SMA Selectively Codes the Active Accumulation of Temporal, Not Spatial, Magnitude

Abstract

Estimating duration depends on the sequential integration (accumulation) of temporal information in working memory. Using fMRI, we directly compared the accumulation of information in temporal versus spatial domains. Participants estimated either the duration or distance of the dynamic trajectory of a moving dot or, in a control condition, a static line stimulus. Comparing the duration versus distance of static lines activated an extensive cortico-striatal network. By contrast, comparing the duration versus distance of dynamic trajectories, both of which required sequential integration of information, activated SMA alone. Indeed, activity in SMA, as well as right inferior occipital cortex, increased parametrically as a function of stimulus duration and also correlated with individual differences in the propensity to overestimate stimulus duration. By contrast, activity in primary visual cortex increased parametrically as a function of stimulus distance. Crucially, a direct comparison of the parametric responses to duration versus distance revealed that activity in SMA increased incrementally as a function of stimulus duration but not as a function of stimulus distance. Collectively, our results indicate that SMA responds to the active accumulation of information selectively in the temporal domain.

A Bayesian Perspective on Accumulation in the Magnitude System

“…the spatial dimension). Consistent with our current results, a recent study 28 showed that a longer duration yielded an underestimation of length. In this experiment, the environment was dark (fMRI study) and participants had no access to visual cues to constrain their spatial estimate of the moving dot.…”

“…Specifically, we replicated the findings of Lambrechts and colleagues 13 by demonstrating that number and surface estimates are under- and overestimated when presented for long and short durations, respectively. These results complement the findings that duration can be resilient to numerosity interference 27 , and that the direction of the interference between space and time may go in opposite direction when using dynamic displays 13, 28 . Although prior findings have reported asymmetrical effects in magnitude estimation, our findings differ in several ways.…”

“…Several neuroimaging studies also suggest the possibility of a common neural code for quantity estimations mostly implicating parietal cortices 33–39 but see ref. 28, 40. However, while a variety of interactions between time, space and number has been reported, the directionality of these interactions is not always consistent in the literature 13, 14 suggesting the need to moderate the claim for a common magnitude system: for instance, manipulating the duration of events has seldom been reported to affect numerical and spatial magnitudes 12, 13, 26, 29 whereas numerosity 6 and size 5, 7 typically influence duration.…”

Time to Move Again: Does the Bereitschaftspotential Covary with Demands on Internal Timing?

“…This result may well be related to timing functions of the SMA (Schubotz et al, 2000; Coull et al, 2004, 2015; Merchant et al, 2011, 2015), in particular to the notion that the SMA has time-accumulator functions (Merchant et al, 2011). However, this result does not invalidate other accounts of the BP.…”

“…The BP may represent this activity. Indeed, relations between both SMA and pre-SMA and time estimation have been well established in fMRI studies (Schubotz et al, 2000; Coull et al, 2004, 2015) and have also been suggested long ago with respect to the relation between (pre-)SMA and BP (Deecke et al, 1985). There is also impressive work on timing characteristics of SMA neurons in monkeys, most recently in a series of studies by Merchant and colleagues where monkeys had to continue a regular beat (e.g., Merchant et al, 2011, 2015) although in intervals ≤1 s, i.e., well below the intervals used in BP studies, and the intervals were explicitly regular, in stark contrast to the BP situation.…”