A MEG Study on the Processing of Time and Quantity: Parietal Overlap but Functional Divergence

Abstract

A common magnitude system for the processing of time and numerosity, supported by areas in the posterior parietal cortex, has been proposed by some authors. The present study aims to investigate possible intersections between the neural processing of non-numerical (time) and numerical magnitudes in the posterior parietal lobe. Using Magnetoencephalography for the comparison of brain source activations during the processing of duration and numerosity contrasts, we demonstrate parietal overlap as well as dissociations between these two dimensions. Within the parietal cortex, the main areas of overlap were bilateral precuneus, bilateral intraparietal sulci, and right supramarginal gyrus. Interestingly, however, these regions did not equivalently correlated with the behavior for the two dimensions: left and right precuneus together with the right supramarginal gyrus accounted functionally for durational judgments, whereas numerosity judgments were accounted by the activation pattern in the right intraparietal sulcus. Present results, indeed, demonstrate an overlap between the neural substrates for processing duration and quantity. However, the functional relevance of parietal overlapping areas for each dimension is not the same. In fact, our data indicates that the same parietal sites rule differently non-numerical and numerical dimensions, as parts of broader networks.

Keywords: parietal cortex; duration; event related fields (ERFs); magnetoencephalography (MEG); math cognition; numerosity; source estimation.

FIGURE 1

Experimental paradigms and behavioral results. (Left) Scheme for the sequential presentation for the numerosity task. Total duration was kept constant and temporal ratio, rhythm biases and pattern recognition were controlled. The figure provides an example of non-periodic series with five dots and a total duration of 1500 ms (T = total signal duration; o_j = dot onset duration; e_j = dot duration; i_j = interdot duration) (adapted from Dormal et al., 2006). (Right) Behavioral results (RTs and Accuracy proportions).

FIGURE 2

Event related fields (ERFs) for the two dimensions. The upper part of the figure contains a butterfly view off all the gradiometers for each condition and dimension. The bottom part includes the cluster averages for the two dimensions with conditions overlapped. The four or three consecutive latencies bands for analyses are displayed in squares, for numerosity and duration, respectively.

FIGURE 3

Whole brain analyses for the two dimensions. The image depicts the F-test values from the many minus few contrast (numerosity) and from long minus short contrast (duration). p < 0.01; >10 voxels. SFG, superior frontal gyrus; SPL, superior parietal lobule; STG, superior temporal gyrus; PC, precuneus; C, cuneus; PCC, posterior cingulate cortex; ACC, anterior cingulate cortex; LG, lingual gyrus; MOG, middle occipital gyrus; Ang, angular gyrus; SMG, supramarginal gyrus; IPL, inferior parietal lobule.

FIGURE 4

F contrasts after parietal masks for the two dimensions. The picture depicts any difference between the respective conditions for each dimension restricted to the parietal lobes, after collapsing the three windows of analyses. SMG, supramarginal gyrus; ANG, angular gyrus; IPS, intraparietal sulcus; PC, precuneus; SPL, superior parietal lobule; PCG, precentral gyrus.

FIGURE 5

Significant correlations with behavioral effects, between all the clusters showing overlap. The cortex shows an overlaid mask with the overlapped areas between dimensions. The mask was smoothed for visualization. Areas showing a significant correlation with reaction times (RT) or accuracy (Acc) are indexed. N₁, Numerosity (60–180 ms); D₂, Duration (long: 160–400 ms; short: 240–460 ms). D₃, Duration (long: 420–850 ms; short: 490–950 ms).