This paper discusses novel research conducted to study the direct impact of learner's affective changes on the value of a well established EEG-mental engagement index. An acquisition protocol for recording the electrical activity of the human brain, known as electroencephalography or EEG, was implemented in a learning environment specifically constructed for emotional elicitation. Data was collected from 35 healthy subjects using 8 sensors and two video cameras. A correlation analysis of the engagement index with emotional states was conducted. Results have shown that emotional states are strongly correlated with learners' mental engagement index and that the later can be used in an educational setting to reliably predict performance.