Time to converge

[Christof Koch]

Re. the 90 msec response time for face cells in the temporal lobe.
Most of that time is due to retinal elements. Given an average neuronal
time-constant of cortical cells of 10 msec, this does not leave any
time to iterate at all, given support to the idea that for the class
of highly-overlearned patterns, such as faces, letters, etc. the brain
essentially acts like a look-up table and does not compute in any real
sense of the worl. This fits with Poggio's RBF approach or with Bartlet Mel's
sigma-pi neurons.
The almost infinite class of objects which we see only a few times in
our life is much more interesting to investigate. Howvever, since we
can respond to these objects with say, approx. 200 msec, we don;t have
time for a lot of iterations, whether their digital cycles or analog
time-constants. This is one reason the original Marr-Poggio cooperative
stereo algoprith was so interesting, since it converged in 7-10 cycles.
 
Finally, there exists no good physiological experiment (with the exception
of loss of length inhibition in LGN; see Silito in Nature, 1989) showing
that any functional property goes away after inactivation of a higher
area. This is rather embarassing, given, for instance, the fact that
at least 10-20 times more fibers project from layer VI in area 17 to
LGN than from LGN to area 17.
 
Christof
 
P.S. That is no evidence for fast communication not involving action
potentials in the brain. The distances are too big and all relevant
biophysical mechanism except solitons too slow...
C.