Member-Initiated Symposia
Symposia Overview
2008 Symposia
Perceptual expectations and the neural processing of complex images
Cortical organization
and dynamics for visual perception and beyond
Crowding
Visual Memory and the Brain
Bayesian models applied to perceptual
behavior
Action for perception: functional
significance of eye movements for vision
The past,
present, and future of the written word
Surface material perception
Symposia from Past Meetings |
Cortical organization and dynamics for
visual perception and beyond
Friday, May 9, 2008, 1:00 - 3:00 pm
Royal Palm 4
Organizer:
Zoe Kourtzi (University of Birmingham)
Presenters: Martin I.
Sereno (UCL and
Birkbeck, London), Uri Hasson (New
York University), Wim Vanduffel (Athinoula
A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital,
and Laboratorium voor Neurofysiologie en
Psychofysiologie, K.U. Leuven Medical School, Campus Gasthuisberg,
Belgium.), Charles E. Connor (Johns Hopkins University School of Medicine),
Geoffrey M. Boynton (University
of Washington),
Pieter R. Roelfsema (Netherlands
Institute for Neuroscience)
Symposium Description
The symposium aims to
showcase state-of-the-art work and methods for studying the cortical dynamics
that mediate complex and adaptive behaviours.
Extensive work in anatomy,
neurophysiology and brain imaging has approached this challenge by studying the
topography and neural function of discrete cortical structures in the human and
non-human primate brain. This approach has been very successful in generating a
roadmap of the primate brain: identifying a large number
of different cortical areas associated with different functions and cognitive
abilities. However, understanding how the brain generates complex and adaptive
behaviours entails extending beyond isolated cortical centres and investigating
the spatio-temporal dynamics that underlie information processing within and
across cortical networks.
Recent developments in
multi-site neurophysiological recordings and stimulation combined with advances
in brain imaging have provided powerful methods for studying cortical circuits
and novel insights into cortical dynamics.
The symposium will bring
together pioneers in the study of cortical circuits in the human and the monkey
brain and combine evidence from interdisciplinary approaches: physiology,
imaging, computational modelling.
First we will present brain
imaging work that characterizes the common principles of spatial and temporal
organization across and beyond the human visual cortex (Sereno, Hasson). Second,
we will discuss studies that delineate the causal interactions within these
cortical circuits combining fMRI and microstimulation (Vanduffel). Third, we
will discuss neurophysiological evidence for the functional role of these
spatiotemporal interactions in the integration of sensory information to global
percepts for visual recognition and actions (Connor). Fourth, we will present
brain imaging work showing that cortical circuits adapt to the task demands and
the attentional state of the observer (Boynton). Finally, we will present
computational approaches investigating how attention and learning shape
interactions within cortical circuits for adaptive behaviour (Roelfsema).
Thus, the symposium will
serve as a forum for discussing novel evidence on cortical organization and
dynamics emerging from current human and animal research and a tutorial for
interdisciplinary state-of-the-art methods for research in this field. As such,
the symposium will target a broad audience of researchers and students in the
vision sciences society interested in understanding the link between brain and
behaviour.
Abstracts
Finding the parts of the cortex
Martin I. Sereno
Understanding brain
dynamics requires knowing what its parts are.
Human neuroimaging has attempted that using contrasts between high level
cognitive tasks averaged across subjects in 3-D.
Two problems are: (1) higher level tasks generate activity in multiple
cortical areas, some of which adjoin each other, and (2) cross-subject 3-D
averages must use blurring kernels close to the modal size of human cortical
areas (1 cm) to overcome anatomical variation and variation in how subjects
perform tasks. Even liberal statistical thresholds underestimate the area of
cortex involved and activation borders only accidentally represent cortical area
borders.
Another way to subdivide
cortex is to find receptotopic (retinotopic, tonotopic, somatotopic) maps.
Topological retinal maps were expected in V1 and early secondary visual areas
based on non-human primate data.
However, recent work in parietal, temporal, cingulate, and frontal cortex shows
that these maps are present at higher levels, extending to the boundaries
between modalities (e.g., VIP). This was not expected on the basis of work in
animals because higher areas have larger receptive fields with a substantial
degree of scatter. Independent manipulation of stimulus and attention shows that
higher level maps are largely maps of attention. Three possible reasons why
spatial maps might persist at high levels are: (1) intracortical connections are
overwhelmingly local, (2) sensory space (retinal, frequency, skin position) is
the most important feature for distinguishing events, and (3) cortical space remains
a convenient way to allocate processing, even if it is not explicitly spatial.
A hierarchy of temporal receptive windows in
human cortex
Uri Hasson, Eunice Yang,
Ignacio Vallines, David Heeger, and Nava Rubin
Real-world events unfold at
different time scales, and therefore cognitive and neuronal processes must
likewise occur at different time scales. We present a novel procedure that
identifies brain regions responsive to sensory information accumulated over
different time scales. We measured fMRI activity while observers viewed silent
films presented forward, backward, or piecewise-scrambled in time. In a first
experiment, responses to backward presentations were time-reversed and
correlated with those to forward presentations. In visual cortex, this yielded
high correlation values, indicating responses were driven by stimulation over
short time scales. In contrast, responses depended strongly on time-reversal in
the Superior Temporal Sulcus (STS), Precuneus, posterior Lateral Sulcus (LS),
Temporal Parietal Junction (TPJ) and Frontal Eye Field (FEF). These regions
showed highly reproducible responses for repeated forward, but not backward
presentations. In a second experiment, stimulus time scale was parametrically
varied by shuffling the order of segments from the same films. The results show
clear differences in temporal characteristics, with LS, TPJ and FEF responses
depending on information accumulated over longer durations (~ 36 s) than STS and
Precuneus (~12 s). We conclude that, similar to the known cortical hierarchy of
spatial receptive fields, there is a hierarchy of progressively longer temporal
receptive windows in the human brain.
Investigating causal functional interactions
between brain regions by combining fMRI and intracortical electrical
microstimulation in awake behaving monkeys
Wim Vanduffel
Areas of the frontal and
parietal cortex are thought to exert control over information flow in the visual
cortex through feedback signals (Kastner and Ungerleider, 2000; Moore, 2003).
Although a plethora of studies provided correlation data to support this
hypothesis, corroborating causal evidence is virtually absent (but see e.g.
Moore and Armstrong, 2003). Also, several models suggest that the frontal
signals modulating incoming sensory activity are gated by bottom-up stimulation
(van der Velde and de Kamps, 2001; Roelfsema, 2006). To test these models and
examine the spatial organization of any observed modulations, we developed a
combination of fMRI (Vanduffel et al. 2001) and chronic electrical
microstimulation (EM) in awake, behaving monkeys. This approach allowed us to
investigate the impact of increased frontal eye field (FEF) output, using
biologically relevant currents, on visually-driven responses throughout occipito-temporal
cortex.
Activity in higher-order
visual areas, monosynaptically connected to the FEF, was strongly modulated in
the absence of visual stimulation, shwoing that the combination of fMRI with EM
holds great potential as in-vivo tractography tool (see also Tolias et al.
2005). Activity in early visual areas, however, could only be modulated in the
presence of bottom-up stimulation, resulting in a topographically specific
pattern of enhancement and suppression.
This result suggests that bottom-up activation of recurrent connections
is needed to enable top-down modulation in visual cortex. We furthermore
uncovered a potentially new subdivision in many areas of the visual cortex, as
the regions with strong visual responses are largely separate from regions
influenced by feedback.
Spatiotemporal integration of object
structure information
Charles E. Connor
Image representation in
early visual cortex is extremely local.
Object perception depends on spatial integration of this local
information by neurons at later cortical stages processing larger image regions.
We have studied the spatial and temporal characteristics of this
integration process at multiple cortical stages in the macaque monkey.
We have found that neurons in area V4 integrate across local changes in
boundary orientation (a first-order derivative) to derive curvature (a
second-order derivative). V4
neurons also integrate across position and binocular disparity to derive 3D
orientation. At the next processing
stage in posterior inferotemporal cortex (PIT), neurons integrate across
spatially disjoint object boundary regions to derive more complex, larger-scale
shape configurations. At still
higher processing stages in central and anterior IT, neurons derive more
complete boundary configurations with potential ecological relevance. CIT/AIT neurons also
integrate disparity and shading information to derive surface and volumetric
elements of 3D object structure.
These integration mechanisms are largely linear at early time points, producing
ambiguous representations of object structure.
Over the course of approximately 50 ms, presumably through recursive
intracortical processing, nonlinear selectivity gradually emerges, producing
more explicit signals for specific combinations of structural elements.
Feature-Based Attention in Human Visual
Cortex
John Serences and Geoffrey M. Boynton
The spatial resolution of
functional MRI makes it ideal for studying the effects of spatial attention on
responses in the human visual cortex: with fMRI we can trace the
enhancement of the BOLD signal in regions that are retinotopically associated
with the spatial location of the attentional spotlight.
Studying the effects of feature-based attention is more difficult because
the columnar organization of visual features such as direction of motion and
orientation are too small for traditional fMRI experiments.
However, recent developments in pattern classification algorithms by
Kamitani and Tong (2006) have allowed researchers to investigate these
feature-based attentional effects by studying how the pattern of fMRI responses
within a visual area is affected by changes in the physical and attended
feature. I will present the results
of two studies in which we have applied these methods to show that (1) in all
early visual areas, feature-based attention for direction of motion spreads
across to unattended locations of the visual field, and (2) only area MT+ (and
possibly V3A) represent the perceived, rather than the physical direction of
motion. These results provide evidence that the early stages of the visual
system respond more than just to the bottom-up stimulus properties. Instead, the
cortical circuitry adapts to the task demands and attentional state of the
observer.
How attentional feedback guides learning of
sensory representations
Aurel Wannig and Pieter R. Roelfsema
I will describe our new
theory, AGREL (attention-gated reinforcement learning; Roelfsema & van Ooyen,
2005), which proposes a new role for feedback connections in learning. We aim to
understand the neuronal plasticity that underlies learning in classification
tasks and test the predictions of our theory using a multilayer neural network.
Stimuli are presented to the lowest layer representing a sensory area of the
cortex.
Activity is then propagated
to the highest layer representing the motor cortex, which has to choose one out
of a number of actions that correspond to the various stimulus categories.
Neurons in the highest layer engage in a competition for action selection. A
reward is delivered if this action is correct, and no reward is delivered in
case of an error. On erroneous trials the correct action is not revealed to the
network. The distinguishing feature of AGREL is that the neurons that win the
competition in the motor cortex feed back to lower layers, just as is observed
for attentional effects in neurophysiology. This attentional feedback signal
gates synaptic plasticity at lower layers in the network so that only neurons
receiving feedback change their synapses. i.e. the attentional feedback acts as
a credit assignment signal. We show that the feedback signal makes reinforcement
learning as powerful as previous non-biological learning schemes, such as error-backpropagation.
Moreover, we demonstrate that AGREL changes the tuning of sensory neurons in
just the same way as is observed in the visual cortex of monkeys that are
trained in categorization tasks.
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