I have been rummaging around "modern myth" web sites and news groups.
Apparently one main source of this particular myth is the "dormant cortex"
phenomenon: neurologists in the first half of the previous century
discovered that they could remove, or apply electric shocks to, large parts
of a rat's cortex, without causing any discernible behaviour. This let them
to speculate whether the rat was only using a fraction of her cortex.