July 20, 2010
Just the other day I was going up to London on the train to see Dr Bruce Lipton give a lecture on “The Biology Of Belief.” While travelling up on the rather modern – and very quite – locomotive, I couldn’t help but notice a strange and somewhat perplexing phenomena occurring.
Basically I was gazing out of the window, watching the world pass by, as we effortlessly glided over the steal railings sturdily laid on the track below this hurtling juggernaut. Big beautiful corrugated clouds passed languidly overhead, set against the summer azure and vibrant green hues of sky and English country side. The variance of relative speed which was bestowed upon the objects closer to myself whisking by, while the more distant ones slowed down in a tapering fashion as they reached out towards the distant horizon. Raised slightly above the landscape upon the elevated foundations of the track, I had the momentary impression that I was dashing swiftly over the English country side in a fighter jet, prowling lowly over the earth’s topography to avoid any radar detection. Then “SWOOSH!” would go by another train, jolting me back into my bodily awareness. Minutes of peace, followed by sudden momentary tension, followed by peace again… All the while this movement behind the speeding train’s visual panoply ingrained itself inconspicuously in my neural net… Until we’d slow down for the next train station.
It was during these stops at the station that I noticed a curious phenomena occurring regarding the train’s ‘standing.’ I first noticed it at Tonbridge station. When the train had come to a complete stop, I gazed at one stationary traveller who was waiting for another train to arrive. She seemed to be engrossed in a book of some sort… However, she possessed an amazingly calm demeanour in the throng of station jostle, one that I found to be exceptionally engrossing and somewhat soothing in comparison to the past rush of country side imagery. While gazing at her, I noticed that the train felt like it was moving backwards slightly. However, observing the weekend revellers clambering through the open doors behind and in front of me, blasé to this ‘obvious’ point, I then remembering the strict rules that all railway companies adhered to i.e. making sure that all trains have come to a complete stand still before allowing passengers to embark. So I knew this couldn’t be the case… Well, only unless the station master was severely negligent of his duties… And perhaps the train driver hadn’t applied the brakes properly… !?
So I lined up a speck of dirt soiled to the window and held my head steady as I pin-pointed its position on an external cable, running from floor to ceiling, on the station wall. This, I hoped, would allow me to see if any horizontal movement was actually occurring or not. But the speck remained fixedly over the cable. Still, the sensation that we were moving backwards was overwhelming. Certainly none of the passengers boarding the train seemed to mind this insubstantial backward glide. “So,” I thought, “why should I?” And off we went once again, rushing over the earth towards the next station at Hildenborough.
Trees darted in front of my field of vision, breaking up the gloriously sun lit country side behind them into speckled fragments of green pastures and lubricious skies. Again I became transfixed on this imagery… And to be fair, it wasn’t any wonder why… Having been stuck in front of a computer screen most of the past week, this new window on the world enticed my visual cortex with a appeasing pattern that was as mesmerising and primal as the dancing flames of a night fire. Onwards we all hurtled, speeding towards the city limits, lost in the steady flow of time’s passage and the world’s movement.
Slowly we pulled into Hildenborough, greeted with the gleeful smiles and colourful attires of those patiently waiting for the future promise at the end of the line… Here a marvellous oak caught my attention, as its boughs swayed steadily in the light warm breeze, smattering the sun’s light that was breaking through it’s branches into a shimmering display of green and golden warmth. As might eyes rested on this delight, I again had the distinct impression that the train was moving backwards on the lines. This time feeling slightly apathetic towards this sensation and the effect it might have on boarding passengers, I aligned up the same window speck with another external object. And low an behold… The train was not moving in the slightest. In fact, as a young girl sat down next to me, I nodded in greeting and asked her if it felt like the train was moving. To which she placed her baggage overhead in the racks, sat down and proceeded to look out of the windows for a reference. After several moments, she replied that we were “very stationary” and smiled, saying that I probably had “motion residue” from the previous journey.
Motion residue… Wow! I had never heard of it before. And then I remembered all those times I had been on fair ground rides that span one round and round and round. Disembarking from these amusements, the ride attendant would always urge everyone to watch their step as the walked down the steps to a sure footing on solid ground. For moments afterwards the world would spinning slightly, and usually in the other direction to the way the ride was geared. Even spinning around on the spot, swirling faster and faster into a dizzying rush of blurred movement, could induce a similar effect. So I replied to the girl, “What… You mean like the effect one gets after spinning on the spot?” To which she replied, “Exactly!” And then told me about her experience of how the ground wobbled while standing on the shores of France having just ridden across the English Channel in a small boat during a Force 8.
At every stop there after we looked at stationary objects and noted with joyful presence the degree to which the train seemed like it was moving. We even had a little lad of not more than 10, who was sitting in front of us, join in our game.
This got me thinking once again about illusions and how we, as human beings, are so prone to perceiving things that are not really happening. And upon my return home, I looked up this phenomena and came across the following article in Scientific American.
Using aftereffects to probe visual function reveals how the eye and brain handle colors and contours.
Although our perception of the world seems effortless and instantaneous, it actually involves considerable image processing, as we have noted in many of our previous columns. Curiously enough, much of the current scientific understanding of that process is based on the study of visual illusions.
Analysis and resolution of an image into distinct features begin at the earliest stages of visual processing. This was discovered in cats and monkeys by a number of techniques, the most straightforward of which was to use tiny needles—microelectrodes—to pick up electrical signals from cells in the retina and the areas of the brain associated with vision (of which there are nearly 30). By presenting various visual targets to monitored animals, investigators learned that cells in early-processing brain areas are each sensitive mainly to changes in just one visual parameter, not to others. For instance, in the primary visual cortex (V1, also called area 17), the main feature extracted is the orientation of edges. In the area known as V4 in the temporal lobes, cells react to color (or, strictly speaking, to wavelengths of light, with different cells responding to different wavelengths). Cells in the area called MT are mainly interested in direction of movement.
One characteristic of these cells that may seem surprising is that their activity when stimulated is not constant. A neuron that responds to red, for instance, will initially fire vigorously but taper off over time as it adapts, or “fatigues,” from steady exposure. Although part of this adaptation may result from depletion of neurotransmitters, it also likely reflects the evolutionary logic that the goal of the cell is to signal change rather than a steady state (that is, if nothing changes, there is literally nothing for the cell to get excited about).
How do we know that such cells also exist in humans? Simply put, we descended from apelike ancestors, and there is no reason why we would have lost those cells during evolution. But we can also infer the existence (and properties) of feature-detecting cells in humans from the results of psychological experiments in which the short-term viewing of one pattern very specifically alters the perception of a subsequently viewed pattern.
For example, if you watch a waterfall for a minute and then transfer your gaze to the grass on the ground below, the grass will seem to move uphill. This illusion occurs because the brain normally interprets motion in a scene from the ratio of activity among cells responding to different directions of movement. (Similarly, the wide range of hues you see on the screen of your television set is based on the relative activity of tiny dots reflecting just three colors—red, green and blue.) By gazing at the waterfall, you fatigue the cells for downward movement; when you then look at a stationary image, the higher baseline of activity in the upward-motion cells results in a ratio that is interpreted as the grass going up. The illusion implies that the human brain must have such feature-detecting cells because of the general dictum that “if you can fatigue it, it must be there.” (This is only a rule of thumb. One of us “adapted” to the dreadful climate and food in England, but there are no “weather cells” or “food-quality cells” in his brain.)
The waterfall effect (or motion aftereffect, as it is also known) was first noted by Aristotle. Unfortunately, as pointed out by 20th-century philosopher Bertrand Russell, Aristotle was a good observer but a poor experimenter, allowing his preconceived notions to influence his observations. He believed, erroneously, that the motion aftereffect was a form of visual inertia, a tendency to continue seeing things move in the same direction because of the inertia of some physical movement stimulated in the brain. He assumed, therefore, that the grass would seem to move downward as well—as if to continue to mimic the movement of the waterfall! If only he had spent a few minutes observing and comparing the apparent movements of the waterfall and the grass, he would not have made the mistake—but experiments were not his forte. (He also proclaimed that women have fewer teeth than men, never having bothered to count Mrs. Aristotle’s teeth.)
The principle of motion adaptation isn’t all that different from the one illustrated by the color aftereffect. Stare at the fixation spot in ‘a’ between the two vertically aligned squares—the top one red, the bottom one green. After a min ute, look at the blank gray screen in ‘b.’ You should see a ghostly bluish-green square where the red used to fall in your visual field and a reddish square where the green used to be. The effect is especially powerful if you blink your eyes.
This color-adaptation aftereffect occurs mainly in the retina. The eye has three receptor pigments–for red, green and blue—each of which is optimally (but not exclusively) excited by one wavelength. Light that contains all wavelengths and thereby stimulates all three receptors equally yields a ratio that the brain interprets as white. If your red color receptors become fatigued from staring at a red square, then when you look at a field of white or light gray, the ratio of activation shifts in favor of greenish blue, which is what you see.
Orientation adaptation, discovered by Colin Blakemore, then at the University of Cambridge, is another striking example of this phenomenon, except that (like the waterfall effect) it occurs in the brain, not the eye. Stare at the anticlockwise-tilted lines in ‘c’ for a minute (while moving fixation within the central disk) and then transfer your gaze to the vertical lines in ‘d.’ You will be startled to find the vertical lines tilted in the opposite direction, clockwise. This perception allows the inference that orientation-specific cells do exist in the human brain: the adaptation to tilt “tilts” the balance of activity among the orientation-specific neurons, favoring those that are attuned to the opposite, clockwise direction.
Even more exciting was Celeste McCollough’s discovery during the early 1960s, while on sabbatical from Oberlin College, of “double duty” cells in humans. Her experiments showed that in addition to cells that respond specifically to a color or an orientation, there are cells that respond only to a line that is both tilted and colored appropriately (that is, a cell for “red line tilted 45 degrees clockwise” or for “green line tilted 10 degrees anticlockwise,” and so on).
Look at ‘e’ (horizontal black and red bars) for 10 seconds, moving your eyes around the central fixation (don’t keep staring just at the fixation) and then at ‘f’ (vertical green and black bars) for 10 seconds. Alternate between them about 10 times each. By doing so, you tire all the color receptors in your retina about equally. If you then look at white paper, you see white—no colors. But an astonishing thing happens if you look at ‘g’ and ‘h,’ which consist of black and white horizontal or vertical bars. (Move your eyes back and forth betweeen them.) The white horizontal lines now look tinged green and the vertical ones red! The effect is even more striking if you look at the patchwork quilt (‘i’).
Why does this happen? The McCollough effect suggests that subsequent to the retinal processing, some cells in the brain’s visual pathway extract two features along independent dimensions simultaneously. For simplicity, assume there are just four types of these cells: red-vertical, green-vertical, red-horizontal and green-horizontal. Because ‘e’ fatigues only the red-horizontal cells, you are left with nonfatigued green-horizontal cells, which are then relatively active when you look at white horizontal stripes. Consequently, the white horizontal stripes look greenish; ‘f’ has the reverse effect on the cells: because green-vertical cells have been selectively adapted, white vertical stripes now appear reddish. But none of these aftereffects occurs when you look at blank white paper because your eye movements ensure that all color receptors are equally stimulated on the retina, whereas cortical cells that have an orientation specificity are not stimulated.
Therefore, with a 10-minute experiment, we have shown the existence of neurons in the brain that require the joint presence of a specific color and orientation to fire. The adaptation effects that result from fatiguing them are called contingent aftereffects. The McCollough effect is an orientation-contingent color aftereffect.
A peculiar aspect of the McCollough effect is that once it has been generated in your brain, it can survive for a long period. Look again next week, and the stripes may very well continue to look red- or green-tinged. (The strength of the aftereffect normally ebbs gradually over time, unless you are submerged in darkness, in which case it endures undiminished!) It has therefore been suggested that contingent aftereffects have more in common with memory and learning than with purely visual adaptation. It is as though during the initial adaptation (or exposure) phase, the brain were saying, “Every time I see horizontal stripes, there’s too much red in the world, so let’s pay less attention to red. Whereas every time I see vertical stripes, I see too much green. So let me damp down the green when I am shown vertical white stripes and damp down red when I see horizontal white.” (In the same way, your brain says, “Any time I set foot into the hot tub, it’s hot, so let me recalibrate my temperature judgment accordingly. I’ll expect it to be hot and won’t withdraw my foot in surprise.”)
It has been shown that certain drugs (including caffeine) can enhance the persistence of the McCollough effect. The phenomenon deserves further study as a way of approaching the neurochemistry of perceptual mechanisms. Visual aftereffects may thus give us insights not only into the neural channels that mediate perception but also into the neural—and possibly pharmacological—basis of memory and learning.
By Vilayanur S. Ramachandran and Diane Rogers-Ramachandran
No doubt other illusions use similar effect with regards to the colour receptors in the eye…
It still amazes me just how easily this body of ours can be deceived, so as to perceive and deduce one fact, while ‘really’ something rather different is actually happening. !?!?
To find out where I sourced this article from, please click here.