Trends in Cognitive Sciences
Volume 8, Issue 9, September 2004, Pages 426-433
Journal home page for Trends in Cognitive Sciences

Adaptive neural models of queuing and timing in fluent action

https://doi.org/10.1016/j.tics.2004.07.003Get rights and content

In biological cognition, specialized representations and associated control processes solve the temporal problems inherent in skilled action. Recent data and neural circuit models highlight three distinct levels of temporal structure: sequence preparation, velocity scaling, and state-sensitive timing. Short sequences of actions are prepared collectively in prefrontal cortex, then queued for performance by a cyclic competitive process that operates on a parallel analog representation. Successful acts like ball-catching depend on coordinated scaling of effector velocities, and velocity scaling, mediated by the basal ganglia, may be coupled to perceived time-to-contact. Making acts accurate at high speeds requires state-sensitive and precisely timed activations of muscle forces in patterns that accelerate and decelerate the effectors. The cerebellum may provide a maximally efficient representational basis for learning to generate such timed activation patterns.

Section snippets

Three levels of temporal structure in skilled performance

A surprisingly demanding problem is the genesis of skilled behavior using complex effectors like a human's arm or speech articulators. Skilled behavior emerges in temporally structured episodes, and brain areas that use distinct representations contribute to this temporal structuring. This review examines computational models of neural circuits contributing to three levels of temporal structure in behavior. Level one is the fluent succession of acts prepared collectively as a sequence. This

Fluent succession of acts via competitive queuing

Fifty years ago, Lashley [1] used data on sequencing errors – in which early and later elements of a sequence mistakenly exchange positions – to infer that neural representations for all elements of a planned sequence are simultaneously active before sequence production. The proposal that sequences are represented by simultaneous parallel activation of representations of their elements differs from many classical and contemporary proposals. In most recurrent-state network models 2, 3, 4,

Neurophysiological evidence for the CQ model

Until 2002 there was no compelling electrophysiological evidence that the brain used the parallel sequence code and iterative choice cycle postulated by CQ theorists. New cell recordings by Averbeck et al. [8] plugged that evidential gap. They trained monkeys to draw a copy of a static geometric form using a routine, prescribed stroke sequence. Thus a form cued recall of sequence-representing information from long term memory. Recordings from area 46 of prefrontal cortex showed that before the

Progress of CQ models in explaining sequencing and timing

To motivate normalized CQ models, Grossberg [5] stressed that neurons exhibit finite activation ranges and noise. Both constrain the ability of neurons to use relative activations to reliably code the relative priority of a large number of sequence elements. Brains using this analog code should exhibit a small upper bound on the number of elements that can be reliably recalled in correct sequential order without secondary strategies, such as reloading chunks from long-term memory 5, 6, 12.

Coordination of rates and completion times in voluntary action

Many movement models, such as Equilibrium Point (EP) models ([31], Box 1), treat the temporal structure of actions from a biomechanical perspective. By contrast, some central pattern generation models, such as Vector Integration To Endpoint (VITE) models ([32], Box 1), treat timing from a cognitive dynamics perspective, with a focus on voluntary gating of plan execution and voluntary control of movement rates. VITE models have successfully simulated both the discharge patterns of diverse motor

Timed anticipatory responses

In a successful ball catch, the arm flicks out and ‘stops on a dime’ at whatever degree of arm extension enables the hand to catch the ball. Newtonian mechanics implies that an arm set in motion by extensor muscles would (disastrously) continue ‘past the mark’ unless braked by precisely timed, anticipatory action of opposing muscles. When driving a car, stomping the accelerator and hitting the brake are separate voluntary actions. When ‘driving’ our bodies, the braking contractions are

Conclusions

This review has focused on neural circuit models of fluent performance of discrete actions, and the implicit claim was that at least three kinds of temporal structuring must be acknowledged to exist as distinct factors in most episodes of skilled action. Competitive queuing theorists who endorsed Lashley's inference that some sequence planning involves parallel activation of all sequence elements can now point to compelling electrophysiological support, but this does not imply that other

Acknowledgements

Preparation of this article was partially supported by NIH R01 DC02852.

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