Sound-localization performance in the cat: the effect of restraining the head

In oculomotor research, there are two common methods by which the apparent location of visual and/or auditory targets are measured, saccadic eye movements with the head restrained and gaze shifts (combined saccades and head movements) with the head unrestrained. Because cats have a small oculomotor range (approximately +/-25 degrees), head movements are necessary when orienting to targets at the extremes of or outside this range. Here we tested the hypothesis that the accuracy of localizing auditory and visual targets using more ethologically natural head-unrestrained gaze shifts would be superior to head-restrained eye saccades. The effect of stimulus duration on localization accuracy was also investigated. Three cats were trained using operant conditioning with their heads initially restrained to indicate the location of auditory and visual targets via eye position. Long-duration visual targets were localized accurately with little error, but the locations of short-duration visual and both long- and short-duration auditory targets were markedly underestimated. With the head unrestrained, localization accuracy improved substantially for all stimuli and all durations. While the improvement for long-duration stimuli with the head unrestrained might be expected given that dynamic sensory cues were available during the gaze shifts and the lack of a memory component, surprisingly, the improvement was greatest for the auditory and visual stimuli with the shortest durations, where the stimuli were extinguished prior to the onset of the eye or head movement. The underestimation of auditory targets with the head restrained is explained in terms of the unnatural sensorimotor conditions that likely result during head restraint.     

Sensorimotor integration in the primate superior colliculus. I. Motor convergence

Orienting movements of the eyes and head are made to both auditory and visual stimuli even though in the primary sensory pathways the locations of auditory and visual stimuli are encoded in different coordinates. This study was designed to differentiate between two possible mechanisms for sensory-to-motor transformation. Auditory and visual signals could be translated into common coordinates in order to share a single motor pathway or they could maintain anatomically separate sensory and motor routes for the initiation and guidance of orienting eye movements. The primary purpose of the study was to determine whether neurons in the superior colliculus (SC) that discharge before saccades to visual targets also discharge before saccades directed toward auditory targets. If they do, this would indicate that auditory and visual signals, originally encoded in different coordinates, have been converted into a single coordinate system and are sharing a motor circuit. Trained monkeys made saccadic eye movements to auditory or visual targets while the activity of visual-motor (V-M) cells and saccade-related burst (SRB) cells was monitored. The pattern of spike activity observed during trials in which saccades were made to visual targets was compared with that observed when comparable saccades were made to auditory targets. For most (57 of 59) V-M cells, sensory responses were observed only on visual trials. Auditory stimuli originating from the same region of space did not activate these cells. Yet, of the 72 V-M and SRB cells studied, 79% showed motor bursts prior to saccades to either auditory or visual targets. This finding indicates that visual and auditory signals, originally encoded in retinal and head-centered coordinates, respectively, have undergone a transformation that allows them to share a common efferent pathway for the generation of saccadic eye movements. Saccades to auditory targets usually have lower velocities than saccades of the same amplitude and direction made to acquire visual targets. Since fewer collicular cells are active prior to saccades to auditory targets, one determinant of saccadic velocity may be the number of collicular neurons discharging before a particular saccade.     

Spatial memory following shifts of gaze. I. Saccades to memorized world-fixed and gaze-fixed targets

During a shift of gaze, an object can move along with gaze or stay fixed in the world. To examine the effect of an object's reference frame on spatial working memory, we trained monkeys to memorize locations of visual stimuli as either fixed in the world or fixed to gaze. Each trial consisted of an initial reference frame instruction, followed by a peripheral visual flash, a memory-period gaze shift, and finally a memory-guided saccade to the location consistent with the instructed reference frame. The memory-period gaze shift was either rapid (a saccade) or slow (smooth pursuit or whole body rotation). This design allowed a comparison of memory-guided saccade performance under various conditions. Our data indicate that after a rotation or smooth-pursuit eye movement, saccades to memorized world-fixed targets are more variable than saccades to memorized gaze-fixed targets. In contrast, memory-guided saccades to world- and gaze-fixed targets are equally variable following a visually guided saccade. Across all conditions, accuracy, latency, and main sequence characteristics of memory-guided saccades are not influenced by the target's reference frame. Memory-guided saccades are, however, more accurate after fast compared with slow gaze shifts. These results are most consistent with an eye-centered representational system for storing the spatial locations of memorized objects but suggest that the visual system may engage different mechanisms to update the stored signal depending on how gaze is shifted.     

Effects of eye position on saccades evoked electrically from superior colliculus of alert cats

Electrical stimulation was carried out in the intermediate and deep gray layers of the superior colliculus in alert cats. The heads of the animals were fixed, and their eye movements were recorded with the scleral search coil method. Stimulation in the anterior two-thirds of the colliculus with long-duration pulse trains produced multiple saccades, as in the primate (45, 51), but their directions and amplitudes were influenced significantly by the initial position of the eye. Stimulation in the posterior part of the colliculus evoked saccades that appeared to be "goal-directed," whereas stimulation at the extreme caudal edge of the colliculus yielded centering saccades. These observations confirm previous reports of Roucoux and Crommelinck (48) and Guitton et al. (24). Saccades evoked during bilateral simultaneous stimulation of the superior colliculi were also dependent on the initial position of the eye. At certain relative intensities of stimulation on the two sides, saccades failed to occur when the eye was within a particular part of the oculomotor range. When the eye was outside this region, the same stimuli triggered an eye movement that drove the eye toward the zone of saccade failure. These findings indicate that saccadic commands resulting from focal collicular stimulation in the cat can be modified by information about current eye position. It is not certain where in the brain this occurs or by what neural mechanisms, but a local feedback model of the saccadic control system (46) can account for the main observations. The functional significance of these findings depends in large measure on the degree to which focal collicular stimulation reproduces naturally occurring patterns of neural activity.     

Express saccades in cat: effects of task and target modality

Saccadic eye movements to visual, auditory, and bimodal targets were measured in four adult cats. Bimodal targets were visual and auditory stimuli presented simultaneously at the same location. Three behavioral tasks were used: a fixation task and two saccadic tracking tasks (gap and overlap task). In the fixation task, a sensory stimulus was presented at a randomly selected location, and the saccade to fixate that stimulus was measured. In the gap and overlap tasks, a second target (hereafter called the saccade target) was presented after the cat had fixated the first target. In the gap task, the fixation target was switched off before the saccade target was turned on; in the overlap task, the saccade target was presented before the fixation target was switched off. All tasks required the cats to redirect their gaze toward the target (within a specified degree of accuracy) within 500 ms of target onset, and in all tasks target positions were varied randomly over five possible locations along the horizontal meridian within the cat's oculomotor range. In the gap task, a significantly greater proportion of saccadic reaction times (SRTs) were less than 125 ms, and mean SRTs were significantly shorter than in the fixation task. With visual targets, saccade latencies were significantly shorter in the gap task than in the overlap task, while, with bimodal targets, saccade latencies were similar in the gap and overlap tasks. On the fixation task, SRTs to auditory targets were longer than those to either visual or bimodal targets, but on the gap task, SRTs to auditory targets were shorter than those to visual or bimodal targets. Thus, SRTs reflected an interaction between target modality and task. Because target locations were unpredictable, these results demonstrate that cats, as well as primates, can produce very short latency goal-directed saccades.     

Discharge patterns of neurons in the rostral superior colliculus of cat: activity related to fixation of visual and auditory targets

Neurons in the rostral superior colliculus (SC) of alert cats exhibit quasi-sustained discharge patterns related to the fixation of visual targets. Because some SC neurons also respond to auditory stimuli, we investigated whether there is a population of neurons in the rostral SC which is active in relation to fixation of both auditory and visual targets. We identified cells which were active with visual fixation and which continued to discharge if the fixation stimulus was briefly extinguished. The population of neurons exhibited similar discharge characteristics when the fixation stimulus was auditory. Few neurons were significantly more active during fixation of visual targets than during fixation of auditory targets. Most fixation neurons showed a diminished discharge rate during spontaneous (self-generated) saccadic eye movements away from a visual fixation stimulus, regardless of the direction of the saccade. this diminished discharge rate (or pause) typically began, on average, 12.2 ms before saccade onset and the duration of the pause was Ionger than the duration of the saccade. These observations are consistent with the hypothesis that increased discharge of these neurons is related to active fixation and that reductions in their activity are important for the generation of saccades. However, the lack of a precise relationship between pause duration and saccade duration implies that these neurons would be unlikely to project directly to the saccadic burst generator. The mean interval from the beginning of the pauses of fixation neurons to be beginning of the saccades away from fixation targets is also shorter than has been found in brainstem omnipause neurons. By analogy with the concept of a receptive field, agaze position error field depicts the range of gaze position error for which a cell is active. Although fixation neurons appear to encode the magnitude and direction of the error between visual targets and the visual axis, visual error fields at the end of fixating eye movements were significantly larger than those at stimulus onset. For auditory stimuli, this difference was not significant. These observations are compatible with a number of recent experiments indicating that neural signals of eye position are damped or delayed with respect to current eye position.     

Frontal eye field contributions to rapid corrective saccades

Visually guided movements can be inaccurate, especially if unexpected events occur while the movement is programmed. Often errors of gaze are corrected before external feedback can be processed. Evidence is presented from macaque monkey frontal eye field (FEF), a cortical area that selects visual targets, allocates attention, and programs saccadic eye movements, for a neural mechanism that can correct saccade errors before visual afferent or performance monitoring signals can register the error. Macaques performed visual search for a color singleton that unpredictably changed position in a circular array as in classic double-step experiments. Consequently, some saccades were directed in error to the original target location. These were followed frequently by unrewarded, corrective saccades to the final target location. We previously showed that visually responsive neurons represent the new target location even if gaze shifted errantly to the original target location. Now we show that the latency of corrective saccades is predicted by the timing of movement-related activity in the FEF. Preceding rapid corrective saccades, the movement-related activity of all neurons began before explicit error signals arise in the medial frontal cortex. The movement-related activity of many neurons began before visual feedback of the error was registered and that of a few neurons began before the error saccade was completed. Thus movement-related activity leading to rapid corrective saccades can be guided by an internal representation of the environment updated with a forward model of the error.     

Predictive remapping of attention across eye movements

Many cells in retinotopic brain areas increase their activity when saccades (rapid eye movements) are about to bring stimuli into their receptive fields. Although previous work has attempted to look at the functional correlates of such predictive remapping, no study has explicitly tested for better attentional performance at the future retinal locations of attended targets. We found that, briefly before the eyes start moving, attention drawn to the targets of upcoming saccades also shifted to those retinal locations that the targets would cover once the eyes had moved, facilitating future movements. This suggests that presaccadic visual attention shifts serve to both improve presaccadic perceptual processing at the target locations and speed subsequent eye movements to their new postsaccadic locations. Predictive remapping of attention provides a sparse, efficient mechanism for keeping track of relevant parts of the scene when frequent rapid eye movements provoke retinal smear and temporal masking.     

Response characteristics of neurons in the pulvinar of awake cats to saccades and to visual stimulation

We studied responses of pulvinar neurons in awake cats that were allowed to execute spontaneous eye movements. Extracellular cell activity during saccades, saccade-like image shifts, and various stationary visual stimuli was recorded together with the animals' eye positions. All neurons analyzed had receptive fields that covered most of the central 80x80 degrees of the animals' visual field and did only respond to large (>20 degrees) visual stimuli. According to their response properties, recorded neurons were divided into three populations. The first group, termed "S neurons" (16%), responded when the animals performed saccades but were unresponsive to any of the visual stimuli tested. These neurons do not seem to receive a visual input that is strong enough to drive them. The second group, termed "V neurons" (51%), responded to various visual stimuli including saccade-like image motion when the eyes were stationary, but not when the animals executed saccades. V neurons therefore distinguish retinal image movements that are generated externally from internally generated image motion. Finally, "SV neurons" (31%) responded when the animals made saccades as well as to saccade-like image motion or to stationary stimuli. Although these neurons do not distinguish self-induced retinal image motion from motion generated by external stimulus movements, they must receive non-retinal motion-related input, because responses elicited by saccades had shorter latencies than responses to saccade-like stimulus movements. Only SV neurons resemble response properties of pretectal neurons that project to the pulvinar and that comprise the major subcortical visual input. The functional significance of pulvinar neuronal populations for visual and visuomotor information processing is discussed.     

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. Received: 24 April 1996 / Accepted: 25 October 1996     

Stimulation of the caudate nucleus induces contraversive saccadic eye movements as well as head turning in the cat.

The effects of stimulation of the caudate nucleus were investigated in alert cats, with special reference to the induction of eye and head movements. Stimulation of caudal portions of the caudate nucleus on one side with trains of current pulses induced gaze shifts towards the contralateral side. When the head of the animal was restrained, the majority of evoked eye movements were single conjugate saccades. The amplitude and direction of the evoked saccade varied depending on the initial eye position. The amplitude of the horizontal component tended to be larger for saccades initiated from more ipsilateral positions, and became gradually smaller as the initial eye position shifted to the contralateral side. If the eye was far into the contralateral positions, no saccades were induced. Furthermore, the saccades tended to have a downward component when the eye was initially focused upward, and an upward component when the eye was focused downward. When the head was made free to move, the same stimulation induced a sequence of contraversive staircase gaze shifts composed of coordinated eye and head movements. The eye movements in the orbit resembled nystagmus, consisting of contraversive saccades followed by reverse compensatory movements. The head turning, though smooth and continuous, was also suggested to consist of a series of movements coupled with saccadic eye movements. This study indicates a potential role of the caudate nucleus in the control of orienting reflexes.     

Smooth-pursuit eye movements elicited by first-order and second-order motion

 The perception of the displacement of luminance-defined contours (i.e., first-order motion) is an important and well-examined function of the visual system. It can be explained, for example, by the operation of elementary motion detectors (EMDs), which cross-correlate the spatiotemporal luminance distribution. More recent studies using second-order motion stimuli, i.e., shifts of the distribution of features such as contrast, texture, flicker, or motion, extended classic concepts of motion perception by including nonlinear or hierarchical processing in the EMD. Smooth-pursuit eye movements can be used as a direct behavioral probe for motion processing. The ability of the visual system to extract motion signals from the spatiotemporal changes of the retinal image can be addressed by analyzing the elicited eye movements. We measured the eye movement response to moving objects defined by two different types of first-order motion and two different types of second-order motion. Our results clearly showed that the direction of smooth-pursuit eye movements was always determined by the direction of object motion. In particular, in the case of second-order motion stimuli, smooth-pursuit did not follow the retinal image motion. The latency of the initial saccades during pursuit of second-order stimuli was slightly but significantly increased, compared with the latency of saccades elicited by first-order motion. The processing of second-order motion in the peripheral visual field was less exact than the processing of first-order motion in the peripheral field. Steady state smooth-pursuit eye speed did not reflect the velocity of second-order motion as precisely as that of first-order motion, and the resulting retinal error was compensated by saccades. Interestingly, for slow second-order stimuli we observed that the eye could move faster than the target, leading to small, corrective saccades in the opposite direction to the ongoing smooth-pursuit eye movement. We conclude from our results that both visual perception and the control of smooth-pursuit eye movements have access to processing mechanisms extracting first- and second-order motion. Received: 26 August 1996 / Accepted: 8 November 1996     

The dorsomedial frontal cortex of the macaca monkey: fixation and saccade-related activity

Summary The activity of 249 neurons in the dorsomedial frontal cortex was studied in two macaque monkeys. The animals were trained to release a bar when a visual stimulus changed color in order to receive reward. An acoustic cue signaled the start of a series of trials to the animal, which was then free to begin each trial at will. The monkeys tended to fixate the visual stimuli and to make saccades when the stimuli moved. The monkeys were neither rewarded for making proper eye movements nor punished for making extraneous ones. We found neurons whose discharge was related to various movements including those of the eye, neck, and arm. In this report, we describe the properties of neurons that showed activity related to visual fixation and saccadic eye movement. Fixation neurons discharged during active fixation with the eye in a given position in the orbit, but did not discharge when the eye occupied the same orbital positions during nonactive fixation. These neurons showed neither a classic nor a complex visual receptive field, nor a foveal receptive visual field. Electrical stimulation at the site of the fixation neurons often drove the eye to the orbital position associated with maximal activity of the cell. Several different kinds of neurons were found to discharge before saccades: 1) checking-saccade neurons, which discharged when the monkeys made self-generated saccades to extinguish LED's; 2) novelty-detection saccade neurons, which discharged before the first saccade made to a new visual target but whose activity waned with successive presentations of the same target. These results suggest that the dorsomedial frontal cortex is involved in attentive fixation. We hypothesize that the fixation neurons may be involved in codifying the saccade toward a target. We propose that their involvement in arm-eye-head motor-planning rests primarily in targeting the goal of the movement. The fact that saccaderelated neurons discharge when the saccades are self initiated, implies that this area of the cortex may share the control of voluntary saccades with the frontal eye fields and that the activation is involved in intentional motor processes.     

Effect of eye position on saccades and neuronal responses to acoustic stimuli in the superior colliculus of the behaving cat

We examined the motor error hypothesis of visual and auditory interaction in the superior colliculus (SC), first tested by Jay and Sparks in the monkey. We trained cats to direct their eyes to the location of acoustic sources and studied the effects of eye position on both the ability of cats to localize sounds and the auditory responses of SC neurons with the head restrained. Sound localization accuracy was generally not affected by initial eye position, i.e., accuracy was not proportionally affected by the deviation of the eyes from the primary position at the time of stimulus presentation, showing that eye position is taken into account when orienting to acoustic targets. The responses of most single SC neurons to acoustic stimuli in the intact cat were modulated by eye position in the direction consistent with the predictions of the "motor error" hypothesis, but the shift accounted for only two-thirds of the initial deviation of the eyes. However, when the average horizontal sound localization error, which was approximately 35% of the target amplitude, was taken into account, the magnitude of the horizontal shifts in the SC auditory receptive fields matched the observed behavior. The modulation by eye position was not due to concomitant movements of the external ears, as confirmed by recordings carried out after immobilizing the pinnae of one cat. However, the pattern of modulation after pinnae immobilization was inconsistent with the observations in the intact cat, suggesting that, in the intact animal, information about the position of the pinnae may be taken into account.     

Interactions between eye movement systems in cats and humans

Eye movements can be broadly classified into target-selecting and gaze-stabilizing eye movements. How do the different systems interact under natural conditions? Here we investigate interactions between the optokinetic and the target-selecting system in cats and humans. We use combinations of natural and grating stimuli. The natural stimuli are movies and pictures taken from the cat’s own point of view with a head-mounted camera while it moved about freely in an outdoor environment. We superimpose linear global motion on the stimuli and use measurements of optokinetic nystagmus as a probe to study the interaction between the different systems responsible for controlling eye movements. Cats display higher precision stabilizing eye movements in response to natural pictures as compared to drifting gratings. In contrast, humans perform similarly under these two conditions. This suggests an interaction of the optokinetic and the pursuit system. In cats, the natural movies elicit very weak optokinetic responses. In humans, by contrast, the natural movie stimuli elicit effectively stabilizing eye movements. In both species, we find a unimodal distribution of saccades for all stimulus velocities. This suggests an early interaction of target-selecting and gaze-stabilizing saccades. Thus, we argue for a more integrated view in humans of the different eye movement systems.     

Primate frontal eye fields. I. Single neurons discharging before saccades

We studied the activity of single neurons in the frontal eye fields of awake macaque monkeys trained to perform several oculomotor tasks. Fifty-four percent of neurons discharged before visually guided saccades. Three different types of presaccadic activity were observed: visual, movement, and anticipatory. Visual activity occurred in response to visual stimuli whether or not the monkey made saccades. Movement activity preceded purposive saccades, even those made without visual targets. Anticipatory activity preceded even the cue to make a saccade if the monkey could reliably predict what saccade he had to make. These three different activities were found in different presaccadic cells in different proportions. Forty percent of presaccadic cells had visual activity (visual cells) but no movement activity. For about half of the visual cells the response was enhanced if the monkey made saccades to the receptive-field stimulus, but there was no discharge before similar saccades made without visual targets. Twenty percent of presaccadic neurons discharged as briskly before purposive saccades made without a visual target as they did before visually guided saccades, and had weak or absent visual responses. These cells were defined as movement cells. Movement cells discharged much less or not at all before saccades made spontaneously without a task requirement or an overt visual target. The remaining presaccadic neurons (40%) had both visual and movement activity (visuomovement cells). They discharged most briskly before visually guided eye movements, but also discharged before purposive eye movements made in darkness and responded to visual stimuli in the absence of saccades. There was a continuum of visuomovement cells, from cells in which visual activity predominated to cells in which movement activity predominated. This continuum suggests that although visual cells are quite distinct from movement cells, the division of cell types into three classes may be only a heuristic means of describing the processing flow from visual input to eye-movement output. Twenty percent of visuomovement and movement cells, but fewer than 2% of visual cells, had anticipatory activity. Only one cell had anticipatory activity as its sole response. When the saccade was delayed relative to the target onset, visual cells responded to the target appearance, movement cells discharged before the saccade, and visuomovement cells discharged in different ways during the delay, usually with some discharge following the target and an increase in rate immediately before the saccade. Presaccadic neurons of all types were actively suppressed following a saccade into their response fields.(ABSTRACT TRUNCATED AT 400 WORDS)     

Electrophysiological correlates of human intrasaccadic processing

Visual discrimination performance is thought to be suppressed during saccades in order to contribute to space constancy. However, under certain experimental conditions, visual inhibition may not take place, suggesting a more complex underlying mechanism. We tested the discrimination ability of 20 healthy subjects during visually guided horizontal saccades and recorded simultaneously the evoked brain activity from 30 channels over occipital, parietal, and temporal areas. During the execution of saccadic eye movements, visual stimuli were presented for 30 ms. In order to prevent retinal afterimages, stimuli were followed by a visual mask. In a control condition, the same stimuli were presented with stationary eyes. Electro-oculogram (EOG) and electroencephalogram (EEG) signals were recorded continuously together with information about the stimuli and the subject's response. Evoked potentials were computed offline, and component latency, field strength (global field power), and topography were compared between conditions. During saccades, subjects showed only slightly reduced discrimination performance which remained very high above the chance level; thus, there was no evidence for strong saccadic suppression with the supra-threshold stimuli employed. However, the cortical activation patterns exhibited large alteration when a physically identical stimulus was presented during the eye movement: around 130 ms latency, field strength was significantly smaller than when stationary targets were processed, and scalp topography was also different. These effects on evoked field distributions may be attributed to neural interactions of an efference copy signal (linked to the oculomotor command) with the afferent excitation following the visual stimulus.     

Response properties of relay cells in the A-laminae of the cat's dorsal lateral geniculate nucleus after saccades

Responses of relay cells in the A-laminae of the dorsal lateral geniculate nucleus (LGNd) during spontaneous saccades and saccade-like visual stimulation were extracellularly recorded in awake cats. Ninety-six out of 137 cells recorded (42 X and 54 Y cells) were responsive during spontaneous saccadic eye movements. All Y cells and 67% of the X cells responded with burst activity, i.e. with either one or two activity peaks during and after saccades. Thirty-three percent of the X cells were inhibited during saccades. Excitatory peaks occurred at mean latencies of 33 ms and 31 ms for X and Y cells, respectively. Comparable burst responses were obtained when retinal image shifts similar to those during saccades were induced by external saccade-like stimulus movements. However, the latencies of excitatory peak activity were significantly longer to external stimuli than to the onsets of saccades. This indicates the existence of an eye movement-related input which activates LGNd relay cells in addition to the visual input. We propose that the pretectogeniculate projection may contribute to the responses of LGNd relay cells following saccadic eye movements via a disinhibitory input and that this input could be involved in intra- and postsaccadic modulations of the transfer of visual signals to visual cortex.     

The location marker effect. Saccadic latency increases with target eccentricity

Previous studies have shown that the retinal eccentricity of target stimuli has surprisingly little effect on the latency of visually driven saccades. But up until now researchers have addressed this issue by presenting saccadic targets in an unstructured visual field. This contrasts with everyday vision in which eye movements are initiated to stimuli within a cluttered environment. The present experiment compared latencies for target onsets in an empty visual field with a condition in which continuously illuminated location markers "tagged" the possible target locations. Previous reports of no effect of eccentricity on latencies in an unstructured field were replicated. However, a significant effect of eccentricity was found when location markers were used. Interestingly this did not reflect a lengthening of latencies as would be predicted by a reduction in target discriminability. Instead, latencies were relatively facilitated to near-visual onsets in the location marker condition. It is concluded that under more natural viewing conditions the latency of saccades is likely to be modulated by the eccentricity of target stimuli. This effect can be explained by competitive attentional interactions in saccade target selection processes.     

The influence of vertically and horizontally aligned visual distractors on aurally guided saccadic eye movements

Eye movements towards a new target can be guided or disrupted by input from multiple modalities. The degree of oculomotor competition evoked by a distractor depends on both distractor and target properties, such as distractor salience or certainty regarding the target location. The ability to localize the target is particularly important when studying saccades made towards auditory targets, since determination of elevation and azimuth of a sound are based on different processes, and these processes may be affected independently by a distractor. We investigated the effects of a visual distractor on saccadic eye movements made to an auditory target in a two-dimensional plane. Results showed that the competition evoked by a vertical visual distractor was stronger compared with a horizontal visual distractor. The eye movements that were not captured by the vertical visual distractor were still influenced by it: a deviation of endpoints was seen in the direction of the visual distractor. Furthermore, the interference evoked by a high-contrast visual distractor was stronger compared with low-contrast visual stimuli, which was reflected by a faster initiation of an eye movement towards the high-contrast visual distractor and a stronger shift of endpoints in the direction of the high-contrast visual distractor. Together, these findings show that the influence of a visual distractor on aurally guided eye movements depends strongly on its location relative to the target, and to a lesser extent, on stimulus contrast.