Left lateralized white matter microstructure accounts for individual differences in reading ability and disability

A dual DTI approach to analyzing white matter in children with dyslexia

Using voxel-based (VBA) and region-of-interest (ROI) diffusion tensor imaging (DTI) analyses, we examined white matter (WM) organization in seven children with dyslexia and six age-matched controls. Both methods demonstrated reduced fractional anisotropy (FA) in the left superior longitudinal fasciculus (SLF) and abnormal orientation in the right SLF in dyslexics. Application of this complementary dual DTI approach to dyslexia, which included novel analyses of fiber orientation, demonstrates its usefulness for analyzing mild and complex WM abnormalities.

Developmental differences of neurocognitive networks for phonological and semantic processing in Chinese word reading

Developmental differences in the neurocognitive networks for phonological and semantic processing in Chinese word reading were examined in 13 adults and 13 children using functional magnetic resonance imaging (fMRI). Rhyming and semantic association judgments were made to two-character words that were presented sequentially in the visual modality. These lexical tasks were compared with a nonlinguistic control task involving judgment of line patterns. The first main finding was that adults showed greater activation than children in right middle occipital gyrus on both the meaning and rhyming task, suggesting adults more effectively engage right hemisphere brain regions involved in the visual-spatial analysis of Chinese characters. The second main finding was that adults showed greater activation than children in left inferior parietal lobule for the rhyming as compared with the meaning task, suggesting greater specialization of phonological processing in adults. The third main finding was that children who had better performance in the rhyming task on characters with conflicting orthographic and phonological information relative to characters with nonconflicting information showed greater activation in left middle frontal gyrus, suggesting greater engagement of brain regions involved in the integration of orthography and phonology.

Simple developmental dyslexia in children: alterations in diffusion-tensor metrics of white matter tracts at 3 T

PURPOSE

To determine whether there are detectable differences in tensor metrics between children who read normally and children with simple developmental dyslexia and/or differences between the right and left hemispheres in these groups by using 3.0-T diffusion-tensor (DT) magnetic resonance (MR) imaging focused on the superior longitudinal fasciculus (SLF), inferior fronto-occipital and inferior longitudinal fasciculi (IFO-ILF), and posterior limb of the internal capsule (PLIC).

MATERIALS AND METHODS

This was a prospective, HIPAA-compliant institutional review board-approved investigation with written informed parental consent. Nineteen English-speaking, right-handed children with a normal IQ and developmental dyslexia (16 male, three female; age range, 6-16 years; mean age, 9.9 years) and 18 normal-reading, age-matched pediatric control subjects (13 male, five female; age range, 6-15 years; mean age, 10.0 years) underwent DT imaging (30 directions, three signals acquired, voxel size of 2 mm). Regions of interest were placed on the SLF, IFO-ILF, and PLIC, and tensor metrics were calculated. Statistical analyses of differences in cognitive function between the dyslexic and control groups were performed by using the two-sample t test. Differences in tensor metrics were examined by using analysis of covariance models.

RESULTS

In the control subjects, the fractional anisotropy (FA) of all tracts studied increased with age. In the dyslexic subjects, the age-related increases in FA in the SLF were most similar to those in the control subjects (P = .504), while mean FA values for the IFO-ILF (P = .009) and PLIC (P < .0001) were higher than those in the control subjects up to around 11 years of age, after which they were lower. Apparent diffusion coefficients consistently decreased in both groups. There was a nonsignificant increase in mean axial diffusivity in the IFO-ILF in the control group but not in the dyslexia group. Increases in axial diffusivity seen in the PLIC in the control group were not seen in the dyslexia group. There were no marked differences in tensor metrics between the left and right hemispheres within or between the two groups.

CONCLUSION

Findings at 3.0-T DT imaging suggest that white matter differences in dyslexic children are not limited to the portion of the brain traditionally considered to be integral to word recognition and processing.

Brain connectivity in non-reading impaired children and children diagnosed with developmental dyslexia

Diffusion Tensor Imaging (DTI) was used to investigate the relationship between white matter and reading abilities in reading impaired and non-reading impaired children. Seventeen children (7 non-reading impaired, 10 reading impaired) participated in this study. DTI was performed with 2mm isotropic resolution to cover the entire brain along 30 noncollinear directions. Voxelwise analyses were conducted on data processed through Tract Based Spatial Statistics (TBSS). The data replicated previous results seen across multiple studies and extended findings to include measures of both real word and pseudoword decoding. Negative correlations were observed in the left posterior corpus callosum between fractional anisotropy (FA) values and both measures of decoding. Positive correlations between FA values and real word and pseudoword decoding were observed in the left superior corona radiata. This extension of findings regarding correlations between the corona radiata and reading skills suggests an important direction for future research into the neurological substrates of reading.

Splenium microstructure is related to two dimensions of reading skill

Inconsistent differences in the corpus callosum (CC) structure between dyslexic readers (DRs) and typical readers (TRs) have been reported. We examine differences in CC splenium microstructure and the association of splenium microstructure with reading-related skills. Nine DRs and 18 TRs completed a reading skills battery and diffusion tensor imaging. DRs had higher splenium fractional anisotropy (FA) and axial diffusivity (LA) as compared with TRs. Retrieval of orthographic information from the language lexicon was negatively associated with FA and LA within both reading groups. Phonological awareness was positively associated with splenium FA and LA in TRs but not DRs. This study suggests two white matter pathways that may be differentially associated with reading skills in the CC splenium.

Structural dissociation of attentional control and memory in adults with and without mild traumatic brain injury

Memory and attentional control impairments are the two most common forms of dysfunction following mild traumatic brain injury (TBI) and lead to significant morbidity in patients, yet these functions are thought to be supported by different brain networks. This 3 T magnetic resonance diffusion tensor imaging (DTI) study investigates whether microstructural integrity of white matter, as measured by fractional anisotropy (FA) within a small set of individually localized regions of interest (ROIs), is associated with these cognitive domains in normal adults and adults with mild TBI. Results in a sample of 23 normal controls reveal a significant correlation between attentional control and FA within a ROI in the left hemisphere anterior corona radiata. Furthermore, the controls demonstrate a correlation between memory performance and FA in a ROI placed in the uncinate fasciculus. Next, to examine whether these relationships are found in the pathological ranges of attention, memory and microstructural white matter integrity associated with mild TBI, these analyses were applied to a group of 43 mild TBI patients. Results, which generally demonstrated a wider range of attention, memory and FA scores, replicated the correlation between attentional control and FA in left hemisphere anterior corona radiata, as well as the correlation between memory performance and FA in the uncinate fasciculus. These two sets of brain-behaviour relationships were highly specific, as shown by a lack of correlation between attention and uncinate fasciculus FA and the lack of correlation between memory performance and anterior corona radiata FA. Furthermore, a 'correlational double dissociation' was demonstrated to exist between two distinct frontal structures independently associated with attention and memory, respectively, via a series of multiple regression analyses in both normal controls and adults with mild TBI. The results of the multiple regression analyses provide direct evidence that tract-specific variation in microstructural white matter integrity among normal controls and among mild TBI patients can account for much of the variation in performance in specific cognitive domains. More generally, such findings suggest that diffusion anisotropy measurement can be used as a quantitative biomarker for neurocognitive function and dysfunction.

White matter microstructures underlying mathematical abilities in children

The role of gray matter function and structure in mathematical cognition has been well researched. Comparatively little is known about white matter microstructures associated with mathematical abilities. Diffusion tensor imaging data from 13 children (7-9 years) and two measures of their mathematical competence were collected. Relationships between children's mathematical competence and fractional anisotropy were found in two left hemisphere white matter regions. Although the superior corona radiata was found to be associated with both numerical operations and mathematical reasoning, the inferior longitudinal fasciculus was correlated with numerical operations specifically. These findings suggest a role for microstructure in left white matter tracts for the development of mathematical skills. Moreover, the findings point to the involvement of different white matter tracts for numerical operations and mathematical reasoning.

Reading impairment in a patient with missing arcuate fasciculus

We describe the case of a child ("S") who was treated with radiation therapy at age 5 for a recurrent malignant brain tumor. Radiation successfully abolished the tumor but caused radiation-induced tissue necrosis, primarily affecting cerebral white matter. S was introduced to us at age 15 because of her profound dyslexia. We assessed cognitive abilities and performed diffusion tensor imaging (DTI) to measure cerebral white matter pathways. Diffuse white matter differences were evident in T1-weighted, T2-weighted, diffusion anisotropy, and mean diffusivity measures in S compared to a group of 28 normal female controls. In addition, we found specific white matter pathway deficits by comparing tensor-orientation directions in S's brain with those of the control brains. While her principal diffusion direction maps appeared consistent with those of controls over most of the brain, there were tensor-orientation abnormalities in the fiber tracts that form the superior longitudinal fasciculus (SLF) in both hemispheres. Tractography analysis indicated that the left and right arcuate fasciculus (AF), as well as other tracts within the SLF, were missing in S. Other major white matter tracts, such as the corticospinal and inferior occipitofrontal pathways, were intact. Functional MRI measurements indicated left-hemisphere dominance for language with a normal activation pattern. Despite the left AF abnormality, S had preserved oral language with average sentence repetition skills. In addition to profound dyslexia, S exhibited visuospatial, calculation, and rapid naming deficits and was impaired in both auditory and spatial working memory. We propose that the reading and visuospatial deficits were due to the abnormal left and right SLF pathways, respectively. These results advance our understanding of the functional significance of the SLF and are the first to link radiation necrosis with selective damage to a specific set of fiber tracts.

Quantifying development: Investigating highly myelinated voxels in preadolescent corpus callosum

Multi-echo T(2) relaxation decay data was acquired for 5 preadolescent males (age range 9-12 years). A signal to noise filter appropriate for multi-exponential T(2) analysis was then applied to remove voxels which did not accurately fit the modelled curve. The remaining voxels were designated "highly myelinated" if their myelin water fraction (MWF) was greater than a critical value. We investigated a range of signal to noise filter cut-off values and highly myelinated critical values and demonstrated a significant correlation (r=0.990, p<0.001) between verbal intelligence quotient scores on the Wechsler Intelligence Scale for Children - Revised and myelination of the corpus callosum in children. This relationship is supported by a growing number of studies showing a correlation between white matter development and cognitive ability.

Developmental dyslexia: gray matter abnormalities in the occipitotemporal cortex

Functional neuroimaging studies have consistently demonstrated less activation of the left occipitotemporal cortex in dyslexic readers. This region is considered critical for skilled reading and damage to it in adult readers leads to severe deficits in reading ability. In contrast to these findings, structural abnormalities in the occipitotemporal cortex were not consistently found to date. We used optimized Voxel Based Morphometry with T1 weighted MR images to investigate gray matter volume in 13 dyslexic and 15 nonimpaired reading adolescents (age 14-16). Less gray matter volume for dyslexic readers was found in the left and right fusiform gyrus, the bilateral anterior cerebellum and in the right supramarginal gyrus. Decreased gray matter volume in the left and right fusiform gyrus of dyslexic readers highlights the importance of this brain region for developmental dyslexia. The structural abnormalities in the right occipitotemporal cortex suggest that dyslexia may be such a persistent disorder because an occipitotemporal reading area, critical for skilled reading, cannot develop in any hemisphere. The extended areas of reduced gray matter volume in dyslexic readers in the cerebellum suggest that structural abnormalities in the cerebellum are also strongly associated with dyslexia and warrant further investigation.

White matter in learning, cognition and psychiatric disorders

White matter is the brain region underlying the gray matter cortex, composed of neuronal fibers coated with electrical insulation called myelin. Previously of interest in demyelinating diseases such as multiple sclerosis, myelin is attracting new interest as an unexpected contributor to a wide range of psychiatric disorders, including depression and schizophrenia. This is stimulating research into myelin involvement in normal cognitive function, learning and IQ. Myelination continues for decades in the human brain; it is modifiable by experience, and it affects information processing by regulating the velocity and synchrony of impulse conduction between distant cortical regions. Cell-culture studies have identified molecular mechanisms regulating myelination by electrical activity, and myelin also limits the critical period for learning through inhibitory proteins that suppress axon sprouting and synaptogenesis.

Tract-based spatial statistics of diffusion tensor imaging in adults with dyslexia

BACKGROUND AND PURPOSE

Diffusion tensor imaging is a tool that can be used to study white matter microstructure in dyslexia. We tested the hypothesis that dyslexics have a white matter structural change (as measured by directional diffusion of water, which can be affected by disruption in white matter tracts) between brain regions that previous functional connectivity studies showed were associated with phonologic processing.

MATERIALS AND METHODS

Diffusion tensor imaging (DTI) scans were acquired from 7 healthy adult normal readers and from 14 adults with dyslexia on a 1.5T scanner. Voxelwise statistical analysis of the fractional anisotropy data were carried out by using Tract-Based Spatial Statistics to compare dyslexic subjects versus control subjects in white matter tracts.

RESULTS

Significant group difference map clusters (comparing adults with and without dyslexia) occurred in specific bilateral white matter tracts within the frontal lobe, temporal lobe, occipital lobe, and parietal lobe.

CONCLUSION

The DTI fractional anisotropy results in the bilateral white matter showing higher fractional anisotropy in adult control subjects compared with adults with dyslexia (relating to white matter fiber tract integrity) are consistent with our previous functional connectivity results from seed points in the bilateral inferior frontal gyrus.

The teen brain: insights from neuroimaging

Few parents of a teenager are surprised to hear that the brain of a 16-year-old is different from the brain of an 8-year-old. Yet to pin down these differences in a rigorous scientific way has been elusive. Magnetic resonance imaging, with the capacity to provide exquisitely accurate quantifications of brain anatomy and physiology without the use of ionizing radiation, has launched a new era of adolescent neuroscience. Longitudinal studies of subjects from ages 3-30 years demonstrate a general pattern of childhood peaks of gray matter followed by adolescent declines, functional and structural increases in connectivity and integrative processing, and a changing balance between limbic/subcortical and frontal lobe functions, extending well into young adulthood. Although overinterpretation and premature application of neuroimaging findings for diagnostic purposes remains a risk, converging data from multiple imaging modalities is beginning to elucidate the implications of these brain changes on cognition, emotion, and behavior.

Microstructural correlates of infant functional development: example of the visual pathways

The development of cognitive functions during childhood relies on several neuroanatomical maturation processes. Among these processes is myelination of the white matter pathways, which speeds up electrical conduction. Quantitative indices of such structural processes can be obtained in vivo with diffusion tensor imaging (DTI), but their physiological significance remains uncertain. Here, we investigated the microstructural correlates of early functional development by combining DTI and visual event-related potentials (VEPs) in 15 one- to 4-month-old healthy infants. Interindividual variations of the apparent conduction speed, computed from the latency of the first positive VEP wave (P1), were significantly correlated with the infants' age and DTI indices measured in the optic radiations. This demonstrates that fractional anisotropy and transverse diffusivity are structural markers of functionally efficient myelination. Moreover, these indices computed along the optic radiations showed an early wave of maturation in the anterior region, with the posterior region catching up later in development, which suggests two asynchronous fronts of myelination in both the geniculocortical and corticogeniculate fibers. Thus, in addition to microstructural information, DTI provides noninvasive exquisite information on the functional development of the brain in human infants.

Extent of microstructural white matter injury in postconcussive syndrome correlates with impaired cognitive reaction time: a 3T diffusion tensor imaging study of mild traumatic brain injury

BACKGROUND AND PURPOSE

Diffusion tensor imaging (DTI) may be a useful index of microstructural changes implicated in diffuse axonal injury (DAI) linked to persistent postconcussive symptoms, especially in mild traumatic brain injury (TBI), for which conventional MR imaging techniques may lack sensitivity. We hypothesized that for mild TBI, DTI measures of DAI would correlate with impairments in reaction time, whereas the number of focal lesions on conventional 3T MR imaging would not.

MATERIALS AND METHODS

Thirty-four adult patients with mild TBI with persistent symptoms were assessed for DAI by quantifying traumatic microhemorrhages detected on a conventional set of T2*-weighted gradient-echo images and by DTI measures of fractional anisotropy (FA) within a set of a priori regions of interest. FA values 2.5 SDs below the region average, based on a group of 26 healthy control adults, were coded as exhibiting DAI.

RESULTS

DTI measures revealed several predominant regions of damage including the anterior corona radiata (41% of the patients), uncinate fasciculus (29%), genu of the corpus callosum (21%), inferior longitudinal fasciculus (21%), and cingulum bundle (18%). The number of damaged white matter structures as quantified by DTI was significantly correlated with mean reaction time on a simple cognitive task (r = 0.49, P = .012). In contradistinction, the number of traumatic microhemorrhages was uncorrelated with reaction time (r = -0.08, P = .71).

CONCLUSION

Microstructural white matter lesions detected by DTI correlate with persistent cognitive deficits in mild TBI, even in populations in which conventional measures do not. DTI measures may thus contribute additional diagnostic information related to DAI.

Development of neural systems for reading

Functional and structural neuroimaging studies of adult readers have provided a deeper understanding of the neural basis of reading, yet such findings also elicit new questions about how developing neural systems come to support this learned ability. A developmental cognitive neuroscience approach provides insights into how skilled reading emerges in the developing brain, yet also raises new methodological challenges. This review focuses on functional changes that occur during reading acquisition in cortical regions associated with both the perception of visual words and spoken language, and it examines how such functional changes differ within developmental reading disabilities. We integrate these findings within an interactive specialization framework of functional development and propose that such a framework may provide insights into how individual differences at several levels of observation (genetics, white matter tract structure, functional organization of language, cultural organization of writing systems) impact the emergence of neural systems involved in reading ability and disability.

Temporal-callosal pathway diffusivity predicts phonological skills in children

The development of skilled reading requires efficient communication between distributed brain regions. By using diffusion tensor imaging, we assessed the interhemispheric connections in a group of children with a wide range of reading abilities. We segmented the callosal fibers into regions based on their likely cortical projection zones, and we measured diffusion properties in these segmented regions. Phonological awareness (a key factor in reading acquisition) was positively correlated with diffusivity perpendicular to the main axis of the callosal fibers that connect the temporal lobes. These results could be explained by several physiological properties. For example, good readers may have fewer but larger axons connecting left and right temporal lobes, or their axon membranes in these regions may be more permeable than the membranes of poor readers. These measurements are consistent with previous work suggesting that good readers have reduced interhemispheric connectivity and are better at processing rapidly changing visual and auditory stimuli.

The contribution of neuroimaging to the study of language and aphasia

New structural and functional imaging methods continue to be developed at a rapid pace. In the last 25 years, advanced imaging techniques have provided insights into how language is represented and processed in the brain and how it can be disrupted by damage to, or dysfunction of, various parts of the brain. Imaging studies have also yielded new information regarding how individuals recover language after stroke. We briefly review the strengths and weaknesses of the various radiological methods currently used to study language and aphasia.

White matter pathways in reading

Skilled reading requires mapping of visual text to sound and meaning. Because reading relies on neural systems spread across the brain, a full understanding of this cognitive ability involves the identification of pathways that communicate information between these processing regions. In the past few years, diffusion tensor imaging has been used to identify correlations between white matter properties and reading skills in adults and children. White matter differences have been found in left temporo-parietal areas and in posterior callosal tracts. We review these findings and relate them to possible pathways that are important for various aspects of reading. We describe how the results from diffusion tensor imaging can be integrated with functional results in good and poor readers.

Functional and morphometric brain dissociation between dyslexia and reading ability

In functional neuroimaging studies, individuals with dyslexia frequently exhibit both hypoactivation, often in the left parietotemporal cortex, and hyperactivation, often in the left inferior frontal cortex, but there has been no evidence to suggest how to interpret the differential relations of hypoactivation and hyperactivation to dyslexia. To address this question, we measured brain activation by functional MRI during visual word rhyme judgment compared with visual cross-hair fixation rest, and we measured gray matter morphology by voxel-based morphometry in dyslexic adolescents in comparison with (i) an age-matched group, and (ii) a reading-matched group younger than the dyslexic group but equal to the dyslexic group in reading performance. Relative to the age-matched group (n = 19; mean 14.4 years), the dyslexic group (n = 19; mean 14.4 years) exhibited hypoactivation in left parietal and bilateral fusiform cortices and hyperactivation in left inferior and middle frontal gyri, caudate, and thalamus. Relative to the reading-matched group (n = 12; mean 9.8 years), the dyslexic group (n = 12; mean 14.5 years) also exhibited hypoactivation in left parietal and fusiform regions but equal activation in all four areas that had exhibited hyperactivation relative to age-matched controls as well. In regions that exhibited atypical activation in the dyslexic group, only the left parietal region exhibited reduced gray matter volume relative to both control groups. Thus, areas of hyperactivation in dyslexia reflected processes related to the level of current reading ability independent of dyslexia. In contrast, areas of hypoactivation in dyslexia reflected functional atypicalities related to dyslexia itself, independent of current reading ability, and related to atypical brain morphology in dyslexia.

Diffusion tensor imaging: Application to the study of the developing brain

OBJECTIVE

To provide an overview of diffusion tensor imaging (DTI) and its application to the study of white matter in the developing brain in both healthy and clinical samples.

METHOD

The development of DTI and its application to brain imaging of white matter tracts is discussed. Forty-eight studies using DTI to examine diffusion properties of the developing brain are reviewed in the context of the structural magnetic resonance imaging literature. Reports of how brain diffusion properties are affected in pediatric clinical samples and how they relate to cognitive and behavioral phenotypes are reviewed.

RESULTS

DTI has been used successfully to describe white matter development in pediatric samples. Changes in white matter diffusion properties are consistent across studies, with anisotropy increasing and overall diffusion decreasing with age. Diffusion measures in relevant white matter regions correlate with behavioral measures in healthy children and in clinical pediatric samples.

CONCLUSIONS

DTI is an important tool for providing a more detailed picture of developing white matter than can be obtained with conventional magnetic resonance imaging alone.

Diffusion tensor imaging segmentation of white matter structures using a Reproducible Objective Quantification Scheme (ROQS)

Reproducible Objective Quantification Scheme (ROQS) is a novel method for regional white matter measurements of diffusion tensor imaging (DTI) parameters that overcomes the limitations of previous approaches for analyzing large cohorts of subjects reliably. ROQS is a semi-automated technique that exploits the fiber orientation information from the diffusion tensor in conjunction with a binary masking and chain-linking algorithm to segment anatomically distinct white matter tracts for subsequent quantitative analysis of DTI parameters such as fractional anisotropy and apparent diffusion coefficient. When applied to 3-T whole-brain DTI of normal adult volunteers, ROQS is shown to segment the corpus callosum much faster than manual region of interest (ROI) delineation, and with better reproducibility and accuracy.

Auditory processing, speech perception and phonological ability in pre-school children at high-risk for dyslexia: A longitudinal study of the auditory temporal processing theory

This study investigates whether the core bottleneck of literacy-impairment should be situated at the phonological level or at a more basic sensory level, as postulated by supporters of the auditory temporal processing theory. Phonological ability, speech perception and low-level auditory processing were assessed in a group of 5-year-old pre-school children at high-family risk for dyslexia, compared to a group of well-matched low-risk control children. Based on family risk status and first grade literacy achievement children were categorized in groups and pre-school data were retrospectively reanalyzed. On average, children showing both increased family risk and literacy-impairment at the end of first grade, presented significant pre-school deficits in phonological awareness, rapid automatized naming, speech-in-noise perception and frequency modulation detection. The concurrent presence of these deficits before receiving any formal reading instruction, might suggest a causal relation with problematic literacy development. However, a closer inspection of the individual data indicates that the core of the literacy problem is situated at the level of higher-order phonological processing. Although auditory and speech perception problems are relatively over-represented in literacy-impaired subjects and might possibly aggravate the phonological and literacy problem, it is unlikely that they would be at the basis of these problems. At a neurobiological level, results are interpreted as evidence for dysfunctional processing along the auditory-to-articulation stream that is implied in phonological processing, in combination with a relatively intact or inconsistently impaired functioning of the auditory-to-meaning stream that subserves auditory processing and speech perception.

White matter growth as a mechanism of cognitive development in children

We examined the functional role of white matter growth in cognitive development. Specifically, we used hierarchical regression analyses to test the unique contributions of age versus white matter integrity in accounting for the development of information processing speed. Diffusion tensor imaging was acquired for 17 children and adolescents (age range 6-17 years), with apparent diffusion coefficient (ADC) and fractional anisotropy (FA) calculated for 10 anatomically defined fiber pathways and 12 regions of hemispheric white matter. Measures of speeded visual-spatial searching, rapid picture naming, reaction time in a sustained attention task, and intelligence were administered. Age-related increases were evident across tasks, as well as for white matter integrity in hemispheric white matter. ADC was related to few measures. FA within multiple hemispheric compartments predicted rapid picture naming and standard error of reaction time in sustained attention, though it did not contribute significantly to the models after controlling for age. Independent of intelligence, visual-spatial searching was related to FA in a number of hemispheric regions. A novel finding was that only right frontal-parietal regions contributed uniquely beyond the effect of age in accounting for performance: age did not contribute to visual-spatial searching when FA within these regions was first included in the models. Considering we found that both FA in right frontal-parietal regions and speed of visual-spatial searching increased with age, our findings are consistent with the growth of regional white matter organization as playing an important role in increased speed of visual searching with age.

Aging: compensation or maturation?

Neuroimaging studies of healthy aging often reveal differences in neural activation patterns between young and elderly groups for episodic memory tasks, even though there are no differences in behavioral performance. One explanation typically offered is that the elderly compensate for their memory deficiencies through the recruitment of additional prefrontal regions. The present study of healthy aging compared magnetoencephalographic (MEG) time-courses localized to specific cortical regions in two groups of subjects (20-29 years and >or=65 years) during a visual delayed-match-to-sample (DMS) task. MR morphometrics and neuropsychological test results were also examined with the hope of providing insight into the nature of the age-related differences. The behavioral results indicated no differences in performance between young and elderly groups. Although there was a main effect of age on the latency of the initial peak in primary/secondary visual cortex, these longer latencies were not correlated with the performance of elderly on the DMS task. The lateral occipital gyrus (LOG) revealed qualitatively different patterns of activity for the two age groups corroborated by neuropsychological test results. Morphometric results for the young versus elderly groups revealed less white (WM) and gray matter (GM) volumes in the frontal lobes of the elderly. When a group of middle-aged subjects (33-43 years) was included in the morphometric analyses, the middle-aged subjects revealed statistically greater WM volumes in frontal and parietal cortex suggesting immature WM tracts in the young. Perhaps our elderly utilized a different strategy compared to the young due to the different brain maturation levels of these groups.