I N PRESS , P ROC . N EURAL I NFORMATION P ROCESSING S YSTEMS (NIPS 2005).

Bayesian Surprise Attracts Human Attention

Laurent Itti
Department of Computer Science
University of Southern California
Los Angeles, California 90089-2520, USA
itti@usc.edu

Pierre Baldi
Department of Computer Science
University of California, Irvine
Irvine, California 92697-3425, USA
pfbaldi@ics.uci.edu

Abstract
The concept of surprise is central to sensory processing, adaptation,
learning, and attention. Yet, no widely-accepted mathematical theory
currently exists to quantitatively characterize surprise elicited by a stimulus or event, for observers that range from single neurons to complex
natural or engineered systems. We describe a formal Bayesian definition of surprise that is the only consistent formulation under minimal axiomatic assumptions. Surprise quantifies how data affects a natural or artificial observer, by measuring the difference between posterior and priorbeliefs of the observer. Using this framework we measure the extent towhich humans direct their gaze towards surprising items while watching
television and video games. We find that subjects are strongly attracted
towards surprising locations, with 72% of all human gaze shifts directed
towards locations more surprising than the average, a figure which rises
to 84% when considering only gaze targets simultaneously selected by
all subjects. The resulting theory of surprise is applicable across different spatio-temporal scales, modalities, and levels of abstraction.
Life is full of surprises, ranging from a great christmas gift or a new magic trick, towardrobe malfunctions, reckless drivers, terrorist attacks, and tsunami waves. Key to survival is our ability to rapidly attend to, identify, and learn from surprising events, to decideon present and future courses of action [1]. Yet, little theoretical and computational understanding exists of the very essence of surprise, as evidenced by the absence from oureveryday vocabulary of a quantitative unit of surprise: Qualities such as the “wow factor”
have remained vague and elusive to mathematical analysis.
Informal correlates of surprise exist at nearly all stages of neural processing. In sensory
neuroscience, it has been suggested that only the unexpected at one stage is transmitted
to the next stage [2]. Hence, sensory cortex may have evolved to adapt to, to predict,and to quiet down the expected statistical regularities of the world [3, 4, 5, 6], focusing
instead on events that are unpredictable or surprising. Electrophysiological evidence forthis early sensory emphasis onto surprising stimuli exists from studies of adaptation invisual [7, 8, 4, 9], olfactory [10, 11], and auditory cortices [12], subcortical structures
like the LGN [13], and even retinal ganglion cells [14, 15] and cochlear hair cells [16]:neural response greatly attenuates with repeated or prolonged exposure to an initially novel
stimulus. Surprise and novelty are also central to learning and memory formation [1], tothe point that surprise is believed to be a necessary trigger for associative learning [17, 18],

as supported by mounting evidence for a role of the hippocampus as a novelty detector [19,20, 21]. Finally, seeking novelty is a well-identified human character trait, with possible
association with the dopamine D4 receptor gene [22, 23, 24].
In the Bayesian framework, we develop the only consistent theory of surprise, in terms ofthe difference between the posterior and prior distributions of beliefs of an observer over
the available class of models or hypotheses about the world. We show that this definition
derived from first principles presents key advantages over more ad-hoc formulations, typically relying on detecting outlier stimuli. Armed with this new framework, we provide
direct experimental evidence that surprise best characterizes what attracts human gaze inlarge amounts of natural video stimuli. We here extend a recent pilot study [25], adding
more comprehensive theory, large-scale human data collection, and additional analysis.

1 Theory
Bayesian Definition of Surprise. We propose that surprise is a general concept, which
can be derived from first principles and formalized across spatio-temporal scales, sensory
modalities, and, more generally, data types and data sources. Two elements are essential
for a principled definition of surprise. First, surprise can exist only in the presence of
uncertainty, which can arise from intrinsic stochasticity, missing information, or limited
computing resources. A world that is purely deterministic and predictable in real-time fora given observer contains no surprises. Second, surprise can only be defined in a relative,
subjective, manner and is related to the expectations of the observer, be it a single synapse,
neuronal circuit, organism, or computer device. The same data may carry different amountof surprise for different observers, or even for the same observer taken at different times.
In probability and decision theory it can be shown that the only consistent and optimal
way for modeling and reasoning about uncertainty is provided by the Bayesian theory ofprobability [26, 27, 28]. Furthermore, in the Bayesian framework, probabilities correspondto subjective degrees of beliefs in hypotheses or models which are updated, as data is acquired, using Bayes’ theorem as the fundamental tool for transforming prior belief distributions into posterior belief distributions. Therefore, within the same optimal framework, theonly consistent definition of surprise must involve: (1) probabilistic concepts to cope withuncertainty; and (2) prior and posterior distributions to capture subjective expectations.
Consistently with this Bayesian approach, the background information of an observer iscaptured by his/her/its prior probability distribution {P (M )}M ∈M over the hypotheses
or models M in a model space M. Given this prior distribution of beliefs, the fundamental effect of a new data observation D on the observer is to change the prior distribution {P (M )}M ∈M into the posterior distribution {P (M |D)}M ∈M via Bayes theorem,
wherebyP (D|M )∀M ∈ M,
P (M |D) =P (M ).
(1)P (D)In this framework, the new data observation D carries no surprise if it leaves the observer
beliefs unaffected, that is, if the posterior is identical to the prior; conversely, D is surprising if the posterior distribution resulting from observing D significantly differs fromthe prior distribution. Therefore we formally measure surprise elicited by data as somedistance measure between the posterior and prior distributions. This is best done using therelative entropy or Kullback-Leibler (KL) divergence [29]. Thus, surprise is defined by
the average of the log-odd ratio:Z
P (M |D)dM(2)
S(D, M) = KL(P (M |D), P (M )) =P (M |D) log
P (M )
M
taken with respect to the posterior distribution over the model class M. Note that KL is not
symmetric but has well-known theoretical advantages, including invariance with respect toFigure 1: Computing surprise in early sensory neurons. (a) Prior data observations, tuning preferences, and top-down influences contribute to shaping a set of “prior beliefs” a neuron may have overa class of internal models or hypotheses about the world. For instance, M may be a set of Poissonprocesses parameterized by the rate λ, with {P (M )}M ∈M = {P (λ)}λ∈IR+∗ the prior distributionof beliefs about which Poisson models well describe the world as sensed by the neuron. New data
D updates the prior into the posterior using Bayes’ theorem. Surprise quantifies the difference between the posterior and prior distributions over the model class M. The remaining panels detail
how surprise differs from conventional model fitting and outlier-based novelty. (b) In standard iterative Bayesian model fitting, at every iteration N , incoming data DN is used to update the prior{P (M |D1 , D2 , ..., DN −1 )}M ∈M into the posterior {P (M |D1 , D2 , ..., DN )}M ∈M . Freezing thislearning at a given iteration, one then picks the currently best model, usually using either a maximum likelihood criterion, or a maximum a posteriori one (yielding MM AP shown). (c) This bestmodel is used for a number of tasks at the current iteration, including outlier-based novelty detection. New data is then considered novel at that instant if it has low likelihood for the best modelba(e.g., DN
is more novel than DN). This focus onto the single best model presents obvious limitations, especially in situations where other models are nearly as good (e.g., M∗ in panel (b) is entirely
ignored during standard novelty computation). One palliative solution is to consider mixture models, or simply P (D), but this just amounts to shifting the problem into a different model class. (d)Surprise directly addresses this problem by simultaneously considering all models and by measuring
how data changes the observer’s distribution of beliefs from {P (M |D1 , D2 , ..., DN −1 )}M ∈M to{P (M |D1 , D2 , ..., DN )}M ∈M over the entire model class M (orange shaded area).
reparameterizations. A unit of surprise — a “wow” — may then be defined for a single
model M as the amount of surprise corresponding to a two-fold variation between P (M |D)and P (M ), i.e., as log P (M |D)/P (M ) (with log taken in base 2), with the total numberof wows experienced for all models obtained through the integration in eq. 2.
Surprise and outlier detection. Outlier detection based on the likelihood P (D|M best ) of
D given a single best model Mbest is at best an approximation to surprise and, in somecases, is misleading. Consider, for instance, a case where D has very small probability
both for a model or hypothesis M and for a single alternative hypothesis M. Although D
is a strong outlier, it carries very little information regarding whether M or M is the bettermodel, and therefore very little surprise. Thus an outlier detection method would strongly
focus attentional resources onto D, although D is a false positive, in the sense that it carries
no useful information for discriminating between the two alternative hypotheses M and M.
Figure 1 further illustrates this disconnect between outlier detection and surprise.

2 Human experiments
To test the surprise hypothesis — that surprise attracts human attention and gaze in natural scenes — we recorded eye movements from eight naı̈ve observers (three females andfive males, ages 23-32, normal or corrected-to-normal vision). Each watched a subset
from 50 videoclips totaling over 25 minutes of playtime (46,489 video frames, 640 × 480,
60.27 Hz, mean screen luminance 30 cd/m2 , room 4 cd/m2 , viewing distance 80cm, fieldof view 28◦ × 21◦ ). Clips comprised outdoors daytime and nighttime scenes of crowded
environments, video games, and television broadcast including news, sports, and commercials. Right-eye position was tracked with a 240 Hz video-based device (ISCAN RK-464),with methods as previously [30]. Two hundred calibrated eye movement traces (10,192
saccades) were analyzed, corresponding to four distinct observers for each of the 50 clips.
Figure 2 shows sample scanpaths for one videoclip.
To characterize image regions selected by participants, we process videoclips through computational metrics that output a topographic dynamic master response map, assigning inreal-time a response value to every input location. A good master map would highlight,
more than expected by chance, locations gazed to by observers. To score each metric wehence sample, at onset of every human saccade, master map activity around the saccade’s
future endpoint, and around a uniformly random endpoint (random sampling was repeated
100 times to evaluate variability). We quantify differences between histograms of masterFigure 2: (a) Sample eye movement
traces from four observers (squares denote saccade endpoints). (b) Our data
exhibits high inter-individual overlap,
shown here with the locations where
one human saccade endpoint was nearby(≈ 5◦ ) one (white squares), two (cyansquares), or all three (black squares)
other humans. (c) A metric where themaster map was created from the three
eye movement traces other than that being tested yields an upper-bound KL
score, computed by comparing the histograms of metric values at human (narrow blue bars) and random (wider greenbars) saccade targets. Indeed, this metric’s map was very sparse (many random
saccades landing on locations with nearzero response), yet humans preferentially saccaded towards the three active
hotspots corresponding to the eye positions of three other humans (many human saccades landing on locations withnear-unity responses).

map samples collected from human and random saccades using again the Kullback-Leibler(KL) distance: metrics which better predict human scanpaths exhibit higher distances fromrandom as, typically, observers non-uniformly gaze towards a minority of regions withhighest metric responses while avoiding a majority of regions with low metric responses.
This approach presents several advantages over simpler scoring schemes [31, 32], including agnosticity to putative mechanisms for generating saccades and the fact that applying
any continuous nonlinearity to master map values would not affect scoring.
Experimental results. We test six computational metrics, encompassing and extending thestate-of-the-art found in previous studies. The first three quantify static image properties(local intensity variance in 16 × 16 image patches [31]; local oriented edge density as
measured with Gabor filters [33]; and local Shannon entropy in 16 × 16 image patches[34]). The remaining three metrics are more sensitive to dynamic events (local motion[33]; outlier-based saliency [33]; and surprise [25]).
For all metrics, we find that humans are significantly attracted by image regions with higher
metric responses. However, the static metrics typically respond vigorously at numerous visual locations (Figure 3), hence they are poorly specific and yield relatively low KL scores
between humans and random. The metrics sensitive to motion, outliers, and surprising
events, in comparison, yield sparser maps and higher KL scores.
The surprise metric of interest here quantifies low-level surprise in image patches overspace and time, and at this point does not account for high-level or cognitive beliefs of ourhuman observers. Rather, it assumes a family of simple models for image patches, eachprocessed through 72 early feature detectors sensitive to color, orientation, motion, etc.,
and computes surprise from shifts in the distribution of beliefs about which models better
describe the patches (see [25] and [35] for details). We find that the surprise metric significantly outperforms all other computational metrics (p < 10−100 or better on t-tests forequality of KL scores), scoring nearly 20% better than the second-best metric (saliency)
and 60% better than the best static metric (entropy). Surprising stimuli often substantiallydiffer from simple feature outliers; for example, a continually blinking light on a staticbackground elicits sustained flicker due to its locally outlier temporal dynamics but is only
surprising for a moment. Similarly, a shower of randomly-colored pixels continually excites all low-level feature detectors but rapidly becomes unsurprising.
Strongest attractors of human attention. Clearly, in our and previous eye-tracking experiments, in some situations potentially interesting targets were more numerous than inothers. With many possible targets, different observers may orient towards different locations, making it more difficult for a single metric to accurately predict all observers. Hence
we consider (Figure 4) subsets of human saccades where at least two, three, or all four
observers simultaneously agreed on a gaze target. Observers could have agreed based onbottom-up factors (e.g., only one location had interesting visual appearance at that time),
top-down factors (e.g., only one object was of current cognitive interest), or both (e.g., asingle cognitively interesting object was present which also had distinctive appearance).
Irrespectively of the cause for agreement, it indicates consolidated belief that a location
was attractive. While the KL scores of all metrics improved when progressively focusing
onto only those locations, dynamic metrics improved more steeply, indicating that stimuli
which more reliably attracted all observers carried more motion, saliency, and surprise.
Surprise remained significantly the best metric to characterize these agreed-upon attractorsof human gaze (p < 10−100 or better on t-tests for equality of KL scores).
Overall, surprise explained the greatest fraction of human saccades, indicating that humans
are significantly attracted towards surprising locations in video displays. Over 72% of all
human saccades were targeted to locations predicted to be more surprising than on average.
When only considering saccades where two, three, or four observers agreed on a common
gaze target, this figure rose to 76%, 80%, and 84%, respectively.

Figure 3: (a) Sample video frames, with corresponding human saccades and predictions from theentropy, surprise, and human-derived metrics. Entropy maps, like intensity variance and orientation
maps, exhibited many locations with high responses, hence had low specificity and were poorly
discriminative. In contrast, motion, saliency, and surprise maps were much sparser and more specific,
with surprise significantly more often on target. For three example frames (first column), saccades
from one subject are shown (arrows) with corresponding apertures over which master map activity
at the saccade endpoint was sampled (circles). (b) KL scores for these metrics indicate significantly
different performance levels, and a strict ranking of variance < orientation < entropy < motion< saliency < surprise < human-derived. KL scores were computed by comparing the number ofhuman saccades landing onto each given range of master map values (narrow blue bars) to the numberof random saccades hitting the same range (wider green bars). A score of zero would indicate equality
between the human and random histograms, i.e., humans did not tend to hit various master map values
any differently from expected by chance, or, the master map could not predict human saccades better
than random saccades. Among the six computational metrics tested in total, surprise performed best,in that surprising locations were relatively few yet reliably gazed to by humans.

Figure 4: KL scores when considering
only saccades where at least one (all10,192 saccades), two (7,948 saccades),three (5,565 saccades), or all four (2,951
saccades) humans agreed on a common
gaze location, for the static (a) and dynamic
metrics (b).
Static metrics improved
substantially when progressively focusing
onto saccades with stronger inter-observer
agreement (average slope 0.56 ± 0.37
percent KL score units per 1,000 pruned
saccades). Hence, when humans agreed
on a location, they also tended to be
more reliably predicted by the metrics.
Furthermore, dynamic metrics improved
4.5 times more steeply (slope 2.44 ± 0.37),
suggesting a stronger role of dynamic eventsin attracting human attention. Surprising
events were significantly the strongest(t-tests for equality of KL scores between
surprise and other metrics, p < 10−100 ).

3 Discussion
While previous research has shown with either static scenes or dynamic synthetic stimuli
that humans preferentially fixate regions of high entropy [34], contrast [31], saliency [32],flicker [36], or motion [37], our data provides direct experimental evidence that humans
fixate surprising locations even more reliably. These conclusions were made possible bydeveloping new tools to quantify what attracts human gaze over space and time in dynamic
natural scenes. Surprise explained best where humans look when considering all saccades,
and even more so when restricting the analysis to only those saccades for which humanobservers tended to agree. Surprise hence represents an inexpensive, easily computableapproximation to human attentional allocation.
In the absence of quantitative tools to measure surprise, most experimental and modelingwork to date has adopted the approximation that novel events are surprising, and has focused on experimental scenarios which are simple enough to ensure an overlap between
informal notions of novelty and surprise: for example, a stimulus is novel during testing if
it has not been seen during training [9]. Our definition opens new avenues for more sophisticated experiments, where surprise elicited by different stimuli can be precisely comparedand calibrated, yielding predictions at the single-unit as well as behavioral levels.
The definition of surprise — as the distance between the posterior and prior distributionsof beliefs over models — is entirely general and readily applicable to the analysis of auditory, olfactory, gustatory, or somatosensory data. While here we have focused on behavior
rather than detailed biophysical implementation, it is worth noting that detecting surprise inneural spike trains does not require semantic understanding of the data carried by the spiketrains, and thus could provide guiding signals during self-organization and development ofsensory areas. At higher processing levels, top-down cues and task demands are known to
combine with stimulus novelty in capturing attention and triggering learning [1, 38], ideas
which may now be formalized and quantified in terms of priors, posteriors, and surprise.
Surprise, indeed, inherently depends on uncertainty and on prior beliefs. Hence surprise
theory can further be tested and utilized in experiments where the prior is biased, for ex-ample by top-down instructions or prior exposures to stimuli [38]. In addition, simple
surprise-based behavioral measures such as the eye-tracking one used here may prove useful for early diagnostic of human conditions including autism and attention-deficit hyperactive disorder, as well as for quantitative comparison between humans and animals which
may have lower or different priors, including monkeys, frogs, and flies. Beyond sensory
biology, computable surprise could guide the development of data mining and compression systems (giving more bits to surprising regions of interest), to find surprising agents incrowds, surprising sentences in books or speeches, surprising sequences in genomes, surprising medical symptoms, surprising odors in airport luggage racks, surprising documentson the world-wide-web, or to design surprising advertisements.
Acknowledgments: Supported by HFSP, NSF and NGA (L.I.), NIH and NSF (P.B.). We thank UCI’s
Institute for Genomics and Bioinformatics and USC’s Center High Performance Computing andCommunications (www.usc.edu/hpcc) for access to their computing clusters.

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