Limits...
The neural representation of Arabic digits in visual cortex.

Peters L, De Smedt B, Op de Beeck HP - Front Hum Neurosci (2015)

Bottom Line: In this study, we investigated how Arabic digits are represented in the visual cortex, and how their representation changes throughout the ventral visual processing stream, compared to the representation of letters.However, the activity in this region might have been confounded by string length-number words contain more characters than digits.We found an alteration in representations throughout the ventral processing stream from clustering based on amount of visual information in primary visual cortex (V1) towards clustering based on symbolic stimulus category higher in the visual hierarchy.

View Article: PubMed Central - PubMed

Affiliation: Parenting and Special Education Research Unit, KU Leuven Leuven, Belgium, Europe.

ABSTRACT
In this study, we investigated how Arabic digits are represented in the visual cortex, and how their representation changes throughout the ventral visual processing stream, compared to the representation of letters. We probed these questions with two functional magnetic resonance imaging (fMRI) experiments. In Experiment 1, we explored whether we could find brain regions that were more activated for digits than for number words in a subtraction task. One such region was detected in lateral occipital cortex. However, the activity in this region might have been confounded by string length-number words contain more characters than digits. We therefore conducted a second experiment in which string length was systematically controlled. Experiment 2 revealed that the findings of the first experiment were task dependent (as it was only observed in a task in which numerosity was relevant) or stimulus dependent (as it was only observed when the number of characters of a stimulus was not controlled). We further explored the characteristics of the activation patterns for digit and letter strings across the ventral visual processing stream through multi-voxel pattern analyses. We found an alteration in representations throughout the ventral processing stream from clustering based on amount of visual information in primary visual cortex (V1) towards clustering based on symbolic stimulus category higher in the visual hierarchy. The present findings converge to the conclusion that in the ventral visual system, as far as can be detected with fMRI, the distinction between Arabic digits and letter strings is represented in terms of distributed patterns rather than separate regions.

No MeSH data available.


Schematic overview of a possible design of Experiment 2.
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Figure 3: Schematic overview of a possible design of Experiment 2.

Mentions: Although we controlled the stimuli from Experiment 1 for the amount of visual information (i.e., number of black pixels in the stimulus), it is evident that number words always consist of more visual elements (i.e., multiple letters) than digits (i.e., one or two elements). Previous research has shown that visual regions, such as the LOC, can be sensitive to the number of visual elements presented (Xu and Chun, 2006; Xu, 2008). To control for this potential confound of the number of visual elements on the screen, participants performed a second fMRI experiment immediately after Experiment 1. In Experiment 2, both string length (2- or 5-characters) and character format (Arabic digits or letters) were manipulated. Participants performed an order judgment task: They were asked to indicate whether the ordering of the first character relative to the last character was correct or not. In the two digit conditions (both 2- and 5-characters), the ordering was correct if the first character was numerically smaller than the last character. In the letter conditions, alphabetical order was correct. Four blocks per condition alternated with five fixation blocks were presented during this experiment, with each block lasting 12 s. Within each trial block, six stimuli were presented. Total duration of one run was 252 s, and participants performed 4 runs. The design of Experiment 2 is illustrated in Figure 3.


The neural representation of Arabic digits in visual cortex.

Peters L, De Smedt B, Op de Beeck HP - Front Hum Neurosci (2015)

Schematic overview of a possible design of Experiment 2.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4585091&req=5

Figure 3: Schematic overview of a possible design of Experiment 2.
Mentions: Although we controlled the stimuli from Experiment 1 for the amount of visual information (i.e., number of black pixels in the stimulus), it is evident that number words always consist of more visual elements (i.e., multiple letters) than digits (i.e., one or two elements). Previous research has shown that visual regions, such as the LOC, can be sensitive to the number of visual elements presented (Xu and Chun, 2006; Xu, 2008). To control for this potential confound of the number of visual elements on the screen, participants performed a second fMRI experiment immediately after Experiment 1. In Experiment 2, both string length (2- or 5-characters) and character format (Arabic digits or letters) were manipulated. Participants performed an order judgment task: They were asked to indicate whether the ordering of the first character relative to the last character was correct or not. In the two digit conditions (both 2- and 5-characters), the ordering was correct if the first character was numerically smaller than the last character. In the letter conditions, alphabetical order was correct. Four blocks per condition alternated with five fixation blocks were presented during this experiment, with each block lasting 12 s. Within each trial block, six stimuli were presented. Total duration of one run was 252 s, and participants performed 4 runs. The design of Experiment 2 is illustrated in Figure 3.

Bottom Line: In this study, we investigated how Arabic digits are represented in the visual cortex, and how their representation changes throughout the ventral visual processing stream, compared to the representation of letters.However, the activity in this region might have been confounded by string length-number words contain more characters than digits.We found an alteration in representations throughout the ventral processing stream from clustering based on amount of visual information in primary visual cortex (V1) towards clustering based on symbolic stimulus category higher in the visual hierarchy.

View Article: PubMed Central - PubMed

Affiliation: Parenting and Special Education Research Unit, KU Leuven Leuven, Belgium, Europe.

ABSTRACT
In this study, we investigated how Arabic digits are represented in the visual cortex, and how their representation changes throughout the ventral visual processing stream, compared to the representation of letters. We probed these questions with two functional magnetic resonance imaging (fMRI) experiments. In Experiment 1, we explored whether we could find brain regions that were more activated for digits than for number words in a subtraction task. One such region was detected in lateral occipital cortex. However, the activity in this region might have been confounded by string length-number words contain more characters than digits. We therefore conducted a second experiment in which string length was systematically controlled. Experiment 2 revealed that the findings of the first experiment were task dependent (as it was only observed in a task in which numerosity was relevant) or stimulus dependent (as it was only observed when the number of characters of a stimulus was not controlled). We further explored the characteristics of the activation patterns for digit and letter strings across the ventral visual processing stream through multi-voxel pattern analyses. We found an alteration in representations throughout the ventral processing stream from clustering based on amount of visual information in primary visual cortex (V1) towards clustering based on symbolic stimulus category higher in the visual hierarchy. The present findings converge to the conclusion that in the ventral visual system, as far as can be detected with fMRI, the distinction between Arabic digits and letter strings is represented in terms of distributed patterns rather than separate regions.

No MeSH data available.