NARRATOR: INSIDE EVERY HEAD IN EVERY HOME IS THE MOST COMPLEX OBJECT WE'VE DISCOVERED IN THE UNIVERSE, THE HUMAN BRAIN.

I'VE SPENT MANY YEARS OF MY LIFE TRYING TO DECIPHER THE MYSTERIES OF THE BRAIN, AND YET I'M STILL IN AWE EVERY TIME I HOLD ONE.

AND THAT'S BECAUSE ALTHOUGH THIS MARVEL OF BIOLOGY SEEMS SO ALIEN TO US, SOMEHOW, IT IS US.

UNTIL RECENTLY, ACTIVITY COURSED THROUGH THESE CELLS.

THIS WAS BARBARA.

SHE HAD OPINIONS AND PASSIONS, SHE LOVED, SHE HAD HER OWN LIFE, AND THIS IS WHERE ALL OF THAT HAPPENED, JUST AS IT DOES FOR EACH ONE OF US.

THIS 3-POUND ORGAN IS MADE UP OF HUNDREDS OF BILLIONS OF CELLS WITH A QUADRILLION CONNECTIONS BETWEEN THEM.

THESE CELLS FIRE TRILLIONS OF ELECTROCHEMICAL SIGNALS EVERY SECOND OF YOUR LIFE.

SOMEHOW, ALL THIS WET BIOLOGICAL STUFF RESULTS IN THE EXPERIENCE OF BEING YOU.

I'M GOING TO EXPLORE A FUNDAMENTAL QUESTION ABOUT OUR LIVES.

WHAT SHAPES WHO YOU BECOME?

THIS IS THE STORY OF HOW YOUR LIFE SHAPES YOUR BRAIN, AND HOW YOUR BRAIN SHAPES YOUR LIFE.

MALE ANNOUNCER: "THE BRAIN WITH DAVID EAGLEMAN" WHAT MAKES YOU YOU?

FOR A LONG TIME, THE ANSWER WAS AN IMMORTAL SOUL, OR SPIRIT: SOMETHING THAT GOES BEYOND MERE MATTER AND GIVES YOU YOUR LIFE AND YOUR IDENTITY.

BUT THE MODERN STUDY OF THE BRAIN TELLS A DIFFERENT STORY: WHO WE ARE CAN ONLY BE UNDERSTOOD IN TERMS OF THE 3-POUND ORGAN IN OUR HEADS.

THE STORY OF BECOMING YOU BEGINS WITH A REMARKABLE FACT.

WE ARE BORN UTTERLY HELPLESS, AND THIS HELPLESSNESS LASTS LONGER IN HUMANS THAN IN ANY OTHER SPECIES.

COMPARE HUMAN BABIES TO OUR COUSINS ACROSS THE ANIMAL KINGDOM.

WHAT'S MUMMY DOING?

MANY NEWBORN ANIMALS ARRIVE READY FOR THE WORLD.

THEY HAVE LIFE SKILLS BUILT IN RIGHT FROM BIRTH.

A BABY ZEBRA CAN RUN WHEN IT'S JUST 45 MINUTES OLD.

BABY GIRAFFES LEARN HOW TO STAND WITHIN HOURS.

DOLPHINS ARE BORN SWIMMING.

NOW, THAT WOULD SEEM TO BE AN ADVANTAGE, BUT PUT ANY OF THESE ANIMALS IN AN ECOSYSTEM NOT TAILORED TO THEM, AND THEY WON'T SURVIVE.

OH, LOOK AT THAT ANIMAL.

WHAT IS THAT?

IN CONTRAST, MY SON ARRI IS TWO AND HE'S STILL DEPENDENT ON ME, BUT ONE DAY, HE COULD LIVE IN ALASKA OR IN THE SAHARA OR ON THE MOON.

IN OTHER WORDS, HE CAN ADAPT TO ANY ENVIRONMENT.

ALL RIGHT.

THAT WAS FUN, HUH?

AND THAT'S THANKS TO THE UNIQUE AND SPECTACULAR WAY THAT THE HUMAN BRAIN CAN MOLD TO FIT THE WORLD AROUND IT.

HUMANS COME TO THE TABLE PRE-PROGRAMMED FOR CERTAIN THINGS, LIKE ABSORBING LANGUAGE OR MIMICKING FACIAL EXPRESSIONS, BUT THE THING THAT'S REALLY REMARKABLE ABOUT HUMANS IS THE DEGREE TO WHICH THEIR BRAINS ARE UNFINISHED, AND THIS LEADS TO A PERIOD OF PROLONGED HELPLESSNESS.

BUT THE PLAN IS SIMPLE.

INSTEAD OF HARD-WIRING EVERYTHING THE WAY A RHINO DOES, LET LIFE EXPERIENCE WIRE UP THE REST OF THE BRAIN.

ARRIVE WITH SOMETHING THAT'S A LITTLE BIT SLOPPY AND TUNE IT UP ON THE FLY.

[TODDLER BABBLING] EAGLEMAN: WE LEARN ON THE JOB.

IT SOUNDS RISKY, BUT THAT'S EXACTLY WHAT YOUNG HUMAN BRAINS DO.

AND IT GIVES US AN EXTRAORDINARY ADVANTAGE AS WE GROW AND LEARN AND ADAPT.

WHAT'S THE SECRET BEHIND THE FLEXIBILITY OF YOUNG BRAINS?

WELL, IT'S NOT ABOUT GROWING NEW CELLS.

THE NUMBER OF CELLS IS THE SAME IN CHILDREN AND IN ADULTS.

INSTEAD, THE SECRET LIES IN HOW THOSE CELLS ARE CONNECTED.

THIS 5-YEAR-OLD HAS ESSENTIALLY ALL OF THE BRAIN CELLS HE'S GOING TO HAVE.

BUT WHAT'S HAPPENING INSIDE HIS HEAD IS VERY DIFFERENT THAN WHAT'S HAPPENING INSIDE MINE.

IN THE BRAIN OF A NEWBORN BABY, THE NEURONS ARE ONLY STARTING TO COMMUNICATE.

BUT THEN, OVER THE FIRST TWO YEARS OF LIFE, THOSE NEURONS BEGIN CONNECTING EXTREMELY RAPIDLY, FORMING AS MANY AS TWO MILLION NEW CONNECTIONS EVERY SECOND.

BY AGE TWO, A TYPICAL NEURON HAS MORE THAN 15,000 CONNECTIONS.

THAT IS ALMOST TWICE AS MANY AS FOUND IN AN ADULT.

SO WHAT HAPPENS IN BETWEEN?

WELL, AFTER THE AGE OF TWO, THE GROWTH IS HALTED.

THE PROCESS OF BECOMING SOMEONE IS ABOUT PRUNING BACK THE POSSIBILITIES THAT ARE ALREADY PRESENT.

YOU BECOME WHO YOU ARE NOT BECAUSE OF WHAT GROWS IN YOUR BRAIN, BUT BECAUSE OF WHAT IS REMOVED.

AS WE GROW AND LEARN NEW SKILLS, WE REDUCE THE NUMBER OF CONNECTIONS IN OUR BRAIN-- IN FAVOR OF FOCUSING ON A SMALLER NUMBER OF STRONGER CONNECTIONS.

AS YOU LEARN TO READ, YOUR CIRCUITRY GETS CARVED TO INTERPRET SQUIGGLES ON A PAGE.

THE CONNECTIONS GO FROM BEING UNIVERSAL TO BEING SPECIFIC.

THOSE LINKS YOU DON'T USE, YOU LOSE.

OVER THE COURSE OF CHILDHOOD, BRAIN CIRCUITRY IS WIRED UP ACCORDING TO EXPERIENCE AND INTERACTION WITH THE ENVIRONMENT.

BUT THIS DEPENDENCE ON THE OUTSIDE WORLD IS A GAMBLE.

THE OUTSIDE WORLD WON'T ALWAYS PROVIDE WHAT A BRAIN NEEDS.

I'M GOING TO TRY... HA HA HA!

EAGLEMAN: THIS IS THE JENSEN FAMILY FROM MILWAUKEE.

CAROL, BILL, THEIR SONS TOM AND JOHN, AND DAUGHTER VICTORIA.

THESE ARE NO ORDINARY CHILDREN: ALL THREE WERE ADOPTED FROM A ROMANIAN ORPHANAGE WHEN THEY WERE JUST 4 YEARS OLD.

JOHN JENSEN: SO IT WAS 1996 WE CAME TO THE UNITED STATES, AND THEN THAT WAS WHEN I TURNED ALMOST 4, ON AUGUST 5 OF 1996.

SO, YEAH.

EAGLEMAN: IN 1989, AT THE FALL OF NICOLAI CEAUSESCU'S REGIME, THERE WERE 170,000 CHILDREN IN ROMANIAN ORPHANAGES.

THE CHILDREN WERE OFTEN KEPT IN APPALLING CONDITIONS, LEFT TO CRY WITHOUT HUMAN CONTACT.

MAN: YOU'D WALK INTO A ROOM AND YOU'D BE SURROUNDED BY LITTLE KIDS WHO YOU'VE NEVER SEEN BEFORE... AND THEY WANT TO JUMP IN YOUR ARMS OR SIT IN YOUR LAP OR HOLD YOUR HAND OR WALK OFF WITH YOU.

AND THIS SORT OF INDISCRIMINATE BEHAVIOR IS SORT OF THE HALLMARK FEATURE OF KIDS WHO HAVE GROWN UP IN AN INSTITUTION.

IT'S SO OVERWHELMING THAT YOUR TENDENCY IS TO GET VERY EMOTIONAL AND SO WE'D HAVE TO KEEP THAT IN CHECK BECAUSE YOU DIDN'T WANT TO DO THAT IN FRONT OF THE KIDS.

EAGLEMAN: DR. CHARLES NELSON WITNESSED THE CHILDREN'S BEHAVIOR AND WONDERED IF IT WENT BEYOND MERE LONELINESS OR DISTRESS.

HAD ALL THAT NEGLECT IMPACTED THE PHYSICAL STRUCTURE OF THEIR BRAINS?

NELSON: PARTICULARLY POSTNATALLY, BRAINS NEED EXPERIENCE IN ORDER TO DEVELOP.

THE DEVELOPING BRAIN IS SEEKING OUT INFORMATION AND SEEKING OUT EXPERIENCES AND IF THEY DON'T GET THOSE, THE BRAIN DOESN'T KNOW HOW TO GET WIRED UP AND BUILT CORRECTLY.

THE KIDS IN THE INSTITUTION, THEY HAVE IQs IN THE 60s AND 70s, THEIR LANGUAGE IS VERY DELAYED, THEY HAVE SEVERE ATTACHMENT PROBLEMS, AND THEY SHOW SIGNS OF AN UNDERDEVELOPED BRAIN.

THEY HAD SMALL HEADS AND THEIR EEG ACTIVITY WAS VERY REDUCED.

EAGLEMAN: DR. NELSON BEGAN LOOKING AT THE ELECTRICAL ACTIVITY IN THESE CHILDREN'S BRAINS.

HE WAS ASTOUNDED TO DISCOVER THAT THE CHILDREN HAD DRAMATICALLY REDUCED NEURAL ACTIVITY.

BUT THERE WAS MORE.

HE FOUND EVIDENCE THAT CHILDREN PLACED INTO FAMILIES BEFORE THE AGE OF TWO GENERALLY RECOVERED.

BUT FOR THOSE WHO DIDN'T LEAVE THE INSTITUTION UNTIL OLDER THAN TWO, THEIR BRAIN DEVELOPMENT WAS COMPROMISED.

NELSON: THE LACK OF EXPERIENCES LEADS THE BRAIN TO WIRE INCORRECTLY BECAUSE IT DOESN'T HAVE ANY INPUT INTO IT.

AND AS A RESULT, NEURONS DON'T KNOW WHICH OTHER NEURONS THEY SHOULD BE COMMUNICATING WITH.

EAGLEMAN: NELSON'S WORK REVEALED THAT WHEN THE BRAIN IS STARVED OF THE THINGS IT NEEDS, LIKE TOUCH, STIMULATION, LOVE, A CHILD'S DEVELOPMENT IS STUNTED.

THE ONLY RECORD THE JENSEN CHILDREN HAVE OF THEIR EARLY YEARS IS A COUPLE OF POLAROIDS.

TOM JENSEN: IN THE ORPHANAGE, WE REALLY DIDN'T SPEAK ANY, LIKE, A KNOWN LANGUAGE.

SO WHEN, LIKE, MY MOM ASKED THE TAXI DRIVER WHAT WE WERE SPEAKING, THE TAXI DRIVER WAS LIKE, "GIBBERISH."

[INDISTINCT CHATTER] EAGLEMAN: THE JENSENS WERE ALL ADOPTED AFTER THE AGE OF TWO.

TOM JENSEN: IN THE ORPHANAGE, IT'S HARD.

IT WAS ROUGH.

IT WAS, UH, IT WASN'T EASY.

UH, KIDS DIDN'T GET MUCH ATTENTION.

EAGLEMAN: ALMOST 20 YEARS HAVE PASSED SINCE THEY LEFT ROMANIA.

THEY STILL LIVE WITH THE CONSEQUENCES OF THAT EARLY NEGLECT.

CAROL JENSEN: TOM HAS ADHD, AND HE HAS LEARNING DISABILITIES, BUT NOT TO THE SAME EXTENT THAT JOHN HAS.

FOR JOHN, IT--I THINK IT STARTED OUT HARD AND IT CONTINUES TO BE HARD.

JOHN: WE WENT TO DOCTORS, PSYCHIATRISTS, PHYSICIANS, HELPING ME OUT THROUGH THE TOUGH TIMES THAT I HAVE RIGHT NOW, SO IT'S WORKING, SO I LIKE IT.

EAGLEMAN: WITH THE SUPPORT OF A LOVING FAMILY, THE JENSEN CHILDREN ARE FINDING WAYS TO COPE WITH THE SCARS OF THEIR EARLY CHILDHOOD.

WHAT WE EXPERIENCE IN OUR EARLY YEARS GOES A LONG WAY TOWARD DEFINING WHO WE BECOME.

EXPERIENCE PRUNES THE BRAIN, BUT THAT'S NOT THE ONLY THING THAT SHAPES WHO YOU ARE BECAUSE NO MATTER WHAT KIND OF LIFE YOU'VE LED, THE BRAIN IS ALSO ON A PREDETERMINED SCHEDULE DICTATED BY GENETICS.

AND THAT MEANS THERE ARE MAJOR CHANGES IN STORE.

[GIRLS SCREAMING] IN OUR TEENAGE YEARS, HORMONES COURSE AROUND OUR BODIES, CAUSING DRAMATIC PHYSICAL TRANSFORMATIONS.

BUT OUT OF SIGHT, OUR BRAINS ARE UNDERGOING EQUALLY MONUMENTAL CHANGES, ONES THAT PROFOUNDLY AFFECT HOW WE BEHAVE AND REACT TO THE WORLD AROUND US.

WE ALL INTUIT THAT TEENAGERS HAVE A DIFFERENT VIEW ON THE WORLD THAN CHILDREN OR ADULTS.

BUT WHAT'S NOT ALWAYS OBVIOUS IS THAT THE WAY TEENS SEE THE WORLD IS NOT SIMPLY A CHOICE OR AN ATTITUDE.

INSTEAD, IT'S THE CONSEQUENCE OF A CHANGING BRAIN THAT'S RIGHT ON SCHEDULE.

WHO THEY ARE RIGHT NOW IS BIOLOGICAL AND INEVITABLE.

TO GET A SENSE OF THE TEEN BRAIN AT WORK, WE'RE RUNNING AN EXPERIMENT.

WITH THE HELP OF MY GRADUATE STUDENT RICKY SAVJANI, WE'RE GOING TO RIG UP VOLUNTEERS OF DIFFERENT AGES TO A MACHINE THAT MEASURES STRESS LEVELS BY GAUGING CHANGES IN THEIR SWEAT GLANDS.

HI THERE.

I'M DR. EAGLEMAN.

THEN WE HAVE THEM SIT IN A SHOP WINDOW TO BE GAWKED AT BY PASSERSBY.

OK, CUE THE CURTAIN.

FIRST UP, AN ADULT.

LOUISA'S STRESS RESPONSE SEEMS TO BE HOLDING ABOUT STEADY, AT A PRETTY LOW LEVEL.

IT'S CLEAR THAT SHE'S RESPONDING TO PEOPLE BEING THERE BUT HER STRESS RESPONSE IS SIMPLY NOT GOING UP.

BUT IN THE SAME SITUATION, THE TEEN BRAIN RESPONDS VERY DIFFERENTLY.

EAGLEMAN: OH, WOW, THAT'S A BIG RESPONSE.

I'VE GOT TO AUTO SCALE HER HEARTBEAT 'CAUSE IT'S GONE UP SO MUCH.

YEAH.

HER GALVANIC SKIN RESPONSE IS REALLY HIGH NOW.

IT JUST KEEPS GOING UP.

IT SUGGESTS SHE'S STRESSED OUT.

EAGLEMAN: HI, ZANDER.

HOW ARE YOU?

I'M GOOD.

GOOD, GOOD.

EAGLEMAN: THE MORE THAT HE AVERTS HIS GAZE, THE HIGHER HIS STRESS RESPONSE IS GOING.

SO THIS IS PRESUMABLY HIS RESPONSE, IS TO PRETEND LIKE HE'S NOT THERE.

EAGLEMAN: WHAT WORDS WOULD YOU USE TO DESCRIBE HOW IT FELT?

GIRL: AWKWARD.

WEIRD, PRETTY MUCH.

ZANDER: IT WAS DIFFERENT, HAVING PEOPLE JUST, LIKE, STARE AT YOU AND NOT KNOWING WHAT THEY WERE THINKING.

EAGLEMAN: SO WHY THE BIG DIFFERENCE IN RESPONSE BETWEEN THE ADULTS AND THE TEENAGERS?

THE ANSWER INVOLVES AN AREA OF THE BRAIN CALLED THE MEDIAL PREFRONTAL CORTEX.

IT BECOMES ACTIVE WHEN YOU THINK ABOUT YOURSELF, ESPECIALLY THE EMOTIONAL SIGNIFICANCE OF A SITUATION TO YOURSELF.

AS ONE GROWS FROM CHILDHOOD INTO ADOLESCENCE, THE ACTIVITY IN THIS AREA RISES, PEAKING AROUND 15 YEARS OLD.

NOW, SOCIAL SITUATIONS CARRY A LOT OF EMOTIONAL WEIGHT.

IN ADULTS, THE STRESS RESPONSE FROM THAT FEELING OF BEING LOOKED AT IS RELATIVELY MODEST.

BUT IN TEENAGERS, THAT SAME EXPERIENCE CAUSES SOCIAL EMOTIONS TO GO INTO OVERDRIVE.

THE RESULT IS A STRESS RESPONSE OF HIGH INTENSITY.

IT'S NOT JUST ABOUT SELF-CONSCIOUSNESS.

THE DEVELOPMENT OF THE TEEN BRAIN HAS OTHER CONSEQUENCES...

INCLUDING POOR IMPULSE CONTROL.

PARTS OF THE PREFRONTAL CORTEX ARE STILL DEVELOPING.

THOSE PARTS ARE INVOLVED IN SIMULATING THE CONSEQUENCES OF ACTIONS, AND THAT TRANSLATES INTO GREATER RISK-TAKING.

BUT WHAT HAPPENS WHEN THINGS CALM DOWN?

ONCE THE RUSH OF OUR TEENAGE YEARS IS OVER, DO OUR BRAINS STOP CHANGING?

IS WHO WE ARE FIXED IN STONE ONCE WE REACH ADULTHOOD?

WELL, IT'S TRUE THAT MOST OF THE DRAMATIC SHIFTS IN BRAIN STRUCTURE ARE DONE BY OUR EARLY TWENTIES... AND FOR A LONG TIME RESEARCHERS THOUGHT THAT WAS THAT.

BUT I'VE COME TO LONDON TO LOOK AT A PIONEERING STUDY THAT REVEALS HOW EVEN IN ADULTHOOD, OUR BRAINS CAN UNDERGO RADICAL PHYSICAL CHANGES.

THE STUDY OF THE KNOWLEDGE IS 640 QUARTER MILE RADIUS AREAS... 24,000 STREETS AND ROADS THAT NEED TO BE LEARNT... 50,000 PLACES OF INTEREST.

EAGLEMAN: WHAT IS KNOWN AS "THE KNOWLEDGE" IS A TEST OF RECALL... OF ALL THE STREETS IN LONDON.

THE DRIVER OF EVERY BLACK CAB HAS TO PASS IT TO GET A LICENSE.

MAN: BAYSWATER ROAD FROM MARBLE ARCH, ROUND PARK LANE... EAGLEMAN: IT'S ONE OF THE WORLD'S MOST DIFFICULT FEATS OF MEMORY, AND COMPLETING IT USUALLY TAKES OVER 4 YEARS OF INTENSIVE STUDY.

MAN: PEOPLE WILL SPEND 3 TO 4 HOURS A DAY RECITING PRETEND JOURNEYS.

THAT STARTS TO MAKE THEM-- "SEE IT" IS THE TERM WE USE-- HOW TO GET AROUND LONDON.

EAGLEMAN: THE UNIQUE MENTAL CHALLENGE OF PASSING "THE KNOWLEDGE" MADE IT OF PARTICULAR INTEREST TO A GROUP OF NEUROSCIENTISTS.

THEY BEGAN DOING BRAIN SCANS OF THE DRIVERS BEFORE, DURING, AND AFTER THE RIGOROUS TRAINING.

THEY WERE INTERESTED IN AN AREA OF THE BRAIN CALLED THE POSTERIOR HIPPOCAMPUS.

IT UNDERPINS SPATIAL MEMORY.

AT THE BEGINNING, IT LOOKED JUST LIKE EVERYONE ELSE'S, BUT BY THE END OF THE TRAINING, IT HAD GROWN PHYSICALLY LARGER.

ALL THE MAP-MEMORIZING, ALL THE DRIVING, ALL THE SIMULATION OF FUTURE ROUTES-- THIS RESHAPED THEIR BRAIN ANATOMY TO MATCH THE TASK AT HAND.

THE LONGER A CABBIE HAD BEEN DOING HIS JOB, THE BIGGER THE CHANGE IN THAT BRAIN REGION.

EVEN WHEN WE'RE ADULTS, OUR BRAINS CAN STILL CHANGE.

SOMETHING THAT CAN BE SHAPED AND HOLD ON TO THAT SHAPE IS WHAT WE CALL PLASTIC, AND THAT'S HOW IT GOES WITH THE ADULT BRAIN: EXPERIENCE CHANGES IT, AND IT RETAINS THAT CHANGE.

IT'S MALLEABLE, IT HAS PLASTICITY.

THAT MEANS THAT WHO YOU ARE, AND WHO YOU CAN BE, IS A WORK IN PROGRESS.

MANY ACTIVITIES CAN CAUSE THE BRAIN TO TRANSFORM.

FOR EXAMPLE, LEARNING A MUSICAL INSTRUMENT CAN PRODUCE DRAMATIC CHANGES.

MUSICIANS CAN LEARN LANGUAGES MORE QUICKLY AND HAVE IMPROVED MEMORY AS A RESULT OF THE WAY YEARS OF PRACTICE HAVE ALTERED THEIR BRAINS.

ALBERT EINSTEIN'S BRAIN WAS EXAMINED AFTER DEATH BY RESEARCHERS.

THEY WERE HUNTING FOR SIGNS OF GENIUS.

BUT INSTEAD, THEY DISCOVERED THE BRAIN AREA DEVOTED TO OPERATING THE FINGERS OF THE LEFT HAND WAS MUCH LARGER THAN NORMAL-- ALL THANKS TO HIS LESS COMMONLY KNOWN PASSION FOR PLAYING THE VIOLIN.

HI, PADDINGTON STATION.

OK.

IN FACT, EVERYTHING WE EXPERIENCE WILL ALTER THE PHYSICAL STRUCTURE OF OUR BRAIN IN SOME WAY...MEANING THAT FOR AS LONG AS WE'RE ALIVE, OUR IDENTITIES AREN'T FIXED, BUT CONSTANTLY CHANGING.

FROM OUR JOBS TO FALLING IN LOVE, HAVING KIDS, AND SPENDING TIME WITH FRIENDS.

ALL OF THESE CHANGE THE WIRING OF YOUR BRAIN TO MAKE YOU WHO YOU ARE, AND WHO YOU CAN BECOME.

BUT THE BRAIN CAN ALSO CHANGE IN WAYS THAT WE HAVE NO CONTROL OVER... WAYS THAT CAN HAVE A TERRIBLE IMPACT ON OUR PERSONALITY AND ACTIONS.

WE SEE THIS IN A LETTER DISCOVERED BY POLICE IN THE WAKE OF A VIOLENT TRAGEDY THAT HAPPENED IN AUSTIN, TEXAS, IN THE SUMMER OF 1966.

25-YEAR-OLD CHARLES WHITMAN WAS A MODEL CITIZEN.

HE HAD BEEN AN EAGLE SCOUT, HE'D BEEN HONORABLY DISCHARGED FROM THE MILITARY.

HE WAS WORKING AS A BANK TELLER AND STUDYING AS AN ENGINEERING STUDENT, YET WHAT HE'D WRITTEN POINTED TO A DISTURBING CHANGE IN HIS PERSONALITY.

"I DON'T REALLY UNDERSTAND MYSELF THESE DAYS.

"I'M SUPPOSED TO BE AN AVERAGE, "REASONABLE, AND INTELLIGENT YOUNG MAN.

"HOWEVER, LATELY--I CAN'T RECALL WHEN IT STARTED-- "I HAVE BEEN A VICTIM OF MANY UNUSUAL AND IRRATIONAL THOUGHTS."

WHAT WHITMAN WAS DESCRIBING WERE THOUGHTS THAT WOULD LEAD TO KILLING.

NEWSREEL NARRATOR: DEATH AND TERROR STOP THE CAMPUS OF THE UNIVERSITY OF TEXAS IN AUSTIN, AS A SNIPER'S BULLETS FORCE PEOPLE TO SCURRY FOR COVER.

A STREAM OF DEADLY ACCURATE FIRE SENDS BODIES CRUMPLING TO THE GROUND EVERYWHERE... EAGLEMAN: ON AUGUST 1, 1966, WHITMAN TOOK AN ELEVATOR TO THE TOP OF THE UNIVERSITY OF TEXAS TOWER.

NEWSREEL NARRATOR: AN ARMORED TRUCK... EAGLEMAN: THERE, HE STARTED FIRING INDISCRIMINATELY AT THE PEOPLE BELOW.

NEWSREEL NARRATOR: VICTIMS WERE BEING HIT AT A RATE OF MORE THAN ONE EVERY 3 MINUTES.

EAGLEMAN: 13 PEOPLE WERE KILLED AND 33 WOUNDED, UNTIL WHITMAN HIMSELF WAS FINALLY SHOT DEAD BY THE POLICE.

NEWSREEL NARRATOR: HIS TOWER ARSENAL INCLUDED 3 RIFLES, A SHOTGUN, TWO PISTOLS, AND A KNIFE.

EAGLEMAN: WHEN POLICE REACHED HIS HOUSE, THEY DISCOVERED THAT HE'D KILLED HIS WIFE AND HIS MOTHER THE NIGHT BEFORE.

THERE WAS ONLY ONE THING MORE SURPRISING THAN THIS RANDOM ACT OF VIOLENCE, AND THAT IS THERE WAS NOTHING REALLY ABOUT CHARLES WHITMAN THAT SUGGESTED HE WOULD DO SOMETHING LIKE THIS.

IT SEEMED COMPLETELY SENSELESS.

BUT THE LETTER THEY FOUND IN HIS HOME, WRITTEN PRIOR TO THE KILLINGS, SUGGESTED A POSSIBLE EXPLANATION FOR HIS ACTIONS.

IN THE NOTE, HE MADE AN UNUSUAL REQUEST.

"AFTER MY DEATH, I WISH THAT AN AUTOPSY WOULD BE PERFORMED ON ME TO SEE IF THERE IS ANY VISIBLE PHYSICAL DISORDER."

WHITMAN'S WISH WAS GRANTED.

DURING THE AUTOPSY, THE PATHOLOGISTS FOUND THAT WHITMAN HAD A BRAIN TUMOR.

IT WAS ABOUT THE SIZE OF A NICKEL, AND IT WAS PRESSING AGAINST A PART OF HIS BRAIN CALLED THE AMYGDALA, WHICH IS INVOLVED IN FEAR AND AGGRESSION.

THIS PRESSURE ON THE AMYGDALA LED TO A CASCADE OF CONSEQUENCES IN WHITMAN'S BRAIN, RESULTING IN HIM TAKING ACTIONS THAT WOULD OTHERWISE BE COMPLETELY OUT OF CHARACTER.

HIS BRAIN MATTER HAD BEEN CHANGING, AND WHO HE WAS CHANGED WITH IT.

WHITMAN'S EXAMPLE IS EXTREME, BUT OTHER LESS DRAMATIC CHANGES IN THE BRAIN ALSO ALTER THE FABRIC OF WHO WE ARE.

PARKINSON'S DISEASE CAN LEAD SOME PEOPLE-- EVEN THE MOST DEVOUT-- TO LOSE THEIR FAITH, AND A MEDICATION FOR PARKINSON'S CAN LEAD TO COMPULSIVE GAMBLING.

AND IT'S NOT JUST DISEASE OR DRUGS THAT CAN CHANGE US.

FROM THE THINGS WE CONSUME TO THE SIMPLE PROCESS OF AGING, EVERYTHING CONTRIBUTES TO CONTINUALLY RESHAPING THE NEURAL NETWORKS THAT AMOUNT TO US.

SO WHO WE ARE CHANGES IN THE COURSE OF OUR LIFE AS OUR BRAIN CHANGES.

BUT THANKFULLY, THERE'S ONE CONSTANT THAT LINKS IT ALL TOGETHER.

IT'S A PILLAR OF OUR PERSONALITY... MEMORY.

MEMORY SITS RIGHT AT THE CORE OF OUR IDENTITY-- IT GIVES OUR LIVES A NARRATIVE, ONE THAT WE CAN SHARE, ONE THAT HAS MEANING.

BUT THERE'S A PROBLEM-- MEMORY ISN'T ALWAYS RELIABLE.

TO UNDERSTAND HOW IT ALTERS AND WHY, I'VE COME UP WITH A LITTLE THOUGHT EXPERIMENT.

MEET 35-YEAR-OLD DAISY.

SO IMAGINE DAISY COULD MEET HER 5-YEAR-OLD SELF.

SAME PERSON, SAME LIFE EXPERIENCES, A SUBSET OF HER MEMORIES.

OR WHAT IF DAISY COULD MEET HER 85-YEAR-OLD SELF, THE SAME PERSON, BUT EXPERIENCES PLAYED OUT MORE.

IMAGINE THAT DAISY COULD MEET ALL OF HER SELVES ACROSS THE SPECTRUM OF HER LIFETIME.

YOU MIGHT THINK THAT ALL THESE DAISYS WOULD SHARE THE EXACT SAME MEMORIES.

BUT ALTHOUGH THEIR MEMORIES RELATE BACK TO THE SAME EVENTS, IN FACT, WHAT THEY REMEMBER IS LIKELY TO BE QUITE DIFFERENT.

AND THAT'S BECAUSE OF WHAT A MEMORY ACTUALLY IS.

SO A FEW YEARS AGO, I WENT OUT FOR DINNER TO CELEBRATE MY FRIEND CHERYL'S BIRTHDAY WITH HER BOYFRIEND JOE.

I REMEMBER IT DISTINCTLY BECAUSE IT WAS SO ENJOYABLE.

EVERYTHING I EXPERIENCED THAT EVENING TRIGGERED PARTICULAR PATTERNS OF ACTIVITY IN MY BRAIN, LIGHTING UP CONSTELLATIONS OF CELLS.

THE CONVERSATION BETWEEN JOE AND CHERYL.

THE SMELL OF THE COFFEE.

THE TASTE OF THIS LITTLE FRENCH CAKE.

ALL OF THESE CONSTELLATIONS BECAME LINKED WITH ONE ANOTHER IN A VAST ASSOCIATIVE NETWORK OF NEURONS THAT THE HIPPOCAMPUS REPLAYED OVER AND OVER UNTIL THE ASSOCIATION BECAME FIXED, AND THAT WAS THE UNIQUE SIGNATURE OF THIS EXPERIENCE.

THAT WOULD BECOME MY MEMORY OF CHERYL'S BIRTHDAY.

SO IS A MEMORY SIMPLY LIKE WATCHING AN OLD VIDEO RECORDING THAT WE JUST CALL UP AND REPLAY?

IT FEELS LIKE THAT, BUT IN REALITY IT'S QUITE DIFFERENT.

MEMORIES ARE ACTUALLY BRAIN STATES FROM A BYGONE TIME THAT WE HAVE TO RESURRECT.

SO HERE I AM 6 MONTHS LATER, IN A TOTALLY DIFFERENT CITY, AND I TASTE ONE OF THESE LITTLE FRENCH CAKES AGAIN, AND IT'S JUST LIKE THE ONE THAT I HAD AT CHERYL'S BIRTHDAY PARTY.

AND IN MY BRAIN, THIS VERY SPECIFIC TRIGGER LIGHTS UP A WHOLE WEB OF ASSOCIATIONS, LIKE THE LIGHTS OF A CITY COMING ONLINE, AND SUDDENLY I'M BACK IN THAT MEMORY.

BUT IT'S NOT AS RICH AS I WOULD HAVE IMAGINED.

I KNOW THAT JOE AND CHERYL WERE THERE, AND CHERYL, I THINK, WAS WEARING A BLUE SHIRT.

OR MAYBE IT WAS PURPLE.

ACTUALLY, MAYBE IT WAS GREEN.

THE MEMORY OF CHERYL'S BIRTHDAY, IN MY BRAIN, HAS STARTED TO FADE.

OUR MEMORIES FADE GRADUALLY, BECAUSE OUR BRAIN ONLY HAS A FINITE NUMBER OF NEURONS, WHICH OVER TIME GET USED FOR OTHER MEMORIES.

IT MEANS THAT THE DETAILS HAVE NOW BECOME A LITTLE HAZY.

WHAT MATTERS IS THAT I REMEMBER THAT WE HAD A GREAT TIME.

BUT EVEN THAT'S NOT TOTALLY CERTAIN.

BECAUSE IN THE INTERVENING MONTHS, JOE AND CHERYL HAVE BROKEN UP.

AND SO NOW I'M WONDERING--DID I SENSE ANY RED FLAGS THERE?

THE STATE OF MY EMOTIONS RIGHT NOW CHANGES THE NETWORK THAT CORRESPONDS TO THEN.

MY PRESENT COLORS MY PAST.

WHAT THIS MEANS FOR ALL OF US IS THAT THE SAME EVENT WILL BE REMEMBERED DIFFERENTLY, DEPENDING ON WHERE YOU ARE AT THAT POINT IN YOUR LIFE.

SO HOW RELIABLE ARE OUR MEMORIES?

HOW FAR CAN THEY BE ALTERED?

AND WHY DO OUR BRAINS WORK THAT WAY?

THE FIRST SUGGESTION ABOUT HOW VULNERABLE OUR MEMORY IS CAME WITH THE GROUNDBREAKING WORK OF PROFESSOR ELIZABETH LOFTUS.

SHE DEVISED THIS SIMPLE EXPERIMENT IN WHICH VOLUNTEERS WATCHED FILMS OF CAR CRASHES.

SHE THEN ASKED THEM A SERIES OF QUESTIONS TO TEST WHAT THEY REMEMBERED.

SO IF I ASK YOU A QUESTION, YOU KNOW, HOW FAST WERE THE CARS GOING WHEN THEY HIT EACH OTHER, VERSUS HOW FAST WERE THE CARS GOING WHEN THEY SMASHED INTO EACH OTHER, WITNESSES GIVE DIFFERENT ESTIMATES OF SPEED.

THEY THINK THE CARS WERE GOING FASTER IF YOU USED THE WORD SMASHED.

AND SO THIS WAS ONE OF MY EARLIEST EXAMPLES SHOWING THAT LEADING QUESTIONS CAN DISTORT THE ANSWERS AND CAN CONTAMINATE A PERSON'S MEMORY.

EAGLEMAN: THE DISCOVERY THAT EXISTING MEMORIES COULD BE DISTORTED LED LOFTUS TO ASK A MORE RADICAL QUESTION.

WOULD IT BE POSSIBLE TO IMPLANT ENTIRELY FALSE MEMORIES?

TO FIND OUT, SHE DEVISED ANOTHER EXPERIMENT.

SHE RECRUITED VOLUNTEERS AND THEN HAD HER TEAM CALL THEIR FAMILIES FOR INFORMATION ABOUT EVENTS FROM THEIR PAST.

THE RESEARCHERS THEN PUT TOGETHER A NUMBER OF STORIES ABOUT THEIR CHILDHOODS.

3 WERE TRUE.

BUT ONE, WHILE SOUNDING PLAUSIBLE, WAS ENTIRELY MADE UP.

THE STORY INVOLVED THE VOLUNTEER GETTING LOST IN A SHOPPING MALL AS A CHILD... THEN BEING FOUND BY A KIND OLD PERSON... AND THEN REUNITED WITH HER PARENTS.

WHEN TOLD THE 4 STORIES, AT LEAST A QUARTER OF THE PARTICIPANTS CLAIMED THEY COULD REMEMBER BEING LOST IN THE MALL-- EVEN THOUGH IT NEVER HAPPENED.

...6 AT THE TIME.

MAN: ...YOU CRY OR ANYTHING LIKE THAT... YEAH, NO, I DID CRY.

CRIED A LOT.

EAGLEMAN: AND THE EXPERIMENT DIDN'T STOP THERE.

LOFTUS: THEY MAY START TO REMEMBER A LITTLE BIT ABOUT IT.

BUT WHEN THEY COME BACK A WEEK LATER, THEY'RE STARTING TO REMEMBER MORE.

MAYBE THEY'LL TALK ABOUT THE OLDER WOMAN WHO RESCUED THEM.

"I THINK I HEARD MY NAME OVER A LOUDSPEAKER."

EAGLEMAN: OVER TIME, MORE AND MORE DETAIL CREPT INTO THEIR FALSE MEMORY.

THE OLD LADY WAS WEARING THIS CRAZY HAT.

"I HAD MY FAVORITE TOY."

"MY MOM WAS SO MAD."

THE INVENTION OF THESE NEW DETAILS THAT GO WAY BEYOND ANYTHING WE PRESENTED TO THEM AS COMING FROM THEIR MOTHER WERE PRETTY IMPRESSIVE TO US.

EAGLEMAN: LOFTUS HAD DISCOVERED THAT NOT ONLY IS IT POSSIBLE TO IMPLANT ENTIRELY NEW MEMORIES IN THE BRAIN, WE NATURALLY EMBRACE AND EMBELLISH THEM-- UNKNOWINGLY WEAVING FANTASY INTO THE VERY FABRIC OF WHO WE ARE.

AND WE'RE ALL SUSCEPTIBLE, EVEN LOFTUS HERSELF.

LOFTUS: MY MOTHER HAD DROWNED WHEN I WAS 14.

AND YEARS LATER, I HAD GONE TO A BIRTHDAY CELEBRATION, AND ONE OF MY OTHER RELATIVES STARTED TO TALK ABOUT MY MOTHER.

AND THIS RELATIVE STARTED TO TELL ME THAT I WAS THE ONE THAT FOUND MY MOTHER'S BODY IN THE SWIMMING POOL.

AND HE WAS SO CONVINCING THAT I WENT HOME FROM THAT BIRTHDAY AND I STARTED TO THINK, MAYBE I DID.

I STARTED TO THINK ABOUT OTHER THINGS THAT I DID REMEMBER, LIKE WHEN THE FIREMEN CAME, THEY GAVE ME OXYGEN, MAYBE I NEEDED THE OXYGEN 'CAUSE I WAS SO UPSET THAT I FOUND THE BODY, AND I COULD ALMOST VISUALIZE MY MOTHER IN THE SWIMMING POOL.

EAGLEMAN: BUT THEN SOMETHING HAPPENED THAT WOULD MAKE LOFTUS REALIZE HER MEMORY HAD BEEN TRICKED.

LOFTUS: THE RELATIVE CALLED AND SAID, "I MADE A MISTAKE, IT WASN'T YOU, IT WAS THE AUNT, WHO FOUND THE BODY."

AND SO I THOUGHT, BOY, THAT'S-- THAT'S WHAT IT FEELS LIKE, WHEN YOU'RE ON YOUR WAY TO DEVELOPING SUCH A RICH FALSE MEMORY.

EAGLEMAN: OUR PAST IS NOT A FAITHFUL RECORD.

INSTEAD, IT'S A RECONSTRUCTION.

IN PART, IT'S A MYTHOLOGY.

SO WHAT DOES THIS MEAN FOR WHO WE ARE?

WELL, THINK ABOUT YOUR LIFE MEMORIES.

NOT ALL THE DETAILS ARE ACCURATE.

SOME CAME FROM THINGS THAT PEOPLE TOLD YOU ABOUT YOURSELF; OTHER DETAILS WERE FILLED IN BY WHAT YOU THINK MUST HAVE HAPPENED.

BUT THAT'S OK.

IT'S ALL PART OF THE EVOLVING STORY THAT IS YOU.

SO WHY DO WE HAVE MEMORIES THAT ARE SO UNRELIABLE?

WELL, AS STRANGE AS IT SOUNDS, MEMORY ISN'T JUST USED FOR RECORDING OUR PAST.

IT ALSO SERVES ANOTHER IMPORTANT FUNCTION.

THAT FUNCTION WOULD BE REVEALED BY A SINGULAR CASE THAT REVOLUTIONIZED NEUROSCIENCE.

WOMAN: HOW LONG HAVE YOU HAD TROUBLE REMEMBERING THINGS?

MAN: I DON'T KNOW MYSELF.

I CAN'T TELL YOU BECAUSE I DON'T REMEMBER.

WOMAN: BUT DO YOU THINK IT'S BEEN MORE THAN A YEAR THAT YOU'VE HAD THIS PROBLEM?

MAN: I THINK ABOUT THAT 'CAUSE, UH, THIS IS JUST A THOUGHT THAT I'M HAVING MYSELF THAT, UH...

I POSSIBLY HAVE HAD AN OPERATION OR SOMETHING.

EAGLEMAN: HENRY MOLAISON WAS BORN IN 1926.

HIS EARLY LIFE WAS LIKE ANY OTHER YOUNG BOY'S, UNTIL HIS 10th BIRTHDAY, WHEN HE SUFFERED HIS FIRST EPILEPTIC SEIZURE.

BY 16, HIS EPILEPSY HAD WORSENED, AND BY 27, HE COULD NO LONGER FUNCTION NORMALLY.

THESE MAJOR SEIZURES INCREASED IN FREQUENCY, UP TO THE POINT WHERE HIS LIFE WAS PRETTY MUCH ON HOLD.

EAGLEMAN: HIS DOCTORS PROPOSED AN EXPERIMENTAL SURGERY THAT WOULD REMOVE THE HIPPOCAMPUS ON BOTH SIDES OF HENRY'S BRAIN, LEAVING TWO YAWNING BLACK HOLES.

CORKIN: THE OPERATION PROCEEDED WITHOUT INCIDENT, AND WITHIN A FEW DAYS, HE RECOVERED.

EAGLEMAN: HENRY'S EPILEPSY WAS CURED.

THAT WAS WHEN THEY REALIZED THAT HE COULDN'T REMEMBER ANYTHING.

EAGLEMAN: FOR THE REMAINING 55 YEARS OF HIS LIFE, HE NEVER FORMED A SINGLE NEW LONG-TERM MEMORY.

AND THERE WAS SOMETHING MORE.

CORKIN: WHAT DO YOU THINK YOU'LL DO TOMORROW?

MOLAISON: WHATEVER'S BENEFICIAL.

YOU MIGHT THINK WHEN YOU SAID--SAY TO HIM, "WHAT--WHAT DO YOU THINK YOU'LL DO TOMORROW?"

HE MIGHT SAY, "WELL, YOU KNOW, I'LL GET UP, UM, AS USUAL AND, UM, GET DRESSED AND SHAVE AND HAVE BREAKFAST," BUT HE DIDN'T EVEN COME UP WITH THAT.

IT WAS LIKE HE WAS ABSOLUTELY STUCK IN THE PRESENT MOMENT.

EAGLEMAN: HENRY MOLAISON'S MISFORTUNE HAD REVEALED SOMETHING PROFOUND.

THE BRAIN AREAS THAT UNDERLINE MEMORY ARE THE SAME AS THOSE THAT ARE USED TO SIMULATE WHAT'S COMING NEXT.

BOTH THE PAST AND THE FUTURE ARE CREATIONS IN THE BRAIN.

WHO WE ARE AT ANY MOMENT IN TIME IS AN ONGOING NARRATIVE.

AS WE LIVE LONGER THAN EVER BEFORE, THIS PRESENTS REAL PROBLEMS FOR BRAIN HEALTH.

DISEASES LIKE ALZHEIMER'S AND PARKINSON'S ATTACK OUR BRAIN TISSUE AND WITH IT, WHO WE ARE.

BUT IN THE SAME WAY THAT YOUR ENVIRONMENT AND BEHAVIOR SHAPE YOUR BRAIN WHEN YOU'RE YOUNGER, THEY'RE JUST AS IMPORTANT IN YOUR LATER YEARS.

EAGLEMAN: IF YOU DON'T MIND ME ASKING, HOW OLD ARE YOU THIS YEAR?

OH, I'M JUST 94.

OK. FOR HOW LONG HAVE YOU BEEN HERE?

WELL, I'VE BEEN IN THE CONVENT OVER 70 YEARS.

DO PEOPLE LIVE UNTIL THEIR 100s?

WE HAD ONE I KNOW THAT WENT OVER 100.

HOW WAS SHE DOING COGNITIVELY, AT THAT AGE?

SHE WAS VERY SMART.

SHE WAS VERY SMART.

OK. SHE SURE WAS, AND SHE DIDN'T MISS ANYTHING.

HA HA!

QUITE ALERT!

EAGLEMAN: ACROSS THE U.S., MORE THAN 1,200 NUNS, PRIESTS, AND BROTHERS HAVE BEEN TAKING PART IN A UNIQUE RESEARCH STUDY EXPLORING THE EFFECTS OF AGING ON THE BRAIN.

WOMAN: THEY REALLY FIGURED THE SISTERS ARE A GOOD STUDY GROUP BECAUSE WE'RE KIND OF STABLE.

YOU KNOW WHERE TO FIND US.

EAGLEMAN: EACH YEAR, THE PARTICIPANTS HAVE TO PROVIDE DETAILED RECORDS OF HOW THEY SPEND THEIR TIME.

THEY ALSO COMMIT TO EXTENSIVE PHYSICAL, GENETIC, AND COGNITIVE TESTS, AND IT DOESN'T END THERE.

WOMAN: WHEN I FIRST HEARD ABOUT THE STUDY, I SAID...WELL, EVEN AFTER I DIE--HA HA!-- WHATEVER I'M CONTRIBUTING CAN STILL GO ON.

EAGLEMAN: AFTER THEY DIE, ALL THE PARTICIPANTS WILL GIVE UP...THEIR BRAINS.

MY STAFF IN CHICAGO IS ON CALL 24 HOURS A DAY, 7 DAYS A WEEK.

WHEN SOMEONE DIES IN NEW YORK, THEY CALL US.

EAGLEMAN: THE RESEARCHERS CAREFULLY EXAMINE THE BRAINS FOR THE TELL-TALE MICROSCOPIC EVIDENCE OF AGE-RELATED BRAIN DISEASE.

THEY'RE LOOKING TO ESTABLISH LINKS BETWEEN BRAIN DEGENERATION AND COGNITIVE PERFORMANCE.

BUT THE FIRST SET OF RESULTS WAS ENTIRELY UNEXPECTED.

BENNETT: WHEN WE FIRST STARTED PUBLISHING IT, A LOT OF PEOPLE WERE SURPRISED.

EAGLEMAN: IT TURNED OUT THAT NEARLY A THIRD OF THE BRAINS TESTED HAD CHARACTERISTIC SIGNS OF FULL-BLOWN ALZHEIMER'S.

ALL: ♪ WHEN I FALL ON MY KNEES.... ♪ EAGLEMAN: BUT THE COGNITIVE TESTS REVEALED THAT THE BRAIN OWNERS HAD SHOWN NO SYMPTOMS OF THIS TERRIBLE DISEASE.

THEIR BRAINS WERE SICK, YET THEY REMAINED UNAFFECTED.

IT DIDN'T MAKE SENSE.

WHAT HAD HAPPENED?

I HAVE A GAME ON MY SMARTPHONE-- RUZZLE.

HA HA HA!

WOMAN: AND SOME OF OUR SISTERS HAVE BEEN INVOLVED IN TEACHING, NURSING... EAGLEMAN: HAVING RESPONSIBILITIES, LEARNING NEW SKILLS, KEEPING THE BRAIN ACTIVE.

THIS WAS PROTECTING THE NUNS FROM THE COGNITIVE SYMPTOMS OF THE DISEASE.

WOMAN: I AM INTERESTED VERY MUCH IN SCIENCE.

I LOVE SCRABBLE.

I DRIVE THE SISTERS TO THE DOCTORS.

...MASS EVERY DAY, AND IF I DIDN'T HAVE THAT, I'D BE COMPLETELY NUTS.

EAGLEMAN: EVEN AS PARTS OF THE BRAIN TISSUE DEGENERATE, MENTAL AND PHYSICAL ACTIVITY CAN BUILD NEW PATHWAYS FOR SOLVING PROBLEMS.

THIS IS CALLED "COGNITIVE RESERVE."

THINK OF THE BRAIN LIKE A TOOL BOX.

IF IT'S A GOOD TOOL BOX, YOU HAVE ALL THE TOOLS YOU NEED, SO I MIGHT CHOOSE A RATCHET TO DISENGAGE THIS BOLT HERE.

BUT WHAT IF I DIDN'T HAVE ACCESS TO THIS RATCHET?

I MIGHT BE ABLE TO FIND SOMETHING ELSE, LIKE THIS WRENCH WOULD DO THE TRICK.

AND IF I DIDN'T HAVE THE WRENCH, I COULD FIND SOMETHING ELSE, LIKE THIS ADJUSTABLE WRENCH.

AND IT'S THE SAME IDEA IN AN ACTIVE, COGNITIVELY FIT BRAIN.

EVEN AS PARTS OF THE BRAIN DEGENERATE, THE BRAIN CAN FIND OTHER SOLUTIONS.

BY MAKING SURE OUR MENTAL TOOL BOX IS EQUIPPED WITH A VARIETY OF WORKING TOOLS, WE MAY BE ABLE TO SLOW DOWN THE EFFECTS OF OUR AGING BRAINS AND HOLD ON TO WHO WE ARE FOR AS LONG AS POSSIBLE.

BUT WHO WE ARE IS MORE THAN JUST THE TASKS WE CAN ACCOMPLISH.

THERE'S SOMETHING ELSE-- SOMETHING THAT'S PERHAPS THE GREATEST MYSTERY ABOUT HOW THE BRAIN WORKS.

IT'S THE SENSE OF "I," OF "ME."

I AM A CONSCIOUS BEING.

WHEN I THINK ABOUT WHO I AM, IT'S TAKING PLACE INSIDE THIS HEAD, THROUGH THESE EYES, FROM THIS PARTICULAR POINT OF VIEW.

CONSCIOUS EXPERIENCE IS AT ONCE THE MOST FAMILIAR AND THE MOST PERPLEXING ASPECT OF OUR IDENTITY.

HOW DOES THE PHYSICAL STUFF OF THE BRAIN EQUAL THE MENTAL EXPERIENCE OF BEING A CONSCIOUS HUMAN?

HOW DO BILLIONS OF BRAIN CELLS PRODUCE THE EXTRAORDINARY, UNIQUE FEELING OF BEING ALIVE, OF BEING ME?

I'M MADE UP OF 100 BILLION NEURONS.

AND WHEN I DIE, THEY'LL STILL BE THERE.

BUT I WON'T BE ME ANYMORE; I'LL BE DEAD.

SO THAT MEANS WHO I AM IS NOT ABOUT THE EXISTENCE OF THE NEURONS.

IT'S ABOUT WHAT THEY DO AND HOW THEY INTERACT.

IMAGINE THAT THE NEURONS IN YOUR BRAIN ARE LIKE A COLLECTION OF DRUMMERS.

[PLAYING DRUMBEATS] IF EACH DRUMMER PLAYS COMPLETELY INDEPENDENTLY, THE NOISE THAT EMERGES IS JUST THAT--NOISE.

BUT IF THEY START LISTENING TO ONE ANOTHER, SOMETHING MORE EMERGES.

OUT OF CACOPHONY APPEARS A RHYTHM, A PERFORMANCE-- A COMPLEX INTERACTION IN WHICH ALL THE DRUMMERS ARE PLAYING BOTH AS INDIVIDUALS AND AS SOMETHING GREATER.

AND IN THE SAME WAY, THIS IS HOW THE EXPERIENCE OF CONSCIOUSNESS ARISES IN THE BRAIN.

BILLIONS OF INTERACTING NEURONS WORK IN CONCERT-- AND UNDER THE RIGHT CIRCUMSTANCES, THEY HIT A SWEET SPOT, THE PLACE WHERE THE SINGULAR, PRIVATE EXPERIENCE OF BEING YOU EMERGES.

CONSCIOUSNESS IS A PERFORMANCE OUR BRAIN PUTS ON FOR US THROUGHOUT OUR DAY.

BUT THERE COMES A TIME-- IN FACT, ONCE A DAY-- WHEN THE CHARACTER OF THAT DRUMMING CHANGES AND IT TAKES ME WITH IT.

THE BEST WAY FOR ME TO SHOW YOU HOW IS FOR ME TO GO TO SLEEP...

WEARING THIS.

I'M WEARING AN ELECTRO-ENCEPHALOGRAM, OR EEG.

IT RECORDS MY BRAIN'S ACTIVITY, GIVING AN IDEA OF HOW MY NEURONS ARE INTERACTING WHILE I SLEEP.

WHEN YOU GO TO SLEEP, YOUR BODY SEEMS TO SHUT DOWN, SO YOU MIGHT THINK THAT THE DRUM BEAT IN YOUR BRAIN WOULD DO THE SAME.

BUT THE REALITY IS QUITE DIFFERENT.

SO AT THE BEGINNING OF THE NIGHT, THIS IS MY BRAIN ACTIVITY: I WAS STILL AWAKE.

BUT IF I GO A LITTLE BIT LATER, I HAVE ACTIVITY THAT LOOKS AS THOUGH I'M STILL AWAKE, BUT IN FACT I WASN'T; I WAS IN DREAM SLEEP HERE, WHICH IS A FORM OF CONSCIOUSNESS, FULL OF VISION AND SOUNDS AND STRANGE SITUATIONS AND MAGNIFIED EMOTIONS.

IT'S ME, BUT A BIZARRE FORM OF ME.

BUT THINGS GET STRANGER.

AT THIS PART OF THE NIGHT, I'M IN DEEP SLEEP.

MY BRAIN IS STILL THERE, AND IT'S STILL ACTIVE, BUT I AM GONE.

IN DEEP SLEEP, OUR NEURONS BECOME MORE SYNCHRONIZED.

IT'S IMPOSSIBLE FOR A COMPLEX RHYTHM TO EMERGE FROM THIS, WHICH MEANS, IN THIS BRAIN STATE, THERE'S NO HOPE OF CONSCIOUS EXPERIENCE.

NO IDENTITY, NO PERSONALITY, NOTHING.

I AM THE RELATIONSHIP BETWEEN MY NEURONS.

CHANGE THEIR INTERACTION JUST A LITTLE BIT AND I FIND MYSELF IN A DREAM WORLD, OR I DISAPPEAR.

OR I RETURN.

WHEN THE NEURONS FIND THEMSELVES BACK IN THEIR PROPER RHYTHM, I MIRACULOUSLY COME BACK ONLINE.

CONSCIOUSNESS AND THE SENSE OF BEING "ME" SOMEHOW EMERGES FROM THE COMPLEX RHYTHMS OF OUR NEURONS FIRING.

BUT WHY CONSCIOUSNESS EMERGES AT ALL STILL REMAINS ONE OF THE GREAT UNSOLVED MYSTERIES OF MODERN SCIENCE.

AFTER I FINISHED GRADUATE SCHOOL, I HAD THE OPPORTUNITY TO WORK WITH ONE OF MY SCIENTIFIC HEROES, FRANCIS CRICK, WHO HAD CO-DISCOVERED THE STRUCTURE OF DNA.

BY THE TIME I MET HIM, HE HAD TURNED HIS ATTENTION TO THE QUESTION OF CONSCIOUSNESS-- WHY DOES IT FEEL LIKE ANYTHING TO BE ALIVE.

AND I REMEMBER WHEN I WENT INTO HIS OFFICE THAT HE HAD A LOT OF WRITING ON THE CHALKBOARD, BUT THERE WAS ONE WORD THAT WAS WRITTEN IN THE MIDDLE AND IT WAS BIGGER THAN THE REST.

THAT WORD WAS "MEANING."

YOU SEE, WE KNOW A LOT ABOUT THE MECHANICS OF NEURONS AND NETWORKS AND BRAIN REGIONS, BUT WHAT WE DON'T KNOW ABOUT THOSE SIGNALS COURSING AROUND IN THE BRAIN IS WHY WE CARE ABOUT ANY OF THEM, WHY ANYTHING CARRIES MEANING.

HOW CAN THE PHYSICAL CELLS IN MY BRAIN CAUSE ME TO CARE ABOUT ANYTHING?

THE MEANING PROBLEM IS NOT YET SOLVED, BUT HERE'S WHAT I THINK WE CAN SAY: THE MEANING OF SOMETHING TO YOU IS ALL ABOUT YOUR WEB OF ASSOCIATIONS, BASED ON YOUR WHOLE HISTORY OF EXPERIENCES.

JUST IMAGINE I WERE TO TAKE A PIECE OF CLOTH AND PUT SOME COLORED PIGMENTS ON IT, AND THEN PUT THAT IN FRONT OF YOUR VISUAL SYSTEM.

IS THAT LIKELY TO TRIGGER MEMORIES AND FIRE UP YOUR IMAGINATION?

WELL, PROBABLY NOT, BECAUSE IT'S JUST A PIECE OF CLOTH, RIGHT?

HERE IT IS--PIGMENTS ARRANGED ON A CLOTH IN THE PATTERN OF A NATIONAL FLAG.

PRESUMABLY THIS TRIGGERS SOMETHING FOR YOU, BUT THE MEANING IS UNIQUE TO YOUR HISTORY OF EXPERIENCES.

WE DON'T PERCEIVE OBJECTS AS THEY ARE.

WE PERCEIVE THEM AS WE ARE.

EACH OF US IS ON OUR OWN TRAJECTORY-- STEERED BY OUR GENES AND OUR EXPERIENCES--AND AS A RESULT, EVERY BRAIN HAS A DIFFERENT NEURAL REALITY.

BRAINS END UP BEING AS UNIQUE AS SNOWFLAKES.

YOUR STORY PLAYS OUT ACROSS A LIFETIME.

TRILLIONS OF NEW CONNECTIONS ARE CONTINUALLY FORMING AND RE-FORMING AS WE LEARN AND CREATE MEMORIES AND BECOME WHO WE ARE.

THE UNIQUE CONNECTIONS IN YOUR BRAIN MEAN NO ONE LIKE YOU HAS EVER EXISTED... OR WILL EVER EXIST AGAIN.

FOR ME, THE WONDER OF THE HUMAN BRAIN IS THAT FROM A VAST NETWORK OF PHYSICAL PIECES AND PARTS, THE EXPERIENCE OF BEING YOU OR ME EMERGES.

AND BECAUSE THE PHYSICAL STUFF IS CHANGING, WE ARE, TOO.

WE'RE NOT FIXED.

FROM CRADLE TO GRAVE, WE ARE WORKS IN PROGRESS.

ANNOUNCER: "THE BRAIN WITH DAVID EAGLEMAN" DR. EAGLEMAN: Next time on "The Brain," I'm going to investigate the unexpected ways that your brain secretly controls everything that you do.

That was amazing.

How it allows great athletic achievement and makes decisions without your awareness.

This is the story of everything the brain does that remains hidden from us.

It's the story of who is really in control.

"THE BRAIN WITH DAVID EAGLEMAN" IS AVAILABLE ON DVD.

THE COMPANION BOOK IS ALSO AVAILABLE.

TO ORDER, VISIT SHOPPBS.ORG