Perceptual expectations and the neural processing of complex images
and dynamics for visual perception and beyond
Visual Memory and the Brain
Bayesian models applied to perceptual
Action for perception: functional
significance of eye movements for vision
present, and future of the written word
Surface material perception
Symposia from Past Meetings
Friday, May 9, 2008, 1:00 - 3:00 pm
Royal Palm 5
Denis G. Pelli (New York University)
Patrick Cavanagh (Harvard University and LPP, Universit� Paris Descartes),
Brad C. Motter (Veterans
Affairs Medical Center and SUNY Upstate Medical University),
Yury Petrov (Northeastern University), Joshua A.
Solomon (City University,
London), Katharine A. Tillman (New York University)
Crowding is a breakdown of
object recognition. It happens when the visual system inappropriately integrates
features over too large an area, coming up with an indecipherable jumble instead
an object. An explosion of new experiments exploit crowding to study object
recognition by breaking it. The five speakers will review past work, providing a
tutorial introduction to crowding, and will describe the latest experiments
seeking to define the limits of crowding and object recognition. The general
question, including �integration�,
�binding�, �segmentation�, �grouping,� �contour integration�, and �selective
attention�, is a burning issue for most members of VSS.
Crowding: When grouping goes wrong
Early visual processes work
busily to construct accurate representations of edges, colors and other features
that appear within their receptive fields, dutifully posting their details
across the retinotopic landscape of early cortices. Then the fat hand of
attention makes a grab at a target and comes up with an indecipherable stew of
everything in the region. Well, that�s one model of crowding. There are others.
Whatever the model of crowding, it is clear that the phenomenon provides a rare
window onto the mid-level process of feature integration. I will present results
on nonretinotopic crowding and anticrowding that broaden the range of phenomena
we include in the category of crowding.
Correlations between visual search and
Brad C. Motter
Visual search through simple
stimulus arrays can be described as a linear function of the angular separation
between the target and surrounding items after scaling for cortical
magnification. Maximum reading speeds as a function of eccentricity also appear
to be bound by a cortical magnification factor. If crowding can explain these
visual behaviors, what is the role of focal attention in these findings?
Locus of spatial attention determines
inward-outward anisotropy in crowding
I show that the locus of
spatial attention strongly affects crowding, inducing inward-outward anisotropy
in some conditions, removing or reversing it in others.
It appears that under normal viewing conditions attention is mislocalized
outward of the target, which may explain stronger crowding by an outward mask.
Context-induced acuity loss for tilt: If it
is not crowding, what is it?
Joshua A. Solomon and
Michael J. Morgan
When other objects are
nearby, it becomes more difficult to determine whether a particular object is
tilted, for example, clockwise or anti-clockwise of vertical. "Crowding" is
similar: when other letters are nearby, it becomes more difficult to determine
the identity of a particular letter or whether it is, for example, upside down
or mirror-reversed. There is one major difference between these two phenomena.
The former occurs with big objects in the centre of the visual field; the latter
does not. We call the former phenomenon "squishing." Two mechanisms have been
proposed to explain it: lateral inhibition and stochastic re-calibration. Simple
models based on lateral inhibition cannot explain why nearby objects do not
impair contrast discrimination as well as tilt acuity, but a new comparison of
acuities measured with the Method of Single Stimuli and 2-Alternative
Forced-Choice do not support models based on stochastic re-calibration. Lateral
inhibition deserves re-consideration. Network simulations suggest that many
neurones capable of contrast discrimination have little to contribute towards
tilt identification and vice versa.
The uncrowded window for object recognition
Katharine A. Tillman and
Denis G. Pelli
It has been known throughout
history that we cannot see things that are too small. However, it is now
emerging that vision is usually not limited by object size, but by spacing. The
visual system recognizes an object by detecting and then combining its features.
When objects are too close together, the visual system combines features from
them all, producing a jumbled percept. This phenomenon is called crowding.
Critical spacing is the smallest distance between objects that avoids crowding.
We review the explosion of studies of crowding � in grating discrimination,
letter and face recognition, visual search, and reading � to reveal a universal
law, the Bouma law: Critical spacing is proportional to distance from fixation,
depending only on where (not what) the object is. Observers can identify objects
only in the uncrowded window within which object spacing exceeds critical
spacing. The uncrowded window limits reading rate and explains why we can
recognize a face only if we look directly at it. Visual demonstrations allow the
audience to verify key experimental results.