Ho Ling Liu

The temporal response of the brain after eating revealed by functional MRI

After eating, the human brain senses a biochemical change and then signals satiation, but precisely when this occurs is unknown. Even for well-established physiological systems like glucose–insulin regulation, the timing of interaction between hormonal processes and neural events is inferred mostly from blood sampling. Recently, neuroimaging studies have provided in vivo information about the neuroanatomical correlates of the regulation of energy intake. Temporal orchestration of such systems, however, is crucial to the integration of neuronal and hormonal signals that control eating behaviour. The challenge of this functional magnetic resonance imaging study is to map not only where but also when the brain will respond after food ingestion. Here we use a temporal clustering analysis technique to demonstrate that eating-related neural activity peaks at two different times with distinct localization. Importantly, the differentiated responses are interacting with an internal signal, the plasma insulin. These results support the concept of temporal parcellation of brain activity, which reflects the different natures of stimuli and responses. Moreover, this study provides a neuro-imaging basis for detecting dynamic processes without prior knowledge of their timing, such as the acute effects of medication and nutrition in the brain.

Brain activation in the processing of Chinese characters and words: A functional MRI study

Functional magnetic resonance imaging was used to identify the neural correlates of Chinese character and word reading. The Chinese stimuli were presented visually, one at a time. Subjects covertly generated a word that was semantically related to each stimulus. Three sorts of Chinese items were used: single characters having precise meanings, single characters having vague meanings, and two‐character Chinese words. The results indicated that reading Chinese is characterized by extensive activity of the neural systems, with strong left lateralization of frontal (BAs 9 and 47) and temporal (BA 37) cortices and right lateralization of visual systems (BAs 17–19), parietal lobe (BA 3), and cerebellum. The location of peak activation in the left frontal regions coincided nearly completely both for vague‐ and precise‐meaning characters as well as for two‐character words, without dissociation in laterality patterns. In addition, left frontal activations were modulated by the ease of semantic retrieval. The present results constitute a challenge to the deeply ingrained belief that activations in reading single characters are right lateralized, whereas activations in reading two‐character words are left lateralized. Hum. Brain Mapping 10:16–27, 2000.

Lie detection by functional magnetic resonance imaging

The accurate detection of deception or lying is a challenge to experts in many scientific disciplines. To investigate if specific cerebral activation characterized feigned memory impairment, six healthy male volunteers underwent functional magnetic resonance imaging with a block‐design paradigm while they performed forced‐choice memory tasks involving both simulated malingering and under normal control conditions. Malingering that demonstrated the existence and involvement of a prefrontal‐parietal‐sub‐cortical circuit with feigned memory impairment produced distinct patterns of neural activation. Because astute liars feign memory impairment successfully in testing once they understand the design of the measure being employed, our study represents an extremely significant preliminary step towards the development of valid and sensitive methods for the detection of deception. Hum. Brain Mapping 15:157–164, 2002.

Gender differences in neural correlates of recognition of happy and sad faces in humans assessed by functional magnetic resonance imaging

To examine the effect of gender on the volume and pattern of brain activation during the viewing of alternating sets of faces depicting happy or sad expressions, 24 volunteers, 12 men and 12 women, participated in this functional magnetic resonance imaging study. The experimental stimuli were 12 photographs of Japanese adults selected from Matsumoto and Ekmans Pictures of Facial Affect. Four of these pictures depicted happy facial emotions, four sad, and four neutral. Half of the photographs were of men and the other half were of women. Consistent with previous findings, distinct sets of neural correlates for processing happy and sad facial emotions were noted. Furthermore, it was observed that male and female subjects used a rather different set of neural correlates when processing faces showing either happy or sad expressions. This was more noticeable when they were processing faces portraying sad emotions than happy emotions. Our findings provide some preliminary support for the speculation that the two genders may be associated with different areas of brain activation during emotion recognition of happy or sad facial expressions. This suggests that the generalizability of findings in regard to neural correlates of facial emotion recognition should consider the gender of the subjects.

Cerebral blood flow measurement by dynamic contrast MRI using singular value decomposition with an adaptive threshold

Singular value decomposition (SVD) is a promising deconvolution technique for use in dynamic contrast agent magnetic resonance perfusion imaging. Computer simulations, however, show that the selection of the threshold for SVD affects the accuracy of the cerebral blood flow measurements and may distort the shape of the vascular residue function. In this report, a pixel‐by‐pixel thresholding method is proposed based on the signal‐to‐noise ratio of the concentration time curve at maximum concentration (SNRC). Monte Carlo simulations were used to determine the optimal threshold for different SNRC. This technique was used to analyze data from six healthy volunteers, resulting in a mean gray to white matter cerebral blood flow ratio of 2.67 ± 0.07. This value is in excellent agreement with values published in the literature. Magn Reson Med 42:167–172, 1999.

Perfusion-weighted imaging of interictal hypoperfusion in temporal lobe epilepsy using FAIR-HASTE: Comparison with H215O PET measurements

To detect perfusion abnormalities in areas of high magnetic susceptibility in the brain, an arterial spin‐labeling MRI technique utilizing flow‐sensitive alternating inversion recovery (FAIR) and half‐Fourier single shot turbo spin‐echo (HASTE) for spin preparation and image acquisition, respectively, was developed. It was initially tested in a functional study involving visual stimulation, and was able to detect significant activation with an increase (approximately 70%) in relative cerebral blood flow. Subsequently, it was applied in a clinical situation in eight patients with temporal lobe epilepsy (TLE). The perfusion‐weighted images obtained showed no susceptibility artifacts even in the region of the inferior temporal lobe and were able to detect interictal hypoperfusion in TLE. The results were compared with those derived from H215O PET perfusion imaging in each patient. A statistically significant correlation (r = 0.75, P < 0.05) was found between results acquired from these two modalities. Magn Reson Med 45:431–435, 2001.

An investigation of the impulse functions for the nonlinear BOLD response in functional MRI

Functional MRI (fMRI) based on blood oxygenation level dependent (BOLD) contrast can be used to detect hemodynamic responses to a broad range of stimuli. It however remains unclear in what fashion the BOLD response is a linear system, and how the impulse function differs with stimulation of varying duration. To address this question, fMRI using visual stimulation with a wide range of duration (0.5-12 s) was performed in six human volunteers. A strong linear correlation was shown on the full width at half maximum (r = 0.998) of the BOLD response curves and the area under the curves (r = 0.999) to the duration of stimulation. However, comparing the errors of the measured and predicted response curves, our results showed a poorer linearity at stimuli of shorter duration. By examining the impulse functions derived from different stimuli, based on the assumption that a linear convolution relationship existed, a higher differentiation was shown in the experiments with shorter stimuli (<3 s). Compared to the area under the impulse function derived from 12 s stimulation, with that obtained from 0.5, 1, 2, 3, 4, 8 s stimuli resulted in differences of 66.2, 33.5, 15.1, 5.4, 0.9, 7.9%, respectively. This study suggests a higher degree of nonlinearity in the BOLD signal changes due to stimuli of shorter duration, in agreement with earlier work.

Neural activities associated with emotion recognition observed in men and women

Previous studies have suggested that men and women process emotional stimuli differently. In this study, we examined if there would be any consistency in regions of activation in men and women when processing stimuli portraying happy or sad emotions presented in the form of facial expressions, scenes, and words. A blocked design BOLD functional magnetic resonance imaging paradigm was employed to monitor the neural activities of male and female healthy volunteers while they were presented with the experimental stimuli. The imaging data revealed that the right insula and left thalamus were consistently activated for men, but not women, during emotion recognition of all forms of stimuli studied. To further understand the imaging data acquired, we conducted the protocol analysis method to identify the cognitive processes engaged while the men and women were viewing the emotional stimuli and deciding whether they were happy or sad. The findings suggest that men rely on the recall of past emotional experiences to evaluate current emotional experiences. This may explain why the insula, a structure important for self-induced or internally generated recalled emotions, was consistently activated in men while processing emotional stimuli. Our findings suggest possible gender-related neural responses to emotional stimuli.

Cerebral hemodynamic response in Chinese (first) and English (second) language processing revealed by event-related functional MRI

Comparative functional neuroimaging studies using the block design paradigm have previously demonstrated that there are no significant differences in the location of areas of cerebral activation when native Chinese speakers independently process single words or sentences in both the Chinese (first) and English (second) languages. While it has also been documented that significant domains of brain response include the inferior to middle left frontal lobe, the latency, amplitude and duration of the associated hemodynamic changes during isolated neural processing of Chinese and English languages still remain unknown. The aim of this study, therefore, was to examine the characteristics of the hemodynamic alterations in the above-mentioned regions with event-related functional MRI (ER-fMRI) when native Chinese speakers performed verb generation tasks in both the Chinese (first) and English (second) languages. Our results demonstrate the presence of a similar neural activity-induced hemodynamic response in the inferior to middle left frontal lobe during both tasks. Further, there were also no statistically significant differences among the variables that described the hemodynamic response curves. These findings strongly imply that the underlying neural mechanism for Chinese (first) and English (second) language processing may be similar in native Chinese speakers.

Neural systems for word meaning modulated by semantic ambiguity

One important issue in neuroimaging research on language is how the brain processes and represents lexical semantics. Past studies with various paradigms reveal that the left inferior prefrontal and mid-superior temporal regions play a crucial role in semantic processing. Those studies, however, typically utilize words having a precise and dominant meaning as stimuli and have not manipulated lexico-semantic ambiguity, a key feature of human language, as an experimental variable. Here, we used a word generation paradigm to examine whether neuroanatomical networks for meaning are modulated by lexical ambiguity. We found that, compared with semantically precise words, semantically ambiguous words were mediated by strong brain activations in the left dorsal-lateral frontal areas, the anterior cingulate, and the right inferior parietal lobe. Semantically precise words, instead, were associated with the left inferior prefrontal and mid-superior temporal sites. These findings indicate that semantic analysis of written words is a dynamic process involving coordination of widely distributed neural subsystems, which are weighted by semantic ambiguity.