Number Processing in Infants Rebecca Merkley
Research Question • Bilateral intraparietal sulcus is implicated in symbolic and non-symbolic number processing and appears to be specialized for number (Holloway, Price, Ansari, 2010) • Research question: Does number-specialization of the bilateral IPS develop through experience, or is it specialized before exposure to training in symbolic number?
Approximate Number System (ANS) • Research has shown evidence for core number systems that are present in pre-verbal infants • The ANS represents the approximate numerical magnitude of a group of objects
Numerical Ratio Effect (NRE) • The larger the numerical difference is between two numerical magnitudes, the faster and less erroneous judgments of relative numerical magnitude are (Moyer & Landauer, 1967) • Furthermore, the ratio of two stimuli magnitudes is more strongly associated with reaction time than the absolute difference (numerical distance) between the two magnitudes is
Evidence for NRE • Behavioural effects have been replicated in many studies (i.e. Cantlon & Brannon, 2006; Dehaene, Dupoux, & Mehler, 1990) • Neuroimaging also shows brain regions that are modulated by numerical distance and ratio (Pinel, Dehaene, Riviere, LeBihan, 2001) • Eye movement patterns are also affected by numerical ratio (Merkley & Ansari, 2010) • Individual differences in NRE are predictive of children’s math achievement scores (Bugden & Ansari, in press; Halberda, Mazzocco, Feigenson, 2008)
Functional Near-infrared Spectroscopy (fNIRS) • Measures hemodynamic response by shining light into the brain and measuring how much is reflected back • Like pulse oximetry • Hemodynamic response: blood flow in the brain (oxygenated and deoxygenated hemoglobin have different properties)
fNIRS • Advantages • Portable and can tolerate some movement (Karmiloff-Smith, 2010) • Higher temporal resolution than fMRI and higher spatial resolution than EEG • Disadvantages • Lower temporal resolution than EEG and lower spatial resolution than fMRI • fNIRS does not produce structural images, so locating the origin of the hemodynamic response based on the scalp measures can be difficult
Investigations of Number Representation in Infants • Hyde, Boas, Blair, & Carey (2010) used fNIRS to investigate neural correlates of number representation in infants • Hypothesis: non-verbal number representation is continuous over development • If results show that the bilateral intraparietal sulcus is specialized for number in infants, as it is in children and adults, then this would support the hypothesis
Hyde et al., 2010 • Method: • 36 infants (5.5-6.5 months) • Adaptation or Oddball Paradigm • Adaptation Images (16 circles) • Number Change Group: • 8 or 32 circles • Shape Change Group: • 16 squares or 16 triangles • Stimuli were presented in short blocks and participants were given short rests • Deviant images were presented pseudo-randomly
Hyde et al., 2010 • Results: • Significant increase in Oxyhemoglobin concentration in the right parietal region on number-deviant trials • Significant increase in Oxyhemoglobin concentration in the right occipital region on shape-deviant trials • Interaction between brain region and change type
Hyde et al., 2010 • Discussion: • Results support the hypothesis that the right inferior parietal region is specialized for number across development • Brain response was seen in the right inferior parietal region in infants, but is seen in the bilateral intraparietal sulcus in children and adults • Evidence that number processing is initially lateralized in right hemisphere • It is possible that there was activity in other brain regions that were not measured by fNIRS
Current Study • More light detectors – better scalp coverage • Block design instead of event-related design – less rejected trials • More statistically powerful Block Design Event-related design
Methodology Questions • Control – blank screen or video of faces? • Length of blocks – between 15 and 30 seconds? • Xu and Spelke (2000) established a habituation criterion of a decline in looking time over trials that summed to at least 12 s) • Number change within subjects or between subjects design?
Discussion • If the results of this study replicate the results of the previous study they will provide further evidence that the right intraparietal sulcus is specialized for number in infancy, and the bilateral parietal sulcus becomes specialized for number over the course of development • What are some alternative possible results and explanations?
Alternatives • If results show that there is a significant increase in oxygenated hemoglobin in the left and right parietal regions for number change trials • Suggests that the bilateral intraparietal sulcus is specialized for number even in infancy • If results show that there is a significant increase in oxygenated hemoglobin in right parietal region in both shape change and number change trials • Suggests that the bilateral intraparietal sulcus becomes specialized for number as a result of learning
Future Directions • Investigate processing of small numbers • Investigate changes of magnitude in shape size