Pupillary Recording Stuart R. Steinhauer, Ph.D. Biometrics Research Program University of Pittsburgh School of Medicine

1. Pupillary Recording Stuart R. Steinhauer, Ph.D. Biometrics Research Program University of Pittsburgh School of Medicine & VA Pittsburgh Healthcare System Society for Psychophysiological Research Workshop, Boston, September 14, 2011

2. Considerations in Measurement • Most people have equal pupils (isocoria), so either eye may be measured • However, check for significant anisocoria • Are subjects taking any anticholinergic drugs (look for light reaction, miosis/contraction to near vision) • Pupil diameter usually falls within range of 2-10 mm

3. How Bright is the Display • Luminance of the emitting visual field should be measured by a light meter (Photometer) and reported in standard units (1 cd/m2 = 0.292 foot-lamberts) including visual angle and/or distance from eye and size of stimuli for light sources

4. Measurement in Light • Report luminance levels • When changing visual stimuli, you are likely to elicit a light reaction (cf. Clynes, 1962) and need to consider this in analyses. • Dark stimuli on a gray background, rather than white on black, will help minimize effects of light reactions.

5. Measurements in Darkness • When recording in darkness (sound, discrete light stimuli), a fixation point should be at least one meter distant, using red stimuli (e.g., LEDs) that will not stimulate the photopic system. • Autokinetic phenomena: A single fixation light in darkness will appear to be moving– therefore use multiple close fixation lights (e.g., triangle of small LEDs) • Multiple light reactions – initial responses will be larger, interstimulus intervals should be constant except in complex designs

6. Methods of Pupillary Measurement • Entoptic Methods (Archimedes, Galileo) • Comparison Pupillometers (Dots)

7. Simple Pupillometers

8. Methods of Pupillary Measurement • Entoptic Methods (Archimedes, Galileo) • Comparison Pupillometers (Dots) • Calipers, Scales • Photographs and film • Bellarminoff Photogram (1885) • General Precision (“Lowenstein”) electronic pupillograph

10. Methods of Pupillary Measurement • Entoptic Methods (Archimedes, Galileo) • Comparison Pupillometers (Dots) • Calipers, Scales • Photographs and film • Bellarminoff Photogram (1885) • General Precision (“Lowenstein”) electronic pupillograph • Direct Video (video recorders) • Infra-red sensitive TV technology

11. Horizontal Diameter Measured to .025 mm, 60/sec

13. Measuring sustained cognitive load via pupil dilation

20. From Tobii Spec Sheet http://www.tobii.com/Global/Analysis/Downloads/Product_Descriptions/Tobii_TX_Product_description.pdf?epslanguage=en

21. ASL Applied Sciences Laboratories

23. Sources of Measurement Error • Edge Detection • Segmental Anomalies of the Iris • Parallax effects when large saccades occur: Vertical eye movements distort vertical diameter Horizontal eye movements distort horizontal diameter Systems that use estimate of ellipse rather than circle can correct for parallax effects

24. Distortion of observed diameter due to parallax

25. Sources of Measurement Error • Glasses, contacts, reflections from light sources - may cause reflections that interfere with either or both pupil and corneal reflection detection • Partial Eyelid closure – problem for vertical scan systems • Closed Eyes – cannot measure pupil! (but Yoss used an eye crutch in studies of sleepiness and narcolepsy) • Noise in the recording system: sensitivity of cameras, additional noise when analog data must be converted to digital values

26. Experimental Control Ideally, subjects should be in a separate testing room, with light and sound controlled. Communication via intercom. For non-automatic systems, head position should be maintained by a head/chin rest (bite bars were often used in the past) to maintain distance from camera lens.

27. What Temporal Resolution Do You Need? • Most systems are typically 50-60 Hz, can be up to 240 Hz, even over 1000 Hz (more critical for eye movements) • For latency (e.g., start of light reaction), these sampling rates are adequate What is critical? • Pupil maximum frequency is ~ 6-9Hz • Therefore, 20 Hz is actually fine for most amplitude measurements • Exact latency may require higher temporal resolution

28. Diameter vs. Area • Measures of diameter, radius, and area all have been reported • Diameter is preferred, most easily replicated and communicated • Changes in area tend to be large; moreover, when percentages used, they are much larger for small initial pupil diameters and tend to overrepresent effects.

29. Millimeters, pixels, percent% • The preferred reporting is always in mm of either absolute diameter or change • Reporting of % change often is distorting if groups or conditions began with different initial/baseline diameters • Reporting of either pixels or digital counts provides no replicable information; it works for analyses within a subject, but not necessarily across subjects and never across labs

30. Calibration 1: What is your output format? If your system saves data in mm., then bravo (but be sure they know how they measure – anything to the nearest .0001 mm can not make physiological sense). If data are reported in pixels or arbitrary digitial counts, then it must be scaled to known measures. Most typically, data will be reported in digital numbers.

31. Calibration 2: Finding the scaling factor Most systems are linear, but they may still have an intercept that is not equal to 0. Ideally, use a known calibration, preferably two reference points (we usually digitize 4 and 8 mm circles), and record a stable measure of each. The output of your system can be used to find: A) # of digital values equal to 1 mm change (slope) B) # of points in offset (see next slide)

32. Calibration 3: Finding the scaling factor Pupil diameter in mm = (Observed Value – Offset) / #pts per mm For example, suppose an 8 mm pupil results in a digital value = 376, and 4 mm pupil = 176 Slope = (376 - 176) [dig pts]/ (8-4) [mm] = 40 points/mm Offset (intercept) = 16 Thus, Pupil in mm = (P – 16)/40 (for each observation, subtract offset and divide by pts/mm)

33. Resolution of measurement • From the previous example, 40 pts/mm, it seems that the system is accurate to the nearest 1mm/40 points = 0.025 mm. • However, that may not be the case. The resolution is really a function of how many digital points are minimally changed between measures. • Thus, if there are at least differences of 3 digital points as the pupil changes, the true resolution would be 3 x 0.025 = 0.075 mm • Be careful – designers/engineers are not always aware of the physiological limitations of their systems in reporting accuracy.

34. What affects system accuracy? • Number of scan lines or pixels in the system • Magnification of the eye: this is a function of the optics of the lens system and distance from the camera. • A few systems automatically recalibrate magnification as distance changes, using (mostly) proprietary algorithms • In other systems, as either distance or magnification is changed, scaling factors are modified in unknowable ways; therefore, keep focal distance and magnification constant after you calibrate

36. Data from analog-based systems were output as voltages, stored on FM magnetic tape, and ultimately averaged on capacitor based summating computers, such as the Computer of Average Transients developed by Manfred Clynes. The newer systems provide either analog voltages which can be stored along with other electrophysiological signals, or provide direct digital formats that are stored in a variety of file formats directly on digital media.