lateralization of cognitive processes in the brain

Lateralization of cognitive processes in the brain

Kenneth Hugdahl *

Department of Biological and Medical Psychology, University of Bergen, Arstadveien 21, 5009 Bergen,

Norway

Abstract

The lateralization of cognitive processes in the brain is discussed. The traditional view of a

language-visuo/spatial dichotomy of function between the hemispheres has been replaced by

more subtle distinctions. The use of magnetic resonance imaging (MRI) to study brain

morphology has resulted in a renewed focus on the relationship between structural and

functional asymmetry. Focus has been on the role played by the planum temporale area in the

posterior part of the superior temporal gyrus for language asymmetry, and the possible sig-

ni®cance of the larger left planum. The dichotic listening technique is used to illustrate the

di�erence between bottom±up, or stimulus-driven laterality versus top±down, or instruction-

driven laterality. It is suggested that the hemispheric dominance observed at any time is the

sum result of the dynamic interaction between bottom±up and top±down processing ten-

dencies. Stimulus-driven laterality dominance is always monitored and modulated through

top±down cognitive processes, like shifting of attention and changes in arousal. A model of

top±down modulation of bottom±up laterality is presented with special reference to the un-

derstanding of psychiatric disorders. Ó 2000 Elsevier Science B.V. All rights reserved.

1. Introduction

One of the most pervasive ideas in the history of the neurosciences has been theview that the two hemispheres of the brain evolved to be specialized for certaincognitive and behavioral functions. In her review of the history of laterality research,the science historian Anne Harrington (1995) wrote that ``many of the fundamental

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assumptions ± the unquestioned truths ± that provide the tacit grounding for modernlaterality research have their source in an early nineteenth-century approach to vi-sualizing mind±brain relations called localization theory'' (p. 3).

Thus, the history of laterality research, or research on the functional (andstructural) specialization of the two hemispheres is closely tied to the more generalnotion of localization of function in the brain, and the debate that this has createdover the last hundred years. The notion of localization of mental function to distinctareas or networks in the brain is credited to Franz Joseph Gall, the father ofphrenology. Gall is typically dismissed in modern books on neurosciences. However,as Harrington puts it in her tracings of the historical roots of laterality and local-ization of function in the brain, ``Gall . . . helped establish several heuristic principlesstill largely operational today. It was he who helped teach scientists that to locate thematerial base for a piece of mind is to claim it for science. And it was he who insistedon the corollary principle that to break mind down into its brain-based buildingblocks is to know it. This way to truth was not a necessary one-di�erent choicescould have been made . . . but the principles would facilitate a highly practical andproductive style of research and justi®cation that has not yet been superceded'' (p. 4).

It is interesting to note that the introduction of the hemodynamic brain imagingtechniques, positron emission tomography (PET) and functional magnetic resonanceimaging (fMRI) has given a new impetus to di�erent functional modules in the brain,including the specialization between the cerebral hemispheres (cf. Frackowiak,Friston, Frith, Dolan, & Mazziotta, 1997). What is needed today is therefore notmore empirical evidence to the existence of functionally specialized neuronal mod-ules in the brain, but advancement of theory to accomplish all the empirical ®ndingsin a coherent theoretical framework. That still remains to be done. An example ofsuch theoretical work is the notion of functional segregation (Frith, 1997). By this ismeant that the brain consists of many cortical networks or modules, that processinformation more or less independent of each other.

The notion of functional segregation has a long history in neuropsychology, withthe observations of Broca (1861) and Wernicke (1874) as classic examples for thelocalization of language areas in the brain. However, it was not until the develop-ment of PET and fMRI that the concept of functional segregation gained acceptancealso in mainstream cognitive psychology and cognitive neuroscience. Before thisdevelopment, it was not uncommon to ®nd statements like ``where is memory lo-calized in the brain ± everywhere and nowhere'' on standard textbooks on psy-chology and medicine. After the introduction of PET and fMRI imaging, suchstatements have almost totally disappeared from the literature. Frith (1997) providesseveral arguments as to why functional segregation, rather than functional inte-gration, is the guiding principle behind functional organization of the brain, takingarguments from both evolution, economy, and the complexity of the design of thebrain.

Looked at from a more narrow perspective, the study of brain laterality has had along tradition in the neurosciences, both in psychology, biology and medicine (seeDavidson & Hugdahl, 1995 for overview). The fact that the vertebrate nervoussystem is divided into half has attracted the attention and has sparked the specu-

212 K. Hugdahl / Acta Psychologica 105 (2000) 211±235

lation of numerous generations of scientists, actually much further back than just ahundred years. Of particular signi®cance in the history of research on this topic wasthe once held presumption that brain asymmetry was a property of only humanbrains, since asymmetry was linked to handedness and language.

Part of the explanation for this assumption is the fact that so strong was the focuson functional laterality of higher cognitive functions, that the question whether otherspecies displayed structural asymmetries similar to those found in humans was not inthe main focus of interest. This has changed and an example is the study by Gannon,Holloway, Broad®eld, and Braun (1998), where it was shown that the left planumtemporale was larger than the right also in the chimpanzee brain. Gannon et al.(1998) reported that there was a leftward asymmetry in 17 out of 18 studied chim-panzee brains. This ®gure is somewhat larger than what was reported by Geschwindand Levitsky (1968) in humans, where 65 out of 100 brains showed a similarasymmetry. This discrepancy recently led Marshall (2000) to question the signi®-cance of planum temporale area size for language function, when in fact a non-linguistic species has a greater asymmetry than humans. Thus, the ®nal word isprobably not said with regard to the signi®cance of the planum temporale for lan-guage function. Structural asymmetries in other parts of the brain are howeverfrequently reported in non-human species that includes rodents, cats, dogs andprimates (Diamond, 1984; LeMay, 1985; Bradshaw & Rogers, 1993; Hopkins &Mariono, 2000).

The notion of the uniqueness of laterality di�erences to the human species mayalso have been instrumental in promoting theories and arguments regarding edu-cational training and scholastic achievements that have been so abundant over thelast decades, with little or no empirical evidence to support such theories, and withlittle success in the educational and school environment.

Although most of these endeavors have been dead ends, one characteristic thathas been successfully applied to the behavioral and medical sciences is what Da-vidson and Hugdahl (1995) called ``the emphasis on hierarchical integration acrossmultiple levels of the neuraxis and types of processes'' (p. i). A related, and equallyimportant, issue is the notion that asymmetries exist at all levels of the central andperipheral nervous systems, also including the autonomic nervous system. For ex-ample, the right-side nucleus ambiguus contains the origin nuclei for the vagal inputto the heart. Thus, the heart is primarily innervated by the right branch of the vagusnerve, particularly at the sinoatrial (SA) node, which is the pace-maker for thebeating of the heart. This may be of considerable signi®cance with regard to how thebrain controls the heart and how autonomic out¯ow is regulated in situations ofstress and arousal.

The story even goes further, the vagal innervation of the heart is through thesinoatrial node which regulates the beating of the heart in addition to its sponta-neous rhythmicity, and the in¯uence of the sympathetic and endocrine systems. Itappears that the atria and the atrioventrial bundle in the upper part of the heart,which transmits the nerve impulses to the bottom of the heart, are supplied with bothvagal and sympathetic ®bers, whereas the ventricles of the heart are supplied by thesympathetic nerve ®bers only. An important consequence of this is that the right

K. Hugdahl / Acta Psychologica 105 (2000) 211±235 213

vagus nerve innervates the sinoatrial node, and is responsible for changes in heartrate, while the left vagus innervates the atrioventral node and bundle (Schmidt &Thews, 1980; Brodal, 1981). Similarly, it has been found that it is also the rightstellate nerve of the sympathetic interganglionic nerve that innervates the SA node ofthe heart (Randall, Armour, Geis, & Lippincott, 1972). Finally, Rogers, Battit,McPeek, and Todd (1978) found much greater in¯uence on heart rate after blockingthe e�ect of the stellate ganglion nerve on the right, compared to the left side. Thus,one may speculate as to the extent that the psychological in¯uences on heart rate arecontrolled and regulated from higher nerve centers located in the right hemisphereand other structures in the brain.

Not only are there two cerebral hemispheres, there are also right and left halves ofmost brain structures including the thalamus, amygdala, hippocampus, caudate andmany other subcortical sites of major importance to higher mental processes. It istherefore, the more surprising that so few points of correspondence between struc-tural and functional asymmetry have been identi®ed in the literature, with perhapsthe planum temporale area in the superior temporal gyrus as the most well-knownexception (Geschwind & Levitsky, 1968; Steinmetz et al., 1989; Hugdahl, Brùnnick,Law, Kyllingsb�k, & Paulson, 1999b), linking the speci®c function of phonologicalencoding and speech perception to the left planum temporale area (however, seePrice et al., 1992; Binder, Frost, Hammeke, Rao, & Cox, 1996 for exceptions).Moreover, as so elegantly pointed out by Lezak (1994), the brain is not only func-tionally organized along the lateral axis, but also along the longitudinal axis. Thishas perhaps been even more in focus during the last decades than the organization offunction between the hemispheres, with the revelation of the frontal lobes as the sitefor complex, ``executive'' functions (Stuss & Benson, 1984), and the role of thetemporal lobe in memory functions (Ungerleider & Mishkin, 1982) and the site forattention in the posterior parietal lobe (Posner & Driver, 1992).

An important kind of functional organization along the lateral axis is what maybe called lateral symmetry (Lezak, 1994), or mirror-reversal of function whencrossing the longitudinal ®ssure. With this is meant that there are no qualitativedi�erences between the function subserved in the homologous left and right hemi-spheres. Functional organization of the primary motor and sensory functions followthis principle of cortical organization, with the right hemisphere serving the ex-tremities on the left side of the body axis and vice versa. The same applies to thevisual and auditory sensory systems where input from the left visual half-®eld isprocessed in the right occipital lobe, and vice versa. The auditory system also followsthis principle although in a more complex way. Each ear projects to both hemi-spheres, however, the contralateral neuronal pathways are more preponderant thanthe ipsilateral pathways (Rosenzweig, 1951). Fig. 1 shows an example of lateralsymmetry.

The data are taken from Ersland et al. (1996) from our laboratory and were basedon fMRI brain imaging of visual imagery in a subject with an amputated right arm.The subject had lost his right arm in a car accident a few years before he was tested inthe MR scanner. In one condition, he was instructed to move his ®ngers of the intactleft hand. In another condition, he was instructed to imagine moving the non-


existing ®ngers of the right hand. As can be seen in Fig. 1, moving the left hand®ngers, signi®cantly increased activation in the contralateral, right, primary motorcortex (white squre).

In contrast, imagining moving the ®ngers of the non-existing right hand and armsigni®cantly increased activation in the left motor cortex, almost identical to theactivation in the opposite hemisphere when actually moving the ®ngers. Thus,contralateral symmetry may be achieved through di�erent pathways, and it is in-teresting to note that visual imagery produces similar activations as actual motorperformance (cf. Kosslyn, Alpert, & Thompson, 1993).

Lateral asymmetry means that homologous areas in the left and right hemispheressubserve qualitatively di�erent functions. An example of this principle of orga-nization is Broca's area in the left prefrontal cortex, which is the major speechproduction area in the brain, and which has no functional equivalent on the cor-responding right side. Another example is the right inferior parietal lobe, whichfrequently produces neglect of information presented in the contralateral left visualhemi®eld (Heilman, 1995). However, a lesion in the homologous area in the lefthemisphere rarely leads to neglect, or if it occurs, seldom with the same seriousconsequences as when the lesion is in the right hemisphere.

Recent research on brain laterality and asymmetry have shown a tendency to-wards both theoretical and methodological integration across the neurosciences (seeDavidson & Hugdahl, 1995 for examples). There is also a steady stream of inte-gration across basic and applied levels of analysis, with recent theories and modelsof structural and functional asymmetries in the schizophrenic brain as a key

Fig. 1. Increased neuronal activation based on fMRI in the contralateral motor cortex during actual ®nger

movements (left panel), and during visual imagery of moving the ``®ngers'' in an amputated arm (right

panel). Data from Ersland et al. (1996).


example (Green, Hugdahl, & Mitchell, 1994; Crow, 1997; Lùberg, Hugdahl, &Green, 1999). In their preface Davidson and Hugdahl (1995) stated that ``we cannotidentify any other construct that forms the focus of such diverse array of behavioralprocesses''.

As a consequence of the diverse applications of the concept of brain laterality, it isone of the few constructs that have attracted the interest of both psychologists,cognitive scientists, neurologists, anatomists and physiologists, neuropsychologists,psychiatrists, speech therapists and educational experts. The results of studies onbrain laterality e�ects are published in numerous journals covering an equally im-pressive range of disciplines, making it almost impossible for a single scientist, withinany ®eld, to keep up with the literature in this ®eld.

In the present paper, I have no intention of covering the entire ®eld of lateralityresearch on cognitive function, but will present a selective discussion of some keysub-disciplines, where a laterality perspective has been particularly instrumental inadvancing the ®eld. These areas will be language, attention, and the application of alaterality perspective for the understanding of cognitive function in patients withbrain lesions, and in schizophrenia.

2. Search for a basic dichotomy?

A basic question throughout the history of laterality research has been theidenti®cation of a basic dichotomy of functioning between the hemispheres. How-ever, as stated by Hellige (1993) ± ``the quest for a fundamental dimension ofhemispheric asymmetry has been unsuccessful'' (p. 58), despite the fact that ``hun-dreds of behavioral asymmetries have been identi®ed in humans'' (p. 61). Others(e.g., Bertelson, 1981) have questioned the whole idea of a single dichotomy thatunderlies how the two hemispheres of the brain function. Within the visual domain,several basic distinctions related to spatial processing have been suggested. Theseinclude, among others, the concept of hierarchical organization (Robertson, 1986),with local patterns, or aspects, of a stimulus complex being embedded within aglobal pattern. The prevailing view is that the left hemisphere is dominant or spe-cialized for processing of local elements, while the right hemisphere is dominant orspecialized for processing of global elements. Another distinction that has receivedmuch attention is the notion of categorical versus coordinate, or relational, pro-cessing (Kosslyn, 1987), and that the left hemisphere is specialized for categoricaljudgements, and the right hemisphere is specialized for decisions about coordinatejudgements in space. A third, and perhaps most in¯uential, concept is the notion ofspatial frequency as a fundamental dimension of how spatial processing operates inthe two cerebral hemispheres (Sergent, 1983; Christman, 1989; Hellige, 1993). Here,the idea is that the two hemispheres may be di�erentially biased in their use of higherand lower spatial frequencies in a visual stimulus display, with the right hemispherebeing more sensitive to discriminations based on higher frequencies. Still anotherview has been proposed by Ivry and Robertson (1998) in which they considerhemispheric asymmetry as a process where the hemispheres construct an asymmetric


representation from sensory input, favoring either the right or left hemisphere de-pending on the available cognitive resources. Thus, it seems that the notion of ageneral hemisphere speci®c module for processing of visuo-spatial relations shouldbe replaced with more subtle modules based on the sub-processes involved in visuo-spatial function.

However, although the search for a basic dichotomy may be a dead end, thereare arguments in favor of a basic dichotomy that cuts across most aspects ofcognitive processing. With the risk of an overstatement, I will dare the argumentthat despite all attempts at rede®ning the core functional asymmetries in the brain,the basic notion of functional asymmetry in the human brain still centers alongthe language-visuo-spatial dimension. The anatomical division of the two hemi-spheres along the longitudinal ®ssure is one of the most conspicuous landmarks inthe brain. Morphologically, the left±right distinction is a fundamental principlealong the entire neural axis. Acknowledging this, it seems odd that such a pro-found anatomical distinction should not relate to a similarly fundamental func-tional distinction. Suggested functions are perhaps not critical enough for humansurvival, which could have motivated the evolution of a divided brain. The answerto a fundamental dichotomy must therefore be sought among functions that areunique to the evolution of modern man. One such process or function is language,the ability to symbolically represent the environment and to communicate thiswith other members of the species. Thus, language function is one of the fewfunctions that would qualify as a functional ``match'' to the structural division ofthe brain into the left and right hemispheres. Following the same line of argument,I will suggest that the ability to represent the three-dimensional environment as avisuo-spatial map is a similarly basic ability that would qualify as having ahemisphere of its own. The reason why we have a divided brain is not because oneof the hemispheres is specialized for rapidly changing sequences, but becauselanguage is sequential in nature. Similarly, right hemisphere specialization didevolve because orientation in space requires rapid identi®cation of objects andtheir relations.

Empirically, there are no other processes that have produced such reliable dif-ferences between the hemispheres as experiments on language (e.g., lexical decision)and spatial (e.g., mental rotation) tasks. Of all the empirical research that has beenpublished over the last 40 years, from Sperry's (see Sperry, 1974), and his students'(e.g., Levy-Agresti & Sperry, 1968; Zaidel & Sperry, 1977; Gazzaniga & LeDoux,1978) pioneering work on functional asymmetry in the brain to more recent accounts(Harnad, Doty, Goldstein, Jaynes, & Krauthamer, 1977; Bradshaw & Nettleton,1981; Bryden, 1982; Hellige, 1993), no other dichotomy or concept has producedsuch consistent ®ndings.

Thus, from a theoretical perspective, we do not need alternative explanationsregarding the fundamentals of functional symmetries, but rather a better under-standing of why language and visuo-spatial performance were singled out as theprimary division of labor between the hemispheres. It has been written extensively onthe nature of the language-spatial dichotomy (cf. Bradshaw & Nettleton, 1981),without any consensus on the issue. It seems, however, likely that the functional


asymmetry in the brain is the result of evolutionary pressure towards specializationfor these behaviors.

Maybe, it goes back to the need for symbolic communication between members ofthe same species, and the need to orient in unknown environments. The need forsymbolic communication grew with the organization in larger social groups, and theneed to successfully orient oneself in the real world was of course important whenout hunting, and ®nding the way back home. The language-spatial dichotomy is alsothe best predictor of di�erences in mental operations between males and females(Kimura, 1996). Females excel over males on language tasks, like verbal ¯uency,when the task is to name as many objects that begin with a certain letter in a shortperiod of time. Males, on the other hand, excel over females on typical spatial tasks,for example, when having to judge if two three-dimensional ®gures are the di�erentor the same, but with one of them rotated around its own axis (Shepard & Metzler,1971). See below for an example of sex di�erences also in brain function, studied withfMRI, when solving a mental rotation task.

2.1. Language and the planum temporale

The best example of correspondence between structural and functional asym-metries in the brain is the larger left-than-right planum temporale area in the upperposterior temporal lobe and speech production. The structural asymmetry of theplanum temporale has been observed both at autopsy (Geschwind & Levitsky, 1968)and with magnetic resonance imaging (MRI) (Steinmetz, Volkmann, J�ancke, &Freund, 1991). The planum temporale is situated in the superior temporal gyrus,between the Heschl's gyrus anteriorly and the end of the Sylvian ®ssure posteriorly(cf. Steinmetz et al., 1989). In individuals where the Sylvian ®ssure bifurcates in adescending and an ascending branch, the ¯oor of the latter has been named theplanum parietale.

The planum temporale is an auditory association area involved in the processingof verbal and non-verbal stimuli. The planum temporale may be an anatomicalcorrespondence in the brain for the evolution of a phonologically based language inhumans. This asymmetry is found both in humans and in the great apes (LeMay,1985), but not in other species, which may provide evidence for a relation betweenthe development of the planum temporale and the ability to develop an auditorybased language. That the great apes did not develop language may therefore nothave been because they lack the necessary cognitive foundations, but rather becausethey did not evolve the necessary supporting anatomy related to vocal chords andthe vocal trajectories.

Fig. 2 shows the anatomical localization of the planum temporale in the upperposterior part of the temporal lobe, with the Heschl's gyrus outlined as the transversegyrus just in front of the planum temporale. See Hugdahl et al. (1999a), Hugdahl,(1999b) for further details.

The planum temporale is anatomically overlapping the classic Wernicke languagearea, which is a functional area. Thus, the left-sided asymmetry of the planumtemporale may be a structural correspondence to the functional asymmetry found in


Wernicke aphasics who loses the ability to understand the spoken language. Re-cently, however, some investigators have questioned the unique role of the planumtemporale in language perception, instead arguing that the planum temporale maybe more tuned to the analysis of the acoustic content re¯ected in speech perception,such as sound frequency, or amplitude information in the speech signal (Demonet etal., 1992; Zatorre, Evans, Meyer, & Gjedde, 1992; Binder et al., 1996). Using PET orfMRI to visualize function in the planum temporale area, these authors have foundthat the planum temporale is equally activated to speech and non-speech sounds, andthat more ventral areas in the superior temporal gyrus, and parts of the medialtemporal gyrus may be unique to speech sound perception. However, these authorshave used very di�erent stimuli, also loading on a semantic dimension, thus, it isdi�cult to evaluate the exact cause-e�ect in these studies (see also Poeppel, 1996discussing the failure to replicate earlier brain imaging studies for localization in thebrain of speech perception).

Hugdahl et al. (1999b), using the PET blood ¯ow technique found a di�erentpattern of activation in the planum temporale area for consonant-vowel syllablesand short musical chords, matched for amplitude information. The data from theHugdahl et al. (1999b) study are shown in Fig. 3, which shows horizontal imagesacross the mid-section of the planum temporale.

2.2. Visuo-spatial processing: Mental rotation

Turning to the issue of functional laterality for spatial functioning, a recent studyby Thomsen et al. (1999) from our laboratory may serve as an illustration, alsoaddressing the issue of di�erences in cognitive function between males and females.Thomsen et al. (1999) studied changes in brain activation with the fMRI techniquewhen males and females solved a mental rotation task taken from the classic Shepard

Fig. 2. Localization of the planum temporale area in the superior temporal gyrus.


and Metzler (1971) set of stimuli. In the classic Shepard and Metzler (1971) mentalrotation task, subjects are shown pairs of perspective drawings of three-dimensionalregular shapes.

Fig. 3. Increased neuronal activation based on PET in the left and right planum temporale to phono-

logical (CV) and non-phonological (musical instruments) stimuli. Data from Hugdahl et al. (1999b).


The task of the subject on each trial is to decide whether the two shapes areidentical, or if one is a mirror-image of the other. The typical ®nding is that responsetimes increase as the angle of disparity between the two shapes increases, with re-quirements for cognitive processing in order to determine if they are the same shapeor not. A common explanation for these ®ndings is that an image of the shape has tobe mentally ''rotated'' to be superimposed on the reference shape in order for thesubject to commit a decision whether the shapes are identical or not (Shepard &Cooper, 1982; Tagaris et al., 1997).

Some studies have found a right hemisphere laterality e�ect with faster reactiontimes when the shapes are presented in the left visual ®eld (e.g., Ditunno & Mann,1990) indicating a right hemisphere basis for mental rotation (see also Corballis &Sergent, 1988 testing a split-brain patient). However other studies on patients withunilateral, left or right hemisphere lesions (e.g., Kosslyn, Holtzman, Farah, &Gazzaniga, 1985) have found impaired performance particularly for left hemispheredamaged patients, indicating a left hemisphere basis for mental rotation.

Thus, it is at present not entirely clear whether the neural basis for the mentalrotation e�ect is a right hemisphere processing superiority similar to what is typicallyfound in other tasks involving visuo-spatial processing (see Hellige, 1995; Kosslyn &Brown, 1995).

Most studies using hemodynamic neuroimaging techniques (PET and fMRI) havemainly implicated the right parietal lobule as involved in mental rotation, thus, atleast to some extent supporting a right hemisphere processing dominance (e.g.,Cohen et al., 1996; Belin, Moroni, Gelbert, Cordoliani, & Delaporte, 1998). Otherstudies have, however, reported bilateral parietal lobule activation (Tagaris et al.,1996 using a di�erent task; Tagaris et al., 1997), or inconsistent laterality acrosssubjects (Cohen et al., 1996).

Thus, the issue of hemispheric asymmetry in brain activation studies of mentalrotation is also unsolved. Part of the explanation for the variability across studiesand subjects in hemispheric asymmetry for mental rotation may be the kind ofbaseline or control stimulus used. Both Cohen et al. (1996) and Richter, Ugurbil,Georgopoulos, and Kim (1997) used the same Shepard and Metzler (1971) shapesduring the experimental (mental rotation) and control conditions, subtracting fMRIimages between the two conditions. However during the control condition, only non-rotated or completely mirror-rotated shapes were used. Although this controls forthe e�ect of visual perception, it may induce ``carry-over'' processing e�ects from theexperimental to the control stimulus condition, with the subject trying to mentallyrotate also the control stimuli, since they are similar in shape and outline as theexperimental stimuli.

In the Thomsen et al. (1999) study we used di�erent control shapes in order tooptimize di�erences in mental rotation demands between the experimental andcontrol conditions, while controlling for overall visual perception and for motorresponses.

The main ®ndings are presented in Fig. 4.The fMRI activation data for the whole group showed signi®cant increases in

neuronal activation bilaterally in the superior parietal lobule, although predomi-


nantly on the right side. This is in agreement with previous studies on mental ro-tation (e.g., Cohen et al., 1996; Tagaris et al., 1997; Belin et al., 1998). The parietallobule thus seems to be important in mental rotation tasks, which may tap into thedorsal pathway of the visual processing system (cf. Ungerleider & Mishkin, 1982).

Other authors have made similar claims, e.g., Cohen et al. (1996) suggested thatthe superior part of the parietal lobule is involved in encoding of visual stimuli, while

Fig. 4. Increased neuronal activation (darker areas) based on fMRI in males and females when solving a

three-dimensional mental rotation task. Data from Thomsen et al. (1999).


Belin et al. (1998) suggested that the same area is involved in visual search, andColby, Duhamel, and Goldberg (1995) maintained that it is involved in object rec-ognition in space. It is di�cult from the present data to distinguish between thesedi�erent functions, linked to the superior parietal lobule. It is also a well-known factthat the parietal lobules seem to be speci®cally involved in processing of spatialrelations, con®rmed by both animal studies (e.g., Morton & Morris, 1995; Steinmetz,1998), clinical lesion studies (e.g., Heilman, 1995) and studies on healthy individuals(e.g., Michel, Kaufman, & Williamson, 1994). More speci®cally, it has been sug-gested that the areas in the superior parietal lobule are responsible for the mental actof ``rotating'' the object in the visual bu�er (cf. Alivisatos & Petrides, 1997).

As is also seen in Fig. 4, the males mainly showed remaining activation in theparietal lobule. When subtracting activation for the males from activation for thefemales, the females showed remaining activity in the right inferior frontal gyrus.This may address the issue that males and females use di�erent processing strategieswhen solving the mental rotation task. Males seem to process the three-dimensionalstimuli mainly as visual gestalts, using right hemisphere object recognition androtation functions, while females may use more frontal lobe areas that may berelated to the utilization of language functions when solving the task (the rightfrontal activation may represent a ``mirror activation'' from the homologous leftareas).

2.3. Bottom±up versus top±down modulation

Brain laterality can be looked at from both a bottom±up, or stimulus-drivenperspective, and from a top±down, or instruction-driven, perspective. With bottom±up laterality is meant the specialization between the hemispheres for processing ofcertain stimuli. With top±down laterality is meant the dynamic modulation of astimulus e�ect by shifting of attention or arousal to either the left or right visual orauditory hemi-space. Thus, the view advocated in the present paper is that lateralityshould be regarded as the sum of stimulus-driven and instruction-driven e�ects, thatdynamically interact to produce a speci®c laterality pattern for a given cognitivecontext. This can explain why not only do individuals di�er in their response tolateralized stimuli, but also that the same individual may show di�erent responsepatterns to the same stimuli when tested at di�erent points in time.

The issue of bottom±up versus top±down processing in relation to brain lateralitywill be illustrated through performance on a variant of the dichotic listening taskwhich we have used extensively in our research on laterality over the last 15 years(see Hugdahl, 1995; Hugdahl, 1997 for reviews).

Dichotic listening means that two di�erent auditory stimuli are presented exactlyat the same time, one in each ear (see Kimura, 1961b, 1967; Bryden, 1988; Hugdahl,1995). Thus, dichotic listening re¯ects the limited capacity of the brain to handle twothings at the same time, and particularly when the two events draw on the samekinds of processing attributes.

The stimuli typically used in our dichotic listening studies consist of presentationsof pairwise combinations of consonant-vowel (CV) syllables that are made up of the


six stop-consonants /b/, /d/, /g/, /p/, /t/, /k/ and the vowel /a/. Thus, examples ofdichotic listening stimulus-pairs are /ba/ ± /pa/; /ga/ ± /pa/, etc. The typical outcomein a standard DL-test is of greater percentage correct reports from the right ascompared to the left ear. This is called a right ear advantage (REA) and is a robustempirical ®nding in both right- and left-handers (see Fig. 5).

The REA is most easily seen in response to the consonant, and di�cult to observeto the vowel in a CV-syllable. It has thus been argued that the REA might re¯ecthemisphere specialization for rapidly changing auditory stimuli, such as the rapidformant transition seen in the stop consonants.

The REA is believed to be caused by the fact that although auditory input istransmitted to both auditory cortices in the temporal lobes, the contralateral pro-jections are stronger and more preponderant, interfering with the ipsilateral pro-jections. This is shown in Fig. 6.

The advantage for the contralateral auditory projections means that the languagedominant left hemisphere receives a stronger signal from the right ear. The con-tralateral signal from the left ear to the right hemisphere must ®rst pass the corpuscallosum in other to be processed in the left hemisphere. Following the same logic, aleft ear advantage (LEA) indicates the right hemisphere to be language dominant,and a no ear advantage (NEA) indicates a bilateral language dominance.

The dichotic listening technique can be used to study the dynamic interactionbetween stimulus-driven and instruction-driven laterality by instructing the subjectsto attend to only one ear at a time, and to only report from that ear. This has beencalled ``the forced-attention paradigm'' and typically involves three conditions; inone block of trials no speci®c instruction regarding instruction is given, in a secondblock of trials the subject is instructed to attend to and report from only the right

Fig. 5. The distribution of subjects across number of correctly reported items from the left and right ear,

respectively, in the dichotic listening paradigm.


ear, in still another block the subject is instructed to attend to and report only fromthe left ear.

When the subject is required to focus attention either to the right or left ear, the``stimulus-driven'' right ear advantage can be either increased or decreased(sometimes shifted to a left ear advantage) depending on which ear the subject isinstructed to attend (Asbjùrnsen & Hugdahl, 1995). Similarly, Mondor and Bryden(1991) showed that presenting an auditory ``cue'' in either the left or right ear afew milliseconds before the dichotic stimuli, also a�ects the ear advantage on atrial-by-trial basis, by having the subject to shift attention between the ears fromtrial to trial. Typically, the REA is increased when attention is focused on the rightear, and decreased or shifted to a LEA when attention is shifted to the left ear (seeFig. 7).

So far, dynamic modulation of laterality has been looked at from a state per-spective. However, there is evidence that also trait properties interact with how ahemisphere-speci®c stimulus is processed. In a recent study on the psychology ofloneliness and social isolation, Cacioppo et al. (2000) used the CV-syllables dichoticlistening paradigm to study the e�ects of changing attentional instructions to theright and left ear in a group of socially isolated and lonely individuals compared witha control group. Lonely individuals tended to show the strongest REA in the

Fig. 6. Neuronal pathways in the auditory system for the explanation of the REA e�ect in dichotic lis-

tening. PAC�Primary auditory cortex. MGB�Medial geniculate body. IC� Inferior colliculus.

CNC�Cochlear nucleus.


no-instruction condition, presumably re¯ecting the potency of bottom-up (i.e.,stimulus-driven) attentional processing.

Perhaps more interesting, lonely individuals apparently failed to shift to an apriori predicted LEA in the left ear attention condition, despite showing a large REAwhen instructed to shift attention to the right ear. Speci®c planned contrasts con-®rmed that all three groups showed a signi®cant REA during the focus on right earcondition, but only the normal and embedded individuals were able to revert to asigni®cant left ear advantage in the focus on left ear condition.

Thus, attentional control appeared comparable in lonely and embedded individ-uals until voluntary attentional control con¯icted with automatic attentional pro-cesses, at which point lonely individuals showed an attentional de®cit. This resultraises the possibility that lonely individuals feel overwhelmed and withdraw from thesocial environment, especially new or complex social environments, because theyhave less control over the focus of their attention.

The issue of trait di�erences between individuals and the e�ect this may have onperformance on laterality tests may be further illustrated in a study by Davidsonand Hugdahl (1996) where the dichotic listening paradigm was used in subjectswho had been screened for trait-like characteristics of EEG asymmetry patterns.Measures derived from scalp-recorded brain electrical activity have been demon-strated to provide reliable indices of asymmetric activation in di�erent scalp re-gions. For example, Ehrlichman and Wiener (1979) reported that alpha powerasymmetry in parieto-temporal sites was stable over time (r � 0:74 over a three-week interval). Davidson has reported on the test-retest stability and internalconsistency reliability of EEG measures of activation asymmetry derived fromaggregated eyes open and eyes closed resting conditions (presented in 8 one-minutetrials, four of each type) in di�erent scalp regions over a three week interval.Davidson and colleagues have reported that individual di�erences in EEG mea-sures of asymmetry from anterior temporal and prefrontal scalp sites predict im-portant features of emotional reactivity and a�ective style (see Davidson, 1995 forreview).

Fig. 7. The e�ect of shifting attention to either the right or left hemi-space on dichotic listening.

NF� non-forced attention, FR� forced-right attention, FL� forced-left attention. M�Males. F�Fe-

males. Data from Hugdahl & Andersson (1986).


Davidson and Hugdahl (1996) sought to determine whether baseline measuresof electrophysiological asymmetry could predict performance on the standard di-chotic task. The results are shown in Fig. 8 and they demonstrated that individ-uals with more left-sided activation in the temporo-parietal region had a largerREA.

Activation levels in the homologous right hemisphere regions were not signi®-cantly associated with dichotic performance. Of particular interest was the fact thatactivation asymmetry in the prefrontal region also predicted dichotic performance,albeit in the opposite direction. Decreased activation in the left prefrontal region andincreased activation in the right prefrontal were each associated with a greater REA.These ®ndings imply that the neural mechanisms re¯ected in dichotic performanceare complex and include both posterior and anterior components. Moreover, sincethe anterior and posterior measures of activation asymmetry were related to dichoticlistening performance in opposite directions, these ®ndings argue for considerablecaution in interpreting a REA on a verbal dichotic task as re¯ecting a simple lefthemisphere processing advantage.

Another example of trait-e�ects on the laterality of cognitive function is a recentstudy by Beaton, Hugdahl, and and Ray (2000, in press), who compared the abilityto modulate the REA in dichotic listening with increasing age. The results are seen inFig. 9.

Fig. 8. Correlation maps between residualized alpha power and dichotic listening performance asym-

metry. Darker regions indicate that decreased alpha power (i.e., increased activation) is associated with

increased REA in dichotic listening, whereas the grey end of the color spectrum indicates that increased

alpha power (i.e., decreased activation) is associated with increased REA in dichotic listening perfro-

mance. Note that left-sided posterior EEG activation is associated with a greater REA. From Davidson

and Hugdahl (1996).


As can be seen in Fig. 9, there was a reduction with age in dichotic ear asym-metry for CV syllables due to a decline in performance at the right ear. There wasalso a reduction in the ability to strategically direct attention to one or other ear.The right ear e�ect is unlikely to be due to a reduction in inter-hemispheric transferof auditory information since, with CV syllables as stimuli, this would show up atthe left rather than the right ear. It is possible, however, that callosal mechanismsare implicated in what (2000, in press) called as a strategic e�ect. Evidence that thecorpus callosum is related to the capacity to sustain attention has been provided byRueckert, Sorensen, and Levy (1994). It may therefore be suggested that a reductionin callosal e�ciency may lie at the heart of the e�ects of age on dichotic perfor-mance and the ability to modulate a stimulus e�ect through top±down cognitivein¯uences.

As also discussed by (2000, in press) it is possible that a similar explanationmight apply to other phenomena associated with aging, including an apparent re-duction in choice of the left hand for a variety of everyday activities. If there is a bias,for whatever reason, towards attending to the right side of personal or extra-per-sonal space, then it may become more and more di�cult to resist this bias withincreasing age.

In conclusion, the previous section has shown that both intra- and inter-individualdi�erences may exert substantial e�ects on observed lateralities, interacting with

Fig. 9. Age e�ects on dichotic listening performance. NF�non-forced attention, FR�Forced-right at-

tention, FL�Forced-left attention. 20ties� age 20±30, 40ties� age 30±50, 60ties� age 55±70. Rear�Right ear score, Lear�Left ear score. Data from (2000, in press).


stimulus-e�ects to produce, or construct (cf. Ivry & Robertson, 1998) a speci®cdominance pattern.

2.4. Clinical applications I: Patients with arachnoid cysts

Arachnoid cysts are congenital cysts that seem to be caused by a duplication ofthe arachnoid fossa, and formed by splitting the arachnoid membrane (Wester,Svendsen, & Hugdahl, 1999). The localization of a cyst is biased to the temporalregion, but may also appear in both the frontal and parietal lobes.

In collaboration with the neurosurgery department at Haukeland UniversityHospital in Bergen, we have been interested in the correlation between cyst local-ization to the left or right hemisphere and functional outcome on a range of neu-ropsychological tests. I will here review some of these ®ndings, focusing on dichoticlistening, which mean that it is possible to probe the left and right temporal regionsindependently. Wester et al. (1999) (see also Wester, 1992) reported a profound biaswith regard to localization of arachnoid cysts in the left temporal region. Cysts lo-cated in the temporal fossa were most common, present in 68.1% of the patients.There were, in addition, about four times as many males than females in the patientgroup. There was also a signi®cant coupling between gender distribution and sid-edness based on the fact that only males contributed to the predominance of left-sided temporal cysts. In patients with an unilateral temporal cyst, the ratio of left toright localization was 2:1.

Comparing performance on the dichotic listening test has systematically shownthat patients with cysts in the left hemisphere have reduced REA amplitudes, andoften reversed performance, showing a LEA. This has quite often been normalizedafter decompressive surgery (Wester & Hugdahl, 1995).

The interaction with gender and sidedness for the localization of the cysts mayindicate a possible prenatal cascade e�ect that may speci®cally a�ect only onehemisphere. Similar cascade e�ects involving increased levels of testosterone duringpregnancy that may have selectively delayed left hemisphere development has pre-viously been suggested for dyslexia (Geschwind, 1984). One may speculate whetherthere is a similar gender-speci®c developmental failure explaining the side di�erencein arachnoid cysts that may be caused by similar abnormalities with regard toneuroendocrine function in the fetus during pregnancy. An example of a left-sidedcyst is shown in Fig. 10.

2.5. Schizophrenia as abnormal lateralization of function

Schizophrenia is one of the major psychiatric disorders, and about 1% of thepopulation will su�er a schizophrenic illness in the course of their lifetime. Recentresearch on the neurobiology of schizophrenia has focused on abnormality of brainlaterality as a marker of the disease (see Crow, 1997; Gruzelier, 1999; Gur, 1999 foroverview). This is perhaps not new since laterality hypotheses for the explanation ofschizophrenia were proposed already in the 1970s (Gruzelier & Venables, 1973; Flor-Henry, 1976; Gur, 1977). What is new in the recent attempts at seeing schizophrenia


as a failure of normal lateralization is the use of high resolution magnetic resonanceimaging techniques to investigate structural abnormalities, and PET and fMRI toreveal functional abnormalities.

Perhaps the most radical view is taken by Crow (e.g., Crow, 1995, 1997) who hassuggested that schizophrenic individuals show less anatomical and functionalasymmetries related to the left hemisphere and as a consequence they show ahemisphere ``dominance failure'' for language. In particular it has been shown thatthe expected leftward asymmetry in the planum temporale area is reduced inschizophrenic patients, and that schizophrenic patients do not show the expectedREA on the dichotic listening test (Green et al., 1994).

This may be evidence for a speci®c left hemisphere temporal lobe dysfunction,related to failure of asymmetry for language processing. Green et al. (1994) alsoshowed that hallucinating patients fared the worst with regard to performance onthe dichotic listening test, indicating that the language areas in the left temporal lobemay be already ``occupied'' by inner speech and voices, excluding the processing ofan external stimulus. More recently, Lùberg et al. (1999) found that schizophrenicpatients also failed to modulate their performance on the dichotic listening task withinstructions to shift attention to the left or right ear. Lùberg et al. (1999) thusshowed that schizophrenic patients su�ered from both bottom±up and top±downprocessing skills, which they called ``a dual de®cit'' referring to the duality in

Fig. 10. Schematic drawing which illustrates the localization of an arachnoid cyst in the inferior left

temporal lobe region.


laterality de®cits that included both failure of stimulus-driven and instruction-driven processing.

3. Summary and conclusions

In this paper, I have presented a selective overview of some central concepts andnotions in modern research on brain laterality.

The use of magnetic resonance imaging (MRI) to study brain morphology hasresulted in a renewed focus on the relationship between structural and functionalasymmetries. Focus has been on the role played by the planum temporale area in theposterior part of the superior temporal gyrus for language asymmetry, and thesigni®cance of the larger left planum. Although it seems both logical and reasonablethat the larger left planum temporal is signi®cantly related to the specialization of theleft posterior part of the temporale lobe for speech perception, researchers are di-vided in their view of the relationship. Binder et al. (1996) argued, on the basis offMRI neuroimaging data that the planum temporale on the left side is more relatedto processing of the acoustic features of the speech signal rather than the phono-logical aspects. Other data, e.g., Hugdahl et al. (1999b), using the PET techniquefound profound di�erences in neuronal activation in the left planum when subjectsprocessed phonological and non-phonological stimuli, that were matched foracoustical features. Thus, more research is needed before the role of the planumtemporale can be resolved.

In the second part of the paper, I have reviewed data from our laboratory, usingthe dichotic listening technique to illustrate the di�erence between bottom±up, orstimulus-driven laterality versus top±down, or instruction-driven laterality. It issuggested that the hemispheric dominance observed at any time is the sum result ofthe dynamic interaction between bottom±up and top±down processing tendencies.Stimulus-driven laterality dominance is always monitored and modulated throughtop±down cognitive processes.

Finally, I have reviewed research concerned with clinical applications of struc-tural±functional asymmetry, and bottom±up versus top±down laterality, in brainlesioned patients and in schizophrenia.

Acknowledgements

The present research was ®nancially supported by grants from the NorwegianResearch Council (NFR). The contribution of colleagues, students, and collabora-tors in Norway and internationally is greatly acknowledged.

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