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Abstract

The importance of temporal expectation for sensory perception has been demonstrated across diverse paradigms and multiple modalities. Overall, the findings are consistent: temporal expectation results in greater encoding precision, higher perceptual sensitivity, and decreased response times during
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Periodicity versus Prediction in Sensory Perception
XVani G. Rajendran and XSundeep Teki
Department of Physiology, Anatomy and Genetics, University of Oxford, South Parks Road, Oxford OX1 3PT, United Kingdom
Review of Morillon et al.
The importance of temporal expectation for
sensory perception has been demonstrated
across diverse paradigms and multiple mo-
dalities. Overall, the findings are consistent:
temporal expectation results in greater en-
coding precision, higher perceptual sensi-
tivity, and decreased response times during
behavioral tasks. Temporal expectation it-
self can take several forms. “Controlled” ex-
pectation arises when a symbolic cue
indicates that a stimulus will occur at a par-
ticular time in the future (Rohenkohl et al.,
2012). Temporal expectations also arise
through rhythmic sensory stimulation
(McAuley and Jones, 2003). Finally, a form
of “automatic” temporal expectation builds
up over time if a sensory event is expected
but precisely when it will occur is unknown
(Nobre et al., 2007).
By designing a task that explores the
space between symbolic and rhythmic ex-
pectations, Morillon et al. (2016) investi-
gated whether the advantages conferred
by temporal expectation, specifically the
enhancement of perceptual sensitivity
and facilitation of motor responses, arise
as a result of temporal prediction in gen-
eral, or are specific to periodic stimula-
tion. In their first experiment, stimulus
sequences consisted of 12 “target tones”
(six 880 Hz deviants, six 440 Hz stan-
dards) embedded pseudorandomly every
1.5– 6 s in a stream of 440 Hz “reference
tones,” and listeners were tasked with
identifying deviant target tones. Tone se-
quences were embedded in continuous
white noise and presented in three differ-
ent temporal contexts. In the periodic
predictable (PP) context, tone onsets were
equally spaced at one of five chosen values
of stimulus-onset asynchrony (SOA; 255,
290, 345, 445, 770 ms). In the aperiodic
predictable (AP) condition, the five SOAs
were arranged ordinally such that they al-
ternated between progressively increasing
and decreasing intervals. Listeners could
therefore exploit the pattern of time inter-
vals to form temporal predictions about
the onset of the next tone even though the
stream of tones itself was not isochron-
ous. Finally, in an aperiodic unpredictable
(AU) condition, SOAs were chosen in a
pseudorandom manner. Visual cues pre-
sented simultaneously on a gray back-
ground informed participants whether a
given sound was a reference (white cross;
92% of the time) or a target (red circle;
8% of the time).
The authors found that performance,
as measured by d(a criterion-free mea-
sure of perceptual sensitivity derived
from signal detection theory), did not
differ significantly between the two pre-
dictable conditions (PP and AP), but
was worse in the unpredictable condi-
tion (AU). In contrast, reaction times
were fastest in the periodic condition
(PP), and were not significantly differ-
ent between the two aperiodic condi-
tions (AP and AU).
The expectations manipulated in this
experiment were purely temporal, and the
authors went one step further in a second
experiment to examine the main effects
and possible interactions between pre-
dictability of temporal and spectral, or
frequency-based, features. In this para-
digm, reference and target stimuli were
bursts of colored (either blue or pink)
noise, and listeners were tasked with de-
tecting a pure tone (“target tone”) on half
of the target stimuli (“target noise”). Col-
ored noises had symmetrical 1/f power
density spectra that either increased
(blue) or decreased (pink) by 3 dB per oc-
tave and intersected at 2027 Hz, the fre-
quency of the target tone. The authors
manipulated temporal (T) and spectral
(S) predictability in a balanced factorial
paradigm (T
S
,T
S
,T
S
,T
S
,
where and denote predictable and
unpredictable conditions, respectively).
In T
conditions, SOA was fixed at 400
ms, and in T
conditions, SOA was drawn
pseudorandomly from one of five possible
values (200, 300, 400, 500, 600 ms). Refer-
ence and target noises were identical in the
S
context, and of contrasting colors in
S
. The same visual cues described previ-
ously indicated whether a given noise was
a reference (87.5% of the time) or a target
(12.5% of the time).
Consistent with their results from Ex-
periment 1, the authors found that peri-
odic stimulation (T
) yielded faster
reaction times. In addition, they showed
that this was the case regardless of whether
target and reference noises were of the
same color (S
or S
). In contrast, audi-
tory sensitivity was facilitated only by
Received April 22, 2016; revised June 1, 2016; accepted June 6, 2016.
This work was supported by the Wellcome Trust to V.G.R.
(WT099750MA;WellcomeTrust Doctoral Programmein Neuroscience) and
S.T. (WT106084/Z/14/Z; Sir Henry Wellcome Postdoctoral Fellowship).
The authors declare no competing financial interests.
Correspondence should be addressed to either Vani G. Rajendran or
Sundeep Teki, Department of Physiology, Anatomy and Genetics, Uni-
versity of Oxford, South Parks Road, Oxford OX1 3PT, UK. E-mail:
vani.rajendran@univ.ox.ac.uk or sundeep.teki@dpag.ox.ac.uk.
DOI:10.1523/JNEUROSCI.1335-16.2016
Copyright © 2016 the authors 0270-6474/16/367343-03$15.00/0
The Journal of Neuroscience, July 13, 2016 36(28):7343–7345 • 7343
periodic stimulation (T
) if target noises
were also spectrally predictable (S
).
Spectral unpredictability (S
) impaired
auditory sensitivity regardless of temporal
predictability (T
or T
).
The major contribution made by this
study is the behavioral dissociation be-
tween response speed and perceptual acu-
ity, the former resulting from periodic
stimulation only, and the latter from tem-
poral expectation more generally. The im-
plications of these findings are potentially
far-reaching and will be discussed below,
but first it is worth noting a few method-
ological issues. First, the major finding of
the double dissociation could have been
even more compelling if Experiment 2
had included an AP condition because, as
it stands, the Tcondition here is both
predictable and periodic. This under-
scores a more general issue that the results
from Experiments 1 and 2 are difficult to
compare directly, their paradigms differ-
ing to the point that one could consider
Experiment 1 a deviant “detection” task
(detecting an 880 Hz deviant tone in a
stream of 440 Hz standard tones) and
Experiment 2 a “discrimination” task
(discriminating a 2027 Hz target tone in
target-colored noise). An alternative par-
adigm using a common stimulus and task
ina23 factorial design with two kinds
of predictions (temporal and spectral)
and three levels of predictability (PP, AP,
and AU for both features) could have
yielded more clearly interpretable main
effects and interactions. Finally, the justi-
fication for using an audiovisual task with
visual cues for the target is unclear, and
somewhat muddles the interpretation of
the results because the two modalities
might contribute differentially to the
formation of the temporal expectations
probed in this study.
These methodological criticisms
aside, the effects reported, though rela-
tively small, are significant and imply
that temporal expectation through
periodic stimulation affects motor
preparation in a way that other forms of
temporal expectation do not. The role of
motor regions in rhythmic timing is well
established (Grahn and Brett, 2007).
Specifically, functional imaging stud-
ies have shown that striato-thalamo-
cortical areas are more active during
temporally regular sequences, whereas
olivocerebellar circuits are more
strongly activated during irregular se-
quences (Teki et al., 2011,2012). This is
consistent with neurophysiological re-
cordings that have identified beta-band
oscillations in the striatum as a putative
marker for temporal regularity (Bartolo
et al., 2014). Striatal beta activity repre-
sents a general sequence-initiation sig-
nal but persists subsequently only for
regular sequences and is suppressed for
irregular ones (Bartolo and Merchant,
2015). An open question is whether the
motor system is recruited only when
sensory stimuli are strictly periodic.
Though Morillon et al. (2016) show no
motor facilitation in the aperiodic pre-
dictable condition, they chose a form of
aperiodicity that is also arrhythmic. An
interesting follow-up experiment would
be to explore rhythmic (but aperiodic)
patterns, such as those used by Nozara-
dan et al. (2011), which have been
shown to entrain cortical activity, but
whose interaction with the motor sys-
tem is not fully understood.
In Experiment 2, a possible neural
mechanism for the establishment of spec-
tral predictions at the sensory level that
was not particularly emphasized in the
current study is stimulus-specific adapta-
tion (SSA), which manifests as a suppres-
sion of neural responses to a repeated
stimulus (Ulanovsky et al., 2003,2004).
SSA represents a potential single-neuron
correlate of the mismatch negativity and
refers to the observation that neural re-
sponses are typically stronger for “devi-
ant” (ie, relatively rare or unexpected)
acoustic events than for commonly oc-
curring and therefore highly predic-
table “standard” events (for review, see
Näätänen et al., 2007;Nelken and
Ulanovsky, 2007). In Experiment 2, the
target tone in the S
context is a clear de-
viant, potentially evoking a strong mis-
match signal. In the S
context, however,
the color of the noise potentially repre-
sents the most salient deviant, which
could have masked the target tone and re-
sulted in the lower dobserved. Thus, SSA
could possibly account for why spectral
predictability enhanced perceptual sensi-
tivity in the S
context relative to S
. The
formation of spectral predictions may
therefore be explained by neural adapta-
tion, a process that may or may not inter-
act with the formation of temporal
predictions through oscillatory entrain-
ment or active sensing mechanisms dis-
cussed below.
Of special note is the authors’ finding
that perceptual sensitivity in the spectrally
predictable context is further facilitated
by periodic stimulation, despite the ob-
served dissociation between perceptual
acuity and response speed. The authors
interpret this result as suggestive of a
hierarchy of predictive filters in sensory
cortices where predictions about “what”
will occur supersede predictions about
“when” something will occur, suggesting
that “what” and “when” predictions inter-
act to shape perception. Indeed the sup-
pression of neural activity in response to
repeated sounds is strongest when the
sounds occur at regular intervals (Costa-
Faidella et al., 2011). Furthermore, the ac-
tivation of a periodicity detector model
based on low-frequency modulation fil-
ters correlates strongly with perceptual
acuity in temporally jittered sequences
(Rajendran et al., 2016). These observa-
tions are consistent with findings that
entrainment of low-frequency delta oscil-
lations to the temporal structure of an at-
tended stimulus can boost perceptual
sensitivity (Lakatos et al., 2008;Schroeder
and Lakatos, 2009), and furthermore sug-
gest that such response gain might require
that elements of the attended stimulus
(“what”) be spectrally predictable.
The neural substrates underlying
“what” and “when” predictions are not
yet fully understood, but the present
findings are consistent with the prevail-
ing idea that the motor system interacts
with sensory circuits, possibly via cross-
frequency coupling between delta and
beta oscillations, to modulate temporal
predictions for adaptive behavior (Ar-
nal et al., 2015). The authors suggest a
model of active sensing, whereby overt
or covert motor signals may be used to
predictively improve sensory processing
in time (Morillon et al., 2015). This
model is similar to the Action Simula-
tion for Auditory Prediction hypothesis
(Patel and Iversen, 2014), which pro-
poses that simulation of rhythmic
movement in motor planning regions
helps the auditory system predict the
timing of an upcoming musical beat.
The findings by Morillon et al. (2016)
and the aforementioned mechanistic
hypotheses may furthermore relate to
studies probing how sensorimotor cir-
cuits encode temporal structure in
speech and music through rhythmic os-
cillations in the beta-band (Arnal, 2012;
Fujioka et al., 2012;Bartolo et al., 2014,
2015;Teki, 2014;Merchant et al.,
2015).
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Rajendran and Teki Journal Club J. Neurosci., July 13, 2016 36(28):7343–7345 • 7345
... It is worth noting, however, that by design these studies look at differences in predictions of not only ''when" an auditory event is expected, but also ''what" that auditory event should be (Teki and Kononowicz, 2016). Behavioral evidence suggests that these two types of predictions may have distinct neural substrates (Morillon et al., 2016;Rajendran and Teki, 2016), and it is therefore not yet possible to say whether pre-attentive responses are a result of temporal expectation alone or a combination of expectations of ''what" and ''when" (Arnal, 2012;Arnal and Giraud, 2012;Schwartze et al., 2013). ...
... It is worth noting that the rhythmic form of temporal expectation is just one of several forms of temporal expectation, each resulting in subtle differences in perception that may arise from differences in the underlying neural substrates (Nobre et al., 2007;Breska and Deouell, 2017). For example, an enhancement of perceptual sensitivity has been demonstrated in both periodic and non periodic sequences that are temporally predictable, but motor facilitation through faster response latencies were only observed in the periodic condition (Morillon et al., 2016;Rajendran and Teki, 2016). ...
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