Aphasia is the term originally introduced by Trousseau to denote the inability to express thought by means of speech which follows certain diseases of the brain; in recent years, however, it has obtained a wider significance, and may now be defined as the loss of the faculty of interchanging thought, without any affection of the intellect or will. The interchange of thought involves, in the first place, the expression of mental processes by means of conventional symbols, and, in the second place, the comprehension of these symbols. Language, in its widest sense, includes all the methods by which thought may be communicated, and makes use of three classes of symbols—gestures, speech, and writing. The interchange of thought may be interrupted by derangement (1) of the expression of mental processes by their corresponding symbols, or (2) of the comprehension of the meaning of these symbols. In other words, there may, on the one hand, be an interference with the channels through which thought is translated into symbols; and, on the other hand, there may be a derangement of the channels by means of which symbols are interpreted as thought. In the former case, the change affects the mechanism conducting from the mental idea to the symbol, which is often termed the motor symbol process; in the latter, there is an alteration in the mechanism conducting impulses from the symbol to the mental idea, often called the sensory symbol process. There is therefore a motor and a sensory aphasia, and either of these may affect one or more of the three classes of symbols mentioned above.
In motor aphasia, there is a loss of the memory of the co-ordinated movements necessary for the formation of symbols, and this usually includes gestures, speech, and writing. The inability to write is commonly termed agraphia. The patient is unable to originate the name of any object shown to him, although he evidently knows what it is, and he cannot reply to any question although he clearly understands it. Sometimes when he is shown an object and asked if it bears a name mentioned, he may be able to show acquiescence or dissent by means of gestures, but usually even this power is absent. In sensory aphasia, there is loss of the memory of the meaning of symbols. This may affect the recollection of spoken language. The patient can hear every sound, but cannot understand a single spoken word. He can speak himself with more or less fluency, but without understanding his own words, or detecting any errors he may make, and in speaking he usually commits many mistakes; he can also read printed and written language, and he can write himself. This condition is termed word-deafness. Sensory aphasia may also be due to a loss of the recollection of written, printed, or gesture language. The sight is sufficiently good to enable the patient to distinguish objects clearly in most cases (although the extent of the field of vision is often somewhat diminished), and he can name these objects quite correctly. He is able to converse perfectly, and to write either his own thoughts or to dictation, but he cannot read what he has written, or detect any mistakes in it. He can slavishly copy printed and written words, as if he were drawing, without understanding their meaning, and he copies all mistakes. This is what is known as word-blindness.
It must never be forgotten that all these varieties of aphasia are entirely independent of any affection of the intellect. Aphasia must further be carefully distinguished from mere loss of speech caused by difficulty in articulation due to paralysis. This condition is commonly known as aphemia. Turning to the localisation of the diseases which cause the different varieties of aphasia, we find that our knowledge has recently been much extended, not only by the careful observation of clinical and pathological facts, but also by the results of experimental investigation. These two methods of inquiry are both of great use in checking their several facts. While the individual muscular acts required in the motor symbol process are ruled by centres in the spinal cord and medulla oblongata, the seat of the memory of their co-ordinated movements is in the brain. It may be well to mention here that the nerve-impulses cross from one side of the body to the other in their course from centre to periphery, and vice versa. While the majority of mankind therefore are right-handed, they are also left-brained. This greater use of the left half of the brain applies to the communication of ideas in every way. No doubt the right side takes up the work of the left, after a time, in the case of any lesion, and this is more especially the case when the affection has come on in early life. In the lower part of the frontal lobe of the brain are situated the centres concerned in the co-ordination of the muscular acts in articulation, the exact position being the third or lowest convolution, usually called Broca's convolution in honour of the distinguished man who first pointed out the connection between disease in this spot and aphasia. Any lesion in this region causes motor aphasia, and if both sides of the brain are affected, this will be complete and permanent. A little higher in the frontal lobe are the centres for gesture and for writing. The auditory perception of words has its centre in the highest convolution of the temporo-sphenoidal lobe. Disease of this part of the brain causes word-deafness. The visual perception of words depends on the integrity of the angular gyrus of the parietal lobe, and if this convolution is affected, the symptom described as word-blindness ensues. See BRAIN.