SHANNON LECTURE LIVING INFORMATION THEORY

1. SHANNON LECTURELIVING INFORMATION THEORY Toby Berger Cornell University IEEE ISIT – LAUSANNE 4 July 2002

2. PART II NEURONS

3. NEURONS (Neurons are Magnificent!)

4. 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)

5. 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.

6. An Intriguing Recent Psychophysical Experiment A Neuron Responds to Real and Illusory Contours FromT. S. Lee and M. Nguyen, Dynamics of subjective contour formation in the early visual cortex. PNAS 98(4):1907-1911, 2001.

7. 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.

8. ARTIST’S CONCEPTION OF A NEURON

9. Glia (Astrocytes & Oligodendrocytes) Feeding and Caring for a Neuron: (Artist’s Conception)

10. NOW LET’S MEET SOME REAL NEURONS

11. Electronmicrograph of Neuron with Supporting Neuroglia

12. Neuron: Body (nucleus and organelles), dendrites, and axon

13. Adapted from Bartlett W.Mel, "Information processing in dendritic trees", Neural Computation.

14. D. Retinal ganglion cell in postnatal cat (390 um); from Maslim et al., JCN, 254:382, 1986.

15. E. Amacrine cell in retina of larval tiger salamander (160 um); from C.Y.Yang & S. Yazulla, JCN, 248:105, 1986.

16. F. Cerebellar Purkinje cell in human; from Ramony Cajal, 1909, v.1, p.61.

17. The Phases of an Action Potential

18. 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.

19. 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 104recipients before the trailing edge reaches the closest recipient.

20. 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.

21. 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!

22. END OF PART II