# 3D Approaches and Challenges in Facial Expression Recognition Algorithms--A Literature Review
## 2 Basic Emotions and Action Units
\begin{table}
\begin{tabular}{c c l} \hline \hline
**Emotion 1** & **Action Units 2** & **Description 3** \\ \hline Anger & 4, 7, 24 & Brow lowerer, Lid tightener, Lip pressor \\ Disgust & 9, 10, 17 & Nose wrinkle, Upper lip raiser, Chir raiser \\ Fear & 1, 4, 20, 25 & Inner brow raiser, Brow lowerer, Lip stretcher, Lips part \\ Happiness & 12, 25 & Lip corner puller, Lips part \\ Sadness & 4, 15 & Brow lowerer, Lip corner depressor \\ Surprise & 1, 2, 25, 26 & Inner brow raiser, Outer brow raiser, Lips part, Jaw drop \\ \hline \hline \end{tabular}  
* [PERSON]’s six basic emotions. 2 Number of corresponding action units, and 3 related description.  
\end{table} 
Table 1: Prototypical action units (AUs) observed in each basic emotion category [1,17]. them at the beginning of his studies and proposed an expanded list of basic emotions, including a range of positive and negative emotions that are not all encoded in facial muscles. Basic emotion classification is, still today, a more popular research topic than automatic AU recognition, even though the latter is independent of interpretation and more suitable for describing spontaneous facial behaviors [11]. At first, this disparity in favor of basic emotion classification [23; 24; 25; 26; 27; 28; 29; 30] at the expense of AU recognition [31; 32; 33] in both 2D and 3D domains, partly due to lack of FACS-codes databases. The Bosphorus is the first, and the only, 3D publicly available database that contains AU annotations and provides facial action coding. [PERSON] et al. [31] proposed an AU recognition system manually labeling eight AUs in the BU-4 DFE database, but their annotations are private. Returning on the basic emotion method, in 2014 [PERSON] et al. [34] from the University of Glasgow reported new research, published in the journal _Current Biology_, intimating that there are only four basic emotions. They reached this conclusion by studying the facial muscles involved in every emotion, as well as the activation over time of each of them. The results obtained stated that while happiness and sadness are distinct over time, fear and surprise share some common signals, like wide-open eyes. Similarly, anger and disgust share the wrinkled nose. Other scientists have developed the theme of basic emotions. For example, [PERSON] [35; 36] developed a new model, called the \"wheel of emotions\", where basic emotions can be expressed at different intensities and can mix to form several emotions, as shown in Figure 1. [PERSON]'s eight basic emotions here called primary are joy, trust, fear, surprise, sadness, anticipation, anger, and disgust. Additional emotions exist, like aggressiveness, optimism, love and submission: these are all seen as a combination of primary emotions. [PERSON] [37; 38] developed the Circumplex Model of Affect, presented in Figure 2. It is a circular model divided into quadrants, to show the level of valence and arousal of emotional states. The x-axis represents the continuum between pleasant and unpleasant emotions, the y-axis represents the continuum between high and low arousal emotions, while the center of the circle represents a neutral valence and a medium level of arousal. Another theory that has gained notoriety is [PERSON]'s approach [39], in which he identified over 100 emotions and conceptualized them as a tree-structured list, as seen from Figure 3. The first layer is composed of six primary emotions (love, joy, surprise, anger, sadness, and fear) that can be branched out into different forms of feeling, and the secondary emotions are the derivation of the primary ones instead of being a combination of them. Some of these emotions were not categorized by human expression, but rather, emotional states. Figure 1: Wheel of emotions. In recent years, some research groups have tried to enlarge the number of emotions and facial expressions considered, testing their algorithm not only on basic emotions. For example, [PERSON] et al. [40] consider in their study also embarrassment, nervousness, and pain. [PERSON] et al.