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Anything that changes from a fast road to a slow one without thinking must obey a 'law of refraction'.
The law of refraction received its name from optics, but it applies much more universally. It makes no difference whether
we observe a beam of light, an avalanche of snow, or a herd of cattle that move from one kind of terrain to
another where they have to change speed. They all change direction as if they were refracted by the interface. The figure below illustrates the principle.
(The illustration is animated.Click here for a minimal strip of frames.)
The yellow lines represent the front of an object (e.g. the wavefront of the beam of light, the front of the avalanche, or the first row of the
herd of cattle) approaching the interface between 2 substrates. After the left part of the front has passed the interface, the new substrate slows it down while the rest of the front still proceeds with the original speed. As a result the front
turns into a new direction as if is was refracted like a beam of light.
Cells do not obey a 'law of refraction'.
Whatever mechanical force is supposed to explain the guidance of the cells
(e.g. differential adhesion, different local pH which may alter cortex activity, etc.), it will
ultimately have to change the speed of the cells. Let us assume, that the guidance
we observed on gold surfaces with glass 'roads' scratched into them, is due to e.g. a higher adhesion on gold than on glass.
Consequently, cells crossing over from glass to gold should slow down while obeying a law of refraction.
Accordingly, their tracks should turn towards the perpendicular line (see figure below). On the other hand, if gold would accelerate the
speed of the cells, their track would turn away from the perpendicular line. However, either way it is certain
that a cell coming from the lower right quadrant has to advance into the upper left quadrant after crossing
the border between gold and glass.
In contrast, I observed cells that acted quite differently. For example, instead if turning towards or away from the perpendicular line, the cell in the figure below
moved into a totally unexpected quadrant!.
(The illustration is animated.Click here for a minimal strip of frames.)
There are other prediction of the law of refraction that cells disobey. For example, if their angle of incidence
is 90° they would have to continue in the same direction, regardless of the difference between the substrates (see figure below).
Again, this is not what I saw actual cells do. In the example below, the cell approached at right angles but
instead of continuing its direction, it turned at 90° and followed the direction of the interface.
(The illustration is animated.Click here for a minimal strip of frames.)
Significance for cell intelligence:
If cells respond to signals rather than to exogenous forces, the forces that keep or change the direction of their bodies must be controlled from within.
Let me use a metaphor to explain how telling these experiments are. Assume a troop of soldiers marches
for a while along a paved road, and then turns into a grassy field. If the troop
changes direction as if refracted, we know that the soldiers did not use their head. Instead, they responded simply to the more slippery surface of the field and
changed direction automatically. In this case we have also reason to suspect that fences and other
mechanical forces but not their thinking had kept them on the road all along.
However, if the troop maintains its direction upon changing into the field, and even turns
and follows the curb, we know that the mechanical forces of traction do not explain the action of the soldiers
now or earlier when they followed the road.
Instead, we can infer that they were driven by an interplay between endogenous instructions and exogenous signals
that were processed by their signal integration systems. I think, the same logic applies to cells that do not obey
laws of refraction. In the case of the soldiers we know that their actions are dictated by their brain. In the case
of cells, I suppose, we have to conclude that they have one.