Or Taking the [Brain] as the Whole Being

Neuron counts and brain-to-body size per species


1:7 250000 neurons

Tree shrews



1:40 EQ: 7.4 Encephalization Quotient: brain mass relative to the average for animals of that type and size


1:78 EQ: 5 Have spindle cells like great apes


1:125 smaller than wolves

Great white sharks



In 1890, William James proposed that since adult human behavior is plastic (it exhibits adaptability to change or variety in the environment), the adult human brain may be plastic as well.

Broadly speaking, neuroplasticity refers to the ability of the brain to remap its functions to new patterns of activity, new regions or locations, or new neuronal networks throughout the individual's cerebral lifespan.

Hebbian theory

"Let us assume that the persistence or repetition of a reverberatory activity (or "trace") tends to induce lasting cellular changes that add to its stability.[…] When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased." - Donald Hebb, The Organization of Behavior (1949)

  • Also called cell assembly theory
  • Often simplified to "cells that fire together, wire together", however one cell must fire before and cause the firing of the other to fit this theory.

Spike-timing-dependent plasticity

  • In 1973, M. M. Taylor suggested that if synapses were strengthened for which a presynaptic spike occurred just before a postsynaptic spike more often than the reverse (consistent with the theory of Hebbian learning), while with the opposite timing or in the absence of a closely timed presynaptic spike, synapses were weakened (anti-Hebbian learning), the result would be an informationally efficient recoding of input patterns.
  • This suggestion was experimentally confirmed in numerous contexts throughout the 1990s, notably in the lab of Pu Muming, a pioneer of synaptic plasticity and leader of the team that produced the first primate clones.

Mirror neurons

  • neurons that fire whether the animal performs an action or whether it observes another animal performing the action
  • mimicry encoded
  • perhaps the mechanism of empathy
  • origin theory: mirror neurons facilitate action understanding and reproduction, so they are selected for in many animals.

The selfie zones

  • The nucleus accumbens releases dopamine when stimulated by self-reference.

Consciousness light-switch

  • A region in the brain called the claustrum, when stimulated, causes a patient to become medically unconscious.

Neuronal density

Dogs and raccoons have similar numbers of neurons, and cats and bears have half as many. Cats and raccoons have ten times less massive brains than bears, and dogs' brains are three times less massive than bears'. Suzana Herculano-Houzel, Dogs Have the Most Neurons, Though Not the Largest Brain: Trade-Off between Body Mass and Number of Neurons in the Cerebral Cortex of Large Carnivoran Species