Clinical and Experimental Optometry 88.1 January 2005
39
Laughter abolishes binocular rivalry Pettigrew
OPTOMETRY
C L I N I C A L A N D E X P E R I M E N T A L
Clin Exp Optom 2005; 88: 1: 39–45
When different stimuli are viewed on cor-
responding retinal locations, most observ-
ers experience binocular rivalry, a striking
phenomenon involving quasi-periodic
switches between perceptual alternatives
at a rate that varies between ~0.1 Hz and
~1-2 Hz, depending on the stimulus con-
ditions and the individual.1,2 One might
have predicted an experience that was a
ORIGINAL PAPER
Laughter abolishes binocular rivalry
John D Pettigrew BSc (Med) MSc MBBS
FRS
Vision Touch and Hearing Research
Centre, University of Queensland,
Queensland, Australia
Submitted: 1 June 2004
Revised: 3 November 2004
Accepted for publication: 9 November
2004
Key words: binocular rivalry, laughter, retina
mixture of the alternatives, instead of a
distinct switch between them. This naive
expectation can be fulfilled in some sub-
jects and under some stimulus conditions
but is a less common experience than the
striking switch between alternative per-
cepts that characterises binocular rivalry.
Sherrington’s experiments3 with binocu-
lar colour rivalry led him to think that
mixing was the rule, whereas in fact per-
ceptual colour mixing is considerably
more common than mixing of other
exclusive percepts, such as those involv-
ing form and motion.
Nevertheless, the mixed percept even
occurs at times involving ‘strong’ stimuli
involving conflicting forms or motion. The
mixed percept, where both conflicting
Background: Binocular rivalry is an increasingly popular technique for the study of
consciousness, which changes quasi-regularly during rivalry, despite the unchanging
sensory stimuli presented to each eye. For example, if a small patch of horizontal stripes
is presented constantly to the fovea of one eye and a small patch of vertical stripes is
similarly presented constantly to the fovea of the other eye, most subjects experience an
alternation between stimuli rather than a simultaneous mixed percept of both.
Methods: Binocular rivalry was induced, superimposed on normal viewing, using liquid
crystal shutters and a short persistence monitor, which produced a one degree circular
patch of horizontal gratings to the right eye and an identical patch of vertical gratings
in the same location for the left eye. The subject signalled with key presses the three
possible perceptual states that alternated with each other, namely horizontal, vertical
and mixed percept (where horizontal and vertical were simultaneously visible).
Results: The present study builds on an incidental observation that laughter stopped
the rivalry alternations between horizontal and vertical and induced the mixed percept
instead. A physical explanation for this effect was ruled out by using stabilised imagery
in the form of retinal after-images of the rivalling gratings. Under conditions of retinal
stabilisation, laughter also produced the mixed percept.
Conclusions: The results are discussed in the light of recent work that indicates the
inadequacy of low-level explanation of rivalry, with laughter being another complex
multi-level contribution to the neural basis of rivalry, along with other aspects of mood.
The results are discussed in relation to the interesting literature on the neurology and
postulated functions of laughter.

Clinical and Experimental Optometry 88.1 January 2005
40
Laughter abolishes binocular rivalry Pettigrew
alternatives are seen simultaneously, thus
signals that the normal switching of binocu-
lar rivalry has ceased for some reason. For
example, in the paradigm that I use rou-
tinely, where the right eye is viewing a small
patch of horizontal lines and the left eye is
viewing a small superimposed patch of ver-
tical lines, volunteers will occasionally report
that there is an interruption of the usual
switching between the alternate percepts of
horizontal and of vertical. Instead, for a time
they see both horizontal and vertical lines
simultaneously as a ‘patchwork’, ‘gingham’,
‘crosshatch’ or ‘grid’. This mixed percept
is distinct and identifiable and clearly indi-
cates that the usual switching of binocular
rivalry is temporarily in abeyance.
One of my volunteers drew my attention
to a striking link between this mixed per-
cept and her own emotional state. Like
most slow-switching manic depressives, this
volunteer had virtually no mixed percept
under normal circumstances, so its occur-
rence was especially notable to her. With
considerable previous experience of the
test procedure, she was not distracted from
the rivalry task by the conversation I had
with her as the data were being collected.
At one point she exclaimed ‘Wow!’ and I
asked her to explain. She had noticed that
her mirth, elicited by one of my jokes,
seemed to be linked to the sudden cessa-
tion of binocular rivalry and its replace-
ment by the mixed percept of ‘grid’. After
a few more trials, where I provoked laugh-
ter that was always correlated with the ces-
sation of rivalry and the appearance of
‘grid’, we changed places and I was able
to observe the same striking phenomenon
in myself. I had never seen ‘grid’ under
those experimental circumstances, despite
many hours of being tested. I found it strik-
ing to observe the unfailing appearance
of ‘grid’ and cessation of binocular rivalry,
whenever I was provoked to laugh.
The present study presents more detail
from normal subjects concerning this link
between the abolition of binocular rivalry
and laughter. The findings are discussed
in relation to the neurology and postu-
lated function of laughter, which are pre-
sented in the context of the postulated
circuitry for the interhemispheric mid-
brain switch underlying rivalry.4
METHODS
There were 14 subjects of both sexes, be-
tween the ages of 19 and 47, all with visual
acuities of 6/6 or better, matched in both
eyes.
Each subject had at least 30 minutes
prior experience on the rivalry apparatus
and was therefore sufficiently practised to
be able easily to pay attention to the nar-
ration of joke lines, while simultaneously
reporting the three possible percepts
(horizontal, vertical or mixed) by press-
ing one of the three keys (H, V or Space
bar). Each key had an embossed pedestal
glued on top to facilitate finding it in the
dimly-illuminated room. All were naive
about the specific hypothesis linking the
mixed percept to laughter but it was com-
mon knowledge that the experiment in-
volved an investigation of the effects of
laughter on rivalry. The hilarity associated
with the experiment and its preparations,
such as the selection of suitable jokes, were
impossible to hide.
Binocular rivalry
The rivalry apparatus has been de-
scribed.4,5 Briefly, a circular patch 1.5
degrees in diameter was presented to both
eyes using liquid crystal shutters so that
the right eye saw a horizontal square wave
grating moving upwards and the left eye
saw a vertical grating drifting to the right
in the same location. The shutters were
synchronised via infrared to the vertical
refresh signal of an oscilloscope (persist-
ence 500 nanoseconds, P46 Green phos-
phor), on which the vertical and horizon-
tal gratings were alternating at 60 Hz.
Because all other visual targets in the
room, such as the edge of the oscilloscope
monitor and the wall decorations, were
clearly visible and could be fused by both
eyes, this set-up for binocular rivalry re-
quired no training and was tolerated by a
wide range of volunteers with no previous
experience in psychophysics. The ease and
comfort of the set-up facilitated the
present experiments. Laughter was much
easier to elicit in volunteers who were com-
fortable and could easily report their bin-
ocular rivalry alternations. Unlike other
methods of presenting binocular rivalry,
the set-up used here allowed for free head
movements, another important freedom
if one is to study laughter.
Block design
Three blocks of 10 minutes each were used
as previously described.4 Each block was
divided into four trials, each separated by
a 30-second rest. Blocks were separated by
100 seconds of rest.
The first block (no laughter block) in-
volved no communication between the
experimenter and the volunteer, who re-
ported his/her binocular rivalry alterna-
tions and bouts of mixed percept as previ-
ously described (V key for vertical percept,
B key for horizontal percept, space bar for
mixed horizontal and vertical percept).
In the second block (laughter block),
jokes were told to the volunteer, who kept
on reporting the three possible percepts
with the three keys, as in Block 1. Bouts of
laughter were recorded and linked to the
record of key presses.
In the third block (sober block), a sober
mood was created, to contrast with the
jovial second block. Tragic events in the
volunteer’s life, or recent public events of
a tragic or sombre nature were discussed.
Invoking laughter
Provoking a significant amount of genu-
ine mirth and laughter in the time-
constrained experimental design and the
darkened experimental room was chal-
lenging. Several measures were under-
taken. First, an effort was made to become
sufficiently familiar with each volunteer to
make it easier to choose jokes that were
appropriate to their demeanour and per-
sonality. Second, to further lighten the
atmosphere of the laboratory, each volun-
teer was asked to bring a friend to the ex-
perimental session who could assist in tell-
ing jokes or recalling humorous situations.
In some cases, such friends were very suc-
cessful at inducing laughter, often with
only a few well-chosen words. The confed-
erate left the experimental room during
the third, Sober Block. Finally, a wide selec-
tion of jokes was available, selected from
web-based and published collections, with
an emphasis on relatively short jokes to
maximise the number of different punch