近红外专栏第三期：fNIRS经典实验设计之-Block Design

fNIRS Classical Experimental Design - Block Design

With the development of science and technology, more and more researchers have started to pay attention to fNIRS, which is a safe, portable, and motion-inclusive imaging technique, and gradually apply it to their own research. As the quality of research is greatly influenced by the appropriateness of the experimental design, it is important to understand what kinds of experimental designs are available for NIRS research and what their characteristics are. Today, we are going to learn about one of the classic experimental designs for fNIRS - Block Design.

1. fNIRS experimental stimuli for Block presentations

In fNIRS research, it is very important to rationally arrange the presentation of different stimuli under different experimental conditions and the same experimental conditions, because different experimental stimulus presentations will enable the researcher to observe different neural response patterns. Today, we will bring you the content about "Block design", which is one of the classic and most frequently used experimental design methods in fNIRS experiments.

In Block Design, stimuli under the same experimental conditions are gathered together and appear sequentially, presented in the form of blocks, during which our brain repeatedly receives and processes the same type of stimuli over a period of time, and the corresponding neural activities will continuously respond to such stimuli, and eventually reach a more stable state. Therefore, Block Design reflects the activation level of a specific region of the brain under a certain experimental condition, and the process of Block Design is shown in Figure 1 below.

Figure 1

Time

It is easy to see through the flow chart above that Block Design has its own advantages. First, the block design is concise and clearly structured, with the same type of stimuli appearing in a concentrated and sequential manner. Secondly, the detection ability of Block Design is stronger, because the same kind of stimuli appear in a concentrated manner, so that the degree of neural response of a specific brain region to different experimental conditions can be detected when different experimental conditions are set and whether there is a difference between these responses. Finally, it is still the case that since the group block design presents the same kind of stimuli consecutively, it can induce a stronger blood oxygenation response (Zhu Chaozhe, 2020). Below we will go through a few typical cases to understand the Block experimental design more specifically.

2. Block Design Case Sharing

2.1 Emotion Modulates Activation in Auditory Cortex-an fNIRS Study

Visual emotional stimuli evoke enhanced activation in areas of the visual cortex, while there is little neuroimaging evidence for modulation of other sensory modalities through emotion. Therefore, one researcher, Plichta et al. (2011), utilized the fNIRS technique to investigate whether complex emotional sounds modulate activation in auditory cortex. Specifically, the researchers selected standardized pleasant, unpleasant, and neutral sounds from the International Affective Digitizing Sound System (IADS, Bradley and Lang, 1999) and presented them to healthy participants while recording evoked auditory cortex activation using the fNIRS technique.

The experiment consisted of 5 pleasant blocks, 5 unpleasant blocks, and 5 neutral sound blocks, which were presented in pseudo-randomized order, with each block lasting 24 s, and each block followed by a 24 s quiet rest period, and the entire fNIRS recording lasted approximately 15 s. The fNIRS optic poles were arrayed in the bilateral temporal lobe regions of the subjects in a total of 22 channels, with channels:2,3,7,11,12 defined as regions of interest (ROIs) and non-ROIs (non-ROIs) as regions of interest. Channels:2,3,7,11,12 were defined as regions of interest (ROIs), while non-ROIs were defined as channels directly adjacent to ROIs minus ROIs, as shown in Figure 2.

Figure 2

In terms of results it is worth noting that while both O2Hb and HHb were analyzed in this study, previous studies have found that analyzing O2Hb tends to lead to unclear results ( Herrmann et al., 2008), whereas the HHB results are more in line with fMRI data on emotionally regulated brain activation, and thus the results focus on changes in HHb in the bilateral temporal lobes.

Figure 3 shows the auditory cortex activation induced by the three emotional types of sounds.

Figure 3

Figure 4a below shows the mean activation and standard error (SE) of the auditory cortex ROIs and Non-ROIs elicited by different emotional sounds (UNP = Unpleasant; NEU = Neutral; PLE = Pleasant). t-tests showed that pleasant and unpleasant sounds resulted in significantly higher auditory cortex activation than neutral sounds, whereas there was no significant difference between pleasant and unpleasant.

Figure 4a

Figure 4b, on the other hand, shows the mean signal time course and standard error (SE) of HHb for different emotional voice categories

Figure 4b

Left temporal Right temporal

The results of this study are consistent with the original hypothesis that pleasant and unpleasant sounds lead to higher auditory cortex activation compared to the neutral condition. From the experimental design, it is easy to see that this is a typical block design, because the whole experiment contained 5 pleasant sound blocks, 5 neutral sound blocks and 5 unpleasant sound blocks and presented them immediately after each other, so this study is clearer and more structured in terms of the experimental design.

2.2 Measuring hemodynamic responses to leg activity using fNIRS

Helena et al. (2023) measured hemodynamic responses in the primary motor cortex (M1) elicited by leg activity using fNIRS. The study recruited 27 adult subjects and completed a leg and foot tapping task at a rate of 1.5 HZ. Specifically, subjects sat in a chair and tapped a rectangular pad on the floor with their right foot in a sequence of 434141243212, with 1,2,3,4 representing the center, front, back, and side, respectively, as shown in Figure 5 below.

Figure 5

The experiment was conducted in a quiet and dimly lit room, with subjects placing their right foot on the number "1" and resting their head on a chin rest, stepping on a mat on the floor in a sequence as soon as the gaze point on the screen turned red, and being asked to take a break and minimize movement of the rest of their body after a block of the task was completed. Specifically, trials began with an 8-second 1.5 Hz metronome sound to remind participants of the rhythm,. This was followed by a white gaze point without the metronome sound lasting 12-15 s. When the gaze point turned red, participants were instructed to perform the stepping task in sequence, which was followed by another white gaze point lasting 5 s upon completion. The experiment consisted of a total of 11 task blocks, the first of which was considered a test experiment and excluded from subsequent analyses. Figure 6 below.

Figure 6

The researcher placed one set of photopoles in the left M1 and PMC, two sets covered the left and right DLPFCs, and the other two sets lined up in the bilateral PPCs, as shown in Fig. 7 below.

Figure 7

Of the 27 subjects recruited there were: one subject was excluded due to too many errors, two subjects withdrew from the experiment due to feeling headache and anxiety, respectively, and another subject was excluded due to too many channels with a scalp coupling index (SCI) below 0.75. Twenty-three subjects were ultimately retained, and their hemodynamic reflections in the M1 region due to leg movements are shown below in Fig. 8. During stepping, compared to baseline, channels 5, 6, and 7 (p = 0.008 - 0.038) and channels 1 and 2 (p = 0.033 - 0.039), which are located in the intermediate position, showed significant changes in HbO, with intermediate channels 6 and 7 showing the highest activation (HbO: Cohen's d = 0.6 - 1.0; HbR: Cohen's d = 0.4).

Figure 8

From the experimental design of the above study, it is easy to see that this is a typical block experimental design that utilizes the fNIRS technique to measure the hemodynamic response of the M1 region under exercise conditions, and similar block studies commonly used in the tasks of finger tapping and grasping and so on. Researchers can refer to similar experimental designs for their own studies.

2.3 Detecting prefrontal activation during task execution using fNIRS

Michael et al. (2018) utilized the fNIRS technique to examine prefrontal activation in the brain during the execution of an n-back task. 39 healthy adults (10 males, 29 females) participated in this experiment and completed the n-back task in both the 0 and 3 conditions under the Block Design experimental design. Throughout the experiment, the Rest and Task blocks were alternated, with the 0-back and 3-back presented twice each and the order of Task block presentation counterbalanced between participants, e.g., half of the subjects experienced 0-3-0-3 and the other half experienced 3-0-3-0. In addition, the tasks were not presented consecutively for a given load to avoid a habituation effect, and the Task blocks lasted 47 s. The Rest and Task blocks were presented consecutively to avoid a habituation effect, and the Task blocks lasted 47 s. The Rest and Task blocks were presented consecutively to avoid a habituation effect. Task Block lasted 47 s and Rest period lasted 30 s. The total duration of the whole experiment was 338 s. The overall experimental procedure is shown in Figure 9 below.

Figure 9

Throughout the experiment, the researchers utilized the fNIRS technique to detect hemodynamic changes in the participants' bilateral PFCs, and the photopole arrangement is shown in Figure 10 below

Figure 10

It was found that HBO2 in the MedialPFC region hardly changed significantly in either the 0-back or the 3-back condition, and this was true for all regions during the 0-back Block. In contrast, during the 3-back Block, there were significant increases in HBO2 in the bilateral prefrontal regions, and these increases peaked approximately 20 seconds after the start of the 3-back Block. The results are shown in Figure 11.

Figure 11

From the experimental flow chart above, it is easy to see that only one condition of stimulation appears multiple times in each Block, for example, in the first Block a certain subject receives only 0-back, while in the second Block only focuses on the stimulation of this condition of 3-back, this is a very typical Block Design in fNIRS research, which is designed to reflects the degree of activation of specific regions of the brain under a certain experimental condition, and also allows comparison of the differences in activation of specific brain regions under different conditions.

3. Conclusion

Block design has become one of the most commonly used experimental designs in fNIRS research due to its simple design, clear structure and strong detection ability. However, it is worth noting that researchers should take the following points into consideration when conducting block design: first, block design usually presents the stimuli of a certain condition in a certain order alternately, so it may be difficult to avoid the anticipation effect in the experiments and subjects may easily develop psychological stereotypes about a certain block, so researchers should try to randomize the order of the experimental conditions when conducting block design. Therefore, researchers try to randomize the order of occurrence of experimental conditions as much as possible in the block design. Second, although the block design has a strong detection ability, it has a weak estimation ability, which makes it difficult to isolate the dynamics of the hemodynamic response induced by a single stimulus. Finally, too many conditions or too many blocks may make the experiment too long, which may lead to subjects' fatigue, inability to concentrate or boredom in the process. In conclusion, the duration of block design should neither be too long nor too short, and each block can be set at 20-30 seconds, the specific duration of which varies from experiment to experiment, and as much as possible, we should refer to the successful examples of the previous work or compare them with the fMRI study, and arrange the pre-experiment to test the experimental effect initially.

Reference List

Zhu. (2020). Functional Brain Imaging with Near-Infrared Spectroscopy. Science Press.

Juan E. Arco, Carlos González-García, Paloma Díaz-Gutiérrez, Javier Ramírez, María Ruz,Influence of activation pattern estimates and statistical significance tests in fMRI decoding analysis, Journal of Neuroscience Methods, Volume 308, 2018, Pages 248-260, ISSN 0165-0270, https://doi.org/ 10.1016/j.jneumeth.2018.06.017

M.M. Bradley, P.J. Lang International affective digitized sounds (IADS): stimuli, instruction manual and affective ratings (Tech. Rep. No. B-2)

The Center for Research in Psychophysiology, University of Florida, Gainesville, FL (1999)

Herrmann, M. J., Huter, T., Plichta, M. M., Ehlis, A. C., Alpers, G. W., Mühlberger, A., & Fallgatter, A. J. (2008). Enhancement of activity of the primary visual cortex during processing of emotional stimuli as measured with event-related functional near-infrared spectroscopy and event-related potentials. Human brain mapping, 29(1), 28-35. https://doi.org/10.1002/hbm.20368

Helena Cockx, Robert Oostenveld, Merel Tabor, Ecaterina Savenco, Arne van Setten, Ian Cameron, Richard van Wezel, fNIRS is sensitive to leg activity in the primary motor cortex after systemic artifact correction, NeuroImage, Volume 269, 2023,119880,ISSN 1053-8119, https://doi.org/10.1016/j. neuroimage.2023.119880.

Michael K. Yeung, Tsz L. Lee, Yvonne M.Y. Han, Agnes S. Chan.

Prefrontal activation and pupil dilation during n-back task performance: a combined fNIRS and pupillometry study, Neuropsychologia,Volume 159,2021 ,107954,issn 0028-3932,https://doi.org/10.1016/j.neuropsychologia.2021.107954.

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Near Infrared Column No.3: fNIRS Classical Experimental Design - Block Design

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