Headwaters of the flow of time
Authors:   , Hendrikx Eva Helena Hendrica 
Published: January 2025

Events around us unfold over time and therefore encompass timing (duration and frequency) information. Quantifying this event timing is vital for perceiving and interacting with our environments. The current dissertation focuses on the timing of visual, sub-second events. Even without a dedicated sensory system for time, the brain can extract timing information from events: it has several areas of neighboring neural populations with timing-tuned responses, which are topographically organized by the timing preferences of these populations. Using 7-Tesla functional magnetic resonance imaging and neural-model based analyses, this dissertation explores how such timing-tuned responses can arise and what happens after their formation. In this way it builds an understanding of the “headwaters”, i.e., the beginning streams, of the flow of time throughout our brains. The source of this flow is the event in the environment, cascading sensory responses.

We show that timing-tuned responses gradually emerge from the responses in early visual areas, which monotonically increase with event timing. Furthermore, both monotonic and tuned responses show changing properties over the hierarchy of visual processing areas. While these monotonic responses are location-dependent, tuned responses have location-independent components. Using artificial neural network models, we show that these transitions have minimal requirements abundantly present in the brain. Firstly, recurrency is necessary for tuned responses to develop. Secondly, training artificial recurrent networks causes a gradual transition between monotonic and tuned responses over network layers. Thirdly, specifically training on predictable sequences, causes the tuned properties’ progressions over network layers to resemble those over brain areas most closely. For monotonic responses, properties across network layers resemble those across brain areas regardless of training. The gradual transitions we see in the brain and networks suggests that event timing is a relevant dimension of sensory input, which can be extracted without requiring a system that is specifically dedicated to this extraction.

Like event timing, numerosity also shows a transition over visual field maps from monotonic to tuned responses, which are topographically organized into cortical maps. Neural commonalities, and behavioral similarities and interactions have led to the hypothesis that numerosity and event timing may share quantity-general neural responses. We compared tuned numerosity and timing responses to investigate whether such common quantity responses hierarchically emerge. We see that the quantities’ responses are initially derived separately, but then increasingly overlap towards superior brain areas. But the neural populations’ preferences and map organizations for these quantities do not hierarchically become increasingly similar. Therefore, the quantities generally do not become a common quantity response. The flow of time does not extend into a deeper level of abstraction.

Potential generalizations (or the lack thereof) of these findings are discussed. Although timing-tuned responses have been shown for other modalities and time scales, they may rely on different mechanisms for computing tuning and represent different timing information. Generally, neural tuning may have benefits including optimizing information storage or facilitating abstraction away from irrelevant input dimensions. Indeed, tuned responses are prevalent in sensory and quantitative information processing, and excitingly, are even found beyond these domains.