The most important spatial adaptations are so well-designed we could not imagine being without them, and thus we tend to forget that they are there in the first place. I'm thinking of the basic ability to locate objects in three-dimensional space, and of the ability to generate a stable sense of an external world. As the body and the senses move in space, the actual input is continuously changing, yet our experience (unless we become dizzy) is of a stable world.
For domain-specific abilities of spatial reorientation, see Hermer,
Linda, and Elizabeth Spelke.
Modularity and development: The case of spatial reorientation. Cognition, 1996 Dec, 61 (3): 195-232. Abstract.
For hippocampus growth due to spatial learning, see Eleanor Maguire's article in the March 2000 issue of the Proceedings of the National Academy of Sciences. News report: Taxi drivers' brains 'grow' on the job (external).
For differential spatial abilities among men and women, see the entries under conceptual adaptations 1: Objects, Living Kinds, and Minds for botanical and zoological.
Karnath et al. (2001) investigate human brain localization for spatial awareness, finding that it is largely confined to the superior temporal cortex in the right hemisphere. The corresponding areas in the left hemisphere are typically dedicated to language, suggesting that lateralization accompanied the emergence of linguistic capabilities. See abstract.
A large number of adaptations have a temporal element, whose proper domain is to regulate our activities. So natural do these activities seem to us, and so unobtrusively are they regulated, that we tend to forget they represent a complex set of adaptations. During a normal life, a series of events unfold in a predetermined order, as if to the beat of a developmental clock:
Adaptations related to timing are also involved in a large number of motor tasks, such as breathing and walking. For a speculative evolutionary of a particular disruption in breathing, see Why we hiccup (BBC News).
As first determined by Michel Jouvet, sleep occurs in two phases,
the so-called rapid eye-movement (REM) phase and the non-REM phase. In
the REM phase, EEG monitoring reveals a pattern of brain waves very
similar to the waking state; it is associated with vivid dreams. In "The evolution of REM sleep" (1999) JM Siegel at UCLA's Sleep Lab surveys studies of REM sleep in mammals, marsupials, monotremes, and reptiles. Siegel writes,
Prior developmental studies by Jouvet-Mounier had pointed out that "altricial" animals (those that were born too immature to care for themselves, such as the cat, human and rat) had much larger amounts of REM sleep at birth than "precocial" mammals (animals that are relatively independent soon after birth such as the guinea pig and horse). REM sleep amounts decrease with age in altricial mammals and to a lesser extent in precocial mammals. However, altricial mammals continue to have much larger amounts of REM sleep than precocial mammals as adults. Zepelin showed that immaturity at birth is the single best predictor of REM sleep time throughout life.
He finds that the platypus has a higher proportion of REM sleep than
is recorded in any other animal, but its characteristics differ from
mammalian REM sleep. Siegel concludes that "REM sleep in mammals
originated as a brainstem state". In subsequent articles, he argues
that the biological function of REM sleep is to prepare the brain for
the waking state following sleep.
Revonsuo (2000) pursues a more domain-specific tack, suggesting in a special issue of Behavioral and Brain Sciences that dreams are designed to simulate threats. The issue has commentaries from several of the main dream researchers.
Reveonsuo, Antti (2000). The Reinterpretation of Dreams: An evolutionary hypothesis of the function of dreaming. Behavioral and Brain Sciences 23. 6 (Dec): xxx-xxx. Full text (external).
Revonsuo, Antti (2000). Did ancestral humans dream for their lives? Behavioral and Brain Sciences 23. 6 (Dec):1063
Revonsuo, Antti and Katja Valli (2000). Dreaming and Consciousness: Testing the Threat Simulation Theory of the Function of Dreaming. Psyche 6. 8. Full text (external).
Another proposed function of REM sleep is that of consolidating memories. For details, see the bibliography at the Laboratory of Neurophysiology (the Sleep Lab) at Harvard, led by Richard Stickgold (both external). They have some recent results showing that dreams play a role in learning by rehearsal, perhaps by cross-indexing information; see this news report:
... when the brain is filing away the memories it needs to keep, it has to go through a series of steps, and dreaming is a manifestation of one crucial step, Dr. Robert Stickgold, a psychiatrist at Harvard Medical School in Boston, who led the study, said. Dreams are just the body’s way of clearing out the mental “in-box,” Stickgold said. “The trick is to move it to the file cabinet and to file it in the right place,” Stickgold said in a telephone interview. “A lot of REM [rapid eye-movement] dreams, those really quirky, strange, bizarre dreams that we have late at night, is the brain looking for ways to cross-index. It is looking for cross references — does this fit with this? Sometimes it does and sometimes it doesn’t,” he said. When it doesn’t fit, the dream seems weird, he said. When the cross-reference is a good one, the brain can reinforce the memory."
See also Stickgold's essays:
Four or five times a night, roughly every hour and a half, we become clinically insane. We begin to hallucinate. We see things that aren't really there. We hear voices when in reality no one is speaking. Stories unfold before us, around us, including us, and we are deluded, believing it all to be true. We become paranoid or filled with delusions of granduer. Impossible things happen without notice. People appear and disappear and change into other people. One moment we're in Boston, the next in Paris and we accept it all as normal. If we did this while walking on the streets in the daytime, we could only hope that some kind person would lock us up.
See also a November 2000 news report on Stickgold's work showing that sleep enables the formation of long-term memories.
Science magazine did a special issue on sleep, dreams, and memory on 2 November 2001. In it, Jerome Siegel (2001) writes, "It has been hypothesized that REM (rapid eye movement) sleep has an important role in memory consolidation. The evidence for this hypothesis is reviewed and found to be weak and contradictory" (1058). In contrast, Stickgold, Hobson, et al. maintain that "Evidence supports a role for sleep in the consolidation of an array of learning and memory tasks" (1052).
Siegel, Jerome M (2001). The REM Sleep-Memory Consolidation Hypothesis. Science 294 (November 2): 1058-1063. Full text (external). See also UCLA's Center for Sleep Research.
Stickgold, R., Hobson, J. A., Fosse, R., Fosse, M. (2001). Sleep, Learning, and Dreams: Off-line Memory Reprocessing. Science 294: 1052-1057. Full text (external).
A team of Swiss and French researchers have recently worked on the details of the hormonal changes that drive sleep. They argue that neurons responsible for the onset of sleep are located in the preoptic area and more specifically, in the ventrolateral preoptic nucleus (VLPO).
Thierry Gallopin, Patrice Fort, Emmanuel Eggermann, Bruno Cauli, Pierre-Hervé Luppi, Jean Rossier, Etienne Audinat, Michel Mühlethaler & Mauro Serafin. Identification of sleep-promoting neurons in vitro. Nature 404, 992 - 995 (April 2000). Full text and media report (both external).
Hobson, J. Allan (1999). Dreaming as Delirium: How the Brain Goes Out of Its Mind. Publisher's presentation.
For a broader discussion, see the special issue on sleep and dreaming of Behavioral and Brain Sciences 23. 6 (2000). For some background on sleep and dreaming, see States (1999) and Steen (1998), with bibliography.
Additional works on dreaming
Aserinsky, Eugene, and Nathaniel Kleitman (1953). "Regularly Occurring Periods of Eye Motility, and Concomitant Phenomena During Sleep." Science 118: 273-78.
Dement, William (1958). "The Occurrence of Low Voltage, Fast, Electroencephalogram Patterns During Behavioral Sleep In the Cat." Electroencephalography and Clinical Neurophysiology (Amsterdam) 10: 291-96.
Jouvet, Michel, and Delorme, F. (1965) "Locus coeruleus et sommeil paradoxal." Comptes Rendus des Séances et Mémoires de la Société de Biologie 159 : 895-99.
Crick, Francis and G. Mitchison (1983). The function of dream sleep. Nature 304: 111-114.
Symons, Donald. "The Stuff That Dreams Aren't Made of: Why Wake-State and Dream-State Sensory Experiences Differ." Cognition 47 (1993): 181-217.
Ramachandran, Vilayanur S. (1996). The Evolutionary Biology of Self-Deception, Laughter, Dreaming and Depression: Some Clues from Anosognosia. Medical Hypotheses 47: 347-362.
Extensive list of articles and links on sleep and dreaming at Michel Jouvet's dream lab
Light and Darkness
The basic adaptation to time has as its proper domain the diurnal rhythm of light and darkness, as old as life itself. The perceptual categorization cues inlude light of a certain wavelength and intensity. Maladaptive effects can occur in the polar regions during the winter, where the input conditions are not met; this can lead to a general loss of daily rhythm, with an attendant loss of energy and motivation. In a somewhat similar vein, jet lag--the temporal disorientation that comes with rapid travel across time zones--may be considered maladaptive in the global village.
Circadian rhythms are often described in terms of endogenous "biological clocks", with the thrust of research to reduce some particular behavioral or physiological circadian rhythm to biochemical events. These clocks are usually set by environmental cues such as the light-dark cycle, and what is characteristic of an endogenous clock is that if one removes the environmental cue, keeps the organism in constant light, for example, the endogenous rhythm will continue, but will tend to drift out of synch. Restoring the external light-dark cue will reset the clock to its normal intrinsic rhythm.
Susan K. Crosthwaite and collaborators at Dartmouth Medical School ave
shown that certain clock genes in the fungus Neurospora are also involved
in the production of proteins necessary for the
organism's response to light, suggesting that circadian oscillators in more complex organisms evolved from light-regulated pathways in simpler organisms (Science 2 May 1997). Joseph Takahashi and his colleagues at Northwestern report that the first documented mammalian clock gene is chemically related to those already known in the bread mold Neurospora and also in the fruit fly - one domain of the related proteins are identical (Cell 16 May 1997).
In mammals, including humans, the biological clock apparently resides in a group of neuron clusters in a part of the brain called the hypothalamus, a region that responds to many chemical inputs, including the hormone melatonin, an indole derived from the metabolism of serotonin. Melatonin is secreted by another hypothalamic brain structure, the pineal gland, which in turn is stimulated by neurons in a nearby cluster (the suprachiasmatic nucleus) that receives input from the retina of the eye. So this is the apparent pathway in mammals: light on the retina, electrical activity in the retino-hypothalamic tract, activity in a hypothalamic region called the suprachiasmatic nucleus, electrical signals to the pineal gland, secretion of the hormone melatonin, action of melatonin on other neural structures in the hypothalamus and elsewhere.
However, there is also evidence of extra-visual phototransduction. Campbell and Murphy (Cornell University) conducted experiments to measure the response of the human circadian clock to extraocular light exposure involving light pulses presented to the popliteal region (the area behind the knee). They found a systematic relation between the timing of the light pulse and the magnitude and direction of clock phase shifts. The authors suggest their findings challenge the belief that mammals are incapable of extraretinal circadian phototransduction, and that the findings also have implications for the development of more effective treatments of sleep and circadian rhythm disorders (Science 16 Jan 1998).
Contact: Scott S. Campbell, Cornell University Medical College, tel. (212) 746-1067.
See the entries under conceptual adaptations 1: Objects, Living Kinds, and Minds for objective causality and biological causality.
See Michael Kositsky's dissertation on motor learning and skill acquisition
The ontology of science appears to rely heavily on our innate object perception faculty. There are of course other useful senses of the word "objective", notably those that seek to distinguish information about an independently existing reality from information that originates wholly in memory.
The Perceptual Body Schema
Patients that suffer strokes resulting in paralysis on one side of the body frequently deny there is anything wrong with their paralyzed limbs (anosognosia), develop an attitude of aversion to the limb, believe that it belongs to someone else, or develop elaborate delusional beliefs about multiple, oddly shaped, or severed limbs (somatoparaphrenia). These pathologies suggest that an intact body schema will frequently continue to be generated in the face of readily available corrective evidence. In cases where evidence of something wrong is accepted, it is frequently not successfully incorporated into a new body schema. (For a detailed account and a psychological interpretation, see Patrick Haggard's lecture notes on "Control of human action" on the web pages of the Department of Psychology at University College London.) A similar effect is observed in the production of phantom limbs: in this case, the missing limb is felt to be present and to occupy a specific position in space. What is truly remarkable is that even person born with a missing limb may experience phantom limbs, suggesting that the body schema is an innate and highly specified cognitive structure with few open parameters. (For a further description of details on these phenomena and further discussion on their significance, see the work of Vilayanur Ramachandran and Diane C. Rogers-Ramachandran at UCSD's Center for Research on Brain and Cognition on what they call "experimental epistemology" at the Scientific American web site.)
If a body schema is a cognitive adaptation, what is the problem it evolved to solve? Since the body is reliably generated, it represents a stable, recurrent feature of the human environment. When you move, a body image would help you make fast decisions about your whole body's relative position to surrounding objects and to parts of itself. Sensory input may also need to be framed in terms of the body schema: changes in the visual field that result from motion must be effectively edited out, so that we do not experience the world bouncing when we walk, or spinning when we turn, and the details of these decisions would appear to depend on the body schema.
The proper domain of the body schema is thus a normal body; the actual domain is of course the body one actually has, which may not conform to the schema. Since an intact schema is present even in the case of missing or paralyzed limbs, we may conclude that the schema has few open parameters; one would, however, expect absolute lengths of limbs and parts of limbs to be open parameters that are easily set, since this would be necessary to handle different human bodies and the continuing changes within a single body. Rigidity along certain dimensions would thus be accompanied by a very clearly specified flexibility along other dimensions--a strong computational solution. The frequency of delusional attitudes suggests that the schema is to a significant degree informationally encapsulated, or modular in Fodor's sense.
Recent reseach by Flanagan et al. (2000) suggests that ephrin-A5, a protein previously known to establish maps of the visual world in lower brain centers, also plays a prominent role in imprinting a proper body map on the brain's cortex during ontogeny. Flanagan argues that this process was until know thought to be shaped mostly by neural input from the body's periphery; as we've seen, Ramachandran and others have already argued it is genetic. The question then becomes how to build it, and whether some of the construction makes use of sensory information from the growing body.
In one extreme case, the body schema is fully specified in the genes, and people will develop a full homunculus even if they lack some of the normal body parts at birth. At the other extreme, no features of the homunculus are specified in the genes; the neural structures are constructed on the basis of sensory information.
What Flanagan and Vanderhaeghen's work shows is that genetic information can be observed to play a part in allocating neural tissue to the homunculus. They have discovered one of what may be a large number of cortical proteins that form n-dimensional gradients in the growing brain. The model they propose is that these protein gradients interact with neural input to construct the perceptual body schema.
Halligan PW, Marshall JC & Wade Dt (1995). Unilateral somatoparaphrenia after right hemisphere stroke: a case description. Cortex, 31, 173-182.
Cutting J (1978). Study of anosognosia. Journal of Neurology, Neurosurgery and Psychiatry, 41, 548-555.
Flanagan, John (2000). Article title unknown. Nature Neuroscience (April). Press release.
Ramachandran, Vilayanur S.; Levi, L.; Stone, L.; Rogers-Ramachandran, D.; and others. Illusions of body image: What they reveal about human nature. The mind-brain continuum: Sensory processes. Edited by Rodolfo Riascos Llinas, Patricia Smith Churchland, et al. Cambridge, MA: MIT Press, 1996. 29-60 of 315 pp. Abstract.
Ramachandran, V.S. and S. Blakeslee Phantoms in the Brain: Probing the Mysteries of the Human Mind. New York: William Morrow, 1998. Abstract.
The visual system of vertebrates, especially that of the higher vertebrates, is highly organized. The retina of the eye, for example, which is essentially a surface upon which photons impinge to be absorbed by chemical photosensors, is "mapped" by exiting nerve fibers, and light-induced activation of the retinal map are relayed with some transformations to various levels of the central nervous system and finally to what is called the "primary visual projection area" of the cerebral cortex (also called "primary visual cortex"). The processing of this mapping information is the key to how we perceive the world around us.
Evidence of the visual system's processing can be seen in visual illusions. Roger Shepard uses such illusions to argue that the visual system contains sophisticated but unconscious theories of the world. He further argues that it is such unconscious theories that make thought experiments effective; see his recent Hitchcock lectures (1999) at UCSB. This argument implies that the inference engines that operate during perception are also accessed during mental simulations, in itself a remarkable phenomenon.
For examples of visual illusions, see An interactive Guide to Optical Illusions and Illusionworks (both external). See Vision Science Resources for research on human and animal vision.
The Role of Visual Experience in Ontogeny
Crair et al. report a study of the development of cortical maps for orientation and eye preference in the primary visual cortex of normal and binocularly deprived cats. These cortical maps were present by two weeks after birth, and developed until nearly three weeks of age whether or not the eyes were open. With continued visual deprivation, responses to both eyes deteriorated. The authors suggest that the basic structure of visual cortical maps is innate, but experience is essential for specific features as well as for maintaining the responsiveness and selectivity of cortical neurons.
Crair, M.C.; Gillespie, D.C.; Stryker, M.P. The role of visual experience in the development of columns in cat visual cortex. Science 279 (Jan. 23, 1998) 5350:566-570.
Contact: Michael P. Stryker <email@example.com>
Plasticity and Critical Periods
In neurobiology, the term "plasticity" is the name given to the capacity of neural tissue to adjust to change. One variant of this concerns the dependence of the "wiring" of the nervous system on its input. Another variant concerns the degree to which one region can under certain conditions assume the function of another region. Plasticity does not occur everywhere in the nervous system, but it is often evident in the cerebral cortex of the brain, the cortex being the thin layer of cells apparently responsible for higher analysis of sensory input, language, ideation, and other so-called higher functions lumped together in the category "cognitive processes".
One striking example of plasticity concerns the so-called "critical period" for histological development of the visual system in mammals. This is a period between birth and a later time during which the neuron circuits in the visual areas of the cerebral cortex are being developed. Once the system is completely developed, keeping one or both eyes closed has no effect on vision. For example, an adult human can develop a cataract in one eye, not have it removed for years, but once it is removed and a lens substitute in place, the vision in that eye is normal. The critical period for vision in humans lasts from birth to about 6 years, and there is much histological evidence that the visual cortex is changing during that time. A similar critical period exists in other mammals. If both eyes are kept covered from birth throughout the critical period in cats and monkeys, and the eyes are then uncovered, the animals remain functionally blind. It can be shown there is a gradation of the effect of the absence of input from the eyes: the effect is greater during the early part of the critical period than later on. And damage can also be demonstrated by keeping only one eye covered. In all of this, the eyes themselves are optically normal after they are uncovered, the cells in the retina and in the lower visual centers appear to function normally in their response to visual input. But the deprived eye or deprived eyes are "blind", and the changes or lack of development responsible for this loss of visual function are in the visual cortex and are due to its plasticity, its dependence on appropriate input for the formation of working connections. (from Science-Report 26 Sep 97)
Kirkwood (2000) notes that cortical circuitry can be altered with simple manipulations of visual experience. For example, deprivation of vision in one eye (monocular deprivation) shifts the response of cortical neurons toward the nondeprived eye. Such plasticity is directed by three diffusely projecting neurotransmitter systems.
Kirkwood, Alfredo (2000). Serotonergic control of developmental plasticity (Proceedings of the National Academy of Sciences 97. 5 (February 29): 1951-1952. Full text (external).
Cross-modal Plasticity in Blind Humans
Leonardo G. Cohen et al. report the results of studies of cross-modal plasticity in blind humans. These studies involved non-invasive interference with cortical activity by applying transient magnetic stimulation from outside the skull. It has been demonstrated that such stimulation can affect brain activity, and in this study the apparatus threshold for stimulation of the motor cortex was first determined, and then transient magnetic stimulation 10% above that threshold applied to the occipital lobes of the brain through the overlying skull to interfere with electrical activity in the visual cortex. The experiments involved various location and procedural controls, and also a group of sighted individuals. Essentially, what was found is that in people blind from an early age, the visual cortex is apparently involved in somato-sensory function (fingertip reading of individual Braille characters), while the same is not true for sighted subjects.
Cohen, LG; Celnik, P; Pascual Leone, A; Corwell, B; and others. Functional relevance of cross-modal plasticity in blind humans. Nature 389 (Sep. 11, 1997) 6647:180-183.
Gerloff, C; Corwell, B; Chen, R; Hallett, M; and others. Stimulation over the human supplementary motor area interferes with the organization of future elements in complex motor sequences. Brain 120 (Sep. 1997) 9:1587-1602.
Contact: Leonardo G. Cohen <firstname.lastname@example.org>
See Ian Pitchford's informal discussion of
neural plasticity versus domain specificity.
When natural scenes are viewed, a multitude of objects that are stable in their environments are brought in and out of view by eye movements. The posterior parietal cortex is crucial for the analysis of space, visual attention and movement. Neurons in one of its subdivisions, the lateral intraparietal area (LIP), have visual responses to stimuli appearing abruptly at particular retinal locations (their receptive fields). The authors have tested the responses in monkeys of LIP neurons to stimuli that entered their receptive field by saccades. Neurons had little or no response to stimuli brought into their receptive field by saccades, unless the stimuli were behaviourally significant. They established behavioural significance in two ways: either by making a stable stimulus task-relevant, or by taking advantage of the attentional attraction of an abruptly appearing stimulus. Their results show that under ordinary circumstances the entire visual world is only weakly represented in LIP. The visual representation in LIP is sparse, with only the most salient or behaviourally relevant objects being strongly represented.
J P Gottlieb, M Kusunoki & M E Goldberg The representation
of visual salience in monkey parietal cortex (Letter to Nature) Nature
Human color vision is typically trichromatic, relying on retinal cone cells sensitive to red, green, and blue (for a more accurate description, see Jacobs 1996). Old-world primates are all trichromats; new-world monkeys have a bewildering variety of color visions. Insects often outdo chordates: bees are tetrachromats and butterflies pentachromats.
In humans, the genes for green and red pigment are both located on the X-chromosome. Since women have two copies of this chromosome and men only one, deficiencies in green/red color vision is more common among men (around 8%). Lacking the genes for one color yields dichromatic vision. Monochromatic vision or literal colorblindness is rare.
Around 16% of women are carriers of dichromatism. These woman have one X-chromosome with genes that code for the retinal cones that produce the phenomenology red and green, and another X-chromosome with two greens. The son of one of these women, if he inherits the green-green chromosome, would be unable to see red. He would have dichromatic vision, or the capacity to see only two primary colors; in this case, green and blue. The woman herself, however, may be a tetrachomat. If the two genes that code for green are slightly different, she may perceive two different kinds of green. Research into this topic has been pursued by Gabriele Jordan and John Mollon of the University of Cambridge.
It has been proposed that these sex differences in color vision may be adaptive. Tetrachromatism may be useful in gathering, making it easier to spot fruits and berries. Conversely, dichromatism may actually be helpful in tracking game. I am aware of no evidence for this hypothesis. Source.
Rosch (1975), Jacobs (1996) and Shepherd (1992).
Special Behavioral and Brain Sciences issue on color (spring 2000)
Biological motion: interpreting the motion of human beings on the basis of limited cues appears to be handled by specialized systems; see Fox, R. & McDaniel, C. (1982). The preception of biological motion by human infants. Science 218: 486-487, and Neri et al. (1998).
Face recognition: There is a fair amount of evidence that facial cognition is a specialized adaptation with inference systems that are distinct from the rest of the the visual system. Farah et al. (1998) find evidence that facial cognition is holistic, that it uses relatively less part decomposition than the visual cognition of other types of objects.
Farah, Martha J.; Wilson, Kevin D.; Drain, Maxwell; Tanaka, James N. What is "special" about face perception? Psychological Review 105. 3 (July 1998): 482-498. Abstract.
Lincoln Holmes, the face-blind man in the news story "The man who can't recognise faces" (full text), tellingly reports, "When I am asked by people, do faces all look the same, the answer to that question is no -- they don't all look the same, but none of them look like anyone."
Tippet et al. (2000) describe an even narrower case, that of a man who is unable to learn new faces, but is fully able to recognize faces he knew before this impairment set in.
Tippett, Lynette J.; Miller, Laurie A.; Farah, Martha J. Prosopamnesia: A selective impairment in face learning. Cognitive Neuropsychology 17. 1-3 (Feb-May 2000): 241-255. Abstract.
The skill comes on-line very early; children respond selectively to face-like visual stimuli. Farah et al. (2000) examines "a failure of plasticity in the neural substrates of face recognition, which suggests that the distinction between faces and other objects, and the localization of faces relative to other objects, is fully determined prior to any postnatal experience."
Farah, Martha J.; Rabinowitz, Carol; Quinn, Graham E.; Liu, Grant T. Early commitment of neural substrates for face recognition. Cognitive Neuropsychology 17. 1-3 (Feb-May 2000): 117-123. Abstract.
It would be interesting to determine if this is a cross-species trait, or at least shared by other animals with expressive faces. On an evolutionary basis one might predict, for example, that dogs, wolves, and simians would have it, but not ungulates, since these largely use other parts of the body for communication.
A related if different issue is facial attractiveness:
Langlois, J.H., L.A. Roggman, L.S. Vaughn (1991). "Facial Diversity and Infant Preferences for Attractive Faces." Developmental Psychology 27 : 79-84
Langlois, Judith H., Lori A. Roggman and Lisa Musselman (1994). "What is Average and What Is Not Average About Attractive Faces." Psychological Science July 1994: 214
Rhodes, Gillian and Tanya Tremewan (1996). "Averageness, Exaggeration, and Facial Attractiveness," Psychological Science, March 1996: 105-110.
Evidence suggests the same neural structures mediate both perception and mental images.
Kreiman, Gabriel, Itzhak Fried, and Christof Kock (2000). Category-specific visual responses of single neurons in the human medial temporal lobe. Nature Neuroscience 3. 9: 946 - 953.
Kreiman, Gabriel, Itzhak Fried, and Christof Kock (2000). Imagery neurons in the human brain. Nature 408 .6810: 357 - 361.
Motion sickness may seem a wholly physiological phenomenon, but a ship in a storm at the movie theater can produce similar results, demonstrating a computational component. In a study of motion sickness, Parker (1971) showed participants a custom-made 8-minute video segment taken from the point of view of a driver of a car as it traveled a winding mounting road. Several participants became nauseous and could not complete the session.
Parker, D.M. (1971). A psychophysical test for motion sickeness susceptibility. Journal of General Psychology, 85, 87-92.
But what is being computed? Since our ancestors did not spend much time at sea or travelling in automobiles, and throwing up is anyway hardly adaptive in that situation, we need to look for an adaptive problem that belongs in a completely different domain.
The most cogent suggestion is that a moving horizon would have been interpreted not as a sign of something being amiss in the environment, but of the sensory system acting up. Why would the senses be acting up? Because you've eaten something strange--so get rid of it. The senses are prone to disturbances by a variety of neurotoxins, some of which are not uncommon in nature.
Motion sickness is an excellent example of an adaptation that evolved to solve a problem in one domain (in Sperber's terms, its "proper domain"), yet functions mostly in a completely different domain (its "actual domain"). Since the actual domain is mainly traveling in cars and buses and on ships, we may also speak of a "cultural domain"--and unfortunately, this adaptation conveys no benefits at all in its current cultural domain. We can only be grateful it saved our ancestor's lives.
The auditory system appears to have some domain-general features and possibly a large number of domain-specific mechanisms. The irregular shape of the ear is designed to permit computations of directionality in a wide range of circumstances (Pinker (1997). See also the references under music. Specialized inference engines may be associated with animals sounds, and, most spectacularly, with the analysis of human speech by a phonological parser.
The phonological parser
"Six-month olds can distinguish or produce every sound in virtually every human language. But within a mere four months, nearly two thirds of this capacity has been sliced away." (Werker & Desjardins 1995) . See also Conceptual Adaptations 3: Phoneme parsing.
Jay McClelland and associates (1999), however, have discovered a way of very quickly training non-native speakers to distinguish beween sounds that are not distinguished in their native tongue. The technique involved exaggerating the differences initially to promote novel Hebbian learning ("cells that fire together, wire together"), and then gradually bringing the two sounds closer to their normal ranges. Thus, the English sounds "r" and "l" are normally not distinguished by Japanese speakers. By learning to distinguish between exaggeratedly different "r" and "l" sounds, Japanese speakers quickly acquired the ability to distinguish between normal occurrances of these sounds in English speech.
See the report in the New York Times, April 20, 1999.
Naatanen, Risto, Anne Lehtokoski, Mietta Lennes, Marie Cheour, and others.
phoneme representations revealed by electric and magnetic brain responses. Nature 385 (6615):
Werker, Janet F (1989). "Becoming a Native
Listener." American Scientist January-February
Werker, Janet F. and Richard C. Tees (1992). "The Organization and Reorganization of Human Speech Perception." Annual Review of Neuroscience 15: 377-402.
Werker, Janet F. and Renee N. Desjardins (1995). "Listening to Speech in the First Year of Life: Experiential Influences on Phoneme Perception." Current Directions in Psychological Science June 1995: 76-81.
Werker, Janet F. and J.E. Pegg (1997).
"Infant speech perception and phonological acquisition." In Phonological
Development: Research, Models and Implications, edited by C.E. Ferguson,
L. Menn, C. Stoel-Gammon. Parkton, MD: York Press, in press.
Spelke has some interesting work on cross-modal inferences in small children (cf. Object Perception). Benedetti describes what is known as Aristotle's Illusion: if we touch a small ball with crossed finger tips, two balls are experienced instead of one.
Benedetti, F. (1986). Tactile diplopia (diplesthesia) on the human fingers. Perception 15, 83-91.
The role of tactile information in the mouth forms a separate topic, referred
to in the food industry as "mouthfeel" -- the unique combination of sensed textures
that affect how the flavor is perceived. "Mouthfeel can be adjusted through
the use of various fats, gums, starches, emulsifiers, and stabilizers. The aroma
chemicals in a food can be precisely analyzed, but the elements that make up mouthfeel
are much harder to measure. How does one quantify a pretzel's hardness, a french
fry's crispness? Food technologists are now conducting basic research in rheology,
the branch of physics that examines the flow and deformation of materials. A number
of companies sell sophisticated devices that attempt to measure mouthfeel. The
TA.XT2i Texture Analyzer, produced by the Texture Technologies Corporation, of
Scarsdale, New York, performs calculations based on data derived from as many
as 250 separate probes. It is essentially a mechanical mouth. It gauges the most-important
rheological properties of a food -- bounce, creep, breaking point, density, crunchiness,
chewiness, gumminess, lumpiness, rubberiness, springiness, slipperiness, smoothness,
softness, wetness, juiciness, spreadability, springback, and tackiness" (Schlosser,
2000 -- external).
Bekoff, Marc and John Alexander Byers (eds) Animal Play: Evolutionary, Comparative, and Ecological Perspectives. Cambridge: Cambridge University Press, 1998. Content and abstracts.
Owens, Stephanie and Francis Steen (2000). Implicit Pedagogy: From Chase Play to Narrative Worldmaking. Presentation at UCSB. Abstract (external).
Smith, Peter K. (ed). Play in Animals and Humans. Basil Blackwell, Inc; Oxford, England, 1984 Content and abstracts.
The sense of the body's orientation in space is relatively independent of visual cues; we can walk around comfortably blindfolded, and blind people have no difficulties keeping track of what their bodies are doing. The sense of the location of the body's limbs, also known as proprioception, is highly selective; thus, we do not generally have much of a proprioceptive awarenss of the processes of digestion or of the circulation of the blood. This suggests the kinesthetic sense is an adaptation primarily relevant to those parts of the body that interact directly with the external world: the mouth, the sexual organs, the limbs, and the skin.
The sense of smell is perhaps the oldest of the senses, consisting of an outcropping of the brain that protrudes into the roof of the nasal cavity--a chemical analyzer. Olfactory receptors can distinguish between some ten thousand different chemical compounds, generating information utilized in regulating food selection, breathing (by indicating air quality), parasite avoidance, and social behavior.
Smells do not directly detect nutrients, poisons, parasites, or mates, but chemicals that, in our ancestral environment, were reliably associated with them. If the association was particularly reliable, the system might evolve a fully specified parameter; certain kinds of smells, for instance, might universally elicit a disgust response (see Rozin & Fallon, 1987). In other cases, the parameter might be partially specified; for instance, there is evidence that people are sensitive to how potential mates' major histocompatibility complex (MHC) relates to their own--a parameter than cannot be set until the detection system knows the body's own MHC. Finally, in the case of open parameters, the association between a smell and a phenomenon is filled in by experience.
"Flavors are composite sensations, synthesized from primary taste and smell sensations by areas of the brain which represent the oldest parts of the new cortex (the medial temporal cortex, including the hippocampus, with connections to amygdala, and hypothalamus). These brain areas participate in our emotional existence, as parts of the limbic system. The human emotional system emerges in evolutionary terms from the smell brain of earlier animals and remains closely linked to taste, smell, and eating behaviors.
The nose is an important regulator of social life. Pheromones are a class of long-distance chemical messenger hormones which regulate social relations, behavioral and physiological responses in insects and many mammals. Social meanings - identity, dominance, ownership of territory, and sexual status, are conveyed by pheromones in the animal world. The vomeronasal organ, an old brain region, is involved in the detection and interpretation of pheromonal information. Observations of human behavior have linked smell messages to menstrual cycle timing" (Stephen J. Gislason, on the site Smell and Taste). See also Did Humans Lose a Sixth Sense? (external, BBC).
Focus and Memory: The human sense of smell is greatly affected by psychological factors and expectations. The mind focuses intently on some of the aromas that surround us and filters out the overwhelming majority. People can grow accustomed to bad smells or good smells; they stop noticing what once seemed overpowering. Aroma and memory are somehow inextricably linked. A smell can suddenly evoke a long-forgotten moment. The flavors of childhood foods seem to leave an indelible mark, and adults often return to them, without always knowing why. On the remarkable role in memory of smells--consider Proust's madelaine, which was probably smelled more than strictly speaking tasted--see Van Tollen & Kendal-Reed (1995). On taste, see below.
Predator avoidance: The behavioral ecologist Joel Berger found that moose in the Greater Yellowstone Ecosystem, where wolf has been absent for fifty years or a dozen generations, no longer react to the scent of wolf scat, while the response is intact in Alaskan moose, regularly subject to wolf predation. This suggests wolf aversion is a learned response--a partially specified or open parameter. His question was whether feeding rates would drop in response to the odor of long-gone predators, such as Siberian tiger scat. This would be an example of a fully specified parameter. Big cats haven't hunted North American moose for a very long time--possibly more than ten thousand years, or a couple of thousand generations. (This raises the possibility that olfactory predator dectection systems in humans might be able to tell us something about which animals preyed on our ancestors.) So far the results are negative: predator avoidance is an open parameter. See the New Scientist 25 December 1999 writeup (external).
Vomeronasal organ: This ancient chemical-detection system, located inside the nose, may play a part in sexual selection, possibly by identifying a histocompatibility complex of a suitable difference to one's own. The operation of this sense appears to be commonly and perhaps entirely beyond the access of conscious awareness (cognitively impenetrable). For more information see The Vomeronasal Organ at the Neuroscience web site of Florida State University.
Literature on pheromones and related steroids:
Scent of Eros: James Kohl's extensive site on pheromotes and mate selection
Jacob, Suma and Martha K. McClintock (2000). Psychological State and Mood Effects of Steroidal Chemosignals in Women and Men. Hormones and Behavior 37. 1 (February): 57-78. Abstract with link to full text (external); press release (external).
McClintock, Martha, and Suma Jacob (2000). Psychological State and Mood Effects of Steroidal Chemosignals in Women and Men. Hormones and Behavior. Press release (external). Studies the effect of the steroid androstadienone on mood.
In humans, approximately 4000 taste buds, each containing 30 to 100 chemoreceptors or taste receptor cells, are distributed throughout the oral cavity and upper alimentary canal. Chemoreceptors are biological cells specialized to respond to chemical stimuli. They signal to the nervous system a change in the chemical environment. In humans, chemoreceptors function to detect taste (gustatory sense) and smell (olfaction). Taste receptors are found in the epithelium of the tongue, and these receptors are responsible for sour, sweet, salty, and bitter sensations from food applied to the tongue. Taste receptors are also found in the pharynx and the upper part of the esophagus. Taste buds are approximately 50 microns wide at their base and approximately 80 microns long, each bud containing 30 to 100 taste receptor cells. Approximately 75 percent of all taste buds are found on the upper (dorsal) surface of the tongue.
In contrast to olfactory receptors, taste receptors do not have their own output extensions (axons) to send signals to the central nervous system, but instead taste receptors stimulate the endings of nerve fibers that send input to the central nervous system ("afferent fibers"). Taste receptor cells are gathered into groups as "taste buds", and the sensing of taste stimuli occurs in finger-like projections (microvilli) at the surface of these taste buds, with various chemical mechanisms proposed to account for transduction of taste stimuli. In general, sourness depends primarily on the acidity of a chemical stimulus, and salty sensations are evoked by solutions with a high sodium concentration. Sweetness and bitterness, on the other hand, are apparently transduced by specific receptor cell membrane receptors for sugars, amino acids, and other chemicals. For details, see Linda Buck's recent work (April 2000, external).
"A nose can detect aromas present in quantities of a few parts per trillion -- an amount equivalent to about 0.000000000003 percent. Complex aromas, such as those of coffee and roasted meat, are composed of volatile gases from nearly a thousand different chemicals. The smell of a strawberry arises from the interaction of about 350 chemicals that are present in minute amounts. The quality that people seek most of all in a food -- flavor -- is usually present in a quantity too infinitesimal to be measured in traditional culinary terms such as ounces or teaspoons. The chemical that provides the dominant flavor of bell pepper can be tasted in amounts as low as 0.02 parts per billion; one drop is sufficient to add flavor to five average-size swimming pools" (Schlosser, 2000 -- external).
Threshold concentrations for taste sensations produced by ingested substances may be calibrated by their adaptive history. For example, the threshold concentration for sodium chloride is approximately 10 millimolar, for sucrose, 20 millimolar, for citric acid 2 millimolar, reflecting thresholds of significant nutritive value. The threshold is much lower for certain bitter-tasting potentially dangerous plant compounds: the threshold concentration for quinine is 0.008 millimolar, and for strychnine 0.0001 millimolar, reflection natural selection for sensitivity to toxins. Human chemoreceptors thus have two relatively distinct functions: to detect nutritive foods, broadly giving rise to a subjective phenomenlogy of pleasure, and to detect environmental toxins, giving rise to aversive responses.
"The taste buds on our tongues can detect the presence of half a dozen or so basic tastes, including sweet, sour, bitter, salty, astringent, and umami, a taste discovered by Japanese researchers -- a rich and full sense of deliciousness triggered by amino acids in foods such as meat, shellfish, mushrooms, potatoes, and seaweed. Taste buds offer a limited means of detection, however, compared with the human olfactory system, which can perceive thousands of different chemical aromas. Indeed, "flavor" is primarily the smell of gases being released by the chemicals you've just put in your mouth. The aroma of a food can be responsible for as much as 90 percent of its taste. The act of drinking, sucking, or chewing a substance releases its volatile gases. They flow out of your mouth and up your nostrils, or up the passageway in the back of your mouth, to a thin layer of nerve cells called the olfactory epithelium, located at the base of your nose, right between your eyes. Your brain combines the complex smell signals from your olfactory epithelium with the simple taste signals from your tongue, assigns a flavor to what's in your mouth, and decides if it's something you want to eat" (Schlosser, 2000 -- external; for details, see above)
A broad range of cues are utilized to judge if food is appropriate for consumption; the basic one is nevertheless smell. (The wild boy of Aveyron, who grew up in the forest without either human or animal parents, reportedly sniffed everything before he would put it in his mouth.) Visual cues are obviously less reliable, but food visibly infested with parasites is generally felt to be disgusting.
Infants' proclivity to put anything they can put their hands on into their mouths, however, seems on the face of it maladaptive, and parents are typically vigilant at preventing their babies from ingesting soil and other inedibles. The evolutionary account generally provided for this behavior posits that in the ancestral environment, babies were most likely carried by their mothers for the first couple of years. During this period, it would be advantageous for them to eat anything they were offered, since mother was supervising. From the age of two or three, when children in the EEA are more likely to have been moving around by themselves, they would be expected to benefit from a much more discriminative approach to food selection.
Somewhat tongue-in-cheek, Jared Diamond has recently proposed another explanation in an article on geophagy, or the ingestion of soils, in Dirty eating for healthy living. He points out that the ingestion of certain soils helps detoxify plant poisons. While this is not a very convincing explanation of infants' apparently indiscriminate tendency to ingest things, it does at least raise the possibility that experimental ingestion of minerals could have an adaptive function.
The love of sugar is a staple example of phenotypic lag in the evolutionary literature. The argument is that sugar is no longer good for us, but served as a useful -- perhaps vital -- cue of readily available calories in the EEA. The preference does not need to be taught; consider this parent's anecdote:
JXN had his first piece of pumpkin pie today. He went through the whipped cream very quickly....most ended up on his face like a beard....he seemed a bit apprehensive about attacking the actual piece of pie though. Finally, he took a little nibble between his fingers and tasted it. A look of surprise!!!! Eyes wide open, jaw dropped with his mouth closed so that his mouth was a little "O". He then grabbed the entire piece of pie and stuffed it in his mouth. Some of it kept on popping out of his mouth and he would just take his hand and jam it back in. We were worried that he was going to choke himself, but he was having the time of his life.
"Studies have found that the color of a food can greatly affect how its taste is perceived. Brightly colored foods frequently seem to taste better than bland-looking foods, even when the flavor compounds are identical. Foods that somehow look off-color often seem to have off tastes. For thousands of years human beings have relied on visual cues to help determine what is edible. The color of fruit suggests whether it is ripe, the color of meat whether it is rancid. Flavor researchers sometimes use colored lights to modify the influence of visual cues during taste tests. During one experiment in the early 1970s people were served an oddly tinted meal of steak and french fries that appeared normal beneath colored lights. Everyone thought the meal tasted fine until the lighting was changed. Once it became apparent that the steak was actually blue and the fries were green, some people became ill. [...]
A typical artificial strawberry flavor, like the kind found in a Burger King strawberry milk shake, contains the following ingredients: amyl acetate, amyl butyrate, amyl valerate, anethol, anisyl formate, benzyl acetate, benzyl isobutyrate, butyric acid, cinnamyl isobutyrate, cinnamyl valerate, cognac essential oil, diacetyl, dipropyl ketone, ethyl acetate, ethyl amyl ketone, ethyl butyrate, ethyl cinnamate, ethyl heptanoate, ethyl heptylate, ethyl lactate, ethyl methylphenylglycidate, ethyl nitrate, ethyl propionate, ethyl valerate, heliotropin, hydroxyphenyl-2-butanone (10 percent solution in alcohol), a-ionone, isobutyl anthranilate, isobutyl butyrate, lemon essential oil, maltol, 4-methylacetophenone, methyl anthranilate, methyl benzoate, methyl cinnamate, methyl heptine carbonate, methyl naphthyl ketone, methyl salicylate, mint essential oil, neroli essential oil, nerolin, neryl isobutyrate, orris butter, phenethyl alcohol, rose, rum ether, g-undecalactone, vanillin, and solvent" (Schlosser, 2000 -- external).
The basic appetite for food is a computational adaptation, and not simply a raw signal the body is running out of fuel. If we eat as soon as we are hungry, and in moderate amounts, the energy level rises at once--as if the body has computed that it can afford to expend existing resources in the knowledge that, after a few hours of digestion, new nourishment will be available.
Nausea is reliably correlated with smells and tastes that, in the Ancestral
Environment, would have been high in neurotoxins. These toxins could produce
hallucinations, loss of balance, and other disturbances to the central
nervous system. The initial response is aversion; if you have ingested
some, you might feel the urge to get rid of it quickly. A particularly
acute sensitivity to environmental toxins is activated in women in the
early months of pregnancy, when the danger of harm to the fetus by teratogens
is particularly high.