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Biomedical subjects

M A Frens

Publications and source records attributed to M A Frens.

10 recordsLinked to original sources
PubMed1998

The collicular code of saccade direction depends on the roll orientation of the head relative to gravity.

This paper investigates the influence of static head tilt on the relation between activity in the motor layers of the superior colliculus (mSC) and saccadic oculomotor output. Based on single-unit recordings and electrical microstimulation in awake rhesus monkeys, we report that head roll changes the direction of the saccade vector generated by the mSC, with respect to a head-fixed coordinate system. Typically, the vector rotates in a direction that is opposite to the head roll direction. This rotation exceeds the amount of ocular counterroll that is observed as a result of difference in static head roll positions. We find that the mSC works in an oculocentric coordinate system that is biased in the direction of gravity. It is argued that the modification is a result of processes that occur downstream of or parallel to the mSC.

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↗
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↗
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

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↗
PubMed1994

Transfer of short-term adaptation in human saccadic eye movements.

Controversy exists as to the extent to which the saccadic system, adapted in the so-called 'gain-shortening paradigm' for a particular target configuration, transfers the resulting change in saccade metrics to saccades elicited under different circumstances. In order to further assess this problem, we investigated the properties of human saccadic eye movements after visually induced short-term adaptation under a variety of conditions. We observed that saccades both during and after the adaptation did not significantly change their main sequence properties with respect to the pre-adaptation baseline. Saccade velocity profiles remained normal throughout the experiment, and we obtained no evidence that correction saccades were gradually absorbed in the primary saccade. We found that the effect of the short-term adaptation on saccade metrics is not confined to the particular combination of initial eye position and spatial position of the visual target used to induce the adaptation response. Saccades elicited from different initial positions towards targets with the same retinotopic coordinates as in the adaptation phase yield the same level of adaptation. However, our findings indicate that adaptation is confined to a limited range of saccade vectors around the oculocentric coordinates of the adaptation target ('restricted adaptation field'). Smaller and larger saccades are endowed with significantly lower adaptation values. Moreover, two further experiments showed that a retinal stimulus is not a prerequisite for adaptation to express itself: First, in a double-step experiment, we dissociated the retinal stimulus vector from the required oculomotor response. Second, we also investigated the effect of visually induced adaptation on auditory evoked saccades. In both tasks the adaptation was transferred to the required motor response. Based on our findings, we conclude that short-term adaptation is expressed at a multisensory stage, where saccadic eye movements are represented as desired eye displacement vectors (motor error). Possible neurophysiological implications are discussed.

Acoustic Stimulation↗
PubMed1991

Coordination of hand movements and saccades: evidence for a common and a separate pathway.

We studied the reaction times and initial directions of hand movements and saccades of human subjects who fixated and pointed as quickly as possible at eccentric targets which were presented unexpectedly. The targets were positioned on a horizontal bar which was placed in front of the subject. Different stimulus conditions were used in the experiments. Knowledge of the target position or the presence of an auditory co-stimulus slightly affected the reaction times of saccades in response to visual stimuli. Auditory co-stimuli reduced the reaction times considerably when the targets were presented after a delay of 200 ms after extinction of the central fixation point. Similar reductions were observed in the reaction times of the hand movements. However, these reductions were seen in hand responses to undelayed as well as delayed target presentations. The saccades were always made in the correct direction when the target was presented without delay. When the target was delayed about 50% of the saccades were made in the wrong direction. Even for undelayed targets the hand sometimes made mistakes. The number of mistakes increased to 35% when the target presentation was accompanied by the sound pulse. For delayed targets the proportion of wrong hand movements was about 50%. For such targets saccades and hand movements were practically always made in the same direction. If visual information is available, saccades and hand movements are generated independently of each other. However, if visual information is not present at the appropriate time and the target position has to be guessed, saccades and hand movements are generated on the basis of shared information.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗
PubMed1990

Spatial properties of horizontal cell responses in the cat retina.

The spatial properties of horizontal cells in the cat retina have been studied by means of intracellular recordings in the optically intact, in situ, eye. The spread of potentials in the horizontal cell layer and the spatial summation properties have been investigated using "white light" stimuli in several different configurations. Area-response curves were measured with flashing spots carefully centered on the receptive field. The lateral spread of potentials was studied using long, narrow slits of light and circular spots of different sizes at different positions in the receptive field. Two-dimensional receptive field profiles showed that the receptive field structure was, both on a large scale and at higher resolutions, relatively homogeneous and isotropic. The size and shape of the receptive fields have been characterized by applying a simplified version of the model proposed by Naka and Rushton (1967) of electrical coupling in the horizontal cells layer in fish. Results show that the model describes the variation of response amplitude reasonably well both as a function of spot size and spot or slit position. However, deviations were found for area-response curves measured at higher light intensities and for receptive field profiles measured with relatively small spots. Furthermore, the estimated length constants resulting from the different experiments were not in agreement. Different model parameters were needed in order to describe the spatial summation properties and the lateral spread of potentials respectively. It is concluded that passive electrical spread and linear summation of potentials cannot account for the observed spatial properties.

Action Potentials↗