Earthworms have isomorphic brains! One could say, "There is only one
earthworm."
But such one-to-one mappability between individuals' brains disap-
pears very soon as you ascend in the thinking-hierarchy and the number of
neurons increases-confirming one's suspicions that there is not just one
human! Yet considerable physical similarity can be detected between dif-
ferent human brains when they are compared on a scale larger than a
single neuron but smaller than the major suborgans of the brain. What
does this imply about how individual mental differences are represented in
the physical brain? If we looked at my neurons' interconnections, could we
find various structures that could be identified as coding for specific things
I know, specific beliefs I have, specific hopes, fears, likes and dislikes I
harbor? If mental experiences can be attributed to the brain, can knowl-
edge and other aspects of mental life likewise be traced to specific locations
inside the brain, or to specific physical subsystems of the brain? This will be
a central question to which we will often return in this Chapter and the
next.
Localization of Brain Processes: An EnigmaIn an attempt to answer this question, the neurologist Karl Lashley, in a
long series of experiments beginning around 1920 and running for many
years, tried to discover where in its brain a rat stores its knowledge about
maze running, In his book The Conscious Brain, Steven Rose describes
Lashley's trials and tribulations this way:Lashley was attempting to identify the locus of memory within the cortex,
and, to do so, first trained rats to rlln mazes, and then removed various
cortical regions. He allowed the animals to recover and tested the retention of
the maze-running skills. To his surprise it was not possible to find a particular
region corresponding to the ability to remember the way through a maze.
Instead all the rats which had had cortex regions removed suffered some kind
of impairment, and the extent of the impairment was roughly proportional to
the amount of cortex taken off. Removing cortex damaged the motor and
sensory capacities of the animals, and they would limp, hop, roll, or stagger,
but somehow they always managed to traverse the maze. So far as memory
was concerned, the cortex appeared to be equipotential, that is, with all
regions of equal possible utility, Indeed, Lashley concluded rather gloomily in
his last paper "In Search of the Engram", which appeared in 1950, that the
only conclusion was that memory wa~ not possible at all.^2Curiously, evidence for the opposite point of view was being developed
in Canada at roughly the same time that Lashley was doing his last work, in
the late 1940's. The neurosurgeon Wilder Penfield was examining the
reactions of patients whose brains had been operated on, by inserting
electrodes into various parts of their exposed brains, and then using small
electrical pulses to stimulate the neuron or neurons to which the electrodes
had been attached. These pulses were similar to the pulses which come
from other neurons. What Penfield found was that stimulation of certain342 Brains and Thoughts