Biomedical subjects

Valentine L Marcar

Publications and source records attributed to Valentine L Marcar.

8 recordsLinked to original sources

PubMed2006

Maturation of luminance- and motion-defined form perception beyond adolescence: a combined ERP and fMRI study.

Abilities to discriminate forms defined by motion continue to develop throughout childhood. To investigate late development of the visual motion system, we measured brain activity with event-related EEG potentials (ERPs) and functional magnetic resonance imaging (fMRI) in groups of adolescents (15-17 years) and adults (20-30 years) during a visual form discrimination task--with forms being either defined by motion or luminance contrast. We further explored whether possible developmental changes varied with the degree of motion coherence reflecting maturation specific to global motion processing. Both the fMRI activation patterns and ERP topographies were very similar between adolescents and adults, suggesting that the basic visual networks for processing motion and form are established by the age of 15-17. The ERP response to luminance- and motion-defined forms was dominated by a posterior negativity (N1: 120-270 ms). The N1 of the motion contrast was delayed in adolescents, whereas the N1 of the static condition did not differ between groups. Since the motion-evoked N1 is thought to arise in the middle temporal area MT/V5, our results indicate that visual motion processing in MT continues to get faster, becoming still more efficient during late development. Neither the ERP nor the fMRI results revealed maturation effects specific to motion coherence. This indicates that the specific mechanisms to process global dot motion are already mature in adolescence. The present findings support the view that static perception matures earlier than dynamic perception, and that these visual systems have different developmental courses.

Adolescent↗

PubMed2004

The influence of cortical maturation on the BOLD response: an fMRI study of visual cortex in children.

We performed blood oxygenation level-dependent (BOLD) functional MR imaging in 11 children younger than 5 y of age and 10 children older than 5 y of age. All but five of the children in the older age group were tested under light anesthesia. We examined the cerebral oxidative metabolism (CMRO(2)) associated with the processing of a flashed and a reversing checkerboard stimulus. These stimuli had been shown in a previous study to induce identical vascular responses. The reversing checkerboard activated twice the neuronal population of the flashed checkerboard, doubling the CMRO(2) associated with it. We compared the extent of activation for the positive BOLD response and found that it did not differ between the different age groups. We estimated the oxidative metabolism by examining the change in the local deoxyhemoglobin (HbR) concentration using Delta R2*. Because both stimuli induced the same vascular response, any increase in oxygen requirement would have to be met by the identical blood volume. Increasing CMRO(2) will therefore result in an increase in the oxygen extraction fraction (OEF), which raises the local HbR concentration. In the younger children, both checkerboard stimuli produced identical, high HbR concentrations. In the older children, the HbR concentration to the flashed stimulus was significantly lower than to the reversing stimulus. We conclude that, for identical stimuli, the oxidative energy requirement associated with the cortical processing is higher in young children than in older children because the presence of superfluous synaptic connections in the immature visual system activates a larger neuronal population.

Cerebrovascular Circulation↗

PubMed2004

The BOLD response: a new look at an old riddle.

Functional imaging has struggled for two decades to establish a quantitative link between the neuronal activity and the change in signal obtained. This review proposes a division of neuronal activity into a component relating to the electrical discharge activity and a component relating to the size of the activated neuronal population. These two components are argued to have opposing influences on the BOLD signal. The influence of these two components on the BOLD signal, are summarised in the standard model.

Animals↗

PubMed2004

What effect does measuring children under anesthesia have on the blood oxygenation level-dependent signal? A functional magnetic resonance imaging study of visual cortex.

We performed functional magnetic resonance measurements involving visual stimuli on 10 children. Half of the children were measured awake, the other half were measured under light Sevoflurane anesthesia corresponding to 0.5 mean alveolar concentration. Each child was presented with a flashed and a reversing checkerboard, which previous investigations have shown to induce identical increases in cerebral blood flow. The latter stimulus activated double the number of neurons as the former so that cerebral metabolic rate of oxygen consumption (CMRO(2)) was doubled, leading to an effective rise of the oxygen extraction fraction. We measured the extent of activation by counting the number of activated pixels and assessed the change in CMRO(2) by measuring the change in the local deoxyhemoglobin (HbR) concentration, using change in spin relaxivity. In both groups of children, the extent of activation was larger for the flashed than the reversing checkerboard, although the absolute number of activated voxels was smaller for the children who were measured under anesthesia. The HbR concentration was significantly higher during the presentation of the reversing compared with the flashed checkerboard. The relative change in the HbR concentration to the flashed and reversing checkerboard was the same in the children who were measured under anesthesia as in the children who were measured awake. We conclude that light levels of anesthesia may reduce the extent of activation but does not unduly influence either CMRO(2) or cerebral blood flow, thus preserving the blood oxygenation level-dependent signal amplitude.

Anesthesia↗

PubMed2004

When more means less: a paradox BOLD response in human visual cortex.

The predictions of the 'Linear Transfer Model' (LTM) have been tested only by modulating the frequency of the action potentials while keeping the size of the activated neuronal population constant. The LTM states that the blood oxygenation level-dependent contrast (BOLD) signal is directly proportional to the neuronal activity averaged over milliseconds or seconds. We examined the influence on the BOLD response, of manipulating the size of the activated neuronal population while maintaining the electrical discharge activity constant. We performed functional MR measurements on 30 awake, healthy adult volunteers (15 male and 15 female) using a flashed and reversing checkerboard. These stimuli induced the same vascular response and the same increase in the electrical discharge activity but varied in the size of the neuronal population being activated. The BOLD response measured by the extent of activation and the BOLD signal amplitude, was larger for the flashed than to the reversing checkerboard. An assessment of the local deoxyhemoglobin (HbR) concentration indicated that the neuronal activity was lower during the flashed checkerboard than the reversing checkerboard. Because the checkerboard associated with the lower neuronal activity yielded the larger number of activated voxels and the larger BOLD signal, our results run contrary to the predictions of the 'Linear Transfer Model' and for this reason we refer to them as paradoxical. Stimuli defined by luminance contrast or a chromatic contrast yielded identical results. We conclude that the 'LTM' may apply to stimuli that modulate the electrical discharge activity but not to stimuli that modulate the size of the activated neuronal population.

Adolescent↗

PubMed2004

How much luxury is there in 'luxury perfusion'? An analysis of the BOLD response in the visual areas V1 and V2.

We re-analyzed the functional magnetic resonance imaging data from a study involving awake, adult, human volunteers in order to examine the influence of vascular density on the blood oxygenation level-dependent (BOLD) response. We employed a flashed and reversing stimulus paradigm where the latter stimulated twice the number of receptive fields and with it doubled the neuronal metabolic load (CMRO2) compared to the former stimulus. The blood flow increase to these stimuli was identical, so that differences in the BOLD response are due to differences in the oxygen extraction fraction. By comparing the BOLD response in human striate cortex (V1) and its neighbor, extra-striate area V2 to the two stimuli, we were able to determine the influence of the higher vascular density of striate cortex on the BOLD response. In striate cortex, the extent of activation, as measured by the number of activated voxels, was larger for the flashed than for the reversing stimulus. In extra-striate area V2, no such difference in the extent of activation was noted. Gauging the local concentration of HbR using deltaR2*, we found it to be significantly lower for the flashed than for the reversing checkerboard. We estimated the HbR concentration in extra-striate area V2 to be double that of striate cortex independent of the stimulus presented. A frequency distribution of the deltaR2* values for the flashed and reversing checkerboard revealed a shift consistent with an increase in the HbR concentration between areas V1 and V2. The metabolically most demanding stimulus, the reversing checkerboard was associated with the highest HbR concentration and with the largest number of voxels with a negative BOLD response.

Adult↗

PubMed2004

What the little differences between men and women tells us about the BOLD response.

We analyzed the functional MRI signal of 15 men and 15 women. All had been presented with a flashed and a reversing, radial checkerboard stimulus. We investigated both positive and negative blood oxygenation level-dependent (BOLD) responses. The extent of activation and the change in the neuronal activity were examined. The former, by counting the number of activated voxels, the latter by using deltaR2* as an indicator of the change in the local deoxyhemoglobin (HbR) concentration. We examined both the positive and the negative BOLD response. Positive BOLD response: The flashed checkerboard gave rise to a larger number of activated voxels than for the reversing checkerboard. The mean number of activated pixels did not differ between men and women. The peak deltaR2* was significantly larger to the flashed than the reversing checkerboard, but did not reveal a gender-related difference. We noted an attenuation of the BOLD signal amplitude with time. This attenuation was larger in women than in men. Negative BOLD response: The attenuation was also larger for the flashed than the reversing stimulus and more pronounced in the chromatic contrast compared to the luminance contrast stimulus. The extent of activation was larger for the flashed than the reversing checkerboard, but did not differ between the sexes. The deltaR2* for the chromatic contrast checkerboard was larger in men than in women. No other significant differences were found. We conclude that the difference in the extent of activation between men and women is the result of our ability to detect activated pixels using statistical methods and not the result of a difference in the processing between the sexes.

Cerebrovascular Circulation↗

PubMed2004

An fMRI study of the cerebral macro network involved in 'cue invariant' form perception and how it is influenced by stimulus complexity.

We investigated the influence of stimulus complexity on the macro network of visual areas involved in 'cue invariant' form perception. Functional MRI imaging on 14 healthy, adult volunteers was performed during a two alternative forced choice (2-AFC) form discrimination task. The functional load imposed onto the visual system was varied by using simple and complex shapes. The figures were defined using a luminance, a chromatic or a motion contrast cue. The three cues activated the same visual areas in the ventral pathway, including area 'LO'. Activation of visual area 'V3v' but not area 'KO' in the dorsal pathway was observed to the motion contrast cue. The simple shapes induced a larger BOLD response in BA18 than the complex shapes, reflecting the selectivity of this region for the features in the stimuli such as edges and vertices. The brain regions yielding a larger BOLD signal to the complex shapes were areas know to be selective to the orthographic content of our complex stimuli. The processing requirement was assessed by comparing the subjects' reaction time. We found no significant difference in the reaction times to the simple and complex shapes. The reaction times to the luminance contrast cue and the chromatic contrast cue were identical but that to the motion contrast cue were 200 ms longer. This finding concurs with neurophysiological studies, reporting a longer onset latency for motion contrast stimuli. It further lends support to the idea that the motion contrast cue requires auxiliary processing by the visual areas of the dorsal pathway before entry into the ventral pathway.

Adult↗