Clock speeds

27/08/2009 by admin.

Look at the frequency of some events: the gamma waves that synchronize the thalamus and large parts of the cortex happen between 25 and 100 times per second but typically near 40; the saccadic eye vibrations happen between 30 and 70 times per second; the flicker rate of movie projectors that makes the picture stable is between 48 and 72 times per second. This makes one guess that there is a good probability that the discontinuous nature of the ‘frames’ from the eye, from a movie screen and from consciousness all have the same timing centered on about 50 Hz. A group has now shown that the focus of attention shifts about 18 to 34 times a second and averages about 25 Hz. It looks like two frames per focus.

This attention timing is reported by ScienceDaily (here) in an item on a paper by T. Buschman and E. Miller.

You’re meeting a friend in a crowded cafeteria. Do your eyes scan the room like a roving spotlight, moving from face to face, or do you take in the whole scene, hoping that your friend’s face will pop out at you? And what, for that matter, determines how fast you can scan the room?…. you are more likely to scan the room, jumping from face to face as you search for your friend. In addition, the timing of these jumps appears to be determined by waves of activity in the brain that act as a clock. …the study showed that brain waves act as a kind of built-in clock that provides a framework for shifting attention from one location to the next. … Buschman found that the spotlight of the mind’s eye shifted focus at 25 times a second and that this process of switching was regulated by brain waves. …the speed at which the animals searched was related to the speed of their brain waves. When the clock ticked faster, the animals “thought” faster.

The paper’s summary is below:

Attention regulates the flood of sensory information into a manageable stream, and so understanding how attention is controlled is central to understanding cognition. Competing theories suggest visual search involves serial and/or parallel allocation of attention, but there is little direct, neural evidence for either mechanism. Two monkeys were trained to covertly search an array for a target stimulus under visual search (endogenous) and pop-out (exogenous) conditions. Here, we present neural evidence in the frontal eye fields (FEF) for serial, covert shifts of attention during search but not pop-out. Furthermore, attention shifts reflected in FEF spiking activity were correlated with 18–34 Hz oscillations in the local field potential, suggesting a “clocking” signal. This provides direct neural evidence that primates can spontaneously adopt a serial search strategy and that these serial covert shifts of attention are directed by the FEF. It also suggests that neuron population oscillations may regulate the timing of cognitive processing.

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