Recently, more and more people write reviews and share
opinions on the World Wide Web, which present a wealth of
information on products and services [1]. These reviews will not
only help other users make better judgements but they are also
useful resources for manufacturers of products to keep track and
manage customer opinions [2]. However, there are large amounts
of reviews for every topic, it is difficult for a user to manually learn
the opinions of an interesting topic. Sentiment classification, which
aims to classify a text according to the expressed sentimental
polarities of opinions such as ‘
positive
’or‘
negative
’, ‘
thumb up
’or
‘
thumb down
’, ‘
favorable
’or‘
unfavorable
’ [3], can facilitate the
investigation of corresponding products or services.
In order to learn a good text classifier, a large number of labeled
reviews are often needed for training [4]. However, labeling
reviews is often difficult, expensive or time consuming [5]. On the
other hand, it is much easier to obtain a large number of unlabeled
reviews, such as the growing availability and popularity of online
review sites and personal blogs [6]. In recent years, a new
approach called semi-supervised learning, which uses large
amount of unlabeled data together with labeled data to build
better learners [7], has been developed in the machine learning
community.
There are several works have been done in semi-supervised
learning for sentiment classification, and have get competitive
performance [3,8–10]. However, most of the existing semi-
supervised learning methods are still far from satisfactory. As
shown by several researchers [11,12], deep architecture, which
composed of multiple levels of non-linear operations, is expected
to perform well in semi-supervised learning because of its
capability of modeling hard artificial intelligent tasks. Deep belief
networks (DBN) is a representative deep learning algorithm
achieving notable success for text classification, which is a directed
belief nets with many hidden layers constructed by restricted
Boltzmann machines (RBM), and refined by a gradient-descent
based supervised learning [12]. Ranzato and Szummer [13]
propose an algorithm to learn text document representations
based on semi-supervised auto-encoders that are combined to
form a deep network. Zhou et al. [10] propose a novel semi-
supervised learning algorithm to address the semi-supervised
sentiment classification problem with active learning. Socher et al.
[14] introduce a novel machine learning framework based on
recursive autoencoders for sentence-level prediction of sentiment
label distributions. Socher et al. [15] introduce the recursive neural
tensor network for semantic compositionality over a sentiment
treebank. The key issue of traditional DBN is the efficiency of
RBM training. Convolutional neural networks (CNN), which are
specifically designed to deal with the variability of two dimensional
shapes, have had great success in machine learning tasks and
represent one of the early successes of deep learning [16].
Desjardins and Bengio [17] adapt RBM to operate in a
convolutional manner, and show that the convolutional RBM
(CRBM) are more efficient than standard RBM.
CRBM has been applied successfully to a wide range of visual
and audio recognition tasks [18,19]. Though the success of CRBM
in addressing two dimensional issues, there is still no published research on the using of CRBM in textual information processing.
In this paper, we propose a novel semi-supervised learning
algorithm called active hybrid deep belief networks (AHD), to
address the semi-supervised sentiment classification problem with
deep learning. AHD is an active learning method based on deep
architecture, which the bottom layers are constructed by RBM,
and the upper layers are constructed by CRBM, then the whole
constructed deep architecture is fine tuned by a gradient-descent
based supervised learning based on an exponential loss function.