How does the brain optimize goal-directed movements? (Prof. Kitazawa)
Voluntary goal-directed movements, such as arm reaching, are nearly optimized in terms of smoothness over the entire movement. Such smoothness is lost with cerebellar dysfunction, suggesting the essential role of the cerebellum in optimizing movement. However, it is still not clear how the cerebellum contributes to achieving smoothness over an entire movement. We previously proposed a random walk hypothesis (Kitazawa 2002) that the terminal errors conveyed by climbing fibers in the cerebellum (Kitazawa et al. 1998) serve to reduce not only the mean error, but also the variance of the error, through a process analogous to the random walk through movement control candidates. We are planning electrophysiological experiments for testing this hypothesis (Inoue et al. 2016).
- Inoue M, Uchimura M, Kitazawa S. Error signals in motor cortices drive adaptation in reaching. Neuron 90, 1114-1126 (2016)
- Kitazawa, S, Wolpert, DM. Rhythmicity, randomness and synchrony in climbing fiber signals. Trends Neurosci. 2005; 28(11) 611-619.
- Kitazawa S. Optimization of goal-directed movements in the cerebellum: a random walk hypothesis. Neurosci Res 2002;43(4):289-94.
- Kitazawa S, Kimura T, Yin PB. Cerebellar complex spikes encode both destinations and errors in arm movements. Nature 1998;392(6675):494-7.
Neurophysiology of space and time perception (Prof. Kitazawa)
Space and time that we perceive in our mind are, of course, based on the counterparts in the physical world, but it remains still unknown exactly how the mental space and mental time are constructed in our brain.
Take the mental space for example. We swing our eyes three times a second on average, with a peak angular velocity as fast as 100-500 degrees/s. But the world in our mind are so stable that we do not even notice our own eye movements. Why do blurred images on the retina not appear in our conscious mind? What are we looking at, while the blurred retinal images are blocked from our mind? How are two distinct retinal images, one before and the other after the rapid eye movement, mapped to the same part in the mental space? As for the mental time, it does not flow equably as hypothesized in physics. For example, subjective temporal order of successive stimuli are inverted just by crossing our arms, or just before the onset of each rapid eye movement. A key word for solving these problems is "postdiction", a notion that the mental space and the mental time are constructed by our brain from sensory signals sampled over the past ~100 ms by combining information across multiple areas in the brain. We are searching for the neural basis of "postdiction" by applying techniques in psychophysics and neuroimaging to human participants, and neurophysiological techniques to monkeys.
Why the world remains stable while we move our eyes has been repeatedly questioned by great thinkers like Alhazen, Descartes, and Helmholtz, for more than 1000 years at least. We hope that our research would ultimately yield final answers to the long-lasting historical question, in addition to other important questions regarding the mental space, time, and their interactions.
- Uchimura M, Nakano T, Morito Y, Ando H & Kitazawa S. Automatic representation of a visual stimulus relative to a background in the right precuneus. Eur J Neurosci, vol.42, Issue 1, 1651-1659 (2015).
- Uchimura, M. & Kitazawa, S. Cancelling prism adaptation by a shift of background: a novel utility of allocentric coordinates for extracting motor errors. J Neurosci 33, 7595-602 (2013).
- Takahashi, T., Kansaku, K., Wada, M., Shibuya, S. & Kitazawa, S. Neural correlates of tactile temporal-order judgment in humans: an fMRI study. Cereb Cortex 23, 1952-64 (2013).
- Miyazaki, M., Yamamoto, S., Uchida, S. & Kitazawa, S. Bayesian calibration of simultaneity in tactile temporal order judgment. Nat Neurosci 9, 875-7 (2006).
- Yamamoto, S. & Kitazawa, S. Reversal of subjective temporal order due to arm crossing. Nat Neurosci 4, 759-65 (2001).
- Takahashi, T. & Kitazawa, S." Modulation of illusory reversal in tactile temporal order by the phase of posterior alpha rhythm. J Neurosci 37, 5298-5308, (2017).
Why do we blink? (Associate Prof. Tamami Nakano)
People blink spontaneously about once every 3 seconds. Noting that blinking serves to moisten the eyes and that one blink every 20 seconds would be enough, the purpose of this frequent blinking has remained a mystery. We examined brain activity as well as timing of blinking while participants were observing a video clip or engaged in face-to-face conversation. We eventually found that we share timing of blinking, at a conclusion of a flow of information, and that each blink triggers a transient but dynamic change of activity in the major neural networks (Nakano et al., 2013). These findings have been recognized by both domestic and international media, and the research was selected as a PRESTO project in 2016.
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