  Facial micro-expressions are very brief, spontaneous facial expressions that
appear on the face of humans when they either deliberately or unconsciously
conceal an emotion. Micro-expression has shorter duration than
macro-expression, which makes it more challenging for human and machine. Over
the past ten years, automatic micro-expressions recognition has attracted
increasing attention from researchers in psychology, computer science,
security, neuroscience and other related disciplines. The aim of this paper is
to provide the insights of automatic micro-expressions and recommendations for
future research. There has been a lot of datasets released over the last decade
that facilitated the rapid growth in this field. However, comparison across
different datasets is difficult due to the inconsistency in experiment
protocol, features used and evaluation methods. To address these issues, we
review the datasets, features and the performance metrics deployed in the
literature. Relevant challenges such as the spatial temporal settings during
data collection, emotional classes versus objective classes in data labelling,
face regions in data analysis, standardisation of metrics and the requirements
for real-world implementation are discussed. We conclude by proposing some
promising future directions to advancing micro-expressions research.
