Part 13 (1/2)

How stereopsis is actually accomplished in the brain remains one of the deepest mysteries of visual neuroscience. We know a bit, but relatively little compared to what we know about how other processes, such as motion perception, are accomplished. We know that the information from each eye remains segregated at the level of your optic nerves. We also know that visual information from your two eyes converges onto the same neurons in your primary visual cortex. This means that certain neurons in this brain region can respond to stimuli from either eye or both eyes. They are binocular.

But where in the brain does vision, based on both eyes, come together? Where is the depth of each object in the scene computed? Where do the images fuse into one seamless experience? We know these things must happen. Otherwise we would have double vision instead of depth perception. In our own labs, we have found that the processes used to derive stereoscopic perception must arise several levels above the primary visual cortex in the visual hierarchy. Finding the exact location is an area of active research.

Stereopsis contributes to Vernon's trick, too, because your two eyes see your card pushed into the deck from different angles. Your brain triangulates these two different retinal images to compute the depth of the card within the deck. It's an illusion, but stereopsis confirms that the card is mid-deck.

Tony took advantage: A. S. Barnhart (in press), ”The exploitation of Gestalt principles by magicians,” Perception.

Good continuation is so integral to a plethora of brain mechanisms: Ibid.

saws a woman in half: This trick can be accomplished in other ways as well. But in all of them good continuation plays a role in the effect.

Charles Gilbert and colleagues: M. K. Kapadia, M. Ito, C. D. Gilbert, and G. Westheimer (1995), ”Improvement in visual sensitivity by changes in local context: Parallel studies in human observers and in V1 of alert monkeys,” Neuron 15: 84356.

A second concept behind the spoon illusion: It has been published as the ”Dancing Bar” illusion by Peter Tse and Brown Hsieh at Dartmouth College. The neural basis of this illusion has been shown by Christopher Pack, now at the Montreal Neurological Inst.i.tute. P. U. Tse, P.-J. Hsieh (2007), ”Component and intrinsic motion integrate in 'dancing bar' illusion,” Biological Cybernetics 96(1): 18; C. C. Pack and R. T. Born (2001), ”Temporal dynamics of a neural solution to the aperture problem in visual area MT of macaque brain,” Nature 409: 104042.

To localize the ends of a line: C. C. Pack, M. S. Livingstone, K. R. Duffy, and R. T. Born (2003), ”End-stopping and the aperture problem: Two-dimensional motion signals in macaque V1,” Neuron 39: 67180.

3. The Brother Who Faked a Dome For further discussion on how visual art and visual science interact, see S. Martinez-Conde and S. L. Macknik (2010), ”Art as Visual Research: Kinetic Illusions in Op Art,” Scientific American Special 20(1): 4855.

Susana's results showed instead: X. G. Troncoso, S. L. Macknik, and S. Martinez-Conde (2005), ”Novel visual illusions related to Vasarely's 'nested squares' show that corner salience varies with corner angle,” Perception 34: 40920; X. G. Troncoso, P. U. Tse, S. L. Macknik, G. P. Caplovitz, P.-J. Hsieh, A. A. Schlegel, J. Otero-Millan, and S. Martinez-Conde (2007), ”BOLD activation varies parametrically with corner angle throughout human retinotopic cortex,” Perception 36: 80820; X. G. Troncoso, S. L. Macknik, and S. Martinez-Conde (2009), ”Corner salience varies linearly with corner angle during flicker-augmented contrast: A general principle of corner perception based on Vasarely's artworks,” Spatial Vision 22: 21124.

In 2006 we designed an experiment: X. G. Troncoso, S. L. Macknik J. Otero-Millan, and S. Martinez-Conde (2008), ”Microsaccades drive illusory motion in the Enigma illusion,” Proceedings of the National Academy of Sciences of the United States of America[hereafter PNAS] 105: 1603338.

Her expression is often: M. S. Livingstone (2000), ”Is it warm? Is it real? Or just low spatial frequency?” Science 290: 1299.

The Leaning Tower illusion: F. A. A. Kingdom, A. Yoonessi, and E. Gheorghiu (2007), ”The Leaning Tower illusion: A new illusion of perspective,” Perception 36(3): 47577.

The only difference between these two faces: R. Russell (2009), ”A s.e.x difference in facial pigmentation and its exaggeration by cosmetics,” Perception 38: 121119.

Some stationary patterns: A. Kitaoka, Trick Eyes: Magical Illusions That Will Activate the Brain (New York: Sterling Publis.h.i.+ng, 2005).

We called the new illusion: S. L. Macknik and M. S. Livingstone (1998), ”Neuronal correlates of visibility and invisibility in the primate visual system,” Nature Neuroscience 1(2): 14449; S. L. Macknik and M. M. Haglund (1999), ”Optical images of visible and invisible percepts in the primary visual cortex of primates,” PNAS 96: 1520810; S. L. Macknik, S. Martinez-Conde, and M. M. Haglund (2000), ”The role of spatiotemporal edges in visibility and visual masking,” PNAS 97: 755660; S. L. Macknik and S. Martinez-Conde (2004), ”Dichoptic visual masking reveals that early binocular neurons exhibit weak interocular suppression: Implications for binocular vision and visual awareness,” Journal of Cognitive Neuroscience 16: 104959; P. U. Tse, S. Martinez-Conde, A. A. Schlegel, and S. L. Macknik (2005), ”Visibility, visual awareness, and visual masking of simple unattended targets are confined to areas in the occipital cortex beyond human V1/V2,” PNAS 102: 1717883; S. L. Macknik (2006), ”Visual masking approaches to visual awareness,” Progress in Brain Research 155: 177215; S. L. Macknik and S. Martinez-Conde (2007), ”The role of feedback in visual masking and visual processing,” Advances in Cognitive Psychology 3: 12552; S. L. Macknik and S. Martinez-Conde, ”The Role of Feedback in Visual Attention and Awareness,” in M. S. Gazzaniga, ed., The Cognitive Neurosciences (Cambridge, Ma.s.s.: MIT Press, 2009), pp. 116579.

4. Welcome to the Show One critical clue: T. Moore and M. Fallah (2004). ”Microstimulation of the frontal eye field and its effects on covert spatial attention,” Journal of Neurophysiology 91: 15262; Z. M. Hafed and R. J. Krauzlis (2010), ”Microsaccadic suppression of visual bursts in the primate superior colliculus,” Journal of Neuroscience 30(28): 954247; N. L. Port and R. H. Wurtz (2009), ”Target selection and saccade generation in monkey superior colliculus,” Experimental Brain Research 192(3): 46577; J. W. Bisley and M. E. Goldberg (2010), ”Attention, intention, and priority in the parietal lobe,” Annual Review of Neuroscience 33: 121.

Other times you can s.h.i.+ft your attention around: Study by Keisuke f.u.kada and Edward K. Vogel, ”Human variation in overriding attentional capture,” Journal of Neuroscience, July 8, 2009.

Research shows that: G. F. Woodman and S. J. Luck (2007), ”Do the contents of visual working memory automatically influence attentional selection during visual search?” Journal of Experimental Psychology: Human Perception and Performance 33(2): 36377.

”retinotopic” s.p.a.ce: R. Desimone and J. Duncan (1995), ”Neural mechanisms of selective visual attention,” Annual Review of Neuroscience 18: 193222.

Jose-Manuel Alonso: Our work with Jose-Manuel Alonso also showed that a specific kind of neuron is enhanced during attention in the center of the spotlight, while a different kind of neuron is inhibited during attention in the surrounding regions. The neurons with enhanced firing in the center of the attentional spotlight are known to inhibit other neurons, whereas the neurons with suppressed firing in the surrounding regions are critical to determining the direction of moving objects. These results suggest that the role of top-down attention in the very earliest stages of vision is to suppress the attention-grabbing aspects of objects moving around whatever it is you want to pay attention to. See Y. Chen, S. Martinez-Conde, S. L. Macknik, Y. Bareshpolova, H. A. Swadlow, and J.-M. Alonso (2008), ”Task difficulty modulates the activity of specific neuronal populations in primary visual cortex,” Nature Neuroscience 11: 97482.

Arturo de Ascanio: A. Ascanio, The Magic of Ascanio, vol. 1, trans. R. B. Etcheberry (self-published, 2007).

n.o.bel laureate Eric Kandel: E. Kandel, In Search of Memory: The Emergence of a New Science of Mind (New York: W.W. Norton, 2007).

Ethological studies: One evolutionary advantage of having a spotlight of attention dissociated from your center of gaze is that it enhances your ability to deceive others. Having a roving spotlight of attention that can point away from your direction of gaze allows you to hide what you are paying attention to (a potential food source, a desirable mate) from compet.i.tors. Marc Hauser at Harvard University has shown that monkeys will intentionally look away from hidden food sources in order to mislead other monkeys away from their stash. See M. D. Hauser (1992), ”Costs of deception: Cheaters are punished in rhesus monkeys (Macaca mulatta),” PNAS 89(24): 1213739. The cost of this system is that attending away from the fovea is, by definition, attending to low resolution information. Therefore, hiding your secret interests from those around you must convey an important adaptive edge.

In this sense, both macaques: Many other species use deception to maximize survival and reproductive success. Some birds will feign having a broken wing to lure a predator away from the nest: a form of misdirection. Such pretense of weakness is an old strategy in human warfare. Sun Tzu wrote in The Art of War more than two thousand years ago: ”All warfare is based on deception. Hence, when able to attack, we must seem unable; when using our forces, we must seem inactive; when we are near, we must make the enemy believe we are far away; when far away, we must make him believe we are near. Hold out baits to entice the enemy. Feign disorder, and crush him.” Other animals rely on camouflage and mimicry for deceptive purposes: some nonpoisonous b.u.t.terflies evolved the same wing patterns as poisonous species, giving them the advantage of warning off predatory birds.

5. The Gorilla in Your Midst To overcome adaptation: S. Martinez-Conde and S. L. Macknik (2007), ”Windows on the mind,” Scientific American 297: 5663; S. Martinez-Conde, S. L. Macknik, X. G. Troncoso, and T. Dyar (2006), ”Microsaccades counteract visual fading during fixation,” Neuron 49: 297305.

You cannot predict: For a more in-depth discussion of these ideas, see S. Martinez-Conde and S. L. Macknik (2008), ”Magic and the brain,” Scientific American 299: 7279; S. L. Macknik, M. King, J. Randi, A. Robbins, Teller, J. Thompson, and S. Martinez-Conde (2008), ”Attention and awareness in stage magic: Turning tricks into research,” Nature Reviews Neuroscience 9: 87179.

To describe these methods: Macknik et al., ”Attention and awareness in stage magic.”

Cognitive neuroscientists: A. Mack and I. Rock, Inattentional Blindness (Cambridge, Ma.s.s.: MIT Press, 1998).

Can You Keep Us From Reading Your Mind?: From Martinez-Conde and Macknik ”Magic and the brain.”

Tamariz uses inattentional blindness: Details can be found in his instructional masterpiece, The Five Points of Magic.

Our own research: Y. Chen, S. Martinez-Conde, S. L. Macknik, Y. Bereshpolova, H. A. Swadlow, and J. M. Alonso (2008), ”Task difficulty modulates the activity of specific neuronal populations in primary visual cortex,” Nature Neuroscience 11: 97482.

The Gorilla in Our Midst experiment: For a wonderful and very entertaining in-depth look at this and related effects, see Chabris and Simons's new book, The Invisible Gorilla (New York: Crown Archetype, 2010).

In 2006, Daniel Memmert: D. Memmert (2006), ”The effects of eye movements, age, and expertise on inattentional blindness,” Consciousness and Cognition 15: 62027.

Inattentional blindness: I. E. Hyman Jr., M. Boss, B. M. Wise, K. E. McKenzie, and J. M. Caggiano (2010), ”Did you see the unicycling clown? Inattentional blindness while walking and talking on a cell phone,” Applied Cognitive Psychology 24: 597607.

Another of our colleagues: C. Rosen (2008), ”The myth of mult.i.tasking,” New Atlantis: A Journal of Technology and Society 20: 10510.

In one version: D. J. Simons and D. T. Levin (1998), ”Failure to detect changes to people during a real-world interaction,” Psychonomic Bulletin and Review 5: 64449. See also C. F. Chabris and D. J. Simons, The Invisible Gorilla (New York: Crown Archetype, 2010).