Our experience of the forms and forces in the world is not limited to what we can capture through the physical senses. This data is taken into account, but much more lies behind in the mind’s capabilities and the even deeper Sense that knows without necessarily relying on the physical data, what the Upanishad calls the “Sense behind the sense”. There is also an intermixing of the data of the senses, Sound can also be experienced as a form of touch through the physical pressure on the nervous envelope. It can also be perceived as color. Western psychology began to recognise these interactions during the systematic experimentation with mind-altering drugs such as Psilocybin or LSD when participants in the studies began to report the colors or solidity of sound, once the filter that fragments the sense experience was removed and they could report the complexity of the interactions occurring. Sensitive individuals, without the aid of such drugs also report the shade or feeling associated with certain sounds, and certainly the connection between scent and experience is well known with its ability to call forth memories that include sight, hearing, taste and smell, showing that they are all linked together in the stored memory.
Sri Aurobindo observes: “Even if we examine the physical senses, say, the sense of hearing, if we observe how the underlying mind receives their action, we shall see that in their essence all the senses are in each other. That mind is not only aware of the vibration which we call sound; it is aware also of the contact and interchange between the force in the sound and the nervous force in us with which that intermixes; it is aware of the definition or form of the sound and of the complex contacts or relations which make up the form; it is aware o the essence or outwelling conscious force which constitutes and maintains the sound and prolongs its vibrations in our nervous being; it is aware of our own nervous inhalation of the vibratory discharge proceeding from the compression of force which makes, so to speak, the solidity of the sound. All these sensations enter into the sensitive reception and joy of music which is the highest physical form of this operation of force, — they constitute our physical sensitiveness to it and the joy of our nervous being in it; diminish one of them and the joy and the sensitiveness are to that extent dulled. Much more most there be this complex unity in a higher than the physical consciousness and most of all must there be unity in the highest. But the essential sense must be capable also of seizing the secret essence of all conscious being in action, in itself and not only through the results of the operation; its appreciation of these results can be nothing more than itself an outcome of this deeper sense which it has of the essence of the Thing behind its appearances.”
Sri Aurobindo, The Upanishads, Kena Upanishad and analysis, pg. 102, 142-155