Final Activity Report Summary - NUMERICAL COGNITION (Does two equal 2? Notational representations in the human brain)
The current project aimed to reveal how humans represented numbers in the brain. The study of neuronal specialisation in different cognitive and perceptual domains was important for our understanding of the human brain, its typical and atypical development, and its evolutionary precursors. Central to this understanding was the issue of numerical representation, and the questions of whether:
1. numerical magnitude was represented by specialised neuronal substrates.
2. different neuronal substrates were involved in representing numerical magnitude as a function of the format of presentation, i.e. the issue of abstract representation.
In the first study, I examined both these questions in a single paradigm by using a functional magnetic resonance adaptation paradigm, which enabled me to improve spatial resolution and tap into the neuronal populations that might be more selective to magnitude rather than other processes, while controlling for non-numerical related activation. The results from the functional magnetic resonance adaptation paradigm, effective connectivity analysis and multivariate pattern analysis supported the idea that numerical representation in the parietal lobes was sub-served by overlapping multiple representations that were format dependent.
In a second study I found convergent evidence for this conclusion by using a transcranial magnetic stimulation (TMS) with adaptation paradigm. By using adaptation to manipulate neural activation states prior to the application of TMS, one could control which neural populations were stimulated by TMS, as noted by Silvanto et al. in 2008. In this experiment, the subjects were adapted to the digit 7, which repeatedly appeared on the screen for 45 seconds in different locations and fonts. Following this adaptation period, the subjects had to decide in a same-different task whether two numbers, digits or verbal numbers, on the screen were perceptually the same or different, while we stimulated the intraparietal sulcus (IPS) with TMS. Only digits were affected by TMS to the left IPS, while words were not affected. Moreover, the TMS effect was most effective when the digit 7 appeared and was attenuated as numerical proximity decreased. This was not the case for verbal numbers. In a second experiment, the subjects were adapted to verbal numbers rather than digits. The results were exactly the opposite from the previous experiment, thus completing a double dissociation and supporting the idea that the parietal lobes were equipped with multiple representations for numerical quantity.
In the last study, I examined what was the contribution of the left and right parietal lobes to numerical acquisition and to what extent were fundamental abilities, such as automaticity or mapping numbers in space, affected by the parietal lobes functions. Previous studies used brain stimulation or patients with neurological damage to functionally assess parietal lobes functions in numerical cognition. However, up until now, no study had examined the necessity of the parietal lobes in learning numerical information. In the current study healthy adults learned an artificial number system during six days, while their left and right parietal lobes were stimulated. Their performance was evaluated in a range of numerical tasks that included artificial digits. It was found that the polarity of the brain stimulation modulated the performance. In general, anodal stimulation to the right parietal lobes enhanced the performance, while cathodal stimulation led to impairment, or had no effect as compared to sham stimulation. These results provided a first step into using brain stimulation as a neurorehabilitation intervention tool for improving numerical learning in populations with numerical difficulties.
1. numerical magnitude was represented by specialised neuronal substrates.
2. different neuronal substrates were involved in representing numerical magnitude as a function of the format of presentation, i.e. the issue of abstract representation.
In the first study, I examined both these questions in a single paradigm by using a functional magnetic resonance adaptation paradigm, which enabled me to improve spatial resolution and tap into the neuronal populations that might be more selective to magnitude rather than other processes, while controlling for non-numerical related activation. The results from the functional magnetic resonance adaptation paradigm, effective connectivity analysis and multivariate pattern analysis supported the idea that numerical representation in the parietal lobes was sub-served by overlapping multiple representations that were format dependent.
In a second study I found convergent evidence for this conclusion by using a transcranial magnetic stimulation (TMS) with adaptation paradigm. By using adaptation to manipulate neural activation states prior to the application of TMS, one could control which neural populations were stimulated by TMS, as noted by Silvanto et al. in 2008. In this experiment, the subjects were adapted to the digit 7, which repeatedly appeared on the screen for 45 seconds in different locations and fonts. Following this adaptation period, the subjects had to decide in a same-different task whether two numbers, digits or verbal numbers, on the screen were perceptually the same or different, while we stimulated the intraparietal sulcus (IPS) with TMS. Only digits were affected by TMS to the left IPS, while words were not affected. Moreover, the TMS effect was most effective when the digit 7 appeared and was attenuated as numerical proximity decreased. This was not the case for verbal numbers. In a second experiment, the subjects were adapted to verbal numbers rather than digits. The results were exactly the opposite from the previous experiment, thus completing a double dissociation and supporting the idea that the parietal lobes were equipped with multiple representations for numerical quantity.
In the last study, I examined what was the contribution of the left and right parietal lobes to numerical acquisition and to what extent were fundamental abilities, such as automaticity or mapping numbers in space, affected by the parietal lobes functions. Previous studies used brain stimulation or patients with neurological damage to functionally assess parietal lobes functions in numerical cognition. However, up until now, no study had examined the necessity of the parietal lobes in learning numerical information. In the current study healthy adults learned an artificial number system during six days, while their left and right parietal lobes were stimulated. Their performance was evaluated in a range of numerical tasks that included artificial digits. It was found that the polarity of the brain stimulation modulated the performance. In general, anodal stimulation to the right parietal lobes enhanced the performance, while cathodal stimulation led to impairment, or had no effect as compared to sham stimulation. These results provided a first step into using brain stimulation as a neurorehabilitation intervention tool for improving numerical learning in populations with numerical difficulties.