The problem of contrast metric for
reaction time to aperiodic stimuli
• Angel Vassilev1, 2 Adrian Murzac2, Margarita B. Zlatkova3
& Roger S. Anderson3
• 1Institute of Neurobiology, Bulgarian Academy of
Sciences, Sofia, Bulgaria,
• 2New Bulgarian University, Sofia, Bulgaria
• 3Vision Science Reasearch Group, School of Biomedical
Sciences, University of Ulster, Coleraine, Northern Ireland,
UK
Aim
• We react faster to a strong stimulus than to a weak one. But
how to measure stimulus strength?
• The aim of the present talk is to give an example how the
choice of stimulus metric affects the conclusions drawn
from a reaction time study.
A typical reaction time experiment
Background
Luminance
Fixation
mark
• A stimulus is flashed
briefly and the observer
has to press on a key (or
release a key) as soon as
possible.
Stimulus • The metric of stimulus
strength most commonly
used is Weber contrast,
Stimulus, D I
DI/I: the change in
Background, I
luminance relative to the
background..
Distance
Is always Weber contrast the appropriate measure of
stimulus strength?
Luminance
• Difficulties are faced when comparing performances to
suprathreshold stimuli that share a common physical
metric, but give rise to different threshold sensitivities.
Such is the case with luminance increments and
decrements..
•For equal D Is,
increments and
decrements are of equal
Stimulus, D I
Weber contrast yet the
Background, I
thresholds might differ.
Usually, the threshold of
Distance
decrements is lower.
The work of Pokorny’s group
Luminance
• Using an unique technique, Cao, Zele and Pokorny (2007)
provided a rich RT data set. They measured cone and rod
RTs to stimuli presented within either a Rapid-On or
Rapid-Off ramp temporal window (fast phase:luminance
increment or decrement).
•They were able to
selectively stimulate the
1 sec
rods over a 5 log units range
Background of retinal illumination.
Time
Part of their data are
presented in the next slide.
Cao, Zele & Pokorny (Vision Res. 2007):
Cone and rod RTs compared
Observer AJZ
Reaction time (ms)
Observer DC
Weber contrast (%)
Note that the cone RTs to Rapid-ON and Rapid-OFF stimuli are similar,
while the rod RTs differ systematically
Cao et al.:Rod RT: Observer DC
0.002 Td
700
600
500
700
Reaction Time (ms)
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2 Td
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Weber contrast (%)
Cao et al.:Rod RT:Observer AJZ
700
0.002 Td
600
Reaction Time (ms)
500
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600
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80
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Weber contrast (%)
80 100
2
The work of Pokorny’s group
• In a parallel paper, Zele, Cao & Pokorny (Vision Research,
2007) posed the question regarding the metric of stimulus
strength used to compare performance to suprathreshold
stimuli.
The work of Pokorny’s group
• Contrast sensitivity (expressed in Weber contrast units) to
rod Rapid-Off stimuli was about two times higher than to
rod Rapid-On stimuli. As expected in view of the
difference in sensitivity, reaction time to Rapid-Off stimuli
was shorter than to Rapid-On stimuli over a range of
Weber contrasts. However, expressing stimulus strength in
multiples of threshold did not equate incremental and
decremental RTs. Instead, for stimuli at the same
suprathreshold level, RT to increments turned out shorter
than RT to decrements.
The work of Pokorny’s group
• Two more contrast metrics, tested by them also failed to
account for the differences between increment and decrement
RTs.
• Zele at al. (2007) assumed that the only meaningful
comparison of reaction times is the comparison of asymptotic
RTs.
• Here we show that a simple contrast metric equates their rod
reaction times and allows inferences about the mechanisms of
stimulus detection.
Luminance
Two cues, two types of detection:
two contrast metrics
Stimulus, D I
Ls
Ls
Background, I
(Lb)
Distance
Weber contrast = DI/I
Spatial luminance ratio =
Lmax/Lmin, the larger and the
smaller of Ls and Lb
• The stimulus generates a
temporal gradient DL of the
background illumination Lb as
well as a spatial gradient, (DL
+ Lb) against Lb. The stimulus
might be detected by temporal
(successive) luminance
discrimination or by spatial
(simultaneous) luminance
discrimination (Sperling &
Sondhi, 1968).
Two cues, two types of detection:
two contrast metrics
• Weber fraction DL/ Lb captures the temporal change of
luminance relative to the background. We assume that
spatial discrimination should depend on the ratio between
background luminance and stimulus luminance, Lb and Ls.
• We calculate it as Lmax/Lmin, where Lmax and Lmin are
the larger and smaller of Lb and Ls.
• The next two figures show the results of transforming
Weber contrast into spatial luminance ratio. The rod
reaction time data of Cao et al. (2007) are presented as
functions of Weber contrast and the spatial luminance
ratio.
•
Rod RT: Observer DC
600
600
500
500
700
Reaction Time (ms)
400
2 Td
600
300
40
80
120
160
0.002 Td
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0.002 Td
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500
1
2
3
4 6
7
400
700
0.02 Td
0.02 Td
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300
600
0
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40
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80 100
2 Td
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400
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600
1.0
1.2
1.4
1.6
1.8
2.0
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0
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80 100
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1.0
1.2
1.4
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2.0
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0.2 Td
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700
300
0
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40
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80 100
500
400
400
300
300
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40
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80 100
Weber contrast (%)
1.0
500
400
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600
500
0
0.2 Td
20 Td
1.0
1.2
1.4
1.6
1.8
1.2
1.4
1.6
1.8
2.0
2.0
Spatial luminance ratio
Rod RT: Observer AJZ
Reaction Time (ms)
700
700
0.002 Td
600
600
500
500
400
700
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600
40
80
120
160
2 Td
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80 100
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4 6
7
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20 Td
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1.0
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500
2.0
20 Td
1.4
1.6
1.8
2.0
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700
0.2 Td
20
40
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80 100
500
500
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400
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300
80 100
Weber contrast (%)
1.0
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0
60
1.8
600
1.2
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40
1.6
700
1.0
600
20
1.4
300
0.2 Td
0
1.2
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500
2 Td
0.02 Td
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3
700
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2 Td
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2
2 Td
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500
0.02 Td
0
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700
0.002 Td
1.0
1.2
1.4
1.6
1.8
1.2
1.4
1.6
1.8
2.0
2.0
Spatial luminance ratio
S-cone selective increments and decrements
(Murzac, 2004; Murzac & Vassilev, 2004)
500
500
Observer TST
450
Reaction Time (ms)
450
500
Observer ERT
450
400
400
400
350
350
350
300
300
300
250
0
20
40
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250
80 100
0
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40
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80 100
0
20
Observer KIR
40
60
80 100
Weber contrast (%)
500
500
Observer TST
450
450
500
Observer ERT
450
400
400
400
350
350
350
300
300
300
250
0
1
2
3
250
4 16 017 1
2
3
250
4 16 017 1
Observer KIR
2
Spatial luminance ratio
3
4 16 17
S-cone selective increments and decrements
• Perceptual differences supporting the assumption of two
types of stimulus detection:
• Some observers reported that perception of S-cone selective
stimuli differs from the perception of ordinary achromatic
stimuli. The sense of winking accompanies the onset of
ordinary stimuli but is absent with S-cone selective stimuli.
Two kinds of perception:
• Perception of stimulus onset
• Perception of stimulus presence
Summary of results
• Reaction times to luminance increments and decrements
are compared under several experimental conditions.
• Cao, Zele & Pokorny’s (2007) data:
• A. Photopic (all-cone) reaction times cluster around a
single RT/Weber-contrast function regardless of the
stimulus sign, increment or decrement.
• B. Rod incremental and decremental reaction times form
two distinct RT/Weber-contrast functions but cluster
around a single function when plotted against the spatialluminance ratio.
• Murzac (2004), Murzac & Vassilev’s (2004) data:
• S-cone reaction time tends to behave like rod reaction time
Interpretation
• Physiological and psychophysical data show that photopic
cone vision is faster and predominantly transient while
scotopic rod vision is slower and predominantly sustained
(Pepperberg, 2001). Also this seems to be the case for Scone vision (Reid & Shapley, 2002).
• Weber contrast is a measure of the transient stimulus
component while the spatial luminance ratio is a steadystate measure.
• The fit of both incremental and decremental RTs by a
single Weber-contrast function or by a single spatialcontrast function parallels the properties of the systems
involved in stimulus detection.
Conclusions
• We assume that the type of neural activity, predominantly
transient or sustained, and, respectively, the type of
stimulus detection by temporal (successive) luminance
discrimination or by spatial (simultaneous) luminance
discrimination determines the appropriateness of Weber
contrast or spatial luminance contrast metric for reaction
time.