William G. Webster

TERRITORIALITY AND THE EVOLUTION OF BRAIN ASYMMETRY

When I was first invited t o contribute t o this conference, its very title seemed t o imply an opportunity t o engage in some theorizing and speculation. Accordingly, 1 decided from the outset t o go onto the proverbial limb and t o share some thoughts about the evolutionary origins of brain lateralization. This sort of exercise can on occasion be useful in generating and making explicit conceptual frameworks within which one can consider brain asymmetry in animals and man. One can approach the problem of brain asymmetry in animals from at least two different perspectives. First, one can regard it from a human perspective and ask what sorts of parallels or precursors might one expect to find in animals, given what we suspect about the nature of complementary hemispheric specialization in man. Despite a long history assuming that brain lateralization and language are somehow closely linked together, it is now clear that there are processes or functions other than speech and language that are lateralized in the human brain, and that animals might share with man some of these dimensions. As well, an argument might be made for the position that what is lateralized in the case of the apparently uniquely human behavior of language are underlying mechanisms or modes of information processing, rather than the behaviors them; again, these underlying mechanisms or processes could be lateralized in nonhumans, although manifest in behaviors other than language4 A second approach is t o look at the phenomenon of brain lateralization from a somewhat wider perspective-the biological-and to ask just what biological significance brain lateralization might have, and what sorts of selective pressures might have favored its evolution. It is this latter approach I would like t o adopt in this paper. For purposes of stimulating discussion, I would like t o argue that the biological significance of brain lateralization may be found in problems related t o the analysis and memory of spatial position, and perhaps t o territoriality. I would like to suggest (or really t o reiterate a suggestion made by others5) that brain asymmetry represents a mechanism that facilitates spatial orientation and the memory of spatial position. Furthermore, I would like t o suggest that brain asymmetry evolved because this facilitation in the analysis and memory of spatial location can facilitate reproduction either directly, in the sense of helping t o find a mate, or indirectly, through its association with the definition and defense of territory, upon which successful mating is predicated in so many species.

Speech-Motor Control and Interhemispheric Relations in Recovered and Persistent Stuttering

The neurological basis of stuttering is associated with anomalies of interhemispheric relations and of the neural mechanisms of speech-motor control, specifically those involving the supplementary motor area (SMA). Stuttering typically develops through childhood and adolescence but many children will recover without formal treatment or intervention. The hypothesis that such spontaneous recovery is related to a maturation of the SMA is explored. Four experimental tasks were performed by adults whose stuttering has persisted, adults who reported having stuttered as children, and a control group of adults who reported never having stuttered. A Sequence Reproduction Finger Tapping task (Webster, 1986) and a Bimanual Crank Turning task (Preilowski, 1972) examined the functioning of the SMA, and 2 Divided Visual Field tasks examined asymmetries of hemispheric activation. The overall pattern of results for persistent stutterers compared to nonstutterers was consistent with motor-perceptual anomalies previously reported in the literature. The Bimanual Crank Turning task revealed additionally that the bimanual coordination deficits reported in adults who stutter are kinesthetically based and mediated through anterior callosal systems, including the SMA. Ex-stutterers were similar to nonstutterers in their performance of the motor control tasks, but similar to persistent stutterers in perceptual asymmetries associated with Divided Visual Field tasks. Taken together, the results from the four experimental tasks support the general hypothesis that an anomaly in interhemispheric relations and a deficit in the mechanisms of speech-motor control are each a necessary but not sufficient condition for stuttering and that recovery from childhood stuttering reflects a maturation of the mechanisms of speech-motor control.

Assumptions, conceptualizations, and the search for the functions of the brain

The conditions under which methods of direct neurological manipulation may be of utility in the study of brain-behavior relations and of cerebral localization of function are discussed, with attention being directed in particular to the conflict of positions represented by R. L. Gregory and L. Weiskrantz. The argument is made that such methods are potentially useful only if the nervous system or particular portions of the nervous system can be conceptualized in terms of being a parallel, rather than a serial information processing system. It is further argued that inferences made on the basis of results generated with such methods (and indeed any method) are dependent upon the assumptions and conceptualizations concerning brain function held by the E.

Neural mechanisms underlying stuttering: evidence from bimanual handwriting performance.

Left- and right-handed male and female stutterers were compared with fluent speakers on a bimanual handwriting task. On each trial four words were read to the subject. After repeating the words, subjects had to write the initial letters as quickly as possible using the two hands simultaneously and without visual guidance. As a group, stutterers were slower, made more mirror-reversed letters, and formed letters of poorer quality than fluent speakers. The effects were the same for males and females, and the data for left- and right-handers were mirror-symmetric with respect to left and right hands. Evidence was found for two subgroups of stutterers with respect to scores on the dependent variables. It was suggested that the overall pattern of results implicates the supplementary motor area in the mediation of stuttering, possibly through relatively ungated callosal pathways.

Evidence in bimanual finger-tapping of an attentional component to stuttering

The performance of right- and left-handed male and female stutters was compared with that of non-stutters on a bimanual coordination task that involved tapping a key twice with one hand for each single tap of a key by the other hand. Right-handed non-stutters performed this 2:1 tapping better when it was the right hand that tapped twice (R2/L1 condition) rather than the left hand (L2/R1 condition), but among left-handers performance was similar under the two conditions. This replicated previous findings of Peters10 that were interpreted as indicating the role of attentional mechanisms in the expression of handedness. The performance of the stutters differed in two major respects. First, overall bimanual tapping rates were significantly slower than those of the non-stutters. Second, right-handed stutters did not show the asymmetry in performance between the R2/L1 and L2/R1 conditions. The results are interpreted within the framework of current neuropsychological research that relates stuttering to anomalous mechanisms of interhemispheric communication.

Neuropsychological models of stuttering—II. Interhemispheric interference

The performance of male adult stutterers and fluent speakers was compared on repetitive sequential finger tapping and index finger tapping with one hand while carrying out concurrent paced tasks with the other hand. For the theoretically meaningful condition of right-hand sequential tapping and left-hand concurrent task performance, there was more interference on the tapping task among stutterers than fluent speakers. The data were interpreted as being consistent with a neuropsychological model of stuttering that includes as an element enhanced left-hemisphere vulnerability to interference by concurrent right-hemisphere activity.

Neuropsychological models of stuttering—I. Representation of sequential response mechanisms

Male stutterers and fluent speakers were compared on index finger tapping and sequential finger tapping tasks. For both groups performance was better with the right than the left hand, and was better under conditions of visualization than under conditions of no visualization of the hand. In addition, participants in both groups showed similar variations in performance on various sequences. The pattern of results suggests that stuttering does not result from a general problem in sequencing and timing of behaviour, and is consistent with a neuropsychologial model of stuttering that would propose normal lateralization of neural mechanisms associated with sequential processing including speech.

Response sequence organization and reproduction by stutterers

Male stutterers and fluent speakers were compared on their performance of a task requiring tapping keys as rapidly and as accurately as possible to reproduce different finger movement sequences demonstrated on a visual display panel. Although overall finger tapping rate was the same in the two groups, indicating no difficulty by stutterers in performing simple motor movements, stutterers achieved fewer correct sequences and made more errors than fluent speakers. In addition, their response initiation times were slower. Once a correct response was initiated, however, the time to execute the sequence was similar to that of fluent speakers. Replicating earlier work, the two groups were not found to differ on a repetitive sequential finger tapping task with respect to correct sequences or total presses, although the probability of error was greater for the stutterers. The data were interpreted as indicating that in stutterers sequential response mechanisms are lateralized normally as they are in fluent speakers; these left-hemisphere sequential response mechanisms in stutterers appear unusually susceptible to interference, possibly from on-going right-hemisphere activity; stutterers have special difficulty in organizing and/or initiating new response sequences, but once the sequence is initiated, they can perform the sequence as rapidly (but with greater probability of error) as do fluent speakers; and stuttering reflects not a simple motor problem per se, but a higher level organizational problem of a cognitive nature.

Motor performance of stutterers: a search for mechanisms.

A continuing program of research, which is concerned with identifying brain mechanisms underlying stuttering through an analysis of manual motor control, is described. Clear evidence has been found that the neural mechanisms associated with sequential responding (and, by implication, with speech) are lateralized in stutterers as they are in nonstutterers. Although no gross or general incoordination has been found in motor performance by most stutterers, their left hemisphere mechanisms appear to be inefficient for organizing and initiating new sequences of responses and vulnerable to interference from other neural activities. Results of research on bimanual coordination in stutterers are consistent with a model that attributes the interference, in part, to interhemispheric processes, possibly involving the supplementary motor area. One implication of the research is that the disfluency of stuttering is only one manifestation of a more general disfunction in motor and cognitive organization and planning.

Sequence initiation performance by stutterers under conditions of response competition

Male stutterers and nonstutterers were compared on their performance of a task that required tapping keys as rapidly and accurately as possible to reproduce different sequences of finger movements as soon as they had been demonstrated on a visual display panel. Testing using the right hand alone confirmed previous findings that stutterers are significantly slower than nonstutterers in initiating new movement sequences and make significantly more errors on initial sequences. Subjects were also tested on this sequence reproduction task while they performed concurrently a manual task with the left hand that required turning a knob in response to signal tones. The hypothesis underlying the study was that associated with stuttering is a dysfunction in the mechanisms regulating interhemispheric communication, and it was predicted that the concurrent task would interfere more with the sequence reproduction task performance of stutterers than with that of nonstutterers. A joint analysis of the speed and accuracy aspects of performance of the two tasks confirmed the prediction. Additional research required to clarify whether the effects are in fact due to the hypothesized interhemispheric mechanisms or to a generally greater susceptibility of stutterers to interference from any concurrent motor or cognitive activity is discussed.