Trends in Cognitive Sciences
ReviewHidden cognitive states revealed in choice reaching tasks
Introduction
Progress in understanding how human cognition operates owes much to the information processing approach, which has adopted the computer metaphor of distinct processing units. The theoretical assumption of discrete intermediate stages has inspired cleverly designed experiments to identify sequential stages of perception, cognition, decision-making and motor output. Reaction times and accuracies obtained from discrete responses such as button presses have been viewed as useful and effective behavioral measurements to individuate sub-components, each adding its own processing times 1, 2.
For decades, however, an alternative view of brain processing has also persisted, reappearing under various guises including connectionism and non-linear systems analysis: multiple internal states can be activated in parallel, and coexist 3, 4, 5, 6, 7. Furthermore, these unified perspectives of perception, cognition and action have emphasized a stronger role for action in perception and cognition 5, 7, 8, 9. Although conventional discrete measurements seem adequate under the assumptions of a symbolic-computational approach to cognition, they cannot fully capture dynamic interactions of internal states over time. What has been lacking in human studies on perception and cognition are easily available behavioral paradigms to measure the temporal evolution of relevant internal events.
We shed light on ongoing endeavors to capture the dynamics of multiple cognitive states unfolding over time 4, 10, 11, 12, 13, 14. There has been an attempt to take advantage of overt actions such as saccadic eye movements and manual reaching movements to read out cognitive processes. For instance, the distributions of saccadic landing positions and changes of saccadic movement trajectories have been used to examine language, attention, memory and oculomotor control 15, 16, 17. Yet, saccades, in general, are rapid and discrete, demonstrating relatively weak trajectory modulation. In contrast, reaching movements have numerous advantages as an alternative behavioral measurement (Box 1). The continuity of reaching movements enables each sample point to be modulated by the real-time progress of a wide range of internal processes 4, 18, 19.
Neurophysiological studies have demonstrated the existence of simultaneous multiple motor plans and blurred distinctions between sensory, decision-making and motor processes in the brain 20, 21, 22, 23, 24, 25, 26, 27. This provides physiological plausibility for the view that reaching movements can reflect underlying competing cognitive states. After a brief review of these neural mechanisms, we describe examples in which the direction of curved reach trajectories reveals otherwise hidden states and processes. This new approach using reaching movements demonstrates interactive parallel processes of perception, cognition and action, bridging the gap across different disciplines in which these processes have been studied independently 5, 6, 7, 9, 10, 28.
Section snippets
Perception, cognition and action: interactive parallel processes
For simplicity and convenience, theoretical models often assume separate and discrete neural substrates for perception, cognition and action. Perceptual evidence is gathered, followed by a decision process. Then the subsequent motor output is planned and executed. However, a growing number of studies have raised the question as to whether interpreting neural activity in terms of distinct perceptual, cognitive or motor processes is appropriate for bridging dynamic neural data with behavior. For
Evidence for real-time cognitive events in reach trajectories
Recent studies using target choice reaching tasks have taken advantage of the ‘leakage’ of earlier cognitive states into the final motor output. In these studies, the analyses of reaching movements demonstrate the current locus of attention, the nature of language representations, spatial representation of numbers, and high-level decision-making processes.
Spatial representation of numbers revealed by systematic trajectory shifts
The ‘numerical distance effect’, in which reaction times and error rates systematically decrease as the numerical distance between two numbers increases, suggests that numbers are represented spatially along a continuum. This evidence of spatial number representation has been observed in adults and in infants and animals [47].
However, this is an indirect inference based on the duration for completing the entire sequence of number comparison processes. Recent studies using reaching tasks further
Using reaching to identify interactions between dorsal and ventral streams
The dorsal and ventral visual streams have been identified as comprising two separate processes: an identification system in the ventral stream, and an action system in the dorsal stream 50, 51. Continuous reaching tasks have been informative in understanding how these two visual systems are dissociated. For instance, a patient with damage to the ventral portion of the visual system demonstrates normal temporal dynamics of goal-directed reach-to-grasp movements even without recognizing objects.
Conclusions
The detailed analysis of reaching movement provides not only insights into visuo-motor behavior itself but also reveals new information regarding otherwise hidden internal events. All of the aforementioned examples in choice reaching tasks indicate that action is not always the final product of perception and cognition; reach trajectories can reveal multiple competing representations in addition to intermediate products. This emphasizes a more unified conception of perception, cognition and
Acknowledgements
We thank R.M. McPeek, R. Rafal, A.Z Khan and E.M. Messer for valuable comments. J-H.S. was supported by an R.C. Atkinson Fellowship Award.
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