Imagine you’re trying to count how many apples are on a table. It seems simple, right? But behind the scenes, your brain is doing some pretty incredible work, especially in the occipito-parietal cortex (according to many studies!).
We’ll dive into the science of how our brains process numerosity. The focus being on some cool discoveries made using EEG and functional magnetic resonance imaging (fMRI).
Many fMRI studies show a right-lateralized advantage in the brain for numerosity processing. Some ERP (event-related potential) studies support this as well, with N1 and P2 components being affected. But, there’s still evidence suggesting bilateral or even left-lateralized processing, contributing to the ongoing debate.
This study aims to resolve questions about timing and lateralization of numerosity processing. They hypothesize that the right occipito-parietal region is mainly responsible for numerosity processing. To add, they expect to observe significant correlation between ERPs and task behavior in these regions.
Experimental procedure
By using a VHF (visual-half field) experiment, researchers wanted to isolate hemisphere-specific processing mechanisms. In the VHF test, participants are required to focus on stimuli presented in left or right visual field. 30 participants enrolled in this research.
The task is to count dots in 2 dots arrays and find the one with more dots. The ratio between the two dot arrays had six difficulties -> 1:2, 3:5, 7:10, 3:4, 5:6, and 7:8. You can see one trial in the image below:
We can see dot arrays positioned in one of three ways on the screen – left, right and middle. Participants then had 2s to denote the bigger array, after which there’s a short fixation period. You can read more on dots’ features in the original paper!To synchronize mobile EEG data and stimulus + response triggers, scientists used the wireless LSL protocol.
ERP-behavioral correlation shows right-sided dominance in numerosity processing
To investigate their hypothesis, researchers looked into correlation between ERP waveforms and behavioral data. Behavioral data implies response times (RT) for each correct answer and error rate (ER).
Take a look at the original paper for more detail on correlation calculations.
Correlation between ERP waveform and RT for 3 visual field conditions. We can see the original ERP waveform and correlations on the same plots. Also, they show the time of N1 component in the images. Correlation was calculated either by using single-trial or averaged (mean-trial) ERPs. Electrode sites that were chosen for the analysis are P7 and P8. Finally, in the middle we can see topographic maps for P8 electrode. These represent t-value maps for the correlations between the N1 component and behavior.
Correlation between ERP waveform and ER for 3 visual field conditions. You can see the explanation of represented results in the previous image (ERP correlation with RT).The study found that when showing stimuli in the left visual field, there’s significant correlation between N1 component and RT and ER. The N1 component is in the right occipito-parietal region. This right-lateralized processing was not observed when showing stimuli in the right visual field. It suggests that non-symbolic numerosity processing is done at the right occipito-parietal lobe. Especially at early processing stages.
Previous fMRI studies generally support a right-brain advantage for numerosity processing. But, the exact timing of this lateralization is the question. This study contributes to the discussion by showing a right-sided dominance at early ERP components.
Single-trial analysis also provided extra insights by revealing correlations that are not clear in average ERP responses. They emphasize the importance of accounting for individual differences in brain-behavior correlations.
Behavioral results inconsistent with ERP-behavior correlation results
Participants indicated which array was bigger by pressing the F (up) and J (down) keypads. We can see the Table showing descriptive statistics of the task. :
Despite the right-brain dominance observed in brain activity, it’s not present in these behavioral metrics. Scientist believe this has to do with the Simon effect. A phenomenon where the difference in stimulus location and response buttons location affects RTs. Two dot arrays are up and down, but J and F keypads are to each other’s sides. This effect is evident when stimuli is in the left visual field, complicating the interpretation of behavioral data.
Future research suggested addressing this by using voice responses instead of key presses.
To summarize, this study successfully addressed the timing of numerosity processing
Non-symbolic numerosity processing in the brain is mostly right-lateralized. This is especially true at early processing stages. Showed by a significant correlation between the N1 component in the right occipito-parietal region and behavioral performance. Also, the study addressed behavioral results on their own, which seem influenced by external factors (Simon effect).
The study also acknowledges the need for further research to explore these mechanisms in different populations. Another suggestion is that single-trial analysis can uncover brain-behavior correlations. These can be often overlooked in traditional, averaged ERP analyses.
Take a look at the original paper (linked) for more!
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