Criticality, Working Memory, and Effort
Condition: Healthy
Study type: Interventional

Summary: 
      The project examines electroencephalography, MRI, and behavioral measures indexing
      flexibility (critical state dynamics) in the brain when healthy young adults do demanding
      cognitive tasks, and in response to transcranial magnetic stimulation.
    

Detailed description: 
      The healthy human brain is a complex, dynamical system which is hypothesized to lie near a
      phase transition at rest - at the boundary between order and chaos. Proximity to this
      critical point is functionally adaptive as it affords maximal flexibility, dynamic range, and
      information handling capacity, with implications for working memory function. Divergence from
      this critical point has become correlated with diverse forms of psychopathology and
      neuropathy suggesting that distance from a critical point is both a potential biomarker of
      disorder and also a target for intervention in disordered brains. The Investigators have
      further hypothesized that subjective cognitive effort is a reflection of sub-criticality
      induced by engagement with demanding tasks.

      A key control parameter determining distance from criticality in a resting brain is
      hypothesized to be the balance of cortical excitation to inhibition (the "E/I balance").
      Transcranial magnetic stimulation is a widely used experimental and clinical tool for
      neuromodulation and theta-burst stimulation (TBS) protocols are thought to modulate the E/I
      balance. Here the Investigators test whether cortical dynamics can be systematically
      modulated away from the critical point with continuous theta-burst stimulation (cTBS), which
      is thought to decrease the E/I balance, and thereby impact on working memory function and
      subjective cognitive effort during performance of the working memory tasks.
    

Intervention name(s): transcranial magnetic stimulation
Intervention type(s): Device
Intervention description(s): The study intervention is modulation of cortical excitation to inhibition (E/I) balance in the dorsolateral prefrontal cortex (dlPFC) by means of 2 trains of spaced continuous theta burst stimulation (cTBS) using a transcranial magnetic stimulation device. As prior work (Huang et al 2005; Chung et al. 2018) has shown that cTBS reliably decreases the cortical E/I ratio with diverse cortical targets, the Investigators expect to replicate a reduction in E/I balance when applied. The mechanism of action is thought to be an increase in inhibitory neurotransmission across diverse timescales. The endpoint of this stimulation will be a decrease in the local E/I ratio that should last at least 60 minutes post-stimulation (Chung et al., 2018).
In separate sessions, all participants will receive stimulation to either the dorsolateral prefrontal cortex (dlPFC) or to the angular gyrus (AG). The Investigators will contrast the effects of dlPFC cTBS with control cTBS to the AG.
Intervention model: Single Group Assignment
Intervention model description: The study will involve two sessions of within-subject, sham-controlled continuous theta burst stimulation delivered either at the left dorsolateral prefrontal cortex or angular gyrus.
Masking: None (Open Label)
Primary purpose: Basic Science