Some Higher Functions of Nervous System
Neurophysiology of language and speechLanguage and speech
Neurophysiology of language and speech
Communication through language is a unique faculty which places the humans much above the animals. Language refers to that faculty of nervous system which enables the humans to understand the spoken and printed words, and to express ideas in the form of speech and writing. There are two aspects of communications: language input (the sensory aspect) and language output (the motor aspect). The sensory aspect of language includes the visual, auditory and proprioceptive impulses, while the motor aspect includes the mechanisms concerned with the expression of spoken (sound) language and written language.
Development of speech
Development of speech involves co-ordinated activity of three important areas of cerebral cortex, namely Wernicke's area, Broca's area and motor areas of the categorical (dominant) hemisphere.
Development of speech in a child occurs in two stages:
First stage. In this stage, there occurs association of certain words with visual, tactile, auditory and other sensations, aroused by objects in the external world, which is stored in the memory.
Second stage. This stage of development of speech involves establishment of new neuronal circuits. When a definite meaning has been attached to certain words, pathway between the auditory area (area 41) and motor area for the muscles of articulation, which helps in speech (area 44) is established. And, the child attempts to formulate and pronounce the words, which are learnt.
Mechanism of speech and speech centres
Speech is of two types: spoken and written.
Spoken speech involves both understanding of spoken words as well as expressing ideas in the form of spoken words.
Written speech also involves both understanding of written words as well as expression of ideas in the form of written words. Mechanism of speech involves co-ordinated activities of central speech apparatus and peripheral speech apparatus. The central speech apparatus consists of cortical and subcortical centres. The peripheral speech apparatus includes larynx or sound box, pharynx, mouth, nasal cavities, tongue and lips. All the structures of peripheral speech apparatus work in co-ordination with respiratory system, under the influence of motor impulses from the respective motor areas of the cerebral cortex.
Mechanism of speech and the centres concerned with can be described separately for:
Understanding of speech (sensory aspects of communication)
Understanding of spoken speech
Understanding of the spoken words is accomplished by following activities.
1. Hearing of the spoken words requires an intact auditory pathway from the ears to primary auditory areas.
Primary auditory areas, also called auditory sensory areas, include the Brodmann's area 41 and 42 and form the centre for hearing. Primary auditory areas are located in the middle of superior temporal gyrus on the upper margin and on its deep or insular aspect (Fig. 10.9-1). This area perceives the nerve impulses as sound, i.e. auditory information, such as loudness, pitch, source and direction of sound.
Fig. 10.9-1 Lateral surface of left (categorical) hemisphere showing location of primary areas of language.
2. Recognition and understanding of the spoken words is carried by auditory association areas (21 and 20) located in the middle and inferior temporal gyrus, respectively (Fig. 10.9-1).These areas receive impulses from the primary area and are concerned with interpretation and integration of auditory impulses.
3. Interpretation and comprehension of the speech ideas. It involves the activities of Wernicke's area. Wernicke's area (area 22) is a sensory speech centre located in the posterior part of the superior temporal gyrus behind the areas 41 and 42 (Fig. 10.9-1) in the categorical hemisphere, i.e. dominant hemisphere. Functions of this area are:
Understanding of written speech
Understanding of the written speech is accomplished by following activities (Fig. 10.9-2).
1. Perception of written words requires an intact visual pathway from eyes to primary visual cortex.
Primary visual cortex, also called as striate area (area 17), or the centre of vision lies on the medial surface of occipital lobe. The optic radiations which bring impulses from parts of both retina.
2. Interpretation of written speech. Visual association areas (area 18 and 19) are concerned with the interpretation of written words. These areas are involved in the recognition and identification of the written words in the light of past experience.
3. Generation of thoughts/ideas in response to written speech. Dejerine area (area 38), located in the angular gyrus behind the Wernicke's area in the dominant hemisphere, is involved in the activity of generation of thoughts/ ideas in response to the written speech. This area is also called visual speech centre and along with the Wernicke's areas (auditory speech centre) forms the so-called sensory speech centre.
Expression of speech (motor aspect of communication)
Expression of speech in response to both spoken speech and written speech can be in the form of spoken speech or written speech or both. It involves the activities of motor speech centres, which include Broca's area (area 44) and Exner's area.
1 Expression in the form of spoken speech
Expression in the form of spoken speech involves the activities of motor speech (Broca's area) area (area 44).
Functions. This area, especially in the dominant hemisphere (left hemisphere in right-handed person) processes the information received from the sensory speech centres (Wernicke's area and Dejerine's area) into a detailed and coordinated pattern for vocalization, which is then projected to motor cortex for implementation. Thus, Broca's area is concerned with the movements of those structures which are responsible for the production of voice and articulation of speech, i.e. it causes activation of vocal cords simultaneously with movements of mouth and tongue during speech; lesions of this area cause motor aphasia.
2 Expression in the form of written speech
Expression in the form of written speech is the function of Exner's area (Fig. 10.9-2).
Exner's area (motor writing centre) is situated in the middle frontal gyrus in the categorical (dominant) hemisphere in the premotor cortex. It processes the information received from the Broca's area into detailed and co-ordinated pattern; and then along with the motor cortex (area 4) initiates the appropriate muscle movements of the hand and fingers to produce written speech.
Concept of dominant hemisphere for language
In human cerebral cortex, the interpretive functions of Wernicke's area, the angular gyrus and the frontal motor speech areas (i.e. the ability to understand or express oneself by spoken or written speech) are more highly developed in one hemisphere called the dominant hemisphere. How one hemisphere comes to be dominant is not yet understood. It is important to note that:
• In approximately 95% of all individuals, the left hemisphere is dominant regardless of handedness.
• Since, the motor area concerned with the hand movements is closely associated with the centre for speech, this explains the right handedness in over 90% of the individuals.
• Right hemisphere dominance is seen in only 15% of left handers.
• Seventy percent of left handers also have left hemisphere dominance.
Concept of categorical and representational hemisphere
Presently, it is believed that left hemisphere is not really dominant over the right hemisphere. In fact, the two halves of the brain have independent capabilities of consciousness, memory storage, and control of motor activities and speech. The corpus callosum and anterior commissure connect the two halves of brain. By these connections, information stored in one hemisphere is made available to the other hemisphere and then the activities of two hemispheres are co-ordinated.
As summarized below, some specialized higher functions are allowed to each hemisphere. Therefore, the terms ‘dominant’ and ‘non-dominant’ have been replaced by categorical and representational hemisphere, respectively.
Speech disorders
Dysarthria
Dysarthria is a disorder of speech in which articulation of words is impaired, but the comprehension of spoken and written speech is not affected.
Aphasia
Aphasia refers to the inability to understand spoken or written speech or inability in expressing the spoken or written speech in the absence of mental confusion or motor deficit. The aphasia may be:
Sensory aphasia
Sensory aphasia, also known as Wernicke's aphasia, is the result of lesion in the Wernicke's area.
Characteristic features of sensory aphasia are:
1. Difficulty in understanding the meaning of speech.
2. Motor speech is intact and the patients talk very fluently, that is why, it is also called fluent aphasia. However, the speech does not make much sense.
3. Impairment in reading and writing. Since the patient cannot comprehend the written words (word blindness) he/she is unable to read aloud or copy print into writing.
Learning and memory
Learning
Learning and memory are closely related. Learning is impossible without memory and memory has no meaning without learning. In fact, learning and memory are two sides of a coin. Learning refers to a neural mechanism by which the individual changes his or her behaviour on the basis of the past experience. Two patterns of learning are reflex and incidental learning.
Reflex learning, in which the learning is associated with an immediate behavioural changes and
Incidental learning, in which the behavioural change is not immediately apparent. The individuals acquire information about the world, while attending incidentally to sensory inputs, and thereby develop the potential to behave differently.
The two broad classes of learning are:
A Non-associative learning
In non-associative learning, the subject learns about the properties of a single stimulus. It results when an animal or person is repeatedly exposed to a single type of stimulus. Two forms of non-associative learning are common in everyday life: habituation and sensitization.
Habituation
Habituation refers to a decrease in response to a benign (neutral type) stimulus when the stimulus is presented repeatedly. When the stimulus is applied for the first time, it is novel and evokes reaction. This response is called orientation reflex or ‘what is it’ response. However, due to habituation lesser and lesser response is evoked on repeated stimulation. Eventually, the subject totally ignores the stimulus and thus gets habituated to it.
B Associative learning
In associative learning, the subject learns about the relationship between two stimuli or between a stimulus and a behaviour. Two forms of associative learning have been distinguished based on the experimental procedures used to establish the learning:
Classical conditioning
Classical conditioning involves learning a relationship between two stimuli. Classical conditioning is also termed Pavlovian conditioning, conditioned reflex type I, type-S conditioning.
Characteristic features of a classical conditioned reflex
• A conditioned reflex is reflex response to stimulus that is acquired by repeatedly pairing the stimulus with another stimulus that normally does produce the response. It depends for its appearance on the formation of new functional connections in CNS.
• Reinforcement, i.e. a process of following a conditioned stimulus (CS) with the basic unconditional stimulus (US) is must for retaining a conditioned reflex, otherwise it will get extinct.
Pavlov's experiment to demonstrate classical conditioned reflex is:
• When food, i.e. US is presented to a hungry dog, it produces salivation (an unconditioned response) or
• If a bell is rung (a CS), just before the food (US) is presented, the dog learns to associate the bell (CS) with the food (US).
• Eventually, ringing the bell (CS) alone causes salivation.
• Of course, if the food fails to appear consistently when the bell is rung, the conditioned response fades away, a process called extinction or internal inhibition. Thus, a conditioned reflex needs to be reinforced frequently, otherwise it dies out.
Operant conditioning
Operant conditioning is also termed as instrumental conditioning, type II conditioning, type-R conditioning or trialand- error conditioning.
Operant conditioning is of two types:
• Reward conditioning. In it a naturally occurring response is strengthened by positive reinforcement (reward).
• Adversive conditioning. In it a naturally occurring (innate) response is weakened by a negative reinforcement (punishment).
Experiment to demonstrate operant conditioning
A hungry animal (e.g. rat) is placed in a cage with a lever (bars) protruding in the cage. Because of naturally occurring (innate) response, the rat will randomly press the lever.
• If pressing of lever is not associated with any event, then the pressing of the lever will be at a random rate.
• If pressing a lever is associated with a positive reinforce, i.e. reward (e.g. food) the rate of pressing the lever will be much more than the random rate (reward conditioning).
• If pressing of lever is associated with a negative reinforce, i.e. punishment (e.g. electric shock), the lever-pressing rate will be much less than the random rate (adversive conditioning).
Memory
Memory refers to the acquisition, storage and retrieval of sensory information; while learning is the change in behaviour based on the sensory information stored in the brain. Brain has different sites and mechanisms for handling different types of information.
Types of memory
Memory can be classified in two ways:
I Physiologically, on the basis of how information is stored and recalled
Implicit memory, also called non-declarative or reflexive memory, refers to the information about how to perform something. It is not associated with awareness and does not involve processing in the hippocampus in most instances. Examples of implicit memory include motor skills, habits, behavioural reflexes and the learning of certain types of procedures and rules, which, once acquired, become unconscious and automatic.
Explicit memory. Explicit memory, also termed as declarative or recognition memory, refers to the factual knowledge of people, places, things and what these facts mean. This is recalled by a deliberate conscious effort. Explicit memory is highly flexible and involves the association of multiple bits and pieces of information. In contrast, implicit memory is more rigid and tightly connected to the original stimulus conditions under which the learning occurred.
II Depending upon permanency of storage
Depending upon permanency of storage, memory is shortterm, intermediate and long-term memory.
1. Short-term memory, also termed as primary memory, lasts for seconds to hours. Example of short-term memory includes memory of a new telephone number after calling the operator or after looking into directory. Most of the times such a telephone number is forgotten after few minutes.
2. Intermediate long-term memory (or secondary memory) lasts for days-to-weeks, but is eventually lost.
3. Long-term memory (or tertiary memory), which once stored, can be recalled years later or for a lifetime.
Mechanism (physiological and cellular or molecular basis) of memory
Studies of memory retention and disruption of memory have revealed that both explicit and implicit memory are stored in stages by different mechanisms. Input to the brain is processed into short-term memory before it is transformed through one or more stages (intermediate long-term memory) into more permanent long-term storage.
Mechanism of implicit memory
Mechanism of short-term storage of implicit memory
Mechanism for simple forms of learning results from changes in the effectiveness of synaptic transmission:
Higher intellectual functions of the prefrontal association cortex
Prefrontal cortex refers to the portion of frontal lobes in front of the motor cortex. This area, like other association areas, is better developed in man than in any other species. The functions thought to be performed by prefrontal cortex are:
• Site of working memory and intellectual functions. For details see page 473.