Research Team (alphabetical order): Geoffrey K Aguirre, David Brainard, Aleksandra “Sasha”
Igdalova, John “Jack” Ryan, Manuel Spitschan
Experiment Title: Perceptual Ratings of Light Flux, Cone-Directed and Melanopsin-Directed
Light Pulses
Research Project: Photoreceptor directed light modulation: perception, pupillometry, fMRI
Subjects
We will obtain a complete data set from 20 subjects from the Philadelphia area. Subject
ages will be between 18–75 years of age. Subject screening will follow the procedures
outlined in this preregistration document: https://osf.io/3c38s/. Subjects will be excluded for
a prior history of glaucoma, generalized epilepsy, or a negative reaction to dilating eye
drops. Subjects will be screened for a history of concussion. Saliva samples will be collected
from each subject for DNA extraction. Each subject will be screened for normal color vision
(using the Ishihara plates) and for normal best-corrected visual acuity (20/40 each eye or
better).
The experimental protocol is expected to be completed in a single, primary session of
approximately 1.5 hours of data collection, with a separate period of subject registration and
screening that may take place on a different day. Subjects will be paid $30/hour. Subjects
may have participated in other experiments as part of this larger project, or may be new to
the project. Subjects who have completed screening during another data collection effort
will not be required to complete it again. Research protocols have been approved by the
Institutional Review Board of the University of Pennsylvania.
Purpose
In a recent series of studies, we have examined neural responses to stimuli designed to
produce selective stimulation of the cones and of melanopsin
(http://dx.doi.org/10.17605/OSF.IO/VQADY http://dx.doi.org/10.17605/OSF.IO/YZWM6
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http://dx.doi.org/10.17605/OSF.IO/EUX6C http://dx.doi.org/10.17605/OSF.IO/G6R2Y
http://dx.doi.org/10.17605/OSF.IO/PV3A4). These studies have indicated that the primary
visual cortex responds to pulses of 400% contrast on melanopsin. The primary purpose of
this experiment is to measure qualities of the subjective experience of viewing light flux,
cone-directed and melanopsin-directed light pulses.
Stimuli
We will collect ratings along 9 perceptual dimensions to high-contrast melanopsin-, LMSdirected and light flux pulses (cosine window rise and fall time 500 msecs; total step
duration 3.0 secs) around a constant background light (mean luminance 20-200 cd/m2). The
nominal contrast on the targeted photoreceptor class (melanopsin, LMS, or combined for
light flux) is 400%. The background spectrum used for each of these modulation directions
will differ; different backgrounds are required to place the high contrast modulations within
device gamut. The chromaticity of the background used for each modulation type will be
matched as closely as can be achieved with our device, within the constraints imposed by
the need to generate the different modulation directions.
Specifically, we will record perceptual ratings of the following stimuli:
● Silent Substitution – MaxLMS
○ 3 sec, +400% LMS-directed step around a ~40 cd/m2 background
(chromaticity: x≈ 0.58, y≈ 0.37).
● Silent Substitution – MaxMel
○ 3 sec, +400% melanopsin-directed step around a ~100 cd/m2 background
(chromaticity: x≈ 0.55, y≈ 0.41).
● Light Flux
○ 3 sec, +400% light flux step around a ~39 cd/m2 background (chromaticity:
x≈ 0.54, y≈ 0.38)
Experimental procedure
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The right eye will be pharmacologically dilated, and the left eye will be covered with an eye
patch to prevent image interference.
The experiment will take place in two phases. Phase 1 will be the demonstration/exposure
phase and Phase 2 will be the experimental/ratings phase.
Phase 1 procedure:
Following dilation, the subject will be positioned in a chin rest and stare directly into the
eyepiece placed in front of their right eye. They will be asked to fixate a dot in the center of
the eye piece for 1 minute to adapt to the light flux background. Three stimulus trials will
then be presented. The subject will initiate each trial with a button press on a game pad
controller and will verbally report when they see a pulse. The adaptation and stimulus trial
procedure will then be repeated for the LMS and melanopsin stimulation conditions. The
intent of this phase is to expose subjects to the time course of the pulses and the general
mechanics of the study.
Phase 2 procedure:
Phase 2 will begin immediately following Phase 1. Subjects will adapt to a background
spectrum for 5 minutes. Perceptual rating trials will then be conducted. Prior to the start of
each trial, the experimenter will read to the subject a description of the perceptual property
the subject is to rate. The rating scales are numerical ratings ranging from 1 to 7. Each of
the properties the subject will judge are defined by two opposite words (e.g., cold and
warm). 1 and 7 correspond to the opposing words on either end of the scale, and 2, 3, 4, 5,
and 6 represent intermediate values between the two opposing words. Subjects will be
instructed to attend to the appearance of the light pulse in the periphery of the visual field
and report their perceptual judgment for this area. This is to dissuade subjects from
reporting perceptual changes that occur near the macula as a result of scattered and filtered
light. Following the instruction phase, the subject will press the button on the game pad to
initiate the presentation of the stimulus pulse. At the end of the 3 second pulse, the subject
will be given the opportunity to see the stimulus again if they desire. If they decline to see
the pulse a second time, they will then provide their rating. The experimenter will then
confirm and record the numerical value of the rating before proceeding to the next trial.
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Subjects will be asked to judge the following properties:
Cool – to – Warm
Dull – to – Glowing
Colorless – to – Colored
Focused – to – Blurry
Slow – to – Rapid
Pleasant – to – Unpleasant
Dim – to – Bright
Smooth – to – Jagged
Constant – to – Fading
Subjects will be presented pulses and asked to judge each of these perceptual
characteristics in turn. This procedure will then be repeated for the other two modulation
directions. This entire sequence will then be repeated, resulting in two ratings for each
perceptual property for each modulation direction. Subjects will be randomized to one of
two possible orderings of presentation of modulation direction:
Order A:
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Trial 1: Light Flux | Trial 2: Melanopsin | Trial 3: LMS | Trial 4: Light Flux | Trial 5: Melanopsin |
Trial 6: LMS
Order B:
Trial 1: Light Flux | Trial 2: LMS | Trial 3: Melanopsin | Trial 4: Light Flux | Trial 5: LMS | Trial 6:
Melanopsin
When ratings are complete for a given modulation direction, subjects will be invited to
provide any free-form observations that they wish to offer regarding the appearance of the
stimuli.
The instructions given to subjects are reproduced below in the Appendix.
Visual stimuli and delivery
All of the stimuli are generated with a digital light synthesis engine (OneLight Spectra)
under computer control. The stimuli are presented through a custom-made eyepiece. The
eyepiece contains a 25mm diameter illuminated disc viewed through a lens with a 20mm
focusing length. The subject views a circular, uniform field of 32° diameter. The central 10°
diameter of the field is obscured to block the effects of the foveal macular pigment which
can cause variation in photoreceptor spectral sensitivity. Subjects will view this field with
pharmacologically dilated right eye. The subject will view the stimulus through an 8mm
diameter artificial pupil to control retinal irradiance across subjects. Visual stimuli are
tailored to the age of the observer, taking their predicted lens density into account in the
pre-receptoral filtering (see Spitschan et al. 2014, 2015, 2016 for details).
Calibration
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Details on the calibration and generation of stimuli are found in Spitschan et al. (2014, 2015,
2016). First, background settings will be selected so as to maximize L+M+S contrast (‘midbackground’), or melanopsin-contrast. Then, a 66.66% modulation will be found. The
negative ‘arm’ of this modulation serves as the background for the studies, and the positive
‘arm’ of the modulation provides the pulses. By changing from the –66.66% settings to the
+66.66% settings, 400% contrast is achieved.
Standard operating procedure
Because of uncertainties in the stimulus control and spectral drifts in the output of the
digital light synthesis engine, we will adopt the following procedure:
The device is allowed several hours to reach a stable operating temperature. We then
perform a spectrum-seeking procedure, in which the spectral output predicted for our
stimuli is compared to the measured output. An “error spectrum” is then derived and
converted into stimulus settings, which are then used to correct the nominal settings. This is
performed over 10 iterations. This procedure yields settings which more closely produce the
desired spectra, which are then used in the experiment. This procedure is performed for
every subject as the stimulus spectra differ by subject age due to pre-receptoral filtering.
Validation
Before and after each experimental session, we will take spectroradiometric measurements
of the stimuli and their backgrounds, and calculate the cone and melanopsin contrast. We
will take five subsequent measurements of this so as to derive error bounds for the
validated contrast. We will calculate the mean and standard error of the five validation
measurements of the same stimulus taken before and after each session. If the pre-session
validation measurements do not meet the criteria specified below for the post-validation
measurements, the experimental session will be aborted and the subject rescheduled.
Exclusion criteria
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Psychophysical data from subjects who are registered for the experiment and pass screening
will be excluded from analysis and inclusion in the results for the following reasons:
● If the subject withdraws from the experiment before completion of the entire set of
data collection trials.
● If pre or post-experiment validation measurements for the melanopsin-directed
pulses reveal greater than 20% contrast on the LMS post-receptoral mechanism; 20%
on L–M; 20% contrast on S cones; or if the melanopsin LMS-directed contrast falls
below 350%.
● If pre or post-experiment validation measurements for the LMS-directed stimuli
reveal greater than 20% contrast on melanopsin, or on one or more of the to-besilenced post-receptoral cone mechanisms (L–M or S); or if the LMS-directed contrast
falls below 350%.
Recruitment target and study termination
We intend to collect a complete data set from 20 subjects, recruiting additional subjects as
needed to replace those who do not pass screening or whose data are excluded. We may
unexpectedly discover a flaw in this protocol that impedes data collection (e.g., the
screening or validation criteria are too strict). We will terminate the study if, in the judgment
of the research team, we are unable to complete the study as proposed with reasonable
effort. In the event of study termination, we will not consider any data collected under this
protocol up to the point of termination to be suitable for examining the primary outcomes.
Primary outcomes to be tested
We will test if, across subjects, perceptual ratings differ for the stimuli with different
photoreceptor targets. In particular, we wish to examine if the set of perceptual
measurements, and which perceptual measurements, allow reliable classification between the
melanopsin and LMS stimuli across subjects.
References
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Spitschan M, Aguirre GK, & Brainard DH (2015) Selective stimulation of penumbral cones
reveals perception in the shadow of retinal blood vessels. PLoS One 10(4): e0124328.
doi:10.1371/journal.pone.0124328.
Spitschan M, Data R, Stern AM, Brainard DH, & Aguirre GK (2016) Human visual cortex
responses to rapid cone and melanopsin-directed flicker. J Neurosci 36(5): 1471–82.
doi:10.1523/JNEUROSCI.1932-15.2016.
Spitschan M, Jain S, Brainard DH, & Aguirre GK (2014) Opponent melanopsin and S–cone
signals in the pupillary light response. Proc Natl Acad Sci U S A 111(43), 1556815572. doi:10.1073/pnas.1400942111.
Appendix:
Instructions:
Hello, and thank you for participating. We need your help describing the appearance of pulses
of light. During the experiment, you will look into an eyepiece and watch the light that you see.
On each of many occasions, the light will change, and we will ask you to rate various qualities of
how you see the changed light.
This experiment will take place in two phases. In the first phase we will have your eyes adapt for
a minute to a background color. This involves looking at a black dot in the center of the
eyepiece for one minute. After adapting, we will show you light pulses that differ from the
background. You will adapt to three different backgrounds, one for each of the three types of
light pulses that you will rate in the actual experiment. This phase is to make you familiar with
the types of lights you will see and the general procedure. In the second phase, we will give you
particular properties of the light to consider. You will watch the light pulse, then rate the
appearance of what you see. We will repeat this several times, occasionally pausing to adapt to
a different background.
Do you have any questions?
Exposure explanations:
Please place your face in the chin rest and focus on the center of the black dot in the eyepiece.
For the next minute, just fixate on the black dot. After that minute is finished, I will show you a
pulse of light. Tell me when you see the pulse. I will show you the same pulse several times,
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and each time simply tell me when you have seen it. We’ll then do this twice more, adapting for
a minute to a background, and then observing the appearance of a few light pulses.
CONDUCT THE EXPOSURE
Ratings explanations:
Now that you have seen what the stimuli look like, we will move on to the ratings portion of the
experiment. In this phase we will have you adapt to the background as we did in the first phase
by focusing on the center of the black dot in the eyepiece. This time, however, you’ll adapt for 5
minutes rather than for 1 minute. After you are done adapting you will be presented with a
series of 3 second pulses of the light. Before each pulse, I will give you a property that I’d like
you to use to judge the light. An example property might be how cool or warm the light pulse
appears. You’ll then watch the pulse and give me a rating on a scale of 1 to 7. I can repeat the
pulse of light for you if you would like a second look to help with your rating. We’ll then repeat
this for several other properties. I’ll remind you before and after each pulse what property you
are going to judge.
Each of the properties you will judge are defined by two opposite words, such as cool and
warm. Use your own understanding of what these words mean to make your rating of the light
pulse. We are interested in the appearance of the large area of the light outside of the black dot.
Keep looking at the center of the black dot during the pulse, and use your peripheral vision to
judge the appearance of the light. During the light pulse, you might see a change in the color or
appearance of the area right around the black dot, or in the black dot itself. Please ignore this
and provide your ratings for the area away from the center.
These are the properties that I will ask you to judge
Cool – to – Warm
Dull – to – Glowing
Colorless – to – Colored
Focused – to – Blurry
Slow – to – Rapid
Pleasant – to – Unpleasant
Dim – to – Bright
Smooth – to – Jagged
Constant – to – Fading
Again, I’ll let you know which property you are supposed to judge before and after each light
pulse. During the experiment you will adapt to the background and then judge the light pulses
on each of these properties a total of six times. The whole process should take about XX
minutes. If you like, you can take a brief break before beginning the next round of adapting to
the background. Let me know if you need a break.
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Ratings descriptions:
Every rating scale will be on a scale from 1 to 7. 1 and 7 correspond to the extremes on either
end of the scale, and 2, 3, 4, 5, and 6 represent intermediate values. Before each trial I will
explain to you what the rating means so you can more accurately describe what you are seeing
in the eyepiece. What matters to us most about your rating is that it describes what you see as
well as possible.
____________________________________________________________________________
Please remember to rate the space in the eyepiece surrounding the black dot, in the periphery
of your vision. Do not rate any changes that happen within the black dot itself, or in the space
immediately surrounding the black dot.
Cool -- to -- Warm: Cool is a 1. Warm is a 7.
- Confirm rating
Dull -- to -- Glowing: Dull is a 1, and clearly Glowing is a 7.
- Confirm rating
Colorless – to -- Colored: Please rate how much the color of the light pulse changes from the
background. 1 would mean that the color of the light pulse does not change from the
background, and a 7 would mean that the color of the light pulse was very different from that of
the background. Please ignore any changes in brightness when you make this rating.
- Confirm rating
Focused – to -- Blurry: Focused is a 1. Blurry is a 7.
- Confirm rating
Slow – to -- Rapid: A slow onset of the pulse is a 1, and a rapid onset of the pulse is a 7.
- Confirm rating
Pleasant – to -- Unpleasant: Pleasant to view is a 1. Unpleasant to view is a 7.
- Confirm rating
Dim – to -- Bright: Dim is a 1. Bright is a 7.
- Confirm rating
Smooth – to – Jagged: Smooth is a 1. Jagged is a 7.
- Confirm rating
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Constant -- to -- Fading: Constant is a 1. Fading is a 7.
- Confirm rating
Are there any observations about that set of pulses that you would like to make?
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