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
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 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.
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.