eh

It seems that in the past 100 or so years, science has been able to explain a lot about how our senses work. We know that vision is a result of light entering our eyes and striking the retina. But how far have they come in understanding what happens after that?

What really bogs my mind, is how in the world the brain takes an electrical signal from the optic nerve and interprets it to have spatial meaning. Has much progress in this field been made? I have no education in anything related to neuroscience, so any dumbed down replies would be appreciated.

anakata

Basically, various aspects such as lines, movement, edges, etc. are processed differently before being fed to the higher parts of the brain. I think the visual cortex is pretty well understood by now.

Bnonn

Well, I think "pretty well understood" is a pretty subjective term, in that case.

The visual cortex is understood well enough that we can create very primitive digital interfaces with it. But it's not "well" understood, just like the rest of the brain isn't "well" understood. It's still quite mysterious for many reasons.

Corey Hammer

Among the reasons...because the precise geometry and structure differs quite a bit between individiuals. For example, the precise regions that accept input from the visual fields switch individual to individual, and map techniques are not quite to that levle yet.

eh

Hmmm. From the URL: http://webvision.med.utah.edu/VisualCortex.html

...."The spatial position of the ganglion cells within the retina is preserved by the spatial organisation of the neurons within the LGN layers. The posterior LGN contains neurons whose receptive field are near the fovea. Progressing from posterior to anterior, the receptive field locations become increasingly peripheral in the retina (see Erwin et al., 1999). This spatial layout is called retinotopic organization because the topological organization of the receptive fields in the LGN parallels the organization of the retina. The signals in area V1 are also retinotopically arranged"

What exactly does this mean though? Does it mean the neurons carrying the signal have the same spatial organization as the actual image collected by the retina? And as it reaches the V1 area, where the perception occurs, is the same applied?

tribalbeeyatch

Yes, 'retinotopic' means that the LGN "maps" the retina. V1 is also retinotopic, but as you go farther along the visual stream, the various regions become more specialized such that they still "map" to the retina, but the neurons only respond to particular aspects of the visual information. For example, in area MT (an area that encodes the direction of motion) you might only see responses corresponding to the parts of the visual scene that are moving (with subsets of neurons responding only to the parts of the visual scene that are moving in a particular direction). Also, while it is true that we need V1 for visual perception, it isn't entirely accurate to say that V1 is "where the perception occurs". Different parts of a visual perception are encoded by different areas of the brain, so the perception is distributed across all of these areas.

eh

Interesting. Is it correct to think of the V1 area as some kind of screen, with millions of receptors that are either ON or OFF? Also, how does one end up perceiving a 2D image when there is a 3D region of the brain processing the information? I have a rough idea of why that might be so, but would like some more insight.

tribalbeeyatch

I think that it is accurate enough for most purposes.
V1 and other cortical regions actually have a laminar organization, so they are largely 2D structures (with 3D attributes like orientation columns).

eh

Thanks for the info. Another idea has surfaced though. Is the visual cortex involved at all when we remember or imagine geometric objects? I can easily imagine a big blob of green floating in front of my screen, but am still certain it isn't real. But for a person with mental illness, maybe they can't tell the difference between imagined objects and those that are real. So is there any role for the visual cortex in such cases?

tribalbeeyatch

I do seem to recall that there is decreased visual cortical function in schizophrenics, and some psychometric dysfunction as well (object recognition perhaps?). I don't know if this might lead to confusion between reality and imagination, especially considering that schizophrenics seem more likely to experience auditory than visual hallucinations. I'm really not the one to ask for definitive answers, though. You'd probably do better running a search at PubMed. You might want to try looking up "impaired magnocellular processing" or something along those lines. I know that has been a focus in schizophrenia research recently.