Shedding light on brain function: the event-related optical signal

1. Gratton & Fabiani (2001) Shedding light on brain function: the event-related optical signal

2. Functional neuroimaging • Hemodynamic techniques: • PET and fMRI useful for spatial information about neural activity • Electromagnetic techniques: • EEG and MEG useful for temporal information about neural activity

3. Event-Related Optical Signal • Hemodynamic techniques lack temporal specificity and electromagnetic techniques lack spatial specificity. • EROS provides both, good temporal and spatial information.

4. How does EROS work? • Fiber optic cables act as sources and detectors. • Sources have just one fiber, detectors have many. • Sources carry light from lasers or LEDs. • Photomotopliers function as detectors of photons.

5. How does EROS work?

6. Basic principles in the “physics” of optical imaging. • If photons are emitted from a light-source (fiber optic) against the surface of a semi-infinite, homogenous object they can be modeled using the same equations as those that describe the positive half of a dipole. • Depth in the case of EROS depends partially on the distance between the source and detector.

7. How does EROS work? • As light propagates from the source it gets scattered and absorbed by brain tissue. • Changes in the activity of brain tissue affect the amount of scattering and absorption. • Scattering causes the photons to have a longer transit time from source to detector. • Therefore, scattering (activity) can be estimated from the increase in transit time/phase delay.

8. Phase delay Input Output Intensity Time

9. Experimental Support • Visual stimulation experiment shows transit time increase for activated areas mm apart.

10. Experimental Support • Comparison found good temporal and spatial overlap with ERPs and fMRI.

11. What might explain this response? • Changes in neuronal membrane affect transparency of membrane and diffraction.

12. Overview: neuronal activity • Neuronal membranes distend/shrink as ions + H2O move in/out after an action potential.

13. The Event-Related Optical Signal • Pros: • Good temporal resolution (milliseconds) • Good spatial resolution (<1cm) • Useful for studying neurovascular coupling • Cons: • Penetration from scalp is limited to 3-5cm (cortex) • Low signal-to-noise ratio requires averaging • Marianna’s question #1 • Pulse correction, phase rejection, movement artifact

14. CNL EROS Video

15. Tse, Lee, Sullivan, Garnsey, Dell, Fabiani and Gratton (2007) Imaging cortical dynamics of language processing with the event-related optical signal

16. Motivation • How do the temporal cortex and inferior frontal cortex interact? • fMRI is too slow to view this interaction • Does processing differ for syntactic vs. semantic anomaly? • Can EROS image interactions between cortical areas?

17. Hagoort (2005) • Identifies three functional components: • Memory: store of language information, involved in retrieval • Unification: integration of lexically retrieved information • Control: language to action, such as choose between using one of two languages C U M

18. Hagoort (2005) • Increased response in left inferior frontal when unification load is increased (in response to anomalous critical word). A lesser response also occurs for correct sentences.

19. Hagoort (2005) • Model emphasizes that posterior and dorsal areas integrate syntactic information, while anterior and ventral areas function more for semantic integration. • But, there is a lot of overlap. • Preview of Albert’s question • Little evidence extending this functional specialization to the temporal lobe.

20. Methods: Participants • 16 participants • All right-handed native English speakers • 11 females and 5 females, ages 18-30 • Lucy’s question: • Why the disproportionate # of females? • Control for sinistrality?

21. Methods: Materials • Each participant saw 864 sentences • 336 unacceptable sentences • 144 were semantically anomalous at the final word • “The hungry child ate the floor.” • 48 filler sentences contained semantically anomalous words in medial position to prevent expectations that anomalies only occur in the final position • 96 had grammatical violations of subject verb agreement • “If work isn’t done, it pile…” • 48 had grammatically incorrect pronoun case • “My mother promised to buy I…”

22. Methods: Materials • 528 acceptable sentences • 144 controls for the semantic sentences • 96 for syntactic subject verb agreement • 48 for pronoun case anomaly • 240 sentences to ensure subjects expect most sentences to be acceptable • Length and frequency was accounted for across conditions. • Israel’s questions- • Couldn’t syntactic anomaly be sentence-final? (When it rains, it pour(s)” • How long is the experiment? • Approx. 10 hours total broken up into two 5-hour sessions.

23. Materials: Procedures • Sentences were presented word-by-word at the center of screen and subjects had to judge if sentence was well-formed.

24. ERP recording • ERPs were recorded. • EEG recording used four scalp electrodes (Fz, Cz, Pz, and right mastoid). • Final bandpass filter of 0.1-20Hz • Sampled at 100Hz • Time-locked 1500 ms epochs with 200ms pre-stimulus onset

25. EROS recording • EROS recording used two montages. • Laser diodes emitted 830nm light at 220MHz which was picked up by PMTs modulated at ~220MHz • Sources, detectors, nasion, preauricular points and random points were digitally localized using a Fastrak 3D digitizer. • Coregistration with individual subject’s anatomy provided by MRI.

26. EROS setup • 128 source-detector pairs per montage

27. Result: Behavioral • Most sentences were classified correctly: • Semantically acceptable- 94% • Semantically unacceptable- 95% • Syntactically unacceptable – 86% • Syntactically acceptable- 88% • Analyses were performed only on these correct trials.

28. Results: ERP • Semantic unacceptable –acceptable • Difference waveform computed from 200-500ms peaking at 420ms (p<.001)

29. Results: ERP • Syntactically unacceptable –acceptable • Difference waveform computed from 500-1500 peaking at 860 ms (p<.001)

30. Results: EROS • Significant increase in phase delay for anomalous critical words. • Both conditions elicited S/MTC activation followed by IFC activation. • Pattern occurred a few times for semantic condition suggesting oscillatory behavior. • ROIs analyzed independently

31. Results: EROS

32. Results:EROS

33. Results: EROS • Different areas activated for each condition up to ~665ms. • Semantic = ventral anterior/middle • Syntactic = dorsal posterior temporal • More frontal activation for semantic anomaly, but lots of overlap. • EROS signals predicted the N400 (@179ms, 384ms in S/MTC) and P600 (@ 819ms, 914ms in IFC)in semantic and syntactic condition, respectively, but not vice versa. • Double dissociation in EROS/ERP

34. Results: EROS • Contrasts were 17mm apart along inferior -superior, but not anterior-posterior • After 655ms there was no reliable differences between the activity for contrasts. • Albert’s question • Potentially due to response • Hagoort’s model

35. Discussion • EROS successfully showed interaction between temporal-frontal network involved in language processing. • Supports model in which retrieval occurs in the temporal regions and the integration occurs in the frontal regions.

36. Discussion • Might reflect integration process in IFC in which predictions about upcoming words are generated and sent to temporal areas. • Marianna’s question # 1 • Lucy’s question # 1 & 2 • Lynn’s Question • Since syntactic anomalies were relatively easy to rectify there was little frontal activity. • Semantic anomaly more difficult to correct and hence more back and forth. • More extensive frontal activity • Double checks retrieval? • The location of these networks is consistent with previous fMRI studies.

37. Word position • Syntactic words were in medial position while semantic were final. • Is activation comparable for sentence medial and sentence final positions?

38. Word position Effect is still present just smaller, as is usually the case.

39. Word position