Figure 1.

Freezing is due to decreased input and output of the basal ganglia. In periods of high response conflict, the subthalamic nucleus (STN) increases its firing rate, which subsequently drives increased activity within the internal segment of the globus pallidus (GPi), leading to decreased activity within the thalamus (Thal) and the pedunculopontine nucleus (PPN). While the triggering event in this model is currently unknown, it may be due to: ¹—inherent impairment within the pre-supplementary motor area (pSMA), leading to an inefficient communication with the STN; ²—impairments in the temporal dynamics between the pSMA and STN, possibly due to impaired white matter connectivity; ³—cellular deficits within the striatum that place the nucleus at an increased risk of becoming hyperpolarized; or ⁴—pathology within the PPN, priming the nucleus for hyperpolarization by minimal inhibitory input. Freezing may be due to any combination of these factors. Key: red—increased activity; blue—decreased activity; arrow—excitatory connection; circle—inhibitory connection.