Dr. Ivry, a "mental timing scholar" (aka., IQ Brain Clock doc) has done it again!
In a prior post I drooled over the means by which he explained possible different neural models of temporal processing (the IQ Brain Clock) via visual-graphic diagrams. As a result of his most recent article (see prior post link above), I went and found an earlier 2004 publication (with Spencer) that again presents an excellent visual-graphic explanation of hypothesized different neural timing models (see figure above), but more importantly, presents a very nice visual-graphic explanation of the hypothesized role of the basal ganglia (and dopamaine), which have been repeatedly implicated in mental timing and such clinical disorders as Parkinson's (see figure below). I've always known that the basal ganglia play a prominent role in mental timing, but have never been able to grasp (probably reflecting my limitations) the possible "why" or underlying mechanism. They offer an interesting and understandable hypothesis.
In the Ivry and Spencer (2004) article in Current Opinion in Neurobiology, the authors suggest that the basal ganglia works like a gating mechanism....which I have compared to the switch operator function in a railway system (see PPT slide show). According to the authors, who also prominently feature the cerebellum in brain clock timing system models:
- The current evidence does not preclude distributed models or hypotheses that assign a central role for timing to another specialized system, such as the basal ganglia. As reviewed here, the results of imaging and lesion studies are ambiguous with respect to the role of the basal ganglia in timing short intervals. A clear dissociation between the cerebellar and the basal ganglia contributions on temporal processing tasks remains elusive, primarily because similar deficits have been observed in patients with lesions of either structure ..... The cerebellar hypothesis offers a parsimonious account over a broad set of tasks, and neurobiologically feasible models have been developed. Nonetheless, a specialized system hypothesis must be able to account for similar patterns of performance following damage to distinct systems.
- As a starting point, we propose that the basal ganglia are an integral part of decision processes, operating as a threshold mechanism (Figure 2). Activations into the basal ganglia are gated such that only those reaching threshold are implemented . The activation functions for different decisions can reflect multiple factors, such as goals, sensory inputs, and contextual information. These representations engage in a competitive process for control. According to this view, the basal ganglia ensure that response implementation or working memory updating does not occur until a criterion level of activation is reached. Dopamine inputs to the striatum modulate threshold settings, providing one mechanism by which the competition can be biased. Thresholds for reinforced actions are lowered, increasing the likelihood of implementation, even if the input patterns are unchanged.
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