================================================================================

Jo-Anne Bachorowski
Vanderbilt University

Laugh Acoustics:  Sex Differences, Social Context, and Perceptual Evaluations

  Laugh acoustics were found to vary by both laugher sex and social context in
two laugh production studies.  For both sexes, particular kinds of laughs are
thought to be especially powerful elicitors of positive emotional responses in
listeners.  However, observed sex differences in the social contexts in which
these laughs are produced suggest that male laughter is largely about relation-
ship maintenance and that female laughter is more directly linked to sex-related
behavior.  Highly consistent findings from 5 perception studies support the
notion that variability in laugh acoustics is predictably associated with
evaluative and emotional responses in listeners.

================================================================================

Benjamin Backus
Stanford University

TBA
================================================================================

Randolph Blake
Vanderbilt University

Perception and Neuroimaging of Biological Motion

  People are remarkably adept at recognizing the actions performed by others,
even when the kinematic patterns of their movements are portrayed by nothing
more than a handful of light points attached to the head and major joints of
the body.  We have found that perception of biological motion is remarkably
robust and can be conveyed by signals registered by "second order" motion
mechanisms.  We have used functional MRI to reveal neural activity uniquely
associated with biological motion perception.  The pattern of activation found
during viewing of biological motion was located within a small region on the
posterior superior temporal sulcus (STS), located lateral and anterior to human
MT/MST, and anterior and inferior relative to KO.  A small region in medial
cerebellum was also active when observers viewed biological motion sequences.
Consistent with earlier neuroimaging and single unit studies, this pattern of
results points to the existence of neural mechanisms specialized for analysis of
the kinematics defining biological motion.

================================================================================

Geoffrey M. Boynton
The Salk Institute

Spatial vs. Task-Specific Attention in Human Occipital and Parietal Cortex

  Our goal is to study the functional organization of the human parietal and
occipital lobes with respect to the location of spatial attention and the
behavioral task.  fMR image sequences were acquired while subjects fixated a
spot centered between stimuli presented simultaneously in both the left and
right visual fields.  Subjects were cued to alternate between performing an RSVP
task in one hemifield and a direction of motion task in the other hemifield.
The hemifields containing the motion and RSVP tasks were switched across
separate runs.  Retinotopic visual areas were greater when the subject attended
to the stimuli in the contralateral hemifield, but did not depend greatly on the
task.  Responses in the posterior parietal lobe (PP), however, varied with the
task regardless of which hemifield contained the task.  The results suggest that
attention to stimulus features reveals segregated regions within PP, while
attention to different spatial locations selectively influences responses in
segregated regions of the retinotopically organized visual areas.

===============================================================================

Maurizio Corbetta (tentative)
Washington University

Functional MRI investigations of visual expectancies

  Novel event-related fMRI methods were used to separate signals produced by
cues indicating direction of motion or location, from signals produced by the
detection of target stimuli, and their modulation by attention.  The temporal
profile of activation within the intraparietal cortex is consistent with a role
of this region in maintaining expectations about location or direction during a
delay between a cue and a target stimulus.  These findings will be discussed in
relationship to current ideas about attentional control signals in vision.

===============================================================================

Amy Criss
Indiana University

Testing Context-noise vs. Global-familiarity Models of Memory
Authors:  Amy Criss and Richard Shiffrin

  In recognition memory, confusion may arise from prior instances of the test
item (context-noise models), or from other studied items in proportion to their
similarity (global-familiarity models).  Participants studied a long list of
faces and words containing categories with high inter-item similarity and low
between-category similarity.  Category size varied from 2 to 9.  At test,
participants had two tasks: to report if the test item itself had been studied,
and to estimate the number of similar items on the study list.  The false alarm
rate to related distractors and hit rate to targets increased with category
length, as did the estimated number of similar items.  More importantly, faces
and words showed similar results. The results are consistent with predictions of
global-familiarity models but present serious problems for context-noise models,
which cannot explain the word results without the additional assumptions and
cannot account for the face results at all.

===============================================================================

Barbara Dosher
UC Irvine

TBA
===============================================================================

Ione Fine
UCSD

Mid to high level perceptual learning for pattern discrimination

  Our goal was to examine mechanisms underlying improvements with practice in
the ability of observers to combine information across wide ranges of spatial
frequency and orientation.  We used a complex grating discrimination task.
Stimuli contained two orthogonal signal components of 3 and 9 c/deg.  Observers
discriminated a 25% spatial frequency shift in these components.  Stimuli also
contained four noise components, separated from the signal components by at
least 45 degrees of orientation or ~2 octaves in spatial frequency.
  In Experiment 1 naive observers were trained using a 4-alternative
same-different forced choice judgment with feedback.  Observers showed
significant learning, thresholds dropped to ~1/3 of their original value.  In
Experiment 2 we found that observers showed far less learning when the noise
components were not present.  Experiment 3 found, unlike many other studies,
almost complete transfer of learning across orientation.  The results of
Experiments 2 and 3 suggest that, unlike many other perceptual learning studies,
most learning in Experiment 1 occurs at mid to high levels of processing rather
than within low level analyzers tuned for spatial frequency and orientation.
  Experiment 4 found that performance was more severely impaired by spatial
frequency shifts in noise components of the same spatial frequency or
orientation as the signal components (though there was significant variability
between observers).  This suggests that after training observers based their
responses on mechanisms tuned for selective regions of Fourier space.
Experiment 5 examined transfer of learning from a same-sign task (the two signal
components both increased/decreased in spatial frequency) to an opposite-sign
task (signal components shifted in opposite directions in frequency space).
Transfer of learning from same-sign to opposite-sign tasks and vice versa was
complete, suggesting that observers combined information from the two signal
components independently.

================================================================================

Wilson S. Geisler
University of Texas

Edge Co-occurrence Predicts Human Contour Detection Performance

  We have measured edge co-occurrence probability as a function of differences
in local edge position and orientation for a large number of natural images.
The resulting edge co-occurrence density was thresholded to define a "contour
binding function," which we use as a model for the probability that the visual
system will group together (bind) any two nearby contour elements.  This contour
binding function was then combined with a simple transitive grouping rule (if
edge A binds to edge B, and edge B binds to edge C, then A binds to C) to create
a model of contour detection.  This model does a respectable job of predicting
human performance in detecting line-element contours in noisy line-element
backgrounds.

================================================================================

Erin M. Harley
University of Washington

A universal contrast effect in information-acquisition tasks

  Processing visually degraded stimuli is a common experience:  we struggle to
find house keys on dim front porches, to decipher slides projected in bright
seminar rooms, or to read bad photocopies of bad photocopies.  This research
addresses two questions:  Why is it difficult to process degraded compared to
undegraded stimuli, and is the cause of the difficulty the same (qualitatively
and quantitatively) for two quite different stimulus-task combinations?  In
parallel experiments, we measured recognition of randomly-generated forms and
recall of four-digit strings.  Stimuli were presented at two contrast levels
combined with seven exposure durations.  Lowering contrast slowed processing by
a factor that was virtually identical for the two tasks, and whose magnitude was
about 1.5 times the ratio of the two contrast levels.  We suggest that lowering
contrast exerts a simple, universal effect, that is usually confined to early
perceptual processes and is thus indifferent to stimulus type and task.

================================================================================

David Heeger
Stanford University

The Neuronal Basis of Binocular Rivalry

  During binocular rivalry two incompatible monocular images compete for
perceptual dominance, with one pattern temporarily suppressed from conscious
awareness.  One theory holds that binocular rivalry involves a competition in
primary visual cortex (V1) between neuronal signals originating from the two
eyes.  To test this hypothesis we measured fMRI signals in V1 while subjects
viewed rival dichoptic images of two different contrasts; the contrast
difference served as a "tag" for the V1 representations of the two monocular
images.  V1 activity increased when subjects perceived the higher contrast
pattern and decreased when subjects perceived the lower contrast pattern.
Moreover, this modulation of V1 activity was comparable to that evoked by
alternately presenting the two monocular stimuli without rivalry.  These results
challenge the common view that binocular rivalry takes place primarily in higher
order visual areas.

===============================================================================

Henning, G. B.
University of Oxford

Mach-band Stimuli as Maskers for Increment and Decrement Detection

  Two-alternative forced-choice procedures were used to measure the detect-
ability of bright and dark bars at various locations on luminance patterns that
produced Mach bands.  Detection performance was significantly affected by both
dark and bright Mach bands.  Poor detection performance with either increments
or decrements was observed at locations near, but not in, the Mach bands.
Relatively good detection performance at locations within the Mach bands was
caused by reliable changes in the appearance of the bands that were used as cues
for detection.  Such cues were apparent with signals of lower luminances than
were detectable in the plateaux regions far from the bands, but the cues were
not sufficiently reliable to allow errorless performance resulting in unusually
shaped psychometric functions.  This cue was removed by presenting signals of
random polarity.

===============================================================================

David E. Huber
University of Colorado

Removing irrelevant information in short-term priming
Authors:  David E. Huber and Richard M. Shiffrin

  We present a new model of short-term priming called Responding Optimally with
Unknown Sources of Evidence (ROUSE).  The model is closely tied to a series of
short-term priming experiments in which passively viewing prime words leads to
a preference to choose prime related words whereas actively processing prime
words leads to a neutral preference or even a preference against choosing prime
related words.  In the model we assume there is source confusion for evidence
arising from the brief flash of the target word, the above threshold
presentation of primes, and visual noise.  Knowledge of the features contained
in the primes is used to guide a Bayesian decision process and results in the
discounting of 'irrelevant' prime information.  The model predicts, and our
experiments confirm, that in certain circumstances the Bayesian decision process
is incapable of removing the preference for prime related words.  Our results
and theory offer a new interpretation of the often variable results obtained
with traditional measures of short-term priming.

===============================================================================

Michael Kalish
University of Western Australia

Attention learning and knowledge restructuring

  Many of the concepts we learn, we learn through experience.  Experience serves
as a tutor partly by providing us with feedback about the error of our
judgments.  Learning theory tells us that both associative weights and attention
strengths are adjusted to reduce these errors.  The result of these changes can
be counter-intuitive, and can lead experts to make serious errors in real-world
situations.  I will discuss the recent history of learning theory as it relates
to concept acquisition, and some new results about the structure and flexibility
of knowledge.

================================================================================

Stanley Klein
UC Berkeley

Modeling the modelfest data

What that is about is the consortium effort to develop a database
of experimental data for testing vision models. For year 1, 12 vision
researcher (including me, Carney, Tyler, Watson, Norcia, Levi...) have
been gathering detection data of a wide assortment of stimuli including
lots of Gabors, Bessel, noise, natural scene, multipoles, sum of sinusoids...
Now the modeling phase has begun, and I'd like to talk about those
results. It will provide a wonderful forum for discussing what has
happened in the last 25 years of vision modeling.  It is nice that
Norma Graham gave a similar talk at AIC-1.

================================================================================

Geoffrey R. Loftus
University of Washington

Interactions of different spatial frequencies in digit perception

  The human visual system analyzes information at a variety of spatial scales.
We will report data bearing on the question:  to what degree is information
processing at one spatial scale independent of information processing at other
spatial scales?  We used a simple task in which four-digit strings were briefly
presented to observers who had to report as many of the digits as possible in
their correct positions.  Over several experiments, the stimuli were presented
either normally or high-pass filtered, or low-pass filtered.  In an initial
experiment, replicating E. Olds and S. Engel (Vision Research, 1998) our results
could be accounted for perfectly by assuming that (a) information processing
proceeds independently at different spatial scales, (b) the internal sensory
response to normal stimuli consists of the weighted sum of responses to the
individual spatial-frequency components and (c) the temporal response function
has a threshold that cuts off subsequent processing.  However, subsequent
experiments in which high- and low-spatial frequency versions of the stimulus
were successively presented within a single trial appear to rule out this simple
independence model.  Broader implications of these results will be discussed in
an interesting fashion.

===============================================================================

Zhong-Lin Lu
USC

TBA

===============================================================================

Michael E. J. Masson and Glen E. Bodner
University of Victoria

Fluent Encoding of Probes Guides Masked Word Identification

The concept of processing fluency is defined as a locally determined
discrepancy between experience and expectancy or between multiple
processing events.  Differential fluency of encoding probes in the
two-alternative forced-choice variant of masked word identification is
claimed to affect subjects' selection between the probes.  Experiments are
reported in which inducing differential processing fluency between two
probes without prior study led to bias effects in the forced-choice task
that replicated the pattern of bias effects caused by prior study of one of
the probes.  We propose that processing operations applied to the probes
fundamentally influences what subjects experience as target identification
and drives the bias effect in the forced-choice task.  In our account,
prior study is one of a number of alternative means for inducing
differential processing fluency between probes.
           
===============================================================================
Gail McKoon  (tentative)
Northwestern U.

  TBA

===============================================================================

Tim McNamara
Vanderbilt University, Nashville

Egocentric and allocentric representations of space   

===============================================================================

William Merigan
University of Rochester

Functions of the ventral cortical pathway in macaques

  Visual cortex in the macaque contains at least two processing streams, which
have been variously termed the dorsal and ventral, motion and color/form, or
where and what pathways.  Recent studies in our laboratory, using visual cortex
lesions in macaques, have added to our understanding of the functions of the
ventral cortical pathway. Lesions at one of two levels in this pathway (in area
V4 or in inferotemporal cortex) cause distinctive losses of complex form and
color discriminations, whereas simple shape discriminations are spared by
lesions at either level. These findings challenge traditional views of the role
of the ventral pathway in shape and color processing.

================================================================================

Clark Ohnesorge (withdrawn)
Gustavus Adolphus College

Lexical Competition in the Interactive Activation Model of Word Recognition.

  The interactive activation model of word recognition (McClelland & Rummelhart
1981) employs competitive and cooperative processing between and within several
levels of representation in order to simulate human visual word recognition.
Competition within the lexical level of representation is a particularly
important commitment of this model.  The major focus of the research I will
present is to examine the effect of lexical level competition in word
recognition tasks that are chosen to highlight the effects of this competition.
I will present the results of 3 different studies in which the data from human
observers is contrasted with the output of the interactive activation model
performing an analogous task.  We evaluate the model's output in a variety of
ways including activation functions, percentage correct, probability correct,
and the number of cycles required to reach criterial levels of activation. Our
analyses reveal that the interactive activation model does not produce a pattern
of data consistent with human subjects, and that changing weights on inhibitory
or excitatory connections within the model is unlikely to correct the problem
without fundamentally altering the character of the model.

================================================================================

Tatiana Pasternak
University of Rochester

MICROSTIMULATION OF AREA MT: AN EFFECT ON A MEMORY FOR MOTION TASK

  Neurons in area MT have been shown to play a role in the integration of local
motion signals.  Recent lesion studies have provided evidence for an involvement
of MT in motion integration and short-term storage of motion direction (Rudolph
& Pasternak, 1999; Bisley & Pasternak, ARVO 1999).  In the present studies we
used electrical stimulation of physiologically identified sites in MT to examine
their roles in the performance of a match-to-sample task.  During the task
monkeys viewed two random-dot fields, sample and test, separated by a delay of
1.5 sec.  The sample was a field of random dots moving within a 250o range of
directions; on each trial the mean direction was randomly selected from a set of
4.  The dots in the test stimulus moved coherently in the direction that was the
same or opposite to that of the sample.  Stimulation was applied randomly on one
quarter of the trials for 500 msec, either during the sample (to test the
encoding process), or during the delay (to test storage).  Stimulation during
either the sample or the delay drastically degraded performance, but only for a
subset of the 4 directions tested.  It remains to be determined whether this
directionally selective effect of stimulation results from injecting noise into
retinotopic locations within MT or it was produced by masking due to the
increased activity within the stimulated directional column.  These data are
consistent with a view that MT not only plays a role in integration of
directional information from non-coherent random-dot fields but it may also be
involved in the short-term storage of this information.

===============================================================================

Misha Pavel (+Larry Maloney)
Oregon Graduate Institute

Optimality of Probability Matching and of Superstition
===============================================================================

Diane Pecher
University of California, Riverside

Priming for new associations
(Diane Pecher and Jeroen Raaijmakers)

  A series of experiments investigated how new semantic information is learned.
In all experiments subjects learned new associations between words
(e.g., APPLE-FATHER), followed by a priming task. Three different priming tasks
were used:  lexical decision, masked perceptual identification, and animacy
decision.  In all tasks an automatic priming effect was obtained for new
associations.  However, manipulations of the similarity between study and test
showed that the degree of similarity between study and test greatly influenced
the amount of priming for new associations.  We argue that semantic knowledge is
not abstract as is often assumed, but context dependent.

===============================================================================

Zygmunt Pizlo
Purdue University

Shape perception - 70 years of research

  Systematic study of shape perception started with two papers published by
Thouless in 1931.  The research in the next several dozen years represented the
interaction of several schools:  Structural, Gestalt and Gibsonian.  The
cognitive revolution introduced elements of information theory, which led to
revival and elaboration of Gestalt approach.  Interestingly, the theories of
shape perception have never been strongly influenced by neuroanatomical and
neurophysiological research.  This may, in part, be related to the emphasis that
students of the brain put on the coding, as opposed to the information
processing aspect of perception.

================================================================================

Roger Ratcliff
Northwestern University

TBA

================================================================================

Steven Shevell
University of Chicago

Color perception: Peripheral contrast coding and central gain control

The perceived color of an object in the natural world depends on the
context in which it is seen.  In the laboratory, the hue of an isolated
patch can be explained by retinal responses to the light.  In natural
scenes, however, color perception depends on higher level neural processes
that are influenced by light from throughout the scene. A textbook
example is chromatic induction: the hue of a light depends on other lights
surrounding it.  Recent experiments show that the perceived color of an
object within a surround depends on the magnitude of chromatic variation
outside the surround.  Color perception is accounted for by first-level
encoding of chromatic contrast at edges.  The contrast signal, in turn, is
modulated by a contrast gain control, with gain dependent on chromatic
variation over a large area and in either eye.  Overall, color perception
depends on monocular (probably retinal) encoding of chromatic differences
at edges, and on a central chromatic-contrast gain control after the locus
where signals from the two eyes are combined.


================================================================================

Rich Shiffrin
Indiana University

REMI:  A Bayesian Model for Long-term Repetition Priming
Authors:  Richard M. Shiffrin, Lael Schooler, and Jeroen Raaijmakers

  We fit REMI to long term repetition priming based on threshold detection with
forced choice testing (the data come mostly from Ratcliff & McKoon, 1997).  REMI
is a Bayesian model applied to the threshold forced choice test.  To explain
priming, we assume that study causes some storage of current context in the
lexical trace for a word.  Subsequent threshold presentation produces a
perceived vector which combines with current context and is compared to the
lexical representations for the two choices (including context stored in those
representations).  Priming is produced by the extra context matching.  We
present an extension capable of predicting 1) word frequency effects and 2)
enhanced performance due to study of both low frequency choices.

==============================================================================

George Sperling
UC Irvine

TBA

================================================================================

Mark Steyvers
Indiana University

Creating Semantic Spaces to Predict Memory Performance
Authors:  Mark Steyvers & Rich Shiffrin

  We are working on extending memory models to take into account the placement
of actual words in our language in a high dimensional metric space.  We hope in
this way to increase the precision of present models by enabling the prediction
of performance for individual words in tasks such as retrieval of knowledge,
implicit memory, and episodic recognition and recall.  This research builds on
previous research with the REM model (Shiffrin & Steyvers, 1997; Shiffrin &
Steyvers, 1998) in which we have laid out a framework for a process model of
episodic memory.  The REM model represents words as vectors of feature values.
The present extension involves building a high dimensional representation of
words (in effect representing them by large vectors of feature values), and
replacing the present arbitrary vectors in REM with the word vectors from the
initial part of the project.
  We will discuss word recognition experiments designed to test the present
extension of the REM model.  In a series of "false memory" experiments, subjects
study several blocks of semantically related words.  At test, we attempt to
induce the subject to rely primarily upon semantic rather than low level
features to accomplish the task:  in addition to asking subjects whether a word
has been studied before, we also ask subjects whether semantically related words
were presented at study.  The goal is to predict performance in these "false
memory" memory experiments across a variety of conditions.  Our key interest
lies in the ability to predict differences in performances for different words
within a condition, although at present, it is not clear to what degree this is
possible.

================================================================================

Bosco S. Tjan
NEC Research Institute, Princeton

Zili Liu
Rutgers University, Newark

Symmetry impedes symmetry discrimination: arguing against a special-purpose
symmetry perception mechanism

Bilaterally symmetric patterns appear to be particularly salient to a human
observer.  Does this saliency reflect a specialized visual processing
mechanism, or is it an intrinsic property of the stimuli?  Among other
reasons, support for a specialized mechanism comes from studies showing that
human's ability in symmetry discrimination was at its best when the test
patterns were nearly symmetric.  Contrary to these results, we found that
symmetry perception can actually impede symmetry discrimination when asymmetry
is introduced in an ecologically appropriate form.  Analysis of classical
stimuli suggests that the previous findings were due to noise in the stimuli
and did not necessarily reflect any property of the visual system. We found
that a simple and generic mechanism for computing image difference is
sufficient to account for both our data and the previous results.  We argue
that high saliency to bilateral symmetry is partly a by-product of the visual
system's need to compare across images and partly an inherent property of
being symmetric.  Our analysis also explains why repetitive patterns are less
salient than reflective (mirror symmetric) patterns, and why radial symmetry
is harder to detect than bilateral symmetry.

================================================================================

Christopher W. Tyler
Smith-Kettlewell Eye Research Institute

Specialized Processing for Symmetry in the Human Brain:  Psychophysical and
  fMRI Evidence

  Perception of visual symmetries is of high evolutionary and aesthetic
importance given their ubiquitous role in live organisms (friend or foe).  This
importance is reflected, and reinforced, by the prevalence of symmetry in
objects of human construction that form much of our present environment.
Previous work implied that bilateral symmetry perception occurs mainly near the
symmetry axis.  We show that this result is attributable to the pattern spatial
scale.  Patterns scaled appropriately allow symmetry to be detected equally
across the entire visual field, unimpaired by total removal of the axis region.
These results suggest the involvement of specialized brain mechanisms for
symmetry processing rather than long-range interactions in primary cortex.  The
properties of this symmetry detection process suggested that it encodes the
(second-order) envelope of contrast energy in the patterns rather than strict
local pattern symmetry.
  We have extended our psychophysical results to functional MRI studies of the
brain regions activated by symmetric versus random dot patterns. This comparison
showed little symmetry-related activity in cortical pattern-responsive areas
V1-V5, but a strong response in the Middle Occipital Gyrus, a relatively unknown
region of the occipital lobe. The responses were largely non-retinotopic and
were specific to pattern symmetries in a general sense, not just bilateral
symmetries.  There was, however, little activation for conceptual symmetry in
the stimulus sequence, in the absence of symmetry patterns as such.

===============================================================================

Eric-Jan Wagenmakers
Indiana University

Bias Versus Enhanced Discriminability in Perceptual Forced Choice
Authors:  Eric-Jan M. Wagenmakers, Rene Zeelenberg, and Jeroen G. W. Raaijmakers

  The counter model for perceptual identification (Ratcliff & McKoon, 1997)
assumes that effects of word frequency and prior study are mediated by different
mechanisms.  Moreover, these mechanisms act solely to bias the identification
process (i.e., subjects prefer studied and/or high frequency items, independent
of the flashed word).  Recent findings (e.g., Wagenmakers, Zeelenberg, &
Raaijmakers, 2000) have led to a fundamental modification of the counter model:
Studied and/or high frequency items retain an advantage due to bias, but they
are presumably also better perceived than nonstudied low frequency items.  We
present new data and discuss several predictions of the counter model.  We also
present an alternative model to illustrate that enhanced discriminability can be
predicted without assuming that more information is extracted from a briefly
flashed stimulus.

==============================================================================
Frances Wilkinson

Migraine Aura: A Window on the Visual Cortex
McGill University

        The visual auras associated with migraine headache have long been
viewed as arising from the striate cortex, possibly as a result of cortical
spreading depression.  However, auras take many different forms in addition
to the classic "zig-zag" fortification pattern described by Lashley and
others.  In the present study, I have used a structured observation
procedure to have migraineurs map and describe their visual auras as they
progress.  Data will be presented from migraineurs with classic auras,
projecting their auras into cortical coordinates in order to determine
whether the dynamic features of the aura are consistent with the V1
spreading depression hypothesis.  Data from subjects with a variety of
atypical aura will also be presented, some consistent with a locus in
striate cortex but others requiring other explanations.  Finally the
temporal relationship between visual and somatosensory auras will be
considered in order to address the question of whether these arise from a
single processing of spreading activation and depression engulfing much of
the cortex, or whether they arise independently in different primary
sensory areas.
                  
==============================================================================

Hugh R. Wilson
University of Chicago

Global processes in higher level form vision

  Primate area V4 is a key site in the form vision pathway, as it provides major
input to areas in inferior temporal cortex.  Psychophysical evidence from our
laboratory indicates that the human form vision system contains units that pool
oriented responses from V1 so as to extract information about concentric,
cross-shaped, and X-shaped configurations in the visual image.  Physiological
data and human fMRI studies indicate that these units reside in human V4. A
neural model of these V4 units accounts for the psychophysical data and permits
detection of faces in natural scenes.
  Observations of the Marroquin illusion, in which a static dot pattern gives
rise to percepts of oscillating circular shapes, provide evidence for the
presence of spatially regional "winner-take-all" competitive interactions among
V4 concentric units.  Incorporation of such interactions into the model accounts
for experimental data on circle visibility and its statistical characteristics.
Biasing of this network via modulatory neurotransmitters may explain aspects of
spatial selective attention.

==============================================================================

Ted Wright
University of California, Irvine

Visual Feedback and the Online Correction of Rapid Movements

Meyer et al. proposed that Fitts task movements are composed of an initial
movement and zero or more corrective submovements, usually guided by visual
feedback.  Recently several authors have rejected this formulation, denying
the corrective nature of the apparent submovements.  We report two
experiments in which the mapping of the moving hand to its visual feedback
representation is subtly perturbed during the initial movement. The presence
and nature of the subsequent submovements is altered by these perturbations.

================================================================================

Wei Yang (and Sam Williamson)
University of Pennsylvania

Plasticity and Neural Substrate of Human Iconic Memory

Abstract:  Research in the partial-report paradigm has characterized the decay
of iconic memory as stimulus-driven and autonomic.  Meanwhile, the neural
substrate of the iconic representation has remained to be determined.  Here we
report results showing that intensive practice can substantially increase the
iconic lifetime, a parameter that is specific to the time course of iconic
decay.  More interestingly, the lifetime of an activation trace, deduced from
habituation of a visually evoked response in the early visual sensory cortex
(V1, V2/V3), was found to precisely match iconic lifetime in each of nine
subjects.  This match indicates that this activation trace serves as the signal
source for the iconic decay exhibited in psychophysical performance.
   
==============================================================================
Edgar DeYoe
Medical College of Wisconsin

What Does the "Spot-light" of Visual Attention Look Like?

Recent advances in neuroimaging have made it possible to identify
specifc neural correlates of spatially directed visual attention. By using such
techniques in conjunction with detailed retinotopic mapping of visual cortex,
it is possible to reconstruct the cortical pattern of attentional modulation
as it
would appear within the subject's visual field. The resulting "attentional field
maps" provide a unique visualization of the topography of the effects of spatial
attention. In such a view, visual responses are enhanced not only at the site of
an attended target but also within neighboring regions of the visual
field and within limited  portions of the opposing visual field.
Surprisingly, these
unexpected attentional effects appear to be behaviorally relevant. These results
(and others) provide mounting evidence that spatial attention effects are more
complex than previously appreciated.

==============================================================================

Rene Zeelenberg (tentative)
University of Amsterdam

Repetition Priming in Implicit Memory Tasks:  Prior Study Causes Enhanced
  Processing, Not Just Bias
Authors: ReneŽ Zeelenberg, Eric-Jan M. Wagenmakers, and Jeroen G W. Raaijmakers

  Ratcliff and McKoon (1996, 1997) have argued that repetition priming effects
in implicit memory tasks are solely due to bias.  They showed that prior study
of the target stimulus resulted in a benefit for that stimulus in a later
implicit memory task.  However, prior study of a stimulus similar to the target
resulted in a cost.  These bias effects were obtained in a number of different
tasks, including visual and auditory word identification, word fragment
completion and picture identification.  Using a two-alternative forced-choice
procedure, we investigated the effect of prior study in an unbiased condition:
Both alternatives were studied prior to their presentation in an implicit memory
task.  Contrary to a pure bias interpretation of priming, performance was better
when both alternatives were studied than when neither alternative was studied.
These results show that prior study results in enhanced processing, not just
bias.

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Last revised: 7jan00