Nonverbal representation of time and number in adults.

A wealth of human and animal research supports common neural processing of numerical and temporal information. Here we test whether adult humans spontaneously encode number and time in a paradigm similar to those previously used to test the mode-control model in animals. Subjects were trained to classify visual stimuli that varied in both number and duration as few/short or many/long. Subsequently subjects were tested with novel stimuli that varied time and held number constant (eight flashes in 0.8-3.2s) or varied number and held time constant (4-16 flashes in 1.6s). Adult humans classified novel stimuli as many/long as monotonic functions of both number and duration, consistent with simultaneous, nonverbal, analog encoding. Numerical sensitivity, however, was finer than temporal sensitivity, suggesting differential salience of time and number. These results support the notion that adults simultaneously represent the number and duration of stimuli but suggest a possible asymmetry in their representations.

Coherent spontaneous activity identifies a hippocampal-parietal memory network.

Despite traditional theories emphasizing parietal contributions to spatial attention and sensory-motor integration, functional MRI (fMRI) experiments in normal subjects suggest that specific regions within parietal cortex may also participate in episodic memory. Here we examined correlations in spontaneous blood-oxygenation-level-dependent (BOLD) signal fluctuations in a resting state to identify the network associated with the hippocampal formation (HF) and determine whether parietal regions were elements of that network. In the absence of task, stimuli, or explicit mnemonic demands, robust correlations were observed between activity in the HF and several parietal regions (including precuneus, posterior cingulate, retrosplenial cortex, and bilateral inferior parietal lobule). These HF-correlated regions in parietal cortex were spatially distinct from those correlated with the motion-sensitive MT+ complex. Reanalysis of event-related fMRI studies of recognition memory showed that the regions spontaneously correlated with the HF (but not MT+) were also modulated during directed recollection. These regions showed greater activity to successfully recollected items as compared with other trial types. Together, these results associate specific regions of parietal cortex that are sensitive to successful recollection with the HF.