Vinod Venkatraman

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.

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.

Separate neural mechanisms underlie choices and strategic preferences in risky decision making.

Adaptive decision making in real-world contexts often relies on strategic simplifications of decision problems. Yet, the neural mechanisms that shape these strategies and their implementation remain largely unknown. Using an economic decision-making task, we dissociate brain regions that predict specific choices from those predicting an individuals preferred strategy. Choices that maximized gains or minimized losses were predicted by functional magnetic resonance imaging activation in ventromedial prefrontal cortex or anterior insula, respectively. However, choices that followed a simplifying strategy (i.e., attending to overall probability of winning) were associated with activation in parietal and lateral prefrontal cortices. Dorsomedial prefrontal cortex, through differential functional connectivity with parietal and insular cortex, predicted individual variability in strategic preferences. Finally, we demonstrate that robust decision strategies follow from neural sensitivity to rewards. We conclude that decision making reflects more than compensatory interaction of choice-related regions; in addition, specific brain systems potentiate choices depending on strategies, traits, and context.

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

&NA; 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.

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.

Age and culture modulate object processing and object-scene binding in the ventral visual area.

Behavioral differences in the visual processing of objects and backgrounds as a function of cultural group are well documented. Recent neuroimaging evidence also points to cultural differences in neural activation patterns. Compared with East Asians, Westerners’ visual processing is more object focused, and they activate neural structures that reflect this bias for objects. In a recent adaptation study, East Asian older adults showed an absence of an object-processing area but normal adaptation for background areas. In the present study, 75 young and old adults (half East Asian and half Western) were tested in an fMR-adaptation study to examine differences in object and background processing as well as object—background binding. We found equivalent background processing in the parahippocampal gyrus in all four groups, diminished binding processes in the hippocampus in elderly East Asians and Westerners, and diminished object processing in elderly versus young adults in the lateral occipital complex. Moreover, elderly East Asians showed significantly less adaptation response in the object areas than did elderly Westerners. These findings demonstrate the malleability of perceptual processes as a result of differences in cohort-specific experiences or in cultural exposure over time.

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.

Sleep Deprivation Biases the Neural Mechanisms Underlying Economic Preferences

A single night of sleep deprivation (SD) evoked a strategy shift during risky decision making such that healthy human volunteers moved from defending against losses to seeking increased gains. This change in economic preferences was correlated with the magnitude of an SD-driven increase in ventromedial prefrontal activation as well as by an SD-driven decrease in anterior insula activation during decision making. Analogous changes were observed during receipt of reward outcomes: elevated activation to gains in ventromedial prefrontal cortex and ventral striatum, but attenuated anterior insula activation following losses. Finally, the observed shift in economic preferences was not correlated with change in psychomotor vigilance. These results suggest that a night of total sleep deprivation affects the neural mechanisms underlying economic preferences independent of its effects on vigilant attention.

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.

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.