The arrival of photons on the retina is transduced thanks to rhodopsin in the rods and cones, to yield spike trains in the optic nerve (I’m simplifying, of course). The arrival of pressure waves at the hair cells in the ear are similarly transduced into spike trains in the auditory nerve, heat and pressure are transduced into yet more spike trains by subcutaneous receptors, and the presence of complex molecules in the air we breathe into our noses is transduced by a host of different transducer molecules in the nasal epithelium. The common medium of spike trains in neuronal axons is well understood, but used to be regarded as a baffling puzzle: how could spike trains that were so alike in their physical properties and patterning underlie such “phenomenally” different phenomena as sight, hearing, touch, and smell? (see Dennett 1978, for an exposure of the puzzle.) It is still extremely tempting to imagine that vision is like television, and that those spike trains get transduced “back into subjective color and sound” and so forth, but we know better, don’t we? We don’t have to strike up the little band in the brain to play the music we hear in our minds, and we don’t have to waft molecules through the cortex to be the grounds for our savoring the aroma of bacon or strawberries. There is no second transduction. And if there were, there would have to be a third transduction, back into spike trains, to account for our ability to judge and act on the basis of our subjective experiences. There might have been such triple transductions, and then there would have been a Cartesian Theater Deluxe, like the wonderful control room in the film Men in Black. But biology has been thrifty in us: it’s all done through the medium of spike trains in neurons. (I recognize that dualists of various stripes—a genus thought extinct not so many years ago—will want to dig in their heels right here. I will ignore their howls for the time being, thinking that I can dispatch them later in the argument when I provide an answer to their implied question “What else could it be?”)
So there is no MEdium into which spike trains are transduced. Spike trains are discriminated, elaborated, processed, reverberated, re-entered, combined, compared, and contrasted—but not transduced into anything else until some of them activate effectors (neuromuscular junctions, hormone releasers, and the like) which do the physical work of guiding the body through life. The rich and complex interplay between neurons, hundreds of neuromodulators, and hormones is now recognized, thanks to the persuasive work of Damasio and many others, as a central feature of cognition and not just bodily control, and one can speak of these interactions as transduction back and forth between different media (voltage differences and biochemical accumulations, for instance)—but none of these is the imagined MEdium of subjective experience.
So there just is no home in the brain for qualia as traditionally conceived. My point can be clarified by a simple comparison between two well-understood media: cinema film and digital media. First imagine showing some stone-age hunter-gatherers a movie using a portable Super-8 film projector. Amazing, they would think, but when they were then shown the frames of film up close, they would readily understand—I daresay—that this was not magic, because there were little blobs of color on each frame. (The soundtrack might still be baffling, but perhaps they would hold the film up to their ears and decide, eventually, that the sounds were just too faint for them to hear with their naked ears.) Then show them a film on a portable DVD player, and demonstrate the powers of the removable, interchangeable disks, and let them ponder the question of how such a disk managed to store all the sounds and colors they just observed on the screen. It would probably be tempting for them to declare that it must be magic—dualism, in other words. But with a little instruction, they could no doubt catch on to the idea that you don’t have to represent color with color, sound with sound. You can transduce color, sound—anything, really—into a system of patterns of differences (0s and 1s, spike trains, …) and then transduce the elements of that system back into color and sound with playback equipment. This could lay magic to rest.
I had better make my implicit claim explicit, at the risk of insulting some readers: if you think there has to be a medium in the brain (or in a dualistic mind) in which subjective colors, sounds, and aromas are rendered, you are making the stone-ager mistake. This, I have come to believe, is the stone wall separating my view from wider acceptance. People pay attention to my arguments, and then, confronted with the prospect that qualia, as traditionally conceived, are not needed to explain their subjectivity, they just dismiss the idea as extravagant. “OF COURSE there are qualia!” This thought experiment is meant to shock them: your confidence here, I am saying, is no better grounded than the imagined confidence of the stone-agers that there just have to be colors and sounds on the DVD for it to convey colors and sounds to the playback machine. A failure of imagination mistaken for an insight into necessity. “But when I have a tune running through my head, it has pitch and tempo, and the timbre of the instruments is there just as if I were listening to a live performance!” Yes, and for that to be non-magically the case, there has to be a representation of the tune that progresses more or less in real time, and that specifies pitch and timbre, but that can all be accomplished without transduction, without further rendering, in the sequence of states of neural excitation in auditory cortex.
Vision isn’t television, and audition isn’t radio. We are accustomed, now, to playback devices that do transduce the signals back into the colors and sounds from which they were transduced, but we need to take advantage of our twenty-first century sophistication and recognize that the second transduction is optional! The information is in the signal, and all that information can be processed, discriminated, translated, re-coded, simplified, embellished, categorized, tagged, adjusted, and used to guide behavior without ever being transduced back into colors and sounds (or “subjective” colors and sounds).