Abnormalities of memory

 ABNORMALITIES OF MEMORY

1. Amnesia

Loss of memory is known as amnesia. Amnesia is classified into two types:

i. Anterograde amnesia: Failure to establish new long-term memories. It occurs because of lesion

in hippocampus.

ii. Retrograde amnesia: Failure to recall past remote long-term memory. It occurs in temporal

lobe syndrome.

2. Dementia

Dementia is the progressive deterioration of intellect, emotional control, social behavior and motivation

associated with loss of memory. 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.

Causes

Dementia occurs due to many reasons. Most common cause of dementia is Alzheimer disease. In about

75% of cases, dementia is due to this disease (given below). Other common causes of dementia are hydrocephalus, Huntington chorea, Parkinson disease, viral encephalitis, HIV infection, hypothyroidism, hypoparathyroidism, Cushing syndrome, alcoholic intoxication, poisoning by high dose of barbiturate, carbon monoxide, heavy metals, etc.

Clinical features

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.

3. Alzheimer Disease

Alzheimer disease is a progressive neurodegenerative disease. It is due to degeneration, loss of function and death of neurons in many parts of brain, particularly cerebral hemispheres, hippocampus and pons.

There is reduction in the synthesis of most of the neurotransmitters, especially acetylcholine. Synthesis

of acetylcholine decreases due to lack of enzyme choline acetyltransferase. Norepinephrine synthesis

decreases because of degeneration of locus ceruleus. Dementia is the common feature of this disease.

CONDITIONED REFLEXES

DEFINITION

Conditioned reflex is the acquired reflex that requires learning, memory and recall of previous experience. It is acquired after birth and it forms the basis of learning. Conditioned reflex is different from unconditioned

CLASSIFICATION OF CONDITIONED REFLEXES

Conditioned reflexes are classified into two types:

A. Classical conditioned reflexes

B. Instrumental conditioned reflexes.

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

Various types of classical conditioned reflexes and their properties are demonstrated by the classical belldog experiments (salivary secretion experiments), done by Ivan Pavlov and his associates.

In dogs, the duct of parotid gland or submandibular gland was taken outside through cheek or chin

respectively and the saliva was collected by some special apparatus. Apparatus consisted of a funnel,

which is sealed over the opening of the duct. Salivary secretion was measured in drops by means of an

electrical recorder.

TYPES AND PROPERTIES OF CLASSICAL CONDITIONED REFLEXES

Classical conditioned reflexes are classified into two groups according to the properties of reflexes, namely excitation or inhibition:

I. Positive or excitatory conditioned reflexes

II. Negative conditioned reflexes.

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 the stimuli are repeated for some days. After the development of reflex, the flash of light (conditioned stimulus) alone causes salivary secretion without food (unconditioned stimulus).

2. Secondary Conditioned Reflex

Secondary conditioned reflex is the reflex developed with one unconditioned stimulus and two conditioned stimuli. After 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 development of the reflex, bell sound (second conditioned stimulus) 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.

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

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

i. Extinction of conditioned reflex

ii. Conditioned inhibition

iii. Inhibition by delay or delayed conditioned reflex

iv. Differential inhibition.

Unconditioned reflex is the inborn reflex, which does not need previous experienced

Conditioned reflex Unconditioned reflex

 

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