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December 14, 2001, noon, 489 Minor Hall
Dan Twelker
UC Berkeley (Vision Science graduate student presentation)
Host: Ian Bailey
Risk analysis in pterygium
Abstract
A pterygium is a nonmalignant lesion which invades the superficial
cornea from the limbus. The name pterygium (plural: pterygia) comes from
the Greek root pteros which means wing, because of its typical
wing-shaped appearance. The prevalence of this condition varies from
region to region, group to group. A recent population-based survey of
Hispanics over the age of 40 years old in Southern Arizona reported a
prevalence rate of 17%.
Clinical and epidemiological studies of pterygium have been limited, in
part, due to a lack of evaluation techniques. We started our
investigation by asking corneal specialists what they considered
important in evaluating pterygium. Based on their responses, we have
developed some methods for evaluating its signs and symptoms.
Information concerning risk factors for disease can aide in prevention
efforts and guide future clinical and laboratory research. Using a
case-control study design, which is an efficient epidemiological
technique, we investigated risk factors for pterygium including anterior
eye shape and increased exposure to ultraviolet radiation. We will
present and discuss the results of this study.
11/30/01, noon, 489 Minor Hall
Michael Menz
UC Berkeley (Vision Science graduate student presentation)
Host: Russell DeValois
Functional connectivity of binocular disparity tuned neurons in cat striate cortex
Abstract
Binocular disparity tuned neurons in striate cortex are local disparity
filters that are the first stage of image processing for stereopsis
(i.e., depth perception from the disparate images of the two eyes). The
spatial characteristics of their receptive field structure measured with
single cell recordings using dense and sparse noise stimuli have been
extensively studied. This study offers a more complete description of
their function by studying their dynamics, alteration of their
space-time receptive fields as a function of stimulus energy, and
connectivity between disparity tuned neurons revealed through neural
cross-correlation. In the hierarchical model information flows from LGN
to simple to complex, so all three cell types were recorded from in the
first two studies. In the dynamics study I show that disparity tuning
becomes sharper with time (~50ms). The proposed mechanism that
generates this temporal coarse-to-fine is a combination of feedforward
and feedback. In the stimulus energy study, receptive fields are
measured with both sparse and dense noise, and the shapes of the
space-time receptive fields are compared. The results in cortex are not
consistent with divisive-normalization, but there is evidence for some
cortical gain control. In the neural cross-correlation study between
disparity-tuned cells there is evidence consistent with disparity
averaging. There are a few strong coarse-to-fine connections (i.e.,
lower frequency is pre-synaptic), and many weaker fine-to-coarse
connections. The results of these studies support a generally
hierarchical model with some disparity-specific feedback at the simple
cell level, and pooling across spatial frequency where low frequencies
have shorter latencies.
Tuesday, November 20, 2001, noon, 489 Minor Hall
Michael Herzog
Institute of Human Neurobiology, University of Bremen, Germany
Host: Stan Klein
New insights in feature binding and segmentation
Abstract
We characterize a class of spatio-temporal illusions with two complementary
properties. Firstly, if a vernier stimulus is presented for a short time and
followed immediately by a grating comprising a small number of straight
bars, the grating is perceived as offset though it is not (feature
inheritance). The illusory offset is inherited from the foregoing vernier
which remains invisible. In a short time window features migrate from their
original carriers. Secondly, presenting gratings with a larger number of
elements can render the first stimulus and its features visible as a
shine-through element. Subjects perceive two independent ``objects'' each
carrying its own features. However, this illusion is sensitive to subtle
changes of the spatio-temporal layout of the grating. Differences in temporal
onsets of grating elements of 10ms and smaller, i.e. in the range of only a very
few neuronal spikes, can switch perception and change performance dramatically.
We speculate that segmentation processes, very sensitive to the individual
timing of single grating elements, underlie these phenomena. Very simple models,
employing a layer of excitatory and inhibitory interconnected neurons, can
explain the results. Moreover, contextual modulation might also be explained by
the same segmentation processes.
11/16/01, noon, 489 Minor Hall
Paul Gamlin
Professor Physiological Optics
Director,
Neuroscience Graduate Program & Senior Scientist in the Univ Alabama Vision Science Research Center
Host: Cliff Schor
Neural control of vergence eye movements
Abstract
My laboratory has been studying the neural substrates for vergence eye
movements. These studies have concentrated on investigating the roles played by
specific cerebro-ponto-cerebellar pathways in the control of these eye
movements. We have identified vergence-related pathways that are comparable to
the pathways involved in the control of saccadic and smooth pursuit eye
movements. More specifically, within the deep cerebellar nuclei and nucleus
reticularis tegmenti pontis, we have identified neurons that are related either
to the near-response or far-response (Gamlin and Clarke, 1995; Zhang and Gamlin,
1998). In addition, within the frontal eye field region of frontal cortex, we
have not only characterized neurons related either to the far-response or
near-response, but have also characterized neurons that are related to the
sensorimotor transformations underlying these eye movements (Gamlin and Yoon,
2000).
Gamlin, P.D.R. and R.J. Clarke, J. Neurophysiol., 73:2115-2119, 1995.
Zhang, H and P.D.R. Gamlin, J. Neurophysiol., 79:1255-1269, 1998.
Gamlin, P.D.R. and K.Yoon, Nature, 407:1003-1007, 2000.
11/9/01, noon, 489 Minor Hall
Giorgio Ascoli
Head Computational Neuroanatomy Group, Krasnow Institute for Advanced Study &
Associate Professor, Department of Psychology, George Mason University, Fairfax,
VA
Host: Don Glaser, Jointly sponsored by Neurobiology
The algorthmic beauty of neurons (can we model dendritic morphology?)
Abstract
It is generally assumed that the variability of neuronal morphology affects the
connectivity and the activity of the nervous system. Until now, only few
attempts have been made to characterize dendritic shape in a complete and
intuitive way. Typically, neuroanatomists describe neuronal structure by
statistical summaries (intuitive but not complete) or sets of digital
reconstructions (complete but not intuitive). We are adopting a third,
intermediate level of description, consisting of stochastic algorithms designed
to grow virtual neurons based on a set of measured parameters. If the
simulated morphology is consistently and statistically indistinguishable from
that of real neurons, the model can become a powerful source of intuition and
hypotheses concerning the biophysical mechanisms underlying neuronal structure
and development. Anatomically realistic virtual neurons can be also
appropriately distributed in three dimension to reproduce the arrangement,
orientation, density, and connectivity of real brain regions. Used in
conjunction with more conventional computational neurobiology approaches (cable
and compartmental simulations of cellular electrophysiology), these real scale
anatomical models constitute a new generation of artificial neural networks for
the systematic study of the structure-activity-function relationship in the
nervous system.
Special Neuroscience Lecture
Thursday, November 8th, 3:00 pm, Beach Room, 3rd Floor Tolman Hall
Guy A. Orban
Professor, Laboratorium voor Neuro-en Psychofyiologie, KU Leuven Medical School.
Sponsored by the Henry H. Wheeler, Jr. Brain Imaging Center
Motion processing: from single cells to fMRI in human and awake monkey
Abstract
Human functional imaging has revealed a number of motion sensitive
regions such as the MT/V5 complex, V3A and intraparietal regions. The
relationship with motion selective regions defined in single cell
studies based on direction selectivity, has remained unclear. With
(contrast enhanced) fMRI in the awake monkey, one can compare imaging
directly to single cell studies: motion sensitive regions include MT/V5,
MSTv,FST,V2,V3, VIP, and also FEF. This strategy is now being extended
to the processing of kinetic boundaries, which in humans activate the
kinetic occipital (KO) region. In monkeys this may correspond to dorsal
V4, although other regions are also involved: anterior intraparietal
sulcus, MT/V5, MSTv, and TE. This latter is in agreement with single
cell studies. Finally, single cell studies have implicated MT/V5 in the
extraction of depth from motion. Both in humans and monkeys imaging has
also implicated MT/V5 in this function, along with anterior
intraparietal regions, V3, V4 and FST.
10/31/01, 5 pm, 489 Minor Hall
Carol Lakkis
School of Optometry,UC Berkeley
Host:Ed Revelli
Tear exchange and corneal health
Abstract
Pseudomonas aeruginosa (P. aeruginosa) is one of the most common causative
agents of contact lens-related corneal infections which pose a serious threat to
vision. Whilst the pathogenesis of contact lens-related infections remains
unclear, it is hypothesized that stagnation of the tear film underneath the
contact lens plays a role. This presentation will provide an overview of
investigations into the role of tear exchange in protecting the cornea from
bacteria using in vitro model systems. Particular attention will be focussed on
epithelial cell interactions with cytotoxic P. aeruginosa strains which do not
require an epithelial defect to cause cell death.
10/26/01, noon, 489 Minor Hall
Donald MacLeoz
Department of Psychology, UCSD
Host: Ted Cohn
Does the retinal mosaic limit visual resolution?
Abstract
Vision isn't perfect, in the sense that very fine details are not resolved in
perception. Where, and why, during visual processing are the details lost?
Experiments with inteferometric resolution targets show that preretinal optical
losses account for only part of the loss. Filtering and sampling by the retinal
mosaic, and post-receptoral neural filtering, may account for the rest. I
consider evidence about each of these in turn. First, certain phenomena due to
non-linearity in early neural responses show that the photoreceptor mosaic can
resolve and represent test gratings of very high spatial frequency, up to about
twice the perceptual resolution limit. Filtering by the retinal mosaic is not
important. A barely resolvable grating target has a period of about twice the
foveal cone spacing, allowing adjacent photoreceptors to be in register with
light and dark bars. This has been viewed as a Nyquist limit situation, where
higher spatial frequencies could not be represented veridically but would be
perceived only as aliases. The perceived luminance profile across such a grating
may, however, be based on output from many rows of cones, as the extent of
cortical receptive fields implies; this would make the effective sampling
density and the sampling limit many times greater than the observed resolution
limit. We find that spatial integration does indeed range, minimally, over
several rows of cones. The retinal mosaic sampling limit on grating resolution,
given the observed extent of spatial integration along the bars, is therefore
hundreds of cycles per degree. So resolution is not limited by sampling by the
photoreceptor mosaic. These results contradict the standard view that the
retinal mosaic limits resolution. Instead, the limit (for interferometric
targets) may reflect a lack of cortical receptive fields sensitive to high
spatial frequencies, the pass-band of the neural spatial filter being roughly
matched to that of the eye's optics (for external targets). Neural spatial
filtering appears to be distributed over a range of stages in visual processing:
retina, geniculo-striate projection and cortex. Supported by NIH grant EY-01711
Thursday, 10/18/01, 1 PM, 101 Life Science Addition
Edward Callaway
Salk Institute (MCB hosted seminar)
Cell Type Specificity of Neural Circuits in Visual Cortex.
Tuesday 10/16/01, 4:00 PM, 3105 Tolman (Beach room)
Robbie Jacobs
Center for Visual Science, University of Rochester
(Psychology Department hosted seminar)
Learning to see in three dimensions
Abstract
Why is seeing the world in three dimensions so easy? We believe that this ease
is due to the fact that the visual world is highly redundant; there are many
cues to perceptual properties such as depth and shape. However, combining
information from multiple cues in an effective manner is non-trivial. We argue
that people must learn their cue combination strategies on the basis of
experience. Three experiments are reported whose results suggest that observers
can indeed adapt their cue combination strategies in an experience-dependent
manner, though their learning abilities seem to be biased in an interesting way.
Next, we address the question of whether or not observers can adapt their visual
cue combination strategies on the basis of consistencies between visual and
haptic (touch) percepts. Berkeley (1709),Piaget (1952), and many others
speculated that people calibrate their interpretations of visual cues on the
basis of their motor interactions with objects in the world. Despite the
intuitive appeal of this hypothesis, it has never been adequately tested. Using
a novel virtual reality environment, we have conducted three experiments whose
results suggest that observers adapt their visual cue combination strategies
based on correlations between visual and haptic percepts.
10/12/01, noon, 489 Minor Hall
Dennis Levi
Dean,
School of Optometry, UC Berkeley
Host: Tony Adams
Are there high level deficits in strabismic amblyopia?
Abstract
Abnormal visual development in strabismic amblyopia is known to have drastic
effects on visual perception and on the properties of neurons in primary visual
cortex (V1). We set out to test the notion that amblyopia may also have
consequences for higher visual areas by asking humans with amblyopia to count
briefly presented features. Our results show that strabismic amblyopes cannot
count accurately with their amblyopic eye - they markedly underestimate the
number of features. This is not due to low level considerations (blur,
visibility, and crowding, undersampling or topographical jitter) because they
also underestimate the number of missing features. Rather, the deficits of
counting in strabismic amblyopes reflect a higher level limitation in the number
of features that the amblyopic visual system can individuate.
Friday, October 5, 2001 noon, 489 Minor Hall
John Michon, MD
Clinical Faculty, Dept. Ophthalmology
Stanford University School Medicine & Research Fellow, Section on Biomedical Informatics, SUMS
Host: Tony Adams
Genomics and the Eye: A Research Agenda
Abstract
The tools of computation and information technology have had a substantial
impact on biomedicine and promise to add even further to our understanding of
biological phenomena in the future. Databases of gene and protein sequence,
structure, function and interaction are the raw material by which we can
elucidate the molecular physiology of disease.
I will discuss the application of these tools to ophthalmology and visual
science, and suggest a research agenda that might allow this new knowledge to be
rapidly incorporated into clinical practice.
9/26/01, 5:00 pm, 489 Minor Hall
Nancy McNamara, PhD
Department of Anatomy, UCSF
Host: Suzanne Fleizig
Innate defense against corneal infection
Abstract
In their quest to colonize and infect host tissues, bacteria cause considerable
damage to their hosts. The mucosal epithelium is often the first line of
defense against potential pathogens and uses mechanisms similar to those used by
lymphocytes. An advantage mucosal cells have over lymphocytes is the speed at
which they can react to pathogens since their pathogen-recognition strategy is
more general. Furthermore, eradication of the impending pathogen directly by
mucosal epithelial cells enables the host to avoid lymphocyte-mediated
inflammation and its associated morbidity. There is convincing evidence that
the epithelial cell can detect the presence of pathogens and respond by altering
gene expression. Epithelial gene products aimed at killing bacteria include
lysozyme, lactoferrin, and defensins. Another important innate defense molecule
is mucin. In the eye, mucin effectively augments the barrier between host and
bacteria while bacteria-laden mucus is cleared by blinking of the eyelid. Our
goal is to understand the intracellular signaling and gene regulation mechanisms
that mediate the production of both epithelial-derived antimicrobials and mucin
in response to clinically important insults.
9/21/01, noon, 489 Minor Hall
Michael Webster
Dept of Psychology, University of Nevada, Reno
Host: Marty Banks
Neural adjustments to image blur
Abstract
Blur is an important and salient dimension of image quality, and one that the
visual system continuously adjusts to. Accommodative changes of the eye's optics
are thought to be the primary mechanism by which the visual system responds to
blur, but there is evidence that acuity and contrast sensitivity are also
affected by neural adjustments to blur. We examined these adjustments by
measuring how the perception of blur depends on the blur of images that
observers are exposed to. Briefly viewing blurred or sharpened images leads to
pronounced changes in the perceived focus of images. These adaptation effects do
not depend on the global amplitude spectra of the images, and instead appear to
adjust to the explicit feature of blur. Such adjustments may play an important
role in calibrating the responses of cortical mechanisms, to provide a form of
perceptual constancy for the expected spatial structure of scenes. Similar
changes in perceived focus are also induced by presenting images within blurred
or sharpened surrounds, suggesting that these adjustments also calibrate
variations in spatial sensitivity across the visual field. These adaptation and
induction effects have important implications for understanding normal spatial
vision and how it varies with refractive errors or during development.
9/7/01, noon, 101 LSA
Matteo Carandini
Institute of Neuroinformatics,
Swiss Federal Institute of Technology & University of Zurich, Switzerland
(MCB hosted seminar)
Suppression without inhibition in primary visual cortex
Abstract
The responses of neurons in the primary visual cortex to an optimal stimulus can
be suppressed by the simultaneous presence of other visual stimuli. This
suppression arises from mechanism that divides the responses of the neurons by a
measure of stimulus strength. The input to this divisive mechanism is
traditionally thought to arise from pools of cortical neurons operating through
feedback inhibition. New results from my laboratory, however, suggest that the
mechanism is feed-forward, and may be local to the synapses connecting the
thalamus to the cortex.
Friday Aug 10, 2001, noon, Room 100 Minor Hall
Heinrich Buelthoff
Director of Max-Planck-Institute for Biological Cybernetics, Tuebingen, Germany
Host: Marty Banks
Object and Face Recognition in Man and Machines
Abstract
Theories of visual object recognition must solve the problem of recognizing 3D
objects given that perceivers only receive 2D patterns of light on their
retinae. Recent findings from human psychophysics, neurophysiology and machine
vision provide converging evidence for image-based models in which objects are
represented as collections of viewpoint specific local features. This approach
is contrasted with structural-description models in which objects are
represented as configurations of 3D volumes or parts.
I will report on recognition experiments with familiar, unfamiliar and dynamic
objects and cross-modal transfer between visual and haptic recognition. All
these experiments show strong viewpoint effects and speak in favor of an
image-based representation of objects in which the physical similarity can
account for recognition with small viewpoint changes. Recently, together with
Guy Wallis we started to look at the importance of temporal similarity on the
representation and recognition of objects. Temporal similarity can link many
views to one object identify, because different views of objects are usually
seen in close succession. To test this hypothesis subjects were presented
sequences of unfamiliar faces in which the identity of the face changed as the
head rotated. The subjects showed a tendency to treat the views as if they were
of the same person. Our results counter the proposal that object views are
recognized simply on the basis of objective, structural components. Instead,
they suggest that we are continuously associating views of objects to support
later recognition, and that we do so not only on the basis of their physical
similarity, but also their correlated appearance in time. The application of
this image-based approach in computer graphics and computer vision will be
described.
Wednesday, July 11, 2001, noon, Room 100 Minor Hall
Dae-Shik Kim
Assistant Professor
Center for Magnetic Resonance Research & Graduate Program in Neuroscience
University of Minnesota Medical School, Minneapolis
Host: Ralph Freeman
Steps towards neuroscientifc MRI
Abstract
The rapid progress of BOLD-based fMRI has raised the hope that the
functional architecture of the living brain could be visualized
non-invasively at high resolution, thus avoiding many limitations of the
more traditional brain mapping techniques. However, many pivotal
questions remain to be answered before MRI can be applied for addressing
systems and developmental neuroscientific questions. To list only the
most crucial ones: a) What is the neural correlate of functional fMRI?
b) What is the ultimate spatial and temporal resolution of fMRI? And c)
How does the balance of excitatory and inhibitory neural events
influence the fMRI activity pattern? I will present recent results from
my laboratory, suggesting that the questions of neural correlates and
spatial specificity might be within the reach of a tentative solution.
Furthermore, I will present some initial studies demonstrating that
Diffusion Tensor based MRI can be used to provide axonal fiber
reconstructions in vivo, thus yielding genuine in vivo neuroanatomy.
Friday, June 22, 2001, noon, 101 Minor Hall
Elise Héon MD, FRCS (C)
Associate Professor & Director of Residents Research
Department of Ophthalmology, Toronto Western Hospital
Host: John Flannery
A molecular perspective on glaucoma: practical implications
Abstract
Despite therapeutic advances, glaucoma remains a leading cause of permanent
blindness worldwide. A major difficulty in management of this condition resides
in early diagnosis before the condition leads to irreversible optic nerve and
visual function damage. The genetic approach to the study of glaucoma has
currently identified at least eighteen glaucoma-related loci. The
identification of an increasing list of glaucoma-related genes allows us to now
identify a number of those at risk of developing the disease and direct them
towards earlier sight-saving therapy. The identification of more genes and the
elucidation of the molecular pathway involved will likely lead to the
development of novel therapies and sight saving approaches. This talk will
discuss the implications of molecular testing in pediatric and adult onset
glaucoma.
Friday, June 1st, 2001, noon, 100 Minor Hall
Dimitri Chernyak, PhD
UCB School of Optometry
Host: Lawrence Stark
Scene recognition by a sequential knowledge-based information
gathering algorithm based on human eye movements
Abstract
Eye movements are an important aspect of human visual behavior. The temporal and
space-variant nature of sampling a visual scene requires frequent attentional
gaze shifts, saccades, to fixate onto different parts of an image. Experimental
evidence suggests that fixations are often directed towards the most informative
regions in the visual scene. We develop a model and its simulation that can
select such regions based on prior knowledge of similar scenes. Having
representations of scene categories as a probabilistic combination of
hypothetical objects, i.e., prototypical regions with certain properties, it is
possible to assess the likely contribution of each image region to the
successive recognition process. Using conditional probabilities for each region
given the scene category, the model can then predict its informative value and
initiate a sequential spatial information-gathering algorithm analogous to an
eye movement saccade to a new fixation. This algorithm establishes the most
likely scene category for a given image.
Friday, May 18, 2001, noon, Room 100 Minor Hall
Stacey Choi
UC Berkeley
The relationship between the Stiles-Crawford effect and myopia
Friday, May 11, 2001, noon, Room 5101 in Tolman Hall
(Rock lecture)
Walter Gerbino
University of Trieste, Italy
Amodal completion: Another microcosm of visual science
Abstract
Phenomenology, visual ecology, psychophysics, physiology, computational
modelling, and more are captured by amodal presence of objects. As recent
research shows, amodal completion is ubiquitous and complex. Its ubiquity is
grounded in the ecology of occlusion and in the spatiotemporal fragmentation of
the optical input. Its complexity depends on the action of various processes,
acting at different levels of visual perception.
Thursday, May 10, 2001, noon, 100 Minor Hall
Günter Niemeyer, MD
Director, Neurophysiology Laboratory & Clinical ERG laboratory
Department of Ophthalmology, University Hospital, CH8091 Zürich Switzerland
Host: Sheldon Miller
Retinal research using the perfused mammalian eye
Abstract
The effort to isolate and maintain alive in vitro an intact mammalian eye is
rewarded by the full control provided over the arterial input and exclusion of
systemic regulatory or compensatory mechanisms. Electrical recording of typical
light-evoked field potentials from retina and optic nerve can be complemented
by single cell recording. Thus light-induced electrical activity reflects the
function of the retinal pigment epithelium, of the layers of the retina and of
the ganglion cells or their axons. Retinal function in vitro is documented by
electrophysiological and morphological methods revealing subtle features of
retinal information processing as well as optic nerve signals that approach -
at threshold stimulus intensity - the human psychophysical threshold. Such
sensitivity of third order retinal neurons is described for the first
time.
This well controlled in vitro preparation has been used successfully for
biophysical, metabolic and pharmacological studies. Examples are provided
that demonstrate the marked sensibility of the rod system to changes in glucose
supply. Moreover, histochemical identification of glycogen stores revealed
labeling of the second and third order neurons subserving the rod system, in
addition to labeling of Müller (glial) cells in the cat retina.
The glycogen content of the cat retina is augmented by prolonged anesthesia,
largely depleted by ischemia after enucleation and enhanced by insulin.
Pharmacological experiments using agonists and antagonists of putative retinal
neurotransmitters are summarized and outlined using the muscarinic cholinergic
agonist QNB as an example. Actions and uptake of the neuromodulator adenosine
are presented in detail, including inhibitory effects on physiologically
characterized ganglion cells. Neuronal effects of adenosine are distinguished
from those resulting from vasodilatation and from glycogenolysis induced by the
neuromodulator.
To open the blood-retina barrier, a hyperosmotic challenge can be applied
transiently. This process is monitored histochemically using FITC-albumin and
with electrophy-siological parameters. Changes in vitreo-scleral resistance and
in the amplitude of the EOG-light peak appear to reflect the open/closed status
of the barrier.
This overview of the uses of the isolated perfused mammalian eye in retinal
research concludes with a discussion of potential implications for clinically
relevant topics.
Friday, April 20, 2001, noon, Room 489 Minor Hall
Yang Dan, PhD
UC Berkeley
Host: Ralph Freeman
Stimulus-timing-dependent plasticity in the visual cortex
Abstract
Relative timing of pre- and postsynaptic spikes on the order of milliseconds
plays a critical role in activity-induced, long-term synaptic modification, but
the functional significance of such synaptic plasticity in vivo is only
beginning to be examined. Since the timing of visual stimuli can directly
affect spike timing in visual neurons, it may play an important role in the
plasticity of visual circuits. In this talk, I will present our recent studies
on stimulus-timing-dependent plasticity of the response properties of cortical
neurons in the orientation and space domain.
Monday, April 16, 2001, 1:00 pm, 489 Minor Hall
Judith Hirsch, PhD
Laboratory of Neurobiology, Rockefeller University, New York
Host: Yang Dan
Synaptic integration in the cat's visual cortex in vivo
Abstract
Each stage of striate cortical processing extracts new information about
the visual environment. We explore the integrative capacity of the
cortical circuit by studying its anatomical structure and the physiology
of its component connections. Our approach is to combine the techniques
of whole-cell recording and intracellular labeling with visual
stimulation. This talk will focus on the synaptic integration of two
basic features of the visual scene, stimulus orientation and
movement.
At the first stage of cortical processing, in layer 4, neurons become to
able resolve stimulus orientation and maintain this selectivity even as
stimulus contrast increases. The push-pull model of orientation
selectivity holds that the ability to detect stimulus angle is based on
the layout of the simple receptive field, in whose adjacent subregions
bright and dark stimuli evoke responses of the opposite sign. Ascending
input from the thalamus is thought to provide the push, or excitation; a
view our results support. The proposed substrate of the pull is a group
of inhibitory simple cells. We have been able to identify such
inhibitory neurons; as predicted, their receptive fields resembled those
of excitatory simple cells. In addition, we found an unanticipated
class of inhibitory cells that were neither simple nor orientation
selective. These cells could help establish the contrast invariance of
orientation tuning and other forms of gain control.
While cells at the first cortical stage respond vigorously to stationary
stimuli, those at later stages do not. To ask if this difference in
sensory response reflects laminar differences in synaptic physiology, we
compared records from the thalamus, layer 4 and its postsynaptic target,
layer 2+3. All cells in layer 4 transmitted the patterns of thalamic
activity that static stimuli evoked. By contrast, at the second
cortical stage in layer 2+3, postsynaptic responses to the stationary
stimuli were brief and intermittent. Only richer stimuli such as those
including motion had substantial effect. Thus, at the first cortical
stage, a large investment is made to incorporate ascending input. After
that, the effort made to transmit information is reduced unless stimuli
meet new standards: the gate between layers 4 and 2+3 operates
economically.
Friday, April 6, 2001, noon, 489 Minor Hall
Mark D'Esposito, MD
Professor Neuroscience and Psychology
Director, Henry H. Wheeler Jr. Brain Imaging Center
University of California at Berkeley
Host: Ralph Freeman
Functional MRI: Implications for neuroscience
Abstract
Initially, the advent of structural brain imaging paved the way for more
precise testing of hypotheses regarding brain-behavior relationships. In
recent years, functional neuroimaging, broadly defined as methods that
provide data on brain activity, has further increased our ability to
study the neural basis of behavior. One such method, functional MRI
(fMRI) has recently emerged as an extremely powerful technique that
affords excellent spatial and temporal resolution. After a brief review
of the basic principles of fMRI, and the physiological factors that
influence the resolution of this method, I will describe and critique a
new class of imaging experimental designs called event-related fMRI.
Event-related fMRI exploits the temporal resolution of fMRI by modeling
fMRI signal changes associated with individual behavioral trials, or
behavioral events within a trial, as opposed to blocks of behavioral
trials. Thus, this new method has the potential to address a number of
neuroscientific questions with a degree of inferential and statistical
power not previously available.
Friday, March 23, noon, 489 Minor Hall
Ward M. Peterson, PhD
Inspire Pharmaceuticals
Host: Sheldon Miller
P2Y2 Receptor Agonists for Dry Eye and Retinal Detachment From
Laboratory to Clinical Development
Abstract
Many ocular conditions result in fluid imbalance either on the ocular
surface (dry eye) or within the subretinal space (retinal detachment). A
pharmacological mechanism for restoring fluid homeostasis on the ocular
surface or in the subretinal space could be therapeutically useful in
the treatment of these conditions. Activation of P2Y2
receptors is coupled to fluid transport in many epithelia, including
conjunctival epithelia and retinal pigment epithelium. Endogenous
ligands for these receptors are extracellular ATP and UTP. However,
rapid hydrolysis of extracellular ATP and UTP by nucleotidases limits
their physiological activity. Hydrolysis-resistant agonists for
P2Y2 receptors were therefore developed and tested both on
the ocular surface and intravitreally. In animal models, these agonists
stimulate conjunctival fluid and mucin secretion when administered on
the ocular surface, and enhance subretinal fluid reabsorption across RPE
when administered as an intravitreal injection. The efficacy profiles of
these P2Y2 receptor agonists from preclinical studies warrant
their development as clinical candidates for the treatment of a variety
of ocular conditions comprising dry eye disease and retinal
detachment.
Friday, March 9, noon, 489 Minor Hall
Michael Kubovy
Depart Psychology, University of Virginia
Host: Marty Banks
Perceptual organization and perceptual transformations
Abstract
After a discussion of perceptual emergent properties, I will talk about
perceptual simplicity, with particular attention to Garner's and
Palmer's ideas about the relation between simplicity and groups. Finally
I will discuss the relation between the rich domain of the perception of
patterns (as defined and classified by Gruenbaum and Shepard, in their
book Tilings and Patterns) and the Gestalt laws of grouping.
Friday, March 2, 2001, noon, 489 Minor Hall
Jamie Mazer, PhD
Department of Psychology, UC Berkeley
Host: Ralph Freeman
Neuronal activity in extrastriate cortex during natural vision
Abstract
The activity of visual neurons in awake animals is typically studied by
training animals to fixate a small visual target and then flashing or
drifting bars and gratings in and around the classical receptive field.
These types of experiments often ignore both the complex temporal
dynamics of natural eye movements and the complex spatial statistics of
natural scenes.
In order to better understand the operation of visual cortex during
natural vision, we have examined the responses of both striate and
extrastriate cortical neurons during performance of a free viewing
visual search task using high resolution photographs natural scenes as
stimuli. We simultaneously recorded single neuron activity and eye
movements during performance of the visual search task and then used a
novel reverse correlation method to compute linear spatiotemporal
receptive fields (STRFs) for neurons in V1, V2 and V4.
Our data indicate that spatiotemporal receptive fields can be reliably
estimated, at least up to area V4, during natural vision. In many,
cases, but not all, natural vision STRFs are well correlated with tuning
measured using bar and grating stimuli during fixation. This
combination of reverse correlation and free viewing visual search
provides a window allowing use to view the operating of the visual
system during natural vision.
Wednesday, February 28, 2001 5pm 489 Minor Hall
Christopher Furmanski
Department of Psychology, UCLA
Host: Ralph Freeman
Models of vision: What fMRI of striate cortex can tell us
about the mechanisms of perception
Abstract
The focus of my research is to understand the links between neural
physiology and human perception. In this talk, I will present
psychophysical and neuroimaging (fMRI) data that suggest neural
responses in human primary visual cortex (V1) underlie some fundamental
asymmetries in perception. A century of research indicates that human
perception is better for horizontal and vertical stimuli than for
stimuli at other, oblique orientations. My fMRI measurements of V1
revealed that cortical responses were smaller for foveal oblique
gratings than for horizontal gratings, however, this difference
diminished in the periphery. This pattern of results matched threshold
measurements of contrast detection but differed from judgements of
supra-threshold orientation identification. Such results support models
of V1 that contain fewer units that prefer oblique orientations than
prefer horizontal and vertical orientations. My current work further
examines this link between detection performance and V1 activity. Here,
extensive training was used to alter perceptual performance.
Preliminary fMRI results indicate that increases in V1 responses for
trained stimuli paralleled an orientation- and location-specific
improvement found in post-training behavioral thresholds. Together,
these results demonstrate that fMRI can serve as powerful tool for
constraining models of the human visual system.
Monday, February 26, 2001, noon, 489 Minor Hall
Yves Frégnac
Unité de Neurosciences Intégratives et Computationnelles,
CNRS, Gif-sur-Yvette, France
Host: Ralph Freeman
Everything you wanted to know on shunting inhibition and
orientation selectivity, but were afraid to ask
Abstract
A prevailing concept in the role of thalamocortical pathways in sensory
processing is the dominant influence of feedforward connectivity. In the
case of the mammalian visual system, it is well established that
topographic maps, the organization of visual receptive fields in ON and
OFF discharge zones, and possibly the genesis of orientation selectivity
at the cortical level result from the strong imprint of the feedforward
input. The spatial convergence of afferents from one relay to the next
decides of the functional architecture of the target structure.
The aim of this talk is to examine challenging evidence - based on
intracellular recordings, patch and sharp, in cat area 17 in vivo -
which suggests that visual input evokes a transient shunting
inhibitory signal, acting concomitantly with the excitatory drive. This
shunting effect is observed for stimulus features that suppress spike
activity (null-direction, cross-orientation), but also for the stimulus
eliciting the strongest firing, which indicates that the primary visual
cortex works more as an overdamped system rather than as a canonical
amplifier.
It is classically accepted that the functional selectivity of cortical
neurons to orientation results from the bias provided by the feedforward
input. I will present current clamp and voltage clamp intracellular data
in vivo, showing that similar orientation preference of cortical cells
expressed at the spiking level can be produced through various
orientation/direction dependent combinations of excitatory and
inhibitory inputs (Monier et al, 2000). Our results suggest that
canonical excitatory-inhibitory iso-oriented models are largely
oversimplified, and that silent shunting inhibition expressed for
non-preferred directions and/or orientations should play a critical
computational role. We propose the existence of a functional map
conformation process which tends to realign the orientation preference
of a cortical cell with that dictated by its immediate neighborhood in
the orientation map, irrespectively of the individual bias provided by
the feedforward input.
Subthreshold depolarizations in area 17 cortical neurons in response to
flashed bars can be evoked with onset latencies as early as 18-20 ms
whereas the first spike occurrence is generally observed at much longer
delays (>35 ms). Our intracellular studies also show the existence of
an early shunt signal dominated by GABAa receptor activation often
visible after the start of the initial postsynaptic depolarization
(Borg-Graham et al, 1998). Thus a pause of several tens of ms delay
exists between the earliest signs of cortical activation and the mean
latency of the output V1 signal. Taken into account the slow propagation
of visual activity along excitatory and inhibitory lateral connections
(Bringuier et al, 1999), one may extrapolate that local inhibitory
circuits put the cortical cells on hold, waiting for the contextual
confirmation or invalidation of the relevance of the input by the rest
of the network.
References
Borg-Graham, L. J., Monier, C. & Frégnac, Y. (1998). Visual input
evokes transient and strong shunting inhibition in visual cortical
neurons. Nature .393, 369-373.
Bringuier, V., Chavane, F., Glaeser, L. & Frégnac, Y. (1999).
Horizontal propagation of visual activity in the synaptic integration
field of area 17 neurons. Science .283, 695-699.
Chavane, F., Monier, C., Baudot, P., Borg-Graham, L.J. and
Frégnac, Y. (2000). Early inhibitory control of spiking evoked
activity in area 17 neurons. ANA New Orleans, USA: 1968.
Chavane, F., Monier, C., Bringuier, V., Baudot, P., Borg-Graham, L.,
Lorenceau, J. and Frégnac, Y. (2000). The visual cortical
association field: a Gestalt concept or a physiological entity? J.
Physiol. (Paris). 94, 333-342.
Frégnac, Y. & Bringuier, V. (1996). Spatio-temporal dynamics of
synaptic integration in cat visual cortical receptive fields. In Brain
Theory: Biological Basis and Computational Theory of Vision, eds.
Aertsen, A. & Braitenberg, V., pp. 143-199. Springer-Verlag,
Amsterdam.
Friday, February 23, 2001, noon, 489 Minor Hall
Geraint Rees, PhD
Institute of Cognitive Neuroscience, University College London
Host: Stan Klein
Parietal Cortex and the Neural Correlates of Consciousness
Abstract
The simplicity with which we open our eyes and consciously experience
the world belies the complexity of the underlying neural mechanisms,
which remain incompletely understood. One currently popular view is that
neurons in the ventral visual pathway alone mediate conscious visual
perception. This presentation will directly challenge such a notion, by
suggesting a crucial and often neglected contribution from other
cortical structures. Empirical data from our neuroimaging studies of
visual extinction, binocular rivalry, inattentional and change blindness
converge with other data to suggest an association of superior parietal
cortex activity with conscious visual experience. While activity in
ventral visual cortex measured by fMRI may be necessary for visual
awareness, it appears to be insufficient without a contribution from
parietal and prefrontal cortex. The physiological and psychological
nature of this contribution will be discussed in the light of
contemporary theories of visual attention. A comprehensive understanding
of the neural correlates of consciousness, particularly in parietal
cortex, will be incomplete without an appreciation of the anatomical and
functional homologies between monkey and human. Our recent work shows a
direct quantitative relationship between single cell activity in monkeys
and fMRI measurements in humans, and the potential future contribution
of neuroimaging technologies to understanding such inter-species
relationships will be discussed.
Friday, February 16, noon, 489 Minor Hall
Jennifer LaVail, PhD
University of California San Francisco
Host: Bridgette Cowell
Herpes simplex virus infections of the cornea
Friday, February 9, noon, 489 Minor Hall
Josef F. Bille, PhD
Kirchhoff-Institut for Physics, University of Heidelberg,
Heidelberg, Germany
Host: Lawrence Stark
Perfect vision: Wavefront-technology, adaptive optics and custom ablations
Abstract
Measurements of wavefront of light reflected from the retina of the
human eye can be used to determine optical aberrations of the human eye
for large pupils. An instrument based on the Hartmann-Shack principle
was developed. The wavefront is refracted by a microlens array and
detected by a CCD camera. In first clinical studies human volunteer eyes
and preoperative and postoperative refractive surgical patient eyes have
been examined. An adaptive optical closed loop system has been devised
for preoperative simulation of refractive outcomes of aberration free
refractive surgical procedures.
In a first clinical study, custom ablation treatments using the
Wave-ScanTM wavefront technology and the VISX STAR S3 ActivTrakTM
excimer laser system were performed in Heidelberg, Germany. Based on the
patient's WavePrint data, the laser created a PreVue TM lens for each
eye. This lens allowed the patients to preview their corrected vision
before the surgery took place. A preliminary analysis of the study
results shows that 100% of the patients achieved 20/20 or better
vision. Of those patients, 89% achieved vision better than 20/20 and 44% achieved 20/16 or better vision at the one month followup.
Thursday February 8, 7 pm, Location TBA
Jane Gwiazda
Center for Myopia Research, New England College of Optometry, Boston
Host: Christine Wildsoet
Lens treatment options for myopia control
Wednesday, February 7, 5 pm, 489 Minor Hall
Susana Marcos
Institute of Optics (CSIC), Madrid, Spain
Host: Christine Wildsoet
Aberrations in the normal eye and how they change after refractive surgery
Abstract
The eye is not a perfect optical system, as it suffers from
imperfections known as optical aberrations. This has been known for more
than a century. However, there has been recently a renewed interest in
studying the ocular aberrations, stimulated by the development of new
instrumentation and by refractive surgery. LASIK surgery has become a
popular alternative to treat refractive errors, and future developments
aim at correcting not only conventional refractive errors but higher
order aberrations. It then becomes crucial to understand the aberrations
in the normal eye and how current techniques modify the aberration
pattern of those eyes. In this seminar, I will talk about the
aberrations in the normal eye: how they change with accommodation, with
wavelength, with myopia, and with age, as measured using a spatially
resolved refractometer, and laser ray tracing. I will also discuss how
these aberrations (particularly spherical aberration) increase following
standard myopic LASIK surgery, and how this relates to visual
performance.
Friday, February 2, noon,489 Minor Hall
Christoph Zetzsche
University of Munich, Germany
Host: Stanley Klein
Higher-order statistics of natural images and nonlinear operations in the visual cortex
Abstract
The independence of local frequency components is a key feature of
current vision models and of recent statistical optimization schemes
(independent component analysis - ICA). Our measurements indicate that
the hypothesis of statistical independence of the responses of linear
frequency-selective mechanisms to natural scenes is substantially
violated. The exploitation of the remaining statistical dependencies
requires specific nonlinear interactions, and these turn out to be
closely related to cortical gain control and to extra-classical
receptive field properties of cortical neurons.
Wednesday January 31, 5 pm, Room 489 Minor Hall
Peter Lennie
Center for Neural Science, New York University, New York
Host: Russel DeValois
Sensitivity regulation in striate cortex
Abstract
Normal eye-movements ensure that the visual world is seen episodically,
as a series of often stationary images. Neurons in striate cortex give
distinctive transient responses to stationary images. The transients
have high contrast sensitivity, high response gain, and
disproportionately low variability. These factors together make the
onset transient an information-rich component of response, such that the
detectability and discriminability of stationary gratings grows rapidly
to an early peak, within 150 msec of the onset of the response in most
neurons. The initial high discharge rate that decays rapidly, and the
change of contrast sensitivity over time, seem to be associated with a
rapid adaptation that in complex cells is pattern-specific. This
adaptation improves the discriminability of patterns resembling a
neuron's preferred pattern, and reduces the redundancy of
representation.
Friday, January 26, 2001, noon, 489 Minor Hall
Juan I. Korenbrot
Department of Physiology, School of Medicine
University of California at San Francisco
Host: John Flannery
Retinal development and repair: teleost fish as an accessible model
Abstract
In the new age of molecular physiology, long standing questions from the
old age remain unanswered. The new tools, however, offer new
possibilities. Retinal development can be described to have two
principal and distinct phases: birth and maturation. Molecular
insights are only beginning to be discovered as to birth, and maturation
is understood even less. We have embarked on a research plan to define
and better understand the role of functional cell-cell interactions in
the developmental maturation of the retina. To this end, we have
developed a novel experimental model: a physiologically active tissue
slice of the developing fish retina. The seminar will present
anatomical and electrophysiological results of studies using this
preparation in which we investigated events associated with retinal
development and injury repair.
Friday, January 19, 2001, noon, 489 Minor Hall
Thai D. Nguyen, PhD
Glaucoma Research, Department of Ophthalmology, UCSF
Host: Chris Wildsoet
Glaucoma and the TIGR gene
Abstract
Applications of genetic mapping and molecular biology research in the past decade
have been instrumental to important discoveries of candidate genes for glaucoma.
There are over ten genetic loci suggested for different forms of the disease and
the number is on the rise. Candidate genes for some of these loci have been
identified and more of them are expected to be achieved in the future with rapid
pace along with the recent near completion of the human genome project. Among
these candidates, the TIGR (for Trabecular meshwork Inducible Glucocorticoid
Response) gene originally cloned by our laboratories (1,2) (also known as MYOC,
GLC1A) is the most studied gene for glaucoma, due to its association with two
most known form of the disease including adult and juvenile onset glaucoma (3).
Interest in the TIGR gene has rapidly expanded in the recent years and involved
various aspects of glaucoma research ranging from mutations, structure and
functions to help provide a possible explanation for glaucoma pathogenesis.
In brief, the gene has been reported to have different forms including the
cellular and extracellular forms; there are over 30 mutations associating with
glaucoma identified within its structural domain; the gene is expressed in
various tissues and by various conditions including steroid hormone, oxidative
and shear stress. In our investigations of the TIGR gene in the trabecular
meshwork cell system, we have found the gene to have a number of attractive
features that could help to explain the outflow resistance commonly seen in
glaucoma. In addition to its genetic aspect, there are also evidences to suggest
that the gene could potentially involve in other glaucoma etiologies including
hormone, environment, and possibly aging factors. In this seminar, we review the
discovery of the TIGR gene and describe its biochemical, cellular properties and
its functions that we uncovered in the trabecular meshwork cells. Other
properties of the TIGR gene recently emerged from various glaucoma research
laboratories and its potential applications for glaucoma diagnosis/prognosis as
well as therapy are also discussed.
Reference
1) Polansky,J.R., Kurtz,R.M., Fauss,D.J., Kim,R.Y. and Bloom,E. (1991). In vitro
correlates of glucocorticoid effects on intraocular pressure. In Krieglstein,
G.K.(ed.), Glaucoma Update IV. Springer-Verlag, Berlin Heidelberg, 20-29.
2) Nguyen TD; Chen P; Huang WD; Chen H; Johnson D; Polansky JR. Gene structure
and properties of TIGR, an olfactomedin-related glycoprotein cloned from
glucocorticoid-induced trabecular meshwork cells. Jr. of Biol. Chem., 1998 Mar
13, 273(11):6341-50.
3) Stone EM; Fingert JH; Alward WLM; Nguyen TD; Polansky JR; Sunden SLF;
Nishimura D; Clark AF; Nystuen A; Nichols BE; et al. Identification of a gene
that causes primary open angle glaucoma. Science, 1997 Jan 31,
275(5300):668-70.
Wednesday, January 17, Noon, 489 Minor Hall
Fred A Miles
Chief of the Section on Oculomotor Control
Laboratory of Sensorimotor Research, NEI Intramural program, Bethesda MD
Host: Clifton Schor
(This presentation is co-sponsored by the Minerva foundation and School of Optometry)
Population Coding of Vergence Eye Movements in the MST Area of Cortex.
Abstract
Single unit discharges were recorded in the medial superior temporal
area (MST) of 5 behaving monkeys. Disparity tuning curves, resembling
the derivative of a Gaussian, were derived from the dependence of
initial vergence responses on the magnitude of step disparity stimuli.
Tuning curves were sorted into four groups based on their shapes that
had features in common with four classes of disparity-selective neurons
in striate cortex. The magnitude, direction and time course of the
initial vergence velocity responses associated with disparity steps
applied to large patterns are all encoded in the summed activity of
disparity-sensitive cells in MST. We suggest that this population
activity in MST plays an active role in the generation of vergence eye
movements with short latencies.
Tuesday January 16, 2001, 5:00 PM, 145 Dwinelle Hall
Semir Zeki
Professor of Neurobiology
University College, London
The Platonic Ideal in Art and Neurology
Cosponsored by the Departments of the History of Art, Philosophy and Art
Practice, as well as The School of Optometry and The Minerva Foundation.
Reference:
Zeki, Semir
Inner vision : an exploration of art and the brain
Semir Zeki. Oxford; New York : Oxford University Press, c1999.
UCB Main N71 .Z45 1999
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