Excerpts

Sample excerpt. Preview only – please do not copy

Chapter 3

The Frontal Lobes

The frontal lobes of the cerebral cortex are traditionally considered to be the seat of the “highest” mental functions and the center of those activities that make us characteristically human. This is largely because in evolutionary terms the frontal cortex has been the most recent to evolve, and humans happen to possess particularly large frontal lobes. However, it may also be that because of the relative difficulty of ascribing a clear set of functions to these areas, they have been attributed with intelligence by default. The large proportion of the cerebral cortex described as frontal lobe, up to about half of the total area of the cortex and an even higher proportion of the association cortex, makes it likely that significant aspects of intellectual activity are performed there, but, as we shall see, it is necessary to be a little more cautious about what functions we can with confidence ascribe to this region, which nonetheless remains one of the most interesting for neuropsychologists.

Some Methodological Issues

Before considering just what aspects of intelligence may be associated with the frontal lobes, some points must be made about the specific problems that arise in carrying out research studies on the effects of focal damage to the cerebral cortex. These problems appear because we have to work with clinical material, which does not arise in a random way, and the points made here therefore apply not only to this chapter, but to all the chapters in this section on clinical studies. The logic of the research design is to collect cases in which there is an identified lesion of some area, let us say in the frontal lobes, and to compare the performance of these patients with the performance of patients who have lesions in areas outside the frontal lobes. This determines whether the functions being studied are affected only by frontal lesions. However, the essential point is that we have to control in some way for all the factors apart from the site of the damage that could contribute to any deficit observed in performance. These other factors include the type of lesion: what caused it, whether it is developing (“progressive”) or stable (“static”), and whether it was recently caused (“acute”) or is long-standing (“chronic”). For example, tumors are usually progressive, and may develop slowly or rapidly depending on type, while a gunshot wound can be considered, after the initial period following the injury, to be static. The age of the patient is also important, as is the extent or “mass” of the lesion and how far it extends below the cortex into subcortical tissue.

The main problem is that lesions of different types tend to occur in different areas, and in patients of different ages. Tumors of certain types grow in particular sorts of tissue, but may be fairly evenly distributed across age groups, while missile wounds obviously occur most frequently in young males injured during war or urban violence. Vascular accidents, in which either the blood supply to some region of the cortex is lost (as in a stroke) or some failure results in bleeding into the brain, tend to occur more commonly in older subjects. Studies that compare lesions of the frontal and parietal regions without controlling for the type of lesion may then end up by confounding the site of the lesion with its cause.

Even if the study is restricted to a comparison of lesions of one particular type, for example those caused by gunshot wounds, the lesions occurring at less usual sites may be in some way atypical. Wounds from modern high-velocity projectiles yield perhaps the best clinical material for the neuropsychologist, for the bullet, if not at close range, tends to punch a very neat hole straight through the head, causing remarkably little disturbance to regions not immediately affected, and producing a clean wound that is self-sterilized and cauterized by the heat generated as the bullet passes through. In such cases, the important issue for survival is whether the bullet passes through important central subcortical centers essential to life or fundamental aspects of behavior. If the entry and exit points are around the temporal and parietal regions, death is much more likely than if they are in the frontal and occipital regions. As a result, more soldiers arrive for neuropsychological assessment with frontal or occipital wounds than with temporal and parietal wounds, and the lesions of those with temporal and parietal injuries who do sur- vive may be less extensive than those of their colleagues and, in a variety of ways, less serious.

An alternative example is studies that examine differences between the left and right members of a particular pair of lobes. Here the confounded variable may be the mass of the lesion. Someone with a developing tumor in the left or right frontal lobe will sooner or later notice some of its effects and will probably consult his or her general practitioner (GP). However, because of the much greater importance of verbal as opposed to spatial abilities in everyday life in our society, these patient are more likely to notice that they cannot remember the contents of the day’s paper or an address just given to them, than that they cannot remember some drawing or route to be taken to a particular place. Since the failure in verbal memory usually results from a left lesion and in spatial memory from a right lesion, patients typically arrive for surgery with smaller tumors in the left than in the right hemisphere, where they have been allowed to grow unnoticed for longer. This can naturally confound the results of any study that compares the effects of tumors in the left and right sides of the head, because any differences found may not be due to the lateral site of the tumor but due to the mass of the lesion. These examples illustrate the considerable difficulty of constructing sound scientific studies when it is necessary to work with incidentally occurring clinical material. The ideal study would involve equal amounts of the same kind of damage occurring in each cortical area, but the data are just not available for such a study. There are additional problems in that it is often assumed that the deficits observed are a reflection of more specific deficits in complex tasks that involve several basic unitary functions in their performance. The factors that contribute to methodological difficulties are summarized in Table 3.1.

It should also be realized that studies of the highest methodological standard are rather uncommon, owing to deficiencies in design and theoretical interpretation, and that many of the findings reported below are subject to difficulties of interpretation that follow from research problems of the type just described.

Intelligence

From the latter part of the 19th century the frontal lobes have been associated with intelligent abilities, but a controversy raged through much of the 20th century as to whether these abilities may be associated exclusively with the frontal lobes. It may simply be that the frontal lobes are large, subserve many functions, and are as a result likely to affect “intelligent” behavior more than other lobes of the brain. Alternatively, there may be some general factors such as attention, or motivation, associated with the frontal lobes that have an impact upon all “intelligent” tasks. (Many psychologists would in any case say that “intelligence” is no more than the abilities that determine performance on intelligence tests.) To evaluate the arguments presented in this controversy, it is important to distinguish between quantitative and qualitative changes in intelligence. In terms of quantitative deficits in intelligence, case reports from the beginning of the 20th century reported reduced intelligence following frontal lesions, and these findings were largely confirmed by the first important research studies by Rylander in 1939 and Halstead in 1940. The finding was simply that measured general intelligence was reduced after damage to the frontal lobes. The view was expressed most clearly in Halstead’s description of “biological intelligence” in 1947. He had formulated this concept from the results of a statistical analysis of a battery of tests that had been administered to a large sample of subjects with various focal cortical lesions. Among these tests, and showing the highest “loading” on biological intelligence, was the Category Test, which is a test of concept formation or categorization in which sets of graphical items are presented, and the patient has to indicate which of the numbers 1 to 4 may be associated with the set from the other three (see Figure 3.1). Patients with frontal lobe damage do badly on this test.

Although Halstead’s theory commanded much support through the 1940s and 1950s, it was criticized by Hebb, who, largely by studying the effects of deliberately placed experimental lesions in animals on abilities such as maze learning, argued that the mass of the lesion was more significant than its location. This view was confirmed in 1959 by Chapman and Wolff.