Perceptual prediction: Rapidly making sense of a noisy world
Clare Press* and Daniel Yon
Department of Psychological Sciences, Birkbeck, University of London
Prior knowledge shapes what we perceive. A new brain stimulation study in Current Biology
suggests that this shaping is achieved by changes in sensory brain regions before the input
arrives, with common mechanisms operating across different sensory areas.
Our brains have to make sense of the vast quantities of information bombarding our senses.
The information reaching our eyes, ears and other receptors changes rapidly across space
and time, and the signals are imperfect . For example, when we listen to a friend on the
metro the sound of their voice is masked by the noise of the train. Our brains must rapidly
generate a best guess about what we heard to guide our behaviour effectively – we will be a
poor conversation partner if it takes us several seconds to work out what they said. A new
study  shows how the brain can generate this best guess by sending predictive signals to
brain regions involved in processing sensory input.
Work from the cognitive sciences across the last few decades has demonstrated that we likely
use our expectations to help shape what we perceive. There are many statistical regularities
within our environment and we can combine these with the sensory input to inform the likely
state of the world. If our conversational partner is a fellow academic, it is more likely that they
said ‘I love computers’ than ‘I love reviewers’, and biasing our perceptual experiences in line
with these likelihoods will tend to increase their accuracy [1,3]. Biased perceptual decisions
have been shown across a number of disciplines and with a number of methods. For example,
we are faster to identify everyday household objects (e.g., loaves of bread) when they are
preceded by observation of contexts in which they are typically seen (kitchen counters; ),
and we are more likely to report the presence of stimuli expected on the basis of arbitrary,
probabilistically-paired cues . Such biasing is also demonstrated through perceptual errors
that occur when typical regularities are disrupted. For example, we report concave faces to
have the more typical convex structure when shading cues are ambiguous , and that
sensations last for a similar length of time to concurrently performed actions – likely because
they typically last for comparable durations .
While cognitive scientists have reported for some time that perception is biased by our
expectations, the precise mechanisms realising these influences have remained elusive.
Indeed, some have even queried whether top-down knowledge really alters what we perceive
at all or rather just the decisions we make about our experiences . For example, producing
slow actions may make us hallucinate that simultaneous events last for longer, because we
typically experience slow actions to be accompanied by long sensations. Alternatively, this
knowledge could just bias us to report that events have lasted for longer because we believe
they should have done, while our perceptual experiences remain unchanged. We can
disentangle these possibilities partly by using rigorous behavioural experiments that
manipulate these processes  and constructing computational models of the decision
process . Neuroimaging methods have also been used to understand the underlying
mechanisms, e.g., examining pattern classification accuracy of sensory signals when
sensations were expected or not [9–12]. These findings have prompted suggestions that
expectations indeed influence perceptual experiences themselves via ‘pre-activation’ of
sensory units tuned to expected events before the input is received . This pre-activation is
thought to lead to competitive interactions that inhibit units tuned to the unexpected, ‘turning
up the volume’ (relative sensory gain) on expected inputs and thereby biasing perception
towards what we expect (‘sharpening’ theories; see Fig. 1).
However, it remains debated whether expectations really alter perception, partly because
these changes in sensory brain areas may not in fact play a causal role in changing perception
. Gandolfo and Downing  addressed this question in a clever study using transcranial
magnetic stimulation (TMS). In their task, participants made rapid judgements about observed
bodies or visual scenes (e.g. is this body slim?). Stimuli were preceded by written cues to
establish expectations about which particular stimulus would be shown (e.g. ‘m’ predicted a
male body). In line with previous work, the participants were faster and more accurate when
their expectations were valid. More importantly, the authors applied TMS at the time of the
cues – disrupting activity in either the extrastriate body area (EBA) or the occipital place area
(OPA). They revealed a compelling double dissociation whereby disrupting activity in body-
selective EBA abolished behavioural expectation effects for body stimuli but not scenes, and
disrupting scene-selective OPA activity had the converse effect. Such a pattern provides
convincing evidence that effects of expectations on perceptual decisions are indeed mediated
by changes in specific sensory processing. It also provides evidence to support the idea that
these modulations are realised through pre-activating units tuned to expected inputs before
the sensory information even hits the receptors.
One particularly interesting feature of this study is the specific regions where effects are found.
EBA and OPA are considered higher level sensory processing regions encoding the complex
configurations of information that characterise bodies and scenes, respectively. Predictive
sharpening effects have sometimes been observed predominantly in primary visual cortex
[9,10], prompting suggestions that predictive influences are only realised through interactions
at the earliest points in the cortical hierarchy. However, the predictive influence identified by
Gandolfo and Downing in these late visual brain areas suggests this is unlikely to be the case,
raising the alternative possibility that previous effects have been confined to early processing
regions because these areas are most sensitive to the stimuli used in these studies, i.e.,
gratings and edges [9,see also 14].
These findings suggest that regardless of the particular sensory region, expectations may
modulate processing in a similar way. Although EBA and OPA encode different kinds of visual
information, influences of prediction appeared to be mediated through similar pre-activation
processes. In other words, the same domain-general pre-activation mechanism may sharpen
representations similarly in different domain-specific sensory regions. This finding concurs
with recent results from our lab revealing that sensory predictions operate via common
mechanisms across domains. In this instance, we demonstrated that the precise nature of the
predictive (not predicted) information did not alter the nature of effects. Specifically, visual
predictions made on the basis of action sharpened visual brain activity just like when the
predictions are furnished by arbitrary sensory cues . This finding in fact conflicted with
previous reports that action expectations have a distinct influence on perception – i.e.,
dampening rather than sharpening processing of predicted inputs (; it had been thought to
be for this reason that we cannot tickle ourselves [e.g., 16]). If predictive mechanisms work
similarly across domains – regardless of the particular nature of the predictive or predicted
information – then it seems logical that Gandolfo and Downing’s findings would have
implications for any domain where observers can rely on probabilistic knowledge. For
example, as well as implications for action perception and normative sensory cognition, similar
principles may explain findings from language  and social cognition  – with effects of
expectations realised through pre-activation of relevant representations in different parts of
the cortical hierarchy.
However, the idea that sensory-specific pre-activation drives our enhanced ability to identify
expected events leaves open questions about the mechanisms that generate predictive
dampening effects when these are found. Why do predictions sometimes attenuate rather than
sharpen perception, e.g., why can’t we tickle ourselves? These findings of attenuated rather
than enhanced processing of the expected are prominent in action control literatures but in
fact are also found elsewhere [17,19]. Similar temporally-tuned methods to those employed
by Gandolfo and Downing may prove useful in disentangling the precise nature of mechanisms
operating across the sensory hierarchy [see 20].
In conclusion, Gandolfo and Downing’s new work contributes to a lively debate about the role
of prior knowledge in shaping what we perceive. Their findings provide compelling evidence
that expectations alter perception through influences realised in specific sensory areas before
the sensory events are presented, and contribute to an emerging view that a common set of
domain-general principles may account for the effects of prediction across a host of
1. Bar, M. (2004). Visual objects in context. Nat. Rev. Neurosci. 5, 617–629.
2. Gandolfo, M., and Downing, P. (2019). Causal evidence for expression of perceptual predictions
in category-selective extrastriate regions. Curr. Biol.
3. de Lange, F.P., Heilbron, M., and Kok, P. (2018). How do expectations shape perception? Trends
Cogn. Sci. 22, 764-779.
4. Palmer, S.E. (1975). The effects of contextual scenes on the identification of objects. Mem. Cogn.
5. Wyart, V., Nobre, A.C., and Summerfield, C. (2012). Dissociable prior influences of signal
probability and relevance on visual contrast sensitivity. Proc. Natl. Acad. Sci. U.S.A. 109, 3593–
6. Gregory, R.L. (1970). The Intelligent Eye. New York, McGraw-Hill.
Figure 1. Combining noisy sensory input with our expectations is a powerful way to generate largely accurate
representations of our environment efficiently. Gandolfo and Downing suggest that this is achieved by pre-
activating sensory representations of expected stimuli, e.g., those of particular bodies within extrastriate body
area, such that perception is biased towards what is expected and therefore more likely to be there.
7. Yon, D., Edey, R., Ivry, R.B., and Press, C. (2017). Time on your hands: Perceived duration of
sensory events is biased toward concurrent actions. J. Exp. Psychol. Gen. 146, 182–193.
8. Firestone, C., and Scholl, B.J. (2016). Cognition does not affect perception: Evaluating the
evidence for “top-down” effects. Behav. Brain Sci. 39, e229.
9. Kok, P., Jehee, J.F.M., and de Lange, F.P. (2012). Less is more: expectation sharpens
representations in the primary visual cortex. Neuron 75, 265–270.
10. Smith, F.W., and Muckli, L. (2010). Nonstimulated early visual areas carry information about
surrounding context. Proc. Natl. Acad. Sci. U.S.A. 107, 20099–20103.
11. Kok, P., Mostert, P., and Lange, F.P. de (2017). Prior expectations induce prestimulus sensory
templates. Proc. Natl. Acad. Sci. U.S.A. 114, 10473–10478.
12. Yon, D., Gilbert, S.J., de Lange, F.P., and Press, C. (2018). Action sharpens sensory
representations of expected outcomes. Nat. Commun. 9, 4288.
13. Bang, J.W., & Rahnev, D. (2017). Stimulus expectation alters decision criterion but not sensory
signal in perceptual decision making. Sci. Rep. 7, 17072.
14. Alilović, J., Timmermans, B., Reteig, L.C., van Gaal, S., and Slagter, H.A. (2019). No evidence
that predictions and attention modulate the first feedforward sweep of cortical information
Processing. Cereb. Cortex 29, 2261–2278.
15. Brown, H., Adams, R.A., Parees, I. Edwards, M., and Friston, K. (2013). Active inference, sensory
attenuation and illusions. Cogn. Process. 14, 411-427.
16. Blakemore, S.J., Wolpert, D.M., and Frith, C.D. (1998). Central cancellation of self-produced
tickle sensation. Nat. Neurosci. 1, 635–640.
17. Blank, H., and Davis, M.H. (2016). Prediction errors but not sharpened signals simulate
multivoxel fMRI patterns during speech perception. PLoS Biol. 14, e1002577.
18. Hudson, M., McDonough, K.L., Edwards, R., and Bach P. (2018). Perceptual teleology:
expectations of action efficiency bias social perception. Proc. Royal Soc. Lond. [Biol.] 285,
19. Richter, D., Ekman, M., & de Lange, F.P. (2018). Suppressed sensory responses to predictable
object stimuli throughout the ventral visual stream. J. Neurosci. 38, 7452-7461.
20. Yon, D., and Press, C. (2017). Predicted action consequences are perceptually facilitated before
cancellation. J. Exp. Psychol. Hum. Percept. Perform. 43, 1073–1083.