Azido- and amino-substituted dipicolinates for the sensitization of the luminescent lanthanides EuIII and TbIII

Article abstract of DOI:10.1016/j.ica.2020.120003

Dipicolinate-based ligands substituted with azido- or amino- functional groups were synthesized and characterized. Crystal structures of E_N3, C_N3, C_NH2, and E_NH2 were obtained, and the ability of these compounds to sensitize Ln^III luminescence (Ln^III = Eu^III and Tb^III) was explored. Luminescence efficiencies of Eu^III and Tb^III were determined and are ?_Eu = 0.11%, 0.15%, and 13.8% for [Ln(C_N3)₃]³⁺, [Ln(E_N3)₃]³⁺, [Ln(C_NH2)₃]³⁺, respectively, and ?^Tb = 0.30%, 0.27%, and 28.9% for [Ln(C_N3)₃]³⁺, [Ln(E_N3)₃]³⁺, [Ln(E_NH2)₃]³⁺. The highest luminescence efficiencies are ?_Eu = 14.4% for [Eu(E_NH2)₃]³⁺ and ?_Tb = 29.7% for [Tb(C_NH2)₃]³⁺. Similar ¹S and ³T excited state energies were observed for all ligands and did not explain the observed discrepancies in luminescence efficiency of their Tb^III and Eu^III complexes. Further analysis revealed that the azido-derivatives were not photostable, which likely contributed to the low luminescence efficiency.

Full text of DOI:10.1016/j.ica.2020.120003

Vision Research 83 (2013) 76–81
Contents lists available at SciVerse ScienceDirect
Vision Research
journal homepage: www.elsevier.com/locate/visres
Effects of attention on visual experience during monocular rivalry
a
b
a
a
Eric A. Reavis a, , Peter J. Kohler , Gideon P. Caplovitz , Thalia P. Wheatley , Peter U. Tse
⇑
a Department of Psychological & Brain Sciences, Dartmouth College, 6207 Moore Hall, Hanover, NH 03755, United States
b Department of Psychology, University of Nevada at Reno, 1664 North Virginia Street, Psychology Department Mailstop 0296, Reno, NV 89557-0296, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
There is a long-running debate over the extent to which volitional attention can modulate the appearance
of visual stimuli. Here we use monocular rivalry between afterimages to explore the effects of attention
on the contents of visual experience. In three experiments, we demonstrate that attended afterimages are
seen for longer periods, on average, than unattended afterimages. This occurs both when a feature of the
afterimage is attended directly and when a frame surrounding the afterimage is attended. The results of
these experiments show that volitional attention can dramatically influence the contents of visual
experience.
Received 20 November 2012
Received in revised form 4 February 2013
Available online 13 March 2013
Keywords:
Visual attention
Monocular rivalry
Consciousness
Afterimages
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Logothetis, 2002; Leopold & Logothetis, 1999). It has been shown
to exhibit a characteristic distribution of dominance durations
Can volitional attention alter the appearance of visual stimuli?
Disagreement over this question has persisted for over a century,
with, for example, Hermann von Helmholtz arguing in the affirma-
tive and Gustav Fechner in the negative (Helmholtz, 1866/1924). In
recent years, evidence has begun to accumulate in favor of the po-
sition that volitional attention can indeed alter the appearance of
visual stimuli (Abrams, Barbot, & Carrasco, 2010; Liu, Abrams, &
Carrasco, 2009; Tse, 2005). Volitional attention can also alter the
contents of visual experience by biasing perception toward a par-
ticular percept in binocular rivalry and ambiguous figure percep-
tion, where visual experience alternates between several
different percepts (Chong, Tadin, & Blake, 2005; Hancock &
Andrews, 2007; Meng & Tong, 2004; Ooi & He, 1999; van Ee, van
Dam, & Brouwer, 2005). Here we demonstrate for the first time
an effect of volitional attention on another type of perceptual mul-
tistability: monocular rivalry.
In monocular rivalry, a single image of two spatially overlapping
transparent surfaces can lead to the experience of perceptual alter-
nations between the two surfaces (Breese, 1899). Qualitatively,
monocular rivalry resembles binocular rivalry, where different
images presented to the two eyes alternate in visibility. Indeed,
there is evidence that the alternation dynamics and suppression ef-
fects which occur in monocular rivalry are similar to those of binoc-
ular rivalry (Maier, Logothetis, & Leopold, 2005; O’Shea et al., 2009).
Binocular rivalry has been characterized far more extensively
than monocular rivalry (for reviews, see Blake, 2001; Blake &
and alternation rates that is similar to those observed in ambigu-
ous figure perception (Brascamp et al., 2005; van Ee, 2005; Zhao
et al., 2004). Based largely on the similar properties of binocular
rivalry and ambiguous figure perception, some have proposed that
a single, supra-modal mechanism for resolving perceptual ambigu-
ity could drive the alternations in all forms of perceptual multista-
bility (Leopold & Logothetis, 1999). If this is true, then it would
follow that manipulations which affect the alternation process in
one form of multistability would likely have similar effects on
the alternation process in other forms of multistability.
Binocular rivalry and ambiguous figure perception can both be
influenced by volitional attention (Chong, Tadin, & Blake, 2005;
Hancock & Andrews, 2007; Meng & Tong, 2004; Ooi & He, 1999;
van Ee, van Dam, & Brouwer, 2005). If shared mechanisms are
responsible for the perceptual switching that occurs in monocular
rivalry, binocular rivalry, and ambiguous figure perception, then
attention might have a similar effect on perceptual switching in
monocular rivalry as it does in other types of multistable percep-
tion. To our knowledge, it remains an open question whether
attention influences monocular rivalry (although O’Shea (2006),
published an abstract reporting possible effects of attention on
monocular rivalry).
Here, we examine the influence of volitional attention on the vi-
sual experience of color afterimages engaged in monocular rivalry.
Afterimages were induced by stimuli like those shown in Fig. 1.
Color afterimages typically occur when opponent-coded retinal
ganglion cells undergo a rebound in firing following prolonged
exposure to an unchanging stimulus (Zaidi et al., 2012). To the
viewer, the afterimages appear like a weak version of the color
⇑
Corresponding author.
E-mail address: Eric.A.Reavis@Dartmouth.edu (E.A. Reavis).
0042-6989/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.visres.2013.03.002

E.A. Reavis et al. / Vision Research 83 (2013) 76–81
77
an outlier who, in several analyses, was more than three standard
deviations from the group mean. Exclusion of the outlier did not
change the direction or significance of any of the reported results.
In this and subsequent experiments, all participants gave informed
consent to participate, and all experimental procedures were ap-
proved by the Dartmouth Committee for the Protection of Human
Subjects.
Participants viewed stimuli on a Mitsubishi Diamond Pro
2070SB CRT monitor from a distance of 57 cm, in a darkened room.
Stimuli were generated and presented using the Psychophysics
Toolbox, version 3, on a PC running MATLAB R2010a in Linux
(Brainard, 1997; Pelli, 1997; MathWorks, Natick, MA).
Each participant completed 100 experimental trials. On each
trial, participants fixated on a dot in the center of the screen and
pressed a key to initiate the trial. While fixating, they passively
viewed an adapting stimulus for 10 s (example in Fig. 1a). The
adapting stimulus contained two overlapping rectangular frames
forming
a
plus-sign (12 Â 12 deg vis. ang., area of overlap
8 Â 8 deg). Each frame was filled with a transparent texture of blue
and yellow or red and green stripes. The background had a mean
luminance of 102 cd/m2, and the adapting stimuli had a mean
luminance of 64 cd/m2 in the overlapping region. The color of the
texture within each frame was randomly assigned for each trial.
The orientation of the stripes was counterbalanced: half of the tri-
als contained stripes perpendicular to the rectangles, half parallel
to the rectangles.
After the adapting period of each trial, all stimuli except the fix-
ation point disappeared, and a tone sounded for 50 ms as a non-vi-
sual attentional cue. Participants were instructed to pay attention
to the vertical frame on trials when they heard a high tone, and
to the horizontal frame on trials when they heard a low tone.
Immediately after the tone, the rectangular frames reappeared
for 10 s. Participants reported what they saw at each moment
within the square region of overlap between the two rectangles.
They held one key if they saw vertical stripes, another if they
saw horizontal stripes, and a third if they saw a mixture of both
orientations. They were instructed to hold the third key anytime
the overlapping region was not exclusively filled by just one of
the two afterimage patterns (i.e., if there was any amount of piece-
meal rivalry). Participants switched keys as often as necessary to
accurately reflect their visual experience, and released all the keys
when they no longer saw an afterimage.
Fig. 1. Example stimuli. Please refer to the web version of the article for color
stimuli. Panel A corresponds to a ‘parallel stripes’ trial in Experiment 1. Panel B
shows an example from Experiment 2. Panel C illustrates a trial from Experiment 3.
The reader can experience a demonstration of the effects by fixating in the center of
one of the three colored stimuli for ten or more seconds, then shifting fixation to the
empty frames at right, and attending to one of the two frames (or stripe
orientations, in Panel C) The afterimage corresponding to the attended surface
should tend to be seen for longer periods than the unattended surface.
inverse of the adapting stimulus, but the exact contents of the per-
cept can be influenced by contextual cues presented during after-
image perception (Daw, 1962; van Lier, Vergeer, & Anstis, 2009).
For post-retinal visual processes such as attention, afterimages
are likely equivalent to direct retinal input. The main advantage
of using afterimages to study monocular rivalry is that afterimage
percepts are stabilized on the retina, and are therefore conve-
niently immune to the confounding effects of eye movements that
can otherwise influence monocular rivalry alternations (Crassini &
Broerse, 1982; Georgeson, 1984).
In three experiments, we show that the mean dominance dura-
tion of attended afterimage surfaces engaged in monocular rivalry
is longer than that of unattended afterimage surfaces. In Experi-
ment 1, we establish the basic effect of volitional attention on
rivaling afterimages. Experiment 2 controls for a potential stimulus
confound in Experiment 1 and replicates the effect. Experiment 3
demonstrates that the effect holds when attention is directed to
a feature of the afterimage itself, rather than a static afterimage
boundary as in Experiments 1 and 2.
2.2. Results
The average probability of every possible response at each mo-
ment of the response period can be seen in Fig. 2a. Responses con-
gruent with the attended stimulus are in green, those congruent
with the unattended stimulus are in red, and mixed responses
in black. Shading indicates 95% confidence intervals based on the
t-statistic. Hence, non-overlapping regions have significantly
different response probabilities. Responses congruent with the
attended stimulus are significantly more probable than those
congruent with the unattended stimulus between 820 and
1910 ms post-cue, and both types of single-percept response are
significantly more probable than mixed percepts until very late
in the reporting period when percepts of any kind were quite rare
because afterimages had faded. Since mixed percepts accounted for
only 10.7% of the total time participants reported any percept, they
were excluded from later analyses.
2. Experiment 1
On average, observers reported 3.25 dominance epochs per trial
(SEM = 0.25). On average, for the first 3 dominance epochs of the
alternation process, the attended stimulus tended to be seen for
longer than the unattended stimulus. In a repeated-measures AN-
OVA with factors attention (attended, unattended) and dominance
epoch (1, 2, 3), there was a significant main effect of attention
2.1. Methods
Twenty-one naïve observers (5 males, mean age 19.9 years,
SEM = 0.60 years) participated for payment or course credit. One
additional participant was tested but excluded from analysis as

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E.A. Reavis et al. / Vision Research 83 (2013) 76–81
Fig. 3. Differences in dominance period length. In Experiment 1, the overall effect of
attention is a significant increase in dominance duration for the attended stimulus
(solid bar). This effect is driven by the trials where the two surfaces contain stripes
parallel to the overlapping frames (vertical pattern), and greatly reduced in trials
containing perpendicular stripes (horizontal pattern). Experiments 2 replicates this
effect of attention with afterimages oriented obliquely to the frames. While the
effect across all trials (solid bar) is similar to Experiment 1, there is no significant
effect of stripe orientation in Experiment 2 (diagonally-striped bars). Experiment 3
replicates the difference between attended and unattended afterimages with
completely overlapping afterimage patterns and attention cued to a feature of the
afterimages themselves. All error bars represent SEM.
as can be seen in Fig. 3a, the effect of attention on dominance dura-
tion was greater in the parallel conditions (mean differ-
ence = 273 ms, SEM: 64 ms) than in the perpendicular conditions
(mean difference = 53 ms, SEM: 78 ms). The interaction was driven
primarily by decreased dominance durations for the unattended
parallel stimuli (M = 1156 ms), while the mean dominance dura-
tions for the other three conditions were similar (attended parallel
M = 1429 ms, attended perpendicular M = 1400 ms, unattended
perpendicular M = 1346 ms).
In perpendicular trials, attended stimuli were significantly less
likely to be the first reported percept than unattended stimuli
(35.91% of trials and 64.10% of trials, respectively) (t(20) = À3.94,
p < 0.001). In parallel trials, the reverse was true: attended stimuli
were significantly more likely to be the first reported percept
(64.38% of trials) than unattended stimuli (35.62%) (t(20) = 3.58,
p = 0.002).
Fig. 2. Probability of each possible response, over time. Lines indicate the
probability of every possible response at each moment of the 10-s response period.
The green line indicates the probability that the surface congruent with the
attentional cue was perceived. The red line indicates the probability that surface
incongruent with the attentional cue was perceived. The black line indicates the
probability that a mixed percept was perceived. Shading indicates 95% confidence
intervals based on the t-statistic. Therefore, response probabilities in areas of non-
overlap are significantly different. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
2.3. Discussion
Experiment 1 demonstrates that volitional attention modulates
the contents of visual experience during monocular rivalry. In the
central overlapping region, observers perceived the afterimage
consistent with the unattended rectangular frame for shorter dura-
tions than the one consistent with the attended frame. Percepts
consistent with the attended frame were significantly more fre-
quent early in the average perceptual timecourse, but this effect
diminished over time. However, when the durations of the first
three dominance epochs of each participant were computed for
each trial, and the duration of attended versus unattended percepts
were compared, attended percepts were perceived for longer
regardless of when they occurred in time. Why are significant dif-
ferences in the probability of perceiving the attended versus unat-
tended surface not apparent later in the perceptual timecourse,
averaged across subjects, if differences in attended versus unat-
tended surfaces are apparent in the average durations of the first
three perceptual dominance periods for each participant? This
can be explained by inter-subject differences in the dominance
durations: subject-wise differences in alternation rate would be
expected to produce a compounding decline in the likelihood that
(F(1,20) = 11.67, p = 0.003, g2P = .037), but no main effect of domi-
nance epoch (F(2,19) = 1.61, p = 0.23, g2P = 0.15), and no interaction
(F(2,19) = 0.34, p = 0.72, g2P = 0.03).
Exploration of the data suggested that the effects of attention on
afterimage perception were largely dependent upon trials where
the stripes of the afterimage were parallel to the rectangular
frames. To quantify this, we performed a 2 Â 2 repeated measures
ANOVA on all the data (i.e., including information from dominance
epochs later than the third) with factors attention (attended, unat-
tended) and stripe orientation (parallel, perpendicular). This anal-
ysis confirmed the significant main effect of attention found in the
previous analysis with data from the first three dominance periods,
such that the mean dominance duration for the attended stimuli
was on average 147 ms longer than the unattended stimuli (see
Fig. 3a) (F(1,20) = 9.67, p = 0.006, g2P = 0.33). There was no signifi-
cant effect of orientation (F(1,20) = 0.927, p = 0.35, g2P = 0.04), but
the analysis revealed a significant interaction between attention
and orientation (F(1,20) = 5.07, p = 0.036, g2P = 0.20). Specifically,

E.A. Reavis et al. / Vision Research 83 (2013) 76–81
79
different subjects would be in the same cycle of perceptual alterna-
tion as time progressed.
ter dominance periods being longer than early ones (dominance
epoch main effect F(2,7) = 4.12, p = 0.07, g2P = 0.54). However, there
was no interaction between attention and epoch (F(2,7) = 0.58,
p = 0.58, g2P = 0.14).
Attention also significantly influenced mean dominance dura-
tions when the mean dominance duration of all dominance epochs
(including those beyond three) was analyzed. Across all dominanc-
es, afterimages consistent with the attended frame were perceived
for 151 ms longer, on average, than those consistent with the unat-
tended frame (SEM = 64 ms, t(8) = 2.34, p = 0.047, two-tailed) (see
Fig. 3b).
In order to ensure that the change in afterimage stripe orienta-
tion did indeed abolish the interaction between orientation and
attention seen in Experiment 1, we conducted a 2 Â 2 repeated
measures ANOVA on the full dataset in exactly the same fashion
as in Experiment 1 (with factors orientation and attention). In con-
trast to Experiment 1, there was no significant interaction between
orientation and attention (F(1,8) = 0.32, p = 0.59, g2P = 0.04) (see
Fig. 3b).
Surprisingly, the attentional effect in Experiment 1 was driven
almost exclusively by trials in which the stripes of the adapting
stimuli were parallel to the orientations of the rectangles. In paral-
lel trials, attended stimuli were also more likely to be reported as
the first percept, while the reverse was true for perpendicular tri-
als. Is the effect of volitional attention on rivaling afterimages lim-
ited to stimulus configurations in which the orientations of the
afterimage and the attended frame are congruent? Experiment 2
tests the generalizability of the effect by modifying the stimulus
configuration to remove consistencies in orientation between the
afterimage and attended frame. In Experiment 2, the afterimage
stripes were rotated 45° within the frames, while all other factors
were kept consistent. Though in principle the frames could have
been rotated and the inducers kept in cardinal orientations, the
frame orientations were maintained so that the attentional cues
(‘attend vertical’ vs. ‘attend horizontal’) could be kept constant,
eliminating the possible confound that participants might find it
easier to attend to certain orientations than others.
Furthermore, reorientation of the afterimages controls for pos-
sible response bias in Experiment 1, wherein participants might
simply be more likely to respond with the key corresponding to
the attentional cue. Rotation of the afterimages makes the re-
sponse criterion (afterimage tilted clockwise/counterclockwise)
orthogonal to the attentional cue (attend to the vertical/horizontal
frame).
3.3. Discussion
The results of Experiment 2 confirm and extend the results of
Experiment 1: volitional attention alters the experience of rivaling
afterimages, even when the orientations of the afterimage textures
are neither parallel nor perpendicular to the attended frames. Fur-
thermore, as expected, the interaction between frame and afterim-
age orientations seen in Experiment 1 are not observed when these
parallels are removed in Experiment 2. These differences allow us
to conclude that although the attentional effects on the visibility of
an afterimage can be influenced by stimulus configuration, these
effects do not depend upon a specific configuration.
3. Experiment 2
3.1. Methods
In the following experiment we further investigate the effects of
stimulus configuration on attentional modulation of monocular
rivalry. In both Experiments 1 and 2, the adapting surfaces had re-
gions of non-overlap. It could be that these non-overlapping re-
gions are critical for the effect, allowing an unambiguous region
to be attended and filling-in of the ambiguous central region to
proceed from there. Thus, the question remains: is the ability of
attention to influence the perceived afterimage dependent upon
these non-overlapping regions? This question is addressed by
Experiment 3, wherein the sections of non-overlap are eliminated
and attention is directed directly to a feature of one of the compet-
ing afterimages (orientation).
Nine observers (6 males, mean age 25.3 years, SEM = 1.09 years)
naïve to the purpose of the experiment participated for payment or
course credit.
The methods in Experiment 2 were identical to Experiment 1,
except that the stripes in the adapting surfaces were rotated 45 de-
grees within the existing frames, so their orientations were differ-
ent than either of the frames (see Fig. 1b). Observers responded
that the afterimage perceived was tilted clockwise, counterclock-
wise, or both, from vertical. As before, they held a separate key
anytime there was any amount of piecemeal rivalry or a mixture
of the two stimuli perceived.
3.2. Results
4. Experiment 3
The probability of each possible response at each moment of the
10-s response period can be observed in Fig. 2b. As in Experiment
1, percepts congruent with the attended afterimage were signifi-
cantly more probable than percepts congruent with the unat-
tended afterimage early in the response period, between 630 and
1270 ms post-cue, and these differences diminished over time.
Again, mixed percepts were quite rare. In fact, the probability of
a mixed percept was generally not significantly different from 0.
Since subjects only reported mixed percepts for 3.8% of the total
time in which they reported any percept, those responses were ex-
cluded from further analysis. Initial responses consistent with the
cued condition were not significantly more frequent (55.89% of tri-
als) than uncued responses (44.11%) (t(8) = 1.54, p = 0.16).
Subjects reported an average of 3.77 dominance epochs before
the afterimage faded (SEM = 0.43). As in Experiment 1, a 2 Â 3 re-
peated-measures ANOVA with factors attention and dominance
epoch (1,2,3) indicates that attended stimuli were perceived for
longer, on average, than unattended (attention main effect
F(1,8) = 5.77, p = 0.04, g2P = 0.42). There was also a trend toward la-
4.1. Methods
Eight observers (3 males, mean age 26.6 years, SEM = 1.02 years)
participated for payment or course credit. All observers were naïve
to the purpose of the experiment, but four had participated in Exper-
iment 2, and one had participated in Experiment 1.
The methods were again nearly identical to those employed in
Experiments 1 and 2. However, the adapting stimulus consisted
of a single square (8 Â 8 deg. vis. ang.) corresponding to just the
overlapping region of the stimuli used in the previous experiments,
as shown in Fig. 1c. Thus, the two surfaces – cropped versions of
those used in Experiment 1 – were completely overlapping. The
same auditory tones were used to cue attention as in the previous
two experiments, but participants were instructed to attend to the
vertical or horizontal stripes following the cue, rather than the ver-
tical or horizontal frames, as in the two previous experiments.
Observers held a separate key when any amount of piecemeal riv-
alry or a mixed percept was perceived.

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E.A. Reavis et al. / Vision Research 83 (2013) 76–81
4.2. Results
the stimulus. Certain configurations (e.g., parallel afterimage and
frame orientations) appear to facilitate the effects of attention
while others diminish it (e.g., perpendicular afterimage and frame
orientations). A fuller exploration of what types of stimuli facilitate
attentional influences on rivalry and why is an important area for
future study.
The probability of each of the three possible percepts, over time,
is shown in Fig. 2c. The general pattern of results is similar to that
of Experiments 1 and 2. Percepts congruent with the attended sur-
face were significantly more probable than incongruent percepts
early in the response period (520–910 ms post-cue), and mixed
percepts were significantly less probable than either single-surface
percept for the majority of the response period (although the dif-
ference between mixed and unattended responses was not signif-
icant between 1620 and 3820 ms post-cue). Again, due to the
relative rarity of mixed percepts (only 11.5% of all reported per-
cepts), those responses were excluded from further analysis. As
in the previous experiments, afterimages generally faded from
view well before the end of the response period. Initial responses
consistent with the cued condition were marginally more frequent
(56.88% of trials) than uncued responses (43.13%) (t(7) = 2.11,
p = 0.07).
Participants reported an average of 4.03 dominance periods be-
fore the afterimage faded (SEM = 0.67). As in the previous two
experiments, we performed a 2 Â 3 repeated-measures ANOVA
with factors attention (attended, unattended) and dominance
epoch (1–3). There was a marginally significant main effect of
attention (F(1,6) = 4.80, p = 0.07, g2P = 0.44), but no main effect of
dominance epoch (F(2,5) = 2.43, p = 0.18, g2P = 0.49), and no inter-
action (F(2,5) = 0.16, p = 0.86, g2P = 0.06).
Intriguingly, the significant interaction between stripe orienta-
tion and attention in Experiment 1 is consistent with the possibil-
ity that spatial and feature-based volitional attention could have
independent influences on dominance duration. It is possible that
allocation of attention to an oriented frame involves both spatial
attention to the region and feature-based attention to the orienta-
tion itself. Were that the case, in Experiment 1, parallel trials
would have elicited the allocation of spatial and feature-based
attention to the same surface, while perpendicular trials would eli-
cit spatial attention to one surface and feature-based attention to
its rival. In other words, the results of Experiment 1 could have
been caused by independent, additive effects of spatial and fea-
ture-based attention on dominance duration, which cancelled each
other out in the perpendicular condition. This would be consistent
with existing physiological data showing that spatial and feature-
based attention have additive effects on neural activity (e.g.,
Andersen, Fuchs, & Müller, 2011; Hayden & Gallant, 2005, 2009;
Treue & Martinez-Trujillo, 1999).
Our results also have implications for present theories of per-
ceptual multistability. We demonstrate that dominance durations
in monocular rivalry can be influenced by attention. This parallels
findings from studies of binocular rivalry and ambiguous figure
perception (Chong, Tadin, & Blake, 2005; Meng & Tong, 2004; Ooi
& He, 1999). Indeed, the dominance durations and attentional ef-
fects we report are of similar order of magnitude to those reported
in the binocular rivalry literature (e.g., van Ee, van Dam, & Brouwer,
2005), though this similarity must be interpreted with caution be-
cause of the known dependence of such durations on size, contrast,
and other basic stimulus characteristics that differ across experi-
ments. Our results are consistent with the theory that various
forms of perceptual multistability are driven by a common mech-
anism of alternation. We show that monocular rivalry is affected
by volitional attention in much the same way as binocular rivalry
and ambiguous figure perception, which could mean that attention
is influencing a common mechanism of alternation in all three
instances.
Because our experiments did not include a ‘no attention’ condi-
tion, it is not possible to determine from our results whether the
effects of attention resulted from an enhancement of processing
of the attended stimulus, or from inhibition of processing of the
suppressed stimulus. The attentional effects observed in binocular
rivalry appear to result from enhancement of processing of the at-
tended stimulus, equivalent to an increase in contrast for that
stimulus during periods of dominance (Chong, Tadin, & Blake,
2005). Attention might have a similar effect of increasing the effec-
tive contrast of the attended stimulus during dominance periods in
monocular rivalry, but additional experiments must be performed
to test this empirically.
In summary, we identify a new domain in which volitional
attention modulates the contents of visual experience. Our results
demonstrate that endogenous, volitional factors such as attention
can dramatically influence the perception of stimuli undergoing
monocular rivalry. Thus, we provide new support for the theory
that visual experience can be influenced by volitional attention.
The effect of attention was evident when the data from all dom-
inance epochs (not just the first three) were analyzed. Afterimages
of the stripes matching the attended orientation were seen for
170 ms longer, on average, than afterimages of the stripes match-
ing the unattended orientation (t(7) = 2.61, p = 0.04, two-tailed,
SEM = 65 ms) (see Fig. 3c).
4.3. Discussion
Experiment 3 replicates and extends the findings of the previ-
ous two experiments. Again, attended afterimages are significantly
more likely to be perceived than unattended ones. Experiment 3
demonstrates that this continues to hold true when a feature of
the afterimage itself (orientation) is attended, rather than the bor-
dering outline attended in the previous two experiments. Further-
more, Experiment 3 demonstrates that attention influences rivalry
dominance durations even when the competing stimuli are com-
pletely overlapping. This refutes the hypothesis that areas of
non-overlap are necessary to provide an unambiguous ‘handle’ to
which attention can be directed that might enable filling-in to
ambiguous overlapping regions.
5. General discussion
The results of the three experiments reported here demonstrate
that volitional attention can alter the contents of experience. Spe-
cifically, volitional attention can alter the appearance of overlap-
ping transparent surfaces engaged in monocular rivalry. Attended
surfaces are perceived for longer durations than unattended sur-
faces across a variety of stimulus configurations. The effect is
demonstrable within a single trial, yet quantifiable by averaging
the influence of attention on rivalry dominance period length over
many trials. Attention modulates visual experience whether it is
allocated to surrounding frames or to the orientation of the after-
images. Thus, the results speak strongly in favor of the view that
volitional attention influences the subjective appearance of visual
stimuli engaged in monocular rivalry.
Acknowledgments
The interaction between afterimage and frame orientation in
Experiment 1 demonstrates that the effects of attention on appear-
ance in monocular rivalry can be modulated by the properties of
This work was supported by a grant from the Templeton
Foundation (to P.U.T.), and a National Science Foundation Graduate
Research Program fellowship (to E.A.R.). G.P.C. was supported by

E.A. Reavis et al. / Vision Research 83 (2013) 76–81
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