When might one use a scale-free neural network? - machine-learning

The structure of a feed-forward neural network is a directed acyclic network (DAG). The neural network structures, such as in a MLP, we typically see in practice have a fixed structure, where the nodes in each layer are linked to each node in the next layer.
When might a general DAG structure outperform a MLP-style structure that is comparable in some sense (eg. an MLP with the same number of weights)?
This question is inspired by biology, where neural pathways, or cell signaling pathways, often have a feed-forward topology that is more like a scale-free network than a network of stacked layers. I am certainly not the first to realize this, so I am wondering -- Where I might learn about the research and types of problems in this area?

One artificial example which you can imagine easily is when your input consists of two independent and highly complexed parts (e.g. concatenated inputs from two different sources) where you want to use this two kinds of information in order to obtain a better estimation of some output which depends on both sources. You may imagine then - that in a first few layers there is no need for connections between the parts of your network which are responsible for computations performed on different parts of your input because it will simply add some number of useless parameters what can harm your training.

Related

Methods to feed multiple images of same object to neural network for object detection

I am thinking about a toy project that would use a neural network for object recognition. Some of my objects are quite similar when viewed from one specific angle but easily distinguishable when viewed from a different angle. Thus my question:
What are methods to feed multiple images of the same object into a network? Or which network architectures exist that can take advantage of multiple images taken at different angles?
I have a good understanding of machine learning techniques but only basic understanding of neural networks. So what I am looking for here is both names of methods, techniques and other jargon that would be relevant for a google search as well as links to specific papers or articles that could be of interest.
The most common ones using multidimensional data use either multidimensional convolutions (https://keras.io/layers/convolutional/#conv3d), recurrent networks (http://www.deeplearningbook.org/contents/rnn.html) or multiple inputs, which is kinda similar to multidimensional convolutions.
Recurrent Networks handle sequences of data and the stacks of images can be seen a sequence. In contrast the multidimensional convolutions mostly exploit nearby data. Therefore it is important that the same space is highly correlated across your image stack. If this is not the case, you might want to consider using multiple inputs into your neural network.

Should the neurons in a neural network be asynchronous?

I am designing a neural network and am trying to determine if I should write it in such a way that each neuron is its own 'process' in Erlang, or if I should just go with C++ and run a network in one thread (I would still use all my cores by running an instance of each network in its own thread).
Is there a good reason to give up the speed of C++ for the asynchronous neurons that Erlang offers?
I'm not sure I understand what you're trying to do. An artificial neural network is essentially represented by the weight of the connections between nodes. The nodes themselves don't exist in isolation; their values are only calculated (at least in feed-forward networks) through the forward-propagation algorithm, when it is given input.
The backpropagation algorithm for updating weights is definitely parallelizable, but that doesn't seem to be what you're describing.
The usefulness of having neurons in a Neural Network (NN), is to have a multi-dimension matrix which coefficients you want to handle ( to train them, to change them, to adapt them little by little, so as they fit well to the problem you want to solve). On this matrix you can apply numerical methods (proven and efficient) so as to find an acceptable solution, in an acceptable time.
IMHO, with NN (namely with back-propagation training method), the goal is to have a matrix which is efficient both at run-time/predict-time, and at training time.
I don't grasp the point of having asynchronous neurons. What would it offers ? what issue would it solve ?
Maybe you could explain clearly what problem you would solve putting them asynchronous ?
I am indeed inverting your question: what do you want to gain with asynchronicity regarding traditional NN techniques ?
It would depend upon your use case: the neural network computational model and your execution environment. Here is a recent paper (2014) by Plotnikova et al, that uses "Erlang and platform Erlang/OTP with predefined base implementation of actor model functions" and a new model developed by the authors that they describe as “one neuron—one process” using "Gravitation Search Algorithm" for training:
http://link.springer.com/chapter/10.1007%2F978-3-319-06764-3_52
To briefly cite their abstract, "The paper develops asynchronous distributed modification of this algorithm and presents the results of experiments. The proposed architecture shows the performance increase for distributed systems with different environment parameters (high-performance cluster and local network with a slow interconnection bus)."
Also, most other answers here reference a computational model that uses matrix operations for the base of training and simulation, for which the authors of this paper compare by saying, "this case neural network model [ie matrix operations based] becomes fully mathematical and its original nature (from neural networks biological prototypes) gets lost"
The tests were run on three types of systems;
IBM cluster is represented as 15 virtual machines.
Distributed system deployed to the local network is represented as 15 physical machines.
Hybrid system is based on the system 2 but each physical machine has four processor cores.
They provide the following concrete results, "The presented results evidence a good distribution ability of gravitation search, especially for large networks (801 and more neurons). Acceleration depends on the node count almost linearly. If we use 15 nodes we can get about eight times acceleration of the training process."
Finally, they conclude regarding their model, "The model includes three abstraction levels: NNET, MLP and NEURON. Such architecture allows encapsulating some general features on general levels and some specific for the considered neural networks features on special levels. Asynchronous message passing between levels allow to differentiate synchronous and asynchronous parts of training and simulation algorithms and, as a result, to improve the use of resources."
It depends what you are after.
2nd Generation of Neural Networks are synchronous. They perform computations on an input-output basis without a delay, and can be trained either through reinforcement or back-propagation. This is the prevailing type of ANN at the moment and the easiest to get started with if you are trying to solve a problem via machine learning, lots of literature and examples available.
3rd Generation of Neural Networks (so-called "Spiking Neural Networks") are asynchronous. Signals propagate internally through the network as a chain-reaction of spiking events, and can create interesting patterns and oscillations depending on the shape of the network. While they model biological brains more closely they are also harder to make use of in a practical setting.
I think that async computation for NNs might prove beneficial for the (recognition) performance. In fact, the result might be similar (maybe less pronounced) to using dropout.
But a straight-forward implementation of async NNs would be much slower, because for synchronous NNs you can use linear algebra libraries, which make good use of vectorization or GPUs.

How do multiple hidden layers in a neural network improve its ability to learn?

Most neural networks bring high accuracy with only one hidden layer, so what is the purpose of multiple hidden layers?
To answer you question you first need to find the reason behind why the term 'deep learning' was coined almost a decade ago. Deep learning is nothing but a neural network with several hidden layers. The term deep roughly refers to the way our brain passes the sensory inputs (specially eyes and vision cortex) through different layers of neurons to do inference. However, until about a decade ago researchers were not able to train neural networks with more than 1 or two hidden layers due to different issues arising such as vanishing, exploding gradients, getting stuck in local minima, and less effective optimization techniques (compared to what is being used nowadays) and some other issues. In 2006 and 2007 several researchers 1 and 2 showed some new techniques enabling a better training of neural networks with more hidden layers and then since then the era of deep learning has started.
In deep neural networks the goal is to mimic what the brain does (hopefully). Before describing more, I may point out that from an abstract point of view the problem in any learning algorithm is to approximate a function given some inputs X and outputs Y. This is also the case in neural network and it has been theoretically proven that a neural network with only one hidden layer using a bounded, continuous activation function as its units can approximate any function. The theorem is coined as universal approximation theorem. However, this raises the question of why current neural networks with one hidden layer cannot approximate any function with a very very high accuracy (say >99%)? This could potentially be due to many reasons:
The current learning algorithms are not as effective as they should be
For a specific problem, how one should choose the exact number of hidden units so that the desired function is learned and the underlying manifold is approximated well?
The number of training examples could be exponential in the number of hidden units. So, how many training examples one should train a model with? This could turn into a chicken-egg problem!
What is the right bounded, continuous activation function and does the universal approximation theorem is generalizable to any other activation function rather than sigmoid?
There are also other questions that need to be answered as well but I think the most important ones are the ones I mentioned.
Before one can come up with provable answers to the above questions (either theoretically or empirically), researchers started using more than one hidden layers with limited number of hidden units. Empirically this has shown a great advantage. Although adding more hidden layers increases the computational costs, but it has been empirically proven that more hidden layers learn hierarchical representations of the input data and can better generalize to unseen data as well. By looking at the pictures below you can see how a deep neural network can learn hierarchies of features and combine them successively as we go from the first hidden layer to the one in the end:
Image taken from here
As you can see, the first hidden layer (shown in the bottom) learns some edges, then combining those seemingly, useless representations turn into some parts of the objects and then combining those parts will yield things like faces, cars, elephants, chairs and ... . Note that these results were not achievable if new optimization techniques and new activation functions were not used.

How to find dynamically the depth of a network in Convolutional Neural Network

I was looking for an automatic way to decide how many layers should I apply to my network depends on data and computer configuration. I searched in web, but I could not find anything. Maybe my keywords or looking ways are wrong.
Do you have any idea?
The number of layers, or depth, of a neural network is one of its hyperparameters.
This means that it is a quantity that can not be learned from the data, but you should choose it before trying to fit your dataset. According to Bengio,
We define a hyper-
parameter for a learning algorithm A as a variable to
be set prior to the actual application of A to the data,
one that is not directly selected by the learning algo-
rithm itself.
There are three main approaches to find out the optimal value for an hyperparameter. The first two are well explained in the paper I linked.
Manual search. Using well-known black magic, the researcher choose the optimal value through try-and-error.
Automatic search. The researcher relies on an automated routine in order to speed up the search.
Bayesian optimization.
More specifically, adding more layers to a deep neural network is likely to improve the performance (reduce generalization error), up to a certain number when it overfits the training data.
So, in practice, you should train your ConvNet with, say, 4 layers, try adding one hidden layer and train again, until you see some overfitting. Of course, some strong regularization techniques (such as dropout) is required.

Neural Network / Machine Learning memory storage

I am currently trying to set up an Neural Network for information extraction and I am pretty fluent with the (basic) concepts of Neural Networks, except for one which seem to puzzle me. It is probably pretty obvious but I can't seem to found information about it.
Where/How do Neural Networks store their memory? ( / Machine Learning)
There is quite a bit of information available online about Neural Networks and Machine Learning but they all seem to skip over memory storage. For example after restarting the program, where does it find its memory to continue learning/predicting? Many examples online don't seem to 'retain' memory but I can't imagine this being 'safe' for real/big-scale deployment.
I have a difficult time wording my question, so please let me know if I need to elaborate a bit more.
Thanks,
EDIT: - To follow up on the answers below
Every Neural Network will have edge weights associated with them.
These edge weights are adjusted during the training session of a
Neural Network.
This is exactly where I am struggling, how do/should I vision this secondary memory?
Is this like RAM? that doesn't seem logical.. The reason I ask because I haven't encountered an example online that defines or specifies this secondary memory (for example in something more concrete such as an XML file, or maybe even a huge array).
Memory storage is implementation-specific and not part of the algorithm per se. It is probably more useful to think about what you need to store rather than how to store it.
Consider a 3-layer multi-layer perceptron (fully connected) that has 3, 8, and 5 nodes in the input, hidden, and output layers, respectively (for this discussion, we can ignore bias inputs). Then a reasonable (and efficient) way to represent the needed weights is by two matrices: a 3x8 matrix for weights between the input and hidden layers and an 8x5 matrix for the weights between the hidden and output layers.
For this example, you need to store the weights and the network shape (number of nodes per layer). There are many ways you could store this information. It could be in an XML file or a user-defined binary file. If you were using python, you could save both matrices to a binary .npy file and encode the network shape in the file name. If you implemented the algorithm, it is up to you how to store the persistent data. If, on the other hand, you are using an existing machine learning software package, it probably has its own I/O functions for storing and loading a trained network.
Every Neural Network will have edge weights associated with them. These edge weights are adjusted during the training session of a Neural Network. I suppose your doubt is about storing these edge weights. Well, these values are stored separately in a secondary memory so that they can be retained for future use in the Neural Network.
I would expect discussion of the design of the model (neural network) would be kept separate from the discussion of the implementation, where data requirements like durability are addressed.
A particular library or framework might have a specific answer about durable storage, but if you're rolling your own from scratch, then it's up to you.
For example, why not just write the trained weights and topology in a file? Something like YAML or XML could serve as a format.
Also, while we're talking about state/storage and neural networks, you might be interested in investigating associative memory.
This may be answered in two steps:
What is "memory" in a Neural Network (referred to as NN)?
As a neural network (NN) is trained, it builds a mathematical model
that tells the NN what to give as output for a particular input. Think
of what happens when you train someone to speak a new language. The
human brain creates a model of the language. Similarly, a NN creates
mathematical model of what you are trying to teach it. It represents the mapping from input to output as a series of functions. This math model
is the memory. This math model is the weights of different edges in the network. Often, a NN is trained and these weights/connections are written to the hard disk (XML, Yaml, CSV etc). Whenever a NN needs to be used, these values are read back and the network is recreated.
How can you make a network forget its memory?
Think of someone who has been taught two languages. Let us say the individual never speaks one of these languages for 15-20 years, but uses the other one every day. It is very likely that several new words will be learnt each day and many words of the less frequent language forgotten. The critical part here is that a human being is "learning" every day. In a NN, a similar phenomena can be observed by training the network using new data. If the old data were not included in the new training samples, then the underlying math model will change so much that the old training data will no longer be represented in the model. It is possible to prevent a NN from "forgetting" the old model by changing the training process. However, this has the side effect that such a NN cannot learn completely new data samples.
I would say your approach is wrong. Neural Networks are not dumps of memory as we see on the computer. There are no addresses where a particular chunk of memory resides. All the neurons together make sure that a given input leads to a particular output.
Lets compare it with your brain. When you taste sugar, your tongue's taste buds are the input nodes which read chemical signals and transmit electric signals to brain. The brain then determines the taste using the various combinations of electric signals.
There are no lookup tables. There is no primary and secondary memories, only short and long term memory.

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