Kaya’s Silhouette Dancer:
The assignation of the dancer’s direction of motion to a particular cerebral hemisphere is an Australian proposal. It first appeared in Australian newspapers, which took Kaya’s invention and applied it to hemispheric function. I would like to offer some comments about the popular response to the proposal, as one of the team that first showed that the switches of perceptual rivalry are coincident with interhemispheric switches (www.uq.edu.au/nuq/jack/CBrivalry.pdf).

1. Split-brains are not the only way to study hemispheres:
One does not need to have a split-brain subject to investigate hemispheric specialisations. Split-brain subjects are rare, are often highly trained professionals whose responses may be affected by the experimenter’s expectations, and they have developed over the years many strategies for overcoming the lack of communication between the hemispheres. In addition, the midline transection is not always complete, with free communication between the inferotemporal cortex on each side via unsevered pathways between the hemispheres. Results on the kind of rivalry experienced by split-brain volunteers have been widely quoted to reject our thesis that the switches of rivalry are based on  interhemispheric switches. These miss the mark because of the problems with the interpretation of split-brain raised above, along with many others such as the philosophical problem of positing that consciousness resides in cortex when the right-left-integrated brainstem is the likely site of many of these phenomena.

2. One Hemifield / One Hemisphere is Wrong Concept:
It is wrong to cite the well-recognised partial decussation of the optic chiasm to infer that one can limit input to one hemisphere by restricting the stimulus to the opposite hemifield of visual space. While it is true that the primary visual cortex has almost perfect splitting of the visual field into two hemifields, each represented by the opposite hemisphere, this is not true for the rest of the “higher” visual areas that contribute to higher-level perception like the spinning dancer. For example, at the level of the inferotemporal visual cortex there is complete overlap between the fields of each eye, with the cortex on one side “seeing” the whole of visual space. http://www.uq.edu.au/nuq/jack/searchswitch.pdf

3. Many UniHemispheric Activation/Inactivation Tests:
 There are a number of different techniques for activating or inactivating one hemisphere, such as the Amytal test, caloric vestibular stimulation and trans-cranial magnetic stimulation. Using such methods one can show that perceptual rivalry is an interhemispheric phenomenon. www.uq.edu.au/nuq/jack/CBrivalry.pdf

4. The Dancer is an Example of Perceptual Rivalry:
The silhouette dancer is an example of perceptual rivalry,………… where a constant sensory input results in an inconstancy: viz:- switches between opposing perceptual states. This may not be apparent in the dancer at first, because this complex figure switches very slowly (simpler examples of rivalry switch much more quickly). If one watches the dancer as one changes one’s mood or action state, it will become apparent that she changes direction, sometimes even in a way that is consonant with that state. Although my team has not done any hemispheric manipulations on the switches seen in the dancer, we have many examples of similar rivalry being associated with interhemispheric switching, some where we are fairly confident which switch is associated with which hemisphere (see www.uq.edu.au/nuq/jack/rivalry.html).

5. Are Her Turns Really Linked to Hemispheres?
One can use indirect methods to infer the hemisphere associated with a particular phase of the perceptual switch. For example, a depressed mood state is associated with excess activity of the Right hemisphere and so results in a bias towards the phase of the switch associated with that hemisphere. In severe depression, the switch may become stuck in that phase. Examples of this can be found on the website given with 4 above. Another indirect method involves a familiarity with the bias on the switch in a particular individual. In my own case, for example, I have a well-documented bias in my switch toward the left hemisphere. When I look at the dancer I do see plenty of rightward (clockwise) rotations, perhaps because she is a very attractive figure (many commentators on the web refer to her nipples!) that may encourage a Right hemisphere cognitive style. Yet if I measure the time spent seeing the clockwise vs. the anti-clockwise rotations, there is a bias for the anti-clockwise, consonant with my usual Left hemisphere bias and supporting the interpretation whizzing around the web. I have had similar results with colleagues and so feel that the popular interpretation is probably correct. There are 2 others pieces of support for this interpretation. i. Studies of single neurons responses in the nucleus of the optic tract of monkeys observing an ambiguously rotating image showed a preference for leftward ('clockwise") rotation on the Right side of the brain and vice versa for the nucleus on the right. ii. as one moves in the usual forward direction through a field of convex objects, like tree trunks, those on our left will show a leftward, "clockwise" rotation and vice versa for those on our right. In other words the preferred directions of rotation have been habitually associated with particular hemispheres as we move about the world (in our lifetimes and during evolution).
 The people who felt that their “result” with the dancer did not live up to expectations should appreciate that their mood state can influence the switch. They should give enough observation time that a number of switches are possible, as well as some variety in mood state.

6. Intellectual Snobbery:
In many of the comments about the dancer on the web, there was the following suspicious sentiment. How could a simple idea like a dichotomous division of labour between the two hemispheres be correct when it is easily understood by a lay audience?  I have found this same suspicion among some of my professional colleagues, which has earned me the epithet “dichotomaniac”. It is strange that a proposal is rejected just because it is simple and can be grasped by most people. Parsimony is an important guiding principle in science so these critics need to come up with a more cogent argument against the hemispheric switching proposal than its simplicity. In fact, neurophysiologists studying the cortex have often noted complementarity between homologous regions on each side. Now there is increasing experimental evidence for a complementary form of cognitive processing in corresponding locations in each hemisphere (V.S. Ramachandran and Alvaro Pasqual-Leone are two creative experimenters who are worth looking up in this context). Some of the skeptics went so far as to criticise the brilliant, original, work of Roger Sperry, who used split brain patients to show the contrasting cognitive styles of the two hemispheres, work for which, in part, he received the Nobel Prize.  If you want to read an account which is cited as evidence against a hemispheric link to the direction of rotation of the dancer, but actually provides plenty of support for it ......see this piece.

7. Switch Rate:
Many commentators have drawn attention to the fact that they can switch the dancer percept voluntarily. This is true for all cases of perceptual rivalry, not just the dancer , and becomes easier as one familiarises oneself with the rivalrous stimulus. It would be a mistake to think that this voluntary control is a general ability, since it is confined to the part of the population with a fast switch. Slow switchers like myself cannot will a switch to occur, although we may be able to set up a mood state that facilitates a switch if we have the direction of change right. In my studies I try to use a form of rivalry that is quite unfamiliar to get the a free-flowing switch, which tends to get stuck if the volunteer is too familiar with it, or which may have a very irregular pattern if the volunteer can voluntarily initiate a switch. It is remarkable that the switch rate on the same rivalrous pattern varies by a factor of 20 in a large sample of the population! Individual variation is a key feature of perceptual rivalry and can be studied conveniently using such displays, just as a number of observers have used the dancer on blogs, in classes or in volunteers. http://www.abc.net.au/catalyst/stories/s1063853.htm
http://freakonomics.blogs.nytimes.com/2007/10/15/which-way-is-the-dancer-spinning/#comments

8. Is the Dancer a Valid Hemispheric Test?
Many observers fail to recognise that the dancer switches back and forth. They hence rely on the the first observation. This may or may not reflect the overall bias on the interhemispheric switch, so a better test would involve a long viewing time and a measurement of the relative time spent in each phase, preferably with the potential for variety of action states or mood states. Appreciating this dynamic aspect will make the experience of the dancer more rewarding, particularly when one sees a switch in direction that coincides with a change in one’s mood or action state, a phenomenon I have observed with self, colleagues and friends many times. In some perceptual rivalries, such as plaid motion rivalry, it has been shown that the duration of the percept up until the first switch is highly correlated with the mean switch time, raising the possibility that the first switch is an indicator of the overall switch. I doubt that this is true for the dancer, because she is so sexy and less likely to engage the Left hemisphere on first viewing, in the way that most other, more neutral, rivalries do .

9. Overall Population Bias for Clockwise (Right Hemisphere) Rotation:
There has been such a strong popular response to the dancer that one can obtain a sample of the preferred directions of rotation in the population. This shows a clear bias toward the clockwise rotation (R hemisphere if one accepts that assignment, as I have already discussed) in the ~500 whose preferences can be counted on various blogs. This apparent Right hemisphere bias was also found by my team in a sample of more than 2000 volunteers. We used a number of hemispheric measures, including the nasal cycle, where the interhemispheric switch is  well-documented by modern scanning techniques such as  MRI. These measures showed an overall population bias toward more time spent  "in Right".  This is not surprising, since the Right's viewpoint is veridical and therefore necessary for survival, compared with  the more "delusional" style of the Left.  Depressives, who are characterised by a strong Right bias, often say that they have a better grasp of reality. My experience bears this out, as I have found that they are the most accurate markers of exam papers (giving perfect outcomes even though they are overly modest about this) compared with Left-biased exam markers (who tend to be self-congratulatory despite the many errors they have committed). Since SSRI anti-depressants tend to reverse the Right bias, based on our experimentsl manipulations of the  interhemispheric switches of the sandlance, one can see that there is a vast field for manipulation and profit-taking in a population that is Right-biased overall! On the other hand, there are real risks of suddenly reversing, with SSRIs, the element of caution that is inherent in the Right hemisphere bias, as we see in the recent tragic suicide of the accomplished newsreader, Charmaine Dragun.