negative peak at ?200 msec
referred to as the N2 wave that was present only when
the A and B tones differed in frequency. Following the
transient responses, there was a sustained potential (SP)
that was negative and maximal over the frontal regions.
The N1 and SP showed larger amplitude during active
than passive listening, F(1,9) = 43.59 and 70.82, respec-
tively, p < .001. The effect of attention was not sig-
nificant for the P1, P2, and N2 waves. As shown in
Figure 3C, the effect of ?f on the P1, N1, and P2 waves
was not significant. However, the N2 showed a signifi-
cant amplitude increase as a function of ?f, F(3,27) =
12.29, p < .001, with a marginal effect for SP, F(3,27) =
2.95, p = .054. There was no significant interaction be-
tween attention and ?f for any ERP deflections elicited
by the onset of the sequence.
A close examination of the SP revealed periodic fluc-
tuations in amplitude that corresponded closely with
rate of stimulus presentation. These smaller fluctuations
were more easily assessed in smaller epochs. Figure 4A
shows 2-sec ERPs in the attend condition with all four
levels of ?f superimposed at FCz. In Figure 4B are single
cycles of the ERPs at FCz and the left and right temporal
sites (T7 and T8). The neural activity associated with
increasing ?f is best illustrated by subtracting ERPs
elicited by stimuli with constant frequency (i.e., 0 semi-
tone condition) from those obtained when the A and
B tones differed in frequency.
This subtraction procedure isolated a series of time-
locked ERP waves elicited by the B tone, which included
P1 (?60 msec), N1 (?115), and P2 (?175 msec) de-
flections at frontocentral scalp regions and an N1c
(?160 msec) that was maximal over the right temporal
electrode (i.e., T8). The results of this subtraction are
shown in Figure 5A for attend and ignore conditions.
Figure 3. Group mean ERPs
to the beginning of the trial.
(A) ERP response to the first
2 sec of each trial for the
attend and ignore conditions
collapsed across ?f at the
midline frontocentral (FCz)
electrode. Horizontal bars
above the time scale represent
pure tones in the stimulus
pattern. (B) Scalp distribution
of voltage for the P1, N1,
P2, N2, and SP waves at 72,
124, 168, 220, and 620 msec,
respectively in the attend
condition collapsed across ?f.
Darker regions indicate more
activity, with polarity labeled
by + and ? signs. Isocontour
lines represent 0.4 AV/step
for P1, N1, P2, and N2 and
0.8 AV/step for SP. (C) Same
as (A) for the four ?f levels
in the attend condition.
4Journal of Cognitive Neuroscience Volume 18, Number 1
The effects of ?f and attention were examined on the
P1, N1, and P2 peak latencies at the nine frontocentral
electrodes (Fz/1/2, FCz/1/2, Cz/1/2). We also quantified
P1–N1 and N1–P2 peak-to-peak amplitudes, allowing us
to examine transient changes in neural activity while
controlling for other changes in sustained activity that
may overlap with the P1, N1, and P2 waves. The means
across the nine frontocentral electrodes were entered
into analyses of variance (ANOVAs) to test for effects
of attention, ?f, and time. We quantified the peak la-
tency and amplitude of the N1c at the left and right
temporal electrodes (T7 and T8). Quantifying the N1c,
which arises from current sources in the auditory cor-
tex with a radial orientation (Picton, Alain, Woods, et al.,
1999), allowed us to test for hemispheric differences
in ?f-based segregation processing. We also examined
whether the latency and amplitude of these responses
varied as a function of time by dividing the 10.8-sec se-
quences into five 2-sec periods and averaging the re-
sponses within that period. This allowed us to examine
whether the neural activity associated with the process-
ing of ?f varied as a function of time.
The P1, N1, and P2 latencies decreased, F(2,18) =
11.09, 17.21, and 13.44, p < .005, and the P1–N1 and
N1–P2 amplitudes increased, F(2,18) = 6.87 and 18.75,
p < .025, as a function of ?f. All linear trends of ?f on
peak latencies and peak-to-peak amplitudes were signif-
icant ( p < .025). There were significantly longer laten-
cies for the N1 and P2, F(1,9) = 5.16 and 13.24, p < .05,
and a larger P1–N1 amplitude, F(1,9) = 6.08, p < .05,
when participants attended the stimuli. A significant
Attention ? ?f interaction occurred only for P2 latency,
Figure 4. ERP time course
as ?f increased in the attend
condition. (A) ERP response at
FCz to 5 cycles (2000 msec) of
the ABA? pattern with a box
around a single repetition of
the ABA? pattern for the four
?f levels. (B) Single-cycle ERPs
at T7 (left temporal), FCz
(frontocentral midline), and T8
(right temporal) showing the
effect of varying ?f. Horizontal
bars above the time scale
represent pure tones in the
Figure 5. Difference waves
between ERPs elicited by 0
semitone ?f and those elicited
by 4, 7, and 12 semitone ?f for
attend and ignore conditions.
(A) Difference waves to the
0.4-sec ABA? pattern averaged
at T7, FCz, and T8 for attend
and ignore conditions.
Horizontal bars above the
time scale represent pure
tones in the stimulus pattern.
(B) Normalized average
amplitude in the P2 time
region (244–300) across nine
frontocentral channels (top)
and the N1c time region
(232–300) at T8 (bottom)
in the attend and ignore
conditions plotted against
?f along with the behavioral
data from Figure 2.
Snyder, Alain, and Picton5
F(2,18) = 4.30, p < .05, with a larger decrease in latency
for the ignore condition than in the attend condition.
There were no other main effects of attention or interac-
tions between attention and ?f, suggesting that ignoring
the stimuli had minimal effects on ?f-based segregation
processing. A significant main effect of time occurred for
the P1 latency, F(4,36) = 5.24, p < .01, with later peaks
as time progressed within the trial. There were no other
effects of time on latency or peak-to-peak amplitudes.
Finally, a three-way interaction between attention, time,
and ?f occurred for P1 latency, F(4,36) = 3.09, p < .05,
with larger increases over time for the attend condition
for only the 4 and 7 semitone conditions.
We also quantified latency and amplitude of the N1c
at T7 and T8 to test for hemispheric difference in pro-
cessing ?f as a function of attention and buildup. As for
the N1 at frontocentral sites, the N1c latency decreased,
F(2,18) = 3.90, p < .05, and its amplitude increased,
F(2,18) = 5.83, p < .025, as ?f increased. The linear
trends of ?f on N1c latency and amplitude were also
significant ( p < .025). The N1c amplitude was larger at
T8 than at T7, F(1,9) = 21.11, p < .001, and it increased
more with larger ?f at T8 than T7, F(2,18) = 7.52,
p < .01. N1c was larger when participants attended the
stimuli, F(1,9) = 5.58, p < .05, and this attention-related
increase was larger at T8 than at T7, F(1,9) = 12.84,
p < .01. There was no interaction between attention
and ?f, again suggesting that attention did not influ-
ence ?f-based segregation processing. The N1c results
are thus consistent with the P1–N1–P2, additionally
showing a right-hemisphere (RH) dominance for ?f-
As shown on the top of Figure 5B, behavioral judg-
ments of streaming during the attend condition cor-
related significantly with P2 amplitude in both attend,
r(2) = .87, t(9) = 21.63, p <.001, and ignore conditions,
r(2) = .86, t(9) = 17.67, p < .001. P1 amplitude also
correlated significantly with behavioral judgments, but
only for the passive condition, r(2) = .59, t(9) = 3.98,
p < .005. The N1 amplitude did not correlate with be-
havioral judgments in either the active or passive con-
ditions, likely due to relatively poor signal-to-noise ratio.
As shown on the bottom of Figure 5B, behavioral judg-
ments of streaming correlated significantly with N1c
amplitude at T8 for both attend, r(2) = ?.72, t(9) =
?4.95, p < .001, and ignore conditions, r(2) = ?.57,
t(9) = ?3.72, p < .005.
Neural Activity Reflecting Buildup of Streaming
To isolate buildup of neural activity as a function of time
during the 10.8-sec trials, we measured the ERP at five
different 2-sec time bins, collapsing across all five repe-
titions of the ABA? pattern within each bin (excluding
the first and last ABA? cycle of each trial). Figure 6A
shows the 2-sec ERPs for each time bin. Figure 6B shows
the activity collapsed across all five repetitions within the
2-sec time bins. As shown in Figure 7, subtracting the t1
condition from each of the other conditions isolated
buildup-related activity. In contrast to the effect of ?f,
the effect of time during the trial is a temporally broad
positive enhancement at FCz that peaked between 150
and 250 msec after the ABA? onset, reversing in polarity
at the right temporal electrode (T8) for both attend and
The mean activity across the 150–250 msec time range
was quantified at the nine frontocentral electrodes and
the mean across these electrodes was entered into an
ANOVA to test for effects of attention, ?f, and time.
There was a main effect of time, F(4,36) = 23.96,
p < .001, and attention, F(1,9) = 13.66, p < .005. The
interaction between time and attention showed a non-
significant trend, F(4,36) = 2.68, p < .10. The interac-
tion between time and ?f was significant, F(12,108) =
4.96, p < .005, reflecting less buildup in the 0 semitone
?f condition. There was also a significant Attention ?
?f interaction, F(3,27) = 9.28, p < .001, due to an in-
Figure 6. ERPs at different
time bins within the trial in the
attend condition. (A) ERP
response at FCz to 5 cycles
(2000 msec) of the ABA?
pattern with a box around a
single repetition of the ABA?
pattern for the five time bins
(t1–t5). (B) Single-cycle ERPs
at T7, FCz, and T8 showing
the effect of time. Horizontal
bars above the time scale
represent pure tones in the
6Journal of Cognitive Neuroscience Volume 18, Number 1
creasing negativity as ?f increased that was larger in the
As described earlier, the first ABA? pattern of each
trial was excluded in order to control for transient ERPs
that occurred at the beginning of the trial. To further
rule out influences of transient responses, we repeated
the previous ANOVA excluding the first time bin (i.e.,
the first six ABA? patterns at the start of the trial).
Time, F(3,27) = 9.54, p < .005, attention, F(1,9) =
10.41, p < .025, and the Attention ? ?f interaction,
F(3,27) = 7.28, p < .005, remained significant and
the interaction between attention and time, F(3,27) =
4.17, p < .05, became significant. The interaction be-
tween time and ?f was no longer significant, F(9,81) =
2.29, p < .1. Thus, the main effects of time and attention
and their interaction cannot be attributed to transient
activity at the beginning of the trial.
We also quantified buildup-related activity from 150 to
250 msec at T7 and T8 to test for effects of hemisphere,
attention, ?f, and time. There was a negative displace-
ment in the ERP as a function of time, F(4,36) = 5.37,
p < .05, and attention, F(1,9) = 8.62, p < .025, but no
interaction between time and attention. The negative
responses related to buildup were larger at T8 than at
T7, F(1,9) = 16.01, p < .005, and there was a larger neg-
ative increase as a function of time at T8 than at T7,
F(4,36) = 7.27, p < .005. This is therefore consistent with
the results at frontocentral sites, with the additional
finding that buildup processing showed RH dominance.
Brain Electrical Source Analysis
We used brain electrical source analysis (BESA 5.0) to
determine how well sources in the primary auditory
cortex (i.e., Heschl’s gyrus) could account for ?f-based
segregation and buildup activity measured at the scalp
in attend and ignore conditions. For the ?f-related
activity, we modeled the difference waves at all 65
electrodes collapsed across participants and ?f from
140 to 400 msec (P1–N1–P2 and N1c modulations).
Using a broad time window was justified because the
major peaks in this interval had similar source loca-
tions when modeled separately. For the buildup-related
activity, we modeled the difference waves at all 65 elec-
trodes collapsed across participants and ?f from 150
to 250 msec. Collapsing across participants and ?f lev-
els enhanced the signal-to-noise ratio. The analysis
assumed a four-shell ellipsoidal head model with rela-
tive conductivities of 0.33, 0.33, 0.0042, and 1 for the
head, scalp, bone, and cerebrospinal fluid, respectively,
and sizes of 85 mm (radius), 6 mm (thickness), 7 mm
(thickness), and 1 mm (thickness). As an initial step,
two symmetrical regional sources were placed at the
Talairach coordinates of Heschl’s gyrus in the RH and
left hemisphere (LH) (x = ±47, y = ?26, z = 13).
Each source contained three orthogonal dipoles repre-
senting three dimensions of current flow at the source
location (tangential, radial, and anterior/posterior).
Maintaining the locations and orthogonality of the three
dipoles in each regional source, the orientation of the
first dipole was aligned with the maximum direction of
Figure 8A and B (top) shows the RH and LH locations
and orientations for the ?f-related and buildup-related
sources, respectively, separately for attend and ignore
conditions. The models yielded residual variances of
3.94% and 3.09% for the ?f-related activity in attend
and ignore conditions and 8.98% and 6.71% for the
buildup-related activity in attend and ignore conditions.
Figure 8A and B (bottom) shows the source activity for
the ?f- and buildup-related sources, respectively. For
the ?f-related activity, the tangential sources accounted
for most of the P1–N1–P2 peaks, with an additional con-
tribution of the anterior/posterior source for the P1.
The radial source accounted for the N1c in the attend
condition and was larger in the RH than the LH, whereas
in the ignore condition the radial source did not strongly
reflect the N1c. For the buildup-related activity, the tan-
gential and radial sources accounted for most of the
activity, with a later peak of activity for the radial source.
Similar results were obtained for the buildup activity
Figure 7. Buildup difference waves for attend and ignore conditions
at T7, FCz, and T8. Horizontal bars above the time scale represent pure
tones in the stimulus pattern.
Snyder, Alain, and Picton7
when waveforms from the 0 semitone ?f condition were
excluded. The radial sources accounted for the effects
of attention more than the tangential sources for both
the ?f- and buildup-related activity.
The likelihood of reporting two streams in the repeating
tone patterns increased as ?f increased, consistent with
previous studies (for a review, see Moore & Gockel,
2002). At the beginning of each trial, a transient neural
response occurred, followed by an attention-dependent
sustained negative potential that was similar to the re-
sponse that occurs for long-duration sounds (Picton,
Woods, & Proulx, 1978) and trains of repeated sounds
(Picton, Campbell, Baribeau-Brown, & Proulx, 1978).
The N2 was larger with increasing ?f likely due to
a larger N1 to the second tone of the ABA? pattern.
Similarly, the sustained response was larger with increas-
ing ?f likely due to increased activity required to
monitor two sustained streams, or to broader activation
of auditory cortices when the stimuli spanned a wider
Superimposed on the SP was activity that tracked the
rhythm of the ABA? patterns. Neural activity following
onsets of the B tone showed a P1–N1–P2 enhancement
as ?f increased, which was successfully modeled as bi-
lateral sources in Heschl’s gyrus. The source analysis
likely reflects a large area of activation in the auditory
cortex centered near Heschl’s gyrus. This is consistent
with previous studies showing enhanced neural re-
sponses following changes in pure-tone frequency
(Na ¨a ¨ta ¨nen et al., 1988; Picton, Woods, et al., 1978;
Butler, 1968) and fundamental frequency and timbre of
musical instrument tones (Jones, Longe, & Vaz Pato,
1998). A similar process may be responsible for the N2
modulation that was present in the transient response
at the beginning of the trial. The particularly large P2
modulation in the current study correlated strongly with
behavioral judgments of streaming. An additional mod-
ulation of the N1c that correlated with behavior oc-
curred at the right temporal electrode but not at the
left temporal electrode, suggesting an RH dominance
Figure 8. Brain electrical source analysis. (A) Symmetrical intracerebral sources of difference ERPs for ?f-related activity in Heschl’s gyrus in
attend (left) and ignore (right) conditions. Left and back views of the head showing source locations of orthogonal dipoles with tangential,
radial, and anterior/posterior orientations relative to the temporal portion of the scalp. Source amplitude time courses of the tangential, radial,
and anterior/posterior dipoles for ?f-related activity in the left hemisphere (LH) and right hemisphere (RH). Horizontal bars above the time
scale in the tangential panel represent pure tones in the stimulus pattern. (B) Same as (A) but for the buildup-related activity. Note that effects
of attention were mainly reflected in the radial sources.
8 Journal of Cognitive NeuroscienceVolume 18, Number 1
for ?f-based stream segregation. This asymmetry also
occurred in the source analysis waveforms (Figure 8).
The ?f-related modulations occurred even when partic-
ipants ignored the sounds, implicating the auditory-
evoked response as an index of automatic ?f-based
Behavioral studies have shown that streaming takes
several seconds to buildup (Anstis & Saida, 1985;
Bregman, 1978) and that this process is sensitive to top-
down controlled processes (Cusack et al., 2004; Carlyon
et al., 2001). An ERP wave from 150 to 250 msec follow-
ing the onset of the ABA? cycle that was larger for
non-0 ?f increased with time over the 10.8-sec trial.
Although we did not measure perceptual buildup con-
comitantly with the ERPs, the timing of this ERP change
is consistent with the timing of perceptual buildup of
streaming that occurs over several seconds. Unlike the
?f-related activity, this neural activity was substantially
reduced when participants ignored the ABA? patterns,
consistent with behavioral studies showing effects of
attention on perceptual buildup of streaming (Cusack
et al., 2004; Carlyon et al., 2001).
The buildup activity we observed began its slow time-
course shortly after the first A tone. This suggests that
the buildup-related activity might respond to the onset
of the repeating ABA? pattern, perhaps reflecting an
increase in the likelihood of hearing two streams or a
decrease in the likelihood of hearing a galloping rhythm.
A negative polarity reversal of the positive frontocentral
buildup occurred at the right temporal electrode as a
function of time but not at the left temporal electrode,
suggesting an RH dominance for stream formation.
Another possible explanation for the buildup activity is
that a negative difference (Nd) wave (Hansen & Hillyard,
1980) related to attending was present at the beginning
of the 10.8-sec trial and as perceptual buildup of stream-
ing took its course, participants no longer attended as
much to the ABA? pattern. This would suggest that the
positive difference waves we observed were actually the
inverse of an Nd wave. Further experiments would be
necessary to determine whether the buildup-related
activity reflected an actual positive wave or the inverse
of a negative wave. The source modeling of buildup-
related activity was consistent with bilateral generators
in or near Heschl’s gyrus. The relatively high residual
variances might reflect in part the activation of multiple
sources over a relatively wide area of the superior and
lateral temporal surfaces.
Support for a Place Model of Stream Segregation
The central idea of a place model is that when partic-
ipants hear two streams of tones, spatially distinct popu-
lations of neurons are activated (Hartmann & Johnson,
1991). In the present study, increases in ERP amplitude
with larger ?f could have arisen from the segregation of
distinct activations corresponding to the A and B tones
in tonotopically organized structures. As two distinct
populations of active neurons become farther apart in
their frequency tuning, the less they will interact with
each other, leading to a larger summed activation at the
scalp. This interpretation is consistent with studies of
streaming using single- and multi-unit activity (Bee &
Klump, 2004; Fishman et al., 2004; Kanwal et al., 2003)
and computational models (McCabe & Denham, 1997;
Beauvois & Meddis, 1996).
Additional support for the involvement of low-level
mechanisms in segregation processes come from be-
havioral studies in infants as young as 3 days old
(McAdams & Bertoncini, 1997), and nonhuman animals
(e.g., MacDougall-Shackleton, Hulse, Gentner, & White,
1998). One study that tested European starlings’ percep-
tion of tone sequences provided evidence for perception
that closely corresponded to adult human perception
of streaming (MacDougall-Shackleton et al., 1998). The
birds were first trained to peck one key when listening
to a constant frequency ABA? tone pattern in a gallop-
ing rhythm (similar to the 0 semitone ?f condition in
the current study), and to press a different key when
listening to a single stream of tones either at the tempo
of the A tones (i.e., A?A?...) or at the tempo of the
B tones (i.e., ?B??...). When presented with ABA?
tone patterns, the birds were more likely to press the
key corresponding to the streaming patterns when ?f
increased. This mirrors human perceptual reports of the
change in rhythm that accompanies streaming. These
behavioral data were recently correlated with the differ-
ence in multi-unit responses to A and B tones in
starlings’ auditory forebrains (Bee & Klump, 2004).
The current data and those of previous studies thus
support stream segregation as a basic function of audi-
tory processing across species.
Evidence for Distinct Mechanisms
of Stream Segregation
ERP modulations related to ?f and buildup in the cur-
rent study were differentially affected by attention, pro-
viding evidence for distinct mechanisms. Cusack et al.
(2004) came to a similar conclusion in postulating low-
level automatic segregation processes and separate
buildup processes that are involved in the formation
of perceptual objects and streams. Despite the evi-
dence for dissociation between segregation and build-
up processes, it remains unclear what neural structures
and mechanisms are responsible for these two types of
process. According to the peripheral channeling hy-
pothesis, segregation of tone patterns depends on acti-
vation along a tonotopic representations in the cochlea
and other subcortical auditory structures (Hartmann &
Johnson, 1991). Given that tonotopic representations
are retained up to the level of the primary auditory cor-
tex (Kaas & Hackett, 2000), it is likely that segregation
of tone sequences in the cochlea is transferred up
Snyder, Alain, and Picton9
the ascending auditory system. Additional inhibi-
tory processes between neurons responsive to dif-
ferent frequencies in the auditory cortex have been
proposed to enhance frequency-based segregation
(Bee & Klump, 2004; Fishman et al., 2004; Kanwal
et al., 2003; McCabe & Denham, 1997), and recent evi-
dence shows that inhibition operates on the N1 (Pantev
et al., 2004; Sable, Low, Maclin, Fabiani, & Gratton,
2004). Inhibition between neurons that are tuned to
different frequencies in the auditory cortex might en-
hance frequency-based segregation by sharpening the
tuning curves in individual neurons, thus leading to less
overlap in the populations of neurons that are respond-
ing to the alternating A and B tones.
The neural mechanisms underlying buildup of stream-
ing (Anstis & Saida, 1985; Bregman, 1978) and the neural
buildup observed in the current study are not as well
understood as the mechanisms of frequency-based seg-
regation. Given the strong influence of attention ob-
served in the current study and previous behavioral
studies (Cusack et al., 2004; Carlyon et al., 2001), it is
likely that buildup of streaming is cortical in nature. This
is consistent with an account of object perception in
which subcortical and early cortical stages of processing
extract features whereas later stages build more complex
object representations. Further study is necessary to
provide a deeper understanding of such higher-level
aspects of auditory scene analysis.
Relation to Other Neurophysiological Paradigms
The current study provided information showing
auditory cortical activity over time during streaming,
demonstrating distinct neural processes related to
frequency-based segregation and buildup of streaming.
In addition to the source analysis performed in the cur-
rent study, investigations using functional neuroimaging
techniques might allow us to define networks of brain
areas that are active during stream segregation. For
example, a study using functional magnetic resonance
imaging (fMRI), a measure of cerebral blood flow that
correlates with neural activity, showed that posterior
regions of the left auditory cortex were modulated by
listening to alternating organ and trumpet tones when
compared to a single stream of either organ or trumpet
tones presented at the same rate (Deike, Gaschler-
Markefski, Brechmann, & Scheich, 2004). This is consist-
ent with the current study showing ERP modulations
near the auditory cortex although the right temporal
lobe appeared to be dominant in the current study.
Another fMRI study using stimuli similar to those in the
current study showed differential activity in the intra-
parietal sulcus depending on whether participants
heard one or two streams (Cusack, 2005). Activations
in such nonauditory regions might index higher-level
processes such as object formation or auditory attention
The present study demonstrated enhancements of the
P1–N1–P2 and N1c components of the auditory-evoked
potential that correlated with behavioral reports of
stream segregation. An additional modulation reflected
an increase in neural activity as a function of time while
listening to the extended ABA? patterns that showed
a similar time course as the buildup of streaming re-
ported in behavioral studies (Anstis & Saida, 1985;
Bregman, 1978). These two modulations were differen-
tially affected by attention with a stronger reduction of
the buildup-related activity while ignoring the sounds,
compared to the frequency-related activity. These find-
ings provide neurophysiological evidence for distinct
mechanisms of streaming, one related to frequency-
based segregation of tones in the auditory cortex and
another related to the process of forming auditory
Ten young adults (6 men and 4 women, age range =
23–38 years, mean age = 29.5 years) participated after
giving written informed consent according to the guide-
lines of the Baycrest Centre for Geriatric Care and
the University of Toronto. All participants were right-
handed except one, all had normal pure-tone thresholds
(<20 dB HL from 250 to 8000 Hz in both ears), and all
were screened for neurological and psychiatric illness.
Five of the 10 participants were musically experienced
(mean = 9.4 years formal training).
Materials and Procedure
As shown in Figure 1, stimuli were pure-tone patterns
of alternating A and B tones with every other B tone
omitted (taking the form ABA?ABA?ABA?...). These
stimuli were presented binaurally through Sennheiser
HD 265 headphones (Sennheiser Electronic, Old Lyme,
CT) at 85 dB SPL. Within each trial, the A-tone frequency
was always 500 Hz and the B-tone frequency was 500,
625, 750, or 1000 Hz. This corresponds to ?f levels of 0,
4, 7, and 12 semitones. Tone duration was 20 msec with
5.0-msec rise and fall times. The stimulus onset asyn-
chrony (SOA) was 100 msec between adjacent A and B
tones within each ABA? cycle. The silent duration (?)
between ABA triplets was 100 msec (10 Hz). The A tones
repeat every 200 msec (5 Hz) and the B tones repeat
every 400 msec (2.5 Hz).
of the ABA? pattern (27 ABA? repetitions). Within a
10 Journal of Cognitive NeuroscienceVolume 18, Number 1
block of trials, 80 trials were presented in which ?f
varied pseudorandomly from trial to trial during elec-
trophysiological recording (20 per ?f level). In the
attend condition, participants indicated at the end of
the sequence by pressing a button if they heard the
pattern as one stream and another button if they heard
the pattern as splitting into two streams by the end of
the trial. The next trial began 2000 msec after the
response. Participants were instructed to focus on the
rhythm as a cue (i.e., not galloping or galloping) to
indicate whether the pattern had split into two streams.
They were also instructed to let their perception take
a natural time course rather than biasing themselves
towards hearing the patterns in one way or another.
Each participant performed four blocks for a total of
320 trials in each experimental condition (80 per ?f
level). Prior to the experiment, participants completed
eight practice trials with two examples of each ?f level.
In the ignore condition, the procedure was the same
except participants watched a muted movie of their
choice presented on a computer monitor with English
subtitles. Instead of each successive trial being acti-
vated by a button press, each trial began 2000 msec
following the completion of the previous trial. For all
participants, the ignore condition took place at least
1 month after the attend condition. Each session lasted
around 75 min.
Electrophysiological Recording and Analysis
In the attend condition, the electrophysiological re-
sponses were continuously collected and digitized
(250 Hz sampling rate; band-pass filtered 0.05–50 Hz)
from an array of 64 electrodes using NeuroScan Syn-
Amps (Compumedics USA, El Paso, TX) and stored for
off-line analysis. Eye movements were monitored with
electrodes placed at the outer canthi and at the superior
and inferior orbit. During recording, all electrodes were
referenced to Cz but they were re-referenced to an aver-
age reference for off-line analysis. ERP recording in the
ignore condition was identical to the attend condition
except that the electrophysiological responses were
digitized at a 1000-Hz sampling rate (band-pass filtered
0.05–200 Hz). The ERP data for the ignore condition
were decimated to 250 Hz following averaging and were
otherwise processed identically to the attend condition.
ERP activity was analyzed in the following three ways.
The first analysis included 200 msec prior to the onset of
the 10.8-sec trial and 2000 msec after the onset of the
trial to test for effects of ?f and attention on the
transient auditory-evoked response. There were 80 such
2200-msec epochs for each ?f level for each participant.
The second and third analyses used epochs that includ-
ed 48 msec before and 400 msec after the onset of each
ABA? pattern, excluding the first and last ABA? patterns
of each trial. These 448-msec epochs were sorted into
five time bins taken from successive 2-sec periods within
the 10.8-sec trial, each bin containing five repetitions
of the ABA? cycle. For each time bin, there were thus
400 epochs (80 trials ? 5 ABA? cycles) for each ?f level
for each participant. We examined the effects of ?f in
each time bin by subtracting the 0 semitone ?f condi-
tion from the non-0 semitone conditions at each time
bin. Finally, to test effects of time, we compared activa-
tions at different time bins.
Trials contaminated by excessive peak-to-peak deflec-
tion (±150 AV) at the channels not adjacent to the
eyes were automatically rejected before averaging. ERPs
were averaged separately for each level of ?f and
electrode site for each participant. For each individual
average, ocular artifacts (e.g., blinks, saccades, and lat-
eral movements) were corrected by means of ocular
source principal components using BESA 3.0 (Picton,
van Roon, et al., 2000; Berg & Scherg, 1994). ERPs were
digitally filtered to attenuate frequencies outside 0.1–
30 Hz for the 2200-msec epoch and 1–20 Hz for the
For the first epoch corresponding to the onset of the
trial, ERP amplitude was measured relative to a 200-msec
baseline. For the 448-msec epoch, ?f-related modula-
tions were quantified using a baseline of 48 msec prior to
the B tone because this tone was the one varying in
frequency. Because of the ongoing nature of the ABA?
patterns,ERP amplituderelated tobuildup wasmeasured
relative to the mean activity across the entire 448-msec
epoch. The epochs were further analyzed by quantify-
ing peak amplitudes at nine frontocentral electrodes
(Fz, F1/2, FCz, FC1/2, Cz, C1/2) and the left and right
temporal electrodes (T7 and T8).
The proportions of trials in which participants reported
hearing two streams in the attend condition were ana-
lyzed using a repeated-measures ANOVA with ?f (0, 4, 7,
and 12 semitones) as the lone factor. ERP latency and
amplitude averaged across nine frontocentral electrodes
were analyzed using three-factor repeated-measures
ANOVAs with Attention (attend vs. ignore), ?f, and Time
bin (t1–t5) as factors. ERP latency and amplitude at tem-
poral electrodes (T7 and T8) were analyzed using four-
factor repeated-measures ANOVAs with Hemisphere
(left and right), Attention, ?f, and Time bin as factors.
When appropriate, the degrees of freedom were ad-
justed with the Greenhouse–Geisser epsilon (>). All
reported probability estimates are based on the reduced
degrees of freedom although the original degrees of
freedom are reported. Post hoc comparisons were per-
formed using a Bonferroni correction for multiple com-
parisons. We report p values less than .05 as significant.
ERP peak amplitudes were related to behavioral judg-
ments of streaming by simple correlations for individual
participants in each condition. These simple correlations
were submitted to one-sample t tests, testing the
Snyder, Alain, and Picton11
hypothesis that the correlations were different than 0,
with Bonferroni corrections for multiple tests.
This research was funded by grants from the Canadian Insti-
tutes of Health Research, the Natural Sciences and Engineering
Research Council of Canada, and the Premier’s Research Excel-
lence Award. We thank Chenghua Wang, Jimmy Chen, Yu He,
and Kelly McDonald for technical assistance.
Reprint requests should be sent to Joel S. Snyder, Department
of Psychiatry - 116A, VA Boston Healthcare System, Harvard
Medical School, 940 Belmont St., Brockton, MA 02301, or via
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