Biomedical subjects

A J Van Opstal

Publications and source records attributed to A J Van Opstal.

At least 19 recordsLinked to original sources

PubMed2006

Stimulus intensity modifies saccadic reaction time and visual response latency in the superior colliculus.

Performance in a reaction time task can be strongly influenced by the physical properties of the stimuli used (e.g., position and intensity). The reduction in reaction time observed with higher-intensity visual stimuli has been suggested to arise from reduced processing time along the visual pathway. If this hypothesis is correct, activity should be registered in neurons sooner for higher-intensity stimuli. We evaluated this hypothesis by measuring the onset of neural activity in the intermediate layers of the superior colliculus while monkeys generated saccades to high or low-intensity visual stimuli. When stimulus intensity was high, the response onset latency was significantly reduced compared to low-intensity stimuli. As a result, the minimum time for visually triggered saccades was reduced, accounting for the shorter saccadic reaction times (SRTs) observed following high-intensity stimuli. Our results establish a link between changes in neural activity related to stimulus intensity and changes to SRTs, which supports the hypothesis that shorter SRTs with higher-intensity stimuli are due to reduced processing time.

Animals↗

PubMed2005

Dynamic ensemble coding of saccades in the monkey superior colliculus.

The deeper layers of the midbrain superior colliculus (SC) contain a topographic motor map in which a localized population of cells is recruited for each saccade, but how the brain stem decodes the dynamic SC output is unclear. Here we analyze saccade-related responses in the monkey SC to test a new dynamic ensemble-coding model, which proposes that each spike from each saccade-related SC neuron adds a fixed, site-specific contribution to the intended eye movement command. As predicted by this simple theory, we found that the cumulative number of spikes in the cell bursts is tightly related to the displacement of the eye along the ideal straight trajectory, both for normal saccades and for strongly curved, blink-perturbed saccades toward a single visual target. This dynamic relation depends systematically on the metrics of the saccade displacement vector, and can be fully predicted from a quantitative description of the cell's classical movement field. Furthermore, we show that a linear feedback model of the brain stem, which is driven by dynamic linear vector summation of measured SC firing patterns, produces realistic two-dimensional (2D) saccade trajectories and kinematics. We conclude that the SC may act as a nonlinear, vectorial saccade generator that programs an optimal straight eye-movement trajectory.

Action Potentials↗

PubMed2002

Auditory-visual interactions subserving goal-directed saccades in a complex scene.

This study addresses the integration of auditory and visual stimuli subserving the generation of saccades in a complex scene. Previous studies have shown that saccadic reaction times (SRTs) to combined auditory-visual stimuli are reduced when compared with SRTs to either stimulus alone. However, these results have been typically obtained with high-intensity stimuli distributed over a limited number of positions in the horizontal plane. It is less clear how auditory-visual interactions influence saccades under more complex but arguably more natural conditions, when low-intensity stimuli are embedded in complex backgrounds and distributed throughout two-dimensional (2-D) space. To study this problem, human subjects made saccades to visual-only (V-saccades), auditory-only (A-saccades), or spatially coincident auditory-visual (AV-saccades) targets. In each trial, the low-intensity target was embedded within a complex auditory-visual background, and subjects were allowed over 3 s to search for and foveate the target at 1 of 24 possible locations within the 2-D oculomotor range. We varied systematically the onset times of the targets and the intensity of the auditory target relative to background [i.e., the signal-to-noise (S/N) ratio] to examine their effects on both SRT and saccadic accuracy. Subjects were often able to localize the target within one or two saccades, but in about 15% of the trials they generated scanning patterns that consisted of many saccades. The present study reports only the SRT and accuracy of the first saccade in each trial. In all subjects, A-saccades had shorter SRTs than V-saccades, but were more inaccurate than V-saccades when generated to auditory targets presented at low S/N ratios. AV-saccades were at least as accurate as V-saccades but were generated at SRTs typical of A-saccades. The properties of AV-saccades depended systematically on both stimulus timing and S/N ratio of the auditory target. Compared with unimodal A- and V-saccades, the improvements in SRT and accuracy of AV-saccades were greatest when the visual target was synchronous with or leading the auditory target, and when the S/N ratio of the auditory target was lowest. Further, the improvements in saccade accuracy were greater in elevation than in azimuth. A control experiment demonstrated that a portion of the improvements in SRT could be attributable to a warning-cue mechanism, but that the improvements in saccade accuracy depended on the spatial register of the stimuli. These results agree well with earlier electrophysiological results obtained from the midbrain superior colliculus (SC) of anesthetized preparations, and we argue that they demonstrate multisensory integration of auditory and visual signals in a complex, quasi-natural environment. A conceptual model incorporating the SC is presented to explain the observed data.

Acoustic Stimulation↗

PubMed2001

A spatial hearing deficit in early-blind humans.

An important issue in neuroscience is the effect of visual loss on the remaining senses. Two opposing views have been advanced. On the one hand, visual loss may lead to compensatory plasticity and sharpening of the remaining senses. On the other hand, early blindness may also prevent remaining sensory modalities from a full development. In the case of sound localization, it has been reported recently that, under certain conditions, early-blind humans can localize sounds better than sighted controls. However, these studies were confined to a single sound source in the horizontal plane. This study compares sound localization of early-blind and sighted subjects in both the horizontal and vertical domain, whereas background noise was added to test more complex hearing conditions. The data show that for high signal-to-noise (S/N) ratios, localization by blind and sighted subjects is similar for both azimuth and elevation. At decreasing S/N ratios, the accuracy of the elevation response components deteriorated earlier than the accuracy of the azimuth component in both subject groups. However, although azimuth performance was identical for the two groups, elevation accuracy deteriorated much earlier in the blind subject group. These results indicate that auditory hypercompensation in early-blind humans does not extend to the frontal target domain, where the potential benefit of vision is maximal. Moreover, the results demonstrate for the first time that in this domain the human auditory system may require vision to optimally calibrate the elevation-related spectral pinna cues. Sensitivity to azimuth-encoding binaural difference cues, however, may be adequately calibrated in the absence of vision.

Acoustic Stimulation↗

PubMed2001

Two-dimensional sound-localization behavior of early-blind humans.

To investigate whether the visual system is crucial for adequate calibration of acoustic localization cues, sound-localization performance of early blind humans was compared with that of sighted controls. Because a potential benefit of vision is mainly expected for targets within the two-dimensional (2D) frontal hemifield, localization was tested within this target range, while using sounds of various durations and spectral content. Subjects were instructed to point, in separate experimental sessions, either with their left arm, or with their nose, in the direction of the perceived target position as accurately as possible. The experiments required the use of all available sound-localization cues such as interaural differences in phase and intensity, as well as the complex spectral shape cues provided by the pinnae. In addition, for long-duration stimuli, subjects could have had access to head motion-induced acoustic feedback. Moreover, the two pointing methods allowed us to assess different response strategies for the two groups. In an additional series, subjects were instructed to respond as quickly as possible. The results show that, in general, 2D sound-localization performance of blind subjects was indistinguishable from that of sighted subjects, both for broad-band noise and for pure tones. In the fast head-pointing task, the latency distributions of both groups were equal. These findings suggest that visual feedback is not required to calibrate the available localization cues--including the idiosyncratic and complex spectral shape cues for elevation. Instead, the localization abilities of blind people show that the putative supervising role of vision may be supported, or taken over, by other non-visual feedback systems. The results do not provide support for the hypothesis that blind people can hypercompensate for the loss of vision in the frontal hemifield by developing superior sound-localization abilities. Despite the general correspondence in localization behavior, some specific differences related to pointing strategies as well as to those between blind and sighted subjects were apparent. Most importantly, the reconstructed origin (bias) of arm pointing was located near the shoulder for the blind subjects, whereas it was shifted and located near the cyclopean eye for the sighted subjects. The results indicate that both early blind and sighted humans adequately transform the head-centered acoustic target coordinates into the required reference frame of either motor system, but that the adopted response strategy may be specific to the subject group and pointer method.

Adult↗

PubMed2000

Blink-perturbed saccades in monkey. I. Behavioral analysis.

Saccadic eye movements are thought to be influenced by blinking through premotor interactions, but it is still unclear how. The present paper describes the properties of blink-associated eye movements and quantifies the effect of reflex blinks on the latencies, metrics, and kinematics of saccades in the monkey. In particular, it is examined to what extent the saccadic system accounts for blink-related perturbations of the saccade trajectory. Trigeminal reflex blinks were elicited near the onset of visually evoked saccades by means of air puffs directed on the eye. Reflex blinks were also evoked during a straight-ahead fixation task. Eye and eyelid movements were measured with the magnetic-induction technique. The data show that saccade latencies were reduced substantially when reflex blinks were evoked prior to the impending visual saccades as if these saccades were triggered by the blink. The evoked blinks also caused profound spatial-temporal perturbations of the saccades. Deflections of the saccade trajectory, usually upward, extended up to approximately 15 degrees. Saccade peak velocities were reduced, and a two- to threefold increase in saccade duration was typically observed. In general, these perturbations were largely compensated in saccade mid-flight, despite the absence of visual feedback, yielding near-normal endpoint accuracies. Further analysis revealed that blink-perturbed saccades could not be described as a linear superposition of a pure blink-associated eye movement and an unperturbed saccade. When evoked during straight-ahead fixation, blinks were accompanied by initially upward and slightly abducting eye rotations of approximately 2-15 degrees. Back and forth wiggles of the eye were frequently seen; but in many cases the return movement was incomplete. Rather than drifting back to its starting position, the eye then maintained its eccentric orbital position until a downward corrective saccade toward the fixation spot followed. Blink-associated eye movements were quite rapid, albeit slower than saccades, and the velocity-amplitude-duration characteristics of the initial excursions as well as the return movements were approximately linear. These data strongly support the idea that blinks interfere with the saccade premotor circuit, presumably upstream from the neural eye-position integrator. They also indicated that a neural mechanism, rather than passive elastic restoring forces within the oculomotor plant, underlies the compensatory behavior. The tight latency coupling between saccades and blinks is consistent with an inhibition of omnipause neurons by the blink system, suggesting that the observed changes in saccade kinematics arise elsewhere in the saccadic premotor system.

Algorithms↗

PubMed2000

Blink-perturbed saccades in monkey. II. Superior colliculus activity.

Trigeminal reflex blinks evoked near the onset of a saccade cause profound spatial-temporal perturbations of the saccade that are typically compensated in mid-flight. This paper investigates the influence of reflex blinks on the discharge properties of saccade-related burst neurons (SRBNs) in intermediate and deep layers of the monkey superior colliculus (SC). Twenty-nine SRBNs, recorded in three monkeys, were tested in the blink-perturbation paradigm. We report that the air puff stimuli, used to elicit blinks, resulted in a short-latency ( approximately 10 ms) transient suppression of saccade-related SRBN activity. Shortly after this suppression (within 10-30 ms), all neurons resumed their activity, and their burst discharge then continued until the perturbed saccade ended near the extinguished target. This was found regardless whether the compensatory movement was into the cell's movement field or not. In the limited number of trials where no compensation occurred, the neurons typically stopped firing well before the end of the eye movement. Several aspects of the saccade-related activity could be further quantified for 25 SRBNs. It appeared that 1) the increase in duration of the high-frequency burst was well correlated with the (two- to threefold) increase in duration of the perturbed movement. 2) The number of spikes in the burst for control and perturbed saccades was quite similar. On average, the number of spikes increased only 14%, whereas the mean firing rate in the burst decreased by 52%. 3) An identical number of spikes were obtained between control and perturbed responses when burst and postsaccadic activity were both included in the spike count. 4) The decrease of the mean firing rate in the burst was well correlated with the decrease in the velocity of perturbed saccades. 5) Monotonic relations between instantaneous firing rate and dynamic motor error were obtained for control responses but not for perturbed responses. And 6) the high-frequency burst of SRBNs with short-lead and long-lead presaccadic activity (also referred to as burst and buildup neurons, respectively) showed very similar features. Our findings show that blinking interacts with the saccade premotor system already at the level of the SC. The data also indicate that a neural mechanism, rather than passive elastic restoring forces within the oculomotor plant, underlies the compensation for blink-related perturbations. We propose that these interactions occur downstream from the motor SC and that the latter may encode the desired displacement vector of the eyes by sending an approximately fixed number of spikes to the brainstem saccadic burst generator.

Animals↗

PubMed1998

Visual-auditory interactions modulate saccade-related activity in monkey superior colliculus.

This paper reports on single-unit activity of saccade-related burst neurons (SRBNs) in the intermediate and deep layers of the monkey superior colliculus (SC), evoked by bimodal sensory stimulation. Monkeys were trained to generate saccadic eye movements towards visual stimuli, in either a unimodal visual saccade task, or in a bimodal visual-auditory task. In the latter task, the monkeys were required to make an accurate saccade towards a visual target, while ignoring an auditory stimulus. The presentation of an auditory stimulus in temporal and spatial proximity of the visual target influenced neither the accuracy nor the kinematic properties of the evoked saccades. However, it had a significant effect on the activity of 90% (45/50) of the SRBNs. The motor-related burst increased significantly in some neurons, but was suppressed in others. In visual-movement cells, comparable bimodal interactions were observed in both the visually evoked burst and the movement-related burst. The large differences observed in the movement-related activity of SRBNs for identical saccades under different sensory conditions do not support the hypothesis that such cells encode dynamic motor error. The only behavioral parameter that was affected by the presentation of the auditory stimulus was saccade latency. Auditory stimulation caused saccade latency changes in the majority of the experiments. Meanwhile, the timing of peak collicular motor activity and saccade onset remained tightly coupled for all stimulus configurations. In addition, saccade latency varied as function of the distance between the stimuli in 36% of the recordings. Interestingly, the occurrence of a spatial latency effect covaried significantly with a similar spatial influence on the SRBNs firing rate. These cells were always most active in the bimodal task when both stimuli were in spatial register, but activity decreased with increasing stimulus separation.

Acoustic Stimulation↗

PubMed1998

Relearning sound localization with new ears.

Because the inner ear is not organized spatially, sound localization relies on the neural processing of implicit acoustic cues. To determine a sound's position, the brain must learn and calibrate these cues, using accurate spatial feedback from other sensorimotor systems. Experimental evidence for such a system has been demonstrated in barn owls, but not in humans. Here, we demonstrate the existence of ongoing spatial calibration in the adult human auditory system. The spectral elevation cues of human subjects were disrupted by modifying their outer ears (pinnae) with molds. Although localization of sound elevation was dramatically degraded immediately after the modification, accurate performance was steadily reacquired. Interestingly, learning the new spectral cues did not interfere with the neural representation of the original cues, as subjects could localize sounds with both normal and modified pinnae.

Adaptation, Physiological↗

PubMed1998

Spectro-temporal factors in two-dimensional human sound localization.

This paper describes the effect of spectro-temporal factors on human sound localization performance in two dimensions (2D). Subjects responded with saccadic eye movements to acoustic stimuli presented in the frontal hemisphere. Both the horizontal (azimuth) and vertical (elevation) stimulus location were varied randomly. Three types of stimuli were used, having different spectro-temporal patterns, but identically shaped broadband averaged power spectra: noise bursts, frequency-modulated tones, and trains of short noise bursts. In all subjects, the elevation components of the saccadic responses varied systematically with the different temporal parameters, whereas the azimuth response components remained equally accurate for all stimulus conditions. The data show that the auditory system does not calculate a final elevation estimate from a long-term (order 100 ms) integration of sensory input. Instead, the results suggest that the auditory system may apply a "multiple-look" strategy in which the final estimate is calculated from consecutive short-term (order few ms) estimates. These findings are incorporated in a conceptual model that accounts for the data and proposes a scheme for the temporal processing of spectral sensory information into a dynamic estimate of sound elevation.

Adult↗

PubMed1997

Human eye-head coordination in two dimensions under different sensorimotor conditions.

The coordination between eye and head movements during a rapid orienting gaze shift has been investigated mainly when subjects made horizontal movements towards visual targets with the eyes starting at the centre of the orbit. Under these conditions, it is difficult to identify the signals driving the two motor systems, because their initial motor errors are identical and equal to the coordinates of the sensory stimulus (i.e. retinal error). In this paper, we investigate head-free gaze saccades of human subjects towards visual as well as auditory stimuli presented in the two-dimensional frontal plane, under both aligned and unaligned initial fixation conditions. Although the basic patterns for eye and head movements were qualitatively comparable for both stimulus modalities, systematic differences were also obtained under aligned conditions, suggesting a task-dependent movement strategy. Auditory-evoked gaze shifts were endowed with smaller eye-head latency differences, consistently larger head movements and smaller concomitant ocular saccades than visually triggered movements. By testing gaze control for eccentric initial eye positions, we found that the head displacement vector was best related to the initial head motor-error (target-re-head), rather than to the initial gaze error (target-re-eye), regardless of target modality. These findings suggest an independent control of the eye and head motor systems by commands in different frames of reference. However, we also observed a systematic influence of the oculomotor response on the properties of the evoked head movements, indicating a subtle coupling between the two systems. The results are discussed in view of current eye-head coordination models.

Acoustic Stimulation↗

PubMed1997

Monkey superior colliculus activity during short-term saccadic adaptation.

This article concerns the neural mechanisms that underlie short-term saccadic adaptation in the rhesus monkey. By means of a consistent intrasaccadic target displacement, the relation between visual input and motor output was gradually changed in three monkeys, such that they made hypometric saccades. During this process, the activity of saccade-related burst neurons in the intermediate and deep layers of the Superior Colliculus (SC) was recorded in two of the monkeys. Our findings show that, like in humans, only saccades evoked within a restricted field around the adaptation target were adapted. However, unlike in humans, the kinematic properties of adapted saccades also changed systematically during the adaptation process. Typically, adapted saccades were slower and had a longer duration than would be expected on the basis of the main sequence for nonadapted visually guided movements. During adaptation, saccade-related activity of units in the SC remained appropriate for the saccade that was required to foveate the initial target, rather than for the saccade that was actually made. This means that adaptation caused a dissociation between SC activity and the ensuing saccade. Thus, the activity of the colliculus was better described in "required eye displacement coordinates" than in "actual eye displacement coordinates." Our data provide further evidence for the hypothesis that short-term saccadic adaptation acts at a level downstream from the SC, presumably at a stage that determines the kinematics of saccadic eye movements.

Adaptation, Physiological↗

PubMed1997

Local feedback signals are not distorted by prior eye movements: evidence from visually evoked double saccades.

Recent experiments have shown that the amplitude and direction of saccades evoked by microstimulation of the monkey superior colliculus depend systematically on the amplitude and direction of preceding visually guided saccades as well as on the postsaccade stimulation interval. The data are consistent with the hypothesis that an eye displacement integrator in the local feedback loop of the saccadic burst generator is gradually reset with a time constant of approximately 45 ms. If this is true, similar effects should occur during naturally evoked saccade sequences, causing systematic interval-dependent errors. To test this prediction in humans, saccades toward visual single- and double-step stimuli were elicited, and the properties of the second saccades were investigated as a function of the intersaccadic interval (ISI). In 15-20% of the saccadic responses, ISIs fell well below 100 ms. The errors of the second saccades were not systematically affected by the preceding primary saccade, irrespective of the ISI. Only a slight increase in the endpoint variability of second saccades was observed for the shortest ISIs. These results are at odds with the hypothesis that the putative eye displacement integrator has a reset time constant >10 ms. Instead, it is concluded that the signals involved in the internal feedback control of the saccadic burst generator reflect eye position and/or eye displacement accurately, irrespective of preceding eye movements.

Algorithms↗

PubMed1996

Task-dependence of saccade-related activity in monkey superior solliculus: implications for models of the saccadic system.

Current models assign a crucial role to the deep layers of the Superior Colliculus (SC) in the dynamic feedback control of saccadic eye movements. However, if the SC is to be part of the local feedback loop for saccades, it is expected that the movement-related firing patterns of deep layer SC cell maintain a fixed relation with the instantaneous saccade trajectory, regardless of the conditions that evoked the saccade. In this paper we provide three different lines of evidence, suggesting that the movement activity of SC burst cells may change as a function of the sensory conditions evoking the saccade. First, it is shown that bimodal (visual-auditory) stimulation may markedly enhance (up to about 350%) or suppress (on average down to 70%) SC motor bursts when compared to the activity for unimodal visual stimulation. Second, the movement activity associated with auditory-evoked saccades appeared to be reduced by almost 60% relative to visually-evoked saccades of the same metrics (tested in one monkey). However, for both paradigms, these relatively large changes in movement activity went without a concomitant change in the saccade properties. Third, a short-term saccadic adaptation paradigm produced saccades with a smaller amplitude (gain about 0.7) upon presentation of the adapting visual stimulus. However, we found that the movement-related activity of SC burst cells did not change in this paradigm. These findings suggest that the SC cells do neither encode saccade kinematics, nor the precise components of the saccade vector. Rather, we propose that the motor SC issues a crude desired eye displacement signal to the brainstem that is transformed into the appropriate movement signals by downstream or parallel mechanisms.

Animals↗

PubMed1995

A quantitative study of auditory-evoked saccadic eye movements in two dimensions.

We investigated the properties of human saccadic eye movements evoked by acoustic stimuli in the two-dimensional frontal plane. These movements proved to be quite accurate, both in azimuth and in elevation, provided the sound source spectrum had a broad bandwidth and a sufficiently long duration. If the acoustic target was a tone, the azimuth of the saccadic end points remained equally accurate, whereas the elevation of the response was related to the frequency of the tone, rather than to the physical position of the target. Saccade elevation accuracy also declined substantially for short-duration noise bursts, although response elevation remained highly correlated with target elevation. The latencies of auditory saccades depended on the amplitude, but not on the direction of the eye movement, suggesting a polar coordinate origin of auditory saccade initiation. We also observed that the trajectories of auditory saccades were often substantially curved. Both a qualitative and a model-based analysis showed that this curvature corrected for errors in the initial direction of the saccade. The latter analysis also suggested that the kinematic properties of auditory saccades could be described by the superposition of two overlapping saccadic eye movements, hypothesized to be based on binaural difference cues and monaural spectral cues in the auditory signal, respectively. It is argued that, although the audio-oculomotor system has to operate in a feedforward way, it must nevertheless have access to an accurate representation of actual and desired eye position. Different models underlying the generation of auditory saccades are discussed.

Adult↗

PubMed1995

Influence of eye position on activity in monkey superior colliculus.

1. Most recording studies on the role of the monkey superior colliculus (SC) in eye movement generation have so far indicated that the code of the recruited population of cells is a fixed vector command representing the desired saccadic eye displacement vector, irrespective of the position of the eyes in the orbit. Experimental evidence from microstimulation, lesions, and neuroanatomy, however, suggests that the SC may have access to an eye position signal. 2. In this paper we have tested the hypothesis that SC activity is influenced by eye position, by recording from presaccadic burst neurons while monkeys made rapid eye movements in the light covering a large part of the oculomotor range. 3. In four alert rhesus monkeys, we obtained sufficient data from 57 SC single units. The activity of a substantial part of these cells (30/57) appeared to be significantly influenced by eye position. Although the tuning properties of these cells for saccade amplitude and direction remained invariant for changes in eye position, the peak firing rate of these units was systematically influenced by the position of the eyes in the head. 4. We have characterized this eye position dependence of a neuron's activity by a qualitative, model-independent, as well as by a quantitative model description (gain field), which takes into account both the tuning properties of the cell for eye displacement vectors and the dependence of eye position. 5. Although a majority of gain fields had their eye position sensitivity vector roughly aligned with the optimal saccade vector direction (colinear gain field, 17/30), a substantial part of the gain fields had their eye position sensitivity vectors in quite different directions, approximately homogeneously distributed with respect to the cell's ON direction. 6. We conclude that the SC has access to a signal related to the position of the eyes in the orbit. Several hypotheses on the possible functional role of this signal, in relation to the neural code of the motor map, are discussed.

Animals↗

PubMed1995

Spatial and temporal factors determine auditory-visual interactions in human saccadic eye movements.

In this paper, we show that human saccadic eye movements toward a visual target are generated with a reduced latency when this target is spatially and temporally aligned with an irrelevant auditory nontarget. This effect gradually disappears if the temporal and/or spatial alignment of the visual and auditory stimuli are changed. When subjects are able to accurately localize the auditory stimulus in two dimensions, the spatial dependence of the reduction in latency depends on the actual radial distance between the auditory and the visual stimulus. If, however, only the azimuth of the sound source can be determined by the subjects, the horizontal target separation determines the strength of the interaction. Neither saccade accuracy nor saccade kinematics were affected in these paradigms. We propose that, in addition to an aspecific warning signal, the reduction of saccadic latency is due to interactions that take place at a multimodal stage of saccade programming, where the perceived positions of visual and auditory stimuli are represented in a common frame of reference. This hypothesis is in agreement with our finding that the saccades often are initially directed to the average position of the visual and the auditory target, provided that their spatial separation is not too large. Striking similarities with electrophysiological findings on multisensory interactions in the deep layers of the midbrain superior colliculus are discussed.

Adult↗

PubMed1993

Three-dimensional analysis of strongly curved saccades elicited by double-step stimuli.

Earlier recordings of eye position in three dimensions have revealed that Listing's law is obeyed in reasonable approximation, both statically and dynamically. This implies that all eye positions are confined to a plane when using a rotation vector or quaternion representation. The orientation of the angular velocity axis is crucial in order to preserve the law. For a single-axis rotation, the eye's angular velocity axis has to tilt out of Listing's plane, otherwise the law cannot be preserved in eccentric saccades. Experimental work has confirmed that normal, visually-guided saccades resemble single-axis rotations whose angular velocity axis tilts by the right amount. We investigated how well the saccadic system implements Listing's law when the trajectory of the eyes is more complicated, as in a non-single-axis rotation where the angular velocity vector depends on instantaneous eye position. Eye position was measured in three dimensions using the magnetic scleral search coil method for five subjects. Non-single-axis rotations of the eye were evoked with a double-step paradigm. We found that Listing's law is obeyed equally well during fixations, single-axis saccades and in non-single-axis saccades. Some deviations from the law were found in both curved and single-axis eye movements, but we demonstrated that the net torsional component of eye position of these saccades is negligible compared to that expected if the angular velocity axis did not tilt at all. In addition, analysis of the angular velocity signals in the curved movements showed strong similarity to the computed signal required for implementing Listing's law. Our results show that the observed deviations from Listing's law reflect only minor failures in the mechanism underlying its dynamic implementation. We conclude that single-axis rotations are not a necessary condition for the implementation of Listing's law in saccades. Our results are compatible with the notion that the implementation of Listing's law relies upon internal feedback. Various suggestions of how models can be reconciled with recent data on the three-dimensional control of saccades are discussed.

Adult↗