Anatomical brain asymmetries in New World and Old World monkeys Stages of temporal lobe development in primate Evolution.код для вставкиСкачать
AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 76:39-48 (1988) Anatomical Brain Asymmetries in New World and Old World Monkeys: Stages of Temporal Lobe Development in Primate Evolution PETER L. HEILBRONER AND RALPH L. HOLLOWAY Department ofdnthropology, Columbia Uniuersity New York, New York 10027 KEY WORDS Brain, Cerebral cortex, Asymmetry, Sylvian fissure, Sylvian point, Central fissure, Wernicke’s area, Cercopithecoid, New World monkey, Hominoid, Endocast ABSTRACT Relatively large (n = 20-30) samples of formalin-fixed brain specimens from five Old and New World monkey species were examined in a study measuring anatomical temporal-lobe asymmetries. Linear measurements of the length of the Sylvian fissure were taken on each cerebral hemisphere to evaluate lateral differences related to development of auditory association cortex. The results indicate significantly greater Sylvian fissure length on the left hemisphere than on the right hemisphere in four of these species. Measurements of a different parameter on Saimiri sciureus brain specimens (length of anterior portion of the Sylvian fissure) also suggested temporal-lobe asymmetry favoring the left hemisphere. Other measurements Oength of the Sylvian fissure lying posterior to the central sulcus, and dorsoventral position of the Sylvian point) in Macaca mulatta and M. fascicularis did not reveal significant rightAeft-hemisphere differences. Sylvian-fissure length determined from photographs of M. mulatta hemispheres in contrast to results of direct measurements did not yield significant right/left-hemisphere asymmetry. We mention possible reasons why previous anatomical studies of brains from monkeys did not discern temporal-lobe asymmetry, and we also discuss whether or not certain of these asymmetries in monkeys foreshadowed the evolution of language-processing areas of the cerebral cortex in hominids. “Results of several behavioral studies suggest that in certain macaque species auditory discrimination abilities often are better developed in the left-hemisphere temporal lobe of the brain than in the right temporal lobe. Experimental studies of the Japanese macaque (Macaca fuscatu) (Petersen et al., 1978, 1984;Beecher et al., 1979;Zoloth et al., 1979) have provided most of the evidence for the existence of this brain asymmetry. These investigations have evaluated lateralized auditory discrimination function by measuring macaques’ ability to distinguish between two semantically distinct fragments from this species’ vocalizations, presented either to the right or the left hemisphere via the contralateral ear. Presentation of the vocalization excerpts to the macaques’ left hemispheres appears to elicit significantly better discrim- 0 1988 ALAN R.LISS. INC ination between the two syllables than righthemisphere presentation. This finding has been corroborated by Heffner and Heffner (1984); these researchers ablated either right- or left-hemisphere temporal-lobe auditory association cortex from several M. fuscata prior to conducting the same test of auditory discrimination function. Only left-hemisphere-ablated monkeys showed impaired ability to distinguish between the two syllables. This result corroborated the greater importance of left- than right-hemisphere temporal-lobe cortex for auditory discrimination functions in macaques. Furthermore, results of an earlier Received June 18, 1987; revision accepted September 8, 1987. 40 P.L. HEILBRONER AND R.L. HOLLOWAY study (Dewson, 1977) indicate that in Macaca rnulatta left-hemisphere temporal-lobecortex plays a more important role than right-hemisphere cortex in mediating auditory-visual matching, a related association function. During much of the ten-year period in which these behavioral findings were reported, studies of nonhuman primate brain morphology (LeMay, 1976; Yeni-Komshian and Benson, 1976; Falk, 1978) failed to identify an anatomical correlate of the lateralized auditory discrimination abilities reported in functional studies. These research efforts, in examining brains or endocranial casts from macaques or other Old World monkey species, looked for righaeft-hemisphere differences in the anatomy of regions of the temporal lobe mediating auditory association operations. Only nonsignificant morphological asymmetries were demonstrated.” Recently, however, Falk et al. (1986) have presented evidence for anatomical temporallobe asymmetry in the brain of Macaca mulatta. Using a new measurement technique (image-analysis of composite photographs) these workers found mean length of the lefthemisphere Sylvian fissure to be significantly greater than mean length of the right Sylvian fissure, in a sample of cranial endocasts (n = 10)from this species. This finding is of great interest as the Sylvian fissure borders cortex subserving auditory association abilities, suggested by behavioral studies to be better developed in the left temporal lobe than in the right hemisphere in Macaca. In the present study we investigated a number of questions to improve our understanding of temporal-lobe asymmetry in nonhuman primates: 1. Why did anatomical studies previous to Falk et al. (1986) fail to discern temporal-lobe brain asymmetry in Old World monkeys? 2. Are significant morphological asymmetries between the right- and left-hemisphere temporal lobe present in monkey taxa other than Macaca? 3. In what ways does the pattern of temporal-lobe asymmetry in monkeys resemble the pattern of asymmetry found in Homo sapiens? 4 . What morphological and functional changes might have taken place during the evolution of these brain asymmetries in primates? MATERIALS AND METHODS Linear measurements designed to assess anatomical development on the temporal lobe auditorycortex region were taken on the right and left cerebral hemispheres of formalin-fixed brain specimens from five New World and Old World monkey species: Macaca fmcicularis, Macaca mulatta, Saguinus oedipus, Callithrix jacchus, and Saimiri sciureus. Taxonomic diversity and specimen availability for attaining large sample size were primary considerations in choosing these five taxa. Measurements were taken on the largest possible brain samples obtainable (n = 20-30 brain specimens per species; see Table 1). With the exception of S. sciureus, all brain specimens were from adults or older juvenile animals, as monkeys from this age range show fully developed cerebra with respect both to size and external morphology (Connolly, 1950; Chi et al., 1977). Because of a scarcity of adult specimens, however, some brain specimens from infants were by necessity included in the S. sciureus sample. Separate statistical tests were performed to distinguish findings €or S. sciureus TABLE 1. Brain specimens examined in study Species M. mulatta M. fascicularis S. oedipus C. jacchus S. sciureus S. sciureus (ad.) Total N Males Females Source’ 30 30 27 26 20 11 14 18 14 14 8 3 16 12 13 12 12 9 WRPC; NE; B G OR NE; BG; OR OKRG OKRG USAL; BG USAL; BG ‘Sources of brain tissue are abbreviated as follows: WRPC: Wisconsin Regional Primate Center; NE: New England Regional Primate Center; BG: Bowman-Gray School of Medicine (N.C.); OR: Oregon Regional Primate Center; OKRG Oak Ridge Assoc. Univ. (TN); USAL: Univ. of S. Alabama School of Medicine (Mobile, AL). 41 ANATOMICAL BRAIN ASYMMETRIES INTERPARIETAL S. LUNATE S. CENTRAL FISSURE O N S.F. POSTERIOR P O I N T O N S.F. CLOSEST P O I N T O N S.F. TO CENTRAL FISSURE Fig. 1. Sylvian fissure landmarks:Macaca. adults from results for the mixed-age (adults + infants) S. sciureus sample. All brain specimens came from monkeys that had died from a non-neurological cause and showed no discernable signs of pathology on the cortex. In each sample the number of brain specimens representing males and females was as close to equal as specimen availability permitted. Cerebral asymmetry in several species, including humans, has been shown to vary as a function of sex (McGlone, 1980; Diamond, 1984). We therefore attempted to account for this factor; in addition, a study by the authors is currently investigating sex differences in cerebral asymmetry in monkeys. Brain specimens were midsagitally sectioned, and slide photographs of the lateral surfaces were taken for subsequent measurements. Except for two assessments (2 and 5 below) linear measurements of sulcus length were made by hand directly on the cerebral cortex of specimens with flexible (plasticcoated fabric) measuring tape. The pia mater was left on the brain to prevent the measuring tape from falling deeply into the sulcus being measured. Straight pins were affixed parallel to each of the first six every centimeter marks on the tape. During each measurement the points became anchored in the pia mater, preventing slippage of the tape along the sulcus. The measuring tape was fitted to the contour of landmarks (portions of the Sylvian fissure) rostrocaudally between designated endpoints; length was estimated to the nearest 0.5 mm. Each measurement taken on each brain hemisphere was repeated, and the repeats were averaged (to ensure accuracy) to provide a final figure used in statistical operations. The following estimates were taken on both the right and left cerebral hemisphere (see Figs. 1-3): 1. The first estimate was of the length of the Sylvian fissure. This parameter was defined as the distance along the Sylvian fissure from the most anterior visible point on the fissure (determined by the investigator from a vantage point lateral to the temporal pole) posterior to the caudal endpoint of the fissure. 2. A second method for measuring length of the Sylvian fissure was also used. Our purpose here was to compare the accuracy of linear measurements taken on photographs of brain specimens with measurements made on brain specimens directly by hand. This comparison was made using Macaca mulatta specimens;right- and left-hemisphere lateral cortical surfaces of these specimens were 42 P.L. HEILBRONER AND R.L. HOLLOWAY CENTRAL SULCUS ANTERIOR P O I N T INTERPARIETAL SULCUS CLOSEST POINT T O CENTRAL SULCUS Fig. 2. Sylvian fissure landmarks: Saimiri. Fig. 3. Position of Sylvian point Macaca. photographed with slide film, and the course of the Sylvian fissure and border of the cerebral hemisphere were traced onto paper from the projected image of each slide (see below for description of method used for photography and tracing). The length of the sinuous outline in the tracing representing the Sylvian fissure was then measured by hand with the flexible measuring tape. 3. The Sylvian fissure in Saimiri sciureus (and not the other four species) posterior to the central sulcus comprises the interparietal sulcus. This landmark extends to the longitudinal fissure, bordering for the most part visual association cortex, mediating functions not relevant to this study. The length of the part of this fissure complex lying ante rior to the central sulcus, adjacent mostly to cortical auditory fields, was therefore measured on brain specimens from S. sciureus. 4. On the 60 Macaca mulatta and Macaca fmcicularis brain specimens, length of the part of the Sylvian fissure posterior to the closest point to the central sulcus (the Sylvian-fissure segment showing the most pronounced rightAeft-hemisphere difference in length in Homo sapiens: Shellshear, 1937) was measured. This measurement was not taken on specimens from the other three species: the central sulcus is not present in C. jacchus and S. oedipus, and in S. sciureus the Sylvian fissure posterior to the central sulcus is referred to as the parieto-occipitalfissure. 5. Dorsoventral position of the posterior endpoint of the Sylvian fissure (Sylvian point) was also determined on M. mulatta and M. fmcicularis specimens. These were the only brains examined on which the Sylvian fissure terminates before reaching the longitudial fissure and that were also large enough to allow for an accurate measurement of Sylvian-point height. To measure Sylvian point position, a slide of the lateral surface of each Macaca mulatta or Macaca fmcicularis brain hemisphere was taken. The method used here was the following: first, the hemisphere was 43 ANATOMICAL BRAIN ASYMMETRIES TABLE 2. Length of Sylvian fissure (ern)’ Left hemisuhere Species Mean S si Mean Right hemisphere S si t P< M.fascicularis 3.218 0.338 0.063 3.093 0.322 0.060 2.82 ,008 4.045 0.243 0.046 3.920 0.236 0.046 3.12 ,004 1.285 0.139 0.026 1.224 0.120 0.023 2.10 ,046 1.177 0.118 0.024 1.117 0.120 0.024 2.23 .035 3.528 0.403 0.092 3.517 0.298 0.068 0.21 .839 3.638 0.401 0.111 3.563 0.281 0.078 1.22 ,250 (n = 30) M. mulatta (n = 29) S. oedipus (n = 27) C. jacchus (n = 26) S.sciureus (mixed ages) (n = 20) S. sciureus (adults) (n = 14) lS = standard deviation; sx = standard error;t = Student’s t-statistic (paired samples). TABLE 3. S. sciureus: Length of Sylvian fissure anterior to central sulcus (cm)’ Sample n Mean Left hemisphere S sir Mean Right hemisphere sir S Mixedages Adults 17 11 1.832 1.918 0.107 0.064 0.026 0.020 1.285 1.759 0.106 0.079 0.027 0.024 t 3.75 2.93 P< .002 .015 ‘See Table 2 for abbreviations. placed medial surface down on a level platform. An Olympus OM-F 35-mm camera with macrolens adaptor, mounted on a tripod raised to a standard height for all specimens that was sufficient to allow the entire lateral surface of the brain hemisphere to appear in focus in the viewfinder, was positioned above the specimen. The camera back was leveled. The brain hemisphere was positioned so that in the viewfinder the Sylvian point appeared in the very center of the focusing circle. A slide was then exposed. Each developed slide was projected onto a piece of 8” x 11” white paper, using a darkroom enlarger kept at a fixed position (for all slides). The course of the Sylvian fissure and border of the cerebral hemisphere were traced by hand onto the paper from the projected image. The distance to the posterior endpoint of the Sylvian fissure, perpendicular from a straight line drawn connecting frontal and occipital poles, was measured on each tracing with Vernier calipers to estimate Sylvian point position. These tracings were also used to estimate length of the Sylvian fissure in M. mulatta, as described in point 2 above. Two-tailed paired-sample t-test (Tables 2, 3 , 6 , 7 , 10) and chi-square test (Tables 4,5,8, 9) values evaluating the statistical significance of asymmetry for each measurement in each sample were calculated using the Statistical Package for the Social Sciences (SPSS, 1985). The &Testsassessed the significance of the rightAeft-hemisphere differences in mean length or height. Two chisquare statistics were calculated. “Chisquare (3)” compared frequences considering three cells (numbers of cases showing greater left-hemisphere, greater right-hemisphere, or equal anatomical development); “chi-square (L-R)” compared numbers of cases showing asymmetry only (in other words, not the L = R cases). RESULTS In four of the six brain samples (all species except S. sciureus) mean length of the lefthemisphere Sylvian fissure was significantly greater than mean length of the right-hemisphere Sylvian fissure (see Table 2). A tendency toward a disparity between the lengths of these landmarks, in these species, was also suggested by chi-square statistics. These values assessed relative numbers of brain specimens showing greater Sylvian fissure length on the right, or left hemisphere (See Table 6); most Chi-square values were significant at the P < .05 level, left-hemisphere predominance being evident in all samples. 44 P.L. HEILBRONER AND R.L. HOLLOWAY TABLE 4. Length of Syluian fissure from tracings (cm) in M. mulatta only (n = 28)' Mean Left hemisphere S s, Mean S Si t 2.030 0.390 12.819 2.172 0.418 0.001 12.807 Right hemisphere P< ,999 IProjected images from enlarged tracings. See Table 2 for abbreviations. TABLE 5. Frequency data: length of Syluian fissure from tracings in M. mulatta' No cases L=R No cases L>R No cases R>L 2 13 Chi-square (2) 0.00 13 'L > R, No. of cases of greater left hemisphere value; L = R, NO.of cases of equal right- and left-hemisphere value; R > L, No. of cases of greater right hemisphere value; Chi-square(2), chi-squarevalue considering all of above cases (df = 2). TABLE 6. Frequencies and chi-square statistics: length of Syluian fissure (all species)' Species M. fascicularis M. mulatta S. oedipus C. jacchus S.sciureus (mixed-age) S.sciureus (adults) L>R No. of cases L=R R>L Chi-square(3) Chi-square (L-R) 18 18 16 13 9 3 4 5 9 3 9 7 6 4 8 11.40* 12.20* 8.72* 4.69 3.10 3.00 4.84* 4.54* 4.76* 0.06 7 1 4 1.50 0.81 'Chi-sq.(3), chi-square value considering all of above cases (df = 2); Chi-sq. (L-R), chi-square value considering all but L = R cases (df = 1). See table 5 for other abbreviations. *Significant at P < .05 level. TABLZ Z Frequency statistics: length of Syluian fissure anterior to central sulcus in S.sciureus' No. L > R S.sciureus (mixed-age) S.sciureus No. R = L No. R > L Chi-square (3) Chi-square (L-R) 12 2 3 10.71* 5.40* 8 1 2 7.83* 3.60 *Significant at P < .05 level. 'See Tables 5 and 6 for abbreviations. Statistics on length of the Sylvian fissure from tracings made from slide photographs of M. mulatta specimens did not indicate the presence of statistically significant rightAeftside asymmetry (see Tables 4,5). Mean righthemisphere Sylvian-fissure length as determined by the photographyltracing method was almost identicaI with mean left-hemisphere fissure length, and the number of brain specimens showing greater Sylvian-fissure length in right-hemisphere tracings was the same as that showing greater fissure length in left-hemisphere tracings. In both S. sciureus samples mean length of the Sylvian fissure in the left hemisphere, determined from measurements made by hand on the cortex, did not differ significantly from the comparable right-hemisphere statistic (Tables 2,6). Chi-square statistics also did not indicate a significant difference between the length of the two Sylvian fissures in these samples. Length of the segment of the Sylvian fissure lying anterior to the central sulcus was, however, also determined on S. sciureus specimens (see Tables 3,7). It will be recalled that 45 ANATOMICAL BRAIN ASYMMETRIES TABLE 8. Length of Sylvian fissure posterior to central sulcus (cm) in M. fascicularis and M. mulatta' Left hemisphere Right hemisphere Mean S SDecies Mean S Si SC t P< M. fascicularis 1.597 0.300 0.055 1.525 0.309 0.245 1.32 ,197 2.181 0.198 0.037 2.095 0.275 0.051 1.59 .123 t P< (n = 30) M. mulatta (n = 29) 'See Table 2 for abbreviations. TABLE 9. Dorsoventral position of Sylvian point (cm in tracings)' Left hemisphere S sir S sir Right hemisphere Mean SS sx Mean sx Species Species Mean Mean M. fascicularis (n = 30) M. mulatta (n = 30) 1.880 0.309 0.057 1.679 0.362 0.067 1.59 .124 1.802 0.434 0.081 1.842 0.376 0.070 -0.56 .583 'See Table 2 for abbreviations. TABLE 10. Frequency and chi-square statistics: length of Sylvian fissure posterior to central sulcus, dorsoventral position of Sylvian p o d Species M. fascicularis (SF from CS) M. mulatta (SF from CS) M. fascicularis (Sylvian point) M. mulatta (Sylvian point) L>R No. of cases L=R R>L Chi-square(3) 15 4 11 6.20* 0.62 18 4 7 11.25* 4.84* 17 2 11 11.90* 1.20 13 4 13 5.40 0.00 Chi-square(L-R) 'See Tables 5 and 6 for abbreviations. *Significant at P < .05 level. the anterior part of the Sylvian fissure, in this species, borders mostly auditory fields. In both the adult and mixed-age sample from this species, mean length of the left-hemisphere anterior Sylvian fissure was significantly greater than mean right-side length. Three of four chi-square statistics for S. sciureus also suggested a tendency toward greater length of this fissure segment in the left cerebral hemisphere than in the right. Results of other measurements did not suggest morphological righaeft-hemisphere asymmetry. Most statistics for length of the portion of the Sylvian fissure lying posterior to the central sulcus (Tables 8, 10) did not reveal the existence of a statistically significant right/left hemisphere anatomical difference in the M. mulatta or in the M. fascicularis sample; likewise measurements of dorsoventral position of the Sylvian point (Tables 9,lO) did not indicate significant asymmetry in Macaca. DISCUSSION In four of the six monkey brain samples examined in the present study, mean length of the left-hemisphere Sylvian fissure was significantly greater than mean length of the right-hemisphere Sylvian fissure. This asymmetry was present in the M. mulatta, M. fascicularis, C. jacchus, and S. oedipus samples. The two S. sciureus brain samples also exhibited a significant temporal-lobe asymmetry favoring the left hemisphere, as revealed by data for measurements of length of the part of the Sylvian fissure lying anterior to the central sulcus. These findings are consistent with results reported by Falk et al. (1986) that indicated Sylvian-fissure asymmetry, favoring the left hemisphere, in Macaca mulatta. The fact that both the present study and Falk et al. (who used a computer image-analyzing system to evaluate configuration of sulci from composite photographs) discerned 46 P.L. HEILBRONER AND R.L. HOLLOWAY anatomical temporal-lobe brain asymmetry in brains of monkeys, while earlier studies (Yeni-Komshian and Benson, 1976; Falk, 1978) failed to find this asymmetry, may reflect measurement techniques and study design utilized. Although they were statistically significant, mean rightAeft-hemisphere sulcus-length differences in the present study were small (on the order of 1-2 mm). Data on petalias (asymmetrical lateral or anteroposterior elongations of the right or left cerebral hemisphere; Gundara and Zivanovic, 1968; Holloway and DeLacoste-Lareymondie,1982; LeMay et al., 1982) show that gross anatomical asymmetries of this order of magnitude are common-even typical-in nonhuman primate and human brains. These characteristic departures from cerebral symmetry in the primate brain necessarily cause right1 left-hemisphere differences in the curvature of the cortex. Cortical curvature can foreshorten the length of a sulcus as it appears in a photograph; in this respect, we believe it significant that data for length of the Sylvian fissure from photographs of M.mulatta hemispheres did not reveal statistically significant asymmetry. Falk’s 1978 anatomical study of monkey endocasts (Falk, 19781, which relied on photographs, is also noteworthy for the absence of rightAeft-hemisphere asymmetries reported. Aside from unreliable measurement techniques, problems in some previous studies include conclusions drawn from small samples and the tendency to lump together data for species in the same genus (presenting scaling difficulties caused by increased variation in brain size, and obscuring speciesspecific characteristics). Moreover, findings of a study by the authors that is in preparation suggest that sexual dimorphism in temporal-lobe asymmetry may be evident in some nonhuman primate species. This observation suggests another possible explanation for nonsignificant results in previous studies, as none of the above research efforts accounted for numbers of brain specimens representing females and males in samples examined. In summary we would suggest that the present study succeeded in demonstrating the existence of a morphological temporal-lobe brain asymmetry in monkeys by 1) examining adequately large samples of brain specimens 2) analyzing data at the level of the species, not the genus, 3) using better measurement techniques, and 4) examining sam- ples reasonably well balanced in terms of numbers of specimens representing males and females. While our study is the first to report anatomical temporal lobe asymmetry in New World monkeys and is only the second investigation to observe this asymmetry in Macaca, anatomical temporal-lobe lateral differences have been reported in several studies of pongid brains. Investigators have noted the same rightAeft-hemisphere asymmetry found in the present study-significantly greater mean length for the Sylvian fissure in the left hemisphere than in the right hemisphere-in Pan troglodytes and Pongo pygmaeus brain samples (Cunningham, 1892; Eberstaller, 1884; Yeni-Komshian and Benson, 1976).(It is worth noting that all of these investigators measured Sylvian-fissure length directly on the cortex of brain specimens, using foils shaped to the contour of the cortex or thread tied to pins embedded at intervals along the fissure.) In Homo sapiens the Sylvian fissure is also usually longer on the left cerebral hemisphere than on the right. This asymmetry has been reported in neuroanatomical studies dating back almost a century (Cunningham, 1892; Von Economo and Horn, 1930; Connolly, 1950; Yeni-Komshian and Benson, 1976).Furthermore, it is well known that in humans left-temporal lobe auditoryassociation cortex plays a more important role in language functions than does righthemisphere temporal-lobe cortex (Penfeld and Roberts, 1959). Did morphological and behavioral temporal-lobe auditory cortex asymmetry in nonhuman primates presage language lateralization in humans? Behavioral studies indicate that monkeys may exhibit left-temporal-lobe specialization for recognition of acoustic features of species-specificvocalizations (Petersen et al., 1978, 1984; Beecher et al., 1979; Heffner and Heffner, 1984); crossmodal matching functions also may show greater development in the left- than in the right-hemisphere temporal lobe in monkeys (Dewson, 1977). It might therefore be suggested that these lateralized cognitive abilities in monkeys foreshadowed the evolution of brain operations used in language, such as auditory analysis and verbal conceptualization functions. Caution appears warranted, however, in extrapolating from evidence for temporallobe asymmetry in nonhuman primates. It ANATOMICAL BRAIN ASYMMETRIES will be recalled that a statistically significant rightneft-hemisphere difference in mean length of the portion of the Sylvian fissure lying posterior to the central sulcus was not found in the M. mulatta or in the M. fmcicularis brain sample in the present study. In contrast, in Homo sapiens this segment typically shows the most pronounced rightnefthemisphere difference of any portion of the Sylvian fissure (Shellshear, 1937; Penfeld and Roberts, 1959).This asymmetry in Homo in turn probably reflects a difference between the two hemispheres in the size of the planum temporale, a structure on the supratemporal plane adjacent to the posterior part of the Sylvian fissure, consisting of cortex necessary for auditory analysis and verbal comprehension functions; studies have shown that in most human brains the planum temporale is larger on the left hemisphere than on the right (Penfield and Roberts, 1959; Geschwind and Levitsky, 1968; Wittleson and Pallie, 1973; Chi et al., 1977; Galaburda et al., 1978; Galaburda and Sanides, 1980). It is probably significant that this structure is not found in any other primate species (Cunningham, 1892; Connolly, 1950; Yeni-Komshian and Benson, 1976). Rightneft asymmetry for height of the Sylvian point was not found in the Macaca mulatta and Macaca fmcicularis samples in the present study. This pattern again differs from that in humans: in Homo sapiens the Sylvian point is usually situated more ventrally on the left cerebral hemisphere than on the right (Connolly, 1950; LeMay and Geschwind, 1975; Rubens, 1977), an asymmetry that again may reflect cognitive specialization of the left cerebral hemisphere. The Sylvian point lies adjacent to the inferior parietal lobule, a region on the left hemisphere associated with mediation of verbal conceptualization abilities, naming functions, reading abilities, and other multimodal processes (Penfield and Roberts, 1959). Results of a comparative study of a nerve fibre tract, the arcuate fasciculus (Galaburda and Pandya, 1982), suggest other differences between the temporoparietal area in the human and in the Old World monkey. The arcuate fasciculus connects temporal-lobe cortex to premotor frontal areas, in Homo s a p iens linking the temporal-lobe auditory area with the motor speech (Broca’s) region. In Galaburda and Pandyas’ study the human arcuate fasciculus was shown to include axons originating from the inferior parietal lob- 47 ule as well as efferent fibres of the temporal lobe; in the brain of Macaca, by comparison, the arcuate fasciculus did not make parietallobe connections. Data on arcuate fasciculus connections in pongids is lacking. However, results of anatomical studies show that the characteristic tendency in Homo sapiens for the Sylvian point to show asymmetry is shared by Pan, Gorillq and Pongo (LeMay and Geschwind 1975; LeMay, 1976; LeMay et al., 1982). It can be inferred from these studies that cortex surrounding the left temporoparietal area may have started to change, in comparison to its right-hemisphere analogue, prior to the divergence of hominids from pongids. On the basis of evidence from the present study it can be surmised that this development took place after the appearance of the first hominoids. “Evidence from this and other studies suggests that monkeys may exhibit aspects of the human pattern of temporal lobe asymmetry. Overall, however, temporal lobe lateralization in these primates appears to differ from the asymmetry pattern in Homo. It is of interest that in Heffner and Heffners’ study (1984), anterior temporal-lobe lesions in Macaca caused impairment, whereas posterior lesions did not cause dysfunction. Moreover, preliminary results of a study of cytoarchitectonics in Macaca fuscata suggest that posterior temporal-lobe subfields do not show rightlleft-hemisphere asymmetry in size, as they do in Homo sapiens (Rosen and Galaburda, personal communication; Galaburda and Sanids, and Geschwind, 1978). 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