Is synesthesia a form of mental imagery?
Synesthesia occurs when the activation of one sense modality elicits an experience in another modality. Synesthetic experiences are often referred to as imagery, and indeed both mental imagery and synesthesia occur in many sense modes and can represent forms of internally generated sensory experiences. However, they differ in prevalence, effects on perception, how voluntary they are, and probably in sites of brain activation; moreover, imagery and synesthesia are clearly differentiated by synesthetes themselves. We conclude that they are best considered as related but distinct mental processes.
Craver-Lemley, Catherine and Reeves, Adam, "Is synesthesia a form of mental imagery?" (2014). Faculty Publications. 1166.