Hi Lucy-
As far as I can tell, every sensory event has its own JND, which is a fixed fraction of stimulus level. The JND varies between observers (or between electronic gizmos) depending on viewpoint, past history, the phase of the moon and the color of your shoes. Further, because granularity of reality occurs through creation of hierarchical structures (like one molecule of silicon dioxide on top of another to make clay, then one glob of clay on top of the other to make a brick, then one brick on top of the other etc) each level of the hierchy has a different "natural grain size" and a different "natural time association". Thus a brick lasts a few thousand years, but with a very high JND for changes, since it has to break, while a sequoia or redwood live a few thousand years and are relatively sensitive to soil changes, or the May Fly that lives only 24 hours, and is very sensitive to the motion of its swarming neighbors etc. All of this can be traced backwards to an initial eigenstate (rigorous logical commonality) at the Planck level.
When you do "something", you are involved in tens of dozens of individual component activities. Each has its own JND so that you experience 3-space as a sort of multiply overlapping sequence of minute changes that have a general flavor of seeming continuous.
Now let's look into n-space, where we are free to interact in other ways as well. I was sitting with a friend a few nights back and discussing the way plants "move". If you meditate deeply (or take a number of dangerous mental modifiers) you can reach a state in which you are sensitive to not only the 3-space of everyday experience, but additional spaces as well. (I tried to explain a way to get there by meditation on my site as Seeing in 7-Space
www.mbs-hypnoclinic.com although it's not very clear.) In that state if you relax away your preconceptions and simply look at a plant, a rose, a bromiliad, ferns or various other flora, you get two impressions. The 3-space impression is that it's sitting still, and the n-space perception is that it is growing, uncurling and variously wiggling and moving. At this stage, you now have reached the JND for multi-dimensional vision. It has to be multi-dimensional, because a 3-space camera can't see it, even though you can.
Because the nature of dimensions, for purely formal logical reasons having to do with generating them ex nihilo in the Beginning, is an hierarchical series of Lie congruent eigenstates (heaps of heaps of heaps that all have the Beginning in common) the next eigensystem has 7 dimensions. Three of those are everyday 3-space, and the other 4 are new. We can add one, two or three of the "new dimensions" to everyday experience so as to obtain four 4-space (physical, not space-time) dimensional systems, and we can inhabit all of them all of the time. As an example, my wife told me that when she meditates she sees four quasi-independent realities extending in front of her. I found them too. I also find that I can manipulate these by moving in 3-space, or in other ways, not in 3-space.
So here we are in a 3-space world, from which we are easily capable of extension into numerically higher spaces. In these other spaces we can move in many dimensions at once. Each motion has its own sensory JND, each object in these spaces has a quantum granularity etc. Each of these can be eventually traced back to its logical roots in the Beginning at the Planck level (or some equivalent, if you are in a different space). That's your GMT constant.
I personally think that kids are wide open to all of this when they are born, but get talked out of it as they get older. "Hey, Johnny, don't talk about Gran'pa Max. He's dead, so you don't really see him. Stop that." So they do. In the end all we believe in is what other people tell us is real for them. "You Mom and I live in the same world and we can both see the same things, and what we can't see doesn't exist. So stop telling us that there's a ball of light coming through the doorway. We don't see it, so it doesn't exist. - Stop talking about those daydreams and go see what's bothering the dog, making it bark so much."
My wife asked my grandadughter, "Where'd you come from?" She replied, "Mommy's tummy." My wife continued, "And before that?" The child responded, "Oh, I was a cook." Since she's a dead ringer for one of the grandparent chefs of years gone by, that kinda stopped all further conversation. Now? She has no interest in looking there.
I strongly suspect that if we were to recover the awareness of childhood we'd live in a far richer and more varied world!
d