4. Retinotopic maps Flashcards
Henschen (1895):
Perimetry test (fixating on a point, then dots of light flash around the visual field - survey map of one’s sensitivity to different visual field locations)
- Tested individuals who suffered brain damage (postmortem analysis to determine what damage)
- Damage to the back of the brain impacted on vision
- Damage in upper regions of occipital lobe resulted in deficits in lower visual field of the contralateral side
(e. g. damage to upper/superior occipital lobe in left hemi resulted in deficits in right lower visual field) - opposite for the lower/inferior occipital lobe
Visual field: eccentricity coordinate
Distance from the centre of the visual field
- looking at an image spreading from the middle as a circle is reflected in that same pattern of activity in the occipital lobe
Visual field: polar angle
e. g. the silce of the visual field at 12 corresponds to a region in the occipital lobe at 6, slice at 6 corresponds to a region at 12
- Think of it like a face of a clock where the slices go around
About ___% of V1 represents the central 20 degrees of the visual field which is a tiny portion of our visual field (this is called ____)
60%
- cortical magnification
What is the Holmes map/primary visual cortex?
Where a full map of the hemi fields is (one half per hemisphere)
(V1)
In individuals with macular degeneration the ____ of the visual field is…
centre
…is compromised
- leading cause of blindness in the western world (1/10 chance to have it if over 80yo)
McKeefry et al (2008):
Using TMS on V3 visual area
- Disruption to V3A = perceived slowing of visual stimuli
- Disruption to nearby V3D = did not affect perceived timing of visual stimuli
Baselere et al (2003):
Activation of the Primary Visual Cortex in Controls vs Rod Achromats
Rod Achromats:
Center of the visual field (where typically only cones cause activity) is partially active in rod achromats who lack cones
- These visual maps set up at birth, may be plastic during a developmental period
What are Rod Achromats?
Entirely colour blind (only have the rod system)
Effects of Macular Lesions
Resolution:
- Without the resolving power of the fovea, the best resolution is reduced (the larger the lesion, the more degraded the image)
Scotoma:
- The lesion itself gives rise to dense regions of blindness (these blind regions move with the eyes)
Congenital vs acquired retinal lesion
Acquiring a retinal lesion in adulthood does not lead to remapping that is found when lesions are present at birth (age DOES matter)
- visual maps can be reconfigured if the (retinal) deficit is present at birth
Visual Field Maps
Interested in functionality of:
- LO1 / LO2 and TMS
- It has been speculated that LO1 responds to orientation and LO2 responds to shape
Orientation:
- Presenting slanted lines (45 degrees)
- Presenting slanted lines for test (these are bit more horizontal or bit more vertical)
- Asked participants if test lines are more vertical or more horizontal (the test lines are changed so that it is very hard to tell)
During answers TMS to LO1 or LO2 (rTMS 20Hz)
Found Double Dissociation:
!!LO1 involved in ORIENTATION, not shape
!!LO2 involved in SHAPE, not orientation
Bridge and Clare (2006):
Striate Cortex (Striated = marked with long, thin parallel streaks)
This is the study which looked at the anatomical feature of the brain and correlates it with activity that we can measure with fMRI
It is an anatomical feature which can tell us where an individual’s Primary Visual Cortex (V1) is
Enables us to ask questions such as how physically big is the Primary Visual Cortex in someone who cannot see!
And many other questions
An artifact caused by the dural sinus occurs in the same place as ___
V4(what was thought to be a new V8 area in one study)
- Sinus can invert fMRI signals
Does fMRI have equal sensitivity everywhere?
No
