Control of fixation and saccades during an anti-saccade task: an investigation in humans with chronic lesions of oculomotor cortex

Fourteen patients with a chronic, unilateral lesion restricted to the frontal lobe (twelve involving the frontal eye field (FEF)), nine patients with a chronic, unilateral lesion restricted to posterior association cortex (eight involving the intraparietal sulcus (IPS)), and twelve neurologically normal control subjects were studied in an anti-saccade task. A combination of manipulating cuing and fixation offset enabled us to examine the effects of chronic oculomotor lesions on both saccade preparation and voluntary control over ocular fixation. Patients with lesions of the FEF made more errors (reflexive glances) toward contralesional targets, whereas patients with IPS lesions made fewer errors toward contralesional targets. Patients with IPS lesions had increased latencies to initiate saccades away from contralesional targets. For FEF patients, the presence of a fixation point inhibited the initiation of contralesionally directed saccades less than those directed ipsilesionally. Saccade preparation in response to a cue did not reduce the inhibitory effect of a fixation point on initiating anti-saccades directed either ipsilesionally or contralesionally for either patient group. We conclude that chronic IPS lesions result in a reduced contralesional visual grasp reflex (VGR) and delayed utilization of visual signals in the contralesional field for planning voluntary eye movements. In contrast, patients with chronic FEF lesions are impaired in inhibiting the VGR toward contralesional signals, and manifest an asymmetry in the balance between fixation and saccade activity. Moreover, voluntary control of fixation is compromised after chronic damage to either frontal or parietal oculomotor cortex.     

Are somatosensory saccades voluntary or reflexive?

The present study examines whether the distinction between voluntary (endogenous) and reflexive (stimulus-elicited) saccades made in the visual modality can be applied to the somatosensory modality. The behavioural characteristics of putative reflexive pro-saccades and voluntary anti-saccades made to visual and somatosensory stimuli were examined. Both visual and somatosensory pro-saccades had much shorter latency than voluntary anti-saccades made in the direction opposite to a peripheral stimulus. Furthermore, erroneous pro-saccades were made towards both visual and somatosensory stimuli on approximately 11-13% of anti-saccade trials. The observed difference in pro- and anti-saccade latency and the presence of pro-saccade errors in the anti-saccade task indicates that a somatosensory stimulus can elicit a form of reflexive saccade comparable to pro-saccades made in the visual modality. It is proposed that a peripheral somatosensory stimulus can elicit a form of reflexive saccade and that somatosensory saccades do not depend exclusively on higher level endogenous control processes for their generation. However, a comparison of the underlying latency distributions and of peak-velocity profiles of saccades made to visual and somatosensory stimuli showed that this distinction may be less clearly defined for the somatosensory modality and that modality-specific differences (such as differences in neural conduction rates) in the underlying oculomotor structures involved in saccade target selection also need to be considered. It is further suggested that a broader conceptualisation of saccades and saccade programming beyond the simple voluntary and reflexive dichotomy, that takes into account the control processes involved in saccade generation for both modalities, may be required.     

Curved saccade trajectories: Voluntary and reflexive saccades curve away from irrelevant distractors

In this study we examined the impact of irrelevant distractors upon trajectories of reflexive and voluntary saccades. Observers made saccades to visual targets above and below fixation as directed by target appearance (reflexive) or by a central directional cue (voluntary) in the presence of an irrelevant distractor stimulus (a cross) whose appearance was simultaneous with target onset. We recorded saccade latency, amplitude and the magnitude of saccade curvature relative to the direct route from the start-to-end of the saccade. Previous studies of saccades curvature have used distractors to provide information about the saccade task and, as a result, have only examined trajectories of voluntary saccades. However, we have shown that both reflexive and voluntary saccades curved away from irrelevant distractors. The effect of irrelevant distractors indicates that observers do not need to attend to distractors in a voluntary fashion for distractors to modify saccade trajectories. Furthermore, it highlights an important parallel in curvature of saccades and reach trajectories, namely that both curve away from irrelevant distractors. The second important observation was that reflexive, as well as voluntary, saccades curved away from distractors. This suggests that curvature is not solely a consequence of voluntary control. These results have been considered within the context of inhibition-based theories of curvature derived from studies of saccade and manual reach trajectories. Electronic Publication     

Effects of age on latency and error generation in internally mediated saccades

Most studies on the effects of ageing on saccades have examined reflexive saccades; the only commonly studied volitional task has been the antisaccade task, with contradictory results. We examined in both young and elderly normal subjects the latency of anti-, memory-guided, and predictable saccades and the timing of self-paced saccades; we also evaluated errors made on the first two tasks. We expected errors to be correlated between tasks; we also expected antisaccade latencies and errors to be inversely correlated. We also expected antisaccade and memory-guided saccade latencies to be longer in individuals with a high self-paced rate. Except for predictable saccades, mean latencies were significantly higher in the elderly. However, their performance was more variable. Errors were also significantly more frequent on anti- and memory-guided saccade tasks. Most of the hypothesised correlations were not observed. Analysis of error latencies showed that whilst most antisaccade errors were reflexive, for memory-guided saccades both express errors and errors with latencies between 0.4 and 2.5 s were observed. The latter appeared to be a premature release of what would otherwise have been a properly planned response. Age thus impaired all but the predictable saccade task; nevertheless, there were few relationships between measures across tasks. This suggests that a range of processes mediate peoples' performance on these saccade paradigms.     

Multisensory interactions in saccade target selection: Curved saccade trajectories

In a series of experiments, we examined the change in saccade trajectories observed when distractors are presented at non-target locations. The primary aim of the experiments was to examine multisensory interaction effects between the visual, auditory and somatosensory modalities in saccade generation. In each experiment observers made saccades to visual targets above and below fixation in the presence of visual, auditory or tactile stimuli to the left or right of fixation. In experiment 1 distractor location indicated which of two stimuli was the target for the saccade. Saccade trajectories showed strong leftward curvature following right-side distractors and showed rightward curvature following left-side distractors. The largest effects on trajectories were observed for visual distractors, but significant curvature was observed with auditory and somatosensory distractors. In experiment 2 saccades were made following the onset of a visual target (reflexive) or following presentation of an arrow at fixation (voluntary), and task-irrelevant crossmodal distractors were presented simultaneously with target onset. Both voluntary and reflexive saccades were found to curve away from task-irrelevant visual distractors, but auditory and somatosensory distractors did not modulate saccade trajectories. In experiment 3 task-irrelevant distractors preceded the onset of the target by 100 ms. Reflexive saccades were found to curve away from visual, auditory and somatosensory distractors, but voluntary saccades curved away from visual distractors only. The modulation of saccade trajectories by distractors from different modalities is interpreted in terms of inhibitory processes operating in neural structures involved in saccade generation. Our findings suggest that visual, auditory and somatosensory distractors can all modulate saccade trajectories. Such effects could be related to the inhibition of populations of neurons, in a common motor map, for the selection of a saccade target.     

Control of voluntary and reflexive saccades

The latency of 'reflexive' saccades (made in response to peripheral visual stimuli) was compared to that of 'voluntary' saccades performed in anti-saccade and symbolically cued paradigms. Manipulation of visual events at fixation was carefully controlled across all conditions. Reflexive saccade latency was significantly faster than the latency of all forms of voluntary saccades. Importantly, the latency of saccades made after presentation of a symbolic cue at central fixation (voluntary arrow-cue condition) was greater than that made in the anti-saccade paradigm that requires suppression of a reflexive response. It is suggested that the increase in latency of saccades made in the voluntary arrow-cue condition may reflect differences in programming a 'When' trigger signal for saccades made in the absence of a peripheral stimulus.     

Saccades to the seeing visual hemifield in hemidecorticate patients exhibit task-dependent reaction times and hypometria

In three patients who had one cortical hemisphere removed surgically (hemidecortication), we studied visually-triggered saccades directed contralateral to the intact cortical hemisphere (i.e., ipsilesional saccades). Both saccade reaction times (SRTs) and accuracy of these saccades have been reported as abnormal in hemidecorticate patients, but not monkeys. One explanation for this difference is that deficits in hemidecorticate patients may not have been directly caused by removal of cortical oculomotor structures themselves, but may have been a manifestation of compensatory strategies used to cope with contralesional hemianopia. We hypothesized that deficits in saccade performance to the ipsilesional (seeing) visual hemifield would be directly linked to how easily patients could localize targets in their blind hemifield with searching saccades. To test this hypothesis, we examined how deficits in our patients varied when targets were: (1) randomly presented to either the seeing or blind hemifield for long durations thereby permitting searching saccades in the blind hemifield; (2) presented as in Experiment 1, but briefly flashed thereby removing visual feedback prior to saccade onset thereby rendering searching saccades useless; (3) briefly flashed as in Experiment 2, but at random locations in only the seeing hemifield (blind hemifield irrelevant). Mean SRTs to the seeing hemifield were 165 ms longer than normal in Experiment 2, but only about 40 ms longer in Experiments 1 and 3. Saccade accuracy was characterized by task-dependent hypometria in all three experiments with a mean undershoot of about twice the amplitude variance. The largest undershoots were in Experiments 2 and 3. Our data suggest that deficits resulted from the direct effects of the lesions themselves coupled with context-dependent strategies used to cope with contralesional hemianopia.     

Effect of unilateral cerebral cortical lesions on ocular motor behavior in monkeys: saccades and quick phases

Saccades and quick phases of vestibular and optokinetic nystagmus were quantitated using the magnetic-field search coil technique before and during 1 yr after unilateral decortication in three rhesus monkeys. Saccades were examined during several different behavioral conditions: spontaneous saccades made in the light and in the dark; intentional saccades including visually guided saccades to a target light, predictive saccades to a target light stepped to a predictable location, and target-searching saccades when the monkey was rewarded to find and fixate the target light located in the defective visual hemifield; and reflexive saccades made to novel visual, auditory, and tactile stimuli. Quick phases of nystagmus and spontaneous saccades could be initiated immediately postoperatively, although those initiated away from the side of the lesion were reduced in amplitude and rarely moved the eyes into contralateral craniotopic space. Intentional and reflexive saccades could not be initiated away from the side of the lesion during the first postoperative week. Visually guided saccades and reflexive saccades to stationary or moving visual stimuli in the defective visual hemifield never recovered. Target-searching and predictive saccades directed away from the lesioned side recovered but were generated in a staircase pattern; those saccades from orbital positions further into craniotopic space on the side opposite the lesion had progressively higher latencies and smaller amplitudes. The amplitudes of visually guided saccades to targets stepped into the normal visual hemifield were increased in amplitude by approximately 15% but slowly returned to near preoperative values by 20 wk. Pure vertical visually guided saccades to targets stepped or moved in the vertical direction were not generated throughout the postoperative period. Instead, the animals generated oblique saccades, tilted 10-15 degrees toward the side of the lesion. Velocities of saccades and quick phases were significantly reduced at all amplitudes both away from (approximately 37%) and toward (approximately 22%) the side of the lesion. This deficit diminished with time but velocities were still low one yr postoperatively. Our results suggest that cortical areas in one hemisphere are involved in the initiation of contralaterally directed intentional and reflexive saccades but not in the initiation of spontaneous saccades or quick phases. In time, other structures can initiate contralaterally directed intentional and reflexive saccades, except those guided by vision.(ABSTRACT TRUNCATED AT 400 WORDS)     

The Saccadic system more readily co-processes orthogonal than co-linear saccades

Real-life visual tasks such as tracking jumping objects and scanning visual scenes often require a sequence of saccadic eye movements. The ability of the ocular motor system to parallel process saccades has been previously demonstrated. We recorded the monocular eye movements of five normal human subjects using the magnetic search coil technique in a double step paradigm. Initial target jumps were always purely horizontal or purely vertical. We were interested in the latency to onset of the second saccade as a function of direction in relation to the first saccade. When the inter stimulus interval (ISI) was 150 or 180 ms orthogonal second saccades were of significantly shorter latency than second co-linear saccades. When the ISI was 250 ms the latencies of orthogonal and co-linear second saccades were statistically indistinguishable. Based on these findings it is postulated that the ocular motor system can more readily co-process orthogonal than co-linear saccades.     

The effect of frontal eye field and superior colliculus lesions on saccadic latencies in the rhesus monkey

Rhesus monkeys were trained to make saccadic eye movements to visual targets using detection and discrimination paradigms in which they were required to make a saccade either to a solitary stimulus (detection) or to that same stimulus when it appeared simultaneously with several other stimuli (discrimination). The detection paradigm yielded a bimodal distribution of saccadic latencies with the faster mode peaking around 100 ms (express saccades); the introduction of a pause between the termination of the fixation spot and the onset of the target (gap) increased the frequency of express saccades. The discrimination paradigm, on the other hand, yielded only a unimodal distribution of latencies even when a gap was introduced, and there was no evidence for short-latency "express" saccades. In three monkeys either the frontal eye field or the superior colliculus was ablated unilaterally. Frontal eye field ablation had no discernible long-term effects on the distribution of saccadic latencies in either the detection or discrimination tasks. After unilateral collicular ablation, on the other hand, express saccades obtained in the detection paradigm were eliminated for eye movements contralateral to the lesion, leaving only a unimodal distribution of latencies. This deficit persisted throughout testing, which in one monkey continued for 9 mo. Express saccades were not observed again for saccades contralateral to the lesion, and the mean latency of the contralateral saccades was longer than the mean latency of the second peak for the ipsiversive saccades. The latency distribution of saccades ipsiversive to the collicular lesion was unaffected except for a few days after surgery, during which time an increase in the proportion of express saccades was evident. Saccades obtained with the discrimination paradigm yielded a small but reliable increase in saccadic latencies following collicular lesions, without altering the shape of the distribution. Unilateral muscimol injections into the superior colliculus produced results similar to those obtained immediately after collicular lesions: saccades contralateral to the injection site were strongly inhibited and showed increased saccadic latencies. This was accompanied by a decrease of ipsilateral saccadic latencies and an increase in the number of saccades falling into the express range. The results suggest that the superior colliculus is essential for the generation of short-latency (express) saccades and that the frontal eye fields do not play a significant role in shaping the distribution of saccadic latencies in the paradigms used in this study.(ABSTRACT TRUNCATED AT 400 WORDS)     

The effect of transcranial magnetic stimulation on the latencies of vertical saccades

In this study, we investigated the effect of transcranial magnetic stimulation (TMS) over the right posterior parietal cortex (PPC) on the latency of two different types of visually-guided vertical saccades: reflexive saccades triggered by the sudden onset of a target, and saccades towards target locations known in advance. For this reason, we used two oculomotor tasks: a gap and a delay task, respectively. Nine normal subjects performed vertical saccades at ±7.5 and ±15°. TMS was applied at 80 and 100 ms after target onset in the gap task, and after fixation offset in the delay task. Without TMS, we confirmed a latency asymmetry in the gap task favouring upward saccades at the lower eccentricity (7.5°), and a latency symmetry in the delay task. TMS increased the latencies of all saccades in the delay task, when delivered at 100 ms. This effect was mostly pronounced for downward saccades at 7.5°. As a result, saccade latencies showed an asymmetry in this condition, similar to the one observed in the gap task without TMS. The gap task with TMS resulted in a variable latency distribution and no significant overall effect on saccade latency. Our results indicate that the right PPC is involved in the initiation of vertical saccades in the delay task, and that this involvement appears to be enhanced for downward saccades. A conclusion for the involvement of this area in the gap task could not be drawn from this study.     

Control of fixation and saccades in humans with chronic lesions of oculomotor cortex

To elucidate the dynamic interactions of cortical and subcortical oculomotor systems, the authors investigated reflexive and strategic control over fixation and saccades in patients with chronic unilateral lesions that involved either frontal or parietal cortex. They measured the effects of indicating the location of the forthcoming target and removing the fixation stimulus on the latencies of eye movements toward a peripheral visual target in 12 patients with frontal eye field (FEF) lesions, 9 patients with lesions restricted to parietal cortex, and 12 neurologically healthy controls. They found that chronic damage to FEF cortex disrupts cortico-collicular interactions, resulting in hypoactivity in the contralesional superior colliculus and a loss of strategic control over the intrinsic collicular circuits that regulate fixation.     

Bilateral interactions in saccade programming

Subjects were required to make a saccade to a target appearing randomly 4° to the left or right of the current fixation position (1280 trials per experiment). Location cues were used to direct visual attention and start saccade preparation to one of the two locations before target onset. When the cue indicated the target location (valid trials), the generation of express saccades (visually guided saccades with latencies around 100 ms) was strongly facilitated. When the opposite location was cued (invalid trials), express saccades were abolished and replaced by a population of mainly fast-regular saccades (latencies around 150 ms). This was found with a peripheral cue independently of whether the fixation point was removed before target onset (gap condition; experiment 1) or remained on throughout the trial (overlap condition; experiment 2). The same pattern also was observed with a central cue that did not involve any visual stimulation at a peripheral location (experiment 3). In the case where the primary saccade was executed in response to the cue and the target appeared at the opposite location, continuous amplitude transition functions were observed: starting at about 60–70 ms from target onset onward, the amplitude of the cue-elicited saccades continuously decreased from 4° to values below 1°. The results are explained by a fixation-gating model, according to which the antagonism between fixation and saccade activity gives rise to multimodal distributions of saccade latencies. It is argued that allocation of visual attention and saccade preparation to one location entails a successive disengagement of the fixation system controlling saccade preparation within the hemifield to which the saccade is prepared and a partial engagement of the opposite fixation system.     

Deficits of cortical oculomotor mechanisms in cerebellar atrophy patients

Commonly, the cerebellum is not associated with cortical components of saccadic eye movement programming. The present study investigates cerebellar effects on visually guided saccades in reflexive tasks (step, gap, overlap) and on internally driven saccades in intentional tasks (anti, memory, short memory sequences of four targets) in five patients with isolated cerebellar atrophy. The cerebellar dysfunction led to impairments in both reflexive and intentional saccades. Cerebellar atrophy patients showed an increase in the gain variability and an increase in the saccade latency. Furthermore, in the memory and anti task, suppression and pro-saccade errors were more frequent in the atrophy group compared to the control group. In the sequence task, patients had difficulties reproducing all four target locations in the order of the displayed sequence. The high variability of the saccade gain is a common observation in cerebellar atrophy patients and can be explained by the general variability present in the saccadic system. The increase in the saccade latency could be due to a cerebellar contribution to cortical processes related to fixation and target selection preceding the initiation of a saccade. Furthermore, the frequent occurrence of saccade errors in the memory and anti task suggests a cerebellar involvement in frontal inhibition of unwanted reflexive saccades. The impaired reproduction of saccade sequences in atrophy patients points to a deficit in short-term memory processes. Thus, this study provides further evidence that the cerebellum is involved in different cortical mechanisms related to the control of saccadic eye movements.     

Shortening and prolongation of saccade latencies following microsaccades

When the eyes fixate at a point in a visual scene, small saccades rapidly shift the image on the retina. The effect of these microsaccades on the latency of subsequent large-scale saccades may be twofold. First, microsaccades are associated with an enhancement of visual perception. Their occurrence during saccade target perception could, thus, decrease saccade latencies. Second, microsaccades are likely to indicate activity in fixation-related oculomotor neurons. These represent competitors to saccade-related cells in the interplay of gaze holding and shifting. Consequently, an increase in saccade latencies would be expected after microsaccades. Here, we present evidence for both aspects of microsaccadic impact on saccade latency. In a delayed response task, participants made saccades to visible or memorized targets. First, microsaccade occurrence up to 50 ms before target disappearance correlated with 18 ms (or 8%) faster saccades to memorized targets. Second, if microsaccades occurred shortly (i.e., <150 ms) before a saccade was required, mean saccadic reaction time in visual and memory trials was increased by about 40 ms (or 16%). Hence, microsaccades can have opposite consequences for saccade latencies, pointing at a differential role of these fixational eye movements in the preparation of saccade motor programs.     

When does action resist visual illusion? The effect of Müller–Lyer stimuli on reflexive and voluntary saccades

The primate visual cortex exhibits two anatomically distinct pathways (dorsal and ventral). According to the “two visual systems hypothesis” (TVSH) of Milner and Goodale (The visual brain in action. Oxford University Press, Oxford, 1995), this anatomical distinction corresponds to a functional division of labor between vision-for-action (dorsal) and vision-for-perception (ventral). This proposal is supported by evidence that, in healthy volunteers, perceptual responses are affected by visual illusions, whereas motor responses to the same illusion-inducing stimuli are not. However, previously we have shown that the amplitude of saccadic eye movements is modified by the Müller–Lyer illusion in a similar manner as perceptual responses. Here we extend this finding to reflexive and voluntary (memory-guided) saccades. We show that both types of saccade can be strongly affected by the illusion. In our studies, the effect on reflexive saccades was comparable to that usually observed with verbal reports (an effect size of 22 ± 8%), whereas the effect on voluntary saccades was smaller (11 ± 11%). In addition, both types of saccade provide evidence for the scaling bias usually observed in perceptual responses. We suggest that previous studies may have employed methods that generally reduced the effect of the illusion. Interpretations of dissociations between reflexive and voluntary saccades in terms of the TVSH appear to be premature.     

Sensory versus motor information in the control of predictive saccade timing

Humans readily make predictive saccades to periodic alternating targets. This predictive behavior depends on internal monitoring of timing error of past saccades in order to determine the time of initiation of future saccades; our earlier studies have confirmed this by finding correlations between latencies of consecutive predictive saccades. It is natural to consider that timing error is determined by visual detection of the difference between the time the target appears and the time the eyes arrive at the target; this in turn implies that saccades must actually be produced in order for their timing errors to be determined and predictive saccade timing to be established. We tested this hypothesis by having subjects view alternating visual targets while fixating a central target in order to eliminate saccade production. After six alternating target presentations, subjects began tracking the alternating targets. Tracking performance was assessed with an error measure that compared saccade latency and inter-saccade interval with desired values (zero and inter-stimulus interval, respectively). Errors in this Prior Viewing paradigm were compared to those from a conventional De Novo paradigm in which saccades began as soon as the alternating targets were presented. Saccades under Prior Viewing reached a low-error steady predictive state more rapidly than under De Novo tracking. The initial saccade under Prior Viewing had a higher latency than the others, suggesting that this saccade was reactive even though the paradigm is predictable; other reasons for this higher latency include time to disengage from the fixation target and time required to pre-program the initial set of saccades. The results show that visual detection of timing error from an actual motor act (saccades) is not necessary to establish predictive saccadic pacing: sensory-only information from viewing the moving targets can help to establish this predictive state.     

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.     

The influence of motivational salience on saccade latencies

Eye movements provide a direct link to study the allocation of overt attention to stimuli in the visual field. The initiation of saccades towards visual stimuli is known to be influenced by the bottom-up salience of stimuli as well as the motivational context of the task. Here, we asked whether the initiation of saccades is also influenced by the intrinsic motivational salience of a stimulus. Face stimuli were first associated with positive or negative motivational salience through instrumental learning. The same faces served as target stimuli in a subsequent saccade task, in which their motivational salience was no longer task-relevant. Participants performed either voluntary saccades, which required the selection of the saccade target out of two simultaneously presented stimuli (experiment 1), or reactive saccades, where only the target stimulus was presented (experiment 2). We found a specific effect of learned positive stimulus value on the latencies of voluntary saccades: For faces with high versus low positive motivational salience, saccadic latencies were significantly reduced. No such difference was observed for previously punished faces. In contrast, reactive saccades to both previously rewarded and punished faces were unaffected by learned stimulus value. Our findings show for the first time that saccadic preparation is susceptible to the acquired intrinsic motivational salience of visual stimuli. Based on the observation that only voluntary saccades but not reactive saccades were modulated, we conclude that the recruitment of neural processes for target identification is required to allow for an influence of motivational stimulus salience on saccadic preparation.     

Ocular motor differences between melancholic and non-melancholic depression

BACKGROUND: Major depressive disorder may be a heterogeneous disorder, yet melancholic depression is the most consistently described subtype, regarded as qualitatively different to non-melancholic depression in terms of cognitive and motor impairments. Eye movement studies in depression are infrequent and findings are inconclusive. METHODS: This study employed a battery of saccadic eye movement tasks to explore reflexive saccades, as well as higher order cognitive aspects of saccades including inhibitory control and spatial working memory. Nineteen patients with major depressive disorder (9 melancholic; 10 non-melancholic) and 15 healthy controls participated. RESULTS: Differences were revealed between melancholic and non-melancholic patients. Melancholia was associated with longer latencies, difficulty increasing peak velocities as target amplitudes increased, and hypometric primary saccades during the predictable protocol. In contrast, the non-melancholic depression group performed similarly to controls on most tasks, but saccadic peak velocity was increased for reflexive saccades at larger amplitudes. LIMITATIONS: Most patients were taking antidepressant medication. CONCLUSIONS: The latency increases, reduced peak velocity and primary saccade hypometria with more severe melancholia may be explained by functional changes in the fronto-striatal-collicular networks, related to dopamine dysfunction. In contrast, the serotonergic system plays a greater role in non-melancholic symptoms and this may underpin the observed increases in saccadic peak velocity. These findings provide neurophysiological support for functional differences between depression subgroups that are consistent with previous motor and cognitive findings.