OK, so on to the next system.
Oh, Ratley, hi. What's that? Timmy's fallen in the well and can't get out?
Oh. Mail? Well, I wasn't planning on doing a mailbag post this week.
Well, if you insist. OK.
Let's see, Chris K. asks about optical illusions. Yeah. OK, I can see where this might be a good time to talk about those, even though it is a function of the association cortices which we'll get to in about a week.
Well, Chris – optical illusions are usually caused by one of two processes. The first is to simply *confuse* the eyes by playing tricks with what we have come to learn is "normal." For example, in typical 3-D vision, left is closer to the left eye, right is closer to the right eye (and hence the cross-over shown yesterday will have two slightly different sizes), close is big, and far away is small. A number of the Escher optical illusions take advantage of violating visual rules and conventions. We follow the line of the Penrose staircase (http://www.optical-illusion-pictures.com/paradox.html) but the artist violates the rules of perspective by using the *same* perspective for up/down and near/far. Likewise the Penrose triangle on the same page violates logic, because instead of consistently shading one surface, Lionel and Roger introduce discontinuities that cannot co-exist, thus creating the illusion.
The second method is to *tease* the eyes by taking advantage of how the retinal ganglion neurons, lateral geniculate nucleus and V1 visual cortex process vision. The text to the right shows the distinction between the real world, and the V1 representation. Because the RGN and LGN are tuned to detect edges, the "fill" in the middle of the text is not represented in V1. That information is not lost, however, color and fill information is transmitted to V2 and V3 second visual areas, which detect shadings, colorations and start to interpret perspective and parallax.
When viewed simply as independent lines, the elements of the Vase optical illusion look distinctly like two faces, or a vase (see the figure at left). V1 has no problem distinctly identifying either feature when presented independently. However, once the lines are put together and shaded, there is conflict between what V2&V3 (the vase) and V1 (the faces) detect due to the interference by the edges of the dark shading. Thus this second type of optical illusion, rely on the brain being presented with two different interpretations – simply because the visual system processes lines and shading separately.
By the way, look up "Necker Cubes" online to see more examples. There are *way* more examples than I can show in this space.
Yes, I was getting to that, Ratley.
One of the more interesting "psychometric" aspects of neuroscience is that it is possible to detect *when* a person's perception of an optical illusion shifts. Most of the motor control are of the brain is in the frontal cortex, just forward of the border with the parietal cortex, and control of the eye muscles is now exception. It may seem that occulomotor control (Cranial Nerve III, the third "O" in yesterday's mnemonic) is a simple matter of pointing the eyes in the right direction. However, the process is *much* more complicated, requiring actual target acquisition and identification – in other words, the full suite of visual cortical processing. Distance and horizontal tracking requires that the eyes move at slightly different angles; focus and lighting changes requires pupil diameter control. The "Frontal Eye Fields" along (with the Edinger-Westphal nucleus of deep thalamus and the superior colliculi and locus coeruleus of the brainstem) is involved in the complex process of integrating actual *vision* with the process of adapting the eye to light and motion. When visual information *changes* it can be revealed as changes in scanning the environment or reacting to light.
In fact, it has been demonstrated that if a person is shown a Necker Cube-style optical illusion, and told to press a button whenever their perception of the cubes changes from the "top" to the "bottom" view, the pupils dilate briefly. This is just one small way in which the operation of the brain (or – dare I say it – The Mind) can be monitored by a physiological reaction.
But more on that rant later, for now I need to shoo these LabRats back into the lab, get YDR out of the peanut butter, and explain to Ratface that we weren't talking about Alaskan Island eyeglass makers.