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Article  |   June 2012
Learning visual saliency by combining feature maps in a nonlinear manner using AdaBoost
Author Affiliations
  • Qi Zhao
    Computation and Neural Systems, California Institute of Technology, Pasadena, CA, USA.
    Now at Department of Electrical and Computer Engineering, National University of Singapore, Singapore
    qzhao@klab.caltech.edu
  • Christof Koch
    Computation and Neural Systems, California Institute of Technology, Pasadena, CA, USA.
    Allen Institute for Brain Science, Seattle, WA, USA
    koch@klab.caltech.edu
Journal of Vision June 2012, Vol.12, 22. doi:10.1167/12.6.22
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      Qi Zhao, Christof Koch; Learning visual saliency by combining feature maps in a nonlinear manner using AdaBoost. Journal of Vision 2012;12(6):22. doi: 10.1167/12.6.22.

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Abstract
Abstract
Abstract:

Abstract  To predict where subjects look under natural viewing conditions, biologically inspired saliency models decompose visual input into a set of feature maps across spatial scales. The output of these feature maps are summed to yield the final saliency map. We studied the integration of bottom-up feature maps across multiple spatial scales by using eye movement data from four recent eye tracking datasets. We use AdaBoost as the central computational module that takes into account feature selection, thresholding, weight assignment, and integration in a principled and nonlinear learning framework. By combining the output of feature maps via a series of nonlinear classifiers, the new model consistently predicts eye movements better than any of its competitors.

Introduction
Humans and other primates shift their gaze to allocate processing resources to a subset of the visual input. Understanding and emulating the way that humans free-view natural scenes has both scientific and economic impact. Thus, a computational model predicting where humans look has general applicability in a wide range of tasks relating to surveillance, in-home healthcare, advertising, and entertainment. 
In the past decade, a large body of computational models (Itti, Koch, & Niebur, 1998; Parkhurst, Law, and Niebur, 2002; Oliva, Torralba, Castelhano, & Henderson, 2003; Walther, Serre, Poggio, & Koch, 2005; Foulsham & Underwood, 2008; Einhäuser, Spain, & Perona, 2008; Masciocchi, Mihalas, Parkhurst, & Niebur, 2009; Chikkerur, Serre, Tan, & Poggio, 2010) have been proposed to predict gaze allocation, many of which were inspired by neural mechanisms (Koch & Ullman, 1985). A common approach is to (a) extract visual features such as color, intensity, and orientation, (b) compute individual feature maps using biologically plausible filters such as Gabor or Difference of Gaussian filters, and (c) linearly integrate these maps to generate a final saliency map. 
Our study focused on the last step—irrespective of the method used for extracting and mapping features, we investigated general principles of how different features are integrated to form the saliency map. In the literature, there are physiological and psychophysical studies in support of “linear summation” (i.e., linear integration with equal weights) (Treisman & Gelade, 1980; Nothdurft, 2000; Engmann et al., 2009), “max” (Li, 2002; Koene & Zhaoping, 2007), or “MAP” (Vincent, Baddeley, Troscianko, & Gilchrist, 2009) type of integration, in which linear summation has been commonly employed in computational modeling (Itti et al., 1998; Cerf, Frady, & Koch, 2009; Navalpakkam & Itti, 2005). Later, under the linear assumption, (Itti & Koch, 1999) suggested various ways to normalize the feature maps. Hu, Xie, Ma, Chia, and Rajan (2004) computed feature contributions to saliency using “composite saliency indicator,” a measure based on spatial compactness and saliency density. Zhao and Koch (2011) analyzed the image features at fixated and nonfixated locations directly from eye tracking data and quantified the strengths of different visual features in saliency using constrained linear regression. 
Lacking sufficient neurophysiological data concerning the neural instantiation of feature integration for saliency, we did not make any assumption regarding the integration stage. Instead, we took a data-driven approach and studied feature integration strategies based on human eye movement data. In particular, the new model aims to automatically and simultaneously address the following issues: (a) select a set of features from a feature pool in the absence of assuming which feature type is needed, (b) find an optimal threshold for each feature, and (c) estimate optimal feature weights according to their contributions to perceptual saliency. 
Our approach
We used AdaBoost as a unifying framework to address these issues. The new model learns the integration of features at multiple scales in a principled manner and with the following key advantages. First, it selects from a feature pool the most informative features that have significant variability. This framework can easily incorporate any additional features and select the best ones in a greedy manner. Second, it finds the optimal threshold for each feature (Li, 2002). Third, it makes no assumption of linear superposition or equal weights of features. Indeed, we explicitly demonstrated that certain types of nonlinear combination significantly and consistently outperform linear combinations. This raises the question of the extent to which the primate brain takes advantages of such nonlinear integration strategies. Future psychophysical and neurophysiological research are needed to answer this question. 
Related work
Different criteria to quantify saliency exist. Itti and Baldi (2006) hypothesized that the information-theoretical concept of spatio-temporal surprise is central to saliency. Raj, Geisler, Frazor, and Bovik (2005) derived an entropy minimization algorithm to select fixations. Seo and Milanfer (2009) computed saliency using a “self-resemblance” measure, in which each pixel of the saliency map indicates the statistical likelihood of saliency of a feature matrix given its surrounding feature matrices. Bruce and Tsotsos (2009) presented a model based on “self-information” after Independent Component Analysis (ICA) decomposition (Hyvarinen & Oja, 2000) that is in line with the sparseness of the response of cortical cells to visual input (Field, 1994). Wang, Wang, Huang, and Gao (2010) defined the Site Entropy Rate as a saliency measure, also after ICA decomposition. In most of the saliency models, features are predefined. Some commonly used features include contrast (Reinagel & Zador, 1999), edge content (Baddeley & Tatler, 2006), intensity bispectra (Krieger, Rentschler, Hauske, Schill, & Zetzsche, 2000), color (Jost, Ouerhani, von Wartburg, Müri, & Hügli, 2005), and symmetry (Privitera & Stark, 2000), as well as more semantic ones such as faces and text (Cerf et al., 2009). On the other hand, various inference algorithms were designed for saliency estimation. For example, Avraham and Lindenbaum (2009) used a stochastic model to estimate the probability that an image part is of interest. In Harel, Koch, and Perona (2007), an activation map within each feature channel was generated based on graph computations. In Carbone and Pirri (2010), a Bernouli mixture model is proposed to capture context dependency. 
In contrast to these models that were built upon particular image features, inference algorithms, or objective functions, another category of saliency models that learn from eye movement data, have become popular. Kienzle, Wichmann, Scholkopf, and Franz (2006) directly learnt from raw image patches whether or not these were fixated. Determining the size and resolution of the patches was not straightforward, and compromises had to be made between computational tractability and generality. In addition, the high dimensional feature vector of an image patch required a large number of training samples. Subsequently, Judd, Ehinger, Durand, and Torralba (2009) constructed a large eye tracking database and learnt a saliency model based on low, middle, and high-level image features. Both Kienzle et al. (2006) and Judd et al. (2009) applied support vector machine (SVM) to learn the saliency models. Different from these approaches, we used an AdaBoost-based model that combines a series of base classifiers to model the complex input data. With an ensemble of sequentially learned models, each base model covered a different aspect of the data set (Jin, Liu, Si, Carbonell, & Hauptmann, 2003). In addition, unlike Kienzle et al. (2006) who used raw image data, and Judd et al. (2009) who included a set of hand-crafted features, the features in the current framework are simple, biologically plausible ones (Itti et al., 1998; Parkhurst et al., 2002; Cerf et al., 2009). 
Alternative approaches employ learning based saliency models based on objects rather than pixels (Khuwuthyakorn, Robles-Kelly, & Zhou, 2010; Liu et al., 2011). To make the object detection step robust and consistent, pixel neighborhood information is considered. Thus, Khuwuthyakorn et al. (2010) extended generic image descriptors of Itti et al. (1998) and Liu et al. (2011) to a neighborhood-based descriptor setting by considering the interaction of image pixels with neighboring pixels. In other efforts, Conditional Random Field (CRF) (Lafferty, McCallum, & Pereira, 2001) that encodes interaction of neighboring pixels effectively detects salient objects in images (Liu et al., 2011) and videos (Liu, Zheng, Ding, & Yuan, 2008), although CRF learning and inference are quite slow. 
The section Features and bottom-up saliency model introduces the architecture of the standard saliency model and the bottom-up features used. Learning nonlinear feature integration using AdaBoost presents the new architecture. Experimental results demonstrates comparative and quantitative results, and General discussions and future work concludes the paper and discusses future work. 
Features and bottom-up saliency model
To focus on the comparisons of feature integration mechanisms for saliency, we used a simple set of biologically plausible features (Itti et al., 1998; Cerf et al., 2009) that included two color channels (blue/yellow and red/green), one intensity channel, and four orientation channels (0°, 45°, 90°, 135°). For each of these channels, (a) six raw maps of spatial scales (2–7) were created using dyadic Gaussian pyramids, (b) six center-surround (c-s) difference maps were then constructed as point-wise differences across pyramid scales to capture local contrasts (center level c = {2, 3, 4}, surround level s = c + δ, where δ = {2, 3}), and (c) a single conspicuity map for each of the seven features was built through across-scale addition and within-feature competition (represented at scale 4). We included a single face channel, generated by running the Viola and Jones face detector (Viola & Jones, 2001). Although different from more primitive features such as color, intensity, and orientation, faces attract gaze in an automatic and task-independent manner (Cerf et al., 2009). The architecture of our model is illustrated in Figure 1
Figure 1
 
Illustration of the bottom-up saliency model. A sample vector for learning from one particular location x (marked in red) is shown on the right.
Figure 1
 
Illustration of the bottom-up saliency model. A sample vector for learning from one particular location x (marked in red) is shown on the right.
Previous models (Itti et al., 1998; Cerf et al., 2009) generated a saliency map based on summation of the c-s maps. We here used information directly from all nf = 42 raw feature maps, ncs = 42 c-s maps, and nc = 4 conspicuity maps to construct the feature vectors for learning. As shown in Figure 1, for an image location x, the values of all the relevant maps at this particular location were extracted and stacked to form the sample vector f(x) = [f1(x) f2(x) ··· fnf+1(x) ··· fnf+ncs+1(x) ··· fnf+ncs+nc(x)]T, where T is the transpose of the vector. 
Each feature vector has a label of either 1 or −1, indicating whether the location is fixated or not. Fixation maps were constructed by convolving recorded fixations with an isotropic Gaussian kernel (Zhao & Koch, 2011); an example is shown in Figure 2b. Formally, for each subject i viewing image j, assuming that each fixation gives rise to a Gaussian distributed activity, all gaze data are represented as the recorded fixations convolved with an isotropic Gaussian kernel KG as where x denotes the two-dimensional image coordinates. xk represents the image coordinates of the kth fixation, and f is the number of fixations. The bandwidth of the kernel, h, is set to approximate the size of fovea, and α normalizes the map. The fixation maps were represented at the same scale as the conspicuity maps (to avoid too large maps, we also limited the largest spatial dimension to 40 [Harel et al., 2007]). We set h = 2 to approximate the size of fovea. 
Figure 2
 
Fixation map illustration. (a) Original image with eye movements of one subject. (b) Fixation map of the same subject free-viewing the image shown in (a) (the first fixation is the center of the image and not included in the fixation map).
Figure 2
 
Fixation map illustration. (a) Original image with eye movements of one subject. (b) Fixation map of the same subject free-viewing the image shown in (a) (the first fixation is the center of the image and not included in the fixation map).
Note that the first fixation in each image is not used as it is—by design—always the center of the image. To assign a label to each sample vector, the continuous fixation map is converted into binary labels by using a sampling technique: locations of positive examples are sampled from the maps (i.e., an image location with a larger value in the fixation map has a higher probability of being sampled as a positive sample), and locations of negative examples are sampled uniformly from nonactivated areas (i.e., with values smaller than a threshold th = 0.001 in our implementation) of the fixation maps. 
Learning nonlinear feature integration using AdaBoost
To quantify the relevance of different features across multiple scales in deciding where to look, we learned—using AdaBoost—nonlinear integration of features G(f) : RdR, where d is the dimensionality of the feature space. The AdaBoost algorithm (Freund & Schapire, 1996; Friedman, Hastie, & Tibshirani, 1998; Schapire & Singer, 1999; Vezhnevets & Vezhnevets, 2005) is one of the most effective methods for object detection (Viola & Jones, 2001; Chen & Yuille, 2004). As a special case of boosting, the final strong classifier is a weighted combination of weak classifiers that are iteratively built. Subsequent weak classifiers are tweaked in favor of the misclassified instances. 
Formally, where gt(·) denotes the weak learner and G(·) the final classifier, here an estimate of saliency. αt is the weight of gt(·), as would be described in Algorithm 1 below. T is the number of weak classifiers. Instead of taking the sign of the AdaBoost output, as conventionally in classification, we use the real value G(f) to form a saliency map. Details are described in Algorithm 1 and in Figure 3
Figure 3
 
Illustration of the AdaBoost-based saliency model. (a) Training stage: using samples from training images, weak classifiers are trained through iterations and combined to form a strong classifier. We use a d-dimensional (a typical value of d in this work is 88) space but plot a two-dimensional one here for illustration. (b) Testing stage: for a new image, the feature vectors of image locations are calculated and input to the strong classifier to obtain saliency scores. For the two output maps shown on the right, the left map is the output of the strong classifier, where brighter regions denote more salient areas; and the right map is the result of the left map after a sigmoid transformation for illustration purposes.
Figure 3
 
Illustration of the AdaBoost-based saliency model. (a) Training stage: using samples from training images, weak classifiers are trained through iterations and combined to form a strong classifier. We use a d-dimensional (a typical value of d in this work is 88) space but plot a two-dimensional one here for illustration. (b) Testing stage: for a new image, the feature vectors of image locations are calculated and input to the strong classifier to obtain saliency scores. For the two output maps shown on the right, the left map is the output of the strong classifier, where brighter regions denote more salient areas; and the right map is the result of the left map after a sigmoid transformation for illustration purposes.
Algorithm 1 Learning A Nonlinear Feature Integration Using AdaBoost 
Input: Training dataset with N images and eye movement data from M subjects. A testing image Im
Output: Saliency map associated with Im
Training stage: 
  1.  
    For all locations in N images, sample {xs}s=1S as a stack of features for the sample at location xs.
  2.  
    Initialize weights to be {ws=1S}s=1S, where S is the number of samples.
  3.  
    For t = 1, … , T (where T is the number of weak classifiers)
     
    •  
      Train a weak classifier gt: Rd → {1, 1}, which minimizes the weighted error function gt=argminguG, where
      εu=s=1Swt(s)[ysgu(fs)].
    •  
      Set the weight of gt as
      αt=12ln1εtεt.
    •  
      Update sample weights
      wt+1(s)=wt(s)exp[αt·ys·gt(fs)]Zt,
      where Z is a normalization factor.
  4.  
    Saliency is defined as
    G(f)=t=1Tαtgt(f).
Testing stage (for a new image Im): for each location x in Im, compute the feature vector f(x), then apply the strong classifier G(f) : RdR (Equation 2) to obtain the saliency value of f(x). 
The algorithm has two stages. During the initial training stage, weak classifiers are built iteratively and combined to form the final strong classifier. In the second, testing stage, new images are applied with the strong classifier, which outputs saliency scores. 
It is worth noting that the AdaBoost algorithm can be interpreted as a greedy feature selection process that selects from a feature set good ones that nevertheless have significant variety (Viola & Jones, 2001). We restrict each weak learner to depend on a single channel. As shown in Step 3.a of Algorithm 1, the algorithm goes over all feature dimensions and picks the feature channel that best separates the positive and negative examples (while testing each of the channels, thresholds are tested in an exhaustive manner, and the threshold with the smallest classification error is used for that particular channel). Iteratively, AdaBoost automatically selects features from a feature pool and finds the optimal threshold and the optimal weight for each feature channel it selects. The total number of weak classifiers, T in Step 3, can correspond to the number of features in a feature pool, or to a much smaller value. 
In Step 3.c, samples are reweighted such that those misclassified by previously trained weak classifiers are weighted more in future iterations. Thus, subsequently trained weak classifiers will place emphasis on these previously misclassified samples. Intuitively, a variety of feature channels is obtained by such a reweighting mechanism since subsequently selected weak learners that best separate the reweighted samples are usually very different from those channels already selected and that misclassified the samples. 
Experimental results
Experimental paradigm
Datasets
This study analyzed eye movements from four recent datasets (Cerf et al., 2009; Bruce & Tsotsos, 2009; Judd et al., 2009; Subramanian, Katti, Sebe, Kankanhalli, & Chua, 2010). 
In the FIFA dataset (Cerf et al., 2009), fixation data were collected from eight subjects performing a 2-sec-long “free-viewing” task on 180 color natural images (28° × 21°). Participants were asked to rate, on a scale of 1 through 10, how interesting each image was. Scenes were colored indoors and outdoors still images. Images included faces in various skin colors, age groups, gender, positions, and sizes. 
The second dataset from Bruce and Tsotsos (2009), referred to here as the Toronto database, contains data from 11 subjects viewing 120 color images of outdoor and indoor scenes. Participants were given no particular instructions except to observe the images (32° × 24°), 4 sec each. One distinction between this dataset and the FIFA one (Cerf et al., 2009) is that a large portion of images here do not contain particularly regions of interest, while in the FIFA dataset most contain very salient regions (e.g., faces, or salient nonface objects). 
The eye tracking dataset published by Judd et al. (2009) (referred to as the MIT database) is the largest one in the community. It includes 1,003 images collected from Flickr creative commons and LabelMe. Eye movement data were recorded from 15 users who free-viewed these images (36° × 27°) for 3 sec each. A memory test was provided at the end to motivate the subjects to pay attention to the images: they looked at 100 images and needed to indicate which ones they had seen before. 
The NUS database published by Subramanian et al. (2010) includes 758 images containing semantically affective objects/scenes such as expressive faces, nudes, unpleasant concepts, and interactive actions. Images are from Flickr, Photo.net, Google, and emotion-evoking IAPS (Lang, Bradley, & Cuthbert, 2008). In total 75 subjects free-viewed part of the image set for 5 sec each (each image of size 26° × 19° was viewed by an average of 25 subjects). 
Similarity measures
Unlike most saliency papers that used solely the area under the ROC curve (AUC) to quantify model performance, Zhao and Koch (2011) showed that in practice, as long as hit rates are high, the AUC is always high regardless of the false alarm rate. Therefore, an ROC analysis is, by itself, insufficient to describe the deviation of predicted fixation patterns from the actual fixation map. We use three complementary similarity measures (Zhao & Koch, 2011) for a more comprehensive assessment—AUC in addition to the Normalized Scanpath Saliency (NSS) (Parkhurst et al., 2002; Peters, Iyer, Itti, & Koch, 2005) and the Earth Mover's Distance (EMD) (Rubner, Tomasi, & Guibas, 2000) that measure differences in value. Both AUC and NSS compare maps primarily at the exact locations of fixation, while EMD accommodates shifts in location and reflects the overall discrepancy between two maps on a more global scale. 
Given the extant variability among different subjects looking at the same image, no saliency algorithm can perform better (on average) than the area under the ROC curve dictated by intersubject variability. The ideal AUC is computed by measuring how well the fixations of one subject can be predicted by those of the other n – 1 subjects, iterating over all n subjects and averaging the result. These ideal AUC values were 78.6% for the FIFA dataset, 87.8% for the Toronto dataset, 90.8% for the MIT dataset, and 85.7% for the NUS dataset (Zhao & Koch, 2011). We express the performance of saliency algorithms in terms of normalized AUC (nAUC) values, which is the AUC using the saliency algorithm normalized by the ideal AUC. 
A strong saliency model should have an nAUC value close to 1, a large NSS, and an EMD value close to 0. 
Performance
For our comparisons, we used both linear models with equal weights (Itti et al., 1998; Cerf et al., 2009) and linear models with optimal weights (Zhao & Koch, 2011), for which the weights of the four conspicuity maps (i.e., color, intensity, orientation, and face maps) were optimized by a linear regression with constraints. 
We divided each dataset into a training set and a testing set, and sampled 10 positive samples (i.e., fixated locations) and the same number of negative samples (i.e., locations that were not fixated) from each image for training and testing. 
FIFA dataset
In the first experiment, we compared linear summation and nonlinear integration on the FIFA dataset. The dataset of 180 images was divided into 130 training and 50 testing images. We trained subject-specific models using eye movement data from one observer, and a subject-independent, general model using data from all eight subjects. We limited the nonlinear integration to the level of conspicuity maps and included three conspicuity maps and a face map in the feature pool (that is, d = 4). 
As illustrated in Figure 4 and Table 1, AdaBoost learning outperformed linear summation. 
Table 1
 
Quantitative comparison when integrating the four channels in a linear summation, optimal linear, or nonlinear manner on the FIFA dataset. The nonlinear model outperforms the linear ones on all three measures.
Table 1
 
Quantitative comparison when integrating the four channels in a linear summation, optimal linear, or nonlinear manner on the FIFA dataset. The nonlinear model outperforms the linear ones on all three measures.
Linear summation Linear integration with optimal weights Nonlinear integration
Subject-specific General Subject-specific General
Mean SD Mean SD
nAUC 0.924 0.945 0.0136 0.944 0.959 0.0161 0.953
NSS 0.845 1.35 0.0720 1.32 1.47 0.0711 1.42
EMD 5.26 4.33 0.236 4.41 2.68 0.150 2.87
Figure 4
 
Comparisons between the linear and nonlinear algorithms. (a) Sample images from the FIFA dataset. (b) Fixation map for one subject. (c) Saliency map from the standard linear summation model. (d) Saliency map from AdaBoost learning using subject-specific data.
Figure 4
 
Comparisons between the linear and nonlinear algorithms. (a) Sample images from the FIFA dataset. (b) Fixation map for one subject. (c) Saliency map from the standard linear summation model. (d) Saliency map from AdaBoost learning using subject-specific data.
Using all three similarity measures, the AdaBoost-based nonlinear integration outperformed the linear integration with optimal weights (Zhao & Koch, 2011), for which the weights for the four conspicuity maps were optimized, although the performance difference was better reflected by NSS and EMD, compared with nAUC. The performance differences are not significant between the average and the subject-specific data, suggesting unremarkable intersubject variability in terms of feature preference. 
Toronto dataset
We divided the image set of 120 images into 80 training images and 40 testing ones. Because there were fewer fixations in the Toronto dataset than in the FIFA dataset, we built only general models in this experiment. When the conspicuity maps and the face map were used as candidate features (d = 4), nonlinearity was performed at a coarser level because conspicuity maps were constructed by linear summations of the c-s maps. In contrast, when all feature channels described in Features and bottom-up saliency model were included (d = 88), we were exploiting nonlinearity at a deeper level. 
Figure 5 illustrates comparative results with linear summation and nonlinear integration. Subjects tended to fixate on regions of trees, houses, and poles, rather than the roads, grasses, or their boundaries that evoked a strong response in the linear summated saliency map (Figure 5c and e). Here, linear summation did not work well. Using a learning based approach, however, the model could learn from the fixation data which featured combinations that are more likely to attract attention. 
Figure 5
 
Comparisons between the linear and nonlinear algorithms. (a and b) Two sample images from the Toronto dataset with fixations (green dots) from all subjects. (c and e) Saliency maps from the standard linear summation model. (d and f) Saliency maps from AdaBoost learning.
Figure 5
 
Comparisons between the linear and nonlinear algorithms. (a and b) Two sample images from the Toronto dataset with fixations (green dots) from all subjects. (c and e) Saliency maps from the standard linear summation model. (d and f) Saliency maps from AdaBoost learning.
A quantitative summary of linear integrations and nonlinear integrations with different levels is shown in Table 2. Comparing the fourth column of Table 2 to the last column of Table 1, in which both use four candidate features, the results on the FIFA dataset are better, consistent with the aforementioned fact that the FIFA dataset was relatively easier due to the presence of large faces and objects in most of the images. 
Table 2
 
Quantitative comparisons of linear and nonlinear integrations on the Toronto dataset. “CBM” stands for Center Bias Modeling.
Table 2
 
Quantitative comparisons of linear and nonlinear integrations on the Toronto dataset. “CBM” stands for Center Bias Modeling.
Linear summation Linear integration with optimal weights Nonlinear integration
Conspicuity level Raw/c-s feature level
4 channels Top 10 of 88 channels 88 channels 88 channels with CBM
nAUC 0.828 0.834 0.836 0.912 0.916 0.982
NSS 0.872 0.920 0.913 1.35 1.37 1.88
EMD 4.85 4.50 3.66 3.28 3.20 2.11
Figure 6 illustrates the test error as a function of the number of weak classifiers. It suggests that the performance does not increase noticeably after around 10 classifiers are included. To further show selected features by AdaBoost, Figures 7 and 8 illustrate the feature selection results as a function of iteration number. Particularly, in Figure 7, we divided all 88 feature maps into four categories of color, intensity, orientation, and face, and results indicated that 8 out of 10 top features were orientation channels (the remaining two were color channels). We broke down all channels but the face channel into their constitutive scale-dependent maps: raw feature maps of six scales and center-surround maps of six combinations, each with two color, one intensity, and four orientation channels, as introduced in Features and bottom-up saliency model. Figure 8 illustrates a relatively even distribution of the top 10 features over different scales, showing the necessity of using a multiscale approach to capture saliency. 
Figure 6
 
Test error as AdaBoost adds up to 88 weak classifiers, tested on the Toronto dataset. Performance does not increase much after 10 iterations.
Figure 6
 
Test error as AdaBoost adds up to 88 weak classifiers, tested on the Toronto dataset. Performance does not increase much after 10 iterations.
Figure 7
 
Selected feature type of AdaBoost as a function of the number of weak classifiers (on the Toronto dataset). Each iteration selects one feature in the four categories of color, intensity, orientation, and face (denoted by * in different colors). The features selected by the first 10 classifiers (shown to be the most important from Figure 6) are mostly orientation channels.
Figure 7
 
Selected feature type of AdaBoost as a function of the number of weak classifiers (on the Toronto dataset). Each iteration selects one feature in the four categories of color, intensity, orientation, and face (denoted by * in different colors). The features selected by the first 10 classifiers (shown to be the most important from Figure 6) are mostly orientation channels.
Figure 8
 
Selected scale as AdaBoost adds more and more classifiers (on the Toronto dataset). “7” is the spatial scale containing the highest spatial frequencies.
Figure 8
 
Selected scale as AdaBoost adds more and more classifiers (on the Toronto dataset). “7” is the spatial scale containing the highest spatial frequencies.
Figure 9
 
Test error as AdaBoost adds in more weak classifiers (on the MIT dataset). Considering more than 10–20 classifiers does not increase performance substantially.
Figure 9
 
Test error as AdaBoost adds in more weak classifiers (on the MIT dataset). Considering more than 10–20 classifiers does not increase performance substantially.
We built saliency maps using the top 10 features selected by AdaBoost from the feature pool, as well as saliency maps using all features. The fifth and sixth columns in Table 2 indicate that after selecting the most discriminative features, the rest did not improve performance much. This, along with Figure 6, demonstrates the feature selection capability of AdaBoost: the algorithm selects the best features without efforts from domain experts. On this dataset, the most informative features selected by AdaBoost were orientation maps at various scales. On the FIFA, MIT, and NUS datasets, the most important feature was the face channel, followed by orientation, color, and intensity. The Toronto dataset ranked the face channel low since the dataset included few frontal faces. 
AdaBoost can act in the same way as the lasso prior (Friedman, Hastie, Rosset, Tibshirani, & Zhu, 2004), and its cost function is different from the linear model. To rule out the probability that the improved performance is due to these two factors, we performed lasso regression and lasso logistic regression, using the same data and features. The lasso type regression was particularly helpful in some cases due to its tendency to prefer solutions with fewer nonzero coefficients, reducing the number of variables. We used it with 88 feature channels rather than 4. We obtained AUC: 0.881; NSS: 1.16; EMD: 3.91 with lasso regression and AUC: 0.886; NSS: 1.20; EMD: 3.85 with lasso logistic regression. The performance was better than the models using four conspicuity maps as features, in which each sub-feature-map belonging to the four broad feature categories (i.e., color, intensity, orientation, face) were inherently linearly added with equal weights to produce one conspicuity map for each feature category. On the other hand, with the same number of feature channels (i.e., 88), the models with various regression methods still underperform the one with AdaBoost, showing that the performance increase of AdaBoost was not due to the lasso prior or the different form of the cost function. 
Under standard testing conditions, a strong center bias is seen (Tatler, 2007; Zhang, Tong, Marks, Shan, & Cottrell, 2008; Zhang, Tong, & Cottrell, 2009; Judd et al., 2009). In Zhao and Koch (2011), both time-varying (Bahill, Adler, & Stark, 1975; Pelz & Canosa, 2001; Parkhurst et al., 2002; Peters et al., 2005; Gajewski, Pearson, Mack, Bartlett, & Henderson, 2005) and constant (Zambarbieri, Beltrami, & Versino, 1995; Fuller, 1996; Vitu, Kapoula, Lancelin, & Lavigne, 2004; Le Meur, Le Callet, Barba, & Thoreau, 2006; Tatler, 2007; Zhang et al., 2008; Zhang et al., 2009; Judd et al., 2009) factors that contribute to this bias were considered. The center bias was modeled as a dynamic process that can be well approximated using a single kernel (Zhao & Koch, 2011). We built a center model that is a Gaussian function learned from the training data and multiplied it by the saliency maps to compensate for the center bias. Performance was boosted by considering such a spatial prior term (last column of Table 2). 
The Toronto dataset has been used as a benchmark in several recent publications. Table 3 compares the performance of these models. 
Table 3
 
Normalized AUC for different saliency models on the Toronto dataset. “CBM” stands for Center Bias Modeling.
Table 3
 
Normalized AUC for different saliency models on the Toronto dataset. “CBM” stands for Center Bias Modeling.
Models Itti et al. (1998) Gao et al. (2007) Bruce & Tsotsos (2009) Hou & Zhang (2008) Our model
Without CBM With CBM
nAUC 0.828 0.880 0.890 0.903 0.916 0.982
MIT dataset
Similar to Judd et al. (2009), we divided the MIT dataset into 903 training images and 100 testing image. Nonlinear integration is learned using d = 88 channels. 
Since the Toronto dataset included few frontal faces, to provide a more complete view of informative features selected by AdaBoost, on this MIT dataset we again visualized the test error as a function of iteration numbers and the feature selection process, i.e., we split the maps into groups based on their feature types and scales, and illustrated the feature type/scale selected as AdaBoost iterates (Figures 10 and 11). Different from the Toronto dataset, the most informative feature was the face channel, followed by orientation, color, and intensity. This was consistent with findings in Cerf et al. (2009) and Zhao and Koch (2011), which demonstrated that faces strongly and rapidly attract gaze, independent of any task. The discrepancy with the Toronto dataset arose from the fact that the Toronto dataset contained few frontal faces, therefore the learning algorithm could not reliably learn face-related information from the limited data. When the training data contained sufficient frontal faces (such as in the MIT, the FIFA, and the NUS datasets), the face channel was always the most important, compared with color, intensity, and orientation. Particularly, the numbers selected in the first 10 iterations were 1, 0, and 8 for the color, intensity, and orientation channels (the remaining one is the face channel). We performed the same experiments on the FIFA and NUS datasets and obtained similar results. 
Figure 10
 
Selected feature type as AdaBoost iterates (on the MIT dataset). Different from the Toronto dataset, the most informative feature is the face channel, followed by orientation, color, and intensity.
Figure 10
 
Selected feature type as AdaBoost iterates (on the MIT dataset). Different from the Toronto dataset, the most informative feature is the face channel, followed by orientation, color, and intensity.
Figure 11
 
Selected scale as AdaBoost iterates (on the MIT dataset). Selected scales are quite evenly distributed across various scales.
Figure 11
 
Selected scale as AdaBoost iterates (on the MIT dataset). Selected scales are quite evenly distributed across various scales.
Quantitative results are reported in Table 4. Though several earlier works had been aware of center bias (Tatler, 2007; Zhang et al., 2008; Judd et al., 2009) for the first time explicitly added a spatial prior and reported improved model performance. The nAUC for our AdaBoost-based models were 0.977 and 0.876, with and without spatial information modeling. In comparison, in Judd et al. (2009), the nAUCs for all features with and without the spatial prior were 0.923 and 0.859, respectively. 
Table 4
 
Performance comparisons of the linear and nonlinear integrations with and without spatial channels on the MIT dataset. Scores of the centered Gaussian model (a pure spatial model) are presented as a reference. “CBM” stands for Center Bias Modeling.
Table 4
 
Performance comparisons of the linear and nonlinear integrations with and without spatial channels on the MIT dataset. Scores of the centered Gaussian model (a pure spatial model) are presented as a reference. “CBM” stands for Center Bias Modeling.
Centered Gaussian Linear summation Linear integration with optimal weights Nonlinear integration
Without CBM With CBM Without CBM With CBM Without CBM With CBM
nAUC 0.869 0.776 0.899 0.792 0.910 0.876 0.977
NSS 1.07 0.635 1.19 0.725 1.24 1.17 1.83
EMD 3.56 4.73 3.04 4.53 2.88 3.56 2.24
NUS dataset
Lastly, we conducted experiments on the NUS dataset, using 500 images for training and the remaining for testing. We again included d = 88 channels for learning nonlinear integration. 
Despite the considerably richer semantic contents in this dataset, the conclusions from the experiments were consistent with those from the previous three datasets (Table 5): the performance of the saliency model was consistently improved by a nonlinear feature integration and a center bias model. 
Table 5
 
Quantitative comparisons of seven models on the NUS dataset. “CBM” stands for Center Bias Modeling.
Table 5
 
Quantitative comparisons of seven models on the NUS dataset. “CBM” stands for Center Bias Modeling.
Centered Gaussian Linear summation Linear integration with optimal weights Nonlinear integration
Without CBM With CBM Without CBM With CBM Without CBM With CBM
nAUC 0.904 0.793 0.922 0.829 0.938 0.842 0.947
NSS 1.06 0.706 1.15 0.858 1.28 0.891 1.33
EMD 3.20 4.85 3.04 4.55 2.97 4.10 2.68
General discussions and future work
Itti and Koch (1999) pointed out that one difficulty in combining different feature maps into a unique scalar saliency map is that these maps are not directly comparable, with different dynamic ranges and extraction mechanisms. For example, salient objects appearing strongly in one orientation map risk being masked by noise or less salient objects in other maps either with larger dynamic ranges or with a larger number of such maps. Several normalization schemes (Itti & Koch, 1999; Le Meur et al., 2006; Ouerhani, Bur, & Hugli, 2006; Onat, Libertus, & König, 2007) were introduced to alleviate this problem. We exploited the strategy that if signals in certain channels are not sufficiently strong, such channels should not contribute to the final saliency map. A principled framework to automatically find the thresholds and weights for different feature channels is the AdaBoost-based algorithm. Our computational experiments demonstrated the superior performance of the nonlinear compared with linear combination schemes. 
There is considerable psychophysical evidence in favor of certain features (e.g., color and luminance) contributing linearly to saliency (Treisman & Gelade, 1980; Nothdurft, 2000; Engmann et al., 2009). This raises the question of the extent to which nonlinear integration is pursued by the human visual system. 
Our work focused on early visual saliency, but the framework could incorporate other features as well. For example, text (Cerf et al., 2009), other interesting objects (Einhäuser et al., 2008), and contextual cues (Torralba, Oliva, Castelhano, & Henderson, 2006) could be added into the framework to see the correlation (Baddeley & Tatler, 2006) and relevance of different features or different categories of features in static images. Furthermore, the current studies can be generalized to dynamic scenes to develop a model to predict “spatiotemporal” deployment of gaze. 
Though not the focus of this paper, we showed, using three complementary measures, the existence of a strong center bias in all datasets. This was largely due to the experimental setup (Tatler, 2007; Zhang et al., 2008, 2009; Judd et al., 2009) and the feature distributions of the image sets (Reinagel & Zador, 1999; Parkhurst et al., 2002; Tatler, Baddeley, & Gilchrist, 2005; Einhäuser, Spain, & Perona, 2008; Judd et al., 2009). To avoid this bias, several pioneering studies (Hayhoe & Ballard, 2005; Land, 2006; Pelz et al., 2000; Schumann et al., 2008) track eye movements when observers move in the real world, avoiding many of the limitations of viewing photos on monitors. A subject ripe for further investigation is to apply the current framework using data from the unrestrained eye-tracking experiments with full-field-of-view (e.g., while subjects are walking). 
Acknowledgments
The authors would like to thank Jonathan Harel for helpful discussions. This research was supported by the NeoVision program at DARPA, by the ONR, by the G. Harold & Leila Y. Mathers Charitable Foundation, and by the WCU (World Class University) program funded by the Ministry of Education, Science and Technology through the National Research Foundation of Korea (R31-10008). 
Commercial relationships: none. 
Corresponding author: Qi Zhao. 
E-mail: qzhao@klab.caltech.edu. 
Address: California Institute of Technology, Pasadena, California, USA. 
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Figure 1
 
Illustration of the bottom-up saliency model. A sample vector for learning from one particular location x (marked in red) is shown on the right.
Figure 1
 
Illustration of the bottom-up saliency model. A sample vector for learning from one particular location x (marked in red) is shown on the right.
Figure 2
 
Fixation map illustration. (a) Original image with eye movements of one subject. (b) Fixation map of the same subject free-viewing the image shown in (a) (the first fixation is the center of the image and not included in the fixation map).
Figure 2
 
Fixation map illustration. (a) Original image with eye movements of one subject. (b) Fixation map of the same subject free-viewing the image shown in (a) (the first fixation is the center of the image and not included in the fixation map).
Figure 3
 
Illustration of the AdaBoost-based saliency model. (a) Training stage: using samples from training images, weak classifiers are trained through iterations and combined to form a strong classifier. We use a d-dimensional (a typical value of d in this work is 88) space but plot a two-dimensional one here for illustration. (b) Testing stage: for a new image, the feature vectors of image locations are calculated and input to the strong classifier to obtain saliency scores. For the two output maps shown on the right, the left map is the output of the strong classifier, where brighter regions denote more salient areas; and the right map is the result of the left map after a sigmoid transformation for illustration purposes.
Figure 3
 
Illustration of the AdaBoost-based saliency model. (a) Training stage: using samples from training images, weak classifiers are trained through iterations and combined to form a strong classifier. We use a d-dimensional (a typical value of d in this work is 88) space but plot a two-dimensional one here for illustration. (b) Testing stage: for a new image, the feature vectors of image locations are calculated and input to the strong classifier to obtain saliency scores. For the two output maps shown on the right, the left map is the output of the strong classifier, where brighter regions denote more salient areas; and the right map is the result of the left map after a sigmoid transformation for illustration purposes.
Figure 4
 
Comparisons between the linear and nonlinear algorithms. (a) Sample images from the FIFA dataset. (b) Fixation map for one subject. (c) Saliency map from the standard linear summation model. (d) Saliency map from AdaBoost learning using subject-specific data.
Figure 4
 
Comparisons between the linear and nonlinear algorithms. (a) Sample images from the FIFA dataset. (b) Fixation map for one subject. (c) Saliency map from the standard linear summation model. (d) Saliency map from AdaBoost learning using subject-specific data.
Figure 5
 
Comparisons between the linear and nonlinear algorithms. (a and b) Two sample images from the Toronto dataset with fixations (green dots) from all subjects. (c and e) Saliency maps from the standard linear summation model. (d and f) Saliency maps from AdaBoost learning.
Figure 5
 
Comparisons between the linear and nonlinear algorithms. (a and b) Two sample images from the Toronto dataset with fixations (green dots) from all subjects. (c and e) Saliency maps from the standard linear summation model. (d and f) Saliency maps from AdaBoost learning.
Figure 6
 
Test error as AdaBoost adds up to 88 weak classifiers, tested on the Toronto dataset. Performance does not increase much after 10 iterations.
Figure 6
 
Test error as AdaBoost adds up to 88 weak classifiers, tested on the Toronto dataset. Performance does not increase much after 10 iterations.
Figure 7
 
Selected feature type of AdaBoost as a function of the number of weak classifiers (on the Toronto dataset). Each iteration selects one feature in the four categories of color, intensity, orientation, and face (denoted by * in different colors). The features selected by the first 10 classifiers (shown to be the most important from Figure 6) are mostly orientation channels.
Figure 7
 
Selected feature type of AdaBoost as a function of the number of weak classifiers (on the Toronto dataset). Each iteration selects one feature in the four categories of color, intensity, orientation, and face (denoted by * in different colors). The features selected by the first 10 classifiers (shown to be the most important from Figure 6) are mostly orientation channels.
Figure 8
 
Selected scale as AdaBoost adds more and more classifiers (on the Toronto dataset). “7” is the spatial scale containing the highest spatial frequencies.
Figure 8
 
Selected scale as AdaBoost adds more and more classifiers (on the Toronto dataset). “7” is the spatial scale containing the highest spatial frequencies.
Figure 9
 
Test error as AdaBoost adds in more weak classifiers (on the MIT dataset). Considering more than 10–20 classifiers does not increase performance substantially.
Figure 9
 
Test error as AdaBoost adds in more weak classifiers (on the MIT dataset). Considering more than 10–20 classifiers does not increase performance substantially.
Figure 10
 
Selected feature type as AdaBoost iterates (on the MIT dataset). Different from the Toronto dataset, the most informative feature is the face channel, followed by orientation, color, and intensity.
Figure 10
 
Selected feature type as AdaBoost iterates (on the MIT dataset). Different from the Toronto dataset, the most informative feature is the face channel, followed by orientation, color, and intensity.
Figure 11
 
Selected scale as AdaBoost iterates (on the MIT dataset). Selected scales are quite evenly distributed across various scales.
Figure 11
 
Selected scale as AdaBoost iterates (on the MIT dataset). Selected scales are quite evenly distributed across various scales.
Table 1
 
Quantitative comparison when integrating the four channels in a linear summation, optimal linear, or nonlinear manner on the FIFA dataset. The nonlinear model outperforms the linear ones on all three measures.
Table 1
 
Quantitative comparison when integrating the four channels in a linear summation, optimal linear, or nonlinear manner on the FIFA dataset. The nonlinear model outperforms the linear ones on all three measures.
Linear summation Linear integration with optimal weights Nonlinear integration
Subject-specific General Subject-specific General
Mean SD Mean SD
nAUC 0.924 0.945 0.0136 0.944 0.959 0.0161 0.953
NSS 0.845 1.35 0.0720 1.32 1.47 0.0711 1.42
EMD 5.26 4.33 0.236 4.41 2.68 0.150 2.87
Table 2
 
Quantitative comparisons of linear and nonlinear integrations on the Toronto dataset. “CBM” stands for Center Bias Modeling.
Table 2
 
Quantitative comparisons of linear and nonlinear integrations on the Toronto dataset. “CBM” stands for Center Bias Modeling.
Linear summation Linear integration with optimal weights Nonlinear integration
Conspicuity level Raw/c-s feature level
4 channels Top 10 of 88 channels 88 channels 88 channels with CBM
nAUC 0.828 0.834 0.836 0.912 0.916 0.982
NSS 0.872 0.920 0.913 1.35 1.37 1.88
EMD 4.85 4.50 3.66 3.28 3.20 2.11
Table 3
 
Normalized AUC for different saliency models on the Toronto dataset. “CBM” stands for Center Bias Modeling.
Table 3
 
Normalized AUC for different saliency models on the Toronto dataset. “CBM” stands for Center Bias Modeling.
Models Itti et al. (1998) Gao et al. (2007) Bruce & Tsotsos (2009) Hou & Zhang (2008) Our model
Without CBM With CBM
nAUC 0.828 0.880 0.890 0.903 0.916 0.982
Table 4
 
Performance comparisons of the linear and nonlinear integrations with and without spatial channels on the MIT dataset. Scores of the centered Gaussian model (a pure spatial model) are presented as a reference. “CBM” stands for Center Bias Modeling.
Table 4
 
Performance comparisons of the linear and nonlinear integrations with and without spatial channels on the MIT dataset. Scores of the centered Gaussian model (a pure spatial model) are presented as a reference. “CBM” stands for Center Bias Modeling.
Centered Gaussian Linear summation Linear integration with optimal weights Nonlinear integration
Without CBM With CBM Without CBM With CBM Without CBM With CBM
nAUC 0.869 0.776 0.899 0.792 0.910 0.876 0.977
NSS 1.07 0.635 1.19 0.725 1.24 1.17 1.83
EMD 3.56 4.73 3.04 4.53 2.88 3.56 2.24
Table 5
 
Quantitative comparisons of seven models on the NUS dataset. “CBM” stands for Center Bias Modeling.
Table 5
 
Quantitative comparisons of seven models on the NUS dataset. “CBM” stands for Center Bias Modeling.
Centered Gaussian Linear summation Linear integration with optimal weights Nonlinear integration
Without CBM With CBM Without CBM With CBM Without CBM With CBM
nAUC 0.904 0.793 0.922 0.829 0.938 0.842 0.947
NSS 1.06 0.706 1.15 0.858 1.28 0.891 1.33
EMD 3.20 4.85 3.04 4.55 2.97 4.10 2.68
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