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Computational Mechanisms Underlying the Voluntary Control of Reach Movement Planning and Execution
No full textIn everyday life we make a variety of reaching movements. Most of these reaching movements have a clear objective of bringing the hand to the spatial location of the object of interest. Although we typically do not explicitly or at least are not aware of formulating a movement plan during reaching, a number of psychophysical studies on simple reaching movements highlight the notion that movement attributes and plans are likely to be represented/programmed in the central nervous system well before execution. In my thesis I have investigated the computational mechanisms underlying change of such movement plans.
I used a REDIRECT task in which targets occasionally changed their locations, to study the control of reaching movements. This task also gives a unique opportunity to address control before movement execution and while the movement is made. I used nine different race model architectures that can explain the redirect behavior of reaching movements. I showed that only the independent GO-STOP-GO model successfully explains the various behavioral measures. Second, using a particular outcome of the task called the hypometric response, EMG measurements and computational modelling I showed that redirecting a movement before initiation and during motor execution shared the same mechanisms.
Based on the race model framework developed in the previous chapter I next addressed whether and how kinematic plans maybe changed online. Reports from various studies suggest the kinematic representation constitutes an important aspect of motor planning. Unlike eye movements for which the kinematics are relatively fixed, hand movements exhibit a large scope for modification of the kinematics. I designed a novel velocity redirect task to understand the computational mechanisms behind the subject’s ability to change the speed of their movement. In two separate tasks the subjects either had to increase or decrease the speed of the movement according to the change in the target color. The applicability of different race model architectures to the velocity redirect task was assessed, similar to the position redirect task. I found that the GO-STOP-GO independent explained all aspects of behavior in the fast to slow velocity condition. The modeling results indicated a peculiar asymmetry in that while the FS model required inhibition, none of the GO-GO or GO-STOP models were able to explain the slow to fast velocity data. Interestingly, a vector averaging model was used to demonstrate the gradual merge of two kinematic plans in the SF task. Further, I undertook a novel approach to investigate the nature of kinematic planning borrowing from a well known paradigm in cognitive control called task switching. Using the logic of the task switch paradigm I showed the existence of a switch cost when subjects switched between slow and fast velocity movements indicating the existence of a kinematic plan well before the target onset, suggesting that such a plan may be part of a motor set. In summary, my work shows how a race model framework can provide a mechanistic understanding of motor control of different aspects of reach movement planning
Computational mechanisms underlying eye-head coordination
No full textGaze or eye-head coordination is an ethologically and physiologically important process that directs the visual system to objects of interest. Due to its relative simplicity, it is an ideal system to study multi-effector coordination. Whereas in the past, gaze control models have been developed on experimental data that focused on the mean responses, I used variability as a tool to study and model temporal coordination, spatial coordination and tested stochastic models of gaze control to test the predictions of the independent, common and interactive models of gaze control. I also used McLaughlin’s task paradigm of gaze adaptation to study gaze coordination under more dynamic conditions and studied its implication for models of gaze control. In the context of temporal coordination, the results suggest a common movement preparation phase underlying eye and head movement initiation. In the case of spatial coordination, I found evidence for the presence of a global gaze error feedback. However, the gaze learning results, which showed effector-specific learning suggested the presence of separate controllers that contribute unequally to gaze adaptation. Taken together, my results collectively suggest that an interactive model, that has a separate internal saccadic feedback controller and a global gaze feedback controller maybe the best model that explains my results
Neural Mechanisms underlying the planning of sequential saccades
No full textSaccades are rapid eye movements that we continually make (about 2-3 times per second) to look around and scan our visual environment. Though we effortlessly execute saccadic eye movements all the time, they are not just reflexive movements; saccades have been shown to involve multifaceted cognitive control mechanisms. This property of saccades, combined with the fact that saccadic parameters are easily measurable, and that the neural circuitry for saccade generation is fairly well established, has established saccadic eye movements to be an excellent tool to study motor planning and decision-making. However, much of the work done on saccade planning has been limited to understanding the production of single saccades. Natural behavior entails making multiple saccadic movements in a sequence to achieve day-to-day tasks such as reading a book. How are sequential saccades planned? This question forms the broad theme of this thesis.
The neural correlates of sequential saccade planning were scouted for in the macaque frontal eye field (FEF), a prefrontal area containing neuronal populations that undertake saccadic decision-making. Visual-salience neurons of the FEF have been shown to encode targets for upcoming saccades and movement-related neurons of the FEF have been shown to control the time of saccade initiation, providing a good link between neural activity and behaviorally measured reaction times. However, much of the neural underpinnings of saccade programming in the FEF have been uncovered using tasks involving single, isolated saccades. Motivated by this, I explored the mechanisms by which FEF neurons contributed to the programming of saccade sequences for this thesis, using single-unit electrophysiological recordings from the FEF of two macaques as they performed a sequential saccade task.
Sequential saccade programming can, in principle, operate through two major modes: serial or parallel. Behavioral measures, like short inter-saccadic intervals, strongly
indicate that multiple saccade plans can proceed in parallel. However, direct neural evidence of parallel programming in the FEF neuronal population that strongly link to behavior, i.e. movement neurons, is lacking. First, I show that FEF movement-related activity can start ramping-up for the second saccade before the first saccade execution is complete, and much before visual feedback from the first saccade can reach FEF, thereby providing neural correlates of parallel programming of sequential saccades. Perceptual processing in the FEF has been shown to precede motor processing in visual search tasks, and consistent with that notion, FEF neurons with visual activity were also able to augment activity related to the second target whilst the first saccade plan was still underway.
After finding neural evidence of parallel programming, I characterized the limits of parallel programming. Numerous studies have shown that when two motor plans overlap closely, processing bottlenecks arise to inhibit the programming of the second plan, and is behaviorally manifested by the progressive lengthening of the second task reaction time, as the temporal gap between the two tasks decreases. This feature of increase in the second task latency has been observed in sequential saccade tasks as well. Neural correlates of processing bottlenecks were found in the responses of FEF movement neurons, wherein for the second saccade plan, the rate of the growth of activity was perturbed and the threshold of saccade initiation was increased, in a degree proportional to the level of concurrence of the two saccade plans. The locus of processing bottlenecks was found to be at the level of FEF movement-related neurons, whereas the activity of the visual neurons indicated that visual processing for perceptually simple tasks might constitute a pre-bottleneck stage. Evidence of activity perturbations was also found for the first saccade plan, supporting capacity-sharing theories of processing bottlenecks, as opposed to single-channel bottleneck theories which postulate that only the second plan is gated by inhibitory control while the first can pass unabated. Together, the results suggest that processing bottlenecks in sequential saccades originate in the partitioning of the brain’s limited processing ‘capacity’ by simultaneously active motor plans, due to which, inhibitory control is applied on both the first and second saccade plans, to prevent straining of the aforesaid capacity.
Finally, I have examined peripheral signatures of sequential saccade planning. Recent studies using single saccade paradigms have shown that the function of FEF as a center of cognitive control is not limited to saccade eye movements, but can be generalized to the control of eye-head gaze shifts. Rapid presaccadic recruitment of dorsal neck muscle activity has been shown to occur after FEF both with single-unit microstimulation and trans-cranial magnetic stimulation, even under head-restrained conditions where no overt head movement is being brought about by the neck muscles. To investigate whether such
presaccadic recruitment occurs during sequential saccade planning or is gated out by inhibitory control, I recorded electromyographic (EMG) activity of motor units of the dorsal neck muscle as macaques performed the same sequential saccade task used for neural recordings. Neck muscle EMG showed leakage of FEF planning signals even for sequential saccades: peripheral correlates of parallel programming and processing bottlenecks were observed, with the activity mirroring that of FEF movement neurons. The correspondence of the results between the FEF and periphery suggest that a tight link exists between the eye and head systems, validating the hypothesis of a common gaze command originating in the FEF. The rapid recruitment of neck muscle activity observed for the second saccade before the completion of the first, also suggested that inhibitory control gates like basal ganglia do not preferentially intercept sequential saccade signals in the FEF-neck muscle circuit.
In summary, the results in this thesis provide direct neurophysiological evidence of behaviorally established features of sequential saccade planning such as parallel programming and processing bottlenecks. The fact that signatures of FEF movement responses can be captured at the level of the dorsal neck muscle suggests that the functional channel connecting FEF and the motor periphery is preserved even during sequential saccade planning and allows central responses to rapidly pass downstream by default, and perhaps prepare for an anticipated head movement in conjunction with the upcoming saccade. In cases where no head movement is elicited or where the head is restrained, inhibitory control mechanisms might come into play later and prevent supra-threshold rise of neck muscle activity
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The Role of Basal Ganglia and Redundancy in Supervised Motor Learning
No full textHuman sensorimotor control can achieve highly reliable movements under circumstances of noise, redundancy, uncertainty, and sensory delays. Our ability to achieve reliable and accurate movements is in the fact we have a nervous system that learns these limitations and continuously compensates for them. The purpose of the thesis is to understand brain mechanisms and computations underlying supervised motor learning, its interaction with reinforcement learning and study its relation to motor variability. To address these issues, we have investigated factors influencing supervised motor learning such as neurological disease condition, the role of the reinforcement signal, motor variability and motor redundancy.
Traditionally, supervised or error-based learning and reinforcement or reward based learning are thought to be occurring at anatomically different places and have functionally separate mechanisms. By leveraging the performance of human patients with Parkinson disease and cerebellar ataxia disease, we demonstrate how the presence and absence of dopamine medication and subthalamic deep brain stimulation (STN-DBS) influenced supervised learning. Furthermore, we also show that the presence and absence of reinforcement at the end of the trial profoundly affected learning such that the difference in learning as a consequence of medication reduced significantly. These results suggest that the basal ganglia modulate the gain of supervised learning in the cerebellum based on the reinforcement received at the end of the trial.
Furthermore, we explored motor variability (thought to be an unwanted characteristic of the motor system) and investigated its significance and effect on supervised motor learning. We propose that some part of motor variability arises out of the redundancy in the joints in the human arm. We showed that greater uses of redundancy in the arm lead to faster learning across healthy subjects. We observed these both in dynamic perturbation learning and kinematic perturbation learning. Interestingly, we also found differences in the use of redundancy between the dominant hand and non-dominant hand, suggesting that the nervous system actively controls the redundancy. Furthermore, we also observed some directions in reaching are difficult to learn in comparison to others directions. To understand such behavior, we separated direction wise errors and constructed errors ellipses and found out that eccentricity of ellipse change with learning, which suggests brain while reducing errors in learning, is also trying to homogenize the distribution of errors caused by the perturbation. We also found interesting differences between redundancy and motor learning that was selectively impaired in PD patients but not cerebellar patients, possibly pointing to a role of the basal ganglia in processing of the use of redundancy in motor learning.
In summary, the results in the thesis provide experimental support for the hypothesis that the basal ganglia modulate the gain of supervised learning and exploration of redundancy aids in learning and that the redundancy component of the motor variability is not noise. In future, we hope that this relationship between basal ganglia, reinforcement, and redundancy in supervised motor learning can be leveraged to enhance motor rehabilitation and motor skills in patients with motor deficits
Central and Peripheral Correlates of Motor Planning
Full text linkA hallmark of human behaviour is that we can either couple or decouple our thoughts, decision and motor plans from actions. Previous studies have reported evidence of gating of information between intention and action that can happen at different levels in the central nervous system (CNS) involving the motor cortex, subcortical structures such as the basal ganglia and even in the spinal cord. In my research I examine the extent of this gating and its modulation by task context. I will present results obtained by data collected from (a) neck muscles and neural recording from frontal eye field (FEF) in macaque monkeys and (b) putative motor units (MUs) from high density electrode arrays using surface EMG signals in human to delineate the type of information that leaks into muscles in the periphery when subjects are involved in preparing eye and hand movements, respectively, and its modulation by task context Overall, my results reveal that we can assess some aspects of central planning in the activity of motor units Further, the recruitment of these motor units depend on task context
A Control Systems Approach to Understanding Saccadic Eye Movements
No full textThe oculomotor system is extensively studied using concepts of control theory like optimal control, feedback control, and predictive control. But these studies have largely been restricted to investigating the central tendencies of movements like mean kinematics or the main sequence. But owing to the inherent noise in the motor system, eye movements generated by the oculomotor system exhibit considerable variability. Hence, it is imperative to study variability in these movements and consider noise to be an important aspect of oculomotor control. Therefore, in this thesis, a type of voluntary eye movement called saccades is studied, with emphasis on mean trajectories as well as inter-trial variability in the trajectories. A stochastic modelling approach has been used to gain deeper insights into control architecture of the saccade execution system. Firstly, using the stochastic optimal control framework it is shown that the saccadic system may have an explicit velocity plan. A new trajectory tracking stochastic optimal control model is proposed, which is observed to outperform the existing endpoint model in predicting the mean velocity profiles. Further evidence of this explicit velocity planning in saccade execution was provided using the saccade data obtained from an eye-hand coordination task. This is an important finding given that the dominant view in the field is that saccades are planned based on target displacement only. Secondly, using a stochastic saccade generation model with internal feedback, it is shown that the saccadic system uses both displacement and velocity information. The proposed model had a displacement block and velocity block with separate input and internal feedback loop. This framework was validated using behavioral data of horizontal saccades. Thirdly, the proposed stochastic dual model was extended for oblique saccade generation using an error coupling scheme. In case of oblique saccades too, the behavioral data was best fit by a dual model which used a convex combination of both displacement and velocity information. In nutshell, these results suggest that a dual control based on displacement as well as velocity information best explains the kinematic variability in saccade behaviour. This study emphasizes that variability in behaviour is an important tool for investigating the principles of movement generation in a stochastic system like the oculomotor system
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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