Cortical potentials evoked to response to a signal to make a memory-guided saccade

The difference in parameters of visually guided and memory-guided saccades was shown. Increase in the memory-guided saccade latency as compared to that of the visually guided saccades may indicate the deceleration of saccadic programming on the basis of information extraction from the memory. The comparison of parameters and topography of evoked components N1 and P1 of the evoked potential on the signal to make a memory- or visually guided saccade suggests that the early stage of the saccade programming associated with the space information processing is performed predominantly with top-down attention mechanism before the memory-guided saccade and bottom-up mechanism before the visually guided saccade. The findings show that the increase in the latency of the memory-guided saccades is connected with decision making at the central stage of the saccade programming. We proposed that wave N2, which develops in the middle of the latent period of the memory-guided saccades, is correlated with this process. Topography and spatial dynamics of components N1, P1 and N2 testify that the memory-guided saccade programming is controlled by the frontal mediothalamic system of selective attention and left-hemispheric brain mechanisms of motor attention.     

Linear ensemble-coding in midbrain superior colliculus specifies the saccade kinematics

Recently, we proposed an ensemble-coding scheme of the midbrain superior colliculus (SC) in which, during a saccade, each spike emitted by each recruited SC neuron contributes a fixed minivector to the gaze-control motor output. The size and direction of this 'spike vector' depend exclusively on a cell's location within the SC motor map (Goossens and Van Opstal, in J Neurophysiol 95: 2326-2341, 2006). According to this simple scheme, the planned saccade trajectory results from instantaneous linear summation of all spike vectors across the motor map. In our simulations with this model, the brainstem saccade generator was simplified by a linear feedback system, rendering the total model (which has only three free parameters) essentially linear. Interestingly, when this scheme was applied to actually recorded spike trains from 139 saccade-related SC neurons, measured during thousands of eye movements to single visual targets, straight saccades resulted with the correct velocity profiles and nonlinear kinematic relations ('main sequence properties' and 'component stretching'). Hence, we concluded that the kinematic nonlinearity of saccades resides in the spatial-temporal distribution of SC activity, rather than in the brainstem burst generator. The latter is generally assumed in models of the saccadic system. Here we analyze how this behaviour might emerge from this simple scheme. In addition, we will show new experimental evidence in support of the proposed mechanism.     

On identification of saccades from sinusoidal eye movement signals

Sinusoidal eye movements and potential saccadic eye movements are examined using the syntactic pattern recognition method presented previously. A few computer tests are presented for the verification of potential saccades from signals of sinusoidal eye movements. The technique was developed and tested with electro-oculographic signals. The verification of saccades consists of three tests: the estimation of average noise peaks in an eye movement signal; an angular velocity threshold; and the comparison between a sinusoidal eye movement signal and the corresponding stimulus signal. The technique is also efficient for noisy signals of eye movements, which were stimulated by both predictive and non-predictive sinusoidal stimulus movements.     

A spiking neural network model of the midbrain superior colliculus that generates saccadic motor commands

Single-unit recordings suggest that the midbrain superior colliculus (SC) acts as an optimal controller for saccadic gaze shifts. The SC is proposed to be the site within the visuomotor system where the nonlinear spatial-to-temporal transformation is carried out: the population encodes the intended saccade vector by its location in the motor map (spatial), and its trajectory and velocity by the distribution of firing rates (temporal). The neurons’ burst profiles vary systematically with their anatomical positions and intended saccade vectors, to account for the nonlinear main-sequence kinematics of saccades. Yet, the underlying collicular mechanisms that could result in these firing patterns are inaccessible to current neurobiological techniques. Here, we propose a simple spiking neural network model that reproduces the spike trains of saccade-related cells in the intermediate and deep SC layers during saccades. The model assumes that SC neurons have distinct biophysical properties for spike generation that depend on their anatomical position in combination with a center–surround lateral connectivity. Both factors are needed to account for the observed firing patterns. Our model offers a basis for neuronal algorithms for spatiotemporal transformations and bio-inspired optimal controllers.     

Frontal eye field inactivation alters the readout of superior colliculus activity for saccade generation in a task-dependent manner

Saccades require a spatiotemporal transformation of activity between the intermediate layers of the superior colliculus (iSC) and downstream brainstem burst generator. The dynamic linear ensemble-coding model (Goossens and Van Opstal 2006) proposes that each iSC spike contributes a fixed mini-vector to saccade displacement. Although biologically-plausible, this model assumes cortical areas like the frontal eye fields (FEF) simply provide the saccadic goal to be executed by the iSC and brainstem burst generator. However, the FEF and iSC operate in unison during saccades, and a pathway from the FEF to the brainstem burst generator that bypasses the iSC exists. Here, we investigate the impact of large yet reversible inactivation of the FEF on iSC activity in the context of the model across four saccade tasks. We exploit the overlap of saccade vectors generated when the FEF is inactivated or not, comparing the number of iSC spikes for metrically-matched saccades. We found that the iSC emits fewer spikes for metrically-matched saccades during FEF inactivation. The decrease in spike count is task-dependent, with a greater decrease accompanying more cognitively-demanding saccades. Our results show that FEF integrity influences the readout of iSC activity in a task-dependent manner. We propose that the dynamic linear ensemble-coding model be modified so that FEF inactivation increases the gain of a readout parameter, effectively increasing the influence of a single iSC spike. We speculate that this modification could be instantiated by FEF and iSC pathways to the cerebellum that could modulate the excitability of the brainstem burst generator.

     

空间注意转移的初步眼动研究

目的：本研究采用线运动错觉测量作为空间注意转移程度的指标,对内源性眼跳与空间注意转移的关系进行探讨。方法：被试以提示刺激的形状和颜色为线索进行相应的眼跳,通过对线运动错觉的分析探索时间间隔、眼跳位置与提示刺激空间方位一致性对空间注意转移的影响以及二者是否存在交互作用。结果：时间间隔效应显著;提示刺激位置与眼跳目标位置的空间一致性对眼跳前空间注意转移有显著影响。结论：内源性眼跳与注意的关系密切;我们可以假设注意与眼跳程序通常是相关的,但需要一个启动信号激活眼跳程序。     

Sensory processing of motor inaccuracy depends on previously performed movement and on subsequent motor corrections: a study of the saccadic system

When goal-directed movements are inaccurate, two responses are generated by the brain: a fast motor correction toward the target and an adaptive motor recalibration developing progressively across subsequent trials. For the saccadic system, there is a clear dissociation between the fast motor correction (corrective saccade production) and the adaptive motor recalibration (primary saccade modification). Error signals used to trigger corrective saccades and to induce adaptation are based on post-saccadic visual feedback. The goal of this study was to determine if similar or different error signals are involved in saccadic adaptation and in corrective saccade generation. Saccadic accuracy was experimentally altered by systematically displacing the visual target during motor execution. Post-saccadic error signals were studied by manipulating visual information in two ways. First, the duration of the displaced target after primary saccade termination was set at 15, 50, 100 or 800 ms in different adaptation sessions. Second, in some sessions, the displaced target was followed by a visual mask that interfered with visual processing. Because they rely on different mechanisms, the adaptation of reactive saccades and the adaptation of voluntary saccades were both evaluated. We found that saccadic adaptation and corrective saccade production were both affected by the manipulations of post-saccadic visual information, but in different ways. This first finding suggests that different types of error signal processing are involved in the induction of these two motor corrections. Interestingly, voluntary saccades required a longer duration of post-saccadic target presentation to reach the same amount of adaptation as reactive saccades. Finally, the visual mask interfered with the production of corrective saccades only during the voluntary saccades adaptation task. These last observations suggest that post-saccadic perception depends on the previously performed action and that the differences between saccade categories of motor correction and adaptation occur at an early level of visual processing.     

Binocular coupling in chameleon saccade generation

Chameleons are capable of making a saccade with one eye while the other does not move. This virtually unique feature poses questions regarding the organization of the saccadic system of the chameleon. By comparing real data with a simulated test signal, we studied whether the saccade generation of the left and right eye can be considered as truly independent. This appeared not to be the case, since there was an increased likelihood to start saccades in close temporal proximity in the two eyes. However, the coupling does not reflect a common saccadic motor signal for both eyes, since even saccades that were made in close temporal proximity did not have correlated metrics. Received: 21 April 1997 / Accepted in revised form: 15 September 1997     

Human fast negative EEG potentials before express saccades

Fast negative EEG potentials preceding fast regular saccades and express saccades were studied by the method of backward averaging under conditions of monocular stimulation of the right and left eye. "Step" and "gap" experimental paradigms were used for visual stimulation. Analysis of parameters of potentials and their spatiotemporal dynamics suggests that, under conditions of the increased attention and optimal readiness of the neural structures, express saccades appear when the previously chosen program of the future eye movement coincides with the actual target coordinates. We assumed that the saccade latency decreases at the expense of the involvement of the main oculomotor areas of motor and saccadic planning in its initiation; an express saccade can be initiated also by means of direct transmission of the signal from the cortex to the brainstem saccadic generator passing by the superior colliculus. Moreover, anticipating release from the central fixation and attention distraction are necessary for the successful initiation of an express saccade.     

Looking for Discriminating Is Different from Looking for Looking’s Sake

Recent studies provide evidence for task-specific influences on saccadic eye movements. For instance, saccades exhibit higher peak velocity when the task requires coordinating eye and hand movements. The current study shows that the need to process task-relevant visual information at the saccade endpoint can be, in itself, sufficient to cause such effects. In this study, participants performed a visual discrimination task which required a saccade for successful completion. We compared the characteristics of these task-related saccades to those of classical target-elicited saccades, which required participants to fixate a visual target without performing a discrimination task. The results show that task-related saccades are faster and initiated earlier than target-elicited saccades. Differences between both saccade types are also noted in their saccade reaction time distributions and their main sequences, i.e., the relationship between saccade velocity, duration, and amplitude.     

Open-loop simulations of the primate saccadic system using burst cell discharge from the superior colliculus

 Saccade-related burst neurons (SRBNs) in the monkey superior colliculus (SC) have been hypothesized to provide the brainstem saccadic burst generator with the dynamic error signal and the movement initiating trigger signal. To test this claim, we performed two sets of open-loop simulations on a burst generator model with the local feedback disconnected using experimentally obtained SRBN activity as both the driving and trigger signal inputs to the model. First, using neural data obtained from cells located near the middle of the rostral to caudal extent of the SC, the internal parameters of the model were optimized by means of a stochastic hill-climbing algorithm to produce an intermediate-sized saccade. The parameter values obtained from the optimization were then fixed and additional simulations were done using the experimental data from rostral collicular neurons (small saccades) and from more caudal neurons (large saccades); the model generated realistic saccades, matching both position and velocity profiles of real saccades to the centers of the movement fields of all these cells. Second, the model was driven by SRBN activity affiliated with interrupted saccades, the resumed eye movements observed following electrical stimulation of the omnipause region. Once again, the model produced eye movements that closely resembled the interrupted saccades produced by such simulations, but minor readjustment of parameters reflecting the weight of the projection of the trigger signal was required. Our study demonstrates that a model of the burst generator produces reasonably realistic saccades when driven with actual samples of SRBN discharges. Received: 25 October 1994/Accepted in revised form: 20 June 1995     

Is Mislocalization during Saccades Related to the Position of the Saccade Target within the Image or to the Gaze Position at the End of the Saccade?

A stimulus that is flashed around the time of a saccade tends to be mislocalized in the direction of the saccade target. Our question is whether the mislocalization is related to the position of the saccade target within the image or to the gaze position at the end of the saccade. We separated the two with a visual illusion that influences the perceived distance to the target of the saccade and thus saccade endpoint without affecting the perceived position of the saccade target within the image. We asked participants to make horizontal saccades from the left to the right end of the shaft of a Müller-Lyer figure. Around the time of the saccade, we flashed a bar at one of five possible positions and asked participants to indicate its location by touching the screen. As expected, participants made shorter saccades along the fins-in (<–>) configuration than along the fins-out (>–<) configuration of the figure. The illusion also influenced the mislocalization pattern during saccades, with flashes presented with the fins-out configuration being perceived beyond flashes presented with the fins-in configuration. The difference between the patterns of mislocalization for bars flashed during the saccade for the two configurations corresponded quantitatively with a prediction based on compression towards the saccade endpoint considering the magnitude of the effect of the illusion on saccade amplitude. We conclude that mislocalization is related to the eye position at the end of the saccade, rather than to the position of the saccade target within the image.     

The timing of sequences of saccades in visual search

According to the LATER model (linear approach to thresholds with ergodic rate), the latency of a single saccade in response to target appearance can be understood as a decision process, which is subject to (i) variations in the rate of (visual) information processing; and (ii) the threshold for the decision. We tested whether the LATER model can also be applied to the sequences of saccades in a multiple fixation search, during which latencies of second and subsequent saccades are typically shorter than that of the initial saccade. We found that the distributions of the reciprocal latencies for later saccades, unlike those of the first saccade, are highly asymmetrical, much like a gamma distribution. This suggests that the normal distribution of the rate r, which the LATER model assumes, is not appropriate to describe the rate distributions of subsequent saccades in a scanning sequence. By contrast, the gamma distribution is also appropriate to describe the distribution of reciprocal latencies for the first saccade. The change of the gamma distribution parameters as a function of the ordinal number of the saccade suggests a lowering of the threshold for second and later saccades, as well as a reduction in the number of target elements analysed.     

Saccadic Momentum and Facilitation of Return Saccades Contribute to an Optimal Foraging Strategy

The interest in saccadic IOR is funneled by the hypothesis that it serves a clear functional purpose in the selection of fixation points: the facilitation of foraging. In this study, we arrive at a different interpretation of saccadic IOR. First, we find that return saccades are performed much more often than expected from the statistical properties of saccades and saccade pairs. Second, we find that fixation durations before a saccade are modulated by the relative angle of the saccade, but return saccades show no sign of an additional temporal inhibition. Thus, we do not find temporal saccadic inhibition of return. Interestingly, we find that return locations are more salient, according to empirically measured saliency (locations that are fixated by many observers) as well as stimulus dependent saliency (defined by image features), than regular fixation locations. These results and the finding that return saccades increase the match of individual trajectories with a grand total priority map evidences the return saccades being part of a fixation selection strategy that trades off exploration and exploitation.     

Modeling Inter-trial Variability of Saccade Trajectories: Effects of Lesions of the Oculomotor Part of the Fastigial Nucleus

This study investigates the inter-trial variability of saccade trajectories observed in five rhesus macaques (Macaca mulatta). For each time point during a saccade, the inter-trial variance of eye position and its covariance with eye end position were evaluated. Data were modeled by a superposition of three noise components due to 1) planning noise, 2) signal-dependent motor noise, and 3) signal-dependent premotor noise entering within an internal feedback loop. Both planning noise and signal-dependent motor noise (together called accumulating noise) predict a simple S-shaped variance increase during saccades, which was not sufficient to explain the data. Adding noise within an internal feedback loop enabled the model to mimic variance/covariance structure in each monkey, and to estimate the noise amplitudes and the feedback gain. Feedback noise had little effect on end point noise, which was dominated by accumulating noise. This analysis was further extended to saccades executed during inactivation of the caudal fastigial nucleus (cFN) on one side of the cerebellum. Saccades ipsiversive to an inactivated cFN showed more end point variance than did normal saccades. During cFN inactivation, eye position during saccades was statistically more strongly coupled to eye position at saccade end. The proposed model could fit the variance/covariance structure of ipsiversive and contraversive saccades. Inactivation effects on saccade noise are explained by a decrease of the feedback gain and an increase of planning and/or signal-dependent motor noise. The decrease of the fitted feedback gain is consistent with previous studies suggesting a role for the cerebellum in an internal feedback mechanism. Increased end point variance did not result from impaired feedback but from the increase of accumulating noise. The effects of cFN inactivation on saccade noise indicate that the effects of cFN inactivation cannot be explained entirely with the cFN’s direct connections to the saccade-related premotor centers in the brainstem.     

Cortical local field potential encodes movement intentions in the posterior parietal cortex

The cortical local field potential (LFP) is a summation signal of excitatory and inhibitory dendritic potentials that has recently become of increasing interest. We report that LFP signals in the parietal reach region (PRR) of the posterior parietal cortex of macaque monkeys have temporal structure that varies with the type of planned or executed motor behavior. LFP signals from PRR provide better decode performance for reaches compared to saccades and have stronger coherency with simultaneously recorded spiking activity during the planning of reach movements than during saccade planning. LFP signals predict the animal's behavioral state (e.g., planning a reach or saccade) and the direction of the currently planned movement from single-trial information. This new evidence provides further support for a role of the parietal cortex in movement planning and the potential application of LFP signals for a brain-machine interface.     

Saccade generation by the frontal eye fields in rhesus monkeys is separable from visual detection and bottom-up attention shift

The frontal eye fields (FEF), originally identified as an oculomotor cortex, have also been implicated in perceptual functions, such as constructing a visual saliency map and shifting visual attention. Further dissecting the area's role in the transformation from visual input to oculomotor command has been difficult because of spatial confounding between stimuli and responses and consequently between intermediate cognitive processes, such as attention shift and saccade preparation. Here we developed two tasks in which the visual stimulus and the saccade response were dissociated in space (the extended memory-guided saccade task), and bottom-up attention shift and saccade target selection were independent (the four-alternative delayed saccade task). Reversible inactivation of the FEF in rhesus monkeys disrupted, as expected, contralateral memory-guided saccades, but visual detection was demonstrated to be intact at the same field. Moreover, saccade behavior was impaired when a bottom-up shift of attention was not a prerequisite for saccade target selection, indicating that the inactivation effect was independent of the previously reported dysfunctions in bottom-up attention control. These findings underscore the motor aspect of the area's functions, especially in situations where saccades are generated by internal cognitive processes, including visual short-term memory and long-term associative memory.     

Effect of sampling frequencies on computation of the maximum velocity of saccadic eye movements

The maximum velocity of saccades is widely used in the clinical assessment of topographical diagnoses. Several methodological factors affect the maximum velocity results. The sampling frequency, the resolution of the analog-digital converter, and filtering of the signal are the most important factors. The sampling frequency should preferabel be higher than 300 Hz. Frequencies below 200 Hz severily deform the velocity profile. The resolution of the analog-digital converter should be 10 bits or more. A theoretical model was constructed for maximum velocity computation. A case study of electro-oculographic and photoelectric recordings confirmed the theoretical model.     

A minicomputer program for automatic saccade detection and linear analysis of eye movement systems using sine wave stimulus

A FORTRAN IV program is described, which may be run interactively or in batch and which allows a user to obtain the frequency response amplitude ratio and phase resulting from the linear analysis of an eye movement system using sine wave stimuli. The response (eye position) signal may contain components contributed by the saccadic eye movements. The program can digitize analog signals and store data on a magnetic tape. With the aid of digital filters, the program can detect saccades without requiring any input parameters from the user. The program interpolates the saccade interval using a method of least square curve fitting with a sine wave. The interpolation is relatively noise immune and works well regardless of the stimulus frequencies and the width of a saccade interval. Moreover, the program can handle long duration of signals such as 90 min of data which covers about 5 cycles of a 0.001 Hz sine wave signal. Sample runs for the cases of 0.001 and 0.1 Hz are given. The resident driver and the overlayable segments of the program have been implemented on a DEC (Digital Equipment Corp.) LAB-11 minicomputer (PDP 11/20).     

Effect of dopamine deficiency on the preparation of visually guided saccadic eye movements

Parameters of saccadic eye movements were studied in patients with Parkinson's disease and control subjects. In parkinsonian patients, the number of slow regular saccades was shown to be increased, and the number of express saccades was shown to be decreased. As a result the mean of saccade latency in patients was longer than in the control group. Moreover, the percentage of multistep saccades in patients with Parkinson's disease. In this case, not one but two or three saccades were performed with smaller amplitude to the target. We point, that the multistep saccades occurred mainly among the express saccades. Obviously, the dopamine deficiency distinguishing parkinsonian patients takes the primary part in the development of saccadic disorders. Degeneration of the nigrostriatal dopamine pathway results in imbalance in activity of the direct and indirect output pathways of the striatum. We suppose that this leads to inhibition of neurons activity in the superior colliculus during the saccade performance, which results in the early saccade interruption. In support of this reasoning, the mean of saccade latency and the percentage of the multistep saccades decreased in patients with Parkinson's disease after dopamine D2/D3 agonist (piribedil) treatment, due to activity restoration of the indirect pathway.