Midterm 1 Oct. 21 in class Slide 2 Read this article by
Wednesday next week! Slide 3 Visual Pathways Lateral Geniculate
Nucleus maintains segregation: of M and P cells (mango and parvo)
of left and right eyes P cells project to layers 3 - 6 M cells
project to layers 1 and 2 Slide 4 Visual Pathways W. W. Norton
Primary cortex maintains distinct pathways functional segregation M
and P pathways synapse in different layers Slide 5 The Role of
Extrastriate Areas Consider two plausible models: 1.System is
hierarchical: each area performs some elaboration on the input it
is given and then passes on that elaboration as input to the next
higher area 2.System is analytic and parallel: different areas
elaborate on different features of the input Slide 6 The Role of
Extrastriate Areas Different visual cortex regions contain cells
with different tuning properties Slide 7 The Role of Extrastriate
Areas Functional imaging (PET) investigations of motion and colour
selective visual cortical areas Zeki et al. Subtractive Logic
stimulus alternates between two scenes that differ only in the
feature of interest (i.e. colour, motion, etc.) Slide 8 The Role of
Extrastriate Areas Identifying colour sensitive regions Subtract
Voxel intensities during these scans from voxel intensities during
these scans etc. Time -> Slide 9 The Role of Extrastriate Areas
result voxels are identified that are preferentially selective for
colour these tend to cluster in anterior/inferior occipital lobe
Slide 10 The Role of Extrastriate Areas similar logic was used to
find motion-selective areas Subtract Voxel intensities during these
scans from voxel intensities during these scans etc. Time ->
MOVING STATIONARY MOVING STATIONARY Slide 11 The Role of
Extrastriate Areas result voxels are identified that are
preferentially selective for motion these tend to cluster in
superior/dorsal occipital lobe near TemporoParietal Junction Akin
to Human V5 Slide 12 Electrical response (EEG) to direction
reversals of moving dots generated in (or near) V5 This activity is
absent when dots are isoluminant with background Tata, Mason &
Sutherland (2007) The Role of Extrastriate Areas Slide 13 Thus PET
studies doubly-dissociate colour and motion sensitive regions Slide
14 The Role of Extrastriate Areas V4 and V5 are doubly-dissociated
in lesion literature: Slide 15 The Role of Extrastriate Areas V4
and V5 are doubly-dissociated in lesion literature: achromatopsia
(color blindness): there are many forms of color blindness cortical
achromatopsia arises from lesions in the area of V4 singly
dissociable from motion perception deficit - patients with V4
lesions have other visual problems, but motion perception is
substantially spared Slide 16 The Role of Extrastriate Areas V4 and
V5 are doubly-dissociated in lesion literature: akinetopsia (motion
blindness): bilateral lesions to area V5 (extremely rare) severe
impairment in judging direction and velocity of motion - especially
with fast-moving stimuli visual world appeared to progress in still
frames similar effects occur when M-cell layers in LGN are lesioned
in monkeys Slide 17 How does the visual system represent visual
information? How does the visual system represent features of
scenes? Vision is analytical - the system breaks down the scene
into distinct kinds of features and represents them in functionally
segregated pathways but the spike timing matters too! Slide 18
Visual Neuron Responses Unit recordings in LGN reveal a
centre/surround receptive field many arrangements exist, but the
classical RF has an excitatory centre and an inhibitory surround
these receptive fields tend to be circular - they are not
orientation specific How could the outputs of such cells be
transformed into a cell with orientation specificity? Slide 19
Visual Neuron Responses LGN cells converge on simple cells in V1
imparting orientation (and location) specificity Slide 20 Visual
Neuron Responses LGN cells converge on simple cells in V1 imparting
orientation specificity Thus we begin to see how a simple
representation - the orientation of a line in the visual scene -
can be maintained in the visual system increase in spike rate of
specific neurons indicates presence of a line with a specific
orientation at a specific location on the retina Why should this
matter? Slide 21 Visual Neuron Responses Edges are important
because they are the boundaries between objects and the background
or objects and other objects Slide 22 Visual Neuron Responses This
conceptualization of the visual system was static - it did not take
into account the possibility that visual cells might change their
response selectivity over time Logic went like this: if the cell is
firing, its preferred line/edge must be present and if the
preferred line/edge is present, the cell must be firing We will
encounter examples in which neither of these are true! Representing
boundaries must be more complicated than simple edge detection!
Slide 23 Visual Neuron Responses Boundaries between objects can be
defined by color rather than brightness Slide 24 Visual Neuron
Responses Boundaries between objects can be defined by texture
Slide 25 Visual Neuron Responses Boundaries between objects can be
defined by motion and depth cues
