Q:

physiological responses to different rhythms

InspIred by Physiological basis for note durations?, I'm wondering about a more general question of how the body responds to different tempos and beat structures, and how musIic is designed to elicit those responses . I know  contemporary pop music is nearly always 4/4, around 90-100bpm, while lots of older styles, like blues, are slower, and electro/dance tends to be a lot faster.
What is it in the body that responds and causes physical and emotional effects? Is it the heart? Or the general resonant frequency often whole body, or just the chest cavity? Or something else?

A:

As far as I know, the experts say this all has to do with psychology, sensory perception and cognition, and not physiology. This is a question about the intersection of psychology and music, and even the discipline known as "music therapy". There has been a lot of research done on this sort of thing in recent years and a lot of books published on it, but it's an emerging field. There are many theories and not a lot of definitive conclusions.
A famous book that would be a starting point if you want to read up on it is This is Your Brain on Music, by Daniel Levitin, 2006.
I don't have a reference, but I also read about a fascinating study where anthropologists found members of remote tribes in the Amazon who had never heard any outside music, whether live or on recording. The anthropologists played them an assortment of Western instrumental classical and pop music and determined that 100% of the listeners were in complete agreement on which pieces conveyed the emotions of "happy" or "sad" or "peaceful" or "energetic", etc. So the researchers are just coming to grips with universal characteristics of music that seem to be interpreted the same way by all people regardless of their background or life experiences.

A:

Well, the tiny hairlike structures in your cochlea transmit impulses through the vestibocochlear cranial nerve to your cerebral cortex, then some stuff happens that we don't fully understand yet, but you experience it as sound and link it emotionally to similar experiences in your past. The only real exception I can think of is if the music you are listening to is so loud that you can feel the sound waves themselves--then basically the same thing happens except with your sense of touch instead of your sense of hearing.
There is probably some truth to the notion that tempo is linked to our physiology--intuitively, you can think of how different animals have vastly different ranges of hearing and similarly differ in their response time to other impulses. A hummingbird, for example, is clearly operating at a much higher neural "clock speed" than a human being to be able to maneuver in the way that it does. The common housefly is much more adept at evading a swatting hand than a similarly annoying colleague.
I don't have enough of a biology background to tell you if heart rate truly has anything to do with this, (my guess is no) but I can tell you that there are frequencies we can and can't experience as tempo. At the extreme ranges, a low tempo beat (with sufficient amplitude), would just feel like a rush of air in your face every three minutes. At the upper extreme, a funny thing happens: you eventually start to experience tempo as pitch!
Assuming a constant rhythmic impulse at 1,200 BPM, you will experience what is occurring as a pitch at 20 Hz, which is just near the lower range of human hearing. My point with all of this is that the very notion that humans have physiological limits to their ability to discern pitch means that there are tempos we can and can't experience as nonsense. Tempo is subjective, so to my knowledge there have not been studies done to find out the range of human tempo differentiation, but that's probably possible at some level.
Linking emotion to physiology is a slippery topic and usually not very fruitful, so I'm not going to go there. Generally you can write everything off as a social construction (and that's in the field of psychology, not physiology).

