Time is memory and you control it | Lila Davachi

I was in labor for 72 hours with my first child my husband and I were hunkered down in our small Greenwich Village apartment in the moment I thought it would never end but now when I think back on those 72 hours they feel like a blink compared to the two minutes between hearing my daughter cry for the first time and being able to hold her in my arms in those two minutes I remember the sound of her cry I remember the doctors and nurses commenting on her full head of dark hair I remember my husband

leaving my side to film her first layin foot stamp and I gel I remember the clock on the wall 5:32 a.m. I remember seeing her face for the first time and I said honey baby and when she heard my voice she opened her eye now when I think back on those two minutes they seemed limitless now there are many ways people have talked about time time is money time flies like an arrow and this is all meant to capture some aspect of what time means in specific contacts and today today I'm here to share with

you a personal and scientific discovery that for all of us time is memory so before I explain this further it's important to differentiate between objective and subjective time and in order to do that we're going to do a little experiment because that's what I do best I'm going to show you two short videos and I simply want you to watch them and afterwards I'm going to ask you to tell me which video is longer okay so one go and to okay raise your hand II Fidel the first was longer great now raise your hand if

you thought the second was longer okay well both videos were actually objectively the same length they were both five seconds but subjective time is how long it feels like it took to you we all varying our perceptions of time some of you may have thought the first took longer and some of you may have thought the second took longer but the important thing to glean from this example is that time does not exist at least not in the way that we might think if you're looking at your watch but instead it's an extraction based on

reality and it's related to memory so how do we know this one we know that if you lose your memory you can't cobble together people with places or things but every day is new and not in a good way we live or you would live in an eternal presence and this is perhaps best exemplified by the famous story of patient hm-hm was incredibly debilitated by severe epilepsy and after many years of suffering he opted to undergo a surgery to relieve his seizures however after the surgery HM could no longer form any new memories he could

remember all of his years leading up to the surgery but no matter how many objective minutes hours days or even years had passed after the surgery HM had subjectively only lived a few more moments so while this is an extreme example it highlights that memory is our way of reliving our past experiences and reappear Ian Singh our time and it follows them that the more discrete memories you have from a prior experience that that experience becomes expanded in time so there's a tendency almost complete agreement that we as humans want more time there's never enough

time but I don't think it's just time that we want instead we long to lead a full life now what I want to share with you is if time is memory and we want more time than what we really want or more memory leading a full life refers to the idea of having lots of varied experiences one that were later be memorable so so far this might seem straightforward but it gets a little complicated because it's exemplified in the story of my labor and birth how we feel time passing in the moment is different from

how we retrospectively remember time and the interesting finding is that our estimates of time in the moment are negatively related to those estimates in memory so let me unpack that for you a little bit let's start with the in the moment perception we know that every day contains the same number of minutes and hours but our subjective perception of each day can vary widely some days go by quickly and others not so quickly so in the moment what produces is variance and how we experience time passing well it turns out that experiences that contain very

little change over time are the ones that seem to drag on and take subjectively longer than experiences that contain a lot of change but now consider how we reflect back and remember time it actually works in the opposite direction if you're asked to remember a very mundane experience you have little to remember and your estimates of time shrink rapidly even though in the moment it felt like it would never end and by contrast when you're asked to remember a very eventful experience one that felt like it was passing very quickly you would have a lot

to remember and your estimates of that experience would actually expand so stepping back a little what does this tell us about how we remember our time well we remember our time with respect to our experiences what you're doing with your time how much you are doing the more places you've been to people you've spoken to or even thoughts that you've had the longer you remember that experience to have been so how will we study this in the lab actually let's make this a little bit more concrete think of each discrete event as a unit of

experience and these units are later available for recall think of them as memory units in an environment with a lot of variety and change you're forming far more memory units that an environment was very little change and it's these units the number of these units that determine our estimates of time later on more units more to remember and time expands time is memory so how do we study this in the lab in the lab we've come up with very simple ways to try to mimic these ideas of contextual stability and change in experimentally tractable ways

and in several experiments we've studied how people remember time participants will come into the lab and stare and they'll look at a computer screen and will present a series of visual images of objects and faces one at a time a new one on each trial think of these as the people and objects and your experiences now to mimic the idea of a context alongside each image we present a picture of a scene and that scene will either remain the same as in this example across time to mimic the idea of doing the same thing in

the same way or the scene will change now later on we ask participants a simple question we show them a pair of images and was simply asking to tell us how far apart did you experience these two images now unbeknownst to the participants we craft a memory test so that the pairs of images had actually been presented objectively the same distance apart about 20 seconds but some of the pairs of images were presented with the same scene hypothetically forming one memory unit while other pairs of images were paired across the scene change hypothetically forming two

memory units so even though the objective amount of time was exactly the same what we see is a participants reliably rate visual images of having been encountered farther apart in time when they had been experienced across a scene change so remembering two memory units compared to one makes your memory for time expand it's as if the changes in their environment has a consequence of pulling things apart in memory and time and making the intervals between them seems aligned longer so to give you everyday example imagine you go to the supermarket and you run into your

friend Surrey in Aisle three and then some time later you run into another good friend Raphael in another aisle in the supermarket so take the same example that's identical except now you run into Raphael in the parking lot later on you'll remember the two encounters to be farther apart in time if they occurred in the two different contacts okay but the interesting thing is what if I plop down in the moment and met you in the parking lot and asked you to tell me how much time had passed on your trip to the supermarket now

you get the opposite answer the analogy here is I would stop you in the parking lot as you're loading groceries into your car and simply ask you how long have you been here in that case you would rate the changing environment as taking a shorter amount of time then the disable environment so the bottom line is is that in the moment we feel like a busy day is passing very quickly a changing environment feels like we're losing our time it's going by quickly but later when your memory is probed those same experiences feel like they

took a longer amount of time time expands so so far what what the examples I've shown you suggests that our memory for time is actually an interpretation of the events that are unfolding around us and our next question was what do we rely on for this interpretation so far all of the examples I've given you suggests that changes in the external world like we've seen changes in my example are going to determine how you remember time but is it completely up to the external world the answer is no we've shown that how you remember time

is modulated by your internal world your own interpretation of the events that are unfolding so we've done these same kinds of tasks using brain imaging and what we can do is we can measure your internal world your brain while you're viewing the same kinds of images and specifically what we what we do is we can take still pictures of your brain at the beginning of a series of images and then at the end and we can compare the neural activity patterns that are present at the beginning and under these experiences and what we see is

that if your neural activity patterns look very different across time as in this example you remember the associated events as having been encountered farther important time it's as if you're changing brain patterns offer an interpretation that you were in two very different contacts in this case mental context at the beginning and end of this series of images on other trials even though the same objective amount of time has passed your brain patterns might look more similar across time and what we see is that when your neural activity patterns are more similar it has the consequence

of bringing the Associated items into a closer more integrated memory trace and you remember those items as having been encountered closer in time together so the important implication here is that even when the external environment is changing a lot you can maintain a stable internal experience and by contrast if the external world isn't changing very much you can create distinctive experiences in your mind so in the end this offer is an important element of control a good example of this for me is my morning commute I live in New York City and I take the

subway to work every day for better or for worse and my commute contains lots of people the sounds of their music smells emanating from their food and sometimes their bodies and at every stop new folks get on and some folks get off and some mornings I feel like my experience is anchored on the external environment I notice a cute little girl and I smile at her or I cringe that the lack of deodorant on my neighbor this happens a lot and you can bet if you could look at my neural activity patterns in that moment

they'd be varying widely as I attend to all the various elements of that experience but on other mornings I may be trying to figure out a problem at work and I barely notice the stops passing by now that's an example of maintaining a stable internal experience so given that we have some control over time through memory which do we prioritize which will you prioritize well there's no right answer right and our priorities might change from day to day do you prioritize the feeling that time is passing slowly is on a lazy Sunday or a day

at the beach or you do prioritize having lots of memories by leading an action-packed full life again there's no right answer and your priorities will likely change from day to day but I hope what I've shared with you today will let you know that time is memory and with that knowledge and hand know that you control time thank you [Applause]