The Role of “Extrastriate” Areas
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The Role of “Extrastriate” Areas
• Functional imaging (PET) investigations of motion and colour selective visual cortical areas
• Zeki et al.
• Subtractive Logic– stimulus alternates between two scenes that differ only in
the feature of interest (i.e. colour, motion, etc.)
The Role of “Extrastriate” Areas
• Identifying colour sensitive regions
Subtract Voxel intensities during these scans… …from voxel
intensities during these scans
…etc.Time ->
The Role of “Extrastriate” Areas
• result– voxels are identified that are preferentially selective for
colour– these tend to cluster in anterior/inferior occipital lobe
The Role of “Extrastriate” Areas
• similar logic was used to find motion-selective areas
Subtract Voxel intensities during these scans… …from voxel
intensities during these scans
…etc.Time ->
MOVING STATIONARY MOVING STATIONARY
The Role of “Extrastriate” Areas
• result– voxels are identified that are preferentially selective for
motion
– these tend to cluster in superior/dorsal occipital lobe near TemporoParietal Junction
– Akin to Human V5
The Role of “Extrastriate” Areas
• Thus PET studies doubly-dissociate colour and motion sensitive regions
The Role of “Extrastriate” Areas
• V4 and V5 are doubly-dissociated in lesion literature:
– achromatopsia (color blindness): • there are many forms of color blindness• cortical achromatopsia arises from lesions in the area of V4• singly dissociable from motion perception deficit - patients with
V4 lesions have other visual problems, but motion perception is substantially spared
The Role of “Extrastriate” Areas
• V4 and V5 are doubly-dissociated in lesion literature:
– akinetopsia (motion blindness): • bilateral lesions to area V5 (extremely rare)• severe impairment in judging direction and velocity of
motion - especially with fast-moving stimuli• visual world appeared to progress in still frames• similar effects occur when M-cell layers in LGN are
lesioned in monkeys
The Role of “Extrastriate” Areas
• Consider two plausible models:
1. System is hierarchical:– each area performs some elaboration on the input it is given
and then passes on that elaboration as input to the next “higher” area
2. System is analytic and parallel:– different areas elaborate on different features of the input
How does the visual system represent visual information?
How does the visual system represent features of scenes?
• Vision is analytical - the system breaks down the scene into distinct kinds of features and represents them in functionally segregated pathways
• but…
• the spike timing matters too!
Visual Neuron Responses
• Unit recordings in LGN reveal a centre/surround receptive field
• many arrangements exist, but the “classical” RF has an excitatory centre and an inhibitory surround
• these receptive fields tend to be circular - they are not orientation specific
How could the outputs of such cells be transformed into a cell with orientation specificity?
Visual Neuron Responses
• LGN cells converge on “simple” cells in V1 imparting orientation (and location) specificity
Visual Neuron Responses
• LGN cells converge on simple cells in V1 imparting orientation specificity
• Thus we begin to see how a simple representation - the orientation of a line in the visual scene - can be maintained in the visual system– increase in spike rate of specific neurons indicates presence of a line
with a specific orientation at a specific location on the retina
– Why should this matter?
Visual Neuron Responses
• Edges are important because they are the boundaries between objects and the background or objects and other objects
Visual Neuron Responses
• This conceptualization of the visual system was “static” - it did not take into account the possibility that visual cells might change their response selectivity over time
– Logic went like this: if the cell is firing, its preferred line/edge must be present and…
– if the preferred line/edge is present, the cell must be firing
• We will encounter examples in which these don’t apply!
• Representing boundaries must be more complicated than simple edge detection!
Visual Neuron Responses
• Boundaries between objects can be defined by color rather than brightness
Visual Neuron Responses
• Boundaries between objects can be defined by texture
Visual Neuron Responses
• Boundaries between objects can be defined by motion and depth cues
Feed-Forward and Feed-Back Processing in the Visual System
The Feed-Forward Sweep
• What is the feed-forward sweep?
The Feed-Forward Sweep
• The feed-forward sweep is the initial response of each visual area “in turn” as information is passed to it from a “lower” area
• Characteristics:– a single spike per synapse– no time for lateral connections – no time for feedback connections
The Feed-Forward Sweep
• The feed-forward sweep is the initial response of each visual area “in turn” as information is passed to it from a “lower” area
• What does it mean for an area to be “lower” or “higher”
The Feed-Forward Sweep
• Hierarchy of visual cortical areas defined anatomically
Dorsal “where”/”how”
Ventral “what”
The Feed-Forward Sweep
• Hierarchy can be defined more functionaly
• The feed-forward sweep is the initial response of each visual area “in turn” as information is passed to it from a “lower” area
• Consider the latencies of the first responses in various areas
The Feed-Forward Sweep
• Thus the “hierarchy” of visual areas differs depending on temporal or anatomical features
• aspects of the visual system account for this fact:
– multiple feed-forward sweeps progressing at different rates (I.e. magno and parvo pathways) in parallel
• M pathway is myelinated
• P pathway is not
– signals arrive at cortex via routes other than the Geniculo-striate pathway (LGN to V1)
• Will be important in understanding blindsight
The Feed-Forward Sweep
• The feed-forward sweep gives rise to the “classical” receptive field properties– tuning properties exhibited in very first spikes
• Orientation tuning in V1• Optic flow tuning in MST
– think of cortical neurons as “detectors” only during feed-forward sweep
After the Forward Sweep
• By 150 ms, virtually every visual brain area has responded to the onset of a visual stimulus
• But visual cortex neurons continue to fire for hundreds of milliseconds!
After the Forward Sweep
• By 150 ms, virtually every visual brain area has responded to the onset of a visual stimulus
• But visual cortex neurons continue to fire for hundreds of milliseconds!
• What are they doing?
After the Forward Sweep
• By 150 ms, virtually every visual brain area has responded to the onset of a visual stimulus
• But visual cortex neurons continue to fire for hundreds of milliseconds!
• What are they doing?
• with sufficient time (a few tens of ms) neurons begin to reflect aspects of cognition other than “detection”
Extra-RF Influences
• One thing they seem to be doing is helping each other figure out what aspects of the entire scene each RF contains
– That is, the responses of visual neurons begin to change to reflect global rather than local features of the scene
– recurrent signals sent via feedback projections are thought to mediate these later properties
Extra-RF Influences
• consider texture-defined boundaries– classical RF tuning
properties do not allow neuron to know if RF contains figure or background
– At progressively later latencies, the neuron responds differently depending on whether it is encoding boundaries, surfaces, the background, etc.
Extra-RF Influences
• How do these data contradict the notion of a “classical” receptive field?
Extra-RF Influences
• How do these data contradict the notion of a “classical” receptive field?
• Remember that for a classical receptive field (i.e. feature detector):
– If the neuron’s preferred stimulus is present in the receptive field, the neuron should fire a stereotypical burst of APs
– If the neuron is firing a burst of APs, its preferred stimulus must be present in the receptive field
Extra-RF Influences
• How do these data contradict the notion of a “classical” receptive field?
• Remember that for a classical receptive field (i.e. feature detector):
– If the neuron’s preferred stimulus is present in the receptive field, the neuron should fire a stereotypical burst of APs
– If the neuron is firing a burst of APs, its preferred stimulus must be present in the receptive field
Recurrent Signals in Object Perception
• Can a neuron represent whether or not its receptive field is on part of an attended object?
• What if attention is initially directed to a different part of the object?
Recurrent Signals in Object Perception
• Can a neuron represent whether or not its receptive field is on part of an attended object?
• What if attention is initially directed to a different part of the object?
Yes, but not during the feed-forward sweep
Recurrent Signals in Object Perception
• curve tracing– monkey indicates whether a
particular segment is on a particular curve
– requires attention to scan the curve and “select” all segments that belong together
– that is: make a representation of the entire curve
– takes time
Recurrent Signals in Object Perception
• curve tracing– neuron begins to respond
differently at about 200 ms
– enhanced firing rate if neuron is on the attended curve
Feedback Signals and the binding problem
• What is the binding problem?
Feedback Signals and the binding problem
• What is the binding problem?• curve tracing and the binding problem:
– if all neurons with RFs over the attended curve spike faster/at a specific frequency/in synchrony, this might be the binding signal
Feedback Signals and the binding problem
• So what’s the connection between Attention and Recurrent Signals?
Feedback Signals and Attention
• One theory is that attention (attentive processing) entails the establishing of recurrent “loops”
• This explains why attentive processing takes time - feed-forward sweep is insufficient
Feedback Signals and Attention
• Instruction cues (for example in the Posner Cue-Target paradigm) may cause feedback signal prior to stimulus onset (thus prior to feed-forward sweep)
• think of this as pre-setting the system for the upcoming stimulus
• What does this accomplish?
Feedback Signals and Attention
• What does this accomplish?
• Preface to attention: Two ways to think about attention– Attention improves perception, acts as a gateway to memory
and consciousness
– Attention is a mechanism that routes information through the brain
• It is the brain actively reconfiguring itself by changing the way signals propagate through networks
• It is a form of very fast, very transient plasticity
Feedback Signals and Attention
• Put another way:
– It may strike you as remarkable that a single visual stimulus should “activate” so many brain areas so rapidly
– In fact it should be puzzling that a visual input doesn’t create a runaway “chain reaction”
• The brain is massively interconnected• Why shouldn’t every neuron respond to a visual stimulus
Feedback Signals and Attention
• We’ll consider the role of feedback signals in attention in more detail as we discuss the neuroscience of attention