fMRI evidence for sensorimotor transformations in human cortex during smooth pursuit eye movements.

Smooth pursuit eye movements (SP) are driven by moving objects. The pursuit system processes the visual input signals and transforms this information into an oculomotor output signal. Despite the object's movement on the retina and the eyes' movement in the head, we are able to locate the object in space implying coordinate transformations from retinal to head and space coordinates. To test for the visual and oculomotor components of SP and the possible transformation sites, we investigated three experimental conditions: (I) fixation of a stationary target with a second target moving across the retina (visual), (II) pursuit of the moving target with the second target moving in phase (oculomotor), (III) pursuit of the moving target with the second target remaining stationary (visuo-oculomotor). Precise eye movement data were simultaneously measured with the fMRI data. Visual components of activation during SP were located in the motion-sensitive, temporo-parieto-occipital region MT+ and the right posterior parietal cortex (PPC). Motor components comprised more widespread activation in these regions and additional activations in the frontal and supplementary eye fields (FEF, SEF), the cingulate gyrus and precuneus. The combined visuo-oculomotor stimulus revealed additional activation in the putamen. Possible transformation sites were found in MT+ and PPC. The MT+ activation evoked by the motion of a single visual dot was very localized, while the activation of the same single dot motion driving the eye was rather extended across MT+. The eye movement information appeared to be dispersed across the visual map of MT+. This could be interpreted as a transfer of the one-dimensional eye movement information into the two-dimensional visual map. Potentially, the dispersed information could be used to remap MT+ to space coordinates rather than retinal coordinates and to provide the basis for a motor output control. A similar interpretation holds for our results in the PPC region.

Gaze pursuit, 'attention pursuit' and their effects on cortical activations.

A moving object draws our attention to it and we can track the object with smooth pursuit eye movements (SPEM). Gaze and attention are usually directed to the same object during SPEM. In this study we investigated whether gaze and attention can be divided during pursuit. We explored the cortical control of ocular tracking and attentive tracking and the role of focused and divided attention. We presented a sinusoidally moving target for pursuit and simultaneously a stationary target for fixation. Gaze could be directed to the pursuit target and attention to the fixation target or vice versa, or gaze and attention were directed to the same (moving or stationary) target. We found that gaze (overt) and attentive (covert) pursuit similarly activated the cortical oculomotor network. Gaze pursuit showed higher activations than attentive pursuit. Activations, specific to the dissociation of attention from gaze and independent of eye movements, were found solely in the posterior parietal cortex. A cue indicating a forthcoming attention task activated large parts of the cortical SPEM network, as a kind of preparatory mechanism. We did not find any attention-related regions outside the well-known visuo-oculomotor network. We conclude that attention control during gaze pursuit and gaze fixation occur within the cortical SPEM network, supporting the premotor theory of attention [Rizzolatti, G., Riggio, L., Dascola, I. & Umilta, C. (1987) Neuropsychologia, 25, 31-40].

Eye movement abnormalities in spinocerebellar ataxia type 17 (SCA17).

BACKGROUND: Spinocerebellar ataxia type 17 (SCA17) is associated with an expansion of CAG/CAA trinucleotide repeats in the gene encoding the TATA-binding protein. In this quantitative characterization of eye movements in SCA17 mutation carriers, we investigated whether eye movement abnormalities originate from multiple lesion sites as suggested by their phenotypic heterogeneity. METHODS: Eye movements (saccades, smooth pursuit) of 15 SCA17 mutation carriers (mean age 36.9 years, range 20 to 54 years; mean disease duration 7.3 years, range 0 to 20 years; 2 clinically unaffected, 13 affected) were compared with 15 age-matched control subjects using the video-based two-dimensional EYELINK II system. RESULTS: Smooth pursuit initiation (step-ramp paradigm) and maintenance were strongly impaired, i.e., pursuit latency was increased and acceleration decreased, whereas latency and position error of the first catch-up saccade were normal. Visually guided saccades were hypometric but had normal velocities. Gaze-evoked nystagmus was found in one-third of the mutation carriers, including downbeat and rebound nystagmus. There was a pathologic increase in error rates of antisaccades (52%) and memory-guided saccades (42%). Oculomotor disorders were not correlated with repeat length. Smooth pursuit impairment and saccadic disorders increased with disease duration. CONCLUSIONS: Several oculomotor deficits of spinocerebellar ataxia type 17 (SCA17) mutation carriers are compatible with cerebellar degeneration. This is consistent with histopathologic and imaging (morphometric) data. In contrast, increased error rates in antisaccades and memory-guided saccades point to a deficient frontal inhibition of reflexive movements, which is probably best explained by cortical dysfunction and may be related to other phenotypic SCA17 signs, e.g., dementia and parkinsonism.

[Simultaneous presentation of amyotrophic lateral sclerosis and multiple sclerosis]. / Kombiniertes Auftreten von amyotropher Lateralsklerose und Multipler Sklerose.

The concurrence of amyotrophic lateral sclerosis (ALS) and multiple sclerosis (MS) is extremely rare. We report on a 55-year-old female patient presenting with a progressive gait disorder for 6 months and a speech disorder for 3 months. Neurological examination revealed a spastic paraparesis and mild dysarthria and dysphagia. Technical and laboratory investigations met the diagnostic criteria for MS: magnetic resonance imaging showed multiple periventricular white matter and cervical lesions; cerebrospinal fluid showed a typical autoimmune response. Within the following 3 months generalized fasciculations, atrophy of the small hand muscles and bulbar signs were noticed. Nerve conduction studies revealed acute and chronic signs of denervation in all limbs without nerve conduction block. Hence clinical and paraclinical examination met the El Escorial criteria for ALS. Although myelitic lesions in the anterior horn cells may lead to peripheral segmental denervation, the generalized denervation suggested the unusual coincident combination of ALS and MS in this patient. In clinical praxis motoneuron diseases should also be considered in patients with pronounced peripheral denervations once "definite" MS has been diagnosed.

Role of anterior and occipital white matter lesions for smooth eye tracking in myotonic dystrophy.

BACKGROUND: The degradation of smooth pursuit eye movements (SP) in myotonic dystrophy (MD) has been shown to result from lesions in the central nervous system. Imaging and histopathological studies have shown considerable areas of white matter lesions (WMLs) with periventricular emphasis. OBJECTIVE: To discover which of these WMLs are relevant in SP impairment? METHOD: Horizontal sinusoidal SP, the vestibulo-ocular reflex (VOR), and its suppression by fixation (VOR-S) were investigated in 12 patients with MD, and the data obtained were compared with those from a control group. RESULTS: Parallel degradation of SP and VOR-S was found in patients, although the eyes hardly needed to move with VOR-S. VOR in patients was normal. These results indicate a central rather than a peripheral origin for the SP degradation. Magnetic resonance images of the patients' heads were obtained and the WMLs transferred to a standard map. The lesions were mainly located around the occipital and anterior horn of the lateral ventricle. SP performance was then related to lesion site. The largest area of deficit associated lesions appeared to be in the parieto-occipital white matter. The most severe SP impairment, however, was associated with frontal WMLs. CONCLUSION: The study establishes a link between SP deficits and WMLs in patients with MD, in line with previous observations that, not only parieto-occipital regions, but also the frontal cortex has a crucial role in the gain control of SP.

The initiation of smooth pursuit eye movements and saccades in normal subjects and in "express-saccade makers".

A vast knowledge exists about saccadic reaction times (RT) and their bi- or multimodal distributions with very fast (express) and regular RT. Recently, there has been some evidence that the smooth pursuit system may show a similar RT behavior. Since moving targets usually evoke a combined pursuit/saccade response, we asked which processes influence the initiation of pursuit and saccadic eye movements. Furthermore, we investigated whether and how the pursuit and saccadic system interact during the initiation of eye movements to moving targets. We measured the RT of the initial smooth pursuit (iSP) response and of the first corrective saccade and compared the RT behavior of both. Furthermore we compared the behavior of the corrective saccades to moving targets to that of saccades to stationary targets, known from the literature. The stimulus consisted of a target that moved suddenly at constant velocity (ramp). In addition, prior to the movement, a temporal gap, a position step or a combination of both could occur (gap-ramp, step-ramp, gap-step-ramp, respectively). Differently from most previous studies, we chose step and ramp with the same direction to provoke competition between the pursuit and saccade system. For the first time we investigated pursuit initiation in "express-saccade makers" (ES makers), a subject group known to produce an abnormally high percentage of short-latency saccades in saccade tasks. We compared their results with subject groups who were either naive or trained with respect to saccade tasks. The iSP started at approximately 100 ms, which corresponds to express saccade latencies. These short iSP-RT occurred reflex-like and almost independent of the experimental task. A bimodal frequency distribution of RT with a second peak of longer iSP-RT occurred exclusively in the ramp paradigm. The RT of the first corrective saccades in a pursuit task were comparable with that in a saccade task and depended on the stimulus. The ability of ES makers to produce a high number of express saccades was transferred to corrective saccades in the pursuit task, but not to pursuit initiation. In summary, short-latency pursuit responses differ from express saccades with respect to their independence of experiment and subject group. Therefore, a simple analogy to express saccades cannot be drawn, although some mechanisms seem to act similarly on both the pursuit and the saccade system (such as disengagement of attention with the gap effect). Furthermore, we found evidence that the initial pursuit response and the first corrective saccade are processed independently of each other. The first corrective saccades to moving targets behave like saccades to stationary targets. Normal pursuit but abnormal saccade RT of ES makers can be explained by recent theories of superior colliculus (SC) function in terms of retinal error handling.

Relationship between saccadic eye movements and cortical activity as measured by fMRI: quantitative and qualitative aspects.

We investigated the quantitative relationship between saccadic activity (as reflected in frequency of occurrence and amplitude of saccades) and blood oxygenation level dependent (BOLD) changes in the cerebral cortex using functional magnetic resonance imaging (fMRI). Furthermore, we investigated quantitative changes in cortical activity associated with qualitative changes in the saccade task for comparable levels of saccadic activity. All experiments required the simultaneous acquisition of eye movement and fMRI data. For this purpose we used a new high-resolution limbus-tracking technique for recording eye movements in the magnetic resonance tomograph. In the first two experimental series we varied both frequency and amplitude of saccade stimuli (target jumps). In the third series we varied task difficulty; subjects performed either pro-saccades or anti-saccades. The brain volume investigated comprised the frontal and supplementary eye fields, parietal as well as striate cortex, and the motion sensitive area of the parieto-occipital cortex. All these regions showed saccade-related BOLD responses. The responses in these regions were highly correlated with saccade frequency, indicating that repeated processing of saccades is integrated over time in the BOLD response. In contrast, there was no comparable BOLD change with variation of saccade amplitude. This finding speaks for a topological rather than activity-dependent coding of saccade amplitudes in most cortical regions. In the experiments comparing pro- vs anti-saccades we found higher BOLD activation in the "anti" task than in the "pro" task. A comparison of saccade parameters revealed that saccade frequency and cumulative amplitude were comparable between the two tasks, whereas reaction times were longer in the "anti" task than the pro task. The latter finding is taken to indicate a more demanding cortical processing in the "anti" task than the "pro" task, which could explain the observed difference in BOLD activation. We hold that a quantitative analysis of saccade parameters (especially saccade frequency and latency) is important for the interpretation of the BOLD changes observed with visual stimuli in fMRI.

Changes in cortical activation during mirror reading before and after training: an fMRI study of procedural learning.

The neural correlates of procedural learning were studied using functional magnetic resonance imaging (fMRI) and the mirror reading paradigm. The aim of the study was to investigate a presumed learning-related change of activation in cortical areas that are involved in the performance of a nonmotor skill. Changes in cortical blood oxygenation contrast were recorded in 10 healthy subjects while they alternatively read visually presented single mirror script words and normal script words. Responses in naive subjects were compared to those acquired after training of mirror script reading. The acquisition volume included the motor and premotor cortex, the parietal lobe and the occipital lobe including its inferior aspects. Striate and extrastriate visual areas, associative parietal cortex and the premotor cortex were bilaterally active during normal and mirror script reading. Significantly stronger activation during mirror reading was seen in BA7 and 40 (parietal associative cortex) and in BA6 (corresponding to the frontal eye fields). Simultaneous eye movement recordings indicated that activation in BA6 was related to processing components other than saccade frequency. After training, BA6 and BA7 exhibited a decrease of activation during mirror reading that significantly exceeded nonspecific changes observed in the normal script control condition. The present findings confirm the hypothesis of practice-related decrease of activation in task-related cortical areas during nonmotor procedural learning.

MR-eyetracker: a new method for eye movement recording in functional magnetic resonance imaging.

We present a method for recording saccadic and pursuit eye movements in the magnetic resonance tomograph designed for visual functional magnetic resonance imaging (fMRI) experiments. To reliably classify brain areas as pursuit or saccade related it is important to carefully measure the actual eye movements. For this purpose, infrared light, created outside the scanner by light-emitting diodes (LEDs), is guided via optic fibers into the head coil and onto the eye of the subject. Two additional fiber optical cables pick up the light reflected by the iris. The illuminating and detecting cables are mounted in a plastic eyepiece that is manually lowered to the level of the eye. By means of differential amplification, we obtain a signal that covaries with the horizontal position of the eye. Calibration of eye position within the scanner yields an estimate of eye position with a resolution of 0.2 degrees at a sampling rate of 1000 Hz. Experiments are presented that employ echoplanar imaging with 12 image planes through visual, parietal and frontal cortex while subjects performed saccadic and pursuit eye movements. The distribution of BOLD (blood oxygen level dependent) responses is shown to depend on the type of eye movement performed. Our method yields high temporal and spatial resolution of the horizontal component of eye movements during fMRI scanning. Since the signal is purely optical, there is no interaction between the eye movement signals and the echoplanar images. This reasonably priced eye tracker can be used to control eye position and monitor eye movements during fMRI.

Visual object localization through vestibular and neck inputs. 1: Localization with respect to space and relative to the head and trunk mid-sagittal planes.

Object localization in space signals in the absence of an external reference (visual, auditory, haptic) involves a signal of the head in space (vestibular). The present study asks whether signals of body position relative to the support surface (proprioceptive) are involved as well, by investigating the role of vestibular-neck interaction (dissociating head and trunk position). Normal human subjects saw a light spot (object) and continuously nulled displacement steps of the spot. They did so before and after vestibular and/or neck rotational stimulation in the horizontal plane, reproducing a predesignated object localization in space (i), relative to the head mid-sagittal (ii), and relative to the trunk mid-sagittal (iii). The predominant frequency contained in the stimuli was varied (0.05, 0.1, and 0.4 Hz). (I) Object localization in space upon whole-body rotation (vestibular stimulus) at high frequency was veridical, whereas that at low frequency fell short. Almost identical results were obtained for trunk rotation about the stationary head (neck stimulus). In contrast, when combining the stimuli in the form of head rotation on the stationary trunk, the results were veridical, independent of stimulus frequency. Additional findings obtained with a large variety of vestibular-neck stimulus combinations suggest a linear summation of vestibular and neck signals. (II) Object localization with respect to the head was approximately veridical, being independent of vestibular and neck stimulation. However, this only applied if subjects were not biased by a head-in-space motion illusion of neck origin. (III) Object localization with respect to the trunk was veridical in all conditions tested. The findings support a recently developed concept, according to which humans evaluate the kinematic state of a visual object in space by (a) relating it to that of the body support by means of an essentially ideal proprioceptive coordinate transformation, and (b) relating, in turn, the kinematic state of the support to a vestibularly derived notion of space, using a proprioceptive coordinate transformation that "knows" the vestibular transfer characteristics. One important aspect is that object localization in space always is veridical during head and trunk rotation relative to a stationary support (for example, the ground) despite non-ideal vestibular transfer characteristics. Additional findings in patients with chronic loss of vestibular function confirm this concept.

Abnormalities of ocular motility in myotonic dystrophy.

Are the oculomotor disturbances in myotonic dystrophy (MD), i.e. reduced smooth pursuit (SP) gain and reduced saccadic peak velocity (PV), of muscular or central origin? To answer this question the following two approaches were used. (i) The performance of SP was compared with the patient's ability to suppress the vestibulo-ocular reflex (VOR) visually (VOR suppression; VOR-S). In the latter task the SP system is involved, but the eyes hardly move within the orbits. A parallel impairment of SP and VOR-S would indicate a central dysfunction. (ii) Peak saccadic velocity was compared between two saccades performed to and fro in rapid succession. The intention was to measure any myotonic effect which might build up after the first saccade and slow down the second saccade. We studied 15 MD patients and 15 age-matched controls. Stimuli for slow eye responses consisted of sinusoidal horizontal rotations of the SP target and/or the vestibular rotation chair at frequencies between 0.1 and 0.8 Hz. Saccades were analysed in terms of PV. accuracy, duration and latency, comparing centripetal versus centrifugal saccades at short and long intersaccadic intervals (ISI; 400 ms and 900 ms, respectively). The SP gain was reduced in patients compared with the controls, the effect being most pronounced (32% less) at the highest stimulus frequency. Whereas VOR was normal in the patients, VOR-S was clearly impaired (50% worse at 0.8 Hz). Despite normal saccadic accuracy, peak saccadic velocity was significantly lower in the patient group (23% less for saccades of 12 degrees amplitude), similarly for centrifugal and centripetal saccades; all these differences were independent of the ISI. Latency was normal with centrifugal saccades, but was considerably increased with centripetal saccades at short ISI (67% longer compared with controls). The observation of a parallel degradation of SP and VOR-S in the patients is interpreted in terms of a central deficit in the SP pathways. Thus, it appears that slow eye movements were not impaired by muscle dystrophy and myotonia to a considerable degree in our patients. The increase in saccadic latency for centripetal saccades at the short ISI also reflects a central deficit. However, the observed slowing of saccades might have a myopathic or neural origin; a distinction was not possible at present. A myotonic origin of the saccade slowing seems unlikely, because the effect was independent of the presaccadic activation of the relaxing (antagonistic) eye muscle.

Detection thresholds for object motion and self-motion during vestibular and visuo-oculomotor stimulation.

We compared the detection threshold for object motion with that of self-motion in space in healthy human subjects. Stimuli consisted of horizontal rotations of subjects' body with a fixation spot kept in fixed alignment with their heads (vestibular stimulus), rotation of the fixation spot relative to the stationary subjects (visuo-oculomotor stimulus), and a combination thereof by applying rotations of subjects body relative to the stationary object (sinusoidal oscillations, 0.025-0.4 Hz). Two series of experiments were performed. 1) One group of subjects was instructed to attend to, and to indicate the occurrence of, either object or self-motion. 2) A second group was instructed not only to detect the occurrence of a perception, but also to quality it either as object motion or self-motion, depending on which modality dominated perceptually. With either instruction it was found that all three stimulus conditions could evoke both, either an object motion perception or a self-motion perception. The detection thresholds of both perceptions were essentially similar. Thresholds were highest with the vestibular stimulus, intermediate with the stimulus combination, and lowest with the visuo-oculomotor stimulus. The vestibular threshold depended on stimulus frequency, in that it decreased with increasing frequency. Thereby, it became similar to the visuo-oculomotor one, which was essentially constant across frequency. Probability of occurrence of the perceptions in the first experimental series was considerably higher than in the second series, suggesting an important role of attentional mechanisms. In the second series, percent frequency of occurrence of veridical perception (object motion with visuo-oculomotor stimulus, self-motion with stimulus combination) was at chance level (50%) at low stimulus frequency, but was augmented considerably at high frequency. We assume that the latter effect is brought about by a visual-vestibular conflict measure by which the visual stimulus (light spot) is qualified as representing either a moving object or a spatial reference for self-motion. While at suprathreshold stimulus intensities the conflict can determine perception magnitude, at threshold levels its influence is restricted mainly on the probability of occurrence of object and self-motion perception.

Preoperative radiochemotherapy and radical surgery in comparison with radical surgery alone. A prospective, multicentric, randomized DOSAK study of advanced squamous cell carcinoma of the oral cavity and the oropharynx (a 3-year follow-up).

A multicentric, randomized study of squamous cell carcinoma (SCC) of the oral cavity and the oropharynx has been undertaken by DOSAK. The results after radical surgery alone have been compared with the results of combined preoperative radiochemotherapy followed by radical surgery. Patients with primary (biopsy proven) SCC of the oral cavity or the oropharynx with tumor nodes metastasis (TNM) stages T2-4, N0-3, M0 were included in the study. A total of 141 patients were treated by radical surgery alone, whereas 127 patients were treated by radical surgery preceded by preoperative radiochemotherapy. The preoperative treatment consisted of conventionally fractioned irradiation on the primary and the regional lymph nodes with a total dose of 36 Gy (5 x 2 Gy per week) and low-dose cisplatin chemotherapy with 5 x 12.5 mg cisplatin per m2 of body surface during the first week of treatment. Radical surgery according to the DOSAK definitions (DOSAK, 1982) was performed after a delay of 10-14 days. During the follow-up period, 28.2% of all patients suffered from locoregional recurrence, and 27.2% of the patients died. The percentages were higher after radical surgery alone for locoregional recurrence (31% and 15.6%) and for death (28% and 18.6%). The life-table analysis showed improved survival rates of 4.5% after 1 year and 8.3% after 2 years in the group of patients treated with combined therapy. The demonstrated improvement appeared to be significant with the Gehan-Wilcoxon test as well as with the log rank test below a P value of 5%.

Effects of stationary textured backgrounds on the initiation of pursuit eye movements in monkeys.

1. The initial ocular pursuit of small target spots (0.25 degrees diam) that suddenly start to move at constant speed (ramps) was recorded in four rhesus monkeys with the electromagnetic search coil technique. All target motions were horizontal, and both eyes were monitored. 2. In agreement with the observations of Keller and Khan, stationary textured backgrounds substantially reduced the initial eye acceleration achieved during pursuit but did not affect its latency. Correlation techniques were used to assess the changes in the eye speed profiles and indicated that the reduction in eye acceleration due to the background was a linear function of the logarithm of target speed over the range investigated (5-40 degrees/s), averaging 60% with the fastest targets. 3. Selectively excluding the background texture from the path of the target with a horizontal strip of card (vertical width, 4 degrees) reduced the impact of the background only slightly, and, even when the vertical width of the card was increased to 60 degrees, the effect of the background was not entirely eliminated. Thus the effect involves regions of the visual field well beyond the target and is not due simply to the reduced physical salience (contrast) of the target spot. Such spatially remote interactions suggest that the neurons decoding the target's motion have very extensive visual receptive fields. 4. Textured backgrounds also caused similar reductions in the eye acceleration during initial pursuit when, before the ramps, the fixated target spots stepped forward, i.e., stepped in the direction of the subsequent ramps (step ramps). In this situation, as with no steps, initial target ramps were foveofugal. When the fixated target spots were stepped back before moving forward so that initial target ramps were foveopetal, textured backgrounds now also delayed the onset of pursuit, and the reductions in eye acceleration were not seen until some time later when tracking resulted from foveofugal target-ramp motion. Selectively excluding the texture from the path of the target with a narrow strip of card eliminated any delays in the onset of pursuit to step ramps, but the later reductions in eye acceleration were still evident. These step-ramp data indicate that the mechanisms decoding foveofugal and foveopetal target ramps differ markedly in their sensitivity to textured backgrounds. That backgrounds can influence the latency and the initial eye acceleration independently is consistent with the idea that there are independent trigger and drive mechanisms for the decoding of target motions.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of vestibular and neck inputs for the perception of object motion in space.

UNLABELLED: The contribution of vestibular and neck inputs to the perception of visual object motion in space was studied in the absence of a visual background (in the dark) in normal human subjects (Ss). Measures of these contributions were obtained by means of a closed loop nulling procedure; Ss fixed their eyes on a luminous spot (object) and nulled its actual or apparent motion in space during head rotation in space (vestibular stimulus) and/or trunk rotation relative to the head (neck stimulus) with the help of a joystick. Vestibular and neck contributions were expressed in terms of gain and phase with respect to the visuo-oculomotor/joystick feedback loop which was assumed to have almost ideal transfer characteristics. The stimuli were applied as sinusoidal rotations in the horizontal plane (f = 0.025-0.8 Hz; peak angular displacements, 1-16 degrees). RESULTS: (1) During vestibular stimulation, Ss perceived the object, when kept in fixed alignment with the moving body, as moving in space. However, they underestimated the object motion; the gain was only about 0.7 at 0.2-0.8 Hz and clearly decreased at lower stimulus frequencies, while the phase exhibited a small lead. (2) During pure neck stimulation (trunk rotating relative to the stationary head), the object, when stationary, appeared to move in space counter to the trunk excursion. This neck-contingent object motion illusion was small at 0.2-0.8 Hz, but increased considerably with decreasing frequency, while its phase developed a small lag. (3) Vestibular, neck, and visuo-oculomotor effects summed linearly during combined stimulations. (4) The erroneous vestibular and neck contributions to the object motion perception were complementary to each other, and the perception became about veridical (G approximately 1, phi approximately 0 degree), when both inputs were combined during head rotation with the trunk stationary. The results are simulated by an extended version of a computer model that previously had been developed to describe vestibular and neck effects on human perception of head motion in space. In the model, the perception of object motion in space is derived from the superposition of three signals, representing "object to head" (visuo-oculomotor; head coordinates), "head on trunk" (neck; trunk coordinates), and "trunk in space" (vestibular-neck interaction; space coordinates).

The role of fixation and visual attention in the occurrence of express saccades in man.

The differential influence of fixation and directed visual attention on reaction times of goal-directed saccades and especially on the occurrence of express saccades was investigated. In all the experiments the subjects were instructed first to keep their direction of gaze at the center of a translucent screen with or without a central fixation point. When a new stimulus appeared, the subjects had to look at it as soon as possible. In some control experiments the subjects had to direct their gaze to the screen center and simultaneously direct their attention to a peripheral light spot before the target for the saccade appeared. Many express saccades occurred when either active fixation of a central fixation point or attention directed to a peripheral visual target (regardless of its position) was interrupted 200 ms before the target for the saccade appeared. Express saccades were almost completely abolished in the presence of fixation and/or directed visual attention at the moment in which the saccade target appeared. We conclude that express saccades occur if visual attention has already been released at the moment when the target for the saccade appears. This disengagement needs some time which adds to the reaction time.