Academic literature on the topic 'Sequential saccade programming'

We show that in saccade sequences, saccade trajectory is modulated in the direction of the preceding saccade and away from the following saccade. The magnitude of this effect is correlated with preceding and following saccade amplitude. This confirms that programming of sequential saccades overlaps. Curvature is also correlated with the deviation of saccade start and end points. Thus, we propose a novel benefit for the modulation of saccade trajectories: minimizing end point error in sequential saccades.

Abstract One of the core mechanisms involved in the control of saccade responses to selected target stimuli is the disengagement from the current fixation location, so that the next saccade can be executed. To carry out everyday visual tasks, we make multiple eye movements that can be programmed in parallel. However, the role of disengagement in the parallel programming of saccades has not been examined. It is well established that the need for disengagement slows down saccadic response time. This may be important in allowing the system to program accurate eye movements and have a role to play in the control of multiple eye movements but as yet this remains untested. Here, we report two experiments that seek to examine whether fixation disengagement reduces saccade latencies when the task completion demands multiple saccade responses. A saccade contingent paradigm was employed and participants were asked to execute saccadic eye movements to a series of seven targets while manipulating when these targets were shown. This both promotes fixation disengagement and controls the extent that parallel programming can occur. We found that trial duration decreased as more targets were made available prior to fixation: this was a result both of a reduction in the number of saccades being executed and in their saccade latencies. This supports the view that even when fixation disengagement is not required, parallel programming of multiple sequential saccadic eye movements is still present. By comparison with previous published data, we demonstrate a substantial speeded of response times in these condition (“a gap effect”) and that parallel programming is attenuated in these conditions.

We use sequences of saccadic eye movements to continually explore our visual environments. Previous behavioral studies have established that saccades in a sequence may be programmed in parallel by the oculomotor system. In this study, we tested the neural correlates of parallel programming of saccade sequences in the frontal eye field (FEF), using single-unit electrophysiological recordings from macaques performing a sequential saccade task. It is known that FEF visual neurons instantiate target selection whereas FEF movement neurons undertake saccade preparation, where the activity corresponding to a saccade vector gradually ramps up. The question of whether FEF movement neurons are involved in concurrent processing of saccade plans is as yet unresolved. In the present study, we show that, when a peripheral target is foveated after a sequence of two saccades, presaccadic activity of FEF movement neurons for the second saccade can be activated while the first is still underway. Moreover, the onset of movement activity varied parametrically with the behaviorally measured time available for parallel programming. Although at central fixation coactivated FEF movement activity may vectorially encode the retinotopic location of the second target with respect to the fixation point or the remapped location of the second target, with respect to the first our evidence suggests the possibility of early encoding of the remapped second saccade vector. Taken together, the results indicate that movement neurons, although located terminally in the FEF visual-motor spectrum, can accomplish concurrent processing of multiple saccade plans, leading to rapid execution of saccade sequences. NEW & NOTEWORTHY The execution of purposeful sequences underlies much of goal-directed behavior. How different brain areas accomplish sequencing is poorly understood. Using a modified double-step task to generate a rapid sequence of two saccades, we demonstrate that downstream movement neurons in the frontal eye field (FEF), a prefrontal oculomotor area, allow for coactivation of the first and second movement plans that constitute the sequence. These results provide fundamental insights into the neural control of action sequencing.

Our target article discussed how emerging knowledge of the physiological processes involved in the control of saccadic eye movements provided the basis for a functional framework in which to understand the programming of such movements. The commentators raised many interesting issues in their varied responses that ranged from detailed discussion of the physiological substrate through issues of saccade control in reading. New evidence at the physiological level demonstrates that some elaborations are needed to the framework we proposed. Most clearly, the spatial selection process operates in a manner different from our suggestion of an increase in activity in the salience map. Some commentators make the interesting and welcome proposal that the functional processes we outline may in fact be implemented with an even more unified physiological substrate (continuity between collicular fixation and build-up cells) than we envisaged. Extensions to the framework are discussed involving the planning of sequential saccades, saccades made in crossmodal situations, the influences of learning and memory, and binocular saccades. We consider carefully the commentaries proposing explicit attentional and/or executive processes in the programming of saccades. We integrate the comments of researchers investigating saccade control in neurological and neuropsychiatric patients and finally discuss whether the framework can account for saccades made in the course of reading.

Many neurons in macaque lateral intraparietal cortex (LIP) maintain elevated activity induced by visual or auditory targets during tasks in which monkeys are required to withhold one or more planned eye movements. We studied the mechanisms for such memory activity with neural network modeling. Recurrent connections among simulated LIP neurons were used to model memory responses of LIP neurons. The connection weights were computed using an optimization procedure to produce desired outputs in memory-saccade tasks. One constraint for the training process is the “single-purpose” rule, which mimics the fact that once LIP neurons hold the memory activity of a saccade, they are insensitive to further stimuli until the motor action is completed. After training, excitatory connections were developed between units with similar preferred saccade directions, while inhibitory connections were formed between units with dissimilar directions. This “push-pull” mechanism enables the network to encode the next intended eye movement and is essential for programming sequential saccades. In simulating double saccades, the push-pull connections locked the on-going activity in the network for the first saccade until the saccade was made, then a new population of units became active to prepare for the second saccade. The simulated LIP neurons exhibited sensory responses and memory activities similar to those recorded in LIP neurons. We propose that push-pull recurrent connections might be the basic structure mediating the memory activity of area LIP in planning sequential eye movements.

During movement programming, there is a point in time at which the movement system is committed to executing an action with certain parameters even though new information may render this action obsolete. For saccades programmed to a visual target this period is termed the dead time. Using a double-step paradigm, we examined potential variability in the dead time with variations in overall saccade latency and spatiotemporal configuration of two sequential targets. In experiment 1, we varied overall saccade latency by manipulating the presence or absence of a central fixation point. Despite a large and robust gap effect, decreasing the saccade latency in this way did not alter the dead time. In experiment 2, we varied the separation between the two targets. The dead time increased with separation up to a point and then leveled off. A stochastic accumulator model of the oculomotor decision mechanism accounts comprehensively for our findings. The model predicts a gap effect through changes in baseline activity without producing variations in the dead time. Variations in dead time with separation between the two target locations are a natural consequence of the population coding assumption in the model.

E-Z Reader achieves an impressive fit of empirical eye movement data by simulating core processes of reading in a computational approach that includes serial word processing, shifts of attention, and temporal overlap in the programming of saccades. However, when common assumptions for the time requirements of these processes are taken into account, severe constraints on the time line within which these elements can be combined become obvious. We argue that it appears difficult to accommodate these processes within a largely sequential modeling framework such as E-Z Reader.

Goal-directed behavior involves the transformation of neural movement plans into appropriate muscle activity patterns. Studies involving single saccades have shown that a rapid pathway links saccade planning in frontal eye fields (FEF) to neck muscle activity. However, it is unknown if the rapid connection between FEF and neck muscle is also maintained during sequential saccade planning. Using neural recordings from FEF, and electromyographic (EMG) recordings from the dorsal neck muscle of head-restrained monkeys, we show that neural sequence planning signals are largely preserved in the neck EMG response. Like FEF movement neurons, we found that neck motor unit activity displayed an accumulation-to-threshold response prior to saccade onset. Responses of both neck motor units and FEF neurons displayed similar trends during saccade sequencing: multiple saccadic eye movements could be programmed in parallel, while processing bottlenecks, indexed by reduced accumulation rates, limited the extent of parallel programming. These results suggest that even without the need for overt head movements, neck muscle activity shows signatures of central gaze planning. We propose that multiple upcoming gaze plans are rapidly passed down from the FEF to the neck muscle to initiate recruitment for anticipated gaze movements. Similarities in the neural and neck motor activity may enable synchronous yet controlled eye-head responses to sequential gaze shifts.

Saccades 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.