Conjunction analysis of the four main conditions (processing C/M or tool nouns in classifier phrases with either a numerical C+M1 or a non-numerical M3) showed activation in the bilateral inferior occipital cortex (IOC) including the fusiform gyrus (FFG), bilateral inferior frontal gyrus (IFG, especially in the left hemisphere), left SFG, left MFG (orbital part), and left insula (see Figure 2B and Table 4).
Conjunction analysis of the five conditions involved in number processing (C/M comparison of numerical C+M1, C/M comparison of non-numerical M3, numbers, dots, and number words) showed activation in the IOC including the FFG, bilateral superior parietal lobule (SPL), bilateral inferior parietal lobule, bilateral IFG, right MFG, bilateral SFG, and bilateral insula (see Figure 2C and Table 4).
Conjunction analysis of the three conditions involved in semantic processing (two noun comparison conditions and the tool noun condition) showed activation in the bilateral occipital cortex including the FFG, bilateral superior parietal lobule, bilateral IFG (mostly in the left hemisphere), left SFG, and bilateral MFG (see Figure 2D and Table 4).
Hemisphere Brain regions Peak MNI
x y z t-Value Cluster size CM comparison – Tool noun comparison
Right Inferior parietal lobule (BA 40) 42 -48 44 9.71 3928 Tool noun comparison – CM comparison
None
Numerical C+M1 – Non-numerical M3
None
Non-numerical M3 – Numerical C+M1
None
Table 4
Common brain activation for different types of conditions, relative to rest. (pFWE-corr
< .05; BA, Brodmann’s area)
Hemisphere Brain regions Peak MNI
x y z t-Value Cluster size CM comparison ∩ Tool noun comparison
Left Inferior occipital cortex -18 -94 -12 20.68 17196 Left Precentral gyrus
(Inferior frontal gyrus, BA 9) -44 4 34 10.33 3107 Left Supplementary motor area
(Superior frontal gyrus, BA6) -6 6 58 8.65 533 Right Precentral gyrus
(Inferior frontal gyrus, BA 9) 48 8 34 7.54 509 Left Middle frontal gyrus, orbital part -44 46 -4 5.58 116
Left Insula -30 20 4 5.53 31
CM comparison ∩ Numbers ∩ Dots ∩ Number words
Right Inferior occipital cortex 34 -80 -12 14.51 Left Supplementary motor area
(Superior frontal gyrus, BA 6) -6 6 58 8.39 604 Right Superior frontal gyrus
(Middle frontal gyrus, BA 6) 32 -2 62 6.57 302
Right Insula (BA 45) 32 24 6 5.93 80
Left Insula (BA 45) -30 24 6 5.83 64
Tool noun comparison ∩ Tool nouns
Left Inferior occipital cortex -34 -86 -8 16.07
(Superior frontal gyrus) Right Precentral gyrus
(Inferior frontal gyrus, BA 9) 46 8 34 6.40 194 Left Middle frontal gyrus, orbital part -44 46 -4 5.58 92
Right Middle frontal gyrus 44 28 22 5.39 90
Discussion
We adopted a modified paradigm that included minimal pairs of C/M with fixed mathematical values to investigate the number processing of C/M with fMRI in this study. We found that processing C/M in a semantic distance task elicited higher activations in the bilateral IPL including the IPS, right SFG, bilateral MFG, right mFG, and right MTG than processing tool nouns. As we predicted, the IPS, which has been shown to frequently engage in numerical representation, was more activated for the contrast of C/M comparison versus tool noun comparison (16, 17). Moreover, the brain activations in the IPL, SFG, and mFG largely overlapped with the brain regions that were reported in a very recent meta-analysis study of number processing (22).
Sokolowski et al. (22) revealed that not only the parietal lobule but also the frontal regions play an important role in number processing. Specifically, the SFG was repeatedly activated for symbolic magnitude processing while the right mFG and cingulate gyrus were activated for non-symbolic magnitude processing. Moreover, the right SFG consistently activated during symbolic and non-symbolic number
processing. Taken together, processing C/M than tool nouns engaged in frontal and parietal regions that have been suggested to associate with processing numerical information. This finding was consistent with the mathematical theory of C/M which proposed that C/M represents mathematical values (6). Although the number of strokes, frequency of C/Ms, and frequency of nouns were carefully matched among the four main experimental conditions and the baseline condition, participants still made more errors while processing C/M compared to processing tool nouns, t(20) = -3.281, p = .004. One may argue that the activation in the IPS for processing C/M than tool nouns reflected higher task demand rather than magnitude representation in this study. However, it is worth noting that the bilateral IPL was found activated during number proceesing in both active and passive tasks (22). This suggests that the activation was related to magnitude processing rather than task demands. However, the function of the bilateral MFG and the rMTG for processing C/M than tool nouns remains unclear and needs further research as these regions were not typical regions that were found to be involved in number processing in the literature.
This finding was different from the finding in the study by Cui et al. (14), in which the contrast analyses between classifiers and tool nouns resulted in no
significant activations. The critical reason why we observed different neural activities of processing classifiers may lie on the nature of classifiers. Chinese classifiers not only have a mathematical function but also function as a profiler. That is, Chinese classifiers not only encode the mathematical values but also highlight the inherent semantic attributes of the noun. However, Cui et al. (14) overlooked the potential
possibility that participants make the semantic judgment based on C/M’s semantic attributes which may have confounded their results. As found in the first behavioral experiment that we conducted before this fMRI experiment, participants chose the C/M phrase that had a similar semantic attribute to the target C/M phrase over the C/M phrase that had a similar mathematical value. Therefore, to control for the semantic attributes of C/Ms, we used minimal pairs of C/Ms as our stimuli in this experiment. Adding the same tool nouns in the nominal phrases, i.e. adopting minimal pairs, helped confine the semantic attributes of C/M. Second, we only included the C/M that encode fixed mathematical values, i.e. C, M1, M3, in our study whereas Cui et al. (14) also incorporated C/M with variable mathematical values, i.e. M2 and M4, as experimental stimuli. According to the second behavioral experiment we conducted, the accuracy for the variable mathematical value condition was only around 50% and significantly lower than the accuracy for the fixed mathematical value condition in the semantic distance comparison task (20). In other words, the underlying cognitive mechanism of processing C/M with a variable mathematical value was unclear whereas participants did show that they make semantic judgment based on
mathematical values when facing C/M with fixed mathematical values. Consequently, we only included C/M with fixed mathematical in the current experiment. These amendments enabled us to purely examine the neural underpinnings of quantity processing of C/M in this study. Moreover, we further added the baseline condition, in which participants saw three identical nominal phrases that required similar
perceptual processing, in this study. By contrasting the four main experimental
conditions versus the baseline condition, the resulting brain activations should, at least in part, reveal magnitude representations. In sum, the brain activities for processing the quantity information that C/M encode may only appear for specific stimuli (C/M with a fixed mathematical values) under strictly controlled situation (presented in the form of minimal pairs) using stringent data analysis (contrasting against a baseline condition) as in our experiment. As C/M with fixed mathematical valuesmay be related to exact magnitude cognition and C/M with variable mathematical values may be linked with approximate quantity conception, future research is needed to
investigate the neural correlates of processing C/M with variable mathematical values to better clarify its underlying cognitive mechanism.
We speculated that another reason why Cui et al. (14) could not find the IPS more activated for classifiers than tool nouns was because that they did not
differentiate numerical and non-numerical C/M. Nonetheless, our results of contrast analyses between numerical C+M1 and non-numerical M3 did not reveal any
significant activation, suggesting that processing these two types of C/M involved similar neural activities. In our experiment, participants had to read three nominal
phrases and judge which one of the two phrases was semantically closer to the target phrase. When participants made C/M comparison, they had to represent the quantity information that each C/M carry and then choose the C/M with closer mathematical value to the target C/M. Although M3s encode non-numerical values, they may be represented as a specific numerical value to be compared in the semantic distance comparison task. For example, when participants had to compare yi bang gang ding (one pound of steel nails) and yi ke gang ding (one gram of steel nails), it is possible that they represent one pound as 453 grams to make the semantic judgment. Therefore, it is likely that due to the nature of the semantic distance comparison task in this study, representing C/M as a numerical value was one of the strategies that participants used.
This may explain why we did not observe different brain activations contrasting between numerical C+M1 and non-numerical M3. Future studies are suggested to adopt other active tasks or a passive viewing paradigm to reexamine the neural correlates of numerical and non-numerical C/M and clarify if the underpinning neural activities are similar regardless of experimental paradigms.
In addition to contrast analyses, we conducted conjunction analyses. First, we showed that processing C/M and processing tool nouns commonly induced higher activations in the IOC (including FFG), bilateral IFG (especially in the left
hemisphere), left SFG, left MFG (orbital part), and left insula. These regions have been found to engage in phonological and semantic processing in Chinese words (18).
Second, the conjunction analysis of number processing (C/M comparison of numerical C+M1, C/M comparison of non-numerical M3, numbers, dots, and number words) showed higher activation in the IOC including the FFG, bilateral SPL,
bilateral IPL, bilateral IFG, right MFG, bilateral SFG, and bilateral insula. Replicating previous studies, the bilateral IPS were more activated for representation of numerical magnitude regardless of notations (16, 17). Our findings were also consistent with the recent meta-analysis of number processing that reported the bilateral IPL, left SPL, and the right SFG activated for both symbolic and non-symbolic number processing (22).
Third, the conjunction analysis of semantic processing (two noun comparison conditions and the tool noun condition) showed higher activation in the bilateral occipital cortex including the FFG, bilateral SPL, bilateral IFG (especially the left hemisphere), left SFG, and bilateral MFG, which was consistent with previous findings that conceptual representation engaged a distributed neural network in the brain (23, 24). Crucially, the left IFG has been shown to activate more naming tools than naming animals while participants engaged in viewing and naming these items (25).
It is worth discussing the role that the SPL play in number processing and semantic processing. Cui et al. (14) reported that the angular gyrus, which locates in the SPL, commonly activated for classifiers, tool nouns, numbers, and dot arrays.
Replicating the finding by Cui et al. (14), the angular gyrus was found more activated for both number processing and semantic processing in this study. This suggests that the angular gyrus did not exclusively engage in number processing. However, the activation in the SPL for number processing (18086 voxels) was a larger cluster than the one elicited by semantic processing (14504 voxels). In particular, we found that the anterior part of the bilateral IPL, overlapping with the IPS, specifically activated for number processing than semantic processing.
Combining the literature and the findings in this study, we concluded that, linguistically, C/Ms not only highlight nouns with semantic attributes but also denote quantity with a mathematical value. This suggests that the linguistic system of C/M interactes with categorization and magnitude cognition. Moreover, our finding that processing C/Ms with fixed mathematical values elicit higher activations in frontal and parietal regions that have been shown to engage in numerical processing partially supported the mathematical theory of C/M, which suggests that C/Ms encode
mathematical values (6). We suggest future studies continue to further investigate the number processing of C/M with variable mathematical values and the multiplication function of C/M to examine the theory more thoroughly. Lastly, our results of
conjunction analysis of number processing verified that the IPS represents numerical magnitude independent of notations by providing neural evidence of quantity
processing of C/Ms.
Acknowledgements
We thank the Taiwan Mind & Brain Imaging Center, supported by the Ministry of Science and Technology, Taiwan (R.O.C), and National Chengchi University for consultation and instrumental availability.
Funding
This work was supported by the Ministry of Science and Technology (MOST 103-2410-H-004-136-MY3).