Following up after more thought, we had seen 'tongue motor imagery' (imagination of moving tongue) as clearly discernible in EEG. The motor cortex also has eye muscle mappings nearby. http://mrtnmcc.com/wp-content/uploads/2013/12/motor_imagery....

So it is conceivable that motor control of the eye or imagination of it could be detected. In the case of imagining dark or light perhaps this could affect pupil/iris dilation muscles.

What range of em spectrum is analysed and what's the sensitivity? I suspect that motor commands produce a blunt low frequency signal, while emotions and thoughts produce something barely discernible.