NOVIEMBRE 2015 “Tarjeta de Asistencia Analizada”

An unresolved question regarding motor and sensory timing is whether they are achieved by separate temporal processes or by a mechanism which is common to both. For the purposes of clarity, here motor timing is defined as timing involving movement, either external movement of the target or movement of the organism, and sensory timing as more passive duration judgments which do not involve movement. Until relatively recently, studies pointed to a common mechanism (Ivry et al., 1995, Keele et al., 1985). Experiments comparing performance in perceptual and production tasks found that when the performance of a tapping task was compared with that of a discrimination of empty intervals task across various durations, the functions produced (variance plotted against duration) had almost identical slopes, leading the authors to conclude that both types of timing shared a common mechanism (Ivry et al., 1995). An earlier paper came to the same conclusion following a series of experiments which demonstrated that people who were “good” at judging brief perceptual events performed equally well during production tapping tasks with both fingers and feet. In other words, a subjects’ performance in sensory timing tasks correlated with their performance of a motor timing task (Keele et al., 1985). The same study found that on average skilled piano players were better at both tasks (perceptual and motor) than a control group of non-piano players. However, in a study involving a greater range of tasks and conditions, Merchant et al found that even though there was a linear increase in the variability of performance with increased duration across all task types, variability was

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greater for motor than perceptual tasks and also greater for visual than auditory tasks (Merchant et al., 2008).

The authors seek to explain these results by proposing the existence of two networks, an automatic mechanism which times predictable sub-second durations defined by movement, and a more cognitive mechanism for longer durations not defined by movement. It is suggested that the two systems overlap. They cite a study which reviewed fMRI literature for different timing tasks and came to a similar conclusion. It was found that sub-second and supra-second tasks produce differing patterns of brain activity. However the authors of this paper suggest that the motor or “automatic” system may also be used to measure brief durations even when no movement is present (Lewis et al., 2003). A proposal that motor and sensory timing may be achieved by non-overlapping systems comes from a review paper by (Buonomano et al., 2010). The paper looks at a particular theory of intrinsic timing known as “population clocks”. A population clock is said to be a group of neurons which respond to a stimulus and also allow the timing of the stimulus by producing a spatial pattern of activity which changes over time. At the termination of the stimulus, its duration is judged based upon the pattern of activity of the neurons at the end point of the stimulus. For motor timing these groups of neurons may be recurrent. In other words, they form a loop so that the activity of a neuron may indirectly feed back into itself. (Buonomano et al., 2010) suggest that sensory events are timed using non- recurrent networks. It is argued that the crucial factor with regard to motor and sensory timing is the strength of the connections between the different neurons. If the strength of the connections between neurons is weak then

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activity produced during the timing of an event will cease relatively quickly when the stimulus ends. This, it is proposed, is what happens during “sensory timing”. “Motor” timing on the other hand would involve neurons with strong connections. These groups of neurons would be capable of producing self maintaining activity even when the initial stimulus has ceased. Thus a recurrent network loop could occur. In this way a group of neurons would be able to predict the timing of the next beat in a tapping task for instance or the time taken for a moving object to reach a particular position in space based on previous activity in the neural network (Buonomano et al., 2010). An early population clock theory proposed that interactions between granule (excitatory) cells and Golgi (inhibitory) cells in the cerebellum could encode time and the activity of these cells could be detected by Purkinje cells in order to read out the state of the network and hence time the event they represent (Buonomano et al., 1994, Mauk et al., 1997). However, circuitry in the cerebellum is known to be incapable of sustaining recurrent excitatory activity (Buonomano et al., 2010). This would seem to suggest that although the cerebellum may well be a strong candidate for sensory timing, the population clock theory would rule out its involvement in motor timing. Since cortical networks have the strong excitatory connections necessary for motor timing, the population clock theory would support the idea of motor timing in these areas. The pre and supplementary motor areas have been suggested as likely candidates and are known to be involved in sequence generation (Buonomano et al., 2010). However, since damage to the cerebellum has been shown to produce impaired performance of the timing of movement (Keele et al., 1985, Ivry et al., 2004, Spencer et al., 2005), it is difficult to

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believe that it is uninvolved in motor timing if only because of its role in learning these tasks. One possible explanation which seems to circumvent this problem is proposed in a paper by Zelaznik et al (2005). The authors conducted a series of experiments which supported their proposal that motor timing requires an initial event based representation of a duration in which a temporal goal is externally defined (sensory timing), but that after a few “movement cycles” control processes take over, which allows the timing to be emergent (internally driven) and that it is these processes that are observed at work in motor or predictive timing (Zelaznik et al., 2005). If this is the case then it would be possible for disruption of the cerebellum to produce a reduction in performance for motor tasks due to its involvement in the initial stages of the task.

Taken together, it seems likely that sensory and motor timing may have a common source, but when motor timing is required additional process are brought into play. Intrinsic timing models also suggest that in both cases readout neurons may be necessary in order that an ultimate temporal value may be produced.

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Chapter 7

Psychophysical methods

Historically, psychophysical methods for the investigation of perceived temporal extent may be divided into two main groups: duration scaling, and duration discrimination (Allan, 1979). These are set out in the table below.

Duration Scaling Duration Discrimination

Verbal estimation Method of comparison

Magnitude estimation Forced choice-fixed standard

Category rating Forced choice-roving standard

Production/Reproduction/Ratio Setting Method of single stimulus