Transcript Stuff and things - Memorial University of Newfoundland

The model
good
Cortical Circuitry
Feedforward intracortical
connections V1 (II/III) to
V2 (IV)
Feedback intracortical and
subcortical connections
V2 (VI) to V1 (VI)
V1 (V) to SC,
V1 (VI) to LGN,
V2 (V) to SC, Pons,
Striatum
Histology of Cerebral Cortex 2
• Pyramidal
neurons are large
and complex
• Similar
orientation
• Process input
from many
sources
Stuff and things
Retina, lateral geniculate nucleus and
primary visual cortex produce rich
information about local points,
but properties of more global objects
are not represented (stuff versus
things).
Color and motion
• Need to determine what goes together to
represent a thing. How does it move?
What is its true color? This takes place in
extrastriate cortex. Farah begins
consideration of global (rather than local)
image perception with color and motion.
Color and motion areas
Color
• Color perception begins with wavelength
detection in the retina. Color contrast is
enhanced by center-surround receptive fields in
retina and LGN. Double opponent effects occur
in blob cells of Layers 2 and 3 in visual cortex
and color-selective responses continue in the
thin cytochrome oxidase stripes of V2 and are
projected to V4. Up to V4, responses are
wavelength-selective rather than color selective.
To be color selective means context can
influence color response.
Border of V1/V2 with blobs and
stripes
Color constancy
• Our perception of the color of an object is based
on the light reflected back to us from that object.
That light depends on both the spectral
reflectance (true color) AND the spectral
composition of the incident light (light that bathes
the object). We perceive color accurately
because we can take the color of the incident
light into account. This ability is called color
constancy. The larger context helps us here.
Mondrian colors
• Land (who invented the Land camera or
Polaroid) showed that Mondrian colors
are perceived accurately if the whole
thing is bathed in the same light but if
one light is used on a patch and
another light on the next patch then we
can’t see color accurately.
• In the rosy light of dawn, for instance, a
yellow lemon will reflect more long-wave
light and therefore might appear orange;
but its surrounding leaves also reflect
more long-wave light. The brain compares
the two and cancels out the increases.
V4 had color constancy
• Zeki, recording from visual pathway
neurons in monkeys, showed that while
areas up to V2 only see light from a patch
and can’t compensate for the context light
overall, neurons in V4 are able to
compensate for the incident light and
accurately report the color!
V4 receptive fields support color
constancy
• These neurons have large receptive fields, even
extending into the other visual field.
• The surrounds are inhibitory and sensitive to the
same wave length as the center thus if the same
wavelength is everywhere, it is ‘discounted’ or
dismissed (psychologically speaking) as
ambient light.
• This is consistent with evidence that a split brain
patient has trouble with color constancy for
stimuli that cross the midline since he/she can’t
know what the overall light is on both sides at
once.
Cerebral achromatopsia
• Lost of color vision or color blindness when
acuity, motion, depth perception and object
recognition are good is called ‘achromatopsia’.
• In some cases the other visual functions are
transiently affected too or pattern recognition
may also be a problem. Cases involving artists
are particularly dramatic.
• Unilateral lesions can create loss in only one
hemifield: hemiachromotopsia.
"... as soon as he entered, he found his entire studio, which was hung with
brilliantly colored paintings, now utterly grey and void of color. His canvases, the
abstract color paintings he was known for, were now greyish or black and white.
His paintings--once rich with associations, feelings, meanings--now looked
unfamiliar and meaningless to him. At this point the magnitude of his loss
overwhelmed him.
"He had spent his entire life as a painter; now even his art was without meaning,
and he could no longer imagine how to go on.“
Oliver Sacks, The Case of the Colorblind Painter, 1995
In An Anthropologist On Mars, p.6
• Achromatopsia is
produced by lesions
on the inferior surface
of temporo-occipital
regions, (lingual and
fusiform gyri).
Other color related disorders
• ‘color anomia’ problem in producing the
names of colors;
• color agnosia, loss of knowledge about
colors (hard to define, can’t learn paired
associates where one word is a color and
the other is a name or number);
• impaired color-object association, can’t
tell the typical colors of things.
Color anomia
Color anomia, damage to the temporal
segment of the left lingual gyrus, prevents
you from naming colors even though you
can match and discriminate between
colors non-verbally. So, there seems to be
a very specific locus where color
perception gets coded as color language,
but the perception can go on normally
even when the ability to encode the
perception as language is destroyed
Neuroimaging and color
• Positron emission tomography (PET) and eventrelated potentials (ERPs) are consistent with
lesions. PET showed activation in
lingual/fusiform region when colored Mondrians
rather than gray scale Mondrians were
presented.
• When attending to color, rather than multiple
stimuli, activation was seen in collateral sulcus
(between lingual and fusiform gyri) and also
dorsolateral occipital cortex (new area).
PET Imaging
Upper row: Control PET scans (resting
while looking at static fixation point is
subtracted from looking at a flickering
checkerboard stimulus positioned 5.5° from
fixation point).
Middle row: Subtraction produces a
somewhat different image for each of 5
subjects.
Bottom row: The 5 images are averaged
to eliminate noise, producing the image at
the bottom.
PET Identification of Inferior Occipital
Region Activated by Color
Multicolor abstract display (top)
and version of the same display
in shades of gray (bottom) used
as stimuli
Activation produced by staring at colored stimuli.
Panel A shows the blood flow images before
subtraction. Panel B show activation after subtracting
responses to the gray stimuli. Panel C depicts
statistical significance of the responses. White is
highest significance. Panel D shows the location of
the most significant responses in a sagittal, coronal,
and axial view (Courtesy of Frackowiak and Zeki).
Event related potentials
ERPs are a non-invasive method of measuring brain activity.
Weak electrical fields, representing the activity of neural populations
within the brain, can be detected at the scalp using electrodes
connected to an amplifier. The amplifier enhances the electrical
signal so that it can be reliably recorded. This signal is time-locked
to an event, such as the presentation of a stimulus (like a word or
picture) or production of response (like a button press), then
averaged to reveal changes in brain activity specifically associated
with different aspects of cognitive processing.
The temporal precision of ERPs is superior to all other
currently available neuroimaging techniques.
• ERPs were examined using checkerboard
patterns in color after adapting to same
color or different color. There is a different
ERP response to different colors in lingual
and fusiform gyri and in dorsolateral
occipital cortex. Also using electrical
stimulation in those same places could
alter color perceptions.
Control issues
• Another study found the classic area
(lingual/fusiform area) and also
widespread activation in other regions, but
may not have been well controlled. They
used color random noise patterns, judging
if mostly red, versus black and white
random noise, judging if mostly white.
One versus 4 colors; one versus 2 for
black and white; also stimuli were not
matched for luminance.
A good fit for V4 as the color area,
but some cautions
• In monkey and man color areas are different in
location, monkey V4 is high up on lateral
surface. In man color center is on the inferior
surface and more medial. But that may be ok.
• V4 also inputs to object or form areas of
temporal lobe and cells in V4 have some
response to form.
• Finally, monkeys with V4 lesions can relearn
color discrimination tasks, but have permanent
problems with form discrimination. Same form
tasks ok in human achromatopsics. Need to do
imaging studies with monkeys.
V4 Color Caveat
• V4 has been suggested to be the color
center by Zeki. It is more involved with
perception of colors than other areas, but
it may not be both necessary and sufficient
for color perception or have no other role
than color perception
And please let Mom, Dad, Rex, Ginger, Tucker, me
and all the rest of the family see color.
Motion perception
• Local measurement of motion is also
ambiguous. You need to look at global indices
(multiple local pieces of information). In early
areas, most neurons respond better to moving
than stationary stimuli (habituation).
• M cells are optimized for movement and provide
input to first direction selective cells (striate
visual cortex, 4B). They project to the middle
temporal area (MT) and thick cytochrome
oxidase stripes of V2 that then project to MT.
From local to global:
INTEGRATION
Small receptive fields
(input): LOCAL
information
(output): GLOBAL
perception of motion
Aperture problem
•
With local view can’t tell
which way edge is ‘really’
moving when it passes
through the local field, as
several patterns of motion
look the same to a small
window.
• Need to combine
information. Plaid
patterns have been used
to test for cells that can
extract global, rather than
local, motion.
The motion center MT
• V1 cells respond to local or component
motion of plaid while MT cells can also
respond to pattern or global motion. MT
cells have larger receptive fields. MT
projects to the medial superior temporal
area (MST), which has cells with even
larger receptive fields and more complex
motion detectors like flow field properties
of shinking, enlarging, rotation and
translation.
Compare psychophysics and cells
• Discriminate dot motion when varying
proportions of dots are moving consistently and
others randomly. More consistent dots, the
easier the discrimination.
• Monkey performance and performance of
direction selective cells in MT was more or less
the same. If task is at threshold for making the
correct discrimination, when the cells are
correct, the monkey is correct.
Necessary and sufficient
• If you are working with random dots and
stimulate a column of cells for a particular
direction then the monkey will judge that that is
the direction the dots are moving. Thus MT
activity causes motion perception.
• Newsome lesioned the MT with ibotenic acid.
Monkeys were impaired in motion discrimination
in the contralateral hemifield, but not for color,
acuity or depth. (Recovery issues?)