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Human Language and Our Reptilian Brain: the subcortical bases of speech, syntax, and thought
Philip Lieberman *
For the past 200 years, virtually all attempts to account for the neural bases and the evolution of human language have focused on the neocortex. And in the past 40 years, linguists adhering to Noam Chomsky's theories have essentially equated language with syntax, hypothetically specified by an innate, genetically transmitted "universal grammar." In Human Language and our Reptilian Brain (2000), I attempt to shift the focus. My premise is that speech is the central element of human linguistic ability and both speech and syntax are learned skills, based on a neural "functional language system" (FLS). Although neither the anatomy nor the physiology of the FLS can be specified with certainty at the present time, converging behavioral and neurobiological data point to language being regulated by a distributed network that crucially involves subcortical structures, the basal ganglia, often associated with reptilian brains though they derive from amphibians.
Like other distributed neural systems that regulate complex behavior, the architecture of the FLS consists of circuits linking segregated populations of neurons in structures distributed throughout the brain, cortical and subcortical, including the traditional "language" areas (Broca's and Wernicke's areas) as well as other neocortical areas. The FLS rapidly integrates sensory information [End Page 32] with stored knowledge; it is a dynamic system, enlisting additional neural resources in response to task difficulty. Regions of the frontal lobes of the human neocortex, implicated in abstract reasoning and planning, and other cortical areas are recruited as task difficulty increases. Since natural selection selects for timely responses to environmental challenges, it is not surprising that the FLS also provides direct access to the information coded in a word, i.e., primary auditory, visual, pragmatic, and motoric information. The mental operations carried out in the brain are not compartmentalized in the "modules" proposed by most linguists and many cognitive scientists. The neural bases of human language are intertwined with other aspects of cognition, motor control, and emotion.
The human FLS is unique; no other living species possesses the neural capacity to command spoken language, which serves as a medium for both communication and thought. The FLS appears, however, to have evolved from neural structures and systems that regulate adaptive motor behavior in other animals. In this light, the subcortical basal ganglia structures usually associated with motor control that are key elements of the FLS reflect its evolutionary history: natural selection operated on neural mechanisms that yield adaptive--that is, "cognitive"--motor responses in other species. There is no reason to believe that the basic operations of the human brain differ for motor control and language. Insights gained from the study of the neural bases of motor control apply with equal force to human language; although the neural architecture that regulates motor control and syntax is part of our innate endowment, the details are learned. And the early stages of the evolution of the cortico-striatal neural circuits that regulate human language and thought may have been shaped by natural selection to meet the demands of upright bipedal locomotion, the first defining feature of hominid evolution.
This evolutionary perspective may not be familiar to some cognitive scientists, linguists, and philosophers. I hope, however, that biological linguists working in an evolutionary framework will lead the way to new insights on the nature of language. Paraphrasing Dobzhansky, "Nothing in the biology of language makes sense except in the light of evolution."
Functional Neural Systems
The traditional view of the neural bases of human language derives from 19th-century phrenology. Phrenologists claimed that discrete parts of the brain, which could be discerned by examining a person's cranium, were the "seats" of various aspects of behavior or character. Neo-phrenological theories do not claim that a bump on your skull shows that you are virtuous, but phrenology lives on in the traditional Broca-Wernicke model of the neural bases of language. In 1861, Broca ascribed the...