Maro Machizawa

Human Visual Short-Term Memory Precision Can Be Varied at Will When the Number of Retained Items Is Low:

It has been debated whether human visual working memory is limited by the number of items or the precision with which they are represented. In the research reported here, we show that the precision of working memory can be flexibly and willfully controlled, but only if the number of retained items is low. Electroencephalographic recordings revealed that a neural marker for visual working memory (contralateral delay activity, or CDA) that is known to increase in amplitude with the number of retained items was also affected by the precision with which items were retained. However, willfully enhanced precision increased CDA amplitude only when the number of retained items was low. These results show that both the number and the (willfully controlled) precision of retained items constrain visual working memory: People can enhance the precision of their visual working memory, but only for a few items.

Overlearning hyperstabilizes a skill by rapidly making neurochemical processing inhibitory-dominant

Overlearning refers to the continued training of a skill after performance improvement has plateaued. Whether overlearning is beneficial is a question in our daily lives that has never been clearly answered. Here we report a new important role: overlearning in humans abruptly changes neurochemical processing, to hyperstabilize and protect trained perceptual learning from subsequent new learning. Usually, learning immediately after training is so unstable that it can be disrupted by subsequent new learning until after passive stabilization occurs hours later. However, overlearning so rapidly and strongly stabilizes the learning state that it not only becomes resilient against, but also disrupts, subsequent new learning. Such hyperstabilization is associated with an abrupt shift from glutamate-dominant excitatory to GABA-dominant inhibitory processing in early visual areas. Hyperstabilization contrasts with passive and slower stabilization, which is associated with a mere reduction of excitatory dominance to baseline levels. Using hyperstabilization may lead to efficient learning paradigms.

Corrigendum: Overlearning hyperstabilizes a skill by rapidly making neurochemical processing inhibitory-dominant

Nat. Neurosci. 20, 470–475 (2017); published online 30 January 2017; corrected after print 18 September 2017 In the version of this article initially published, NIH grant R01EY019466 was missing from grants to T.W. in the Acknowledgments. The error has been corrected in the HTML and PDF versions of the article.

The neural mechanism of stabilization of perceptual learning revealed by the concentration of excitatory and inhibitory neurotrasmitter.

Visual perceptual learning (VPL) is regarded as a manifestation of experience-dependent plasticity in the visual/brain system. It has been found that VPL of a task is disrupted or interfered with by training of a new task if the interval between the two training was less than one hour (Seitz et al, PNAS, 2005). This retrograde interference effect suggests that after training is over, it takes up to one hour for VPL to be stabilized. However, another study has found that first training also interferes with VPL of second training (Yotsumoto et al, Vis Res, 2009), indicating that the mechanism of interference and its relationship with stabilization process after VPL training is more complex than it was originally thought. The purpose of the present study is to resolve the puzzle and better clarify the underlying mechanism of stabilization and interference. In the first experiment, a detection task on a different orientation was repeatedly performed in each of the first and second training. There was no interval between the two trainings. The subjects who underwent 8-block training (N=12) showed retrograde interference, whereas the subjects who underwent 16-block training (N=12) showed anterograde interference. In the second experiment, we measured the concentration of excitatory and inhibitory neurotransmitters in the early visual cortex as a function of the length of training on an orientation detection task using magnetic resonance spectroscopy. The E(xicitaotry)/I(nhibitory) ratio defined by the concentration of glutamate divided by that of GABA was significantly higher after the 8-block training than before the training (N=12), whereas it was significantly lower after the 16-block training (N=12). These results are in accord with the hypothesis that a lower E/I ratio is associated with greater resilience against being interfered with and therefore greater stability of VPL. Meeting abstract presented at VSS 2015.