Biomedical subjects

W C Machielsen

Publications and source records attributed to W C Machielsen.

6 recordsLinked to original sources

PubMed2001

Anterior medial temporal lobe activation during attempted retrieval of encoded visuospatial scenes: an event-related fMRI study.

Various studies have shown that the medial temporal lobe (MTL), which consists of the hippocampus and parahippocampal gyrus, is important for episodic memory. Earlier fMRI studies substantiated this role by showing activation upon encoding of visuospatial scenes. In this study we used event-related fMRI to study whether the cognitive process of retrieval of visuospatial scenes, tested with the use of a recognition paradigm, also activates the MTL. Nine subjects (mean age 24 years) were presented previously studied color pictures (old) and pictures they had never seen before (new) in a mixed trial design. Data analysis allowed calculation of the fMRI response of correct judgments on new pictures, old pictures, and false judgments. Since we used previously encoded color pictures as old stimuli, we also included an encoding paradigm in the current set of experiments. This allowed us to compare encoding and recognition activation in the MTL of exactly the same pictures in the same subjects. Correct judgments on new pictures showed an increased activation in the anterior parahippocampus bilaterally and the right anterior hippocampus compared to judgments on old pictures in the recognition experiment. The former judgments took significantly longer, indicating that retrieval of successfully stored information is less demanding than the effort to retrieve nonencoded information. A comparison of the two experimental data sets showed evidence for a functional segregation of encoding and retrieving color pictures. We conclude that the left posterior parahippocampal gyrus responds during encoding, while on the other hand the left anterior parahippocampal gyrus and the right anterior hippocampus were more strongly involved in retrieval.

Adult↗

PubMed2000

FMRI of visual encoding: reproducibility of activation.

fMRI, a noninvasive technique to measure brain activation, is gaining clinical interest, because its sensitivity enables individual assessments. However, more insight in the reproducibility of these measurements during higher cognitive tasks is necessary. We performed an fMRI study involving within- and between-subject reproducibility during encoding of complex visual pictures. Ten healthy subjects were studied on three occasions: twice in the same scanning session (study 1 and 2), and a third time, 3-24 days later (study 3). On all 30 occasions but one, activation was found in areas expected on the basis of previous studies, including the fusiform and lingual gyri, occipital and parietal areas, the (para)hippocampal area, and the frontal inferior sulcus. The reproducibility of the number of activated voxels in the whole brain was 72% and 63% (respectively, studies 1 and 2, and 1 and 3). The reproducibility of anatomical identical pixels that supplement these results was 49% and 36%. These reproducibility measures increase about 5-15% when only areas of expected activation are included. The quantitative measurements indicate that there is substantial variation in the volume of activation. The recognition of pictures as tested afterward explains part of this variation between subjects. Our findings indicate that whereas consistent patterns of activation exist, more insight is needed into what determines the volume of activation, especially to assess cognitive alterations in patients over time.

Adult↗

PubMed2000

Cortico-hippocampal communication by way of parallel parahippocampal-subicular pathways.

The hippocampal memory system, consisting of the hippocampal formation and the adjacent parahippocampal region, is known to play an important role in learning and memory processes. In recent years, evidence from a variety of experimental approaches indicates that each of the constituting fields of the hippocampal memory system may serve functionally different, yet complementary roles. Understanding the anatomical organization of cortico-parahippocampal-hippocampal connectivity may lead to a further understanding of these potential functional differences. In the present paper we present the two main conclusions of experiments in which we studied the anatomical organization of the hippocampal memory system of the rat in detail, with a focus on the pivotal position of the entorhinal cortex. We first conclude that the simple traditional view of the entorhinal cortex as simply the input and output structure of the hippocampal formation needs to be modified. Second, our data indicate the existence of two parallel pathways through the hippocampal memory system, arising from the perirhinal and postrhinal cortex. These two parallel pathways may be involved in separately processing functionally different types of sensory information. This second proposition will be subsequently evaluated on the basis of series of electrophysiological studies we carried out in rats and some preliminary functional brain imaging studies in humans.

Animals↗

PubMed2000

Visualizing brain activation during planning: the tower of London test adapted for functional MR imaging.

BACKGROUND AND PURPOSE: Recent positron emission tomography and single-photon emission CT studies using the Tower of London test have shown that brain activation during planning activities primarily resides in the prefrontal cortex. In this study, we adapted the Tower of London test for functional MR imaging. METHODS: For use with functional MR imaging, a block design of the test was created, in which planning stages were contrasted with counting of colored balls. For nine healthy participants, multisection echo-planar functional MR imaging was performed to assess brain activation based on changes in blood oxygen level. Activation maps for individual participants and a group average map were created. RESULTS: In the group average map, activation in the dorsolateral prefrontal cortex, the anterior part of the cingulate cortex, the cuneus and precuneus, the supramarginal and angular gyrus in the parietal lobe, and the frontal opercular area of the insula was seen. These findings are in agreement with grouped data of previous positron emission tomography results. Functional MR imaging enabled us to investigate brain activation during planning activities with high spatial (and temporal) resolution in individual patients, showing that the dorsolateral prefrontal cortex was activated in all participants studied. CONCLUSION: Presented is a working functional MR imaging version of the planning task. The high sensitivity of functional MR imaging may allow the use of this test for patients with possible (pre)frontal disorders.

Adult↗

PubMed1997

Visual association encoding activates the medial temporal lobe: a functional magnetic resonance imaging study.

The involvement of structures in the medial temporal lobe during the encoding of visual associations was studied with functional magnetic resonance imaging. In 11 out of 12 normal healthy volunteers this task resulted in activation in posterior portions of the parahippocampal region, close to the collateral sulcus. In seven subjects activation was encountered in the hippocampal formation. The visual association task as adapted for this study may provide a sensitive measure to study anterograde amnesia prevalent in Alzheimer's disease. Therefore, the present paradigm enables the study of individual changes in learning and memory capacities over time.

Adult↗

PubMedDate not supplied

Functional MR imaging in Alzheimer's disease during memory encoding.

BACKGROUND AND PURPOSE: We applied functional MR imaging with a learning task in healthy elderly volunteers and in patients with Alzheimer's disease to study brain activation during memory performance. The purpose was to determine the feasibility of functional MR imaging during a learning task in healthy elderly volunteers and in patients with Alzheimer's disease and to test our hypothesis that brain activation is decreased in the medial temporal lobe (MTL) memory system in patients with Alzheimer's disease compared with control volunteers. METHODS: In 12 patients with mild to moderate forms of Alzheimer's disease and 10 elderly control volunteers, activation of the MTL memory system was studied. We used two learning tasks that required the encoding of new information into memory. After the functional MR imaging experiment, participants were tested for recognition of the encoded objects. RESULTS: In the elderly control volunteers, activation during memory encoding was observed in medial and lateral temporal lobe structures (fusiform, parietal and occipital parts, and hippocampal formation) and in the frontal cortex, as reported previously in studies of young control volunteers. Focusing on the MTL, we observed that activation was significantly decreased in patients with Alzheimer's disease compared with control volunteers in the left hippocampus and parahippocampal gyrus bilaterally during the first encoding task but not during the second (P < .05, uncorrected). CONCLUSION: Functional MR imaging with a learning task seems feasible in elderly volunteers and in patients with Alzheimer's disease. The measured functional signal decrease in MTL areas warrants further exploration of the (early) diagnostic usefulness of functional MR imaging in cases of Alzheimer's disease and other dementias.

Aged↗