Neurobiology of Aging 36 (2015) 301e303
Contents lists available at ScienceDirect
Neurobiology of Aging journal homepage: www.elsevier.com/locate/neuaging
Brief communication
Reduced dorsal premotor cortex and primary motor cortex connectivity in older adults Zhen Ni, Reina Isayama, Gabriel Castillo, Carolyn Gunraj, Utpal Saha, Robert Chen* Division of Neurology, Krembil Neuroscience Centre, Toronto Western Research Institute, University Health Network, University of Toronto, Toronto, Ontario, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 December 2013 Received in revised form 12 July 2014 Accepted 13 August 2014 Available online 15 August 2014
Motor functions decline with increasing age. The underlying mechanisms are still unclear and are likely to be multifactorial. There is evidence for disruption of white matter integrity with age, which affects cortico-cortical connectivity. Studies with transcranial magnetic stimulation found both inhibitory and facilitatory connections from dorsal premotor cortex (PMd) to the ipsilateral primary motor cortex (M1) in young adults. We investigated whether aging affects this connectivity in 15 older and 15 young healthy adults. Transcranial magnetic stimulation in a paired-pulse paradigm was used to test the connectivity between left PMd and M1. Motor evoked potential in the right first dorsal interosseous muscle was recorded. We found that both the inhibitory effect with low intensity PMd stimulation and the facilitatory effect with high intensity PMd stimulation observed in young adults were decreased in older adults. We conclude that the connectivity between PMd and ipsilateral M1 is reduced in older adults. Ó 2015 Elsevier Inc. All rights reserved.
Keywords: Aging Dorsal premotor cortex Motor evoked potential Primary motor cortex Transcranial magnetic stimulation
1. Introduction
2. Methods
Human motor functions decline with age. The underlying mechanisms are multifactorial and are still not fully understood (Ward, 2006). Increasing age leads to marked effects on motor cortical morphology (Kolb et al., 1998). However, neuronal loss in primary motor cortex (M1) is not evident in the normal aging process (Haug and Eggers, 1991). On the other hand, disruption of white matter integrity occurs with increasing age, affecting cortico-cortical connectivity. Significant decline in white matter integrity in frontal fibers correlated with decreased motor function in older adults (Sullivan et al., 2010; Ward, 2006; Wu and Hallett, 2005; Zahr et al., 2009). These raised the possibility that connectivity between secondary motor areas and M1 decreased with normal aging. Studies with transcranial magnetic stimulation found both inhibitory and facilitatory connections from dorsal premotor cortex (PMd) to the ipsilateral M1 in young adults (Baumer et al., 2009; Civardi et al., 2001). We investigated whether normal aging affects the connectivity between PMd and M1. We hypothesize that both the inhibitory and facilitatory connectivity between PMd and M1 seen in young adults are decreased in older adults.
2.1. Subjects
* Corresponding author at: Division of Neurology, University of Toronto, 7MC-411, Toronto Western Hospital, 399 Bathurst Street, Toronto, Ontario, M5T 2S8, Canada. Tel.: þ1 416 603 5207; fax: þ1 416 603 5004. E-mail address: [email protected] (R. Chen). 0197-4580/$ e see front matter Ó 2015 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.neurobiolaging.2014.08.017
We studied 15 young (7 women; age range, 20e32 years; mean age, 25.0 3.6 years) and 15 older healthy adults (7 women; age range, 59e77 years, mean age, 65.3 5.1 years). All subjects were right-handed. Subjects with a history of neurologic or psychiatric disorders and those who took central nervous system active drugs including antidepressants, benzodiazepines, and hypnotics were excluded. 2.2. Experimental protocol Motor evoked potentials (MEPs) were recorded using surface electromyograms from right first dorsal interosseous muscle. Transcranial magnetic stimulation in a paired-pulse paradigm was used to test the connectivity between left PMd and left M1 (Fig. 1A). A Magstim 2002 stimulator and a small figure-of-8 shaped coil (5 cm diameter) (Magstim, Dyfed, UK) were used to deliver left M1 stimulation in a posterior-anterior current direction. The stimulation intensity was set at “1 mV” (to produce approximately 1 mV MEP in amplitude when given alone). PMd stimulation was applied to the location 2.5 cm anterior and 1 cm medial to the location for M1 stimulation (Ni et al., 2009). Another figure-of-8 shaped coil (4 cm diameter) was used to deliver anterior-posterior directed PMd stimulation. PMd stimulation followed by M1 stimulation at interstimulus
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were different from MEP amplitudes from M1 stimulation alone. A 2-way analysis of variance was used to test the effects of group (between-subject factor) and ISI or PMd stimulation intensity (within-subject factors). Post hoc testing used unpaired t test with Bonferroni correction for multiple comparisons. The threshold for significance was set at p < 0.05. Further details of the methods are provided in Supplementary Material. 3. Results
Fig. 1. Connectivity between dorsal premotor cortex and primary motor cortex in young and older adults. (A) Experimental design. The connectivity between left PMd and left M1 was tested with a paired pulse transcranial magnetic stimulation paradigm. PMd was measured 2.5 cm anterior and 1 cm medial to the target muscle presentation in M1 from scalp. The arrows indicate the direction of induced current in the brain. (B) Example of recordings obtained from 1 young adult and 1 older adult. The top row shows recordings with M1 stimulation alone (average of 20 trials for each recording). The bottom 3 rows show recordings with paired-pulse stimulation (average of 10 trials for each recording). ISIs of 4, 6, and 8 ms were tested. The left panel shows recordings with PMd stimulation intensity of 90% AMT. The right panel shows recordings with PMd stimulation intensity of 110% AMT. The left column in each panel shows recordings obtained from the young adult. The right column in each panel shows recordings obtained from the older adult. (CeE) Group analysis of the effects of PMd stimulation on MEP amplitude induced by M1 stimulation. Values are shown as mean with standard deviation. Open circles represent young adults and filled circles represent older adults. The ordinate indicates the MEP amplitude. It was normalized as a percentage value of MEP amplitude generated by the paired-pulse stimulation to that generated by M1 stimulation alone (dashed line). Values more than 100% indicate facilitation, those less than 100% indicate inhibition. * p < 0.05, ** p < 0.01, comparing MEP induced by pairedpulse stimulation to MEP induced by M1 stimulation alone. C and D show the data for 3 ISIs with PMd stimulation intensities of 90% and 110% AMT. The abscissa indicates the ISI between PMd and M1 stimulations. E shows the data for PMd intensity input-output curve at ISI of 6 ms. The abscissa indicates the PMd stimulation intensities. Abbreviations: AMT, active motor threshold; ISI, interstimulus interval; M1, primary motor cortex; MEP, motor evoked potential; PMd, dorsal premotor cortex.
intervals (ISIs) of 4, 6, and 8 ms at PMd stimulation intensities of 90% and 110% active motor threshold (AMT) was tested in all subjects. Furthermore, PMd intensity input-output curve from 70% to 140% AMT was tested at ISI of 6 ms in 8 young and 8 older adults. 2.3. Data and statistical analysis MEP amplitudes were measured peak-to-peak. The MEP amplitude evoked by paired-pulse stimulation was expressed as a percentage of the mean MEP amplitude evoked by M1 stimulation alone. Paired t tests were used to determine whether MEP amplitudes from paired pulses under various experimental conditions
Age was significantly different between the young and older adult groups (difference in mean age 40.3 years, t28 ¼ 25.03, p < 0.001). AMT and stimulation intensity for “1 mV” MEP were not different between 2 groups. Fig. 1 (B, C, and D) shows that the test MEP was inhibited by PMd stimulation with 90% AMT intensity at ISIs of 4 and 6 ms and was facilitated by PMd stimulation with 110% AMT intensity at ISI of 4 ms (p < 0.01 for all comparisons, paired t test) in young adults. In older adults, no inhibition was found with PMd stimulation at 90% AMT. With PMd stimulation at 110% AMT, facilitation seen in young adults turned into weak inhibition at ISIs of 4 ms (p < 0.05). Further between-subject analysis of variance revealed significant main group effect (90% AMT, F1,56 ¼ 8.61, p ¼ 0.007; 110% AMT, F1,56 ¼ 15.82, p < 0.001) for PMd stimulation at both stimulus intensities. The main effect of ISI was significant for 90% AMT (F2,56 ¼ 5.35, p ¼ 0.008) but not for 110% AMT PMd stimulation. The interaction between 2 main factors was significant for 110% AMT (F2,56 ¼ 9.27, p < 0.001) but not for 90% AMT PMd stimulation. Post hoc unpaired t test with 110% AMT PMd stimulation showed that older adults were different from young adults at 4 ms (p < 0.001) and 6 ms ISIs (p < 0.05). PMd intensity input-output curve at ISI of 6 ms (Fig. 1E) showed M1 inhibition at 90% AMT and M1 facilitation at 130%e140% AMT in young adults (p < 0.05 for all comparisons), but there was no effect in older adults. Analysis of variance revealed a significant group effect (F1,98 ¼ 4.84, p ¼ 0.045) and interaction between the 2 main factors (F7,98 ¼ 5.92, p < 0.001). The effect of PMd intensity showed a trend toward significance (F7,98 ¼ 2.06, p ¼ 0.056). Post hoc test showed that young and older adults were different at PMd intensities of 90% AMT and 110%e140% AMT (p < 0.05 for all comparisons). 4. Discussion The assessments of age-related motor cortical changes are not only important in the understanding of normal aging but also play a critical role in the understanding of many neurologic disorders such as neurodegenerative diseases and stroke that are common in older individuals. In young adults, we found that PMd stimulation with lower intensity inhibited the M1 at ISIs of 4 and 6 ms, and PMd stimulation with higher intensity facilitated the M1 at ISI of 4 ms, confirming the results of the previous studies that M1 excitability is modulated by PMd with both inhibitory and facilitatory inputs (Baumer et al., 2009; Civardi et al., 2001; Koch et al., 2007). These results were compatible with the anatomic connections and proposed the functional role of PMd in the motor cortical network with motor hierarchy where PMd receives direct inputs from dorsolateral prefrontal cortex, posterior parietal cortex and other cortical motor related areas, and projects its outputs to the M1 for movement execution (Hoshi and Tanji, 2007; Rizzolatti and Sinigaglia, 2010; Rizzolatti et al., 1998; Weinrich and Wise, 1982). In addition, the MEP inhibition and facilitation under various experimental conditions (ISIs and conditioning stimulus intensities) are consistent with previous studies showing that the PMd contains neurons with different structural and functional properties (Chouinard and Paus, 2006; Rizzolatti and Sinigaglia, 2010). Importantly, the novel finding in the present study is that both inhibitory and facilitatory inputs from PMd to M1 are decreased in older adults.
Z. Ni et al. / Neurobiology of Aging 36 (2015) 301e303
This was supported by the altered time course in older adults compared with young adults. Interestingly, decreased inhibition with 90% AMT PMd stimulation in older adults was found at all ISIs tested (Fig. 1C), whereas the decreased facilitation in older adults with 110% AMT intensity was only seen at shorter ISIs (Fig. 1D). This suggests that the effects of the aging processes for inhibitory and facilitatory neurons in PMd may be different. On the other hand, it may be argued that the difference between 2 subject groups is caused by a shift in firing threshold of PMd neurons in older adults with aging. We tested the PMd intensity input-output curve to address this question (Fig. 1E). Over a wide range of PMd stimulation intensities, we confirmed that both inhibition at low stimulation intensity and facilitation at high stimulation intensity were decreased in older adults. The result strongly suggests that the aging process leads to the loss of the complex connectivity between PMd and M1. This is consistent with the findings from a functional magnetic resonance imaging study that in a verbal encoding task there was reduced but less selective activation of the frontal regions in older compared with younger adults (Logan et al., 2002). Similarly, it was reported that the performance of automatic sequential finger movements required greater activity in cortical areas including the PMd in older compared with young adults when temporal information was processed in a complex motor task (Wu and Hallett, 2005). To achieve the same motor performance as young adults, more PMd neurons may be recruited in older adults to compensate for the reduced connectivity between PMd and M1. It has been reported in young adults that the inhibitory and facilitatory PMd-M1 connectivity was modulated during the reaction period, and the changes depend on the nature of the upcoming action, consistent with a role of the PMd in facilitating and suppressing cued movements (Koch et al., 2006). It is likely that the functional modulation of PMd-M1 connection before and during a motor task is also altered in aging and compensatory mechanisms may be activated. An event related potential study reported completely different patterns of PMd activity in older and young adults when spatial information with increasing complexity was being processed (Sterr and Dean, 2008). We cannot exclude the possibility that reduced connectivity between PMd and M1 is secondary to the functional decline of upstream cortical structures. In the normal aging, atrophy with reduction in gray matter volume is most significant in the dorsolateral prefrontal cortex (Good et al., 2001; Jernigan et al., 2001). Changes in neurotransmitter (e.g., dopamine) (Kaasinen and Rinne, 2002) or hormone levels in older adults may also influence our results. Neurobiological aging is a complex process with reduction in cortical connectivity and functions. The electrophysiological approach used in the present study showed that at the systems level, the connectivity between PMd and ipsilateral M1 is decreased in older adults. Decreased PMd and M1 connectivity may be related to impairments in the basic functions of PMd in motor planning, preparation, decision making, and execution of movement (Hoshi and Tanji, 2007), resulting in significant increase in coordination difficulty, variability, and slowness of movement with aging (Ward, 2006). In addition, inputs from PMd plays important role in compensation for the impaired motor functions after stroke (Johansen-Berg et al., 2002) and in preclinical Parkinson’s disease patients (van Nuenen et al., 2009). Reduction in connectivity between PMd and M1 in older adults may lead to less capacity in compensating for neurologic diseases.
Disclosure statement The authors reported no conflicts of interest. The protocol was approved by the University Health Network (Toronto) Research
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Ethics Board. All subjects provided written informed consent in accordance with the Declaration of Helsinki. Acknowledgements This study was supported by the Canadian Institutes of Health Research (MOP 62917). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.neurobiolaging. 2014.08.017. References Baumer, T., Schippling, S., Kroeger, J., Zittel, S., Koch, G., Thomalla, G., Rothwell, J.C., Siebner, H.R., Orth, M., Munchau, A., 2009. Inhibitory and facilitatory connectivity from ventral premotor to primary motor cortex in healthy humans at restea bifocal TMS study. Clin. Neurophysiol. 120, 1724e1731. Chouinard, P.A., Paus, T., 2006. The primary motor and premotor areas of the human cerebral cortex. Neuroscientist 12, 143e152. Civardi, C., Cantello, R., Asselman, P., Rothwell, J.C., 2001. Transcranial magnetic stimulation can be used to test connections to primary motor areas from frontal and medial cortex in humans. Neuroimage 14, 1444e1453. Good, C.D., Johnsrude, I.S., Ashburner, J., Henson, R.N., Friston, K.J., Frackowiak, R.S., 2001. A voxel-based morphometric study of ageing in 465 normal adult human brains. Neuroimage 14, 21e36. Haug, H., Eggers, R., 1991. Morphometry of the human cortex cerebri and corpus striatum during aging. Neurobiol. Aging 12, 336e338. Hoshi, E., Tanji, J., 2007. Distinctions between dorsal and ventral premotor areas: anatomical connectivity and functional properties. Curr. Opin. Neurobiol.17, 234e242. Jernigan, T.L., Archibald, S.L., Fennema-Notestine, C., Gamst, A.C., Stout, J.C., Bonner, J., Hesselink, J.R., 2001. Effects of age on tissues and regions of the cerebrum and cerebellum. Neurobiol. Aging 22, 581e594. Johansen-Berg, H., Rushworth, M.F., Bogdanovic, M.D., Kischka, U., Wimalaratna, S., Matthews, P.M., 2002. The role of ipsilateral premotor cortex in hand movement after stroke. Proc. Natl. Acad. Sci. U.S.A 99, 14518e14523. Kaasinen, V., Rinne, J.O., 2002. Functional imaging studies of dopamine system and cognition in normal aging and Parkinson’s disease. Neurosci. Biobehav. Rev. 26, 785e793. Koch, G., Franca, M., Del Olmo, M.F., Cheeran, B., Milton, R., Alvarez, S.M., Rothwell, J.C., 2006. Time course of functional connectivity between dorsal premotor and contralateral motor cortex during movement selection. J. Neurosci. 26, 7452e7459. Koch, G., Franca, M., Mochizuki, H., Marconi, B., Caltagirone, C., Rothwell, J.C., 2007. Interactions between pairs of transcranial magnetic stimuli over the human left dorsal premotor cortex differ from those seen in primary motor cortex. J. Physiol. 578, 551e562. Kolb, B., Forgie, M., Gibb, R., Gorny, G., Rowntree, S., 1998. Age, experience and the changing brain. Neurosci. Biobehav. Rev. 22, 143e159. Logan, J.M., Sanders, A.L., Snyder, A.Z., Morris, J.C., Buckner, R.L., 2002. Underrecruitment and nonselective recruitment: dissociable neural mechanisms associated with aging. Neuron 33, 827e840. Ni, Z., Gunraj, C., Nelson, A.J., Yeh, I.J., Castillo, G., Hoque, T., Chen, R., 2009. Two phases of interhemispheric inhibition between motor related cortical areas and the primary motor cortex in human. Cereb. Cortex 19, 1654e1665. Rizzolatti, G., Luppino, G., Matelli, M., 1998. The organization of the cortical motor system: new concepts. Electroencephalogr. Clin. Neurophysiol. 106, 283e296. Rizzolatti, G., Sinigaglia, C., 2010. The functional role of the parieto-frontal mirror circuit: interpretations and misinterpretations. Nat. Rev. Neurosci. 11, 264e274. Sterr, A., Dean, P., 2008. Neural correlates of movement preparation in healthy ageing. Eur. J. Neurosci. 27, 254e260. Sullivan, E.V., Rohlfing, T., Pfefferbaum, A., 2010. Quantitative fiber tracking of lateral and interhemispheric white matter systems in normal aging: relations to timed performance. Neurobiol. Aging 31, 461e481. van Nuenen, B.F., Van Eimeren, T., van der Veqt, J.P., Buhmann, C., Klein, C., Bloem, B.R., Siebner, H.R., 2009. Mapping preclinical compensation in Parkinson’s disease: an imaging genomics approach. Mov. Disord. 24, S703eS710. Ward, N.S., 2006. Compensatory mechanisms in the aging motor system. Ageing Res. Rev. 5, 239e254. Weinrich, M., Wise, S.P., 1982. The premotor cortex of the monkey. J. Neurosci. 2, 1329e1345. Wu, T., Hallett, M., 2005. The influence of normal human ageing on automatic movements. J. Physiol. 562, 605e615. Zahr, N.M., Rohlfing, T., Pfefferbaum, A., Sullivan, E.V., 2009. Problem solving, working memory, and motor correlates of association and commissural fiber bundles in normal aging: a quantitative fiber tracking study. Neuroimage 44, 1050e1062.
Contents lists available at ScienceDirect
Neurobiology of Aging journal homepage: www.elsevier.com/locate/neuaging
Brief communication
Reduced dorsal premotor cortex and primary motor cortex connectivity in older adults Zhen Ni, Reina Isayama, Gabriel Castillo, Carolyn Gunraj, Utpal Saha, Robert Chen* Division of Neurology, Krembil Neuroscience Centre, Toronto Western Research Institute, University Health Network, University of Toronto, Toronto, Ontario, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 December 2013 Received in revised form 12 July 2014 Accepted 13 August 2014 Available online 15 August 2014
Motor functions decline with increasing age. The underlying mechanisms are still unclear and are likely to be multifactorial. There is evidence for disruption of white matter integrity with age, which affects cortico-cortical connectivity. Studies with transcranial magnetic stimulation found both inhibitory and facilitatory connections from dorsal premotor cortex (PMd) to the ipsilateral primary motor cortex (M1) in young adults. We investigated whether aging affects this connectivity in 15 older and 15 young healthy adults. Transcranial magnetic stimulation in a paired-pulse paradigm was used to test the connectivity between left PMd and M1. Motor evoked potential in the right first dorsal interosseous muscle was recorded. We found that both the inhibitory effect with low intensity PMd stimulation and the facilitatory effect with high intensity PMd stimulation observed in young adults were decreased in older adults. We conclude that the connectivity between PMd and ipsilateral M1 is reduced in older adults. Ó 2015 Elsevier Inc. All rights reserved.
Keywords: Aging Dorsal premotor cortex Motor evoked potential Primary motor cortex Transcranial magnetic stimulation
1. Introduction
2. Methods
Human motor functions decline with age. The underlying mechanisms are multifactorial and are still not fully understood (Ward, 2006). Increasing age leads to marked effects on motor cortical morphology (Kolb et al., 1998). However, neuronal loss in primary motor cortex (M1) is not evident in the normal aging process (Haug and Eggers, 1991). On the other hand, disruption of white matter integrity occurs with increasing age, affecting cortico-cortical connectivity. Significant decline in white matter integrity in frontal fibers correlated with decreased motor function in older adults (Sullivan et al., 2010; Ward, 2006; Wu and Hallett, 2005; Zahr et al., 2009). These raised the possibility that connectivity between secondary motor areas and M1 decreased with normal aging. Studies with transcranial magnetic stimulation found both inhibitory and facilitatory connections from dorsal premotor cortex (PMd) to the ipsilateral M1 in young adults (Baumer et al., 2009; Civardi et al., 2001). We investigated whether normal aging affects the connectivity between PMd and M1. We hypothesize that both the inhibitory and facilitatory connectivity between PMd and M1 seen in young adults are decreased in older adults.
2.1. Subjects
* Corresponding author at: Division of Neurology, University of Toronto, 7MC-411, Toronto Western Hospital, 399 Bathurst Street, Toronto, Ontario, M5T 2S8, Canada. Tel.: þ1 416 603 5207; fax: þ1 416 603 5004. E-mail address: [email protected] (R. Chen). 0197-4580/$ e see front matter Ó 2015 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.neurobiolaging.2014.08.017
We studied 15 young (7 women; age range, 20e32 years; mean age, 25.0 3.6 years) and 15 older healthy adults (7 women; age range, 59e77 years, mean age, 65.3 5.1 years). All subjects were right-handed. Subjects with a history of neurologic or psychiatric disorders and those who took central nervous system active drugs including antidepressants, benzodiazepines, and hypnotics were excluded. 2.2. Experimental protocol Motor evoked potentials (MEPs) were recorded using surface electromyograms from right first dorsal interosseous muscle. Transcranial magnetic stimulation in a paired-pulse paradigm was used to test the connectivity between left PMd and left M1 (Fig. 1A). A Magstim 2002 stimulator and a small figure-of-8 shaped coil (5 cm diameter) (Magstim, Dyfed, UK) were used to deliver left M1 stimulation in a posterior-anterior current direction. The stimulation intensity was set at “1 mV” (to produce approximately 1 mV MEP in amplitude when given alone). PMd stimulation was applied to the location 2.5 cm anterior and 1 cm medial to the location for M1 stimulation (Ni et al., 2009). Another figure-of-8 shaped coil (4 cm diameter) was used to deliver anterior-posterior directed PMd stimulation. PMd stimulation followed by M1 stimulation at interstimulus
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Z. Ni et al. / Neurobiology of Aging 36 (2015) 301e303
were different from MEP amplitudes from M1 stimulation alone. A 2-way analysis of variance was used to test the effects of group (between-subject factor) and ISI or PMd stimulation intensity (within-subject factors). Post hoc testing used unpaired t test with Bonferroni correction for multiple comparisons. The threshold for significance was set at p < 0.05. Further details of the methods are provided in Supplementary Material. 3. Results
Fig. 1. Connectivity between dorsal premotor cortex and primary motor cortex in young and older adults. (A) Experimental design. The connectivity between left PMd and left M1 was tested with a paired pulse transcranial magnetic stimulation paradigm. PMd was measured 2.5 cm anterior and 1 cm medial to the target muscle presentation in M1 from scalp. The arrows indicate the direction of induced current in the brain. (B) Example of recordings obtained from 1 young adult and 1 older adult. The top row shows recordings with M1 stimulation alone (average of 20 trials for each recording). The bottom 3 rows show recordings with paired-pulse stimulation (average of 10 trials for each recording). ISIs of 4, 6, and 8 ms were tested. The left panel shows recordings with PMd stimulation intensity of 90% AMT. The right panel shows recordings with PMd stimulation intensity of 110% AMT. The left column in each panel shows recordings obtained from the young adult. The right column in each panel shows recordings obtained from the older adult. (CeE) Group analysis of the effects of PMd stimulation on MEP amplitude induced by M1 stimulation. Values are shown as mean with standard deviation. Open circles represent young adults and filled circles represent older adults. The ordinate indicates the MEP amplitude. It was normalized as a percentage value of MEP amplitude generated by the paired-pulse stimulation to that generated by M1 stimulation alone (dashed line). Values more than 100% indicate facilitation, those less than 100% indicate inhibition. * p < 0.05, ** p < 0.01, comparing MEP induced by pairedpulse stimulation to MEP induced by M1 stimulation alone. C and D show the data for 3 ISIs with PMd stimulation intensities of 90% and 110% AMT. The abscissa indicates the ISI between PMd and M1 stimulations. E shows the data for PMd intensity input-output curve at ISI of 6 ms. The abscissa indicates the PMd stimulation intensities. Abbreviations: AMT, active motor threshold; ISI, interstimulus interval; M1, primary motor cortex; MEP, motor evoked potential; PMd, dorsal premotor cortex.
intervals (ISIs) of 4, 6, and 8 ms at PMd stimulation intensities of 90% and 110% active motor threshold (AMT) was tested in all subjects. Furthermore, PMd intensity input-output curve from 70% to 140% AMT was tested at ISI of 6 ms in 8 young and 8 older adults. 2.3. Data and statistical analysis MEP amplitudes were measured peak-to-peak. The MEP amplitude evoked by paired-pulse stimulation was expressed as a percentage of the mean MEP amplitude evoked by M1 stimulation alone. Paired t tests were used to determine whether MEP amplitudes from paired pulses under various experimental conditions
Age was significantly different between the young and older adult groups (difference in mean age 40.3 years, t28 ¼ 25.03, p < 0.001). AMT and stimulation intensity for “1 mV” MEP were not different between 2 groups. Fig. 1 (B, C, and D) shows that the test MEP was inhibited by PMd stimulation with 90% AMT intensity at ISIs of 4 and 6 ms and was facilitated by PMd stimulation with 110% AMT intensity at ISI of 4 ms (p < 0.01 for all comparisons, paired t test) in young adults. In older adults, no inhibition was found with PMd stimulation at 90% AMT. With PMd stimulation at 110% AMT, facilitation seen in young adults turned into weak inhibition at ISIs of 4 ms (p < 0.05). Further between-subject analysis of variance revealed significant main group effect (90% AMT, F1,56 ¼ 8.61, p ¼ 0.007; 110% AMT, F1,56 ¼ 15.82, p < 0.001) for PMd stimulation at both stimulus intensities. The main effect of ISI was significant for 90% AMT (F2,56 ¼ 5.35, p ¼ 0.008) but not for 110% AMT PMd stimulation. The interaction between 2 main factors was significant for 110% AMT (F2,56 ¼ 9.27, p < 0.001) but not for 90% AMT PMd stimulation. Post hoc unpaired t test with 110% AMT PMd stimulation showed that older adults were different from young adults at 4 ms (p < 0.001) and 6 ms ISIs (p < 0.05). PMd intensity input-output curve at ISI of 6 ms (Fig. 1E) showed M1 inhibition at 90% AMT and M1 facilitation at 130%e140% AMT in young adults (p < 0.05 for all comparisons), but there was no effect in older adults. Analysis of variance revealed a significant group effect (F1,98 ¼ 4.84, p ¼ 0.045) and interaction between the 2 main factors (F7,98 ¼ 5.92, p < 0.001). The effect of PMd intensity showed a trend toward significance (F7,98 ¼ 2.06, p ¼ 0.056). Post hoc test showed that young and older adults were different at PMd intensities of 90% AMT and 110%e140% AMT (p < 0.05 for all comparisons). 4. Discussion The assessments of age-related motor cortical changes are not only important in the understanding of normal aging but also play a critical role in the understanding of many neurologic disorders such as neurodegenerative diseases and stroke that are common in older individuals. In young adults, we found that PMd stimulation with lower intensity inhibited the M1 at ISIs of 4 and 6 ms, and PMd stimulation with higher intensity facilitated the M1 at ISI of 4 ms, confirming the results of the previous studies that M1 excitability is modulated by PMd with both inhibitory and facilitatory inputs (Baumer et al., 2009; Civardi et al., 2001; Koch et al., 2007). These results were compatible with the anatomic connections and proposed the functional role of PMd in the motor cortical network with motor hierarchy where PMd receives direct inputs from dorsolateral prefrontal cortex, posterior parietal cortex and other cortical motor related areas, and projects its outputs to the M1 for movement execution (Hoshi and Tanji, 2007; Rizzolatti and Sinigaglia, 2010; Rizzolatti et al., 1998; Weinrich and Wise, 1982). In addition, the MEP inhibition and facilitation under various experimental conditions (ISIs and conditioning stimulus intensities) are consistent with previous studies showing that the PMd contains neurons with different structural and functional properties (Chouinard and Paus, 2006; Rizzolatti and Sinigaglia, 2010). Importantly, the novel finding in the present study is that both inhibitory and facilitatory inputs from PMd to M1 are decreased in older adults.
Z. Ni et al. / Neurobiology of Aging 36 (2015) 301e303
This was supported by the altered time course in older adults compared with young adults. Interestingly, decreased inhibition with 90% AMT PMd stimulation in older adults was found at all ISIs tested (Fig. 1C), whereas the decreased facilitation in older adults with 110% AMT intensity was only seen at shorter ISIs (Fig. 1D). This suggests that the effects of the aging processes for inhibitory and facilitatory neurons in PMd may be different. On the other hand, it may be argued that the difference between 2 subject groups is caused by a shift in firing threshold of PMd neurons in older adults with aging. We tested the PMd intensity input-output curve to address this question (Fig. 1E). Over a wide range of PMd stimulation intensities, we confirmed that both inhibition at low stimulation intensity and facilitation at high stimulation intensity were decreased in older adults. The result strongly suggests that the aging process leads to the loss of the complex connectivity between PMd and M1. This is consistent with the findings from a functional magnetic resonance imaging study that in a verbal encoding task there was reduced but less selective activation of the frontal regions in older compared with younger adults (Logan et al., 2002). Similarly, it was reported that the performance of automatic sequential finger movements required greater activity in cortical areas including the PMd in older compared with young adults when temporal information was processed in a complex motor task (Wu and Hallett, 2005). To achieve the same motor performance as young adults, more PMd neurons may be recruited in older adults to compensate for the reduced connectivity between PMd and M1. It has been reported in young adults that the inhibitory and facilitatory PMd-M1 connectivity was modulated during the reaction period, and the changes depend on the nature of the upcoming action, consistent with a role of the PMd in facilitating and suppressing cued movements (Koch et al., 2006). It is likely that the functional modulation of PMd-M1 connection before and during a motor task is also altered in aging and compensatory mechanisms may be activated. An event related potential study reported completely different patterns of PMd activity in older and young adults when spatial information with increasing complexity was being processed (Sterr and Dean, 2008). We cannot exclude the possibility that reduced connectivity between PMd and M1 is secondary to the functional decline of upstream cortical structures. In the normal aging, atrophy with reduction in gray matter volume is most significant in the dorsolateral prefrontal cortex (Good et al., 2001; Jernigan et al., 2001). Changes in neurotransmitter (e.g., dopamine) (Kaasinen and Rinne, 2002) or hormone levels in older adults may also influence our results. Neurobiological aging is a complex process with reduction in cortical connectivity and functions. The electrophysiological approach used in the present study showed that at the systems level, the connectivity between PMd and ipsilateral M1 is decreased in older adults. Decreased PMd and M1 connectivity may be related to impairments in the basic functions of PMd in motor planning, preparation, decision making, and execution of movement (Hoshi and Tanji, 2007), resulting in significant increase in coordination difficulty, variability, and slowness of movement with aging (Ward, 2006). In addition, inputs from PMd plays important role in compensation for the impaired motor functions after stroke (Johansen-Berg et al., 2002) and in preclinical Parkinson’s disease patients (van Nuenen et al., 2009). Reduction in connectivity between PMd and M1 in older adults may lead to less capacity in compensating for neurologic diseases.
Disclosure statement The authors reported no conflicts of interest. The protocol was approved by the University Health Network (Toronto) Research
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Ethics Board. All subjects provided written informed consent in accordance with the Declaration of Helsinki. Acknowledgements This study was supported by the Canadian Institutes of Health Research (MOP 62917). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.neurobiolaging. 2014.08.017. References Baumer, T., Schippling, S., Kroeger, J., Zittel, S., Koch, G., Thomalla, G., Rothwell, J.C., Siebner, H.R., Orth, M., Munchau, A., 2009. Inhibitory and facilitatory connectivity from ventral premotor to primary motor cortex in healthy humans at restea bifocal TMS study. Clin. Neurophysiol. 120, 1724e1731. Chouinard, P.A., Paus, T., 2006. The primary motor and premotor areas of the human cerebral cortex. Neuroscientist 12, 143e152. Civardi, C., Cantello, R., Asselman, P., Rothwell, J.C., 2001. Transcranial magnetic stimulation can be used to test connections to primary motor areas from frontal and medial cortex in humans. Neuroimage 14, 1444e1453. Good, C.D., Johnsrude, I.S., Ashburner, J., Henson, R.N., Friston, K.J., Frackowiak, R.S., 2001. A voxel-based morphometric study of ageing in 465 normal adult human brains. Neuroimage 14, 21e36. Haug, H., Eggers, R., 1991. Morphometry of the human cortex cerebri and corpus striatum during aging. Neurobiol. Aging 12, 336e338. Hoshi, E., Tanji, J., 2007. Distinctions between dorsal and ventral premotor areas: anatomical connectivity and functional properties. Curr. Opin. Neurobiol.17, 234e242. Jernigan, T.L., Archibald, S.L., Fennema-Notestine, C., Gamst, A.C., Stout, J.C., Bonner, J., Hesselink, J.R., 2001. Effects of age on tissues and regions of the cerebrum and cerebellum. Neurobiol. Aging 22, 581e594. Johansen-Berg, H., Rushworth, M.F., Bogdanovic, M.D., Kischka, U., Wimalaratna, S., Matthews, P.M., 2002. The role of ipsilateral premotor cortex in hand movement after stroke. Proc. Natl. Acad. Sci. U.S.A 99, 14518e14523. Kaasinen, V., Rinne, J.O., 2002. Functional imaging studies of dopamine system and cognition in normal aging and Parkinson’s disease. Neurosci. Biobehav. Rev. 26, 785e793. Koch, G., Franca, M., Del Olmo, M.F., Cheeran, B., Milton, R., Alvarez, S.M., Rothwell, J.C., 2006. Time course of functional connectivity between dorsal premotor and contralateral motor cortex during movement selection. J. Neurosci. 26, 7452e7459. Koch, G., Franca, M., Mochizuki, H., Marconi, B., Caltagirone, C., Rothwell, J.C., 2007. Interactions between pairs of transcranial magnetic stimuli over the human left dorsal premotor cortex differ from those seen in primary motor cortex. J. Physiol. 578, 551e562. Kolb, B., Forgie, M., Gibb, R., Gorny, G., Rowntree, S., 1998. Age, experience and the changing brain. Neurosci. Biobehav. Rev. 22, 143e159. Logan, J.M., Sanders, A.L., Snyder, A.Z., Morris, J.C., Buckner, R.L., 2002. Underrecruitment and nonselective recruitment: dissociable neural mechanisms associated with aging. Neuron 33, 827e840. Ni, Z., Gunraj, C., Nelson, A.J., Yeh, I.J., Castillo, G., Hoque, T., Chen, R., 2009. Two phases of interhemispheric inhibition between motor related cortical areas and the primary motor cortex in human. Cereb. Cortex 19, 1654e1665. Rizzolatti, G., Luppino, G., Matelli, M., 1998. The organization of the cortical motor system: new concepts. Electroencephalogr. Clin. Neurophysiol. 106, 283e296. Rizzolatti, G., Sinigaglia, C., 2010. The functional role of the parieto-frontal mirror circuit: interpretations and misinterpretations. Nat. Rev. Neurosci. 11, 264e274. Sterr, A., Dean, P., 2008. Neural correlates of movement preparation in healthy ageing. Eur. J. Neurosci. 27, 254e260. Sullivan, E.V., Rohlfing, T., Pfefferbaum, A., 2010. Quantitative fiber tracking of lateral and interhemispheric white matter systems in normal aging: relations to timed performance. Neurobiol. Aging 31, 461e481. van Nuenen, B.F., Van Eimeren, T., van der Veqt, J.P., Buhmann, C., Klein, C., Bloem, B.R., Siebner, H.R., 2009. Mapping preclinical compensation in Parkinson’s disease: an imaging genomics approach. Mov. Disord. 24, S703eS710. Ward, N.S., 2006. Compensatory mechanisms in the aging motor system. Ageing Res. Rev. 5, 239e254. Weinrich, M., Wise, S.P., 1982. The premotor cortex of the monkey. J. Neurosci. 2, 1329e1345. Wu, T., Hallett, M., 2005. The influence of normal human ageing on automatic movements. J. Physiol. 562, 605e615. Zahr, N.M., Rohlfing, T., Pfefferbaum, A., Sullivan, E.V., 2009. Problem solving, working memory, and motor correlates of association and commissural fiber bundles in normal aging: a quantitative fiber tracking study. Neuroimage 44, 1050e1062.
