Discrete and analogue quantity processing in the parietal lobe: a functional MRI study

Abstract

The human intraparietal sulcus (IPS) is implicated in processing symbolic number information and possibly in nonsymbolic number information. Specific IPS activity for discrete quantities (numerosities) as compared with continuous, analogue quantity has not been demonstrated. Here we use a stimulus-driven paradigm to distinguish automatic estimation of "how many things" from "how much" and "how long." The discrete analogue response task (DART) uses the perception of hues which can change either abruptly (discrete, numerous stimuli) or smoothly (analogue, nonnumerous stimuli) in space or in time. Subjects decide whether they saw more green or more blue. A conjunction analysis of spatial and temporal conditions revealed that bilateral IPS was significantly more active during the processing of discrete stimuli than during analogue stimuli, as was a parietal-occipital transition zone. We suggest that processing numerosity is a distinct process from processing analogue quantity, whether extended in space or time, and that an intraparietal network connects objects' segmentation to the estimation of their numerosity.

Fig. 1.

Schematic view of DART stimuli arranged in a 2 × 2 design. The same hues are used in all conditions. (Upper Left) In the discrete temporal condition, a sequence of blue and green squares appears at random intervals between 150 and 400 ms in the same place on the screen. (Upper Right) In the corresponding analogue temporal condition, the same hues are linked by intermediate hue values, so that a single square appears to be smoothly changing hue. In the corresponding discrete spatial condition (Lower Left), the same hues are formed into discrete rectangles separated by gray background, whereas, in the analogue spatial condition (Lower Right), a smoothing function blurs the boundaries between the different hues. Every trial of discrete stimuli is transformed into a trial of analogue stimuli.

Fig. 2.

Behavioral results. (a) Subjects’ correct responses for the four tasks and five difficulty levels. Errors bars indicate the range of observed percentages of correct responses (i.e., max = 100%). (b) Subjects’ reaction time for the four tasks and five difficulty levels. Errors bars indicate 1 SD.

Fig. 3.

Conjunction analysis (spatial and temporal presentations). Bilateral areas along the length of the IPS were more activated by numerosity processing (discrete, countable stimuli) than extent processing (analogue, uncountable stimuli). Numerosity processing activates the IPS and the caudal IPS bilaterally.

Fig. 4.

Parametric analysis of task difficulty. The activity in the more horizontal segment of the IPS and in the postcentral sulcus increased as the difference between the number of green and blue stimuli decreased. Figures show task difficulty effect activation during numerosity processing in time (a) and in space (b and c).