Sequences Of Saccades Research Articles

A Cortico-Subcortical Model for Generation of Spatially Accurate Sequential Saccades

This article provides a systems framework for the analysis of cortical and subcortical interactions in the control of saccadic eye movements, A major thesis of this model is that a topography of saccade direction and amplitude is preserved through multiple projections between brain regions until they are finally transformed into a temporal pattern of activity that drives the eyes to the target. The control of voluntary saccades to visual and remembered targets is modeled in terms of interactions between posterior parietal cortex, frontal eye fields, the basal ganglia (caudate and substantia nigra), superior colliculus, mediodorsal thalamus, and the saccade generator of the brainstem. Interactions include the modulation of eye movement motor error maps by topographic inhibitory projections, dynamic remapping of spatial target representations in saccade motor error maps, and sustained neural activity that embodies spatial memory. Models of these mechanisms implemented in our Neural Simulation Language simulate behavior and neural activity described in the literature, and suggest new experiments.

Multi-stepping saccadic sequences in humans

A number of paradigms are reported in which multi-stepping saccadic sequences (MSS) are reliably elicited in normal subjects. It was observed that MSS are consistently associated with a prolongation of the initial saccade latency. Further experiments show that this phenomenon is validated across a number of independent eye movement tasks. However, the probability of MSS occurring is reduced when temporal constraints are imposed on the latency of the response. An interpretation of these task-elicited MSS in terms of the neural mechanisms controlling voluntary saccades is proposed.

Spontaneous Saccades and Gaze-Holding Ability in the Pigmented Rat. II. Effects of Localized Cerebellar Lesions.

We have studied the effects of the ablation of the cerebellar vermal area corresponding to lobules VI - VIII and of the flocculus - paraflocculus of both sides on the spontaneous eye movements performed in the light and in the dark in head-restrained pigmented rats. These effects have been compared with those already described for the inferior olive lesion. The cerebellar lesions were performed 1 week to 6 months in advance. Eye movements were recorded through a phase detection search coil apparatus. Following vermal topectomy, the main characteristics of the spontaneous saccades are unmodified. Following the ablation of the flocculus - paraflocculus there is no change in the saccadic main sequence. However, the spontaneous saccades in the dark present a postsaccadic drift made up of two components with different time courses, the first one being fast and the second one slow. The former is due in part to a mismatch between the phasic (the pulse) and the tonic (the step) components of the eye movements; the latter to the leakage of the neural integrator. In light only the first component is present and the eye maintains a steady position. The time constant of the neural integrator is considerably reduced to approximately 600 - 900 ms from a value of approximately 1600 - 4000 ms in the intact rats. The amplitude of the postsaccadic drift in the light depends on both the mismatch between the pulse and the step of innervation of the extraocular muscles and the increased leakiness of the neural integrator. The gain of the pulse to step transformation is reduced to approximately 0.79 at all saccadic amplitudes and eccentricities and such a reduction is due to a decreased step amplitude, while the pulse amplitude remains unchanged. The contribution of the leakage of the neural integrator to the postsaccadic drift in the light is a function of the eccentricity with a slope of 0.23. The deficits described after flocculus - paraflocculus ablation are also very similar to those described following inferior olive lesion from a quantitative point of view. The possible mechanisms of the visually activated olivocerebellar system in the control of saccadic performance and in maintaining its calibration are discussed.

Impaired performance in a saccadic tracking task in schizophrenic patients

The performance of schizophrenic patients in a task which requires a sequence of saccades guided by visuospatial cues is reported. A previous study recording eye movements determined a highly increased number of fixations necessary for this task in acute schizophrenic patients compared with normal controls. The performance of normal control subjects of different age groups and the correlation between the performance in the tracking task and the results of a clinical psychological test battery are described. Schizophrenic patients in a partly remitted state and in a remitted or mildly chronic state performed this task worse than matched control subjects; this was particularly indicated by the time score. The relation to the lifetime dosage of neuroleptic medication is considered.

Planning sequences of saccades

Subjects used saccades to fixate a sequence of 1–5 stationary targets (separation= 90′) located at the vertices of an imaginary pentagon. The latency of the first saccade in a sequence and the duration of intervals between subsequent saccades increased with sequence length at a rate of about 20 msec/target. Latency also varied with ordinal position in the sequence. These results were not due to directional differences in saccadic latency nor to latency-accuracy or to latency-precision trade-offs. Results were similar when targets were removed and saccades were directed to remembered locations. These effects may be best accounted for by models that have been proposed to account for similar effects of sequence length and ordinal position on other voluntary motor tasks, such as typing, speech and finger-tapping. In these models motor programs for a sequence of responses are planned before execution and then retrieved from memory during execution. These models are fundamentally different from the traditional saccadic models in which visual error signals evoke saccades. Instead, we propose that saccades are controlled by an organized plan for an entire sequence of saccades. Visual error signals may modify or elaborate the plans during the execution of a sequence. Our proposal is consistent with ideas developed by Lashley [ Cerebral Mechanisms in Behavior: The Hixon Symposium. Wiley, New York (1951)] in his general treatment of the central organization that determines voluntary motor performance.

Preparing sequences of saccades under choice reaction conditions: effects of sequence length and context

Three experiments explored whether each saccade of a sequence of saccades is programmed prior to the onset of a sequence of saccades. Using a choice reaction paradigm that has been used to study the programming of sequences of manual responses, subjects were required to perform either one, two, or three saccades under blocked (experiment 1) and randomized (experiment 2) sequence length conditions. Replicating manual performance (Rosenbaum, Saltzman and Kingman 1984a) the time to initiate a sequence of saccades (T1) increased with the number of forthcoming responses in experiment 1 but not in experiment 2. Experiment 3 was analogous to Rosenbaum, Inhoff and Gordon's (1984b) manual choice reaction task in that subjects were instructed to choose between two sequences of saccades that shared either the sequence initial or sequence terminal fixation position. In contrast to manual choice performance, which shows shorter keypress responses for shared response elements, saccadic latencies were unaffected by the sharing of fixation positions. The results are discussed in the framework of exogenous and endogenous motor control systems that are interfaced with structures controlling oculomotor response execution.

Saccadic deterioration in spinocerebellar degeneration

Two frequent findings in cases of inherited ataxias involving the pons are the severe reduction of saccadic velocities and the presence of saccadic hypometria. The patients described here were initially observed to make frequent, severely hypometria refixations, composed of a sequence of small saccades of normal velocity. The saccadic gain was so low that these refixations appeared clinically to be continuous, slow movements. Over time, the small saccades then became slow but the overall movement remained hypometria. Also present initially but becoming more frequent with time were refixations consisting of a single, extremely slow saccade. This longitudinal study revealed a progressive deterioration in the saccadic system, possibly indicating an early defect in the way in which fast eye movements are programmed, followed by dysfunction in the pathways which actually generate them.

Visual and oculomotor functions of monkey substantia nigra pars reticulata. III. Memory-contingent visual and saccade responses.

Visual and oculomotor functions of monkey substantia nigra pars reticulata. III. Memory-contingent visual and saccade responses.

Structural analysis of eye movement response to visual field stimuli

The nature of the saccadic sequence made in response to a near threshold point light stimulus is considered as an index of perception. Features of the response sequence are evaluated by their ability to discriminate between searching responses to sub-threshold stimuli and acquisition responses to supra-threshold stimuli. A structural analysis of the entire sequence is constructed which achieves successful discrimination. The approach is proposed as the basic component of an alternate method of static perimetry.

Overlapping saccades and glissades are produced by fatigue in the saccadic eye movement system

Saccadic eye movements and their neurological control signals change significantly as the human fatigues. Electronic instrumentation with a band-width extending from DC to 1 kHz enabled the recording of anomalous looking saccadic eye movements that occurred as the subject's physiological state changed. Fatigue can produce: overlapping saccades in which the high-frequency saccadic bursts should show large pauses; glissades in which the high-frequency bursts should be much shorter than appropriate for the size of the intended saccades; and low-velocity, long-duration, non-Main Sequence saccades in which the motoneuronal bursts should be of lower frequency and longer duration than normal. As few as 30 saccades of 50 deg magnitude or a longer sequence of saccades as small as 10 deg can produce these aberrant eye movements and their concomitant neuronal firing pattern variations. The effects of fatigue could explain some of the variations between and spread within published data for velocity vs amplitude of human saccadic eye movements. Measuring the resistance to eye movement fatigue could become either a common clinical tool for diagnosing specific or general disease states, or a research tool for studying dyslexia or fatigue.

Sampling or Intermittency in Hand Control System Dynamics

A hand control model is proposed. Investigation of the hand's intermittency synchronization shows it corresponds to an input-synchronized sampler rather than the clock-synchronized sampler more typical of engineering systems. A velocity control mechanism, similar to that in an eye tracking system is shown to be absent in the hand. A quantitative transfer function for predictable inputs serves further to define the hand's input adaptive characteristics. Stability margin adjustments of a linear reduced model enabled us to match the available quantitative data. The most exciting result of this study is the evidence for intermittency: a refractory period shown in the short pulse experiment, peaks in the frequency response experiments, and a saccadic sequence of steps in response to an open loop step input and to a closed loop ramp input.