Lectures 11 & 12 Flashcards
Reference frame
System for representing locations relative to some real or arbitrary standard
Neglect can affect multiple reference frames
2 Main Classes of Reference Frames
- Egocentric Space- locations defined relative to the observer (retina, head, body midline)
- Allocentric Space- locations defined independently of observer (but in relation to gravity, or other objects)
3 types of egocentric reference frames
- Hemispatial- defined relative to body midline
( right of the body means right hemispace) - Somatotopic- defined in relation to parts of the body
( this can move dependent to the hemispace, both arms can be in same hemispace) - Retinotopic- defined in relation to the retina
2 types of allocentric reference frames
- Environmental/gravitational- relative to the environment
Have rubber duck in environmental left and environmental right. Patient will ignore whole duck in the left. We know it is not a problem with egocentric left because we have patient lie down and they still ignore left duck (disentangles the reference frames by having them lie down) - Stimulus-centered- relative to the middle of the object
Patient may ignore the left side of the object despite its orientation. Think of rubber duck being turned sideways and upside down. Will still neglect original left side of it
Implications of reference frames
- Spatial relations (left vs. right) are represented in a variety of reference frames
- Different frames are used for different spatial processes
- Parietal cortex is where the different spatial maps are represented in the brain
- Neglect may affect multiple reference frames
Representation of external space experiment done by Bisiach and Luzzatti
Neglect patients who lived in town for most of their lives told to imagine themselves at Point A facing Point B and to describe what they saw
Then vice versa
Result: neglect of the left image is also in imagination
Patients with unilateral neglect:
When they faced north would ignore features on left. When faced south, once again ignore features on the right that they had just named previously
Left parietal represents the right side of space; right parietal represents the left side of space
Neglect is due to a problem forming visual representations including memory by the hemisphere that is damaged
Attention as acts a mental spotlight
Experiment by Hemholtz
Attention selects information for conscious processing
Attention can be dissociated from overt direction of gaze
Overt- where your attention is focused
Covert- can tend your attention to place where eyes are not looking (peripheral)
Covert attention is disturbed in neglect
Posner Task
Valid Cue: directs attention to target location
Invalid Cue: directs attention to wrong location (20% of cues are invalid)
Results: For normally intact people reaction time is slower for invalid cues on both-sides of space
For people with neglect from right parietal lesions they were slower overall, performance asymmetry, and left target detection was severely impaired after an invalid right cue
This pattern is called “extinction-like reaction time pattern”
Improve performance in neglect
Ask patients to report cues before line bisection task actually improves their performance
( have them read a number on each side of the line)
Why is left neglect more common than right neglect
Could be that the right hemisphere can pay attention to both right and left hemispace, while the left hemisphere can only look at right hemispace
Kinsbourne’s attentional orienting model of left neglect
Orienting- the process that “directs” overt or covert attention to regions in space
-Kinsbourne said that each hemisphere directs orienting contralaterally
-They compete for control
-When one hemisphere is damaged, the other one dominates orienting
-It over-attends in its preferred direction
-Left hemisphere has a stronger bias than right hemisphere in right-handed individuals
(left hemisphere is the base of language)
This all results in left neglect being more common
Compatibility of the 3 theories
They are all complementary of each other
- The left hemisphere has a stronger orienting bias, and that leads to a predominance of left neglect
- Attention can be directed within multiple mental representations:
- images in our head with eyes closes
- external world
- our own bodies - If neglect involves a rightward attentional bias, the same bias will be shown despite the frame of reference we are using
Interhemispheric pathways
Commissures:
projection fibers that connect structures on the left & right sides of the brain
Corpus Callosum:
largest commissure; 225 million commissural fibers
Region-Specific connections of the corpus callosum
- Rostrum: orbital frontal cortex
- Genu: prefrontal cortex
- Rostral body: superior frontal and motor cortex
- Caudal body: parietal/somatosensory and temporal/auditory
- Splenium: occipital cortices-not V1
Types of callosal connections
Interhemispheric connections
1. Homotopic: connects with same regions (left V2 to right V2)
2. Heterotopic: connects different regions (left V2 to right V3)
Interhemispheric connections are contralateral
Intrahemispheric connections are ipsilateral
