Member-Initiated Symposia
Symposia Overview
2008 Symposia
Perceptual expectations and the neural processing of complex images
Cortical organization
and dynamics for visual perception and beyond
Crowding
Visual Memory and the Brain
Bayesian models applied to perceptual
behavior
Action for perception: functional
significance of eye movements for vision
The past,
present, and future of the written word
Surface material perception
Symposia from Past Meetings |
Action for perception: functional
significance of eye movements for vision
Friday, May 9, 2008, 3:30 - 5:30 pm
Orchid 1
Organizers:
Anna Montagnini (Institut de Neurosciences
Cognitives de la Méditerranée) and Miriam Spering (Justus-Liebig University
Giessen, Germany)
Presenters:
Maria Concetta Morrone (Facoltà di
Psicologia, Università Vita-Salute S Raffaele, Milano, Italy),
Tirin Moore (Stanford University School of Medicine, USA),
Michele Rucci (Boston University),
Miriam Spering (Justus-Liebig University Giessen, Germany; New York University),
Ziad Hafed (Systems Neurobiology
Laboratory, Salk Institute),
Wilson S. Geisler (University of Texas, Austin)
Symposium Description
When we view the world
around us, our eyes are constantly in motion.
Different types of eye
movements are used to bring the image of an object of interest onto the fovea,
to keep it stable on this high-resolution area of the retina, or to avoid visual
fading. Moment by moment, eye movements change the retinal input to the visual
system of primates, thereby determining what we see. This critical role of eye
movements is now widely acknowledged, and closely related to a research program
termed “Active Vision” (Findlay & Gilchrist, 2003).
While eye movements improve
vision, they might also come at a cost.
Voluntary eye movements can
impair perception of objects, space and time, and affect attentional processing.
When using eye movements as a sensitive tool to infer visual and cognitive
processing, these constraints have to be taken into account.
The proposed symposium
responds to an increasing interest in vision sciences to use eye movements. The
aims of the symposium are (i) to review and discuss findings related to
perceptual consequences of eye movements, (ii) to introduce new methodological
approaches that take into account these consequences, and (iii) to encourage
vision scientists to focus on the dynamic interplay between vision and
oculomotor behavior.
The symposium spans a wide
area of research on visuomotor interaction, and brings to the table junior and
senior researchers from different disciplines, studying different types of eye
movements and perceptual behaviors. All speakers are at the forefront of
research in vision and brain sciences and have made significant contributions to
the understanding of the questions at hand, using a variety of methodological
approaches.
Concetta Morrone (Università
Vita-Salute, Italy) reviews findings on the perisaccadic compression of space
and time, and provides a Bayesian model for these perceptual phenomena. Tirin
Moore (Stanford University, USA) discusses the neural mechanisms of perisaccadic
changes in visual and attentional processing. Michele Rucci (Boston University,
USA) argues for an increase in spatial sensitivity due to involuntary miniature
eye movements during fixation, which are optimized for the statistics of natural
scenes.
Miriam Spering (University
of Giessen, Germany) focuses on the relationship between smooth pursuit eye
movements and the ability to perceive and predict visual motion. Ziad Hafed
(Salk Institute, USA) discusses the effect of eye movements on object
perception, pointing out an intriguing role of oculomotor control for visual
optimization. Wilson Geisler (University of Texas, USA) uses ideal-observer
analysis to model the selection of fixation locations across a visual scene,
demonstrating the high degree of efficiency in human visuomotor strategy.
The topic of this symposium
is at the same time of general interest and of specific importance. It should
attract at least three groups of VSS attendants – those interested in low-level
visual perception, in motor behavior, and those using eye movements as a tool.
We expect to attract both students, seeking an introduction to the topic, and
faculty, looking for up-to date insights. It will be beneficial for VSS to
include a symposium devoted to the dynamic and interactive link between visual
perception and oculomotor behavior.
Abstracts
Perception of space and time during saccades:
a Bayesian explanation for perisaccadic distortions
Maria Concetta Morrone,
Paola Binda and David Burr
During a critical period
around the time of saccades, briefly presented stimuli are grossly mislocalized
in space and time and both relative distances and durations appear strongly
compressed. We investigated whether the Bayesian hypothesis of optimal sensory
fusion could account for some of the mislocalizations, taking advantage of the
fact that auditory stimuli are unaffected by saccades. For spatial localization,
vision usually dominates over audition during fixation (the “ventriloquist
effect”); but during perisaccadic presentations, auditory localization becomes
relatively more important, so the mislocalized visual stimulus is seen closer to
its veridical position. Both the perceived position of the bimodal stimuli and
the time-course of spatial localization were well-predicted by assuming optimal
Bayesian-like combination of visual and auditory signals. For time localization,
acoustic signals always dominate. However, this dominance does not affect the
dynamics of saccadic mislocalization, suggesting that audio-visual capture
occurs after saccadic remapping. Our model simulates the time-course data,
assuming that position in external space is given by the sum of retinal position
and a noisy eye-position signal, obtained by integrating the output of two
neural populations, one centered at the current point of gaze, the other
centered at the future point of gaze. Only later the output signal is fused with
the auditory signal, demonstrating that some saccadic distortions take place
very early in visual analysis.
This model not only accounts
for the bizarre perceptual phenomena caused by saccades, but provides a novel
vision-based account of peri-saccadic remapping of space.
Neural mechanisms and correlates of
perisaccadic changes in visual perception
Tirin Moore
The changes in visual
perception that accompany saccadic eye movements, including shifts of attention
and saccadic suppression, are well documented in psychophysical studies.
However, the neural basis of these changes is poorly understood. Recent evidence
suggests that interactions of oculomotor mechanisms with visual cortical
representations may provide a basis for modulations of visual signals and visual
perception described during saccades. I will discuss some recent
neurophysiological experiments that address the impact of oculomotor mechanisms,
and of saccade preparation, on the filtering of visual signals within cortex.
Results from these experiments relate directly to the observed enhancement and
suppression of visual perception during saccades.
Fixational eye movements, natural image
statistics, and fine spatial vision
Michele Rucci
During visual fixation,
small eye movements continually displace the stimulus on the retina. It is known
that visual percepts tend to fade when retinal image motion is eliminated in the
laboratory. However, it has long been debated whether, during natural viewing,
fixational eye movements have other functions besides preventing the visual
scene from fading. In this talk, I will summarize a theory for the existence of
fixational eye movements, which links the physiological instability of visual
fixation to the statistics of natural scenes. According to this theory,
fixational eye movements contribute to the neural encoding of natural scenes by
attenuating input redundancy and emphasizing the elements of the stimulus that
cannot be predicted from the statistical properties of natural images. To test
some of the predictions of this theory, we developed a new method of retinal
image stabilization, which enables selective elimination of the motion of the
retinal image during natural intersaccadic fixation. We show that fixational eye
movements facilitate the discrimination of high spatial frequency patterns
masked by low spatial frequency noise, as predicted by our theory.
These results suggest a
contribution of fixational eye movements in the processing of spatial detail, a
proposal originally speculated by Hering in 1899.
Motion perception and prediction during
smooth pursuit eye movements
Miriam Spering, Alexander C.
Schütz and Karl R. Gegenfurtner
Smooth pursuit eye movements
are slow, voluntary movements of the eyes that serve to hold the retinal image
of a moving object close to the fovea. Most research on the interaction of
visual perception and oculomotor action has focused on the question what visual
input drives the eye best, and what this tells us about visual processing for
eye movement control. Here we take a different route and discuss findings on
perceptual consequences of pursuit eye movements. Our recent research has
particularly focused on the interaction between pursuit eye movements and motion
sensitivity in different tasks and visual contexts. (i) We report findings from
a situation that particularly requires the dissociation between retinal image
motion due to eye movements and retinal object motion. A moving object has to be
tracked across a dynamically changing moving visual context, and object motion
has to be estimated. (ii) The ability to predict the trajectory of a briefly
presented moving object is compared during pursuit and fixation for different
target presentation durations. (iii) We compare the sensitivity to motion
perturbations in the peripheral visual context during pursuit and fixation.
Results imply that pursuit consequences are optimally adapted to contextual
requirements.
Looking at visual objects
Ziad Hafed
Much of our understanding
about the brain mechanisms for controlling how and where we look derives from
minimalist behavioral tasks relying on simple spots of light as the potential
targets. However, visual targets in natural settings are rarely individual,
point-like sources of light. Instead, they are typically larger visual objects
that may or may not contain explicit features to look at. In this presentation,
I will argue that the use of more complex, and arguably more "natural", visual
stimuli than is commonly used in oculomotor research is important for learning
the extent to which eye movements can serve visual perception. I will provide an
example of this by describing a behavioral phenomenon in which the visual system
consistently fails in interpreting a retinal stimulus as containing coherent
objects when this stimulus is not accompanied by an ongoing eye movement. I will
then shed light on an important node in the brain circuitry involved in the
process of looking at visual objects. Specifically, I will show that the
superior colliculus (SC), best known for its motor control of saccades, provides
a neural "pointer" for the location of a visual object, independent of the
object's individual features and distinct from the motor commands associated
with this brain structure. Such a pointer allows the oculomotor system to
precisely direct gaze, even in the face of large extended objects.
More importantly, because
the SC also provides ascending signals to sensory areas, such a pointer may also
be involved in modulating object-based attention and perception.
Mechanisms of fixation selection evaluated
using ideal observer analysis
Wilson S. Geisler
The primate visual system
combines a wide field of view with a high resolution fovea and uses saccadic eye
movements to direct the fovea at potentially relevant locations in visual
scenes. This is a sensible design
for a visual system with limited neural resources.
However, to be effective this design requires sophisticated
task-dependent mechanisms for selecting fixation locations.
I will argue that in studying the brain mechanisms that control saccadic
eye movements in specific tasks, it can be very useful to consider how fixations
would be selected by an ideal observer.
Such an ideal-observer analysis provides: (i) insight into the
information processing demands of the task, (ii) a benchmark against which to
evaluate the actual eye movements of the organism, (iii) a starting point for
formulating hypotheses about the underlying brain mechanisms, and (iv) a
benchmark against which to evaluate the efficiency of hypothesized brain
mechanisms. In making the case, I
will describe recent examples from our lab concerning naturalistic visual-search
tasks and scene-encoding tasks.
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