Decoding the Information Encoded in Neural Activity

Transcription

1 Decoding the Information Encoded in Neural Activity Tom M. Mitchell Carnegie Mellon University April 2017

2 How does neural activity encode word meanings?

3 How does neural activity encode word meanings? How does brain combine word meanings into sentence meanings?

4 Neurosemantics Research Team Research Scientists Research Scientists Tom Mitchell Marcel Just Erika Laing Kai-Min Chang Dan Howarth Recent/Current PhD Students Leila Wehbe Dan Schwartz Alona Fyshe Mariya Toneva Mark Palatucci Gustavo Sudre Nicole Rafidi funding: NSF, NIH, IARPA, Keck

5 Functional MRI

6 Typical stimuli

7 fmri activation for bottle : bottle Mean activation averaged over 60 different stimuli: fmri activation high average bottle minus mean activation: below average

8 Classifiers trained to decode the stimulus word Trained Classifier (SVM, Logistic regression, Deep net,bayesian classifier...) Hammer or Bottle (classifier as virtual sensor of mental state)

9 Classification task: is person viewing a tool or building? Classification accuracy p4 p8 p6 p11 p5 p7 p10 p9 p2 p12 p3 p1 Participants statistically significant p<0.05

10 Are neural representations similar across people? Can we train classifiers on one group of people, then decode from new person?

11 Are representations similar across people? YES rank accuracy classify which of 60 items

12 Lessons from fmri Word Classification Neural representations similar across people language word vs. picture Easier to decode: concrete nouns emotion nouns Harder to decode: abstract nouns verbs* * except when placed in context

13 Predictive Model? Predicted fmri activity Arbitrary noun

14 Predictive Model? [Mitchell et al., Science, 2008] vector representing word meaning Input noun: telephone Retrieve text statistics v = 25 i=1 f i (w) c vi Predicted fmri activity trillion word text collection trained on other fmri data

15 Represent stimulus noun by co-occurrences with 25 verbs* Semantic feature values: celery , eat , taste , fill , see , clean , open , smell , touch , drive , wear , lift , break , ride Semantic feature values: airplane , ride , see , say , near , open , hear , run , lift , smell , wear , taste , rub , manipulate * in a trillion word text collection

16 Predicted Activation is Sum of Feature Contributions eat Celery = taste fill f eat (celery) from corpus statistics c 14382,eat learned high prediction v = 25 i=1 f i (w) c vi low 500,000 learned c vi parameters Predicted Celery

17 celery airplane Predicted: fmri activation high average Observed: below average Predicted and observed fmri images for celery and airplane after training on other nouns. [Mitchell et al., Science, 2008]

18 Evaluating the Computational Model Leave two words out during training celery? airplane? 1770 test pairs in leave-2-out: Random guessing à 0.50 accuracy Accuracy above 0.61 is significant (p<0.05)

19 Learned activities associated with meaning components Participant P1 Semantic feature: Eat Push Run Gustatory cortex somato-sensory Biological motion Pars opercularis (z=24mm) Postcentral gyrus (z=30mm) Superior temporal sulcus (posterior) (z=12mm)

20 Alternative semantic feature sets PREDEFINED corpus features Mean Acc. 25 verb co-occurrences verb co-occurrences.79 50,000 word co-occurences Latent Semantic Analysis features corpus features from Collobert&Weston ICML08.78

21 Alternative semantic feature sets PREDEFINED corpus features Mean Acc. 25 verb co-occurrences verb co-occurrences.79 50,000 word co-occurences Latent Semantic Analysis features corpus features from Collobert&Weston ICML features collected using Mechanical Turk.83 Is it heavy? Is it flat? Is it curved? Is it colorful? Is it hollow? Is it smooth? Is it fast? Is it bigger than a car? Is it usually outside? Does it have corners? Does it have moving parts? Does it have seeds? Can it break? Can it swim? Can it change shape? Can you sit on it? Can you pick it up? Could you fit inside of it? Does it roll? Does it use electricity? Does it make a sound? Does it have a backbone? Does it have roots? Do you love it? features authored by Dean Pomerleau. feature values 1 to 5 features collected from at least three people people provided by Amazon s Mechanical Turk

22 Alternative semantic feature sets PREDEFINED corpus features Mean Acc. 25 verb co-occurrences verb co-occurrences.79 50,000 word co-occurences Latent Semantic Analysis features corpus features from Collobert&Weston ICML features collected using Mechanical Turk* features discovered from the data**.86 * developed by Dean Pommerleau ** developed by Indra Rustandi

23 Discovering shared semantic basis 1. Use CCA to discover latent features across subjects specific to study/subject [Rustandi et al., 2009] 20 learned latent features f (w) CCA abstraction f k (w) = v x v c vi CCA abstraction f k (w) = v x v c vi subj 1, word+pict subj 9, word+pict CCA abstraction f k (w) = v x v c vi subj 10, word only CCA abstraction f k (w) = v x v c vi subj 20, word only

24 [slide courtesy of Indra Rustandi] Each column is one fmri image

25 Discovering shared semantic basis 1. Use CCA to discover latent features specific to study/subject [Rustandi et al., 2009] 20 learned latent features f (w) CCA abstraction f k (w) = v x v c vi CCA abstraction f k (w) = v x v c vi subj 1, word+pict subj 9, word+pict CCA abstraction f k (w) = v x v c vi subj 10, word only CCA abstraction f k (w) = v x v c vi subj 20, word only

26 Discovering shared semantic basis 1. Use CCA to discover latent features 2. Train regression to predict them specific to study/subject [Rustandi et al., 2009] independent of study/subject 218 MTurk features b(w) f i (w) = k 20 learned latent features b k (w) c ik f (w) CCA abstraction f k (w) = v x v c vi CCA abstraction f k (w) = v x v c vi subj 1, word+pict subj 9, word+pict word w CCA abstraction f k (w) = v x v c vi subj 10, word only CCA abstraction f k (w) = v x v c vi subj 20, word only

27 Discovering shared semantic basis 1. Use CCA to discover latent features 2. Train regression to predict them 3. Invert CCA mapping independent of study/subject 218 MTurk features b(w) f i (w) = k 20 learned latent features b k (w) c ik f (w) specific to study/subject predict representation v = i f i (w) c vi predict representation v = i f i (w) c vi [Rustandi et al., 2009] subj 1, word+pict subj 9, word+pict word w predict representation v = i f i (w) c vi subj 10, word only predict representation v = i f i (w) c vi subj 20, word only

28 CCA Components: Top Stimulus Words component 1 component 2 component 3 component 4 Stimuli that most activate it apartment church closet house barn screwdriver pliers refrigerator knife hammer telephone butterfly bicycle beetle dog pants dress glass coat chair shelter? manipulation? things that touch my body?

29 Timing?

30 MEG: Stimulus hand (word plus line drawing) [Sudre et al., NeuroImage 2012]

31 word length word length 100 ms 50 ms word length right diagonalness verticality ms (Sudre et al., under review) [Sudre et al., NeuroImage 2012]

32 word length word length 100 ms word length right diagonalness verticality ms (Sudre et al., under review) [Sudre et al., 2012]

33 aspect ratio 150 ms word length internal details ms (Sudre et al., under review) [Sudre et al., 2012]

34 internal details aspect ratio 200 ms internal details IS IT HAIRY? ms (Sudre et al., under review) [Sudre et al., 2012]

35 IS IT HOLLOW? white pixel count horizontalness 250 ms IS IT MADE OF WOOD? IS IT HAIRY? IS IT AN ANIMAL? ms (Sudre et al., under review) [Sudre et al., 2012]

36 IS IT MAN-MADE? IS IT ALIVE? CAN IT BITE OR STING? WAS IT EVER ALIVE? DOES IT GROW? 300 ms CAN YOU PICK IT UP? CAN YOU HOLD IT? IS IT BIGGER THAN A CAR? IS IT ALIVE? DOES IT GROW? IS IT ALIVE? ms (Sudre et al., under review) [Sudre et al., 2012]

37 COULD YOU FIT INSIDE IT? DOES IT HAVE FOUR LEGS? IS IT MAN-MADE? WAS IT EVER ALIVE? CAN YOU PICK IT UP? CAN YOU HOLD IT? 350 ms CAN YOU HOLD IT IN ONE HAND? IS IT ALIVE? CAN IT BEND? ms (Sudre et al., under review) [Sudre et al., 2012]

38 IS IT BIGGER THAN A CAR? CAN YOU PICK IT UP? DOES IT HAVE CORNERS? IS IT MAN-MADE? WAS IT EVER ALIVE? WAS IT INVENTED? 400 ms CAN YOU PICK IT UP? IS IT TALLER THAN A PERSON? IS IT MAN-MADE? WAS IT EVER ALIVE? IS IT MANUFACTURED? ms DOES IT HAVE FEELINGS? IS IT ALIVE? (Sudre et al., under review) [Sudre et al., 2012]

39 IS IT AN ANIMAL? IS IT MANUFACTURED? IS IT ALIVE? IS IT HOLLOW? DOES IT GROW? 450 ms WAS IT INVENTED? IS IT BIGGER THAN A BED? CAN YOU HOLD IT? IS IT HOLLOW? ms (Sudre et al., under review) [Sudre et al., 2012]

40 IS IT TALLER THAN A PERSON? CAN YOU PICK IT UP? DOES IT GROW? CAN YOU PICK IT UP? 500 ms IS IT BIGGER THAN A BED? CAN YOU HOLD IT IN ONE HAND? ms (Sudre et al., under review) [Sudre et al., 2012]

41 CAN IT BE EASILY MOVED? 550 ms IS IT ALIVE? IS IT MAN-MADE? WAS IT EVER ALIVE? ms (Sudre et al., under review) [Sudre et al., 2012]

42 Details

43 Color= decodability* of feature wordlength (peak decodability msec) L,R cuneus L,R Lat. Occipital L,R lingual ß Time (msec.) L,R pericalcarine L,R precuneus L,R Sup. Parietal ß 100 ß Brain regions à * % of feature variance predicted by MEG, mean across 9 subjects

44 Color= decodability of grasping features (initial peak: msec) L Inferior Parietal L, R Postcentral L Supramarginal L,R cuneus L,R Lat. Occip. L,R lingual L,R pericalcarine L,R precuneus L,R Sup. Parietal [Sudre et al., 2012]

45 Color= decodability of animacy features (initial peak: msec) L Fusiform L Inferior Parietall R Midtemporall L, R Postcentrall L, Precentrall L Supramarginall L,R cuneus L,R Lat. Occip. L,R lingual L,R pericalcarine L,R precuneus L,R Sup. Parietal 0 - ß Time (msec.) ß Brain regions à [Sudre et al., 2012]

46 20 most accurately decoded semantic features out of 218 [G. Sudre et al., 2012] size manipulability animacy shelter

47 Sentence Reading Nicole Rafidi, Erika Liang, Dan Schwartz

48 Sentence Reading Nicole Rafidi, Erika Liang, Dan Schwartz How does brain parse sentences? Which word meanings are activated when, as the sentence is being understood?

49 +

50 A

51 student

52 found

53 the

54 hammer.

55 +

56 MEG Experiment [ Nicole Rafidi, 2016] 32 sentences x 15 repe//ons: 16 Ac&ve: A student found the hammer 16 Passive: The hammer was found by a student A dog ate the peach. 500ms per word /me

57 brain locations / MEG sensors Sentence MEG activity: time à A student found the hammer + A student found the hammer +

58 Sentence MEG activity: time à A student found the hammer + brain locations / MEG sensors when is the brain thinking about each earlier word? (e.g., first noun, student?) A student found the hammer +

59 Sentence MEG activity: time à A student found the hammer + brain locations / MEG sensors when is the brain thinking about each earlier word? (e.g., first noun, student? when can we decode first noun ( student ) from sliding 100 msec window of MEG? 100 msec = A student found the hammer +

60 When does brain activate meaning of first noun in sentence? [Nicole Rafidi et al., HBM 2015]

61 thank you!