Pupillary RecordingStuart 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

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

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

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.

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

Methods of Pupillary Measurement

• Entoptic Methods (Archimedes, Galileo)

• Comparison Pupillometers (Dots)

Simple Pupillometers

Methods of Pupillary Measurement

• Entoptic Methods (Archimedes, Galileo)

• Comparison Pupillometers (Dots)

• Calipers, Scales

• Photographs and film

• Bellarminoff Photogram (1885)

• General Precision (“Lowenstein”) electronic pupillograph

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

Horizontal Diameter Measured to .025 mm, 60/sec

Measuring sustained cognitive load via pupil dilation

From Tobii Spec Sheet

http://www.tobii.com/Global/Analysis/Downloads/Product_Descriptions/Tobii_TX_Product_description.pdf?epslanguage=en

ASL Applied Sciences Laboratories

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

Distortion of observed diameter due to parallax

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

Experimental ControlIdeally, 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.

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

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.

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

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.

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)

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)

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.

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

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.