# Deep Learning for EEG-Based Preference Classification in Neuromarketing
## 3 Literature Review
### EEG Indices  
Based on our literature review, we identified four autonomic EEG indices that have been utilized for evaluating the reactions of the people in marketing stimuli: (1) the approach-withdrawal (AW) motivation index; (2) effort index; (3) choice index; and (4) valence. Such indices assist marketers in understanding customer responses to products [30; 31].  
1. **AW Index** The AW index is also known as the frontal alpha asymmetry, which indicates motivation, desire, or approach avoidance. The frontal asymmetry theory, which was initiated in 1985, states that the frontal regions of the left and right hemispheres are responsible for positive feelings (approach motivation) and negative feelings (withdrawal motivation) [29], respectively. This index can be defined as the difference between the two hemispheres in the prefrontal alpha band, that is, the relative engagement of the frontal left hemisphere compared with the right one. Positive AW values correspond to positive motivation (approach behaviors), expressed in terms of the higher activation of the left frontal cortex. In contrast, negative AW values correspond to negative motivation (avoidance behaviors), expressed in terms of the higher activation of the left frontal cortex [29; 30; 31; 32].  
Numerous researchers have demonstrated the reliability and dependability of the frontal alpha asymmetry as an effective marker in the emotion and neuromarketing research [29; 30; 31; 32; 33; 34]. [PERSON] [29] calculated the frontal alpha asymmetry scores by considering the difference  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Reference** & **Rhythms** & **Electrode Channel** \\ \hline
[24] & Beta range (16 – 18 Hz) & AFz, F2, FC1, and FCz. \\
[24] & Theta range (60 – 100 Hz) & F1, F2, F4, FC3, FC1, FCz, FC2, FC4, C5, C3, C1, C4, and CP5. \\
[28] & Theta & Fp1/Fp2, AF7/AF8, F7/F8, and F1/F2. \\
[28] & Alpha & Fp1/Fp2, AF7/AF8, F7/F8, and F1/F2. \\
[29] & Alpha & Fp3, and F4. \\
[27] & N200 & F3, C3, P3, Pz, Fz, Cz, and C4. \\ \hline \hline \end{tabular}
\end{table}
Table 3: Common rhythms/ERP and electrode positions used for the preference detection in neuromarketing.  
between the right and left power spectral densities divided by their sum, as obtained using electrodes F4 and F3.  
\[AW=\frac{alpha(F4)-alpha(F3)}{alpha(F4)+alpha(F3)} \tag{1}\]
2. **Effort Index** The effort index is defined as the frontal theta activity in the prefrontal cortex. A higher theta power in the frontal region has been linked to higher levels of task difficulty and complexity. This index acts as a sign of cognitive processing that results from mental fatigue [33], and it has been investigated extensively in neuromarketing research [3; 24; 28; 33; 35]. This factor demonstrates the importance of positive and negative emotional processing for the creation of the steady memory traces during advertising [30].
3. **Choice Index** The choice index is defined in terms of the frontal asymmetric gamma and beta oscillations, which are mostly linked to the real decision-making stage. It is also the most related element to willingness-to-pay responses, especially in the gamma band, for evaluating consumer preference and choice. Higher values of gamma and beta bands indicate a stronger activation of the left prefrontal region, and lower values are linked to relatively stronger activation of the right region [32]. [PERSON] et al. calculated the choice index for each band individually (gamma and beta) using electrodes AF3 and AF4 according to Equation (2): \[\text{Choice index}=\frac{log(AF3)-log(AF4)}{log(AF3)+log(AF4)}\] (2)
4. **Valence** Frontal asymmetry has been linked to preferences expressed as valence (i.e., the direction of a customer's emotional state). Left and right frontal activation is related to positive and negative valence, respectively. Numerous studies have supported the hypothesis that the frontal EEG asymmetry is an indicator of valence [34].