Object Recognition (2b) Flashcards
(18 cards)
What are our two visual systems
- Ventral: the WHAT system (for perception)
- Dorsal: the WHERE system (for vision-for-action)
These are the two separate routes for recognising objects
What are ‘agnosias’
Agnosia: when the objective recognition system FAILS
- there is visual form agnosia vs associative agnosia
Modularity of the visual system: what are the two pathways
The DORSAL STREAM: where in the world objects are located
–> parietal lobe
The VENTRAL STREAM: what objects are (fine-grain discrimination of objects)
–> occipital/temporal lobe
First processed in V1 - primary visual cortex
Early studies: outline the word by Pohl (1973) what vs where in monkey visual cortex
Pohl (1973)
- Monkeys predicting reward
Task A:
- in this task specific pairs of objects predicted food reward
- lesions to INFEROTEMPORAL (VENTRAL) cortex impair object recognition (WHAT)
Task B:
- in this task the proximity of the cylinder to the foodwell predicts reward
- lesions to parietal cortex (DORSAL) impair spatial recognition (WHERE)
What v Where: Human Neuropsychological Evidence - outline the work by Kohler et al. (1995)
Human studies CONVERGE with monkey studies
Occipitotemporal Cortex Lesions (VENTRAL)
= visual agnosia (deficit in recognising objects)
Parietal cortex lesions (DORSAL)
= deficits of spatial awareness - hemispatial neglect
Subjects had to perform two tasks
- Subjects presented with two displays and this time had to judge whether the LOCATIONS of the objects were the same in both displays
- Subjects were again presented with two displays and this time had to judge whether all the pictures were the same OBJECTS in the two displays
FOUND:
- contrasting the two tasks produced different patterns of activation
- activation was greater for the object task than the spatial task in VENTRAL TEMPORAL CORTEX (primarily fusiform gyrus)
- activation was greater for the spatial task than the object task in DORSAL CORTEX (primarily inferior parietal cortex)
What v Where: Human Neuropsychological Evidence - outline the work by Karnath et al. (2009)
Experiment shows the ‘what’ v ‘where’ distinction is not quite that simple
–> Tested a patient (JS) with a circumscribed lesion to ventral occipitotemporal cortex on a
1. perceptual judgement task
2. motor action task
FOUND:
- patient was impaired on the perception task but NOT the motor task
- shows that ventral lesions impair vision for perception, but not vision for action, suggesting the ventral/dorsal distinction may be more along these lines
the most common patients with object recognition issues - stroke!!
What are the two main types of visual agnosia (state them)
- Visual form agnosia
- Associative agnosia
What is visual form agnosia
Impairment in VISUAL PERCEPTION but ABOVE the level of a basic sensory deficit
–> patient cannot recognise, copy, match, or discriminate simple visual stimuli
–> cannot recognise simple shapes
–> inability to group and integrate objects into a whole
–> deficit in shape processing
What is associative agnosia
A HIGHER LEVEL DEFICIT
- basic perception seems fine, but recognition cannot take place
- patient can make good copies but cannot recognise them
- patients lose associations with objects (can’t say what it is or what it is for)
- not due to deficits in language
- in a ‘matching by function task’ patients cannot find objects with similar functions, instead they choose the two most visually similar objects
What does agnosia tell us about object recognition (3 things)
- Object recognition is MODULAR
- separate brain regions for different processes
- different types of agnosia broadly highlight distinction between:
o perceptual processing (shape analysis) - impaired in visual form agnosia
o semantic processing (activation of associated knowledge) - impaired in associative agnosia - Object Recognition is a CONSTRUCTIVE PROCESS
- brain constructs representations of objects based on many different sources of contextual input
- these representations (not simply just retinal input) are what we are consciously aware of - Object Recognition is a SEMANTIC PROCESS
- information about the meaning of an object is automatically processed when we see it,
eg. its function
- involves not just seeing an object
What does agnosia NOT tell us about object recognition
does NOT tell us specifically WHERE in the ventral visual cortex all of this happens
- lesions can be large, diffuse, and extremely variable (often affecting multiple regions, even encompassing different lobes)
- quite rare to find very ‘pure’ neuropsychological cases
Function to structure in the ventral visual pathway: describe the structure of the V1
Retina –> Optic Nerve and Optic Chiasm –> Optic Tract –> LGN –> Primary Visual Cortex (V1) located in the occipital lobe
Describe what is meant by ‘The Primary Visual Cortex is Retinotopic’
Structured RETINOTOPICALLY: different regions of the visual field (real world) are perceived in different regions in the primary visual cortex
–> There is MAPPING AND MAGNIFICATION
MAPPING: the correspondence between the spatial structure of the primary visual cortex and the spatial structure of the real world
CORTICAL MAGNIFICATION: a disproportionately large of the area of the visual cortex is dedicated to the centre of the visual field (corresponding to the eye’s fovea)
How does processing change moving from V1 to the extra-striate cortex
Processing complexity INCREASES as we move from V1 to extra-striate cortex
V1 –> LO –> V5
V1 - the neurons processing very low-level stimuli
–> as we move forward the neurons are more sensitive to complex features
to the LO
–> begin to see more complex features
to the V5
–> key role in motion processing
What is the Lateral Occipital Complex (LOC) and its properties
Lateral Occipital Complex - a specialised brain region for integrating features into shapes
–> part of the VENTRAL processing stream
–> plays a role in integrating features into whole shapes
–> a largely NON-RETINOTOPIC AREA: doesn’t keep strict spatial correspondence to ‘real-world’
—> activated by contralateral and ipsilateral visual fields
–> seems to ENCODE HIGHER-LEVEL REPRESENTATIONS OF SHAPE
–> studies use visual illusions to show its sensitivity
fMRI evidence of the LOC: outline the work by Vuilleumier et al (2002)
Researchers have used fMRI adaptation to try and understand what the brain is doing
–> adaption: the process by which neurons reduce their response to a constant or repetitive stimulus over time
–> found reduced activation in left fusiform cortex to the same object from a different viewpoint relative to when different objects were presented
–> therefore: region may play a role in OBJECT CONSTANCY (enabling us to recognise objects from multiple viewpoints)
fMRI evidence of the LOC: outline the work by Drucker et al. (2010)
fMRI reveals a region may play a role in object constancy - enabling us to recognise an object under multiple different contexts, viewpoints etc.
Procedure:
- subjects were presented with pairs of words sharing either function/shape/function-and-shape/unrelated/identical
FOUND:
- regions in left temporal cortex showed adaptation to pairs of words with the same function
Summary: outline the key function-to-structure maps in the ventral visual pathway
There are separate ‘modules’ in ventral visual pathway for:
1. Integrating features into shapes (lateral occipital cortex)
2. Viewpoint invariant representation of objects (aka object constancy) (left fusiform cortex)
3. Representation of functions (left medial temporal cortex)
