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noreply@blogger.com (Zekivisao_HM) — Mon, 26 Mar 2007 13:36:00 +0000

A Imagem Visual 1

noreply@blogger.com (Zekivisao_HM) — Fri, 23 Mar 2007 18:53:00 +0000

The Visual Image

noreply@blogger.com (Zekivisao_HM) — Fri, 23 Mar 2007 18:40:00 +0000

The visual stimuli available to the brain do not offer a stable code of information.

The wavelengths of light reflected from surfaces change along with alterations in the illumination, yet the brain is able to assign a constant color to them.

As you see the retinal image produced by my hand is never the same from moment to moment, yet the brain must consistently categorize it as a hand. Also, an object’s image varies with distance, and again the brain can ascertain its true size.

The brain’s task, then, is to extract the constant, invariant features of objects from the the perpetually changing flood of information it receives from them.

To obtain its knowledge of what its visible, the brain cannot therefore merely analyze the images presented to the retina; it must actively construct a visual world.

To do so, the brain as developed an elaborate neural mechanism, with a complex division of labor, so marvelously efficient that it took a century of study before anyone even guessed at its rnany cornponnents.

What, then, is needed to have a good vision?

At the levei of our front we need to have the visual organs in good shape to provide:

• Central vision — which means that we can descrirninate clearly the shape of the objects we are foccusing our attention. It depends particularly on the functioning of some celis (bastonets) behind an area of the retina called macula wich center is named fovea with a central area - fovela.

• Periferical vision — wich means that we are abble to percept the ali field of view presented to our eyes. Depends mostly on the functioning of cells cailed cones, that percept general forrns in white and gray (also responsable for noctive perception).

• Cromatic vision — wich rneans that we have the perfect notion of the colors of whatever we are looking at. It depends firstly on a protein called rodopsin that lays in the bastonets, wich are behind a point of the retina called fovela in the center of macula.

• Binocular vision — saying in another way, we must have a good stereopsy - means that we have notion of deepness and we can see the world in its three dimensions — this happens when the retinal image ofthe two eyes is overlapped.

At the levei of our brain we need to have in good shape several cortical areas and subregions of areas wich are specialized for particular visual functions, such as:

• Lateral Geniculate Nucleus — where most connections between the retina and the visual cortex at the back of the brain pass. This subcortical structure has six cell layers:

. Parvoceliular pathway — consists on 4 layers of cells (smaller than the Magnocelis) wich are sensitive for color stimuli

.Magnoceilular pathway — consists on 2 layers wich respond to stimuli indifferent to color
.Visual cortex — where stands the structural and functional organization of the specialized cortical areas with specific functions concerned to different attributes
visual stimuli, forming 4 parailel systems:

Area V1 - unusualy rich in ceil layers, reveals an even richer methabolic architecture, characterized by columns of celis that extend from the cortical surface to the underlying nerve tissue called white matter. These collumns appear as heavily stained blobs (wavelength-selective cells), separated from one another by more lightly stained interblob (form-selective celis) regions. These collumns, especially prominent in the second and third layers of Vi, receive input from de parvoceilular layers (whose celis respond to stimuli concerned with color). A distinct set of structures is on the layer 4B of Vi, wich receives input from the magoceilular layers ( whose cells respond transiently to stimuli indifferent to color, suggesting that some parts of it are specialized for motion perception).

Area V2 — Receives inputs from Vi and has a special architecture, with the shape of thick stripes (connected to blobs of Vi), thin stripes (related
interblobs of Vi), separated from one another by more staining interstripes (connected to layer 4B of Vi). Cells sensitive to form are distributed
both the thick stripes and the interstripes.

V1 and V2, therefore, assemble different signals before beeing relayed to the specialized visual areas:

Area V3 — concerned with dynamic-form system — the shapes of the objects in motion. It derives its inputs from the magnocelullar layers of the lateral geniculate nucleus through layer 4B of Vi; the signais then proceed to V3 both directly and though the thick stripes of V2.

Area V4 — on wich depends the:
• Color system — its inputs pass trough the parvoceluliar layers of the lateral geniculate nucleus to the bolbs of Vi, then proceed to V4 directly or trough the thin stripes of V2.

• Form system linked to color — derives its inputs from the parvoceluliar layers of the lateral geniculate nucleus, by way of the interblobs of Vi and the intcrstripes of V2.

Area V5 — the pivotal prestriate area for the motion system — its inputs run from the retina, through the magnocclullar layers of the lateral geniculate nucleus, to layer 4B of Vi. From there the signals pass to V5, both directly and through the thick stripes of V2.

For the brain to make spatial sense of the integrated information, the information must somehow be referred to an area that has a more precise topographic map of the retina and hence ofthe visual field.

Of all the visual areas, the one with the most precise map is area Vi followed by V2.
The specialized areas must therefore send information back to Vi and V2 so that the resuits of the comparisons can be mapped back onto the visual field.

There are, then, reentrant connections which allow information to flow both ways between different areas, and are essential for resolving conflicts between celis that have different capabilities and are responding to the sarne stirnulus.

Integration of the visual information is a monumental task that necessitates a vast network of anatomic links between the four paraliei systems at every levei, because each ievel contributes explicitiy to perception.

To understand coherent motion, for example, brain must determine wich features in the fieid of view are moving in the sarne direction and at the sarne speed. The information that goes back from area V5 (motion) to area V2 allows connections with the information that V2 emits to area V3 (dynamic-form).

Semir Zeki and his coileagues have developed a theory of multistage integration. li hypothesizes that integration does not occur in a single step trough a convergence of output onto a master area, nor does integration have to be postponed untii ai! the visuai areas have compieted their individual operations.

Instead the integration of visual information is a process in wich perception and com prehension of the visual world occur simultaneousiy.

The Visual Image

noreply@blogger.com (Zekivisao_HM) — Fri, 23 Mar 2007 18:40:00 +0000

HTM_ A Imagem Visual

noreply@blogger.com (Zekivisao_HM) — Fri, 23 Mar 2007 17:34:00 +0000

Os estímulos visuais com que nos deparamos ao longo de cada dia não mantêm um código de informação estável.

O nosso cérebro é forçado a extrair traços de constância e de invariância dos estímulos presentes nos objectos que observa em permanente mudança de aparência visual.

Portanto, o conhecimento não decorre da mera análise das imagens que atingem a retina, mas de uma constante construção do mundo visual efectuada pelo cérebro.

Para realizar esta actividade o cérebro desenvolveu um elaborado mecanismo neuronal, com uma complexa divisão de tarefas.

Aqui temos o mote que nos leva à criação deste blog...

A Imagem Visual : no cérebro e na mente

O que vemos... o que percebemos...