Perception of Motion, Depth and Form. David M. Waitzman, M.D., Ph.D. Discussion Question. Motion in Depth lecture points:
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Perception of Motion, Depth and Form
David M. Waitzman, M.D., Ph.D.
The POINT of this Demonstration: The brain is wired to perceive motion even though in this demonstration none of the objects were actually moving
Movement of an image on the retina or an eye movement (above)
True movement or apparent (flashed) movement (right)
Top : Grating moving in any of 3 apertures has the same apparent
Bottom: In MT two edge detecting neurons are linked to
permit identification of the overall motion of the entire object
Neurons in V1 encode the direction of individual components of motion (top)
MT neurons encode the perceived speed and direction of the moving visual stimulus (below)
Type of Agnosia
Can’t name use or recognize real objects
Doesn’t recognize drawn objects
Can’t recognize faces
Doesn’t associate a color with an object
Can’t name colors
Can’t distinguish hues
Loss of stereoscopic vision
Can’t see objects moving
Purpose: Track visual stimuli moving at < 100/s
Despite a target appearance on the contralateral side, initial pursuit velocity is in the direction of smooth motion
A binocular disparity signal arises in V1 (Hubel and Wiesel)
Cells in visual (V1, V2, V4) AND extrastriate (MT) cortex respond to targets closer (orange) or further (blue) than fixation point (green)
Both dorsal and ventral pathways have disparity tuning
Tuning for targets closer or further from the monkey (near and far response patches)
Newsome et al, J. Neurosci. 1999
Cells increase in complexity of object recognition, but lose the ability to spatially localize objects (no gaze direction sensitivity)
Face recognition is present
Regions for face recognition have been demonstrated by fMRI studies include the FFA and PPA (temporal lobe) and overlap with object recognition regions Haxby et al, Science 293: 2425-2430 , 2001.
Neural basis of prosopagnosia… N. Hadjikhani and B. de Gelder, Human Brain Mapping, 16(3):176-182, 2002.