Biomedical subjects

Uwe J Ilg

Publications and source records attributed to Uwe J Ilg.

11 recordsLinked to original sources

PubMed2006

Primate area MST-l is involved in the generation of goal-directed eye and hand movements.

The contributions of the middle superior temporal area (MST) in the posterior parietal cortex of rhesus monkeys to the generation of smooth-pursuit eye movements as well as the contributions to motion perception are well established. Here, we present the first experimental evidence that this area also contributes to the generation of goal-directed hand movements toward a moving target. This evidence is based on the outcome of intracortical microstimulation experiments and transient lesions by small injections of muscimol at identified sites within the lateral part of area MST (MST-l). When microstimulation was applied during the execution of smooth-pursuit eye movements, postsaccadic eye velocity in the direction of the preferred direction of the stimulated site increased significantly (in 93 of 136 sites tested). When microstimulation was applied during a hand movement trial, the hand movement was displaced significantly in the same direction (in 28 of 39 sites tested). When we lesioned area MST-l transiently by injections of muscimol, steady-state eye velocity was exclusively reduced for ipsiversive smooth-pursuit eye movements. In contrast, hand movements were displaced toward the contralateral side, irrespective of the direction of the moving target. Our results provide evidence that area MST-l is involved in the processing of moving targets and plays a role in the execution of smooth-pursuit eye movements as well as visually guided hand movements.

Action Potentials↗

PubMed2005

Preparation and execution of saccades: the problem of limited capacity of computational resources.

Saccades are very fast, ballistic movements, which move the eyes from one target to another. Here, we show that the latency, precision and kinematics of saccades directed toward a target presented on a dark homogeneous background do not differ from the parameters of saccades directed toward a target presented on a structured background. However, if the visual background changed either its luminance or orientation simultaneously with the presentation of the saccade target, a significant increase in saccade latency was observed. The saccade kinematics as well as saccade precision, however, was not affected. Likewise, additional auditory stimulation applied simultaneously with the presentation of the target did not increase saccade latency. The increase in saccade latency and the maintenance of saccade kinematics indicate a sensory channel overload caused by the change in background. As a consequence, execution of the saccade was delayed until the computational resources to program the eye movement were available again.

Acoustic Stimulation↗

PubMed2005

Suppression of optokinesis during smooth pursuit eye movements revisited: the role of extra-retinal information.

When our eyes track objects that are moving in a richly structured environment, the retinal image of the stationary visual scene inevitably moves over the retina in a direction opposite to the eye movement. Such self-motion-induced global retinal slip usually provides an ideal stimulus for the optokinetic reflex. This reflex operates to compensate for global image flow. However, during smooth pursuit eye movements it must be shut down so that the reflex does not counteract the voluntary pursuit of moving targets. Here, we asked if retinal information is sufficient for this cancellation of the optokinetic reflex during smooth pursuit eye movements. In a series of experiments, we show that neither the eye movement-induced retinal image motion per se nor the relative motion between the pursuit target and the background are sufficient for suppression of optokinesis. We, therefore, conclude that extra-retinal information about smooth pursuit eye movements is required for the cancellation of the optokinetic reflex.

Blinking↗

PubMed2005

The neural basis of smooth-pursuit eye movements.

Smooth-pursuit eye movements are used to stabilize the image of a moving object of interest on the fovea, thus guaranteeing its high-acuity scrutiny. Such movements are based on a phylogenetically recent cerebro-ponto-cerebellar pathway that has evolved in parallel with foveal vision. Recent work has shown that a network of several cerebrocortical areas directs attention to objects of interest moving in three dimensions and reconstructs the trajectory of the target in extrapersonal space, thereby integrating various sources of multimodal sensory and efference copy information, as well as cognitive influences such as prediction. This cortical network is the starting point of a set of parallel cerebrofugal projections that use different parts of the dorsal pontine nuclei and the neighboring rostral nucleus reticularis tegmenti pontis as intermediate stations to feed two areas of the cerebellum, the flocculus-paraflocculus and the posterior vermis, which make mainly complementary contributions to the control of smooth pursuit.

Animals↗

PubMed2004

Posterior parietal cortex neurons encode target motion in world-centered coordinates.

The motion areas of posterior parietal cortex extract information on visual motion for perception as well as for the guidance of movement. It is usually assumed that neurons in posterior parietal cortex represent visual motion relative to the retina. Current models describing action guided by moving objects work successfully based on this assumption. However, here we show that the pursuit-related responses of a distinct group of neurons in area MST of monkeys are at odds with this view. Rather than signaling object image motion on the retina, they represent object motion in world-centered coordinates. This representation may simplify the coordination of object-directed action and ego motion-invariant visual perception.

Action Potentials↗

PubMed2004

Motion perception without explicit activity in areas MT and MST.

It is widely accepted that middle temporal (MT) and middle superior temporal (MST) cortical areas in the brain of rhesus monkeys are essential for processing visual motion. We asked whether this assumption holds true if the moving stimulus consists of a second-order motion stimulus. In addition, we asked whether neurons in area MT and MST code for moving sound sources. To answer these questions, we trained three rhesus monkeys on a direction-discrimination task. Our monkeys were able to correctly report the direction of all motion stimuli used in this study. Firing rates of directionally selective neurons from area MT (n = 38) and MST (n = 68) were recorded during task performance. These neurons coded only for the stimulus movement if the motion stimulus was separated from the background by luminance or flicker (Fourier and drift-balanced motion). If these segregation cues were absent (in the case of theta motion and of the moving sound source), firing rates did not code for the stimulus' direction. Therefore we conclude that although areas MT and MST are undoubtedly involved in processing a moving stimulus, they are not the final cortical stages responsible for perceiving it.

Action Potentials↗

PubMed2004

Single-neuron evidence for a contribution of the dorsal pontine nuclei to both types of target-directed eye movements, saccades and smooth-pursuit.

The primate dorsolateral pontine nucleus (DLPN) is a key link in a cerebro-cerebellar pathway for smooth pursuit eye movements, a pathway assumed to be anatomically segregated from tegmental circuits subserving saccades. However, the existence of afferents from several cerebrocortical and subcortical centres for saccades suggests that the DLPN and neighbouring parts of the dorsal pontine nuclei (DPN) might contribute to saccades as well. In order to test this hypothesis, we recorded from the DPN of two monkeys trained to perform smooth pursuit eye movements as well as visually and memory-guided saccades. Out of 281 neurons isolated from the DPN, 138 were responsive in oculomotor tasks. Forty-five were exclusively activated in saccade paradigms, 68 exclusively by smooth pursuit and 25 neurons showed responses in both. Pursuit-related responses reflected sensitivity to eye position, velocity or combinations of velocity and position with minor contributions of acceleration in many cases. When tested in the memory-guided saccades paradigm, 65 out of 70 neurons activated in saccade paradigms showed significant saccade-related bursts and 20 significant activity in the memory period. Our finding of saccade-related activity in the DPN in conjunction with the existence of strong anatomical input from saccade-related cerebrocortical areas suggests that the DPN serves as a precerebellar relay for both pursuit and saccade-related information originating from cerebral cortex, in addition to the classical tecto-tegmental circuitry for saccades.

Action Potentials↗

PubMed2003

Visual-tracking neurons in area MST are activated during anticipatory pursuit eye movements.

Here, I report that rhesus monkeys are able to generate anticipatory smooth pursuit eye movements in the transient absence of a moving target, if this target moves periodically. The eye velocity before target reappearance was significantly larger if the target trajectory was predictable compared with the control condition consisting of unpredictable target trajectory. Parallel to the registration of the eye movements, single-unit activity was recorded from neurons in the middle superior temporal (MST) area of the two monkeys. The neuronal activity of visual-tracking neurons resembled the observed eye movements, i.e. these neurons increased their activity earlier if the movement of the target was predictable compared with the unpredictable control. These results provide further evidence for the existence of extra-retinal signals in the activity of visual-tracking neurons located in area MST.

Action Potentials↗

PubMed2003

Visual tracking neurons in primate area MST are activated by smooth-pursuit eye movements of an "imaginary" target.

Because smooth-pursuit eye movements (SPEM) can be executed only in the presence of a moving target, it has been difficult to attribute the neuronal activity observed during the execution of these eye movements to either sensory processing or to motor preparation or execution. Previously, we showed that rhesus monkeys can be trained to perform SPEM directed toward an "imaginary" target defined by visual cues confined to the periphery of the visual field. The pursuit of an "imaginary" target provides the opportunity to elicit SPEM without stimulating visual receptive fields confined to the center of the visual field. Here, we report that a subset of neurons [85 "imaginary" visual tracking (iVT)-neurons] in area MST of 3 rhesus monkeys were identically activated during pursuit of a conventional, foveal dot target and the "imaginary" target. Because iVT-neurons did not respond to the presentation of a moving "imaginary" target during fixation of a stationary dot, we are able to exclude that responses to pursuit of the "imaginary" target were artifacts of stimulation of the visual field periphery. Neurons recorded from the representation of the central parts of the visual field in neighboring area MT, usually vigorously discharging during pursuit of foveal targets, in no case responded to pursuit of the "imaginary" target. This dissociation between MT and MST neurons supports the view that pursuit responses of MT neurons are the result of target image motion, whereas those of iVT-neurons in area MST reflect an eye movement-related signal that is nonretinal in origin. iVT-neurons fell into two groups, depending on the properties of the eye movement-related signal. Whereas most of them (71%) encoded eye velocity, a minority showed responses determined by eye position, irrespective of whether eye position was changed by smooth pursuit or by saccades. Only the former group exhibited responses that led the eye movement, which is a prerequisite for a causal role in the generation of SPEM.

Action Potentials↗

PubMed2002

Commentary: smooth pursuit eye movements: from low-level to high-level vision.

If an object of great interest moves in our environment, we are able to elicit smooth pursuit eye movements that keep the image of the moving object stationary on our fovea. The processing of visual motion underlying the execution of smooth pursuit eye movements is very similar to the processing underlying the perception of visual motion. During initiation of smooth pursuit, an averaging across all available motion information occurs. Cognitive factors including attention, prediction and learning are able to influence the execution of smooth pursuit. The pursuit target trajectory in space is represented in the discharge rates of neurons in the posterior parietal cortex of rhesus monkeys.

Animals↗

PubMed2002

Flicker in the visual background impairs the ability to process a moving visual stimulus.

For the detection of a moving object, segregating the object from the background is a necessary first step. This segregation can be achieved by detection of differences in the spatial, temporal and spatio-temporal properties of the object and background. Here we investigate how flicker influences the perception of a moving object in man and monkey, and we examine the neuronal responses in extrastriate medial temporal and medial superior temporal areas (MT and MST) of two rhesus monkeys. The performance of humans and monkeys in a direction discrimination task was impaired in the presence of flicker in the background compared to the static background condition. A similar effect was found in recordings from 155 single units in areas MT and MST during the discrimination task. The discriminability (d') of the neuronal responses in preferred and nonpreferred directions was reduced by 33% on average in the presence of a flicker background compared to the static background. This reduction in discriminability was not caused by differences in variance of the neuronal activity for the two background conditions, but was due to a reduction of the difference between the activities in preferred and nonpreferred direction. This reduction in directional selectivity could be traced back to two different mechanisms: in 32 out of 155 neurons (21%), the decrease resulted from an increase in the response to the stimulus moving in the nonpreferred direction; in 62 out of 155 neurons (40%), the reduction in directional selectivity was due to a decrease in the response to the preferred direction. These results give deeper insights into how moving stimuli are processed in the presence of background flicker as present in natural visual scenes.

Action Potentials↗