Sense of time

29/10/2009 by admin.

ScienceDaily has an item on research by A. Graybiel, ‘Neural representation of time in corticobasal ganglia circuits’. ( here )

“Keeping track of time is one of the brain’s most important tasks. As the brain processes the flood of sights and sounds it encounters, it must also remember when each event occurred. But how does that happen? … For decades, neuroscientists have theorized that the brain “time stamps” events as they happen, allowing us to keep track of where we are in time and when past events occurred. However, they couldn’t find any evidence that such time stamps really existed — until now … The research team trained two macaque monkeys to perform a simple eye-movement task. After receiving the “go” signal, the monkeys were free to perform the task at their own speed. The researchers found neurons that consistently fired at specific times — 100 milliseconds, 110 milliseconds, 150 milliseconds and so on — after the “go” signal … The neurons are located in the prefrontal cortex and the striatum, both of which play important roles in learning, movement and thought control. … We have sensory receptors for light, sound, touch, hot and cold, and smell, but we don’t have sensory receptors for time. This is a sense constructed by the brain.”

It strikes me that this clock system would be one more or less dedicated to motor processes because: it measures very short durations; it is found in the prefrontal cortex/striatum system; and it is more accurate than flexible. The conscious feeling of the passage of time may be from another system derived from this one or even a completely separate system.

The New Scientist had an article on time perception (here) by D. Fox that reviews a number of experiments.

• R. VanRullen showed that vision is framed rather than continuous and the frame rate is about 13 per second. He found that the visual area of the right inferior parietal lobe generated a 13 Hertz wave. The question then was - is the framing global or independent for each preceived object? Visual illusions showed that framing is not global.

“This implies that there is not a single “film roll” in the brain, but many separate streams, each recording a separate piece of information. What’s more, this way of dealing with incoming information may not apply solely to motion perception. Other brain processes, such as object or sound recognition, might also be processed as discrete packets.”

• Using very weak stimuli he showed that there are windows of perception.

“… found that the likelihood of them noticing the light depended on the phase of another wave in the front of the brain, which rises and falls about 7 times per second. It turned out that subjects were more likely to detect the flash when the wave was near its trough, and miss it when the wave was near its peak. There’s a succession of ‘on’ periods and ‘off’ periods of perception. Attention is collecting information through snapshots … So it seems that each separate neural process that governs our perception might be recorded in its own stream of discrete frames. But how might all these streams fit together to give us a consistent picture of the world?”

• E. Poppel looked at this problem, proposed blocks of frames and found some experimental evidence for the blocks.

“… separate snapshots from the senses may feed into blocks of information in a higher processing stream. He calls these the “building blocks of consciousness” and reckons they underlie our perception of time. … It’s an appealing idea, since patching together a chronological order of events hitting our senses is no mean feat. Sounds tend to be processed faster than images, so without some sort of grouping system we might, say, hear a vase smashing before we see it happen. Pöppel’s building blocks of consciousness would neatly solve this problem: if two events fall into the same building block, they are perceived as simultaneous; if they fall into consecutive buildings blocks, they seem successive. Perception cannot be continuous because the limits of neural processing. …. A space of 30 to 50 milliseconds is necessary to bring together in one time-window the distributed activity in the neural system.”

There is also interesting discussion of whether our sense of time becoming slower or faster is a function of brain speed or of memory density. And finally exploration of the idea that some symptoms of schizophrenia may be due to faulty timekeeping.

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