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OBJECT PROCESSING IN THE VENTRAL VISUAL STREAM :PENCIL2: B+B2 LECTURE 1 -…
OBJECT PROCESSING IN THE VENTRAL VISUAL STREAM
:PENCIL2: B+B2 LECTURE 1
Visual Object Processing Hierarchy
- essential to know the stimulus in front of us so we can categorise it and come up with sensical behavioural choices.
Hierarchical models of Vision
Feed-forward fashion for information flow along a cortical hierarchy of regions.
Analysis is assumed to get more complex along this hierarchy.
RETINAL GANGLION CELLS --> THALAMUS --> PRIMARY VISUAL CORTEX
(Only a small section of the process)
Regions
early
in the hierarchy (
posterior
regions) have
fine-grained, local RFs
.
More
anterior
regions that appear
later
in the hierarchy have
larger RFs
.
Larger RFs
are more useful for evaluating
meaningful object information
. (E.g. "where are boundaries between the object and the background?")
V1, V3, V4
(Early in the hierarchy --> later in the hierarchy)
Regions
higher
up in the visual processing hierarchy (e.g. LO or TO), you will find
bigger RFs across eccentricities
compared to regions earlier in the hierarchy such as V1 or V2.
Two visual systems
Dorsal Stream
"Where" - object location - useful for planning and executing actions towards objects. Object-guided action.
Ventral Stream
"What" - responsible for object recognition and naming.
Mishkin et al (1983)
Made life-sized lesions in the brains of monkeys and got them to complete a visual task.
Object Discrimination Task
- Monkeys learned to associate a shape with a reward.
When
Inferotemporal Cortex
(part of the
ventral
stream) is removed, the monkeys cannot do this task anymore.
DOUBLE-DISSOCIATION
- Qualifies roles of brain regions in object recognition.
Landmark Discrimination Task
- Monkey will find the reward under the box closest to the landmark.
When a chunk of the
parietal cortex
(part of the
dorsal
stream) is removed, monkeys cannot do this task anymore, but they can do the Object Discrimination Task.
Evidence from Humans
Patient DF
Had a lesion in the LO (
Lateral Occipital Cortex
) region in the
ventral
stream, associated with object processing.
Not good at perceptual matching of object quality. Cannot do a task involving tilting a card to match orientation, but she can act on this task like healthy controls. This shows a
dissociation
between
object recognition in the ventral stream
and
vision for action in the dorsal stream.
The Ventral Visual System
Object recognition is mainly associated with ventral stream computations in regions like
V2, V4
or the
inferotemporal cortex
. Study responses along these regions provides a window into the stages of object recognition in the brain.
Spatial Sampling in the Ventral Stream
Different regions
cover different parts
of visual space.
Generally, all visual regions have preference for the
contralateral
(opposite) visual field.
Regions within the visual cortex of each hemisphere have
different sampling properties
.
Regions along the ventral stream have
different receptive field (RF) sizes
.
Receptive Field (RF)
= Regions of the outside world where a stimulus leads to a change in neural processing.
If a stimulus is presented somewhere
within a receptive field,
the
neurone responds
. If the stimulus is presented
somewhere else within the visual field
, the
neurone is silent
.
RF size increases
as you get more
peripheral
.
Feature-tuning
Neurons in early visual regions are driven by simple visual features such as contrast edges, which fall in their small RFs.
Cells respond preferentially to certain rotations.
This helps us to construct feature maps across visual space, but they do not allow meaningful object recognition.
In intermediate regions of the ventral pathway (V4), neurones exhibit preferences to complex feature conjunctions such as combinations of shapes or colours.
They show weaker responses to singular components of these compounds.
These are more diagnostic of objects, but are not too useful for behaviour yet.
Colour-selective cells that respond to movement of red and green things in one particular direction.
Further up the hierarchy = conjunctions of these features.
In the inferotemporal cortex, in regions at late stages of the ventral stream hierarchy, neurons respond to the presence of meaningful objects.
E.g. face-selective neurons
In anterior temporal cortex (end stages of the visual processing hierarchy), some neurons even seem to only respond to meaningful stimuli that we have encountered before, such as familiar people.
E.g. 'Grandmother neurons' or the 'Jennifer Anniston' neuron.
Challenges in Object Recognition
The problem of feature binding
How are the many features of objects combined to create meaningful object representations?
How does the brain know which feature belongs to which of multiple objects?
Each object consists of multiple features (colours, orientations, etc.)
These features may not be unique to one object, but a variety of objects, so how does the brain know which object these features belong to?
Solutions to this are not entirely clear yet...
Combinatory Explosions
With many possible combinations of features, can we have a cell for all possible feature combinations? (e.g. a cell that responds to a striped triangle in a specific orientation)
Solution: Exploit more distributed codes across neurons.
Rely on ensembles of neurons for property representation instead?
Achieving Invariance
Need to generalise across different viewing conditions such as viewpoint, illumination or size.
Across the hierarchy, object representations become more view-invariant.
At early levels of the hierarchy, representations change a lot depending on viewpoint.
Moving from posterior to anterior regions, size invariance index gets smaller and smaller.
The further you go into the hierarchy, the more invariant responses become.
Interpreting Missing Inputs
Visual system needs to infer what is there.
In real-world vision, inputs are not always complete (e.g. due to occlusion, etc.)
Visual cortex 'fills in' the missing information.
World experience and current context help the brain to do this.
Natural Scenes and competition for processing
Objects don't appear in isolation.
Objects in the real world compete for processing resources.
When multiple objects are present, neural processing is reduced, hampering perceptual efficiency.
The more objects compete, the more severely processing is impaired.
Deficits
Visual Agnosia
Inability to recognise objects.
Not a disorder of declarative memory.
Patients can still recognise objects through other modalities.
Not a deficit of basic visual function, not blind or handicapped in visual acuity.
Types of agnosia
Apperceptive Agnosia
Posterior visual cortex lesions.
Results from damage in the posterior occipito-temporal cortex, more often in the RH.
Patients are unable to perceive the visual attributes of objects, but they can often correctly recognise the object.
Often okay in naming objects, but cannot copt a drawing of one - perceptual construction of features is suffering in apperceptive agnostics.
Apperceptive agnosia may disrupt processing stages that underlie perceptual object processing.
Integrative Agnosia
Posterior visual cortex lesions.
Results from damage to the occipital cortex.
Patients are unable to group features into objects.
Not related to patients being unable to produce visual details.
Patients can draw images from memory, but have issues with separating compound shapes.
Deficit is about discerning visual features from sensory inputs rather than knowledge or ability required to produce such features.
Perceptual end of the processing hierarchy.
Associative Agnosia
Anterior lesions in the ventral visual system.
Damage to anterior temporal cortex.
Able to reproduce visual object detail, but cannot name objects.
Perceptual representation is intact, but mapping perceptual representation to concept is disrupted.
Disconnection between perceptual and knowledge systems.
Patients can construct drawings of objects
But these cannot be linked to semantic information.
Can separate compound images into their individual components, but cannot name these components.
Prosopagnosia
Anterior temporal cortex lesions.
Lesions of the right fusiform gyrus (region associated with face processing)
Inability to recognise faces, but generic object recognition is intact.
Spatial separation in the processing of visual categories.
Related to deficits in holistic processing.
Compensate for this by focusing more on the outer features of a face (e.g. hair)