Cognitive Neuroscience Lecture 13: Spatial Representation

''L13: Spatial Representation''

What is the problem to be solved?

Need to know where things are so that:

-         Identification: move our eyes to them

o ID what it is; face/word/unfamiliar

-         Action: interact with objs

o Pick up, sit on them, avoid them, etc.

-         Navigation: find them when they are outside the range of our senses

o Your house, your car, supermarket

o

o   What: STGàIFG (superior temporal gyrus to inferior frontal gyrus)

o   Where: STGàSPLàSFG (STGàsup. Parietal lobeàsuperior frontal gyrus)

-         Visual pathways:

o Superior longitudinal fasciculusàPosteroparietal cortex

o Inferior longitudinal fasciculusàinferior temporal cortex

Representation of location

'''Relative by reference to something else: reference point '''

Anything can be a reference point

Reference point is a defining aspect of a frame of reference

What reference frames used by brain areas that encode locations?

-         Cells in v1?

-         Cells in the doral/parietal (where) pathway?

Reference frames:

-         Retino-centered

-         Viewer-centered (ego centric)

-         Object-centered (allo-centric)

Frame of reference for V1 neurons

-         Calcarine fissure (v1)

-         Fovea most posterior

-         In goes out, up goes down, left goes right

-         V1 neurons only encode in terms of direction and distance from fixation

o '''Retino-centric frame of reference (relative to fixation)'''

-         We know this because:

o Objects in world that stim same location in retina represented by same v1 neurons

§ If a cup is at point A on retina, and then a phone is at point A, these will both activate same v1 neurons

o When fixation moves, same objs represented by diff v1 neurons (ie not obj centric)

Cortical blindness

-         Deficit defined relative to fixation

-         When fixation moves, same location in external space represented differently

Frames of reference: evidence from visuo-spatial neglect

Visuo-spatial neglect/extinction

-         Difficulty in perceiving/attending/being conscious of stim on contralesional side of space

Varieties:

-         Viewer centered

-         Object centered

Provides evidence for viewer-centered and object-centered frames of reference in temporal/parietal areas

Neuropsychological cases: right temporal/parietal subsequent to drug overdose

Line bisection: better for shorter line

Line cancellation: cross out all lines on page

Frames of reference: evidence from visuospatial neglect 

-         Not cortical blindness; V1 is intact

-         Defecit persists despite eye mvt

o Therefore '''attentional defecit '''due to parietal lobe lesion

What frame of reference must we assume is disrupted?

-         Critical evidence:

o V1 is intact

o Neglect persists despite eye mvts

o Left is not retinocentric but rather defined relative to viewer

-         '''Viewer centered neglect: person’s midline '''

-         Difficulty in processing contralesional side of midline regardless of eye position

Neuropsych: half of each obj missing (group of bow is an obj)

-         Location defined with reference to represented object (not retina or viewer): object-centered frame of ref

-         You’ll be able to see it when you flip the obj

-         What counts as an object?

o Grouping/visual attributes

-         Locations defined relative to obj’s right and left side

-         Difficulty processing/perceiving stim of contralesional side of obj regardless of midline/eye pos

Scene copy and gap detectionà viewer centered neglect associated with lesions to inferior lobule 

Superior temporal gyrus: object centerd neglect

Monkey studies show:

To grab mug, refrence frame must be in common frame of ref