Updating target distance across eye movements in depth

We tested between two coding mechanisms that the brain may use to retain distance information about a target for a reaching movement across vergence eye movements. If the brain was to encode a retinal disparity representation (retinal model), i.e., target depth relative to the plane of fixation, each vergence eye movement would require an active update of this representation to preserve depth constancy. Alternatively, if the brain was to store an egocentric distance representation of the target by integrating retinal disparity and vergence signals at the moment of target presentation, this representation should remain stable across subsequent vergence shifts (nonretinal model). We tested between these schemes by measuring errors of human reaching movements (n = 14 subjects) to remembered targets, briefly presented before a vergence eye movement. For comparison, we also tested their directional accuracy across version eye movements. With intervening vergence shifts, the memory-guided reaches showed an error pattern that was based on the new eye position and on the depth of the remembered target relative to that position. This suggests that target depth is recomputed after the gaze shift, supporting the retinal model. Our results also confirm earlier literature showing retinal updating of target direction. Furthermore, regression analyses revealed updating gains close to one for both target depth and direction, suggesting that the errors arise after the updating stage during the subsequent reference frame transformations that are involved in reaching.     

Neural correlates of reafference: evoked brain activity during motion perception and saccadic eye movements

The ability to perceive a stable visual environment despite eye movements and the resulting displacement of the retinal image is a striking feature of visual perception. In order to study the brain mechanism related to this phenomenon, an EEG was recorded from 30 electrodes spaced over the occipital, temporal and parietal brain areas while stationary or moving visual stimuli with velocities between 178 degrees/s and 533 degrees/s were presented. The visual stimuli were presented both during saccadic eye movements and with stationary eyes. Stimulus-related potentials were measured, and the effects of absolute and relative stimulus velocity were analyzed. Healthy adults participated in the experiments. In all 36 subjects and experimental conditions, four potential components were found with mean latencies of about 70, 140, 220 and 380 ms. The latency of the two largest components between 100 and 240 ms decreased while field strength increased with higher absolute stimulus velocity for both stationary and moving eyes, whereas relative stimulus velocity had no effect on amplitude, latency and topography of the visual evoked potential (VEP) components. If the visual system uses retinal motion information only, we would expect a dependence upon relative velocity. Since field strength and latency of the components were independent of eye movements but dependent upon absolute stimulus velocity, the visual cortex must use extraretinal information to extract stimulus velocity. This was confirmed by the fact that significant topographic changes were observed when brain activity evoked during saccades and with stationary eyes was compared. In agreement with the reafference principle, the findings indicate that the same absolute visual stimulus activates different neuronal elements during saccades than during fixation.     

Effect of viewing distance on the generation of vertical eye movements during locomotion

Vertical head and eye coordination was studied as a function of viewing distance during locomotion. Vertical head translation and pitch movements were measured using a video motion analysis system (Optotrak 3020). Vertical eye movements were recorded using a video-based pupil tracker (Iscan). Subjects (five) walked on a linear treadmill at a speed of 1.67 m/s (6 km/h) while viewing a target screen placed at distances ranging from 0.25 to 2.0 m at 0.25-m intervals. The predominant frequency of vertical head movement was 2 Hz. In accordance with previous studies, there was a small head pitch rotation, which was compensatory for vertical head translation. The magnitude of the vertical head movements and the phase relationship between head translation and pitch were little affected by viewing distance, and tended to orient the naso-occipital axis of the head at a point approximately 1 m in front of the subject (the head fixation distance or HFD). In contrast, eye velocity was significantly affected by viewing distance. When viewing a far (2-m) target, vertical eye velocity was 180° out of phase with head pitch velocity, with a gain of 0.8. This indicated that the angular vestibulo-ocular reflex (aVOR) was generating the eye movement response. The major finding was that, at a close viewing distance (0.25 m), eye velocity was in phase with head pitch and compensatory for vertical head translation, suggesting that activation of the linear vestibulo-ocular reflex (lVOR) was contributing to the eye movement response. There was also a threefold increase in the magnitude of eye velocity when viewing near targets, which was consistent with the goal of maintaining gaze on target. The required vertical lVOR sensitivity to cancel an unmodified aVOR response and generate the observed eye velocity magnitude for near targets was almost 3 times that previously measured. Supplementary experiments were performed utilizing body-fixed active head pitch rotations at 1 and 2 Hz while viewing a head-fixed target. Results indicated that the interaction of smooth pursuit and the aVOR during visual suppression could modify both the gain and phase characteristics of the aVOR at frequencies encountered during locomotion. When walking, targets located closer than the HFD (1.0 m) would appear to move in the same direction as the head pitch, resulting in suppression of the aVOR. The results of the head-fixed target experiment suggest that phase modification of the aVOR during visual suppression could play a role in generating eye movements consistent with the goal of maintaining gaze on targets closer than the HFD, which would augment the lVOR response. Received: 23 November 1998 / Accepted: 17 May 1999     

Properties of cerebellar fastigial neurons during translation, rotation, and eye movements

The most medial of the deep cerebellar nuclei, the fastigial nucleus (FN), receives sensory vestibular information and direct inhibition from the cerebellar vermis. We investigated the signal processing in the primate FN by recording single-unit activities during translational motion, rotational motion, and eye movements. Firing rate modulation during horizontal plane translation in the absence of eye movements was observed in all non-eye-movement-sensitive cells and 26% of the pursuit eye-movement-sensitive neurons in the caudal FN. Many non-eye-movement-sensitive cells recorded in the rostral FN of three fascicularis monkeys exhibited convergence of signals from both the otolith organs and the semicircular canals. At low frequencies of translation, the majority of these rostral FN cells changed their firing rates in phase with head velocity rather than linear acceleration. As frequency increased, FN vestibular neurons exhibited a wide range of response dynamics with most cells being characterized by increasing phase leads as a function of frequency. Unlike cells in the vestibular nuclei, none of the rostral FN cells responded to rotational motion alone, without simultaneously exhibiting sensitivity to translational motion. Modulation during earth-horizontal axis rotation was observed in more than half (77%) of the neurons, although with smaller gains than during translation. In contrast, only 47% of the cells changed their firing rates during earth-vertical axis rotations in the absence of a dynamic linear acceleration stimulus. These response properties suggest that the rostral FN represents a main processing center of otolith-driven information for inertial motion detection and spatial orientation.     

The influence of target distance on eye movement responses during vertical linear motion

Summary Studies of the linear vestibulo-ocular reflex (LVOR) suggest that eye movement responses to linear head motion are rudimentary. This may be due to inadequate control of target distance (D). As D approaches infinity, eye movements are not required to maintain retinal image stability during linear head displacements, but must become increasingly large as D shortens. The LVOR in the presence of visual targets (VLVOR) was tested by recording human vertical eye and head movements during self-generated vertical linear oscillation (averaging 2.7 Hz at peak excursion of 3.2 cm) while subjects alternately fixated targets at D=36, 142, and 424 cm. Response sensitivity rose from 0.10 deg/cm (5.8 deg/s/g) for D=424 cm to 0.65 deg/cm (37.5 deg/s/g) for D=36 cm. Results employing optical manipulations, including spherical lenses to modify accommodation and accomodative convergence, and prisms to modify fusional vergence without altering accommodation, imply that the state of vergence is the most important variable underlying the effect. Trials in darkness (LVOR) and with head-fixed targets (visual suppression of the LVOR) suggest that, while visual following and perhaps mental set influences results, the major proportion of the VLVOR response is driven by vestibular (presumably otolith) inputs.     

EEG correlates of eye contact and interpersonal distance.

The EEG of 18 male subjects was monitored while the subject gazed at the eyes of a male experimenter located 2, 4, 8, 16 or 32 ft from the subject. The experimenter either gazed directly at the subject or averted his eyes. EEG arousal was highest when the experimenter was at 2 ft and gazing into the subject's eyes. EEG arousal diminished as a function of distance, while arousal for direct gaze was always higher than for averted gaze, whatever the distance.     

Hand distance modulates the electrophysiological correlates of target selection during a tactile search task

This study investigated whether the N140cc ERP component, described as a possible electrophysiological marker of target selection in touch, was modulated by body posture. Participants performed a tactile search task in which they had to localize a tactile target, presented to the left or right hand, while a simultaneous distractor was delivered to the opposite hand. Importantly, the distance between target and distractor (hands separation) was manipulated in different experimental conditions (near vs. far hands). Results showed reduced errors and enhanced amplitudes of the late N140cc when the hands were far apart than in close proximity. This suggests that the competition between target and distractor is stronger when the hands are close together in the near condition, resulting in a degraded selection process. These findings confirm that the N140cc reflects target selection during the simultaneous presentation of competing stimuli and demonstrate for the first time that the attentional mechanisms indexed by this ERP component are based at least in part on postural representations of the body.     

Short-latency compensatory eye movements associated with a brief period of free fall

The vertical eye movements induced by a brief period of free fall were recorded from three monkeys (Macaca mulatta) using the electromagnetic search-coil technique. Free fall was initiated in total darkness immediately following binocular fixation of one of six target lights located at viewing distances ranging from 20 to 107 cm. Responses consisted of an initial transient downward eye movement (anticompensatory direction) with a latency of a few milliseconds at most followed by a sustained upward (compensatory) eye movement. The early transient was independent of viewing distance and attributed to an artifact, whereas the later component was a linear function of the inverse of the prior viewing distance and attributed to the translational vestibulo-ocular reflex (TVOR). Response latencies for the four nearer viewing distances were determined from the individual eye velocity traces using a computerized algorithm: after removing the initial transient by subtracting the mean response obtained with the most distant viewing, a regression line was fitted to the initial rising phase of the residual response and then extrapolated back to the baseline to determine the onset. When so determined, median latencies for the nearest viewing ranged from 16.4 to 18.5 ms, values appreciably shorter than any in the literature.     

Neural pathways common to vestibular and optokinetic eye movements

Summary To determine how vestibular and optokinetic eye movement signals share the central oculomotor neural organization, optokinetic afternystagmus was superposed on vestibular nystagmus in the monkey. To a first approximation there was algebraic additivity in the velocity domain. This result suggests that vestibular and optokinetic eye movements are integrated at a level of neural organization above the ocular motoneurons, at a level in which neural signals are coded in terms of eye movement velocity rather than eye position.     

Guidance of eye movements during visual conjunction search: local and global contextual effects on target discriminability

The composition of a visual scene influences the ability of humans to select specific details within that scene for discrimination or foveation with saccadic eye movements. With the goal of establishing an animal model to investigate the neural mechanisms underlying the deployment of visual attention and the guidance of saccades during visual search, we studied the visual behavior of three monkeys while they performed a conjunction (color + form) search task similar to those used in human studies. We found that search performance declined when distractors adjacent to the target shared its color, thereby revealing that color was more discriminable than form in these displays and suggesting that monkeys perceptually grouped stimuli by proximity and similarity. Search performance also varied with the overall composition of the display. Most importantly, saccades were biased toward distractors sharing the target color when there were few of them within the display and away from those distractors when they were numerous. Last, the monkeys initiated saccades with a fixed latency, suggesting that their responses to the display were automatic and that search strategies did not involve attentional resources beyond those recruited for regulating saccades. We conclude that monkeys adapt their visual strategies, largely via bottom-up processes, to both the local and the global context of the search. These findings suggest that the visual behavior of monkeys is guided by strategies similar to those observed in humans.     

Delay activity and sensory-motor translation during planned eye or hand movements to visual or tactile targets

To perform eye or hand movements toward a relevant location, the brain must translate sensory input into motor output. Recent studies revealed segregation between circuits for translating visual information into saccadic or manual movements, but less is known about translation of tactile information into such movements. Using human functional magnetic resonance imaging (fMRI) in a delay paradigm, we factorially crossed sensory modality (vision or touch) and motor effector (eyes or hands) for lateralized movements (gaze shifts to left or right or pressing a left or right button with the corresponding left or right hand located there). We investigated activity in the delay-period between stimulation and response, asking whether the currently relevant side (left or right) during the delay was encoded according to sensory modality, upcoming motor response, or some interactive combination of these. Delay activity mainly reflected the motor response subsequently required. Irrespective of visual or tactile input, we found sustained activity in posterior partial cortex, frontal-eye field, and contralateral visual cortex when subjects would later make an eye movement. For delays prior to manual button-press response, activity increased in contralateral precentral regions, again regardless of stimulated modality. Posterior superior temporal sulcus showed sustained delay activity, irrespective of sensory modality, side, and response type. We conclude that the delay activations reflect translation of sensory signals into effector-specific motor circuits in parietal and frontal cortex (plus an impact on contralateral visual cortex for planned saccades), regardless of cue modality, whereas posterior STS provides a representation that generalizes across both sensory modality and motor effector.     

Serial linkage of target selection for orienting and tracking eye movements

Many natural actions require the coordination of two different kinds of movements. How are targets chosen under these circumstances: do central commands instruct different movement systems in parallel, or does the execution of one movement activate a serial chain that automatically chooses targets for the other movement? We examined a natural eye tracking action that consists of orienting saccades and tracking smooth pursuit eye movements, and found strong physiological evidence for a serial strategy. Monkeys chose freely between two identical spots that appeared at different sites in the visual field and moved in orthogonal directions. If a saccade was evoked to one of the moving targets by microstimulation in either the frontal eye field (FEF) or the superior colliculus (SC), then the same target was automatically chosen for pursuit. Our results imply that the neural signals responsible for saccade execution can also act as an internal command of target choice for other movement systems.     

The role of compensatory eye and head movements in the rat for image stabilization and gaze orientation

Compensatory horizontal eye movements of head restrained rats were compared with compensatory horizontal eye-head movements of partially restrained rats (head movements limited to the horizontal plane). Responses were evoked by constant velocity optokinetic and vestibular stimuli (10–60°/s) and recorded with search coils in a rotating magnetic field. Velocity and position components of eye and head responses were analysed. The velocity gains of optokinetic and vestibular responses of partially restrained and of head restrained rats were similarly high (between 0.8 and 1.0). Eye movements in partially restrained rats also contributed most (about 80%) to the velocity components of the responses. At stimulus velocities above 10°/s, the “beating field” of the evoked optokinetic and vestibular nystagmus was shifted transiently in the direction of ocular quick phases. The amplitude of this shift of the line of sight was about 3–10° in head restrained and about 20–30° in partially head restrained rats. Most of this large, transient gaze shift (about 80%) was accomplished by head movements. We interpret this gaze shift as an orienting response, and conclude that the recruitment of the ocular and the neck motor systems can be independent and task specific: head movements are primarily used to orient eye, ear and nose towards a sector of particular relevance, whereas eye movements provide the higher frequency dynamics for image stabilization and vergence movements.     

Maintenance of equilibrium during tracing eye movements

Horizontal tracing movements of eyes modify the type of vertical posture maintenance decreasing the role of the lower segment in the regulation of the position of the pressure center. The relationship between fluctuations of the pressure center in the frontal and sagittal planes increases. Periodicity of eye movements corresponds to fluctuations of the pressure center and these signals were phase shifted relatively to each other. Translated fromByulleten’ Eksperimental’noi Biologii i Meditsiny, Vol. 138, No. 8, pp. 152–158, August, 2004 The study was supported by the Russian Foundation for Basic Research (grant No. 01-04-49489).     

Primate frontal eye fields. II. Physiological and anatomical correlates of electrically evoked eye movements

We studied single neurons in the frontal eye fields of awake macaque monkeys and compared their activity with the saccadic eye movements elicited by microstimulation at the sites of these neurons. Saccades could be elicited from electrical stimulation in the cortical gray matter of the frontal eye fields with currents as small as 10 microA. Low thresholds for eliciting saccades were found at the sites of cells with presaccadic activity. Presaccadic neurons classified as visuomovement or movement were most associated with low (less than 50 microA) thresholds. High thresholds (greater than 100 microA) or no elicited saccades were associated with other classes of frontal eye field neurons, including neurons responding only after saccades and presaccadic neurons, classified as purely visual. Throughout the frontal eye fields, the optimal saccade for eliciting presaccadic neural activity at a given recording site predicted both the direction and amplitude of the saccades that were evoked by microstimulation at that site. In contrast, the movement fields of postsaccadic cells were usually different from the saccades evoked by stimulation at the sites of such cells. We defined the low-threshold frontal eye fields as cortex yielding saccades with stimulation currents less than or equal to 50 microA. It lies along the posterior portion of the arcuate sulcus and is largely contained in the anterior bank of that sulcus. It is smaller than Brodmann's area 8 but corresponds with the union of Walker's cytoarchitectonic areas 8A and 45. Saccade amplitude was topographically organized across the frontal eye fields. Amplitudes of elicited saccades ranged from less than 1 degree to greater than 30 degrees. Smaller saccades were evoked from the ventrolateral portion, and larger saccades were evoked from the dorsomedial portion. Within the arcuate sulcus, evoked saccades were usually larger near the lip and smaller near the fundus. Saccade direction had no global organization across the frontal eye fields; however, saccade direction changed in systematic progressions with small advances of the microelectrode, and all contralateral saccadic directions were often represented in a single electrode penetration down the bank of the arcuate sulcus. Furthermore, the direction of change in these progressions periodically reversed, allowing particular saccade directions to be multiply represented in nearby regions of cortex.(ABSTRACT TRUNCATED AT 400 WORDS)