The The modelmodel

good

Feedforward intracortical connections V1 (II/III) to V2 (IV)

Feedback intracortical and subcortical connectionsV2 (VI) to V1 (VI)V1 (V) to SC,V1 (VI) to LGN,V2 (V) to SC, Pons, Striatum

Cortical Circuitry

Histology of Cerebral Cortex 2Histology of Cerebral Cortex 2

• Pyramidal neurons are large and complex

• Similar orientation

• Process input from many sources

Stuff and thingsStuff and thingsRetina, lateral geniculate nucleus and Retina, lateral geniculate nucleus and

primary visual cortex produce rich primary visual cortex produce rich information about local points,information about local points,

but properties of more global objects but properties of more global objects are not represented (stuff versus are not represented (stuff versus

things).things).

Color and motionColor 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 areasColor and motion areas

ColorColor

• 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 Border of V1/V2 with blobs and stripesstripes

Color constancyColor 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 colorsMondrian 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 constancyV4 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 V4 receptive fields support color constancyconstancy

• 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 achromatopsiaCerebral 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 disordersOther 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 anomiaColor 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 colorNeuroimaging and color

• Positron emission tomography (PET) and event-related 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 ImagingPET 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 PET Identification of Inferior Occipital Region Activated by ColorRegion 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).

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.

Event related potentialsEvent related potentials

• 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 issuesControl 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, A good fit for V4 as the color area, but some cautionsbut 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 CaveatV4 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 perceptionMotion 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:From local to global:INTEGRATIONINTEGRATION

(output): GLOBAL perception of motion

Small receptive fields (input): LOCAL information

Aperture problemAperture 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 MTThe 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 cellsCompare 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 sufficientNecessary 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?)