nips nips2011 nips2011-261 knowledge-graph by maker-knowledge-mining

261 nips-2011-Sparse Filtering


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Author: Jiquan Ngiam, Zhenghao Chen, Sonia A. Bhaskar, Pang W. Koh, Andrew Y. Ng

Abstract: Unsupervised feature learning has been shown to be effective at learning representations that perform well on image, video and audio classification. However, many existing feature learning algorithms are hard to use and require extensive hyperparameter tuning. In this work, we present sparse filtering, a simple new algorithm which is efficient and only has one hyperparameter, the number of features to learn. In contrast to most other feature learning methods, sparse filtering does not explicitly attempt to construct a model of the data distribution. Instead, it optimizes a simple cost function – the sparsity of 2 -normalized features – which can easily be implemented in a few lines of MATLAB code. Sparse filtering scales gracefully to handle high-dimensional inputs, and can also be used to learn meaningful features in additional layers with greedy layer-wise stacking. We evaluate sparse filtering on natural images, object classification (STL-10), and phone classification (TIMIT), and show that our method works well on a range of different modalities. 1

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Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 edu Abstract Unsupervised feature learning has been shown to be effective at learning representations that perform well on image, video and audio classification. [sent-4, score-0.288]

2 In this work, we present sparse filtering, a simple new algorithm which is efficient and only has one hyperparameter, the number of features to learn. [sent-6, score-0.503]

3 In contrast to most other feature learning methods, sparse filtering does not explicitly attempt to construct a model of the data distribution. [sent-7, score-0.455]

4 Instead, it optimizes a simple cost function – the sparsity of 2 -normalized features – which can easily be implemented in a few lines of MATLAB code. [sent-8, score-0.453]

5 Sparse filtering scales gracefully to handle high-dimensional inputs, and can also be used to learn meaningful features in additional layers with greedy layer-wise stacking. [sent-9, score-0.359]

6 We evaluate sparse filtering on natural images, object classification (STL-10), and phone classification (TIMIT), and show that our method works well on a range of different modalities. [sent-10, score-0.481]

7 1 Introduction Unsupervised feature learning has recently emerged as a viable alternative to manually designing feature representations. [sent-11, score-0.362]

8 In many audio [1, 2], image [3, 4], and video [5] tasks, learned features have matched or outperformed features specifically designed for such tasks. [sent-12, score-0.627]

9 For example, the sparse RBM [6, 7] has up to half a dozen hyperparameters and an intractable objective function, making it hard to tune and monitor convergence. [sent-14, score-0.38]

10 In this work, we present sparse filtering, a new feature learning algorithm which is easy to implement and essentially hyperparameter-free. [sent-15, score-0.455]

11 Sparse filtering works by optimizing exclusively for sparsity in the feature distribution. [sent-18, score-0.408]

12 Moreover, the hyperparameter-free approach means that sparse filtering works well on a range of data modalities without the need for specific tuning on each modality. [sent-21, score-0.344]

13 This allows us to easily learn feature representations that are well-suited for a variety of tasks, including object classification and phone classification. [sent-22, score-0.508]

14 Comparison of tunable hyperparameters in various feature learning algorithms. [sent-24, score-0.322]

15 These feature learning approaches have been successfully used to learn good feature representations for a wide variety of tasks [1, 2, 3, 4, 5]. [sent-26, score-0.482]

16 However, they are also often challenging to implement, requiring the tuning of various hyperparameters; see Table 1 for a comparison of tunable hyperparameters in several popular feature learning algorithms. [sent-27, score-0.353]

17 Though ICA has only one tunable hyperparameter, it scales poorly to large sets of features or large inputs. [sent-29, score-0.312]

18 To this end, we only focus on a few key properties of our features – population sparsity, lifetime sparsity, and high dispersal – without explicitly modeling the data distribution. [sent-31, score-0.895]

19 3 Feature distributions The feature learning methods discussed in the previous section can all be viewed as generating particular feature distributions. [sent-34, score-0.362]

20 For instance, sparse coding represents each example using a few non-zero coefficients (features). [sent-35, score-0.464]

21 A feature distribution oriented approach can provide insights into designing new algorithms based on optimizing for desirable properties of the feature distribution. [sent-36, score-0.412]

22 For clarity, let us consider a feature distribution matrix over a finite dataset, where each row is a (i) feature, each column is an example, and each entry fj is the activity of feature j on example i. [sent-37, score-0.564]

23 We consider the following as desirable properties of the feature distribution: Sparse features per example (Population Sparsity). [sent-39, score-0.41]

24 This notion is known as population sparsity [13, 14] and is considered a principle adopted by the early visual cortex as an efficient means of coding. [sent-43, score-0.363]

25 ICA is unable to learn overcomplete feature representations unless one resorts to extremely expensive approximate orthogonalization algorithms [12]. [sent-45, score-0.426]

26 This property is known as lifetime sparsity [13, 14]. [sent-50, score-0.427]

27 Concretely, we consider the mean squared activations of each feature obtained by averaging the squared values in the feature matrix across the columns (examples). [sent-53, score-0.449]

28 While high dispersal is not strictly necessary for good feature representations, we found that enforcing high dispersal prevents degenerate situations in which the same features are always active [14]. [sent-55, score-1.053]

29 For overcomplete representations, high dispersal translates to having fewer “inactive” features. [sent-56, score-0.315]

30 As an example, principle component analysis (PCA) codes do not generally satisfy high dispersal since the codes that correspond to the largest eigenvalues are almost always active. [sent-57, score-0.349]

31 For instance, [14] showed that population sparsity and lifetime sparsity are not necessarily correlated. [sent-59, score-0.762]

32 For example, the sparse RBM [6] works by constraining the expected activation of a feature (over its lifetime) to be close to a target value. [sent-62, score-0.556]

33 , each basis has unit norm) that normalize each feature, and further optimizes for the lifetime sparsity of the features it learns. [sent-65, score-0.753]

34 Sparse autoencoders [16] also explicitly optimize for lifetime sparsity. [sent-66, score-0.369]

35 On the other hand, clustering-based methods such as k-means [17] can be seen as enforcing an extreme form of population sparsity where each cluster centroid corresponds to a feature and only one feature is allowed to be active per example. [sent-67, score-0.824]

36 Sparse coding [11] is also typically seen as enforcing population sparsity. [sent-69, score-0.42]

37 In this work, we use the feature distribution view to derive a simple feature learning algorithm that solely optimizes for population sparsity while enforcing high dispersal. [sent-70, score-0.816]

38 In our experiments, we found that realizing these two properties was sufficient to allow us to learn overcomplete representations; we also argue later that these two properties are jointly sufficient to ensure lifetime sparsity. [sent-71, score-0.336]

39 4 Sparse filtering In this section, we will show how the sparse filtering objective captures the aforementioned principles. [sent-72, score-0.322]

40 Concretely, let (i) (i) fj represent the j th feature value (rows) for the ith example (columns), where fj = wj T x(i) . [sent-74, score-0.524]

41 Specifically, we first normalize each feature to be equally active by dividing each feature by its 2 norm across all examples: ˜j = fj / fj 2 . [sent-76, score-0.797]

42 The normalized features are optimized f f f for sparseness using the 1 penalty. [sent-78, score-0.333]

43 For a dataset of M examples, this gives us the sparse filtering objective (Eqn. [sent-79, score-0.322]

44 2 (1) 1 Optimizing for population sparsity The term ˆ(i) f ˜(i) f ˜(i) f measures the population sparsity of the features on the ith example. [sent-82, score-0.899]

45 Since the normalized features ˆ(i) are constrained to lie on the unit 2 -ball, this objective is f 1 = 2 1 3 minimized when the features are sparse (Fig. [sent-83, score-0.827]

46 Notice that the sparseness of the features (in the 1 sense) is maximized when the examples are on the axes. [sent-89, score-0.286]

47 One property of normalizing features is that it implicitly introduces competition between features. [sent-93, score-0.291]

48 Since we are minimizing ˆ(i) 1 , the objective encourages f the normalized features, ˆ(i) , to be sparse and mostly close to zero. [sent-97, score-0.369]

49 j fj /F This measure is commonly used to characterize the sparsity of neuron activations in the brain. [sent-101, score-0.36]

50 2 Optimizing for high dispersal Recall that for high dispersal we want every feature to be equally active. [sent-104, score-0.723]

51 Specifically, we want the mean squared activation of each feature to be roughly equal. [sent-105, score-0.283]

52 In our formulation of sparse filtering, we first normalize each feature so that they are equally active by dividing each feature by its norm across the examples: ˜j = fj / fj 2 . [sent-106, score-1.096]

53 This has the same effect as constraining each feature to have f (i) the same expected squared value, Ex(i) ∼D [(fj )2 ] = 1, thus enforcing high dispersal. [sent-107, score-0.308]

54 3 Optimizing for lifetime sparsity We found that optimizing for population sparsity and enforcing high dispersal led to lifetime sparsity in our features. [sent-109, score-1.569]

55 To understand how lifetime sparsity is achieved, first notice that a feature distribution which is population sparse must have many non-active (zero) entries in the feature distribution matrix. [sent-110, score-1.277]

56 Therefore, every feature must have a significant number of zero entries and be lifetime sparse. [sent-112, score-0.46]

57 This implies that optimizing for population sparsity and high dispersal is sufficient to define a good feature distribution. [sent-113, score-0.824]

58 4 Deep sparse filtering Since the sparse filtering objective is agnostic about the method which generates the feature matrix, one is relatively free to choose the feedforward network that computes the features. [sent-115, score-0.777]

59 In this way, sparse filtering presents itself as a natural framework for training deep networks. [sent-119, score-0.391]

60 Training a deep network with sparse filtering can be achieved using the canonical greedy layerwise approach [7, 19]. [sent-120, score-0.38]

61 In particular, after training a single layer of features with sparse filtering, one can compute the normalized features ˆ(i) and then use these as input to sparse filtering for learning f another layer of features. [sent-121, score-1.34]

62 In practice, we find that greedy layer-wise training with sparse filtering learns meaningful representations on the next layer (Sec. [sent-122, score-0.571]

63 5 Experiments (i) In our experiments, we adopted the soft-absolute function fj = T T + (wj x(i) )2 ≈ |wj x(i) | as our activation function, setting = 10−8 , and used an off-the-shelf L-BFGS [20] package to optimize the sparse filtering objective until convergence. [sent-125, score-0.548]

64 1 Timing and scaling up Figure 2: Timing comparisons between sparse coding, ICA, sparse autoencoders and sparse filtering over different input sizes. [sent-127, score-0.941]

65 In this section, we examine the efficiency of the sparse filtering algorithm by comparing it against ICA, sparse coding, and sparse autoencoders. [sent-128, score-0.822]

66 For each image size, we learned a complete set of features (i. [sent-131, score-0.355]

67 We implemented sparse autoencoders as described in Coates et al. [sent-134, score-0.393]

68 However, with 32 × 32 image patches (3072-dimensional inputs), sparse coding, sparse autoencoders and ICA were significantly slower to converge than sparse filtering (Fig. [sent-138, score-1.045]

69 For ICA, each iteration of the algorithm (FastICA [12]) requires orthogonalizing the bases learned; since the cost of orthogonalization is cubic in the number of features, the algorithm can be very slow when the number of features is large. [sent-140, score-0.288]

70 For sparse coding, as the number of features increased, it took significantly longer to solve the 1 -regularized least squares problem for finding the coefficients. [sent-141, score-0.503]

71 5 We obtained an overall speedup of at least 4x over sparse coding and ICA when learning features from 32 × 32 image patches. [sent-142, score-0.756]

72 In contrast to ICA, optimizing the sparse filtering objective does not require the expensive cubic-time whitening step. [sent-143, score-0.408]

73 For the larger input dimensions, sparse coding and sparse autoencoders did not converge in a reasonable time (<3 hours). [sent-144, score-0.857]

74 2 Natural images In this section, we applied sparse filtering to learn features off 200,000 randomly sampled patches (16x16) from natural images [9]. [sent-146, score-0.637]

75 The first layer of features learned by sparse filtering corresponded to Gabor-like edge detectors, similar to those learned by standard sparse feature learning methods [6, 9, 10, 11, 16]. [sent-150, score-1.205]

76 More interestingly, when we learned a second layer of features using greedy layer-wise stacking on the features produced by the first layer, it discovers meaningful features that pool the first layer features (Fig. [sent-151, score-1.288]

77 We highlight that the second layer of features were learned using the same algorithm without any tuning or preprocessing of the data. [sent-153, score-0.444]

78 3 Figure 3: Learned pooling units in a second layer using sparse filtering. [sent-156, score-0.395]

79 We show the most strongly connected first layer units for each second layer unit; each column corresponds to a second layer unit. [sent-157, score-0.363]

80 To obtain features from the large image, we followed the protocol of [17]: features were extracted densely from all locations in each image and later pooled into quadrants. [sent-174, score-0.586]

81 For a fair comparison, the number of features learnt was also set to be consistent with the number of features used by [17]. [sent-179, score-0.458]

82 In accordance with the recommended STL-10 testing protocol [17], we performed supervised training on each of the 10 supervised training folds and reported the mean accuracy on the full test set along with the standard deviation across the 10 training folds (Table 2). [sent-180, score-0.325]

83 Random weight baselines have been shown to perform remarkably well on a variety of tasks [23], and provide a means of distinguishing the effect of our divisive normalization scheme versus the effect of feature learning. [sent-185, score-0.451]

84 Test accuracy for phone classification using features learned from MFCCs. [sent-188, score-0.448]

85 Using sparse filtering, we learned 256 features from contiguous groups of 11 MFCC frames. [sent-205, score-0.566]

86 For comparison, we also learned sets of 256 features in a similar way using sparse coding [11, 30] and ICA [12]. [sent-206, score-0.756]

87 3 To evaluate the relative performances of the different feature sets (MFCC, ICA, sparse coding and sparse filtering), we used a linear SVM, choosing the regularization coefficient C by cross-validation on the development set. [sent-208, score-0.919]

88 We found that the features learned using sparse filtering outperformed MFCC features alone and ICA features; they were also competitive with sparse coding and faster to compute. [sent-209, score-1.259]

89 Using an RBF kernel [31] gave performances competitive with state-of-the-art methods when MFCCs were combined with learned sparse filtering features (Table 3). [sent-210, score-0.566]

90 In contrast, concatenating ICA and sparse coding features with MFCCs resulted in decreased performance when compared to MFCCs alone. [sent-211, score-0.719]

91 Indeed, these pipelines are built on top of feature representations that can be derived from a variety of sources, including sparse filtering. [sent-213, score-0.591]

92 Conversely, sparse filtering uses divisive normalization as an integral component of the feature learning process to introduce competition between features, resulting in population sparse representations. [sent-220, score-1.195]

93 2 Connections to ICA and sparse coding The sparse filtering objective can be viewed as a normalized version of the ICA objective. [sent-222, score-0.833]

94 In sparse filtering, we replace the objective with a normalized sparsity penalty, where the response of filters are divided by the norm of the all the filters ( W x 1 / W x 2 ). [sent-227, score-0.546]

95 Similarly, one can apply the normalization idea to the sparse coding framework. [sent-229, score-0.551]

96 In particular, 1 sparse filtering resembles the 2 sparsity penalty that has been used in non-negative matrix factorization [35]. [sent-230, score-0.514]

97 Thus, instead of the usual 1 penalty that is used in conjunction with sparse coding (i. [sent-231, score-0.527]

98 Unsupervised feature learning for audio classification using convolutional deep belief networks. [sent-253, score-0.296]

99 Linear spatial pyramid matching using sparse coding for image classification. [sent-261, score-0.527]

100 Efficient learning of sparse representations with an energy-based model. [sent-357, score-0.338]


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