To study the dynamics of
wave propogation in rivalry, we devised a technique allowing us
to control the location at which a dominance wave originates and,
therefore, to measure the speed at which that wave travels to
other parts of the pattern. Our technique capitalizes on the well-established
fact that an abrupt contrast increment reliably triggers the immediate
dominance of a previously suppressed rival pattern. In our experiments
the observer depressed and held the spacebar on the computer keyboard
when the radial grating (or, on other blocks of trials, the concentric
grating) was phenomenally suppressed, the spiral alone being visible.
Very shortly after this keypress, a brief contrast increment was
introduced at one location of the suppressed rival grating; the
observer monitored the rivalry status of a specified portion of
the grating, releasing the spacebar when that portion achieved
dominance. Over trials, we varied the distance between the perturbing
increment and the monitored portion of the grating.
Propogation time increases approximately linearly with angular
distance, with no evidence for variations in speed with distance
or with the location of the contrast increment. The abrupt appearance
of the contrast increment ignites a wave of dominance that spreads
in both directions away from the location of the increment.
In several ancillary experiments we examined the influence of
stimulus variables on the speed of wave propogation (Table 1).
Observer | c/deg | contrast | condition | time (sec) |
RB | 3.24 | .15 | one pulse | 1.63 (.06) |
RB | 3.24 | .075 | one pulse | 2.05 (.12) |
RB | 3.24 | .0375 | one pulse | 3.84 (.40) |
RB | 3.4 | .15 | Spiral contrast = .3 | 1.59 (.08) |
HRW | 3.24 | .25 | one pulse | |
HRW | 6.48 | .25 | one pulse | |
HRW | 1.62 | .25 | one pulse | |
RB | 3.24 | .15 | Double pulse | 1.05 |
Propogation time does not depend on grating spatial frequency
nor on the contrast of the contralateral, spiral grating. Wave
speed does, however, slow somewhat at low values of grating contrast.
Indeed, at very low contrast levels, the local dominance ignited
by a contrast increment often produces only a weak wave that dwindles
and never reaches regions of the grating in another quadrant of
the annulus. We also tried producing dominance waves using brief
decrements in the contrast of the suppressed grating, but decrements
proved ineffective at breaking suppression locally. Consequently,
the measured durations associated with this condition were primarily
those associated with spontaneous switches from suppression to
dominance at the monitored location.
For all observers speed of propogation is considerably faster
for the concentric grating than for the radial grating. This anisotropy
may be related to long-range lateral connections among orientation-selective
neurons in neighboring cortical columns. These long-range connections
tend to be more extensive among orientation columns that are collinear
in orientation preference compared to orientations parallel to
one another. Collinearity also boosts the incidence of joint predominance
of spatially distributed rivalry figures.
In another experiment, two contrast increments were introduced
simultaneously within the suppressed grating, at locations on
opposite sides of the circular annulus. As expected, dual perturbations
ignited two waves of dominance that spread from their respective
locations. The observer monitored the rival status of the grating
at the midpoint of these two perturbation spots, releasing the
spacebar whenever the first of these two waves reached the monitored
location. The average propogation time for this condition was
faster than that associated with perturbation at either point
alone. In fact, this increase in speed is predicted from probability
summation, the idea being that two stochastic processes will,
on average, yield faster response times than will either process
on its own.
METHODS
Four observers (one naive) participated in these experiments.
Annular rival patterns were generated on a 21" NEC RGB monitor
(P104 phosphor; 1024 x 768 resolution; 100 Hz frame rate) controlled
by a Power PC computer. All stimuli and trial-related events were
programmed using Matlab software in conjunction with the Psychophysics
Toolbox. The two circular patterns, each 3.6 deg visual angle
in diameter, were viewed through a mirror stereoscope with the
head stabilized by a chin and head rest. The central portion of
each circular pattern comprised a concentric grating with a central
fixation point. The peripheral, annular portion of one pattern
comprised a spiral figure generated as the weighted sum of a radial
and a concentric grating, with pitch angle equaling 45 deg. The
corresponding, annular portion of the other rival pattern was
either a radial grating (3.24 c/deg, unless otherwise specified)
or a concentric grating (also 3.24 c/deg). Grating contrast was
adjusted for each observer to find a value at which the spiral
(100% contrast) was completely dominant for the majority of the
viewing period; grating contrast varied among observers from .15
- .25.
For each experiment the observer first carefully adjusted the
stereoscope mirrors to achieve accurate, relaxed binocular alignment.
During the experiment, the observer maintained strict fixation
in the center of the annular rival patterns (a region in which
the two eyes viewed identical contours, which stabilized fusion).
Once the grating was suppressed in its entirety, with the spiral
alone dominant, the observer depressed the space bar on the computer
keyboard. This produced a 100 msec increment (abrupt onset and
offset, time locked to the video retrace) in the contrast of the
grating at one of eight equally spaced locations around the circular
grating. The increment comprised a gaussian spatial envelope the
half-width of which was 18 arc min; the magnitude of the contrast
increment was 0.70, a value sufficient to penetrate suppression
locally on virtually every trial. With fixation maintained in
the center of the pattern, the observer monitored the phenomenal
status of a region of the grating demarcated by two nonius lines.
Once that portion of the grating became dominant, the observer
released the spacebar, thereby providing a measure of the time
elapsing between presentation of the increment and reappearance
of the grating at the monitored site. Trials were run in blocks
of 30, with rest periods taken as needed. In several ancillary
experiments, the location of the contrast increment was fixed
relative to the monitored location of the grating; the purpose
of these experiments was to measure the average propogation time
for a fixed distance, typically as the function of some other
stimulus variable.