Pupillometry for dummies

1. Pupillometry for dummies 10th May 2017 Yunzhe Liu Expert: Eran Eldar

2. Overview • Physiology of the pupil response • Methodology and procedure • Advantages and disadvantages • Task evoked pupillary response • Data analysis

3. Pupillometry: definition • Pupillometry is the measurement of the pupil diameter and its change in psychology. • First study: 1960 by Hess and Polt, pupil response to emotionally relevant visual stimuli. The pupil size of male and female adults increased when they viewed images of half-naked members of the opposite sex, only female subjects shows significant pupil dilations to pictures of babies.

4. Physiology of pupil response Pupil: hole in the centre of the iris of the eye, it allows the light to strike the retina. Iris: coloured part of the eye, has the function of a diaphragm that serves as the aperture stop. Cornea: transparent front part of the eye that covers the iris and the pupil. Pupil dilation and constriction are controlled via the optic and oculomotor cranial nerve. 2 groups of muscle of the iris: Sphincter pupillae, Sympathetic nervous system pupil contraction Dilator pupillae, Parasympathetic nervous system pupil dilation

5. Physiology of pupil response The pupil response is associated with the Locus Coeruleus Norepinephrine (LC-NE) system, which is involved in cognitive tasks and in general functions such as sleep-wake cycle and arousal. Locus coeruleus is a nucleus in the pons (brainstem) involved with physiological responses to stress and panic.It also influences attention, memory and cognitive control.

6. The Locus Coeruleus • Aston-Jones & Cohen (2005) – key paper • LC neurons respond phasically to task-relevant stimuli • Phasic mode increases the gain of units in the network • Could increase exploitation of current sources of reward • May also act as an attentional filter

7. The Locus Coeruleus Tonic LC activity changes baseline noradrenaline • Persistent increase of gain across the network • When tonic activity is high, subjects show higher distractibility and more task errors • Possible that this state is linked to exploration of new sources of reward

8. Physiology of pupil response Light Average changes in diameter: ~2mm Max: 5 mm Emotions, sexual arousal and fatigue Pupil size is sensitive to: Cognitive effort Amount of cognitive resources needed to complete a task (perception, memory, judgment) Average changes in diameter: 0.1 – 0.5 mm

9. Physiology of pupil response

10. Pupillometry: methodology Eye tracker: infrared illuminator + video camera + mathematical algorithms Infrared illumination  creates reflection patterns on the cornea and pupil. Video camera  captures the eye images and the infrared reflection patterns. Mathematical algorithms  find specific details in the subject’s eyes and reflection patterns, then calculate the eye and gaze location, and the size and location of the pupil. Different sampling rates can be used.

11. Pupillometry: pros and cons • Advantages: •  Relatively inexpensive method compared to other physiological measurements (EEG, fMRI, MEG). • Completely non-invasive for the subject. • Can be easily coupled to the acquisition of other neuroimaging (e.g. EEG, MEG, fMRI). • Can be used to test a wide population, including preverbal children or subject normally unable to understand instructions or to provide controlled, verbal responses (e.g. aphasic patients, patients with locked-in syndrome).

12. Pupillometry: pros and cons Disadvantages:  Sensitive to variation in lighting conditions.  A “summative measure”: may reflect the combined effect of any influence on cognitive activation, such as anxiety, interest, incentive, age, intelligence, illness, medication. • Difficulties in choosing a reliable baseline due to a varying noise level in pupil size. How to balance advantages and disadvantages? • Rigorous research methods • Careful research design • Accurate programming for pupillometry(adequate triggers in the script)

13. Task Evoked Pupillary response Task Evoked Pupillary Response (TEPR) changes in pupil dilation linked to stimulus presentation or task demand. • Smaller than light driven responses • Involuntary • Can be time locked to critical information. • Tonic vs Phasic dilation: resting pupil diameter contrasted against changes associated with stimulus presentation. Main TEPR measures: • Peak amplitude. • Average dilation over a selected time window. • Peak latency. • Baseline, or resting pupil size during a period in which no systematic processing occurs.

14. Pupillometry procedure • Procedure: • Measure pupil size for high and low light condition, and find an intermediate light condition for conducting the experiment. • Make sure the participant sit comfortably with his head placed on the head-stabiliser. • Make sure that the eye is on focus on the screen. • Calibrate the eye-tracker (3, 5, 9 or 13 points across the screen). • Validate the information acquired during calibration.

15. Calibration and validation • Calibration: we need to “teach” the computer what the eye looks like when the participant is fixated on a known location on the screen. 2. Validation: we make sure calibration was accurate by re-presenting the target points and determining whether the computer’s estimation of eye position is close to the known position of the target. Calibration is successful if errors found are minimal

16. Calibration assumptions: • Vergence: the eyetracker assumes the participant is always looking at screen dept. Any vergence movements will be treated as the eyes moving on the screen (3D objects presentation or tiredness can be problematic). • Lighting: the eyetracker assumes that the eye is constantly lit. Changes in pupil size are assumed to be due to a change in eye location or in the cognitive effort. Practical hints: • No mascara or heavy make-up: dark part are tracked as pupil! • No glasses. Soft contact lenses are generally okay to wear. Make sure eyes are not too dry and overly watery. • No medication or coffee before. • Allow the participant to rest and take breaks: pupil size is sensitive to stress and fatigue.

17. Analysis - Preprocessing • Similar to other eye-tracking procedures • Segment data into individual trials • Clean up your data

18. http://www.uqtr.ca/~siroiss/pupillometry/

19. Analysis - Preprocessing • Similar to other eye-tracking procedures • Segment data into individual trials • Clean up your data • Missing data (participants’ heads moved, lose pupil/CR data) • Blinks (normal, double, long, partial) • Linear interpolation • Or, discarding trials when necessary

20. http://www.uqtr.ca/~siroiss/pupillometry/

21. Analysis – Baseline Correction • Baseline correction • Necessary to compare across individuals • Excludes residual pupillary reactions

22. http://www.uqtr.ca/~siroiss/pupillometry/

23. Analysis – Baseline Correction • Baseline correction • Necessary to compare across individuals • Excludes residual pupillary reactions • Time window before stimulus onset • Subtract this baseline from data after stimulus onset • Z-scores for individual participants to remove individual differences • Be careful that your baseline correction doesn’t confound your data trial by trial! Correct baseline is important!

24. Analysis • Task-evoked pupil responses • Compare phasic and tonic responses • Phasic: change in pupil diameter in response to a stimulus • Tonic: baseline pupil diameter • Plot curves and choose your time windows

26. Analysis • Baseline to peak

27. Analysis • Peak latency

28. Analysis • Average diameter in time window

29. Analysis • Timepoint-by-timepoint

30. What can pupillometry indicate? • Locus coeruleus function • Gilzenrat et al. (2010), Aston-Jones and Cohen (2005) • Emotionally relevant stimuli • Seminal paper of Hess and Polt (1960) • Mental effort • Load on attention (Kahneman, 1973) • Load on memory (Beatty and Kahneman, 1966) • Interference of stimuli (Laeng et al., 2011) • Unconscious processing of stimuli • Hakerem & Sutton, 1966

31. Where is pupillometry useful? • As a proxy for brain functioning • Growing body of evidence for pupillometry as an index of locus coeruleus functioning – useful, as the LC is difficult to see with neuroimaging methods • Developmental studies • Studies of infants have found greater pupillometric reactions to impossible events (Jackson & Sirois, 2009) • Clinical studies • Measurement of cognitive effort in linguistic processing in aphasic patients (Chapman et al., 2015) • Different responses to looming sounds in Alzheimer’s disease (Fletcher et al., 2015)

32. Useful resources • Laeng, B., et al. (2012). Pupillometry: A Window to the Preconscious? Perspectives on Psychological Science, 7, 18-27 – Review of pupillometry, covering where pupillometry is most useful and areas in which it is commonly used • Aston-Jones, G., & Cohen, J. D. (2005). An integrative theory of locus coeruleus-norepinephrine function: adaptive gain and optimal performance. Annual Reviews Neuroscience, 28, 403-450 • http://www.uqtr.ca/~siroiss/pupillometry/ - A useful walkthrough of practical aspects in pupillometry (with further useful references!)

33. Thank you!