Overall the results indicate that there are differences between the handedness groups in terms of brain structure and brain function including brain sulci, GM volume, WM anisotropy and language processing. Furthermore, there are differences in the neuroanatomical correlates of intelligence including intentionality, fluid and crystallised intelligence.
The novel findings in this thesis are: (i) a relationship between handedness and sex on PO volume asymmetry, (ii) an effect of handedness on the sulcal contours defining the PO and PTR, and (iii) greater anisotropy in right- than left-handers in regions throughout the frontal lobe and in regions of the temporal lobe. This thesis presents for the first time (iv) voxel-wise statistical analysis of asymmetric FA images in a large cohort composed solely of left-handers. Moreover, presented here for the first time is (v) a relationship between language and spatial laterality interaction and the cognitive variables: verbal comprehension and perceptual organisation. This thesis also contributes to the literature in this field by showing for the first time (vi) the neuroanatomical correlates of intentionality in a cohort comprised solely of left-handers, (vii) differences in the neuroanatomical correlates of intentionality between left- and right-handers, and (viii) significant differences in the GM correlates of fluid and crystallised intelligence.
Sulcal contours in the brain
The effect of handedness on sulcal contours in the brain.
In right-handers the inferior frontal sulcus (IFS) is discontinuous more often in the right than the left hemisphere (62% vs. 43%), while in left-handers it is discontinuous more often in the left than the right hemisphere (65% vs. 48%) although the difference in left- handers was not significant. The present study supports that of previous studies which suggest variability in the continuity of the IFS (Keller et al., 2007, 2009b; Ono et al., 1990) and this can be seen in Table 10.1 which shows continuity of the IFS in the left
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and right-hemisphere in each study reviewed. Findings in the left hemisphere in right- handers (i.e. 43%) are similar to previous reports (ranging from 40-46%), whereas a discontinuous IFS in the left-hemisphere in left-handers (i.e. 65%) occurred more often than has previously been reported in right-handers. A greater degree of variability of discontinuous IFS is seen in the right hemisphere (see Table 10.1). Results for either handedness group however, cannot be compared directly to these previous studies as Keller et al (2007) used a sample of left- (n=13) and right- (n=37) handed subjects, Ono et al (1990) do not report the number of right-handed subjects studied in their sample of 2 post-mortem brains and Keller et al (2009b) do not report handedness in their sample of 30 subjects.
Variability is further reported in connection patterns between the IFS and IPCS, the incidence of ‘no connection’ between these two sulci ranging from 12% to 33% (Ono et al., 1990; Keller et al., 2007). The current study reports no connection in 19% left and 29% right hemispheres for right-handers and 30% left and 25% right hemispheres for left-handers, all within the range reported in previous studies.
The second significant association related to the sulcal contours was between presence of the DS and handedness. The probability of presence of the diagonal sulcus (DS) is higher in the right than in the left hemisphere for left-handers (63% vs. 28%), although not significantly so for right-handers (64% vs. 45%). Table 10.1 shows the percentage of left and right hemispheres presenting a DS which have been reported in a number of different studies. It can be seen that the DS was present almost as often in the right hemisphere in left- (63%) and right-handers (64%), a figure which is the same as that reported by Ono et al (1990) i.e. 64% for right-handers at least. The presence of the DS has been associated with increased PO volume (Keller et al., 2007, 2009b), presumably as a result of increased intrasulcal area which the DS creates.
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Table 10.1. Percentage of individuals with a discontinuous IFS and present DS in five different studies including the present study. Figures are given in percentages. RH=right-handers, LH=left-handers, nr=not reported.
Discontinuous IFS Presence of DS left hemisphere right hemisphere left hemisphere right hemisphere Present Study n=42RH, 40LH RH: 43% LH: 65% RH: 62% LH: 48% RH: 45% LH: 28% RH: 64% LH: 63% Keller et al (2009b) n=30, handedness=nr 43% 37% nr nr Keller et al (2007) n=37RH, 13LH 46% 50% 52% 20% Ono et al (1990) n=25, handedness=nr 40% 56% 72% 64% Galaburda (1980) n=102, handedness=nr nr nr 26.5% 12.75%
Whilst differences in the sulcal contours defining the PO and PTR are found the functional role of the sulci is not clear. One suggestion is that the sulci increase the surface area of a region and therefore the neuronal number underlying the surface area. This would then increase the information processing capacity (IPC) of the area (see Section 2.4 for the functional interpretation of surface area). For instance, the presence of the DS in the PO is thought to increase the surface area of the region thereby increasing its IPC (Keller et al., 2007). In this thesis presence of the DS is similar in the right hemisphere in left- and right-handers and both handedness groups show similar right hemisphere PO volume. However a DS is present more often in the left hemisphere PO in right- than left- handers and this may explain the significantly larger left hemisphere PO volume right- than left-handers. An increase in neuronal number would then increase the IPC of the region and may explain the greater involvement of the left hemisphere is language in right-handers than left-handers.
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Relative pars opercularis and pars triangularis volume
The results presented in Chapter 5 showed leftward PO volume asymmetry in right- handed males only, with no significant difference (in fact no numerical mean difference at all) between left and right PO in right-handed females. Male and female left-handers showed rightward PO volume asymmetry. Significant differences in PO asymmetry were found between left- and right-handed males and between left- and right-handed females. This supports previous studies which have shown leftward asymmetry of the PO (e.g. Foundas et al., 1998; Uylings et al., 2006). For example, in a post-mortem study Uylings et al (2006) found greater PO volume in the left hemisphere in all 10 subjects (5 male): asymmetry was only significant for the subgroup of males. These findings resemble the present results in right-handers however, handedness was not reported by Uylings et al (2006).
The findings indicating symmetry of PO volume (i.e. no signficiant difference between left and right PO volume) in right-handed females support previous studies which indicate hemispheric specialisation is less marked in females (Baxter et al., 2003; Gur et al., 2000; Jaeger et al., 1998; Kansaku et al., 2000; Rossell et al., 2002; Shaywitz et al., 1995; Vikingstad et al., 2000).
Significant differences in PO volume asymmetry were found between left- and right- handed males. The effect of handedness seems to be greater for males; this is supported by VBM studies on structural images (Pujol et al., 2002; Watkins et al., 2001), as well as morphology (Witelson and Kigar, 1992), morphometry (Witelson, 1989) and DTI (Hagmann et al., 2006) studies using healthy subjects. For instance, Witelson (1989) found that handedness was a factor in corpus callosum size for males but not females. Witelson and Kigar (1992) documented anatomical details of the Sylvian fissure as a measure of language lateralisation in 67 post-mortem brains (24 males), and found that these correlated with handedness in males but not females: specifically, right-handed males had longer horizontal Sylvian fissure segments in both hemispheres than males who were not consistently right-handed, while the direction and magnitude of asymmetry did not differ between these two groups. No significant difference in PO volume asymmetry was found between left- and right-handed females, which supports the above literature.
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Whilst the results demonstrate significantly larger left PO volume in right-handers than left-handers, no significant difference was found for right PO volume. This suggests that the significant difference in PO asymmetry between the handedness groups is the result of smaller left PO volume as opposed to increased right PO volume. This interpretation is similar to that of Foundas et al (2002), who investigated asymmetry of the planum temporale in 37 right-handers and 30 left-handers and found that only the size of the left planum was significantly related to handedness: in this sample right- handers had significantly larger left planum than left-handers. The present findings also indicate that volume of the right PO is less variable than the left PO across handedness and sex groups.
Right-handed males and right-handed females had larger left-than-right relative PTR volume, whilst left-handed males and left-handed females showed no significant difference between left and right PTR. The present findings appear to be consistent with that of previous studies which show a greater degree of variability in PTR asymmetry in left-handers compared to right-handers (e.g. Foundas et al., 1995, 1998). For instance, Foundas et al (1995) showed that 7/8 (88%) right-handers had a larger PTR on the left, with more variable asymmetry in left-handers. Additionally, Foundas et al (1998) found a significant leftward asymmetry of the PTR in right- and left-handers, although this asymmetry was reduced in left-handers who presented leftward asymmetry in 9 out of 16 cases, compared to 11 out of 16 cases in right-handers. Overall findings from the present study are consistent with studies reporting significant leftward volume or surface area asymmetries of the PTR, particularly in right-handed people (Foundas et al., 1995, 1996, 1998, 2001).
No effect of sex was found on PTR volume asymmetry. A number of studies have reported an effect of sex on brain structure and function (for a review see Cosgrove et al., 2007), however, a meta-analysis of the fMRI data from 2,151 subjects from 26 studies found no effect of sex on language lateralization (Sommer, 2010). Inconsistent findings related to the effect of sex on language lateralization suggest that the difference, if any, in the functional organisation of language and language associated cortex is small. The findings indicate that the effect of sex on brain structure is highly variable and not always consistently observed in language associated cortex.