[From Kent McClelland (2013.09.03.1430 EDT)]
Richard Pfau (2013.09.03 09:32 EDT) regarding Rick Marken's (2013.09.01.1205) reply to Rupert Young (2013.08.31 13.00 BST)
RY:Ch. 6, question 1 asked if you can create a sensation of pressure without effort. Rick and David said no, but I thought if you had a brick resting on your hand you could perceive pressure without effort. Am I misunderstanding something? Perhaps, the meanings of pressure and effort could be clarified..
RM: I think the idea is that you can't vary the pressure felt on the hand when you, say, press the hand against a wall, without perceiving the effort required to produce this this pressure. I'm not sure this is such a great example, though, if the goal is to demonstrate a hierarchical relationship between these perceptions (pressure higher level than effort) since we can experience pressure without effort when an object is placed on our resting hand. But maybe I'm missing something....
RP: Wouldn't effort be required to place the brick on your hand? (Effort expended either by yourself or by someone else when doing so.)
Relatedly, if you asked someone else to place the brick there, you'd be expending effort when asking them.
KM: Let me chime in with some observations.
Some other ways (besides Rupert's brick) that you could perceive pressure on your hand without the perception of effort:
(1) Simply let your hand drop until it comes to rest against some surface (the table? your leg?), and you feel the pressure of gravity pulling your hand downward against the surface. Here you're still controlling effort, I guess, but setting the reference to zero.
(2) The wind is really blowing or it's raining hard or even sleeting, and you feel the pressure of the airflow or water or ice striking your hand. Here you're not necessarily expending any muscular effort except to keep the hand in its current position, whatever that is.
(3) You're wearing a glove, and feel the weight and pressure of the fabric or leather against your hand. In all of these cases, there's some other source of physical disturbance to prompt the tactile sensation besides the actions of your own muscles.
The sensors that register tactile pressure and those that register effort are in slightly different parts of your anatomy. Within skin layers and perhaps also fat layers, for pressure, and within the muscles and tendons, for effort. I would think they must belong to two different branches of the neural hierarchy, on the input side with regard to pressure, and on the motor or output side with regard to effort.
So, while it seems obvious that feelings of pressure and of effort are both very low-order perceptions, there's no particular reason to assume that one is higher or lower than the other, at least not within the same branch of the perceptual hierarchy. In other words, perceptions of effort don't "report" to the control system units for perceptions of pressure, or vice versa. I guess I'd argue that pressure and effort may be perceived independently of each other.
What we feel as perceptions of pressure are probably second-level perceptions, i.e., sensations: a summation of the activity of tactile neurons covering a whole region of the skin. A neural current from a single neuron alone or a very small group of neurons is likely to feel more like a pinprick.
Similarly, perceptions of effort are also likely to be perceptions at the second level: a summation of the input activity of many intra-muscular neurons throughout a given muscle or group of muscles (contrary to what Bill seems to suggest in Chapter 7??).
Perceptions of pressure and perceptions of effort must combine to form certain third-level perceptions, configurations like the perception of squeezing an object.
Getting back to Leading Question 9 from chapter 4, which Rupert and Rick also discussed, about placing a book in an outstretched hand: ""How does the arm respond to the disturbance due to the book's weight? Hint: Watch the tendon in the crook of the elbow."
What Rupert said was that he didn't see anything at all, and Rick responded that he saw a slight movement of the tendon. What I'm seeing, when I do it, is a bob downward of the hand and arm receiving the book, followed by a quick return to the previous arm position. I interpret this movement as a transient loss of control as the arm-position control system encounters the large disturbance of the added weight of the book and then returns the arm to the steady-state position corresponding to its reference signal. I also see a quick stretching of the tendon, but the real action seems to be in the movement of the hand down and then up.
Best to all,
Kent