Changes in the sensory properties of numerosity stimuli have a direct effect on the outcomes of nonsymbolic number tasks. This suggests a prominent role of sensory properties in numerosity processing. However, the current consensus holds that numerosity is processed independent of its sensory properties. To investigate the role of sensory cues in ordinal number processes, we manipulated both dimensions orthogonally. Participants passively viewed the stimuli while their brain activity was measured using EEG. The results revealed an interaction between numerosity and its sensory properties in the absence of main effects. Different neural responses were present for trials where numerosity and sensory cues changed in the same direction compared with trials where they changed in opposite directions. These results show that the sensory cues are expected to change in concert with numerosity and support the notion that the visual cues are taken into account when judging numerosity.

Numbers are known to be processed along the left and right intraparietal sulcus. The present study investigated hemispheric differences between the magnitude representation of small and large symbolic numbers. To this purpose, an fMRI adaptation paradigm was used, where the continuous presentation of a habituation number was interrupted by an occasional deviant number. The results presented a distance-dependent increase of activation: larger ratios of habituation and deviant number caused a larger recovery of activation. Similar activation patterns were observed for small and large symbolic numbers, which is in line with the idea of a more coarse magnitude representation for large numbers. Interestingly, this pattern of activation was only observed in the left parietal cortex, supporting the recently proposed idea of Ansari [Ansari, D. Does the parietal cortex distinguish between “10”, “Ten,” and Ten Dots? Neuron, 53, 165–167, 2007] that the left parietal cortex is specialized in the processing of encultured symbolically presented numbers.

The close behavioral parallels between the processing of quantitative information conveyed by symbolic and non-symbolic stimuli led to the hypothesis that there exists a common cerebral representation of quantity (Dehaene, Dehaene-Lambertz, & Cohen, 1998). The neural basis underlying the encoding of number magnitude has been localized to regions in and around the intraparietal sulcus (IPS) by brain-imaging studies. However, it has never been demonstrated that these same regions are also involved in the quantitative processing of nonsymbolic stimuli. Using functional brain imaging, we explicitly tested the hypothesis of a common substrate. Angles, lines, and two-digit numbers were presented pairwise, one to the left and one to the right of the fixation point. In the three comparison tasks, participants ( n = 18) pressed the key on the side of the largest quantity. In the three control tasks, they indicated the side on which dimming occurred. A conjunction analysis between the three subtractions (comparison task-control task) revealed a site in left IPS that is specifically responsive when two stimuli have to be compared quantitatively, irrespective of stimulus format. The results confirm the hypothesis that quantity is represented by a common mechanism for both symbolic and nonsymbolic stimuli in IPS. In addition, the interaction between task and type of stimulus identified a region anterior to the conjunction site, not specific for quantitative processing, but reflecting general processes loaded by number processing.

The aim of the present positron emission tomography (PET) study was to investigate how visual processing in dorsal and ventral streams depends on the quantitative nature of the task. In the same—different task, participants identified the presence of an orientation difference between two gratings, presented centrally in succession. In the quantification task, participants estimated the magnitude of the difference and compared it to a fixed standard. Detection of dimming of the fixation point was used as a control task. Visual input, motor responses, and performance were equated across tasks. Subtracting same— different from quantification yielded significant activation in the left superior parietal lobule and left ventral premotor cortex, consistent with results obtained in number-processing tasks. The reverse subtraction yielded activation in the right inferior temporal gyrus, in agreement with earlier results. These results demonstrate that a single attribute can be processed either in the ventral or dorsal stream, depending on the cognitive operations required by the tasks.