Model-free fMRI group analysis using FENICA.

Exploratory analysis of functional MRI data allows activation to be detected even if the time course differs from that which is expected. Independent Component Analysis (ICA) has emerged as a powerful approach, but current extensions to the analysis of group studies suffer from a number of drawbacks: they can be computationally demanding, results are dominated by technical and motion artefacts, and some methods require that time courses be the same for all subjects or that templates be defined to identify common components. We have developed a group ICA (gICA) method which is based on single-subject ICA decompositions and the assumption that the spatial distribution of signal changes in components which reflect activation is similar between subjects. This approach, which we have called Fully Exploratory Network Independent Component Analysis (FENICA), identifies group activation in two stages. ICA is performed on the single-subject level, then consistent components are identified via spatial correlation. Group activation maps are generated in a second-level GLM analysis. FENICA is applied to data from three studies employing a wide range of stimulus and presentation designs. These are an event-related motor task, a block-design cognition task and an event-related chemosensory experiment. In all cases, the group maps identified by FENICA as being the most consistent over subjects correspond to task activation. There is good agreement between FENICA results and regions identified in prior GLM-based studies. In the chemosensory task, additional regions are identified by FENICA and temporal concatenation ICA that we show is related to the stimulus, but exhibit a delayed response. FENICA is a fully exploratory method that allows activation to be identified without assumptions about temporal evolution, and isolates activation from other sources of signal fluctuation in fMRI. It has the advantage over other gICA methods that it is computationally undemanding, spotlights components relating to activation rather than artefacts, allows the use of familiar statistical thresholding through deployment of a higher level GLM analysis and can be applied to studies where the paradigm is different for all subjects.

Comparison of two different odorants in an olfactory detection threshold test of the Sniffin’ Sticks.

The olfactory test battery Sniffin' Sticks is a test of nasal chemosensory function that is based on pen-like devices for odour presentation. It consists of three olfactory subtests: threshold, discrimination, and identification. The detection threshold can be measured using two different odorants--n-butanol or PEA (phenylethyl alcohol). Both tasks are commonly applied in published studies, but little is known about the formal comparison of values obtained using them. Unlike the Sniffin' Sticks with n-butanol as odorant, there is poor validation for the threshold subtest with the odorant PEA. The purpose of this study was to compare these two different odorants. Both odorants were applied to 100 normosmic, healthy subjects (50 females). The experiment was divided into two sessions performed on two different days. After each threshold test the discrimination and identification subtests were conducted. We obtained significant differences in detection thresholds of PEA and n-butanol. The mean score of PEA threshold and PEA TDI (sum of threshold, discrimination, identification) was significantly higher compared to n-butanol. No significant correlation between individual PEA and n-butanol thresholds was observed. The differences between both odorants indicate that a formal validation of the Sniffin' Sticks with PEA as odorant for probing olfactory thresholds may be required.

Potential impact of a 32-channel receiving head coil technology on the results of a functional MRI paradigm.

PURPOSE: The authors investigated the potential of a 32-channel (32ch) receiving head coil for functional magnetic resonance imaging (fMRI) compared to a standard eight-channel (8ch) coil using a motor task. MATERIAL AND METHODS: Brain activation was analyzed in 14 healthy right-handed subjects performing finger tapping with the right index finger (block design) during two experimental sessions, one with the 8ch and one with the 32ch coil (applied in a pseudorandomized order). Additionally, a phantom study was performed to compare signal-to-noise ratios (SNRs) of both coils. RESULTS: During both fMRI sessions, analysis of motor conditions resulted in an activation of the left "hand knob" (precentral gyrus). Application of the 32ch coil obtained additional activation clusters in the right cerebellum, left superior frontal gyrus (SMA), left supramarginal gyrus, and left postcentral gyrus. The phantom study revealed a significantly higher SNR for the 32ch coil compared to the 8ch coil in superficial cortical areas located near the surface of the brain. CONCLUSION: The 32ch technology has a potential impact on fMRI studies, especially in paradigms that result in activation of cortical areas located near the surface of the brain.

Bad mood--bad activation? : The influence of emotions on the BOLD signal during finger tapping.

PURPOSE: The aim of this study was to investigate the influence of the subject's emotional state on the BOLD signal during simple finger tapping. MATERIAL AND METHODS: Twenty-nine healthy subjects participated in three functional magnetic resonance imaging (fMRI) sessions each. The sessions differed regarding emotional states, which were induced by standardized pleasant (positive condition, POS), unpleasant (negative condition, NEG), or neutral (neutral condition, NEU) pictures taken from the International Affective Picture System (IAPS) while the subjects performed a finger-tapping task (right index-to-thumb opposition). After each session, the subjects had to rate their actual mood and the pleasantness of the presented pictures. Furthermore, their state anxiety was assessed. Behavioral data were evaluated with SPSS. Functional imaging data were processed using statistical parametric mapping (SPM2) and were analyzed for main effects of emotional stimulation using an analysis of variance (ANOVA). The local maximum of interest was analyzed by a signal change analysis. RESULTS: Compared to the neutral emotional state, the positive and the negative emotional states caused a reduction of signal intensity changes within the primary sensorimotor hand area during simple finger tapping. The behavioral data indicated that the unpleasant pictures had a stronger effect on the emotional state than the pleasant images. According to these data the decrease in signal intensity change was more pronounced (significant; p < 0.001) in the negative condition than in the positive condition. CONCLUSION: This study showed that the emotional state of a test person is indeed influencing fMRI results and that well-balanced subjects in a neutral mood achieve the best fMRI results.

Chemosensory properties of human sweat.

Human sweat contains a mixture of odorants with trigeminal as well as olfactory properties. It has been shown that trigeminal perception is necessary to localize odors and that humans are not able to localize substances that only activate the olfactory system. To analyze the chemosensory properties of human sweat, we studied humans' ability to localize sweat stimuli to the different nostrils. Human sweat was collected during a bicycle workout (20 males) and was then applied to 34 different subjects (17 females) during odor detection and localization experiments by using an olfactometer. During the detection experiment, subjects were instructed to discriminate between sweat stimuli (20) and blanks (10). During the localization experiment, they were assigned to allocate the stimuli to either the right (15) or the left nostril (15). We found that subjects were able to detect the sweat stimuli with moderate to high sensitivity. However, they failed to localize the sweat stimuli to the accurate nostril above chance level. Due to this inability to localize the stimuli, we conclude that human sweat does not activate the intranasal trigeminal system but only the olfactory system.

Effects of laserneedle acupuncture on olfactory sensitivity of healthy human subjects: a placebo-controlled, double-blinded, randomized trial.

The aims of the present study were to investigate the influence of laserneedle acupuncture on olfactory sensitivity and to examine whether the attitude towards laserneedle acupuncture affects the outcome. Olfaction was tested repeatedly on two days using the olfactory detection threshold subtest of the Sniffin' Sticks test battery in sixty-four healthy subjects of which 32 showed a positive attitude towards the effects of laserneedle acupuncture and 32 were sceptic about its effects. Testing was accomplished three times on day one (T1 = 0 min, T2 = 35 min, T3 = 105 min) without laserneedle acupuncture and on day two (T1* = 0 min, T2* = 35 min, T3* = 105 min) when the subjects were randomized in a non-stimulation (placebo) and a stimulation (laserneedle acupuncture) group. Stimulation or non-stimulation was conducted in a double-blinded design. Following laserneedle acupuncture a significant decrease in olfactory detection thresholds was observed at both, T2* and T3*, whereas no significant changes were found in the baseline or placebo group. Effects of laserneedle acupuncture on the olfactory detection threshold did not differ between sceptic and non-sceptic subjects. In conclusion, laserneedle acupuncture is an effective method to improve olfactory sensitivity after one session of stimulation for at least one hour, independently of the attitude of subjects towards the stimulation method.

Olfactory detection thresholds and pleasantness of a food-related and a non-food odour in hunger and satiety.

The primary aim of this study was to investigate whether olfactory detection thresholds are dependent on different states of satiety. Using the threshold test of the Sniffin' Sticks test battery (single-staircase, three alternative forced choice procedure), sensitivity to a non-food odour (n-butanol) and a food-related odour (isoamyl acetate) was investigated. Twenty-four healthy, female subjects (mean age 24.2 years, SD 2.7 years) with normal olfactory function performed the tests when hungry and when satiated. Additionally, they rated their emotional condition, arousal, alertness as well as the intensity and pleasantness of both odorants. No significant change in the detection thresholds for the non-food odour n-butanol, but a significant change in detection threshold for the food-related odour isoamyl acetate was found. The detection threshold for isoamyl acetate was significantly lower in the state of satiety compared to the hungry condition. As expected, the perceived pleasantness of isoamyl acetate was significantly lower in satiety. In summary, the results indicate that in our experimental setting the actual state of satiety has effects on detection thresholds of a food-related odour, but not of a non-food odour. Interestingly, the higher sensitivity was found during the state of satiety challenging the current hypothesis that control of food intake is supported by a decrease in sensitivity to food odours. Instead our findings that satiety decreases the pleasantness of a food-related odour support the hypothesis that both odour threshold as well as pleasantness play an important role in the control of food intake.

Trigeminal perception is necessary to localize odors.

The human ability to localize odorants has been examined in a number of studies, but the findings are contradictory. In the present study we investigated the human sensitivity and ability to localize hydrogen sulphide (H(2)S), which in low concentrations stimulates the olfactory system selectively, the olfactory-trigeminal substance isoamyl acetate (IAA), and the trigeminal substance carbon dioxide (CO(2)). A general requirement for testing of localization was the conscious perception of the applied stimuli by the participants. Using Signal Detection Theory, we determined the human sensitivity in response to stimulation with these substances. Then the subjects' ability to localize the three different substances was tested. We found that humans can detect H(2)S in low concentration (2 ppm) with moderate sensitivity, and possess a high sensitivity in response to stimulation with 8 ppm H(2)S, 17.5% IAA, 50% v/v CO(2). In the localization experiment, subjects could localize neither the low nor the high concentration of H(2)S. In contrast, subjects possessed the ability to localize IAA and CO(2) stimuli. These results clearly demonstrate that humans, in spite of the aware perception, are not able to localize substances which only activate the olfactory system independent of their concentration, but they possess an ability to localize odorants that additionally excite the trigeminal system.

Investigation of breathing parameters during odor perception and olfactory imagery.

Compared with visual and auditory imagery, little is known about olfactory imagery. There is evidence that respiration may be altered by both olfactory perception and olfactory imagery. In order to investigate this relationship, breathing parameters (respiratory minute volume, respiratory amplitude, and breathing rate) in human subjects during olfactory perception and olfactory imagery were investigated. Fifty-six subjects having normal olfactory function were tested. Nasal respiration was measured using a respiratory pressure sensor. Using an experimental block design, we alternately presented odors or asked the subjects to imagine a given smell. Four different pleasant odors were used: banana, rose, coffee, and lemon odor. We detected a significant increase in respiratory minute volume between olfactory perception and the baseline condition as well as between olfactory imagery and baseline condition. Additionally we found significant differences in the respiratory amplitude between imagery and baseline condition and between odor and imagery condition. Differences in the breathing rate between olfactory perception, olfactory imagery, and baseline were not statistically significant. We conclude from our results that olfactory perception and olfactory imagery both have effects on the human respiratory profile and that these effects are based on a common underlying mechanism.

Cerebral differences in explicit and implicit emotional processing--an fMRI study.

The processing of emotional facial expression is a major part of social communication and understanding. In addition to explicit processing, facial expressions are also processed rapidly and automatically in the absence of explicit awareness. We investigated 12 healthy subjects by presenting them with an implicit and explicit emotional paradigm. The subjects reacted significantly faster in implicit than in explicit trials but did not differ in their error ratio. For the implicit condition increased signals were observed in particular in the thalami, the hippocampi, the frontal inferior gyri and the right middle temporal region. The analysis of the explicit condition showed increased blood-oxygen-level-dependent signals especially in the caudate nucleus, the cingulum and the right prefrontal cortex. The direct comparison of these 2 different processes revealed increased activity for explicit trials in the inferior, superior and middle frontal gyri, the middle cingulum and left parietal regions. Additional signal increases were detected in occipital regions, the cerebellum, and the right angular and lingual gyrus. Our data partially confirm the hypothesis of different neural substrates for the processing of implicit and explicit emotional stimuli.