Download SHANNON LECTURE LIVING INFORMATION THEORY

SHANNON LECTURE
LIVING INFORMATION
THEORY
Toby Berger
Cornell University
IEEE ISIT – LAUSANNE
4 July 2002
PART II
NEURONS
NEURONS
(Neurons are Magnificent!)
It all boils down to neurons and the
connections among them.
Everything you see, hear, and feel, every
movement you make, all your thoughts,
desires, and plans - even your emotions
and personality – can be explained
ultimately in terms of patterns of activity in
networks of neurons.
Paraphrased from "The Functional Neuroanatomy of Language“ by Gregory Scott
Hickok, Department of Cognitive Sciences, UC Irvine(MIT Press, ’02, by permission)
• DOES MENTAL EXPERIENCE TRANSCEND
BRAIN TISSUE ACTIVITY?
• MAYBE, BUT SCIENTIFIC FINDINGS TO
DATE SUGGEST THAT MIND = BRAIN.
• A LOFTY, PERHAPS ATTAINABLE GOAL:
TO SOMEDAY UNDERSTAND THE
INFO-BIO OF THE BRAIN.
An Intriguing Recent
Psychophysical Experiment
A Neuron Responds to Real and Illusory Contours
From T. S. Lee and M. Nguyen, Dynamics of subjective contour formation in the early visual cortex. PNAS 98(4):1907-1911, 2001.
NEURON CARDINALITY
There are approximately 1011 neurons in the human brain.
Most of them are formed between the ages of -1/2 and +1.
Each neuron forms synapses with between 10 and 105
others, resulting in a total of circa 1015 synapses.
From age -1/2 to age +2, the number of synapses increases
at net rate of a million per second, day and night; many are
abandoned, too.
It is believed that neuron and synapse formation rates drop
rapidly after age 1 and age 2, respectively, but recent results
show that they do not drop to zero.
ARTIST’S CONCEPTION OF A NEURON
Glia (Astrocytes & Oligodendrocytes) Feeding and Caring for a Neuron:
(Artist’s Conception)
NOW LET’S
MEET SOME
REAL
NEURONS
Electronmicrograph of Neuron with Supporting Neuroglia
Neuron: Body (nucleus and organelles), dendrites, and axon
Adapted from Bartlett W.Mel, "Information processing in
dendritic trees", Neural Computation.
D. Retinal ganglion cell in postnatal cat (390
um); from Maslim et al., JCN, 254:382, 1986.
E. Amacrine cell in retina of larval tiger
salamander (160 um); from C.Y.Yang & S.
Yazulla, JCN, 248:105, 1986.
F. Cerebellar Purkinje cell in human; from
Ramony Cajal, 1909, v.1, p.61.
The Phases of an Action Potential
0
2
4
6
8
10
ms
Refractoriness Following Excitation:
Thick solid line denotes the time-varying threshold.
In absolute refractory period the neuron is unexcitable.
In relative refractory period it is excitable at a threshold
higher than normal.
SPIKE PROPAGATION TIMES
In most cases a neuron’s spikes travel to
the end of its axon in less than 2 ms.
Accordingly, each spike’s leading edge
already has been delivered to the furthest of
its 104 recipients before the trailing edge
reaches the closest recipient.
MULTICASTING
• Viewed as a communication network, the
human brain simultaneously multicasts 1011
messages that have an average of 104 recipients.
• Every 2 ms a new binary digit is delivered to
these 1011 x 104 = 1015 destinations; 2 ms later
another petabit that depends on the outcome of
processing the previous one has been multicast.
• The Internet pales by comparison.
Simplified diagram of interconnections of regions of visual cortex
V3
V4
V5
V2
(I will soon give an
information-theoretic
treatment of regions like V1
and V2, which I call
“coalitions” of neurons.)
V1
“In addition to all of the connections from V1 and V2 to V3, V4
and V5, each of these regions connects back to V1 and V2.
These seemingly backward or reentrant connections are not well
understood … Information, instead of flowing in one direction,
now flows in both directions. Thus, later levels do not simply
receive information and send it forward, but are in an intimate
two-way communication with other modules.”
= Feedback!
END OF PART II