Naïla Boudiaf

Functional MRI evidence for the decline of word retrieval and generation during normal aging

This fMRI study aimed to explore the effect of normal aging on word retrieval and generation. The question addressed is whether lexical production decline is determined by a direct mechanism, which concerns the language operations or is rather indirectly induced by a decline of executive functions. Indeed, the main hypothesis was that normal aging does not induce loss of lexical knowledge, but there is only a general slowdown in retrieval mechanisms involved in lexical processing, due to possible decline of the executive functions. We used three tasks (verbal fluency, object naming, and semantic categorization). Two groups of participants were tested (Young, Y and Aged, A), without cognitive and psychiatric impairment and showing similar levels of vocabulary. Neuropsychological testing revealed that older participants had lower executive function scores, longer processing speeds, and tended to have lower verbal fluency scores. Additionally, older participants showed higher scores for verbal automatisms and overlearned information. In terms of behavioral data, older participants performed as accurate as younger adults, but they were significantly slower for the semantic categorization and were less fluent for verbal fluency task. Functional MRI analyses suggested that older adults did not simply activate fewer brain regions involved in word production, but they actually showed an atypical pattern of activation. Significant correlations between the BOLD (Blood Oxygen Level Dependent) signal of aging-related (A > Y) regions and cognitive scores suggested that this atypical pattern of the activation may reveal several compensatory mechanisms (a) to overcome the slowdown in retrieval, due to the decline of executive functions and processing speed and (b) to inhibit verbal automatic processes. The BOLD signal measured in some other aging-dependent regions did not correlate with the behavioral and neuropsychological scores, and the overactivation of these uncorrelated regions would simply reveal dedifferentiation that occurs with aging. Altogether, our results suggest that normal aging is associated with a more difficult access to lexico-semantic operations and representations by a slowdown in executive functions, without any conceptual loss.

BOLD fMRI of cerebrovascular reactivity in the middle cerebral artery territory: A 100 volunteers’ study ☆

BACKGROUND AND PURPOSE The assessment of cerebrovascular reactivity (CVR) has shown promising results for its use in medical diagnosis and prognosis, especially in patients suffering from severe intracranial arterial stenosis. However, its quantification remains uncertain because of a large variability inherent in brain anatomy and in methodological settings. To overcome this variability, we provide lateralization index (LI) values for CVR within the middle cerebral artery territory to detect CVR impairment. MATERIALS AND METHODS We assessed CVR in 100 volunteers (41 females; 47.52 ± 21.58 years) without cervico-encephalic arterial stenosis using BOLD-fMRI contrast with a block-design hypercapnic challenge. Averaged end-tidal CO2 was used as a physiological regressor for statistical analyses with a general linear model. We measured %BOLD signal-change in segmented gray matter regions of interest in the middle cerebral artery territory (MCA). We calculated a laterality index according to the following formula: LI=(CVRleft-CVRright)/(CVRleft+CVRright). We tested the effects of methodological settings (i.e. hypercapnic gas, gas administration means, MR acquisition and sex) on %BOLD signal change and LI values with analysis of variance. RESULTS No adverse effects of the hypercapnic challenge were reported. LI values were independent of experimental conditions. Mean LI calculated in MCA territories was 0.016 ± 0.031, giving the lower and upper limits of 95% (m ± 2SD) of this population distribution at]-0.05; 0.08[. CONCLUSION LI can effectively help us to overcome measurement variabilities. Therefore, it can be used to detect abnormal asymmetries in CVR and identify patients at higher risk of ischemic stroke.

Behavioral evidence for a differential modulation of semantic processing and lexical production by aging: a full linear mixed-effects modeling approach.

ABSTRACT The effect of normal aging on lexical production and semantic processing was evaluated in 72 healthy participants. Four tasks were used, picture naming (PN), picture categorization (PC), numerical judgment (NJ), and color judgment (CJ). The dependence of reaction time (RT) and correct responses with age was accounted by mixed-effects models. Participants underwent neuropsychological testing for verbal, executive, and memory functions. The RTs increase significantly with age for all tasks. After parceling out the non-specific cognitive decline, as reflected by the NJ task, the RT for the PN task decreases with age. Behavioral data were interpreted in relation with neuropsychological scores. Our results suggest that (a) naming becomes more automatic and semantic processing slightly more difficult with age, and (b) a non-specific general slowdown of cognitive processing occurs with age. Lexical production remained unaltered, based on compensatory automatic processes. This study also suggests a possible slowdown of semantic processing, even in normal aging.

Aging Modulates the Hemispheric Specialization during Word Production

Although older adults exhibit normal accuracy in performing word retrieval and generation (lexical production; e.g., object naming), they are generally slower in responding than younger adults. To maintain accuracy, older adults recruit compensatory mechanisms and strategies. We focused on two such possible compensatory mechanisms, one semantic and one executive. These mechanisms are reflected at inter- and intra-hemispheric levels by various patterns of reorganization of lexical production cerebral networks. Hemispheric reorganization (HR) changes were also evaluated in relation to increase naming latencies. Using functional magnetic resonance imaging (fMRI), we examined 27 healthy participants (from 30 years to 85 years) during an object naming task, exploring and identifying task-related patterns of cerebral reorganization. We report two main results. First, we observed a left intra-hemispheric pattern of reorganization, the left anterior-posterior aging (LAPA) effect, consisting of supplementary activation of left posterior (temporo-parietal) regions in older adults and asymmetric activation along the left fronto-temporal axis. This pattern suggests that older adults recruit posterior semantic regions to perform object naming. The second finding consisted of bilateral recruitment of frontal regions to maintain appropriate response times, especially in older adults who were faster performers. This pattern is discussed in terms of compensatory mechanism. We suggest that aging is associated with multiple, co-existing compensation and reorganization mechanisms and patterns associated with lexical production.

Track-weighted imaging for neuroretina: Evaluations in healthy volunteers and ischemic optic neuropathy: Retinal Tractography in Ischemic Disease

The use of MRI‐tractography to explore the human neuroretina is yet to be reported. Track‐weighted imaging (TWI) was recently introduced as a qualitative tractography‐based method with high anatomical contrast.

Gray Matter Volume and Cognitive Performance During Normal Aging. A Voxel-Based Morphometry Study

Normal aging is characterized by decline in cognitive functioning in conjunction with extensive gray matter (GM) atrophy. A first aim of this study was to determine GM volume differences related to aging by comparing two groups of participants, middle-aged group (MAG, mean age 41 years, N = 16) and older adults (OG, mean age 71 years, N = 14) who underwent an magnetic resonance images (MRI) voxel-based morphometry (VBM) evaluation. The VBM analyses included two optimized pipelines, for the cortex and for the cerebellum. Participants were also evaluated on a wide range of cognitive tests assessing both domain-general and language-specific processes, in order to examine how GM volume differences between OG and MAG relate to cognitive performance. Our results show smaller bilateral GM volume in the OG relative to the MAG, in several cerebral and right cerebellar regions involved in language and executive functions. Importantly, our results also revealed smaller GM volume in the right cerebellum in OG relative to MAG, supporting the idea of a complex cognitive role for this structure. This study provides a broad picture of cerebral, but also cerebellar and cognitive changes associated with normal aging.

Aging modulates fronto-temporal cortical interactions during lexical production. A dynamic causal modeling study

HighlightsOlder adults show increased access to semantic memory during lexical production.The increased access to semantic memory depends on top‐down mechanisms.Significant interaction between cognitive control and semantic retrieval mechanisms may occur during aging. Abstract In this dynamic causal modeling (DCM) study, we evaluated the effect of age on the effective connectivity of a cerebral network involved in lexical production. Younger and older adults performed an object naming task during fMRI. The DCM was used to explore the interactions between four regions of interest: the occipital cortex, OC; the lateral temporal cortex, LTC; the medial temporal cortex, MTC; and the inferior frontal cortex, IFC. We mainly focused on the modulation of the fronto‐temporal interaction, according to the hypothesis that aging requires strategies that modulate the access to the semantic knowledge, either through a neural reserve mechanism (increased MTC‐LTC connectivity) or through a neural compensation mechanism (supplementary IFC‐MTC connectivity). For younger adults, our results indicated a bi‐directional interaction between the left IFC and LTC suggesting a typical activation related to lexico‐semantic representations. For older adults, our results reveal the existence of bi‐directional interaction between the IFC and MTC, but not between the IFC and LTC – which in turn suggests that older adults adapt a new strategy, via supplemental access to conceptual access and semantic retrieval processes. This neural compensation strategy would be facilitated by a top‐down mechanism from the IFC to the MTC. We discuss our results in the context of the possible additional strategies used by older compared to younger adults, to retrieve and produce words.

The Self-Pleasantness Judgment Modulates the Encoding Performance and the Default Mode Network Activity

In this functional magnetic resonance imaging (fMRI) study, we evaluated the effect of self-relevance on cerebral activity and behavioral performance during an incidental encoding task. Recent findings suggest that pleasantness judgments reliably induce self-oriented (internal) thoughts and increase default mode network (DMN) activity. We hypothesized that this increase in DMN activity would relate to increased memory recognition for pleasantly-judged stimuli (which depend on internally-oriented attention) but decreased recognition for unpleasantly-judged items (which depend on externally-oriented attention). To test this hypothesis, brain activity was recorded from 21 healthy participants while they performed a pleasantness judgment requiring them to rate visual stimuli as pleasant or unpleasant. One hour later, participants performed a surprise memory recognition test outside of the scanner. Thus, we were able to evaluate the effects of pleasant and unpleasant judgments on cerebral activity and incidental encoding. The behavioral results showed that memory recognition was better for items rated as pleasant than items rated as unpleasant. The whole brain analysis indicated that successful encoding (SE) activates the inferior frontal and lateral temporal cortices, whereas unsuccessful encoding (UE) recruits two key medial posterior DMN regions, the posterior cingulate cortex (PCC) and precuneus (PCU). A region of interest (ROI) analysis including classic DMN areas, revealed significantly greater involvement of the medial prefrontal cortex (mPFC) in pleasant compared to unpleasant judgments, suggesting this region’s involvement in self-referential (i.e., internal) processing. This area may be responsible for the greater recognition performance seen for pleasant stimuli. Furthermore, a significant interaction between the encoding performance (successful vs. unsuccessful) and pleasantness was observed for the PCC, PCU and inferior frontal gyrus (IFG). Overall, our results suggest the involvement of medial frontal and parietal DMN regions during the evaluation of self-referential pleasantness. We discuss these results in terms of the introspective referential of pleasantness judgments and the differential brain modulation based on internally- vs. externally-oriented attention during encoding.