1 Introduction
Adversarial (testtime) attacks perturb an input item slightly, forming such that (1)
is classified differently than
; (2) the change from to is small. The oftquoted reason for (2) is to make the attack hard to detect szegedy2013intriguing ; goodfellow6572explaining ; moosavi2017universal ; carlini2017adversarial . This assumes an “inspector”, who detects suspicious items before sending them to the classifier salamati2019perception . Our paper focuses on (2) in the context of image classification attacks where the inspector is a human. We ask the question: are current measures of “small change” adequate to characterize visual detection by a human inspector? Answering this question is directly relevant to the efficacy of adversarial learning research.We start with a popular measure: the pixel norm. Perturbations lying inside the norm ball (see Figure 2) are assumed to be imperceptible. However, even with the optimal norm and radius , there can be a mismatch between what an average human perceives as small changes to (schematic green area, which may not be in the norm ball family) and the pixel
norm ball (gray). Without knowledge of human perceptual behavior, adversarial machine learning can’t accurately attack
or defend: adversaries cannot predict which attacks will succeed, and defenders don’t know what is most important to look for. For an adversary, false acceptances like produce futile attacks because the human inspector readily detects the attack, while false rejections like lead to a false sense of security: one may presume the attack at can never escape detection due to its large norm, while in reality the attack will pass the human inspector. The need for human perception knowledge is urgent: Out of 32 papers we surveyed, 27 papers (each with over citations) used pixel norms in attacks. Among these 27, 20% assumed norms are a good match to human perception without providing evidence; 50% used them because other papers did; and the rest used them without justification.Such knowledge requires multiple interdisciplinary studies in adversarial machine learning and cognitive science. The seminal work by Sharif et al. performed a behavioral study on adversarial attack and human perception Sharif2018Bright . They showed that humans may categorize two perturbed thumbnails – of the same pixel norm (for ) distance to the original thumbnail – differently. While valuable, their conclusions are limited due to the study design: they only tested pixel , , norms but not other norms or measures. Their test also required knowledge of the radius , and depended on humans (mis)categorizing a low resolution thumbnail (MNIST lecun1998mnist , CIFAR10 krizhevsky2009learning ), which does not reflect humans’ ability to notice small changes in a normalsized image well before humans’ categorization on that image changes.
Our work significantly extends and complements Sharif2018Bright , and addresses all these issues: Our design enables us to test all pixel
norms, earth mover’s distance, structural similarity (SSIM), and deep neural network representation. It is also agnostic to the true value of
by using the notion of human justnoticeabledifference. We test humans in small imagechange regimes that better match what a human inspector typically faces in an adversarial setting. Our main results caution against the use of pixel norms, earth mover’s distance, structural similarity, or deep neural network representation to define “small changes” in adversarial attacks. In addition, we give quality of approximation for different measures. For instance, we experimentally determined (see Figure 2) that pixel 3norm is the best approximation to human data among the measures studied, see section 6 for details. Our results have the potential to improve the understanding of adversarial attack and defense strategies.We also mention some limitations of our work. We cannot directly answer “what is the correct measure”, because computationally modeling human visual perception is still an open question in psychology simons2005change ; rensink2002change ; wolfe2010visual ; itti2001computational . We used a “show then perturb” experiment paradigm, while in real applications the human inspector may not have access to . We also limit ourselves to the visual domain. These topics remain future work.
2 The pixel norm central hypothesis and its implications
Let the pixel feature space be , where equals the number of pixels times the number of color channels (it is straightforward to generalize to color depth other than 255). Consider a natural image and another image . The pixel norm for any measures the amount of perturbation by . We define the 0norm to be the number of nonzero elements. To facilitate mathematical exposition in this section, we posit an “ideal observer” who has population median human perception. Natural variations in real human observers will be handled in section 4. The central hypothesis of pixel norm is the following.
Definition 1 (The Central Hypothesis).
, threshold , such that the ideal observer perceives any the same as if , and the ideal observer notices the difference if .
The threshold is known as the “Just Noticeable Difference” (JND) in experimental psychology fechner1966elements ; zhang2008just . We further define the set of JustNoticeablyDifferent images with respect to under the central hypothesis: . In other words, is the shell of the norm ball centered at with radius . A main task of the present paper is to test the central hypothesis. To this end, we derive a number of testable implications of the central hypothesis. These implications will be tested through human behavioral experiments in later sections. The first implication follows trivially from the definition of . It states that any JustNoticeablyDifferent images of an has the same norm (note: does not require knowledge of ):
Implication 1.
Suppose is the correct norm for the central hypothesis. Then .
The second implication is more powerful in the sense that it can be tested without knowing the true parameter or . To do so, we utilize special perturbed as follows. As indicated in Figure 2, we consider generated along the ray defined by a perturbation direction with a perturbation scale : . Here is the projection onto ; namely, clipping values to and rounding to integers. Note that as increases, the perturbation becomes stronger. The perturbation direction is important: in our experiments some directions are generated by popular adversarial attacks in the literature, while others are designed to facilitate statistical tests. Specifically, we define perturbation directions as any with the following two properties: (i) Its support (nonzero elements) has cardinality ; in many cases will be sparse with ; (ii) the nonzero elements are either 1 or 1 depending on the value of the corresponding element in : if , and 1 otherwise. For perturbations and integer it is easy to see that the projection is not needed: . This allows for convenient experiment design. More importantly, for such perturbed images any pixel norm has a simple form: . Implication 2 states that two justnoticeable perturbed images with the same perturbation sparsity should have the same perturbation scale . Importantly, it can be tested without knowing or . If it fails then no pixel norm is appropriate to model human perceptions of justnoticeabledifference.
Implication 2.
, , perturbation directions with the same sparsity , suppose such that and . Then .
3 Behavioral experiment design
We conducted a human behavioral experiment under Institutional Review Board (IRB) approval. We release all behavioral data, and the code that produces the plots and statistical tests in this paper, to the public for reproducibility and further research at http://www.cs.wisc.edu/~jerryzhu/pub/advMLnorm/. The figures below are best viewed by zooming in to replicate the participant experience.
Center images and perturbation directions
: We chose three natural images (from the Imagenet dataset
deng2009imagenet ) popular in adversarial research: a panda goodfellow6572explaining , a macaw moosavi2016deepfool and a cat athalye2018obfuscated as in our experiment. We resized the images to to match the input dimension of the Inception V3 image classification network szegedy2016rethinking . For each natural image we considered 10 perturbation directions , see Figure 3. Eight are specially crafted perturbation directions varying in three attributes, and further explained in the caption of Figure 3:# Dimensions Changed (s)  Color Channels Affected  Shape of Perturbed Pixels 

S = 1, M = 288  Red = only the red channel of a pixel  Box = a centered rectangle 
L = 30603, X = 268203  RGB = all three channels of a pixel  Dot = scattered random dots 
(mnemonic: garment size)  Eye = on the eye of the animal 
The remaining two perturbation directions are adversarial directions. We used Fast Gradient Sign Method (FGSM) goodfellow6572explaining and Projected Gradient Descent (PGD) madry2017towards to generate two adversarial images for each , with Inception V3 as the victim network. All attack parameters are set as suggested in the methods’ respective papers. PGD is a directed attack and requires a target label; we choose gibbon (on panda) and guacamole (on cat) following the papers, and cleaver (on macaw) arbitrarily. We then define the adversarial perturbation directions by and . We use the factor based on a pilot study to ensure that changes between consecutive images in the adversarial perturbation directions are not too small or too big.
Experimental procedure: See Figure 4. Each participant was first presented with instructions and then completed a sequence of trials, of which 30 were perturbation or adversarial trials, and 4 were guard trials. The order of these trials was randomized then fixed (see figure). During each trial the participants were presented with an image . They were instructed to increase (decrease) perturbations to this image by using right / left arrow keys or buttons. Moving right (left) incremented (decremented) by 1, and the subject was then presented with the new perturbed image . We did not divulge the nature of the perturbations beforehand, nor the current perturbation scale the participant had added to at any step of the trial. The participants were instructed to submit the perturbed image when they think it became just noticeably different from the original image . The participants had to hold in memory, though they could also go all the way left back to see again. We hosted the experiment using the NEXT platform jamieson2015next ; sievert2017next .
In a perturbation trial, the perturbation direction is one of the eight perturbations. We allowed the participants to vary within to avoid value cropping. If a participant was not able to detect any change even after , then they were encouraged to “give up” (see figure).
In an adversarial trial, the perturbation direction is or . We allowed the participants to increment indefinitely, though no one went beyond , see Figure 5.
The guard trials were designed to filter out participates who clicked through the experiment without performing the task. In a guard trial, we showed a novel fixed natural image (not panda, macaw or cat) for . Then for , a highly noisy version of that image is displayed. An attentive participant should readily notice this sudden change at and submit it. In our main analyses, we disregarded the guard trials.
Participants and data inclusion criterion: We enrolled 68 participants using Amazon Mechanical Turk buhrmester2011amazon master workers. A master worker is a person who has consistently displayed a high degree of success in performing a wide range of tasks. All participants used a desktop, laptop or a tablet device; none used a mobile device where the screen would be too small. On average the participants took minutes to finish the experiment. Each participant was paid . As mentioned before, we use guard trials to identify inattentive participants. While the change happens at exactly in a guard trial, our data indicates a natural spread in participant submissions around 20 with sharp decays. We speculate that the spread was due to keyboard / mouse auto repeat. We set a range for an acceptable guard trial if a participant submitted . A participant is deemed inattentive if any one of the four guard trials was outside the acceptable range. Only out of 68 participants survived this stringent inclusion condition. All our analyses below are on these 42 participants.
To summarize the data: on each combination of natural image and perturbation direction , the participants gave us their individual perturbation scale . That is, the image is the one participant thinks has justnoticeabledifference to . We will call these human JND images. We present box plots of the data in Figure 5. The perturbation directions
are indicated on the xaxis. The box plots (left yaxis) show the median, quartiles, and outliers of the participants’ perturbation scale
.Because our participants can sometimes choose to “give up” if they did not notice a change, we have right censored data on . All we know from a giveup trial is that , but not what larger value will cause the participant to noticed a difference. In Figure 5 the blue bars (right yaxis) show the number of participants who chose to “give up”. Not surprisingly, many participants failed to notice a difference along the S_Red_Box and S_Red_Dot perturbation directions. Because of the presence of censored data, in later sections we often employ the KolmogorovSmirnov test which is a nonparametric test of distribution that can incorporate the censored data. There are 9 tests including the appendix. To achieve a paperwide significance level of e.g. , we perform Bonferroni correction for multiple tests leading to individual test level .
4 Pixel norms do not match human perception
4.1 Humans probably do not use pixel 0norm, 1norm, 2norm, or norm
Let us start by assuming humans use pixel 1norm, i.e. . Implication 1 suggests the following procedure: for all original images , for all perturbation directions , perturb along these two directions separately until the images each become just noticeable to the ideal observer. Denote and on the two resulting images . Then we have . Conversely, if the equality does not hold on even one triple (, ), then implication 1 with , and consequently the central hypothesis with , will be refuted.
Of course, we do not have the ideal observer. Instead, we have participants from the population. Starting from along perturbation direction , the th participant identifies their own justnoticeablydifferent image . Under this produces a number . The numbers from all participants form a sample (there can be identical values). Similarly, denote the sample for direction by . Figure 6(left) shows a box plot for . If implication 1 with were true, the medians (orange lines) would be at about the same height within the plot. Qualitatively this is not the case: the median for is but the median for is merely . We perform a statistical test.
Hypothesis test 1.
The null hypothesis
is: and have the same distribution, where , and . A twosample KolmogorovSmirnov (KS) test on our data () yields a pvalue , rejecting .Exactly the same reasoning applies if we assume humans use pixel 2norm or norm. Figure 6(center) shows 2norm on , where has median but has median ; (right) shows norm on , where has median but has median . The full plots are in appendix Figure 11.
Hypothesis test 2.
: and have the same distribution, where , and . KS test yields pvalue , rejecting .
Hypothesis test 3.
: and have the same distribution, where , and . KS test yields pvalue , rejecting .
Finally, is refuted by noticing in Figure 5 that in the M, L, X directions have vastly different 0norms, yet each direction has its own nonzero scale that induces human JND. This contradicts with implication 1 with , which predicts changes are never noticeable below a 0norm threshold, and always noticeable above it, regardless of . Taken together, we have rejected implication 1 with , or
. This suggests that humans probably do not use pixel
, , , or norm when they judge if a perturbed image is different from its original.4.2 Humans probably do not use any pixel norm
But what if humans use some other norm in ? Implication 1 requires a specific to test, which is not convenient. Instead, we now test implication 2 whose failure can refute any . We take and look at the two perturbation directions and . These two perturbations have the same sparsity . Therefore, implication 2 predicts that the scales , to reach justnoticeabledifference should be the same. However, the perturbation directions differ in their “shape of support”: MRedDot changes random pixels, while MRedEye changes pixels of the eye region which presumably humans pay attention to and thus detect earlier. On perturbation direction , our participants produced scales ; similarly, for the other direction , they produced . See Figure 5(right) for the human behaviors: the median scale is 62 and 37, respectively, as we suspected.
Hypothesis test 4.
: Human JND and have the same distribution, where , and . KS test yields pvalue , rejecting .
We report more statistical tests in the appendix that further refute this and other implications. Taken together, these results indicate that pixel norms are not a good fit for human behaviors regardless of . There are probably other perceptual attributes that are important to humans which are unaccounted for by pixel norms.
5 Some measures other than pixel norms
We seek an alternative distance function (does not need to be a metric) that matches human perception. That is, for human JND images in , ideally satisfies .
1SSIM,  DNN,  EMD, 

Earth mover’s distance
(EMD), also known as Wasserstein distance, is a distance function defined between two probability distributions on a given metric space. The metric computes the minimum cost of converting one distribution to the other one. EMD has been used as a distance metric in the image space also, e.g. for image retrieval
rubner2000earth . Given two images and , EMD is calculated as Here,is the set of joint distributions whose marginals are
and (treated as histograms), respectively. We test which assumes that the same amount of earth moving in images corresponds to the same detectability by human perception. Figure 7(right) shows the box plots of human JND images along different perturbation directions for (the full plots are in appendix Figure 12). It is immediately clear that on perturbation direction XRGBBox humans need to move a lot more earth as measured by EMD before they perceive the image difference. These should not happen: ideally human JND should occur at the same value. The following test implies EMD probably should not be used to define adversarial attack detectability.Hypothesis test 5.
: Human JND images’ on directions , for have the same distribution. KS test yields pvalue , rejecting .
Structural Similarity (SSIM) is intended to be a perceptual similarity measure that quantifies image quality loss due to compression wang2004image , and used as a signal fidelity measure with respect to humans in multiple research works wang2009mean ; sheikh2006statistical . SSIM has three elements: luminance, contrast and similarity of local structure. Given two images and , SSIM is defined by and are the sample means; , and
are the standard deviation and sample cross correlation of
and (after subtracting the mean) respectively. To compute SSIM we use window size without Gaussian weights. Since SSIM is a similarity score, we define . Figure 7(left) shows the box plots of of our participant data for (The full plot is in appendix Figure 13). The following test implies 1  SSIM probably should not be used to define adversarial attack detectability.Hypothesis test 6.
: Human JND on directions , for have the same distribution. KS test yields pvalue , rejecting .
Deep neural network (DNN) representation. Even though DNNs are designed with engineering goals in mind, studies comparing their internal representations to primate brains have found similarities kriegeskorte2015deep . Let denote the last hidden layer representation of input image in a DNN. We may define as a potential distance metric for our purpose. We use Inception V3 representations with . Figure 7(center) shows the box plots of human JND images’ DNN 2norm along different perturbation directions for . The full plot for DNN norms and all animals is in appendix Figure 14. Interestingly, the human JND images along the adversarial perturbation directions (FGSM and PGD) have much larger DNN norm than the perturbation directions. As an example, , human JND images have median DNN 2norm 1.8, while human JND images have median 13.6. The following test implies that norm on DNN representation probably should not be used to define adversarial attack detectability.
Hypothesis test 7.
: Human JND images’ DNN 2norm along and for have the same distribution. KS test yields pvalue , rejecting .
6 But which measure is a better approximation?
We emphasize that our human experiments do not support pixel norm, EMD, 1  SSIM, or DNN representation as the correct measure . Nonetheless, some of them may be useful as computational approximations to human perception. As such, does our data suggest which measure offers the best approximation? While none of the measures exactly satisfies , the equality inspires the following idea: the best measure should minimize the standard deviation of over all human JND images . This is because would have been a constant if the equality were true. However, different measures have vastly different scales (e.g. for
norms alone, the all1 vector in
has 1norm , 2norm , and norm 1), making a direct comparison difficult. Instead, we normalize by the center image in order to find the best approximation: where is the zero vector. The standard deviation is taken over all our human experiment data for a particular center image , pooling all participants and all perturbation directions together, excluding “give ups”. Figure 2 shows of different measures. For pixel norms this is presented as a function of ; EMD and 1  SSIM are constant lines; and DNN has values larger than 0.12 for all and thus not shown (see appendix Figure 15 with DNN).Interestingly, by this criterion the pixel 3norm is the best approximation of human JND judgment among the tested measures. We plot of the human JND ’s in Figure 8. Compared to pixel 1, 2, and norms in Figure 6, EMD, 1  SSIM, and DNN norm in Figure 7, and their full plots in the appendix, the median of pixel 3norm (orange lines) are closer to having the same height. This qualitatively supports pixel 3norm as a better approximation than the other measures.
7 Conclusion
Our behavioral experiment suggests that pixel norms, EMD, 1  SSIM, and DNN representation norms do not match how humans judge justnoticeablydifferent images. Even though pixel 3norm is the closest approximation we tested, Figure 8 still contains significant variability. Future research is needed to identify better measures of cognitive response to image distortion, and to generalize our work to other domains such as audio and text.
Acknowledgments
The authors would like to thank PoLing Loh and Tim Rogers for helpful discussions. This work is supported in part by NSF 1545481, 1561512, 1623605, 1704117, 1836978, the MADLab AF Center of Excellence FA95501810166, American Family Insurance, and the University of Wisconsin.
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Appendix A Supplemental materials
a.1 Further statistical tests
We also take and look at the two perturbation directions and . The directions again have the same sparsity; this time they also share the same “shape of support”: the nonzero elements of are both randomly scattered over pixels. The difference is that changes only the red color channel on 288 random pixels, while changes all three channels but only on random pixels. Implication 2 again predicts that the humans should reach justnoticeabledifference at the same scale. But Figure 5(left) suggest that humans are more sensitive to simultaneous changes to all RGB channels: scales , have median 57.5 and 31, respectively.
Hypothesis test 8.
The null hypothesis is: The sample generated from , and the sample generated from , come from the same distribution. A twosample KolmogorovSmirnov test on our data () yields a pvalue , rejecting .
These two tests refute implication 2. They already indicate that no can make the central hypothesis true.
While implication 2 focuses on perturbation directions of the same sparsity, the next implication states that if one perturbation is changing more dimensions than the other, it should achieve just noticeable difference with a smaller perturbation scale. Again, this is true for all .
Implication 3.
, perturbation directions with sparsity , suppose such that and . Then .
To further strengthen our case, we also test implication 3 which states that it is easier to notice changes if the perturbation has larger support . This is mostly true as seen in Figure 5: the median of generally decreases as support size increases in the order of S, M, L, X. However, there is a curious inversion on , vs : Implication 3 predicts that , but human behaviors have mean 6.5 and 9.9 (and median 6 and 9.5), respectively: the other way around. The human data for these perturbations are not censored; Figure 5 also suggests they are close to normal in distribution. We therefore perform a onetailed twosample
test with unequal variances.
Hypothesis test 9.
The null hypothesis is: Human JND scales generated from , has equal mean as those generated from , . The lefttailed alternative hypothesis is: the former has a smaller mean. A onetailed twosample test with unequal variances on our data yields a pvalue of , rejecting and retaining .
a.2 Additional figures
panda  macaw  cat 

panda  macaw  cat 

a.3 Amazon Mechanical Turk instructions
For reference, screenshots of the instructions displayed to participants are included in this appendix.
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