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Unformatted text preview: Cognitive Neuroscience Dorsal Route and Spatial Representation The dorsal stream Where? Or Perception for action? Why do we need to know where things are? Among other reasons, to reach and grasp them Grasping requires knowing where something is (but also what its shape is) D.F. Goodale & Milner (1992) 34 years old Carbon monoxide poisoning MRI: bilateral diffuse lesions in the occipital lobe (ventral pathway) Severe object recognition disorder (agnosia) Naming line drawings: 0% correct Poor copying of line drawings Poor judgment of line orientation Perceptual matching: Orientation DF control DF: A fairly lowlevel deficit in the object recognition system Action Dorsal pathways: More detail
Dorsal: Perception for action Object Location Object shape for physical interaction Control/movement of attention Spatial reference frames (spatial normalization) The dorsal stream: Control of attention What does attention do? Enhances processing in specific locations (or object properties) Topdown modulation of processing Where do attentional signals orginate? The representation of location What is the problem to be solved? We need to know where things are so that: We can look at them To identify what it is (a face, a word, unfamiliar blobs on your plate) We can interact with them Pick them up, sit on them, avoid them, etc. We can find them when they are outside the range of our senses (navigate to them) Your house, your car, your classroom, your keys, the supermarket The representation of location How could you define my location? Locations are defined in a relative manner, by reference to something else Within some frame of reference Defining spatial locations How are locations defined/encoded at different levels of processing? By reference to what? (In what frame of reference?) For ganglion cells? For cells in V1? For parietal neurons? How is location defined in V1? What makes something location #3 in V1? What's the frame of reference? Retinocentric frame of reference: Location is defined according to location on the retina (relative to fixation) + Which V1 neurons are active? + + + + + Retino centric frame of reference Cortical blindness: Deficit is defined relative to fixation What happens to the "block dots" when fixation moves to the right edge of the screen? What happens if a word is presented briefly in the center of the screen for reading? What happens if a word is presented briefly to the left of fixation? What happens if a word is presented for unlimited viewing? Rijpkema et al (2008) Small, Med, Large .25, 1.77, 9.8 Task: Subjects detected a target letter "F" at specified scale (Small, Med, or Large). b), c) Effect of attending different scales Small > Med, Large Med > Small, Large Large > Small, Med Representation of Fovea Rijpkema et al (2008) Shows that topdown allocation of attention (from parietal cortex) affects occipital cortex Small : Stronger activation in retinotopic areas encoding very center of the fovea Medium : Stronger activation in retinotopic areas encoding rest of fovea Large : Stronger activation in retinotopic areas encoding regions outside the fovea. Defining locations Anything could serve as a reference point (the center of a frame of reference) Question: Are there certain reference frames that are typically used by the brain? Frames of reference: Evidence from visuospatial neglect Visuospatial neglect: Difficulty in perceiving/attending to/being conscious of stimuli on the contralesional side of space Varieties: viewercentered objectcentered Provide evidence for viewercentered and objectcentered frames of reference beyond V1 Neuropsychological Cases CS 66 years old, righthanded Right parietal hemorrhage subsequent to drug overdose Frame of Reference Not cortical blindness, V1 is intact Cases of cortical blindness can adjust fixation thru head and eye movements so that performance is intact CS's deficit persists despite eye movements > attentional deficit Failure of perception/awareness of items on the left side left is defined relative to the viewer (not the retina) Viewercentered neglect NG 79 years old, female, lefthanded Left parietal infarct NG What frame of reference has been affected in this case? Location is defined with reference to the represented object (not the retina or the viewer) objectcentered frame Frames of Reference Purpose of these different frames? Retinacentered (retinocentric) Guide eyemovements Determine an object's location in the world Viewercentered (egocentric) Guide reaching and other movements Objectcentered (allocentric) Object recognition despite changes in orientation, position, location Neural substrates of different references frames Neural substrates The jury is still out, however, early indications: Egocentric (viewercentered) Parietal lobe Frontal lobe Allocentric) Temporal lobe Frontal lobe (including frontal eye fields) Olson & Gettner (1995) Single unit recording Recording:Frontal eyefields Monkey trained to make an eye movement to end of bar where cue appeared. Pairs of stimuli with identical eye movements. How is left defined for this neuron? How is it not defined? ...
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- Spring '08