L7- Perception Vs Action Flashcards
What are the 2 main ways brain functionality is supported?
Brain structures- process particular forms of information (grey matter)
Distinct cortical pathways- pass linked relevant info on for further processing (white matter)
What are the 2 main functionally different pathways in the brain proposed by Ungerleider & Mishkin (1982)?
Ventral (occipito-temporal):
-Specialised for object perception and recognition
-WHAT
Dorsal (occipito-parietal):
-Specialised for spatial perception
-WHERE?
What were the findings of Mishkin, Ungerleider, & Macko (1983)?
-Landmark and Object discrimination tasks
-Monkeys trained on one, and either temporal or parietal lobe lesioned
Landmark test (where):
-Unoperated monkeys- no impairment
-Temporal lesion- minimal impairment
-Parietal lesion- impaired performance
Object task (what):
-Temporal lesion- impaired
-Parietal- minimal impairment
At what point do the dorsal and ventral pathways split?
After area V2
-Almost immediately in occipital lobe
What 2 major fibre bundles (fasciculi) are the output from V1 contained in?
Dorsal- Superior longitudinal fasciculus terminating in posterior parietal lobe
Ventral- Inferior longitudinal fasciculus terminating in temporal lobe
What are some visual properties of ventral stream neurons? (in monkeys)
-Some have high categorical specificity (e.g. facial processing neurons)
-Selectivity/tuning of features of objects irrespective of:
orientation; viewpoint; retinal image size; L/RVF; colour
-Not greatly affected by monkey’s motor behaviour
-Are modulated by repeat presentation & reward
-Receptive fields always encompass fovea (what you’re looking at)
What are the different properties in the dorsal stream?
-Large receptive fields which respond in nonselective way
-60% of neurons respond to peripheral info (excluding fovea)
-Good for detecting appearing objects of optic flow
-Some posterior parietal neurons split, activated by type of response to the visual stimuli (arm reach; grasp; saccade; slow eye pursuit; static ocular fixation) - involved in determining action required
How does the dorsal stream compute more than just spatial location?
-Egocentric locations of visual targets computed separately for eye and hand movements
-Bilateral temporal lesions in monkeys show visual recognition affected but many visuomotor skills retained (e.g. avoiding/catching obstacles)
-Milner & Goodale (1993) - dorsal stream primarily for mediating the visual control and guidance of action
What does Optic Ataxia tell us about brain pathways in humans?
-Optic Ataxia associated with lesions in PARIETAL cortex (dorsal)
-Patients can use non-visual sense for reach & grasp movement
-Errors in contralesional hand for reaching towards an oriented slot (Perenin & Vighetto, 1988)
-Often unable to use vision to form grip during reach (Jeannerod, 1986)
-Little difficulty giving perceptual reports of orientation & location of objects
What does Visual Form Agnosia tell us about human brain pathways?
-Visual form agnosia associated with bilateral lesions to occipital lobes
-Severe deficits in object recognition but intact ability to interact with world
Goodale & Milner (1992) patient DF:
-couldnt indicate size of blocks from a distance, but could reach out and pick them up
-unable to orient a card to match slot, but could actively post the card into the slot
Outline the dorsal component of Milner & Goodale’s (1995) Dual Visual System Theory
-Proposes pathways have separate evolutionary pathways
Dorsal:
-Older, shared with many animal species
-Specialised for quick responses
-Rapid inputs & eye-movements, and has links to motor areas
-Ideal for rapid actions, on objects encoded in egocentric reference frame
-Precise object properties not required for many actions
Outline the ventral component of Milner & Goodale’s (1995) Dual Visual System Theory
Ventral:
- more recent development
-Specialised for fine-grained categorisation
-Useful for identifying objects & scenes, enabling memories of visual events, & visualising new visual scenes
-Represents surrounding scene as a relationship between objects, so object-centred (allocentric)
-Representations have been transformed so objects remain largely constant
-Therefore ideal for constructing sense of conscious experience
What are the problems with neuropsychological evidence for brain pathways?
-Deficits are not usually focal (happen by chance)
-Symptoms are often not clear cut
Outline some behavioural evidence for brain pathways
-Reach to grasp illusions shown to affect perception NOT action
Bhalla & Profitt (1999):
-Judged slant of hill from the bottom
-Perception biased- slant greater for visual or verbal reports. Misperception increased after a run (biased by effort)
-Action unbiased- Most accurate reports using haptic device. No change after run
