Chapter 4 The Neural Bases of Language

 

In this chapter, the author attempts to provide some current view on the nature of the neural bases of human language.  His focus is on the cortical-striatal-cortical circuits which yield the reiterative ability.  The faculty not only plays a critical role in human speech and confers the flexibility of human thought processes but probably derives from elements of neural circuits that regulate motor control.  Besides, he also discuss studies that are exploring the neural bases of the brain’s dictionary, neural plasticity, and the lateralization of the brain.

 

I.            Neuroanatomy

(1.)     Introduction

A.         The brain is divided in two cerebral hemispheres, along the anterior to posterior axis. 

B.         Structures closer to the midline of the brain are “medial,” and those out from it are “lateral.” 

C.         Neocortex has neurons arranged in six layers.

(a)    paleocortex

(b)   basal ganglia

(c)    thalamus

(d)   hippocampus

(e)    cerebellum 

(f)     other subcortical structures in the basal ganglia and other subcortical structures are represented right and left.

D.         The surface of the brain is shown in the lateral view: 


 

(2.)     Neurons

A.         Neurons connect to each other and transmit information by means of dendrites and axons.

B.         A cluster of dendrites is associated with each neuron.

C.         Synapses are structures that determine the degree to which an incoming signal will cause the cell body to generate an electrical pulse, the action potential or spike that is transmits out without decrement on its axon.

D.         The synapse acts as a coupling device in the transfer of a signal from a dendrite to the neuron.

 

 

(3.)     Distributed Neural Networks

A.         Every neuron in setαprojects to every neuron in setβ.

B.         This drawing, where N=6, grossly understates the size and connectivity of the nervous system and single neurons and single synapses have little effect on the discharge patterns of the group as a whole.

(4.)     Understanding Distributed Neural Networks

A.         Large sections of the network can be destroyed without losing memory traces.

B.         When the cortical areas are destroyed, people do not lose their preexisting language or their native dialect.

C.         Transient loss of language may occur, but after a period of months, a person’s premorbid linguistic ability returns.

II.    Electrical Power Grids: A Metaphor for Distributed Networks

(1.)     Electrical Power Grids:

(2.)     The Robustness of Distributed Neural Networks

A.         The robust nature of memory trace follows from the fact that information is coded by the synaptic settings of the entire system.

B.         A distributed network also can be perturbed by events taking place far away from them.

C.         If switches fail to correct the problem in time, an isolated event that overloads a switching point can propagate through the network.

D.         If the network is properly designed and monitored, a single power plant failure or downed transmission line would not interrupt your electrical supply.

E.          The memory traces that code the motor pattern generators that result in speech, the cognitive pattern generators specifying the rules of syntax, and the lexicon clearly have widely distributed cortical representations.

(3.)     Associative Learning

A.         Imitation and associative learning are powerful “general” procedures for acquiring knowledge Pavlov’s dogs

B.         Associative learning has been demonstrated in Aplysia californius a gastropod mollusk.

C.         Associative learning has a selective value to any animal because it allows rapid phenotypical changes in response to new environmental conditions.

III. Cortical Architecture──Cortical Maps

(1.)     The human brain is not a unitary distributed neural network.

(2.)     Different parts of the cortex have different cytoarchitectonic structures, in other words, different distributions of neurons in the layers that form the cortex.

(3.)     The cytoarchitectural maps of brains made by Brodmann generally were thought to reflect discrete functional distinctions.  (It is becoming clear that the situation is more complex than Brodmann thought.)

(4.)     Broca’s and adjacent cortical areas also are implicated in manual motor control.

(5.)     Cortical areas and subcortical structures carry out different local operations and play a part in circuits regulating different aspects of behavior.

(6.)     It clearly is not a “module” dedicated to language and language alone.

(7.)     The feed-forward and back connections that typify all cortical areas manifest themselves in Broca’s area being activated during speech perception and sentence comprehension.

(8.)     Many areas of cortex are malleable and can take in new functions due to damage to the brain or birth defects.

IV.  The Neurophysiologic Toolkit

(1.)     Direct Electrophysiologic Recording

A.         Direct observation of neuronal activity in the brain of a living animal involves the placement of microelectrodes.

B.         These electrodes conduct electrical signals induced by the electrochemical communications between neurons.

(2.)     Tracer studies

A.         One of the properties of axons is that amino acids, the building blocks of proteins, are transmitted from the cell body down the length of an axon.

B.         All traditional tracer techniques involve injecting substances into the brain of a living animal, waiting for the tracer to be transported, and then sacrificing the animal.

C.         A recent advance in tracer techniques employs a variation of MRI technology that allows neural circuits to be traced in living human subjects.

(3.)     MRI, fMRI, and PET Imaging

A.         Functional magnetic resonance imaging (FMRI) is the most recent technique, and it is a variant on the structural magnetic resonance imaging (MRI) systems that are routinely used for imaging brain structures.

B.         FMRI can map out the flow and transport of deoxygenated blood (the BOLD signal) that is an indirect marker of metabolic activity, hence neural activity, in the brain.

C.         Event-relater FMRI involves close synchrony of tasks with the FMRI BOLD signal, permitting investigation of the time course of neural activity.

D.         Positron emission tomography (PET) involves injecting a short-half life radioactive tracer into the bloodstream.

(4.)     CT Scans

Computerized tomography (CT) show slices of brain anatomy reconstructed by computer processing of multiple X-ray exposures taken at different angles and planes.

(5.)     ERP

Event-related potentials (ERPs) can reveal transient electrical activity recorded by means of electrodes placed in contact with a person’s scalp.  The technique involves recording by means of the procedures commonly used to record EEG brain activity in clinical settings.

V.     Brain──Behavior Models

(1.)     Phrenology

(2.)     Modular Theories

VI.  Neural Circuit Models

(1.)     Neurophysiologic activity must be considered at two levels.  First, distinct neuroanatomic structures usually do not regulate an observable behavior.

A.         The neuronal population that carries out the local operation in a given part of the brain projects to anatomically segregated neuronal populations in other neuroanatomic structures.

B.         Circuits linking anatomically segregated populations of neurons form neural systems carrying out processes in different parts of the brain. The systems are the neural bases of complex behaviors.

(2.)     Second, different regions of the neocortex and different subcortical structures are specialized to process particular stimuli, visual or auditory, while other regions perform specific operations that regulate aspects of motor control or holding information in short-term memory.  The local operations form a set of neural computations that link together in complex neural circuits.

(3.)     The neural mechanism that carries out the instruction set manifested in my pecking at my computer’s keyboard is a circuit, linking neuronal populations in different neuroanatomic structures in many parts of the brain.

VII.   Experiments in Nature

(1.)     The traditional Broca-Wernicke Model

A.         Although the Broca-Wernicke model has the virtue of simplicity, it is wrong. Clinical evidence shows that permanent loss language does not occur without subcortical damage even when Broca’s or Wernicke’s areas have been destroyed.

B.         Damage to subcortical structures, sparing the cortex, can produce aphasic syndromes.

C.         Language impairments occurred that ranged from fairly mild disorders in the patient’s ability to recall words to “global aphasia” in which the patient produced very limited nonpropositional speech.

D.         Damage to the internal capsule (the nerve fibers that connect neocortex to subcortical structures0 and basal ganglia structures, the putamen, and caudate nucleus resulted in impaired speech production similar to that of the classic aphasias, as well other cognitive deficits.)

(2.)     The Basal Ganglia and Cortical──Striatal Circuits

A.         Basal ganglia dysfunction can produce the constellation of motor, syntax, and cognitive deficits associated with Broca’s syndrome.

B.         The basal ganglia are subcortical structures located deep within the brain. They are primitive neural structures that can be traced back to anurans.

C.         The basal ganglia in humans and other primates include the caudate nucleus and the lentiform nucleus, which constitute the striatum.

VIII.                Neurodegenerative Diseases

Damage to striatal and associated subcortical structures that support a cortical-striatal-cotrical circuit can result in a behavioral deficit that was thought to derive from damage to the region if the cortex served by the circuit.

The primary deficits of these neurodegenerative diseases are motoric; tremors, rigidity, and repeated movement patterns occur.  However, these subcortical diseases also cause linguistic and cognitive deficits.

(1.)     Probable Basal Ganglia Operations

The functional basal ganglia complex includes the putamen, caudate nucleus, globus pallidus, and close connections to the substantia nigra, thalamus, and other structures.  The system essentially acts as a sequencing engine.  Within the basal ganglia, information is transmitted by inhibitory and excitatory channels.

(2.)     Neuroimaging Studies──Set Shifting and Sequencing

(3.)     Implicit Associative Learning

(4.)     Blobology

A.         This procedure attempts to identify neural structures that are key factors in regulating a particular aspect of behavior by subtracting the pattern of activity pattern associates with the experimental stimuli.

B.         Blobology overlooks the fact that circuits linking activity in many neuroanatomic structures generally carry out complex, observable behaviors.

C.         The neuroanatomic structures implicated in listening to acoustic signals must necessarily be activated when we interpret word.  And there is no guarantee that the part of the brain that supposedly is the seat of word comprehension is not also activated in some other behavior that was not explored in the particular experiment.

IX.      Evidence for Basal Ganglia Activity in CorticalStriatalCortical Circuits

(1.)     Broca’s Syndrome

(2.)     Speech Sequencing

X.     Linguistic Deficits of Broca’s Syndrome

Contrary to the traditional Broca’-Wernicke model that assigns speech production to Broca’s area, higher-level linguistic deficits also occur in Broca’s aphasic syndrome.  They consistently show reduced auditory semantic priming effects, especially when the acoustic signal is degraded.

    Cognitive Deficits of Broca’s Syndrome

XI.  Parkinson’s Disease (PD)

(1.)     VOT Sequencing Deficits

(2.)     Sentence Comprehension Deficits

(3.)     Cognitive Set-Shifting Deficits

(4.)     Cognitive Planning

XII.   Mount Everest as an Experiment in Nature──Hypoxia

XIII.                Cerebellum

XIV.                Verbal Working Memory

XV.   Dynamic Systems: Throwing More Resources at the Problem

XVI.                  The Brain’s dictionary

The Dog Who Did Not Bark: When Procedural Knowledge Becomes Declarative Knowledge

XVII.               Cortical Plasticity

XVIII.            Basal Ganglia Grammar

 Motor Control Programs and the Rules of Syntax

XIX.                  Brain Lateralization and Language