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Title: Effect of top-down connections in Hierarchical Sparse Coding

Paper content:
Hierarchical Sparse Coding (HSC) is a powerful model to efficiently represent multi-dimensional, structured data such as images.
The simplest solution to solve this computationally hard problem is to decompose it into independent layerwise subproblems.
However, neuroscientific evidence would suggest inter-connecting these subproblems as in the Predictive Coding (PC) theory, which adds top-down connections between consecutive layers.
In this study, a new model called Sparse Deep Predictive Coding (SDPC) is introduced to assess the impact of this inter-layer feedback connection.
In particular, the SDPC is compared with a Hierarchical Lasso (Hi-La) network made out of a sequence of Lasso layers.
A 2-layered SDPC and a Hi-La networks are trained on 3 different databases and with different sparsity parameters on each layer.
First, we show that the overall prediction error generated by SDPC is lower thanks to the feedback mechanism as it transfers prediction error between layers.
Second, we demonstrate that the inference stage of the SDPC is faster to converge than for the Hi-La model.
Third, we show that the SDPC also accelerates the learning process.
Finally, the qualitative analysis of both models dictionaries, supported by their activation probability, show that the SDPC features are more generic and informative.
Finding a "efficient" representation to model a given signal in a concise and efficient manner is an inverse problem that has always been central to the machine learning community.
Sparse Coding (SC) has proven to be one of the most successful methods to achieve this goal.
SC holds the idea that signals (e.g. images) can be encoded as a linear combination of few features (called atoms) drawn from a bigger set called the dictionary (Elad, 2010) .
The pursuit of optimal coding is usually decomposed into two complementary subproblems: inference (coding) and dictionary learning.
Inference consists in finding an accurate sparse representation of the input data considering the dictionaries are fixed, it could be performed using algorithms like ISTA & FISTA (Beck & Teboulle, 2009 ), Matching Pursuit (Mallat & Zhang, 1993) , Coordinate Descent (Li & Osher, 2009 ), or ADMM (Heide et al., 2015) .
Once the representation is inferred, one can learn the atoms from the data using methods like gradient descent (Rubinstein et al., 2010; Kreutz-Delgado et al., 2003; Sulam et al., 2018) , or online dictionary learning (Mairal et al., 2009a) .
Consequently, SC offers an unsupervised framework to learn simultaneously basis vectors (e.g. atoms) and the corresponding input representation.
SC has been applied with success to image restoration (Mairal et al., 2009b) , feature extraction (Szlam et al., 2010) and classification (Yang et al., 2011; Perrinet & Bednar, 2015) .
Interestingly, SC is also a field of interest for computational neuroscientists.
Olshausen & Field (1997) first demonstrated that adding a sparse prior to a shallow neural network was sufficient to account for the emergence of neurons whose Receptive Fields (RFs) are spatially localized, band-pass and oriented filters, analogous to those found in the primary visual cortex (V1) of mammals (Hubel & Wiesel, 1962) .
Because most of the SC algorithms are limited to single-layer network, they cannot model the hierarchical structure of the visual cortex.
However, few solutions have been proposed to tackle Hierarchical Sparse Coding (HSC) as a global optimization problem (Sulam et al., 2018; Makhzani & Frey, 2013; .
These methods are looking for an optimal solution of HSC without considering their plausibility in term of neuronal implementation.
Consequently, the quest for reliable HSC formulation that is compatible with a neural implementation remains open.
Rao & Ballard (1999) introduce the Predictive Coding (PC) to model the effect of the interaction of cortical areas in the visual cortex.
PC intends to solve the inverse problem of vision by combining feedforward and feedback connections.
In PC, feedback connection carries prediction of the neural activity of the lower cortical area while feedforward pass prediction error to the higher cortical area.
In such a framework, neural population are updated to minimize the unexpected component of the neural signal (Friston, 2010) .
PC has been applied for supervised object recognition Spratling, 2017) or unsupervised prediction of future video frames (Lotter et al., 2016) .
Interestingly, PC is flexible enough to introduce a sparse prior to each layer.
Therefore, one can consider PC as a bio-plausible formulation of the HSC problem.
This formulation is to confront with the other bio-plausible HSC formulation that consists of a stack of independent Lasso problems (Sun et al., 2017) .
To the best of our knowledge, no study has compared these two mathematically different formulations of the same problem of optimizing the Hierarchical Sparse Coding of images.
What is the effect of top-down connection of PC?
What are the consequences in term of computations and convergence?
What are the qualitative differences concerning the learned atoms?
The objective of this study is to experimentally answer these questions and to show that the PC framework could be successfully used for improving solutions to HSC problems.
We start our study by defining the two different mathematical formulations to solve the HSC problem: the Hierarchical Lasso (Hi-La) that consists in stacking Lasso sub-problems, and the 2-Layers Sparse Predictive Coding (2L-SPC) that leverages PC into a deep and sparse network of bi-directionally connected layers.
To experimentally compare both models, we train the 2L-SPC and Hi-La networks on 4 different databases and we vary the sparsity of each layer.
First, we compare the overall prediction error of the two models and we break it down to understand its distribution among layers.
Second, we analyze the number of iterations needed for the state variables of each network to reach their stability.
Third, we compare the convergence of both models during the dictionary learning stage.
Finally, we discuss the qualitative differences between the features learned by both networks in light of their activation probability.
What are the computational advantages of inter-layer feedback connections in hierarchical sparse coding algorithms?
We answered this question by comparing the Hierarchical Lasso (Hi-La) and the 2-Layers Sparse Predictive Coding (2L-SPC) models.
Both are identical in every respect, except that the 2L-SPC brings inter-layer feedback connections.
This extra-connection forces the internal state variables of the 2L-SPC to converge toward a trade-off between on one hand an accurate prediction passed by the lower-layer and on the other hand a facilitated predictability by the upperlayer.
Experimentally, we demonstrated on 4 different databases and for a 2-layered network that the inter-layer feedback top-down connection
(i) mitigates the overall prediction error by distributing it among layers,
(ii) accelerates the convergence towards a stable internal state and
(iii) accelerates the learning process.
Besides, we qualitatively observed that top-down connections bring contextual information that helps to extract more informative and less over-fitted features.
The 2L-SPC holds the novelty to consider Hierarchical Sparse Coding as a combination of local sub-problems that are tightly related.
This a crucial difference with CNNs that are trained by backpropagating gradients from a global loss.
To the best of our knowledge the 2L-SPC is the first one that leverage local sparse coding into a hierarchical and unsupervised algorithms (the ML-CSC from (Sulam et al., 2018 ) is equivalent to a one layer sparse coding algorithm , and the ML-ISTA from ) is trained using supervised learning).
Moreover, even if our results are robust as they hold for 4 different databases and with a large spectrum of first and second layer sparsity, further work will be conducted to generalize our results to deeper networks and different sparse coding algorithms such as Coordinate Descent or ADMM.
Further studies will show that our 2L-SPC framework could be used for practical applications like image inpainting, denoising, or image super-resolution.

assistant: text: This paper presents a study that compares techniques for Hierarchical Sparse Coding, showing that the top-down term is beneficial in reducing predictive error and can learn faster.