Bogus mathematics, (was Re: L'état de PCT, c'est moi (was ...))

[Rick Marken 2018-08-15_18:54:36]

[From Erling Jorgensen (2018.08.15 1830 EDT)]

EJ:Â I am suggesting a different PCT explanation than the statistical-

artifact/illusion one that Rick is proposing.Â

RM: The fact that the power law is an illusion can be determined without any knowledge of statistics. All you have to know is that movement – the changing positions of the wrist as the arm moves, for example – is a controlled result of action. Once you know that, you know that the power law is an unintended side effect of this controlling. That side effect is considered an “illusion” when it is taken to reveal something about the mechanism that produced the movement. Our statistical analysis just shows why something close to the 2/3 power law is found for most of the trajectories people produce. But once you know that the power law is a side effect of control you can stop studying it as a way of learning about the mechanisms that produce movement and start studying movement as a control phenomenon, which will involve figuring out what perceptual variables are being controlled when people produce movements.Â




Richard S. MarkenÂ

"Perfection is achieved not when you have nothing more to add, but when you
have nothing left to take away.�
                --Antoine de Saint-Exupery

EJ: So when someone scribbles between two points, it’s not known whether the

deceleration and reversal of direction is linear, power law, or some other

function? I’m wondering whether change of direction along a line is a

simplified paradigm for then extrapolating to velocity along a curve. It

sounds like it’s been hard to specify the controlled variables for 2D



Maybe that is a good idea. There is a lot of research on point-to-point movements, and Bill never published his analysis done with the LittleMan simulation.Â


EJ: I suppose it wouldn’t work to have the Little Man follow an ellipse

target, and see what happens. If power-law dynamics are involved in mouse

movements to get the target going in an ellipse, that seems like putting the

dynamics into the reference itself. Maybe Bruce Abbott is the one to ask

here. Can a point moving in an ellipse be programmed as a target in Little

Man V2, for instance at a constant speed rather than a power-law varying one?


Yes, you can also take a position control model (even without arm dynamics simulated) and see what happens. I’ve been doing that, and it seems position control is not enough here, I don’t have a complete solution.


EJ: I don’t understand the “at any overall speed� portion. Your illustration

of the three ellipses with equal time-distant points showed an ellipse on the

left where the points were also equally distant spatially (beta of 1), so it

seems an ellipse can be traveled at a constant speed without it being forced

into a power law relationship. But you also emphasized, in your discussion

with Rick, that the 2/3 power law regularity emerged when the speed of the

drawing was fast enough to not deliberately counteract what may be happening

on the curves.


Movement at overall speed means that there might be variation in local speed, but the average speed of the total movement is relatively constant across cycles. People can traverse ellipses at low overall speed without following the power law, yes. For trajectories generated with orthogonal sinusoids, at any overall speed, speed will be correlated with curvature.


EJ: I’m trying to consider what happens when various repetitive phenomena –

e.g., a line between two points, a sine wave, an ellipse – are produced not

by a formula but by a living control system. The formula for an ellipse comes

out of the two radii. A drawn version of an ellipse may relate to the

endpoints of greatest inflection on the curve. I don’t have equipment to

measure those things myself, so I have to rely on what others may have done.

I am trying (perhaps simplistically) to apply step-changes in Position

control, with two articulation points, to see whether a power-law relationship

‘falls out’ of the model as an un-controlled outcome, i.e., a side effect.

EJ: From what I’ve heard so far in this discussion, this possibility is not

(yet?) ruled out.


If you have only point-to-point movement, then there is no curvature, right? So, you’d need something else controlled to see what falls out.