"You may see an automobile for example with your eyes, and hear its sound through your ears, but it is also within the human capacity, ideally speaking, to hear the sight of the car, and to see the sound of the car. Practically speaking these capacities have been overlooked in human development simply because the ego hit upon the present method of perception, and clung to it."
—TES4 Session 154 May 12, 1965
"Once these physical pathways are opened, the way becomes easier. The pineal, pituitary, and thalamus are important here. The salt mentioned earlier has a connection with the thyroid. The physical brains alone, the unused portions, have it within their ability, for example, to hear color, to smell sound; in other words, these portions contain among other things functions, unused mainly, that would allow you to perceive physical reality in various other fashions. These undifferentiated areas existed first before the specialized sense apparatus was uniformly adopted."
"As a species you could just as easily have smelled color rather than viewed it, you see. These portions of the brain, once activated, then allow you to switch sense impressions from one sense mechanism to another, you see."
—TES7 Session 308 December 12, 1966
"Now: when you hear this music, in your mind try to translate it into color and design. And when you think of color, then, sense its motion."
—TES9 Session 501 September 17, 1969
"Mechanisms and pathways exist, making it quite possible for you to see sound, or hear color, although that is not your primary habit at this time."
—NoPR Chapter 14: Session 653, April 4, 1973
https://synesthesia.com/blog/what-is-synesthesia/ :
"A study points out that meditators may be more aware of synesthetic experiences. It is not a surprise. The goals of meditation are to increase the sensory sensitivity."
https://youtu.be/rkRbebvoYqI