Higher Intellectual Functions Notes

Higher Intellectual Functions

Higher intellectual functions are very essential to make up the human mind. These functions are also called higher brain functions or higher cortical functions. Learning and memory form the principal higher intellectual functions. Other higher functions are conditioned reflexes and language (speech). The extensive outer layer of gray matter in the cerebral cortex is responsible for higher intellectual functions.

Read And Learn More: Medical Physiology Notes

Learning

1. Learning Definition:

Learning is defined as the process by which new information is acquired. It alters the behavior of the person on the basis of past experience. And memory is the ability to recall that information or any other past experience. Both learning and memory are closely related. And, both are to be considered together.

2. Types Of Learning: Learning is of two types:

1. Non-associative learning
2. Associative learning.

1. Non-associative Learning: It involves the response of a person to only one type of stimulus. It is based on two factors:

1. Habituation
2. Sensitization.

1. Habituation:

• Habituation means getting used to something to which a person is constantly exposed. When a person is exposed to a stimulus repeatedly, he starts ignoring the stimulus slowly.
• During the first experience, the event (stimulus) is novel and evokes a response. However, it evokes less response when it is repeated. Finally, the person is habituated to the event (stimulus) and ignores it.

2. Sensitization:

• Sensitization means a state in which the body is made to become more sensitive to a stimulus. It is called the amplification of response. When a stimulus is applied repeatedly, habituation occurs. But if the same stimulus is combined with another type of stimulus, which may be pleasant or unpleasant, the person becomes more sensitive to the original stimulus.
• Sensitization is defined as an increase in response to an innocuous stimulus when that stimulus is applied after another type of stimulus. For example, a woman gets habituated to different sounds around her, and sleep is not disturbed by these sounds. However, she suddenly wakes up when her baby cries because she is sensitized to the crying sound of her baby.

2. Associative Learning: It is a complex process. It involves learning about relations between two or more stimuli at a time. The classic example of associative learning is the conditioned reflex, which is described later in this chapter.

Memory

1. Memory Definition:

Memory is defined as the ability to recall the past experience. It is also defined as the retention of learned materials. There are various degrees of memory. Some memories remain only for a few seconds, while others last for hours, days, months or years together.

2. Types Of Memory:

Memory is classified by different methods on the basis of various factors. Physiologically, memory is classified into two types:

1. Explicit memory
2. Implicit memory.

1. Explicit Memory:

• Explicit memory is otherwise known as declarative memory or recognition memory. It is defined as a memory that involves conscious recollection of past experience. It consists of memories regarding the events which occurred in the external world around us.
• The information stored may be about a particular event that happened at a particular time and place. Examples: Recollection of a birthday party celebrated three days ago; the events taking place while taking breakfast, etc.
• Explicit memory involves the hippocampus and the medial part of the temporal lobe.

2. Implicit Memory:

• Implicit memory is otherwise known as non-declarative memory ot skilled memory. It is defined as the memory in which pos* experience is utilized without conscious awareness. It helps to perform various skilled activities properties- For example, cycling, driving, playing tennis, dancing, typing, etc. are performed automatically without awareness.
• Implicit memory involves the sensory and motor pathways.
• Generally, memory is also classified as:

1. Short term memory
2. Long term memory

1. Short-Term Memory:

• Short-term memory is recalling the events that happened very recently, i.e. within hours or days. It is also known as recent memory. For example, the telephone number that is known today may be remembered till tomorrow. If it is not recalled repeatedly, it may be forgotten on the third day.
• Short-term memory may be interrupted by many factors such as stress, trauma, drug abuse, etc. There is another form of short-term memory called working memory. It is concerned with the recollection of past experience for a very short period on the basis of which an action is executed.

2. Long-Term Memory: It is otherwise called remote memory. It is the recalling of the events of weeks, months, years or sometimes lifetime. Examples are recalling the first day of schooling, the birthday celebration of the previous year, the picnic enjoyed last week, etc. Long-term memory is more resistant and is not disrupted easily.

• Depending upon the duration, memory is classified into three types:

1. Sensory memory
2. Primary memory
3. Secondary memory.

1. Sensory Memory:

• It is the ability to retain sensory signals in the sensory areas of the brain. But, the sensory signals are retained only for a short period after the actual sensory experience, i.e. few hundred milliseconds.
• But, the signals are replaced by new sensory signals in less than one second. It is the initial stage of memory. It. resembles working memory.

2. Primary Memory:

• Primary memory is the memory of facts, words, numbers, letters or other information recalled for a few seconds to a few minutes at a time. For example, after searching and finding a telephone number in the directory, we remember the number for a short while. After appreciating the beautiful scenery, the details of it could be recalled for some time or days. Afterward, it disappears from memory.
• The characteristic feature of this type of memory is that the information is available for recall easily from the memory store itself. One need not search or squeeze through the mind but, this memory is easily replaced by new bits of memory, i.e. by looking into another telephone number, the first one may disappear. It resembles long-term memory.

3. Secondary Memory: Secondary memory is the storage of information in the brain for a longer period. The information could be recalled after hours, days, months or years. It is also called fixed memory or permanent memory. It resembles long-term memory.

3. Anatomical Basis Of Memory:

• The anatomical basis of memory is the synapse in the brain. Synapse for memory coding is slightly different from other synapses. Two separate presynaptic terminals are present here.
• One of the terminals is the primary presynaptic terminal which ends on postsynaptic neurons as in conventional synapses. This terminal is called the sensory terminal because the sensations are transmitted to the postsynaptic neuron through this terminal.

• The other presynaptic terminal ends on the sensory terminal itself. The second terminal is called the facilitator terminal.
• When the sensory terminal is stimulated alone without the fadfMur terminal, the firing from the sensory terminal leads to action; that is, the firing decreases slowly. On the other hand, if both the terminals are stimulated, facilitation occurs and the signals remain strong for a long period, i.e. for a few months to a few years.

4. Physiological Basis Of Memory: Memory is stored in the brain by the alteration of synaptic transmission between the neurons involved in memory. Storage of memory may be facilitated or habituated depending upon many factors such as neurotransmitters, synaptic transmission, functional status of the brain, etc.

• Facilitation:
  • It is the process by which the memory storage is enhanced. It involves an increase in synaptic transmission and increased postsynaptic activity. Often facilitation is referred as positive memory.
  • The process involved in the facilitation of memory is called memory sensitization.
• Habituation: It is the process by which the memory storage is attenuated (attenuation = decrease in strength, effect or value). It involves a reduction in synaptic transmission and a slow stoppage of postsynaptic activity. Sometimes habituation is referred to as negative memory.
• The basis for Short-Term Memory: Basic mechanism of memory is the development of new neuronal circuits by the formation of new synapses and facilitation of synaptic transmission. The number of presynaptic terminals and the size of the terminals are also increased. This forms the basis of short-term memory.
• The basis for Long-Term Memory: When the neuronal circuit is reinforced by constant activity, the memory is consolidated and encoded into different areas of the brain. This encoding makes memory a permanent or long-term memory.
• Sites of Encoding: Hippocampus and the Papez circuit (the closed circuit between the hippocampus, thalamus, hypothalamus, and corpus striatum) are the main sites for memory encoding. Frontal and parietal areas are also involved in memory storage.
• Experimental Studies of Memory – Aplysia:
  • Most of the experimental studies of memory and learning are based on research carried out in the sea hare (sea snail) called Aplysia.
  • This animal is useful in brain research because it has a simple uncomplicated nervous system that can be easily approached in living animals with simple dissection. Another advantage of this snail is that the individual nerve cells are large and brightly colored.
  • Nobel laureate Eric Kandel was the pioneer to use aplysia for the studies of memory and learning.

5. Chemical Or Molecular Basis Of Memory:

• Memory Engram: The molecular basis of memory can be explained by memory engram. Memory engram is a process by which memory is facilitated and stored in the brain by means of structural and biochemical changes. Often it is also called a memory trace.
• Molecular Basis of Facilitation:
• The molecular mechanism of facilitation is given in. In this process, the neurotransmitter serotonin plays a major role. The calcium ions increase the release of serotonin which facilitates synaptic transmission to a great extent leading to memory storage.

• Molecular Basis of Habituation: Habituation is due to the passive closure of calcium channels of the terminal membrane. Hence, the release of the transmitter decreases resulting in a decrease in the number of action potentials in the postsynaptic neuron. So, the signals become weak. The weakening of the signals leads to habituation.

6. Consolidation Of Memory:

• The process by which short-term memory is crystallized into long-term memory is called memory consolidation. Consolidation causes permanent facilitation of synapses.
• It is possible by rehearsal mechanism, i.e. rehearsal of the same information again and again accelerates and potentiates the degree of transfer of short-term memory into long-term memory. This is what happens in memorizing a poem or a phrase.

7. Drugs Facilitating Memory: Several stimulants for the central nervous system are shown to improve learning and memory in animals. The common stimulants are caffeine, physostigmine, amphetamine, nicotine, strychnine, and omeprazole. All the substances mentioned above facilitate the consolidation of memory.

8. Applied Physiology-Abnormalities Of Memory

1. Amnesia:

• Loss of memory is known as amnesia. Amnesia is classified into two types:
  • Anterograde amnesia: It is the failure to establish new long-term memories. It occurs because of the lesion in the hippocampus.
  • Retrograde amnesia: Failure to recall past remote long-term memory is called retrograde amnesia. It occurs in temporal lobe syndrome.

2. Dementia: Progressive deterioration of intellect, emotional control, social behavior, and motivation associated with loss of memory is known as dementia. It is an age-related disorder. Usually, it occurs above the age of 65 years. When it occurs under the age of 65, It Is called presenile dementia.

• Dementia Causes:
  • Dementia occurs due to many reasons. The most common cause of dementia is Alzheimer’s disease. In about 75% of cases, dementia is due to this disease (given below).
  • Other common causes of dementia are hydrocephalus, Huntington’s chorea, Parkinson’s disease, viral encephalitis, HIV infection, hypothyroidism, hypoparathyroidism, Cushing’s syndrome, alcoholic intoxication, poisoning by a high dose of barbiturate, carbon monoxide, heavy metals, etc.
• Dementia Clinical features:
  • The common features are loss of recent memory, lack of thinking and judgment, and personality changes. As the disease progresses, psychiatric features begin to appear. Motor functions are also affected.
  • Finally, the patient has to lead a vegetative life without any thinking power. The person is speechless and is unable to understand anything. There is no effective treatment for this disorder. Physostigmine, which inhibits cholinesterase, causes moderate improvement.
• Alzheimer’s Disease:
  • It is a progressive neurodegenerative disease. It is due to degeneration, loss of function, and death of neurons in many parts of the brain, particularly the cerebral hemispheres, hippocampus, and pons.
  • There is a reduction in the synthesis of most of the neuro¬transmitters, especially acetylcholine. The synthesis of acetylcholine decreases because of the lack of the enzyme choline acetyltransferase. Norepinephrine synthesis decreases because of the degeneration of the locus ceruleus. Dementia is a common feature of this disease.

Conditioned Reflexes

1. Conditioned Reflexes Definition:

A conditioned reflex is a reflex response acquired or learned by experience. A conditioned reflex is the basis of learning. The unconditioned reflex is the inborn reflex. There is no need of previous experience for this reflex. An example is the salivary secretion by placing the food in the mouth. But, the conditioned reflex is acquired after birth and it requires learning, memory, and recall of previous experience.

2. Types Of Conditioned Reflexes: The conditioned reflexes are of two types:

1. Classically conditioned reflexes
2. Instrumental conditioned reflexes.

3. Classical Conditioned Reflexes: Classical conditioned reflexes are those reflexes, which are established by a conditioned stimulus followed by an unconditioned stimulus.

• Method of Study — Pavlov’s Bell-Dog Experiments:
  • The various types of classical conditioned reflexes and their properties are demonstrated by the classical bell-dog experiments (salivary secretion experiments) done by Ivan Pavlov and his associates.
  • In dogs, the duct of the parotid gland or submandibular gland was taken outside through the cheek or chin respectively and the saliva was collected by some special apparatus. The apparatus consisted of a funnel, which is sealed over the opening of the duct. The salivary secretion was measured in drops by means of an electrical recorder.

4. Types And Properties Of Classical Conditioned Reflexes: Classically conditioned reflexes are divided into two groups:

1. Positive or excitatory conditioned reflexes
2. Negative conditioned reflexes.

This classification is according to the properties of the reflexes namely, the excitation or inhibition.

5. Positive Conditioned Reflexes (Excitation Of Conditioned Reflexes): Types of positive conditioned reflexes:

1. Primary conditioned reflex
2. Secondary conditioned reflex
3. Tertiary conditioned reflex.

1. Primary Conditioned Reflex:

• Primary conditioned reflex is the reflex developed with one unconditioned stimulus and one conditioned stimulus. This reflex is established in the following way. The animal is fed with food (unconditioned stimulus).
• Simultaneously a flash of light (conditioned stimulus) is also shown. Both stimuli are repeated for some days. After the development of the reflex, the flash of light (conditioned stimulus) alone causes salivary secretion without food (unconditioned stimulus).

2. Secondary Conditioned Reflex:

• The secondary conditioned reflex is the reflex developed with one unconditioned stimulus and two conditioned stimuli. After the establishment of a conditioned reflex with one conditioned stimulus, another conditioned stimulus is applied along with the first one.
• For example, the animal is fed with food (unconditioned reflex), and simultaneously a flash of light (first conditioned stimulus), and a bell sound (second conditioned stimulus) are applied. After the development of the reflex, the second conditioned stimulus-the bell sound alone can cause salivary secretion.

3. Tertiary Conditioned Reflex: In this reflex, a third conditioned stimulus is added and, the reflex is established. But, the reflex with more than three conditioned stimuli is not possible. Many types of conditioned stimuli associated with sight and hearing were employed by Pavlov.

6. Negative Conditioned Reflexes (Inhibition Of Conditioned Reflexes): The established conditioned reflexes can be inhibited by some factors. The inhibition is of two types:

1. External or indirect inhibition
2. Internal or direct inhibition.

1. External or Indirect Inhibition:

• The established conditioned reflex is inhibited by some form of stimulus, which is quite different from the conditioned stimulus. It is not related to the conditioned stimulus.
• For example, some disturbing factors like the sudden entrance of a stranger, sudden noise, or a strong smell can abolish the conditioned reflex and inhibit salivary secretion. The extra stimulus evokes the animal’s curiosity and distracts its attention. According to Paviov, it evokes an investigatory reflex. If the extra (inhibitory) stimulus is repeated for some time, its iriiNNiory effect gets weakened or abolished.

2. internal or Direct Inhibition: There are four ways in which, the established conditioned reflex is abolished by direct or internal factors, which are related to the conditioned stimulus.

1. Extinction of conditioned reflex
2. Conditioned inhibition
3. Delayed conditioned reflex or inhibition by delay
4. Differential inhibition.

1. Extinction of conditioned reflex:

• Extinction is the failure of conditioned reflexes. It occurs if the established conditioned reflex is not reinforced by an unconditioned stimulus. After establishing a conditioned reflex, the conditioned stimulus must be coupled with an unconditioned stimulus now and then, i.e. the conditioned stimulus must be reinforced by an unconditioned stimulus.
• If a conditioned stimulus is given repeatedly several times without reinforcing it by an unconditioned stimulus, there is the failure of the conditioned reflex. However, the reflex is not abolished if the unconditioned reflex is also used in between.

2. Conditioned inhibition:

• Conditioned inhibition is the failure of a conditioned reflex due to the introduction of an unknown (new) conditioned reflex. When a conditioned stimulus like a flash of light is effective, if another conditioned stimulus like a bell sound is applied along with this stimulus suddenly, the response does not occur.
• Of course, if these two conditioned stimuli are given with unconditioned stimulus (food) repeatedly, the secondary conditioned reflex is developed.

3. Inhibition by delay or delayed conditioned reflex:

• It is the absence of a response or delayed response that occurs while eliciting a conditioned reflex by delaying the unconditioned reflex. While establishing a conditioned reflex, the conditioned stimulus (light or sound) must be followed by an unconditioned stimulus (food) immediately.
• If the unconditioned stimulus is applied after a long period, the response may be absent or delayed. The reflex is called delayed conditioned reflex.

4. Differential inhibition:

• Differential inhibition is the failure of a conditioned reflex that occurs when the conditioned stimulus is altered. When an animal is trained or conditioned for a particular type of conditioned stimulus, and if this stimulus is altered even slightly, the response does not occur.
• The animal is able to discriminate the difference. For example, the alteration in the frequency of sound or intensity of light abolishes the conditioned reflex.

7. Instrumental Or Operant Conditioned Reflexes:

• The instrumental conditioned reflexes are those reflexes in which the behavior of the person is instrumental. This type of reflex is developed by the conditioned stimulus followed by a reward or punishment. The instrumental conditioned reflexes are also called operant conditioned reflexes or Skinner conditioning.
• During the development of this type of reflexes, the animal is taught to perform some task in order to obtain a reward or to avoid punishment. Accordingly, the instrumental conditioned reflexes are of several types such as:

1. Conditioned avoidance reflex
2. Food avoidance reflex
3. Conditioned reward reflex

1. Conditioned Avoidance Reflex: It is the reflex by which the animal is trained to avoid an electric shock by pressing a bar.

2. Food Avoidance Conditioning: If the animal is given tasty food along with the injection of a drug, which produces nausea or sickness, the animal starts avoiding or hating that food. It is called food aversion conditioning.

3. Conditioned Reward Reflex: If the animal is rewarded by a banana by pressing a bar, the animal repeatedly presses the bar. It is the conditioned reward reflex. The instrumental conditioned reflexes play an important role during the learning processes of a child. These conditioned reflexes are also responsible for the behavior pattern of an individual.

8. Physiological Basis Of Conditioned Reflexes: Learning and memory form the physiological basis of the conditioned reflexes.

Speech

1. Speech Definition:

• Speech is defined as the expression of thoughts by the production of articulate sound, bearing a definite meaning. It is one of the highest functions of the brain. It is brought about by the coordinated activity of different parts of the brain, particularly the motor, sensory and psychic areas.
• When a sound is produced verbally, it is called the speech. If it is expressed by visual symbols, it is known as writing. If visual symbols or written words are expressed verbally, that becomes reading.

2. Mechanism Of Speech:

• Speech depends upon the coordinated activities of central speech apparatus and peripheral speech apparatus. The central speech apparatus consists of higher centers, i.e. the cortical and subcortical centers.
• The peripheral speech apparatus includes the larynx or sound box, pharynx, mouth, nasal cavities, tongue, and lips. All the structures of the peripheral speech apparatus function in coordination with respiratory system with the influences of the motor impulses from the respective motor areas of the cerebral cortex.

3. Development Of Speech:

• First Stage: The first stage in the development of speech is the association of certain words with visual, tactile, auditory, and other sensations, aroused by objects in the external world. The association of words with other sensations is stored as memory.
• Second Stage:
  • New neuronal circuits are established during the development of speech. When a definite meaning has been attached to certain words, the pathway between the auditory area (Heschl area – 41) and motor area for the muscles of articulation which helps in speech (Broca’s area 44) is established. And, the child attempts to formulate and pronounce the learned words.
  • The development of speech involves the integration of three important areas of the cerebral cortex namely, Wernicke’s area, Broca’s area, and the motor area of the dominant hemisphere.
    • Role of Wernicke’s area – Speech understanding:
      • The understanding of speech begins in Wernicke’s area which is situated in the upper part of the temporal lobe. It sends fibers to Broca’s area through a tract called the arcuate fasciculus.
      • Wernicke’s area is responsible for understanding the visual and auditory information required for the production of words. After understanding the words it sends the information to Broca’s area.
    • Role of Broca’s area – Speech synthesis:
      • Speech is synthesized in Broca’s area. It is situated adjacent to the motor area responsible for the movements of the tongue, lips, and larynx which are necessary for speech.
      • By receiving information required for the production of words from Wernicke’s area, Broca’s area develops the pattern of motor activities required to verbalize the words. The pattern of motor activities is sent to the motor area.
    • Role of the motor area – Activation of peripheral speech apparatus:
      • By receiving the pattern of activities from Broca’s area, the motor area activates the peripheral speech apparatus. It results in the initiation of movements of the tongue, lips, and larynx required for speech.
      • Later, when the child is taught to read, auditory speech is associated with visual symbols. Then, there is an association of the auditory and visual areas with the motor area for the muscles of the hand. Now, the child is able to express auditory and visual impressions in the form of written words.

4. Nervous Control Of Speech:

• Speech is an integrated and well-coordinated motor phenomenon. So, many parts of cortical and subcortical areas are involved in the mechanism of speech.
• The subcortical areas concerned with speech are controlled by cortical areas of the dominant hemisphere. In about 95% of human beings the left cerebral hemisphere is functionally dominant and those persons are right-handed. Following are the motor and sensory cortical areas concerned with speech.

1. Motor Areas:

• Broca’s area: It is area 44. It is also called the speech center, motor speech area or lower frontal area. It is situated in the lower part of the lateral surface of the prefrontal cortex. This area controls the movements of structures concerned with vocalization. So, this area is known as a speech center.
• Upper frontal motor area: It is situated in the paracentral gyrus over the medial surface of the cerebral hemisphere. It controls the coordinated movements concerned with writing.

2. Sensory Areas:

• Auditopsychic area: Auditopsychic area is situated in the superior temporal gyrus. It is concerned with the storage of memories of spoken words.
• Visuopsychic area: It is present in the angular gyrus of the parietal cortex. It is concerned with the storage of memories of the visual symbols.

3. Wernicke’s Area: This area is situated in the upper part of the temporal lobe. It also plays an important role in speech. It is responsible for understanding the auditory and visual information about any word and sending the information to Broca’s area.

5. Applied Physiology – Disorders Of Speech:

The speech disorder is a communication disorder characterized by disrupted speech. It is of four types:

1. Aphasia
2. Anarthria or Dysarthria
3. Dysphonia
4. Stammering.

6.  Aphasia:

• Aphasia is defined as the loss or impairment of speech due to brain damage (in Greek aphasia = without speech). It is an acquired disorder and it is distinct from developmental disorders of speech or other speech disorders like dysarthria. Aphasia is not due to paralysis of muscles of articulation. It is due to damage of speech centers.
• Damage to the speech centers impairs the expression and understanding of spoken words. It also affects reading and writing. The speech function is localized to the left hemisphere in most of people.
• Aphasia may be associated with other speech disorders which also occur due to brain damage.
  • Causes for Aphasia: Usually aphasia occurs due to damage of one or more speech centers which are situated in the cerebral cortex. Damage of speech centers occurs due to:
    • Stroke
    • Head injury
    • A severe blow to the head
    • Cerebral tumors
    • Brain infections
    • Degenerative diseases.

Usually, in conditions like head injury aphasia occurs suddenly. And, in conditions like infections or cerebral tumors, it develops slowly. In children, traumatic aphasia can develop by exposure to a horrifying event without any brain damage. It may be cured with psychological treatment.

• • Types of Aphasia: Aphasia is classified by different methods. The simple and convenient clinical classification divides aphasia into five types:

1. 1. 1. Broca’s aphasia
      2. Wernicke’s aphasia
      3. Global aphasia
      4. Nominal aphasia
      5. Other types of aphasia

1. Broca’s aphasia:

• Broca’s aphasia is a nonfluent speech problem. It occurs due to damage of the left frontal lobe of the cerebral cortex. It is also known as expressive aphasia or anterior aphasia.
• The affected persons do not complete the sentences because of their inability to construct the sentences. They often talk in short phrases by omitting small words such as ‘and’, ‘is’, ‘for’ etc. They make great efforts even to initiate speech.
• Persons with Broca’s aphasia are able to understand spoken or written words. Often they are affected by weakness or paralysis of the right arm or leg. It is due to damage of the frontal lobe which is also responsible for motor activities.

2. Wernicke’s aphasia:

• Wernickes’ aphasia is speech without any meaning, it is also called receptive aphasia or posterior aphasia. Wernickes’ aphasia occurs due to damage of the left temporal lobe. It is characterized by fluent speech. Toe-affected persons speak long sentences but without any meaning.
• They use incorrect or nonexistent words and cannot speak sensibly. This type of speech is known as jargon speech. These individuals are unable to understand others’ speech. Because of this weakness, they are unaware of their own mistakes while speaking. Often they are mistaken as psychiatric patients.
• Wernicke’s aphasia is not associated with paralysis or weakness of muscles because the injury does not involve the centers concerned with movements.

3. Global aphasia:

• It is the type of aphasia characterized by combined features of Broca’s aphasia and Wernicke’s aphasia. It is due to widespread damage of speech areas of the brain.
• It is the most common type of aphasia that develops due to infarction of the left cerebral hemisphere. The affected persons can neither speak nor understand the spoken words. They cannot read and write also. So they have severe communication problems.

4. Nominal aphasia: Nominal aphasia is a speech disorder characterized by the inability in naming familiar objects. It is also called anomic aphasia or amnestic aphasia. It is due to damage of the posterior temporal and inferior parietal gyri.

5. Other types of aphasia:

1. Motor aphasia: It is a speech disorder caused by a defect in the pathway between the left speech center and the excitomotor cortex. It is also known as verbal aphasia or dyspraxia or apraxia of speech. It is characterized by difficulty in uttering individual words due to a lack of coordination between the central speech apparatus (higher cortical centers) and peripheral speech apparatus. The affected persons are able to decide what to talk. But they cannot pronounce all the words. They are able to pronounce only a few monosyllables such as ‘yes’ or ‘no’.

2. Sensory aphasia: It is the inability to understand words or symbols. It is of two types:

• Auditory aphasia: It is the inability to understand spoken words. It is also called the word deafness. It is due to the lesion in the auditopsychic area.
• Visual aphasia: It is the difficulty in understanding written symbols (difficulty in reading). It is also called word blindness or dyslexia and it occurs due to a lesion in the visual psychic area.

3. Agraphia: Agraphia means inability to write. There is no defect in the muscles of the hand concerned with writing. The subject can read and speak. Agraphia is due to a defect in the connection between the writing center and the excitomotor center.

• Head’s Classification of Aphasia: Henry Head was the pioneer scientist in the field of speech disorders and he was the first one to classify aphasia. In 1926, he classified aphasia into four types:

1. 1. Verbal aphasia: Disability in the formation of words
   2. Syntactical aphasia: Inability to arrange words in proper sequence
   3. Semantic aphasia: Inability to recognize the significance of words
   4. Nominal aphasia: Difficulty in naming the object due to failure in recognizing the meaning of words.

7. Dysarthria Or Anarthria: The term dysarthria refers to disturbed articulation. Anarthria means inability to speak. Dysarthria or anarthria is defined as the difficulty or inability to speak because of paralysis or ataxia of muscles involved in articulation. The psychic aspect of speech is not affected. The spoken and written words are understood.

• Causes of Dysarthria: Dysarthria is caused by damage of the brain or the nerves that control muscles involved in speech. It occurs in conditions like stroke, brain injury, and degenerative diseases like Parkinson’s disease and Huntington’s disease.

8. Dysphonia: Dysphonia is a voice disorder. Often it is characterized by hoarseness and a sore or dry throat. Hoarseness means difficulty in producing sound while trying to speak or a change in the pitch or loudness of voice. The voice may be weak, breathy, scratchy or husky. Dysphonia occurs due to:

• Traurna of vocal cords
• Paralysis of vocal cords
• Lumps (nodules) on the vocal cord
• Inflammation of larynx
• Hypothyroidism
• Stress – psychological dysphonia

9. Stammering

• Stammering or shuttering is a speech disorder characterized by hesitations and involuntary repetitions of certain syllables or words. It is also described as a speech disorder in which the normal flow of speech is disturbed by repetitions, prolongations or abnormal block or stoppage of sound and syllables. It is due to the neurological incoordination of speech and it is common in children.
• Stammering is associated with some unusual facial and body movements. The exact cause for stammering is not known. It is thought that stammering may be due to genetic factors, brain damage, neurological disorders or anxiety.