Neurology Center

What's the difference between walking down the street yesterday, and walking down the street tomorrow?

It's nothing to do with the walking, or the street: that's the same. When seems to be something external to the what, how, and where of the situation. But this creates a problem for neuroscientists.

We think we know how the fact that the brain could store the concept of "walking down the street" (or "walking" and "street"). Very roughly, simple sensory impressions are thought to get built up into more and more complex combinations, and this happens as you move away from the brain's primary visual cortex (V1) and down the so-called ventral visual stream.

In area V1, cells respond mostly to nothing more complex than position and the orientations of straight lines: / or \ or _ , etc. Whereas once you get to the temporal lobe, far down the stream, you have cells that respond to Jennifer Aniston. In between are progressively more complex collections of features.

Even if the details are wrong, the fact that complex objects are composed of simpler parts and ultimately raw sensations, means that our ability to process complex scenes doesn't seem too mysterious, given that we have senses.

But the fact that we can take any given scene, and effortlessly think of it as either "past", "present", or "future", is puzzling under this view because, as I said, the scene itself is the same in all cases. And it's not as if we have a sense devoted to time: the only time we're ever directly aware of, is "right now".

Swedish neuroscientists Nyberg et al used fMRI to measure brain activity associated with "mental time travel": Consciousness of subjective time in the brain. They scanned volunteers and asked them imagine walking between two points, in 4 different situations: past, present, future, or remembered (as opposed to imagined in the past). This short walk was one which they'd really done, many times.

What happened?
Compared to a control task of doing mental arithmetic, both remembering and imagining the walk activated numerous brain areas and there was very strong overlap between the two conditions. No big surprise there.

The crucial contrast was between remembering, past imagining and future imagining, vs. imagining in the present. This revealed a rather cute little blob:

This small nugget of the left parietal cortex represents an area where the brain is more active when thinking about times other than the present, relative to thinking about the same thing, but right now. They note that this area "partly overlaps a left angular region shown to be recruited during both past and future thinking and with parietal regions implicated in self-projection in past, present, or future time."

So what? This is a nice study, but like most fMRI it doesn't tell us what this area is actually doing. To know that, we'd need to know what would happen to someone if that area were damaged. Would they be unable to imagine any time except the present? Would they think their memories were happening right now? Maybe you could use rTMS could temporarily inactivate it - if you could find volunteers willing to lose their sense of time for a while...

Nyberg L, Kim AS, Habib R, Levine B, & Tulving E (2010). Consciousness of subjective time in the brain. Proceedings of the National Academy of Sciences of the United States of America PMID: 21135219