Biomedical subjects

Mathieu Brodeur

Publications and source records attributed to Mathieu Brodeur.

6 recordsLinked to original sources

PubMed2006

Effect of connectivity and bistability on the visual potentials evoked by illusory figures.

The present study aimed at testing functional hypotheses regarding two brain potentials elicited by illusory figures. Accordingly, the N1 potential indexes mechanisms connecting the separate parts of the illusory form, whereas a subsequent negative potential indexes compensatory processes triggered by perceptual difficulty. Here, perceptual difficulty was induced by bistability; that is, by equating the probability of perceiving the illusory form to that of perceiving the independent separate parts. We compared the brain potentials evoked by a strongly connected illusory square, with almost no bistability, with those evoked by a weakly connected illusory square presenting strong bistability. Consistent with our hypotheses, the latter figure evoked the smallest N1 and a larger negative component peaking at 360 ms (N360). These results strengthen the link between N1 and connection and between negativity to perceptual difficulty and perceptual difficulty.

Adult↗

PubMed2005

The effect of interpolation and perceptual difficulty on the visual potentials evoked by illusory figures.

Completion is the process by which the brain unifies and segregates the parts of an incomplete form. It is qualified as amodal when the form is placed behind an obstacle and modal when the form is at the foreground and closed by illusory contours. The N1, and sometimes the N2, deflections of the visual evoked potentials are known to be larger for modal figures, such as the Kanizsa triangle, than for control figures. This result is generally linked to completion or illusory contours, but it could also be related to a third process: the interpolation of the form by connecting its separate parts. To test the influence of interpolation, a modal triangle, an amodal triangle, a figure with outlined inducers, and a no-triangle figure were randomly presented to 26 subjects. The N1 evoked by the three triangle figures were all larger than the N1 to the no-triangle figure. These results suggest that the N1 amplitude is largely determined by the possibility of interpolating a form in the figure. The greatest N1 to the modal figure further suggests that interpolation may be increased by modal completion and decreased by the features that diminish the saliency of triangle in the amodal figure and the figure with outlined inducers. On the other hand, the largest N2 was evoked by the amodal figure. This effect may index processes activated in response to the great difficulty in perceiving the triangle in the amodal figure, a difficulty that is initially caused by a conflict of perceptions characterizing this figure.

Adult↗

PubMed2004

The interfering effect of related events on recognition memory discriminability: a functional magnetic resonance imaging study.

Retrieval of information from memory often involves the selection of an event among competing related events, a process that frequently gives rise to interference effects. The present study used a forced-choice recognition test to identify neural correlates of the interfering effect of related events on recognition memory discriminability. Participants encoded landscape pictures divided into three segments. One segment was presented during encoding, and a forced-choice recognition task contrasted a studied and a nonstudied segment for each landscape. For half of the landscapes, the third segment was presented between encoding and recognition tasks to induce associative interference by reducing recognition discriminability. A behavioral study with 40 subjects yielded a significant difference in the correct recognition rate between control and interference trials (76% and 64%, respectively, p<0.001). A subsequent event-related fMRI study with 16 subjects yielded significant activations for correct interference recognition trials relative to control trials in left superior parietal regions, which suggests that these regions play a role in the representation of stimuli and associated information. The opposite contrast yielded significant activations in inferior prefrontal regions bilaterally, right dorsolateral prefrontal cortex and right parahippocampal cortex. Since this contrast was conducted using only correctly recognized trials, these findings could reflect an index of memory discriminability or saliency which could influence conscious recollection.

Adult↗

PubMed2003

Prefrontal cortex contribution to associative recognition memory in humans: an event-related functional magnetic resonance imaging study.

The present event-related fMRI study examined in ten healthy participants neural correlates of recognition memory for intact, rearranged, and new pairs of visual stimuli. The correct categorization of both intact and rearranged pairs relative to pairs of new, never presented before stimuli relies on the retrieval of associative information whereas the discrimination of rearranged pairs from intact pairs additionally involves a recall-to-reject process by which subjects retrieve additional information to reach a recognition decision. Relative to new pairs, both intact and rearranged pairs yielded activation in inferior prefrontal cortex bilaterally and left dorsolateral prefrontal cortex. Relative to intact pairs, rearranged pairs were associated with increased activity in left dorsolateral prefrontal cortex. The latter could reflect the neural correlates of a recall-to-reject process, a pivotal process of associative recognition memory.

Adult↗

PubMed2003

The role of human and mouse hepatic scavenger receptor class B type I (SR-BI) in the selective uptake of low-density lipoprotein-cholesteryl esters.

Low-density lipoprotein (LDL)-cholesteryl ester (CE) selective uptake has been demonstrated in nonhepatic cells overexpressing the scavenger receptor class B type I (SR-BI). The role of hepatic SR-BI toward LDL, the main carrier of plasma CE in humans, remains unclear. The aim of this study was to determine if SR-BI, expressed at its normal level, is implicated in LDL-CE selective uptake in human HepG2 hepatoma cells and mouse hepatic cells, to quantify its contribution and to determine if LDL-CE selective uptake is likely to occur in the presence of human HDL. First, antibody blocking experiments were conducted on normal HepG2 cells. SR-BI/BII antiserum inhibited (125)I-LDL and (125)I-HDL(3) binding (10 microg of protein/mL) by 45% (p < 0.05) and CE selective uptake by more than 85% (p < 0.01) for both ligands. Second, HepG2 cells were stably transfected with a eukaryotic vector expressing a 400-bp human SR-BI antisense cDNA fragment. Clone 17 (C17) has a 70% (p < 0.01) reduction in SR-BI expression. In this clone, (3)H-CE-LDL and (3)H-CE-HDL(3) association (10 microg of protein/mL) was 54 +/- 6% and 45 +/- 7% of control values, respectively, while (125)I-LDL and (125)I-HDL(3) protein association was 71 +/- 3% and 58 +/- 5% of controls, resulting in 46% and 55% (p < 0.01) decreases in LDL- and HDL(3)-CE selective uptake. Normalizing CE selective uptake for SR-BI expression reveals that SR-BI is responsible for 68% and 74% of LDL- and HDL(3)-CE selective uptake, respectively. Thus, both approaches show that, in HepG2 cells, SR-BI is responsible for 68-85% of CE selective uptake. Other pathways for selective uptake in HepG2 cells do not require CD36, as shown by anti-CD36 antibody blocking experiments, or class A scavenger receptors, as shown by the lack of competition by poly(inosinic acid). However, CD36 is a functional oxidized LDL receptor on HepG2 cells, as shown by antibody blocking experiments. Similar results for CE selective uptake were obtained with primary cultures of hepatic cells from normal (+/+), heterozygous (-/+), and homozygous (-/-) SR-BI knockout mice. Flow cytometry experiments show that SR-BI accounts for 75% of DiI-LDL uptake, the LDL receptor for 14%, and other pathways for 11%. CE selective uptake from LDL and HDL(3) is likely to occur in the liver, since unlabeled HDL (total and apoE-free HDL(3)) and LDL, when added in physiological proportions, only partially competed for LDL- and HDL(3)-CE selective uptake. In this setting, human hepatic SR-BI may be a crucial molecule in the turnover of both LDL- and HDL(3)-cholesterol.

Animals↗

PubMed2003

Visual evoked potentials to line- and luminance-defined triangles.

Processing of line- and luminance-defined triangles was investigated by studying visual potentials (VEPs) evoked by triangles. Twenty-six subjects were randomly presented with line, grey, and illusory triangles. Relative to VEPs elicited by grey and illusory triangles, VEPs to line triangles included P220s that were smaller at frontal sites but larger at occipital sites, and N260s that were smaller over both temporal and occipital sites. It is proposed that, in contrast to triangle defined by a line, illusory and luminance-defined triangles include information involved in the processing of surface.

Adult↗