2.13 Identify the parts of the cortex that are responsible for higher forms of thought, such as language.
Association areas are made up of neurons in the cortex that are devoted to making connections between the sensory information coming into the brain and stored memories, images, and knowledge. In other words, association areas help people make sense of the incoming sensory input. Although the association areas in the occipital and temporal lobes have already been mentioned, much of the brain’s association cortex is in the frontal lobes. Furthermore, some special association areas are worth talking about in more detail.
In the left frontal lobe of most people is an area of the brain devoted to the production of speech. (In a small portion of the population, this area is in the right frontal lobe.) More specifically, this area allows a person to speak smoothly and fluently. It is called Broca’s area after nineteenth-century neurologist Paul Broca, who first provided widely-accepted clinical evidence that deficits in fluent and articulate speech result from damage to this area (Finger, 1994). Damage to Broca’s area causes a person to be unable to get words out in a smooth, connected fashion. People with this condition may know exactly what they want to say and understand what they hear others say, but they cannot control the actual production of their own words. Speech is halting and words are often mispronounced, such as saying “cot” instead of “clock” or “non” instead of “nine.” Some words may be left out entirely, such as “the” or “for.” This is called Broca’s aphasia. Aphasia refers to an inability to use or understand either written or spoken language (Goodglass et al., 2001). (Stuttering is a somewhat different problem in getting words started, rather than mispronouncing them or leaving them out, but may also be related to Broca’s area.)
In the left temporal lobe (again, in most people) is an area called Wernicke’s area, named after the physiologist and Broca’s contemporary, Carl Wernicke, who first studied problems arising from damage in this location. This area of the brain appears to be involved in understanding the meaning of words (Goodglass et al., 2001). A person with Wernicke’s aphasia would be able to speak fluently and pronounce words correctly, but the words would be the wrong ones entirely. For example, Elsie suffered a stroke to the temporal lobe, damaging this area of the brain. In the emergency room the nurse tried to take her blood pressure, and when the cuff inflated, Elsie said, “Oh, that’s so Saturday hard.” Elsie thought she was making sense. She also had trouble understanding what the people around her were saying to her.
A condition that can sometimes occur following a stroke is spatial neglect, or unilateral neglect, in which a person with damage to the right parietal and occipital lobes of the cortex will ignore everything in the left visual field. Unilateral refers to one side of the body, and this type of brain damage most often only affects attention to objects in the left visual field. Damage to areas of the frontal and temporal lobes may also play a part along with the parietal damage. Spatial neglect can affect the left hemisphere, but this condition occurs less frequently and in a much milder form than right-hemisphere neglect (Corbetta et al., 2005; Heilman et al., 1993; Springer & Deutsch, 1998).
Dr. V. S. Ramachandran reported an interesting case of spatial neglect in his book, Phantoms in the Brain (Ramachandran & Blakeslee, 1998). A woman who was typically neat and conscientious about her appearance was discovered by her son on a visit to look messy and totally odd. Her hair was uncombed on the left side. Her green shawl was hanging neatly over her right shoulder but hanging onto the floor on the left. Her lipstick was neatly applied to the right side of her lips, and only to the right side—the left side of her face was completely bare of makeup! Yet her eyeliner, mascara, and rouge were all neatly applied to the right side of her face. This woman had suffered a stroke that left her with unilateral spatial neglect.