It is about time

It's about time…

about time

I remember there was a period in my Medical training, when I asked my Lecturer in Neurology, "How do we sense time?" Specifically, "how do we begin an activity and carry it out in an orchestrated fashion, e.g. taking a step, throwing a ball or even typing a paper, like I am doing now.
There were certain things I grasped about our brain's involvement that seemed easily understood, because of my experience with electronics and wiring circuits from a technical class in high school. I could easily apply how basic movements of small muscle groups in the fingers become a group process and coordinate with hands and arms and shoulders through skill sets weaved together for a targeted action. Just as small apps or macros can carry out a stepwise process in the right moment which is called forth from your skill set library in the cerebellum, it seemed similar to programmed circuits on a board. But how does it even begin in the first place? Sure, it starts as a plan like most great feats of practice training in sports, but how does it keep time? After all, we would have to have a time t0 to trigger before the series of programmed actions followed.

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My lecturer did not provide an answer that addressed my curiosity. He shared " there is so much about our brains we do not know, than what we do". But an article which I have just read seems to hint at an answer about how this timing mechanism is possible.

What the author discloses is that there seems to be a pacing circuit located in the region of the brain above the ears near the upper surface on the hairline, known as the lateral intraparietal region (LIP).

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Through various animal studies, it has been found that this region kicks off a frequency pattern of firing, that begins at a rapid pace and then diminishes over a short time. If you were to plot the rate over a short period it would look something like the drawing below.

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Intraparietal circuit

What this article reports is how the timing we estimate is not an absolute reference. But it does become more refined when we prepare for a ready- set - go phenomenon. For example, if we are about to do a sprint and we are lined up waiting for the starting shot, the initial pulses from the LIP seem to become more intense and more frequent. This makes the slope of diminishing pulses to become more abbreviated, thereby a more steep slope.

Let's say we practice this conditioning for a sprint, we become more accurate in our response time for the ready-set -go stimuli. This will result in the runner becoming quicker at his sprint by improving start time.

What seems interesting is how a steep firing slope from the LIP can, in fact, increase reaction times.It seems that somehow, we are able to read the steep slope backwards, by reacting to an established rate slowing pulse at the end of the slope once we are signaled "Go!".

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The internal timing

There is much to understand about how we become very effective at optimizing our skills sets through the internal timing. At first blush, it may seem rough and imperfect as a gauge by our atomic timekeeping standards. But after witnessing incredible feats of human activity, hearing an incredible performance by a concert violinist, or watching the first steps of a toddler, we have to confess that there is far more about this brain of ours than we can truly understand. I continue to be amazed by our design and the designer.

You can read more from the article below.
… for now, it is TIME, for me to get some sleep. 🙂

Thanks for joining me in this stroll…

Greg

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How the Brain Keeps Time

Study reveals neuron-firing patterns that underlie time measurement.

Keeping track of time is critical for many tasks, such as playing the piano, swinging a tennis racket, or holding a conversation.

Neuroscientists at MIT and Columbia University have now figured out how neurons in one part of the brain measure time intervals and accurately reproduce them.

The researchers found the lateral intraparietal cortex (LIP), which plays a role in sensorimotor function, represents elapsed time, as animals measure and then reproduce a time interval. They also demonstrated how the firing patterns of population of neurons in the LIP could coordinate sensory and motor aspects of timing.

LIP is likely just one node in a circuit that measures time, says Mehrdad Jazayeri, the lead author of a paper describing the work in the Oct. 8 issue of Current Biology.

"I would not conclude that the parietal cortex is the timer, " says Jazayeri, an assistant professor of brain and cognitive sciences at MIT and a member of the McGovern Institute for Brain Research. "What we are doing is discovering computational principles that explain how neurons' firing rates evolve with time, and how that relates to the animals' behavior in single trials. We can explain mathematically what's going on."

The paper's senior author is Michael Shadlen, a professor of neuroscience and member of the Mortimer B. Zuckerman Mind Brain Behavior Institute at Columbia University.

As time goes by

Jazayeri, who joined the MIT faculty in 2013, began studying timing in the brain several years ago while a postdoc at the University of Washington. He began by testing humans' ability to measure and reproduce time using a task called "ready, set, go." In this experiment, the subject measures the time between two flashes ("ready" and "set") and then presses a button ("go") at the appropriate time — that is, after the same amount of time that separated the "ready" and "set."

From these studies, he discovered that people do not simply measure an interval and then reproduce it. Rather, after measuring an interval they combine that measurement, which is imprecise, with their prior knowledge of what the interval could have been. This prior knowledge, which builds up as they repeat the task many times, allows people to reproduce the interval more accurately.

"When people reproduce time, they don't seem to use a timer, " Jazayeri says. "It's an active act of probabilistic inference that goes on."

To find out what happens in the brain during this process, Jazayeri recorded neuronal activity in the LIP of monkeys trained to perform the same task. In these recordings, he found distinctive patterns in the measurement phase (the interval between "ready" and "set"), and the production phase (the interval between "set" and "go").

During the measurement phase, neuron activity increases, but not linearly. Instead, the slope of activity begins as a steep curve that gradually flattens outas time goes by, until the "set" signal is given. This is key because the slope at the end of the measurement interval predicts the slope of activity in the production phase.

When the interval is short, the slope during the second phase is steep. This allows the activity to increase quickly so that the animal can produce a short interval. When the interval is longer, the slope is gentler and it takes longer to reach the time of response.

"As time goes by during the measurement, the animal knows that the interval that it has to produce is longer and therefore requires a shallower slope, " Jazayeri says.
During the measurement phase, neuron activity increases, but not linearly. Instead, the slope of activity begins as a steep curve that gradually flattens out as time goes by, until the "set" signal is given. This is key because the slope at the end of the measurement interval predicts the slope of activity in the production phase. Credit: MIT News.

Using this data, the researchers could correctly predict, based on the slope at the end of the measurement phase, when the animal would produce the "go" signal.

"Previous research has shown that some neurons exhibit a ramping up of their firing rate that culminates with the onset of a timed motor response. This research is exciting because it provides the first hint as to what may control the slope of this 'neural ramping, ' specifically that the slope of the ramp may be determined by the firing rate at the beginning of the timed interval, " says Dean Buonomano, a professor of behavioral neuroscience at the University of California at Los Angeles who was not involved in the research.

"A highly distributed problem"

All cognitive and motor functions rely on time to some extent. While LIP represents time during interval reproduction, Jazayeri believes that tracking time occurs throughout brain circuits that connect subcortical structures such as the thalamus, basal ganglia, and cerebellum to the cortex.

"Timing is going to be a highly distributed problem for the brain. There's not going to be one place in the brain that does timing, " he says.

His lab is now pursuing several questions raised by this study. In one follow-up, the researchers are investigating how animals' behavior and brain activity change based on their expectations for how long the first interval will last.

In another experiment, they are training animals to reproduce an interval that they get to measure twice. Preliminary results suggest that during the second interval, the animals refine the measurement they took during the first interval, allowing them to perform better than when they make just one measurement.

About this neuroscience research
Source: Anne Trafton – MIT
Image Source: The image is credited to MIT News
Original Research: Abstract for "A Neural Mechanism for Sensing and Reproducing a Time Interval" by Mehrdad Jazayeri and Michael N. Shadlen in Current Biology. Published online October 8 2015 doi:10.1016/j.cub.2015.08.038

Abstract

A Neural Mechanism for Sensing and Reproducing a Time Interval

Highlights
•Non-human primates use a Bayesian strategy to reproduce time intervals
•Parietal cortex conveys distinct timing signals for measurement and production
•Parietal neurons represent an estimate of elapsed time on a trial-by-trial basis
•The brain encodes time prospectively during time interval reproduction

Summary
Timing plays a crucial role in sensorimotor function. However, the neural mechanisms that enable the brain to flexibly measure and reproduce time intervals are not known. We recorded neural activity in parietal cortex of monkeys in a time reproduction task. Monkeys were trained to measure and immediately afterward reproduce different sample intervals. While measuring an interval, neural responses had a nonlinear profile that increased with the duration of the sample interval. Activity was reset during the transition from measurement to production and was followed by a ramping activity whose slope encoded the previously measured sample interval. We found that firing rates at the end of the measurement epoch were correlated with both the slope of the ramp and the monkey's corresponding production interval on a trial-by-trial basis. Analysis of response dynamics further linked the rate of change of firing rates in the measurement epoch to the slope of the ramp in the production epoch. These observations suggest that, during time reproduction, an interval is measured prospectively in relation to the desired motor plan to reproduce that interval.

"A Neural Mechanism for Sensing and Reproducing a Time Interval" by Mehrdad Jazayeri and Michael N. Shadlen in Current Biology. Published online October 8 2015 doi:10.1016/j.cub.2015.08.038

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