Biomedical subjects

Vinod Venkatraman

Publications and source records attributed to Vinod Venkatraman.

11 recordsLinked to original sources

PubMed2006

Cortical effects of anticipation and endogenous modulation of visceral pain assessed by functional brain MRI in irritable bowel syndrome patients and healthy controls.

Visceral pain processing is abnormal in a majority of irritable bowel syndrome (IBS) patients. Aberrant endogenous nociceptive modulation and anticipation are possible underlying mechanisms investigated in the current study. Twelve IBS patients and 12 matched healthy controls underwent brain fMRI scanning during the following randomised stimuli: sham and painful rectal distensions by barostat without and with simultaneous activation of endogenous descending nociceptive inhibition using ice water immersion of the foot for heterotopic stimulation. Heterotopic stimulation decreased rectal pain scores from 3.7+/-0.2 to 3.1+/-0.3 (mean+/-SE, scale 0-5) in controls (p<0.01), but not significantly in IBS. Controls differed from IBS patients in showing significantly greater activation bilaterally in the anterior insula, SII and putamen during rectal stimulation alone compared to rectal plus heterotopic stimulation. Greater activation during rectal plus heterotopic versus rectal stimulation was seen bilaterally in SI and the right superior temporal gyrus in controls and in the right inferior lobule and bilaterally in the superior temporal gyrus in IBS. Rectal pain scores were similarly low during sham stimulation in both groups, but brain activation patterns differed. In conclusion, IBS patients showed dysfunctional endogenous inhibition of pain and concomitant aberrant activation of brain areas involved in pain processing and integration. Anticipation of rectal pain was associated with different brain activation patterns in IBS involving multiple interoceptive, homeostatic, associative and emotional areas, even though pain scores were similar during sham distension. The aberrant activation of endogenous pain inhibition appears to involve circuitry relating to anticipation as well as pain processing itself.

Adult↗

PubMed2006

The neural basis of interindividual variability in inhibitory efficiency after sleep deprivation.

Sleep deprivation results in the loss of our ability to suppress a prepotent response. The extent of decline in this executive function varies across individuals. Here, we used functional magnetic resonance imaging to study the neural correlates of sleep deprivation-induced differences in inhibitory efficiency. Participants performed a go/no-go task after normal sleep and after 24 h of total sleep deprivation. Regardless of the extent of change in inhibitory efficiency, sleep deprivation lowered go/no-go sustained, task-related activation of the ventral and anterior prefrontal (PFC) regions bilaterally. However, individuals better able to maintain inhibitory efficiency after sleep deprivation could be distinguished by lower stop-related, phasic activation of the right ventral PFC during rested wakefulness. These persons also showed a larger rise in such activation both here and in the right insula after sleep deprivation relative to those whose inhibitory efficiency declined.

Adult↗

PubMed2006

Dissociating response conflict from numerical magnitude processing in the brain: an event-related fMRI study.

Functional neuroimaging studies of numerical cognition have repeatedly associated activation of the intraparietal sulcus (IPS) with number processing. During number comparison, the IPS has been found to be modulated by the numerical distance. This has lead to the contention that the IPS houses the internal representation of numerical magnitude. However, this theory has been challenged by the argument that IPS activation may reflect domain-general response selection. In the present study, we used the numerical size congruity paradigm to further elucidate the role played by the IPS in number comparison. In an event-related, functional magnetic resonance imaging (fMRI) study, participants judged which of two number words was numerically larger. In addition to the numerical distance, physical stimulus size was varied such that physical size and numerical magnitude were either (a) congruent (e.g., numerically smaller number printed in smaller font) or (b) incongruent (e.g., numerically larger number printed in smaller font). This allowed for the study of both the main effects and the interaction of numerical distance and stimulus congruency. A main effect of numerical distance was found in bilateral regions of the IPS. However, these parietal areas were not significantly modulated by congruency or the interaction of distance and congruency. Instead, the main effect of congruency and an interaction of distance and congruency were observed in anterior cingulate and prefrontal cortices. These findings suggest some degree of independence between the processing of numerical distance and size congruity, lending support for the hypothesis that distance effects in IPS reflect the underlying representation of numerical magnitude.

Adolescent↗

PubMed2006

Functional imaging of working memory following normal sleep and after 24 and 35 h of sleep deprivation: Correlations of fronto-parietal activation with performance.

Working memory was evaluated after normal sleep, and at 24 and 35 h of sleep deprivation (SD) in 26 healthy young adults to examine the neural correlates of inter-individual differences in performance. The extent of performance decline was not significantly different between the two SD test periods although there was greater variability in performance at SD35. In both SD sessions, there was reduced task-related activation (relative to normal sleep) in both superior parietal regions and the left thalamus. Activation of the left parietal and left frontal regions after normal sleep was negatively correlated with performance accuracy decline from normal sleep to SD24 thus differentiating persons who maintained working memory performance following SD from those who were vulnerable to its effects.

Adult↗

PubMed2006

Effect of language switching on arithmetic: a bilingual FMRI study.

The role of language in performing numerical computations has been a topic of special interest in cognition. The "Triple Code Model" proposes the existence of a language-dependent verbal code involved in retrieving arithmetic facts related to addition and multiplication, and a language-independent analog magnitude code subserving tasks such as number comparison and estimation. Neuroimaging studies have shown dissociation between dependence of arithmetic computations involving exact and approximate processing on language-related circuits. However, a direct manipulation of language using different arithmetic tasks is necessary to assess the role of language in forming arithmetic representations and in solving problems in different languages. In the present study, 20 English-Chinese bilinguals were trained in two unfamiliar arithmetic tasks in one language and scanned using fMRI on the same problems in both languages (English and Chinese). For the exact "base-7 addition" task, language switching effects were found in the left inferior frontal gyrus (LIFG) and left inferior parietal lobule extending to the angular gyrus. In the approximate "percentage estimation" task, language switching effects were found predominantly in the bilateral posterior intraparietal sulcus and LIFG, slightly dorsal to the LIFG activation seen for the base-7 addition task. These results considerably strengthen the notion that exact processing relies on verbal and language-related networks, whereas approximate processing engages parietal circuits typically involved in magnitude-related processing.

Attention↗

PubMed2006

Age-related changes in object processing and contextual binding revealed using fMR adaptation.

Using fMR adaptation, we studied the effects of aging on the neural processing of passively viewed naturalistic pictures composed of a prominent object against a background scene. Spatially distinct neural regions showing specific patterns of adaptation to objects, background scenes, and contextual integration (binding) were identified in young adults. Older adults did not show adaptation responses corresponding to binding in the medial-temporal areas. They also showed an adaptation deficit for objects whereby their lateral occipital complex (LOC) did not adapt to repeated objects in the context of a changing background. The LOC could be activated, however, when objects were presented without a background. Moreover, the adaptation deficit for objects viewed against backgrounds was reversed when elderly subjects were asked to attend to objects while viewing these complex pictures. These findings suggest that the elderly have difficulty with simultaneous processing of objects and backgrounds that, in turn, could contribute to deficient contextual binding.

Adult↗

PubMed2005

Dissociation of cortical regions modulated by both working memory load and sleep deprivation and by sleep deprivation alone.

Working memory is an important mental capacity that is compromised following sleep deprivation (SD). To understand how working memory load interacts with state to influence brain activation in load-sensitive regions, and the extent to which SD-related changes are common across different loads, we used fMRI to study twelve healthy subjects following 24 h of SD using a verbal n-back task with three load levels. Performance decline was observed by way of reduced accuracy and slower response times following SD. The left prefrontal region and thalamus showed load dependent activity modulation that interacted with state. The right parietal and anterior medial frontal regions showed load dependent changes in activity as well as an effect of state. The anterior cingulate and occipital regions showed activation that displayed state effects that were independent of working memory load. These findings represent a step toward identifying how different brain regions exhibit varying vulnerability to the deleterious effects of SD on working memory.

Adult↗

PubMed2005

Neural correlates of symbolic and non-symbolic arithmetic.

Recent evidence suggests that areas in and around the intraparietal sulcus (IPS) represent magnitude in a stimulus-independent format. However, it has not been established whether the same is true for mental arithmetic or whether activation for higher level numerical processing diverges as a function of stimulus format. We addressed this question in a functional imaging study by presenting participants with simple addition problems using both symbolic (Arabic numerals) and non-symbolic (arrays of dots) stimuli. Conjunction analysis revealed common neural substrates for symbolic and non-symbolic addition in the anterior IPS bilaterally, left posterior IPS, medial frontal gyrus and left precentral gyrus. Right parietal and frontal cortex showed greater activation for non-symbolic addition. Our results demonstrate that mental arithmetic, studied using addition problems, is processed within the IPS independent of stimulus form. Additionally we examined whether exact and approximate addition conditions activated different neural substrates as a function of stimulus format. We did not find any differences between exact and approximate addition using symbolic and non-symbolic stimuli. This could be due to the inability of the participants to suppress exact calculation for single-digit addition problems. In contrast to recent findings, we found no significant activation for exact addition condition in left, language-related areas.

Adult↗

PubMed2003

Comparison of block and event-related fMRI designs in evaluating the word-frequency effect.

Printed word frequency can modulate retrieval effort in a task requiring associative semantic judgment. Event-related fMRI, while avoiding stimulus order predictability, is in theory statistically less powerful than block designs. We compared one event-related and two block designs that evaluated the same semantic judgment task and found that similar brain regions demonstrated the word frequency effect. Although the responses were lower in amplitude, event-related fMRI was able to detect the word frequency effect to a comparable degree compared to the block designs. The detection of a frequency effect with the event-related design also suggests that stimulus-order predictability may not be as serious a concern in block designs as might be supposed.

Adult↗

PubMed2003

Stimulus repetition and hemodynamic response refractoriness in event-related fMRI.

We investigated the extent of hemodynamic recovery following the paired presentation of either identical or different faces at two different inter-stimulus intervals (ISI). Signal recovery was consistently better at an ISI of 6 sec compared to 3 sec. Significantly less signal recovery was associated with identical faces compared to different faces in bilateral mid-fusiform and right prefrontal regions but not in the calcarine and posterior fusiform regions. Repetition suppression effects contributed significantly to incomplete signal recovery in a region-specific manner. Simulations using empirically derived data suggest that experiments with shorter ISI (average 4.5-6.0 sec) are as sensitive as experiments with intermediate ISI (average 9 sec) in detecting response differences if experimental duration is equivalent. However, designs using intermediate ISI may be more appropriate if the expected difference in responses is small and if the number of suitable stimuli is limited.

Adult↗

PubMed2003

Reproducibility of the word frequency effect: comparison of signal change and voxel counting.

We determined the reproducibility of both the direction and the effect size of the word frequency effect (WFE) as it relates to associative semantic judgments. Sixteen volunteers were scanned twice. At the group level of analysis, signal change and voxel counting could both reproducibly detect the existence of a WFE. However, signal change data showed less intersession variation, particularly in the left inferior frontal gyrus. The effect size of WFE was well reproduced only with signal change measurements. In consideration of the signal change data, statistical threshold did not have a major effect on the detection or determination of the effect size. In general, while the direction of the WFE was reasonably reproducible at the individual level, the effect size was far less well reproduced. These findings suggest that with existing techniques, fMRI may be used to track changes in brain activation stemming from improvement in language proficiency at the group level but not at the individual level.

Adult↗