Ocular Fixation Research Articles

Defining the temporal threshold for ocular fixation in free-viewing visuocognitive tasks

Patterns of ocular fixation in free-viewing tasks reflect aspects of visual attention and cognition. To quantify these patterns, fixations must be identified in raw eye movement data by applying explicit spatial and temporal thresholds. A temporal threshold of 200 ms is commonly used in clinical visuocognitive research, despite having been originally derived from a study of eye movements in reading. We systematically explored temporal fixation thresholds below 200 ms, using biologically relevant (human face) and abstract (complex geometric) stimuli. Short fixations (<200 ms) significantly altered spatiotemporal patterns of fixation for both the face and geometric stimuli, by increasing the number of fixations and the scanpath length. A threshold of 100 ms was found to discriminate fixations from other oculomotor activity effectively, and was consistent with current physiological and visuocognitive models. Statistical examination of fixation duration data for each subject suggested the median was a less biased and more robust measure of central tendency than the mean.

Unilateral pallidotomy for Parkinson’s disease disrupts ocular fixation

Although some motor functions of the basal ganglia have been well studied, the oculomotor functions are not well established. We studied eye movements in patients with Parkinson’s disease (PD) undergoing pallidotomy to assess the role of the globus pallidus interna (GPi) in oculomotor control. Horizontal visually guided, gap and predictive saccades as well as ocular fixation were studied in patients with advanced PD before and 1 month after unilateral pallidotomy, and in healthy controls on two occasions 1 month apart. There was no difference in saccadic latency or accuracy, the number of saccadic anticipations or the ability to generate predictive saccades between the two assessments for either patients or controls. The number and amplitude of square wave jerks during ocular fixation however increased significantly in patients after pallidotomy. The results imply altered function of frontal or prefrontal cortical regions involved in ocular fixation resulting from a disruption to inhibitory pallidal influences on thalamocortical projections. The posteroventral GPi however appears not to be involved in externally controlled or predictive saccadic function.

Ocular fixation with quadriceps tendon allograft.

Management of difficult strabismus, such as strabismus fixus and paralytic strabismus, in order to maintain the alignment is complicated. There are many surgical approaches described in the current literature, together with notes on the materials used to stabilize these deformities. We present a new surgical approach using quadriceps tendon allograft for the correction of difficult strabismus such as strabismus fixus and paralytic strabismus. Our idea for using deep frozen quadriceps tendons developed from the fact that this is the most stable and strongest tendon in the whole body. Six patients with strabismus fixus (n = 1), paralytic strabismus secondary to extraocular muscle damage (n = 1) and combined cranial nerve palsy (n = 4) were operated on using a strip of deep frozen quadriceps tendon allograft sutured onto both the globe and the periosteum. The mean age of the patients was 44.33 years. (range 17-71 years) All the patients were followed up for six months. The mean preoperative deviation in the six cases was 60 prism diopters (PD) (range 30-123 PD) The mean change in horizontal alignment at 1 month, 3 months and 6 months postoperatively was 54 PD, 53 PD and 49.16 PD respectively. We had only one case of undercorrection. This may possible be due to the remaining function of the antagonist and/or the fact that we had used the distal end of the quadriceps tendon. When the patient (patient No. 5) who had 2 operations was excluded, the mean change in horizontal alignment was 48.6 PD, 47.4 PD and 43.6 PD. Post-operatively there was no infection or any other complications. According to our study of existing literature, these cases of deep frozen quadriceps tendon allograft application for the indications mentioned above are the first reported cases of its type. We conclude that ocular fixation with quadriceps tendon to the periosteum is a safe and effective option for the management of difficult strabismus. Further research on a larger cohort of patients and longer follow-up time are needed.

Altered performance on an ocular fixation task in attention-deficit/hyperactivity disorder

Background: Attention-deficit/hyperactivity disorder (ADHD) is a common psychiatric disorder without validated objective markers. Eye movement studies may be useful in providing objective criteria for characterizing the disorder.Methods: We compared 53 children (29 girls) with ADHD to 44 healthy control children (18 girls) on a 21-sec fixation task. Large saccades (> 4°) away from the fixation point were analyzed.Results: Children with ADHD made more large saccades that interrupted fixation than did control children (p = .001). Mean scores of the ADHD group did not change significantly with subsequent retesting on placebo (p = .11); however, there was poor intrasubject correlation (r = .16).Conclusions: Both boys and girls with ADHD made significantly more intrusive saccades during fixation than did control subjects, possibly reflecting intrinsic neurologic dysfunction; however, a probable “floor effect” obviates conclusions about the reliability of this measure.

Representation of the visual field in the lateral intraparietal area of macaque monkeys: a quantitative receptive field analysis.

The representation of the visual field in the primate lateral intraparietal area (LIP) was examined, using a rapid, computer-driven receptive field (RF) mapping procedure. RF characteristics of single LIP neurons could thus be measured repeatedly under different behavioral conditions. Here we report data obtained using a standard ocular fixation task during which the animals were required to monitor small changes in color of the fixated target. In a first step, statistical analyses were conducted in order to establish the experimental limits of the mapping procedure on 171 LIP neurons recorded from three hemispheres of two macaque monkeys. The characteristics of the receptive fields of LIP neurons were analyzed at the single cell and at the population level. Although for many neurons the assumption of a simple two-dimensional gaussian profile with a central area of maximal excitability at the center and progressively decreasing response strength at the periphery can represent relatively accurately the spatial structure of the RF, about 19% of the cells had a markedly asymmetrical shape. At the population level, we observed, in agreement with prior studies, a systematic relation between RF size and eccentricity. However, we also found a more accentuated overrepresentation of the central visual field than had been previously reported and no marked differences between the upper and lower visual representation of space. This observation correlates with an extension of the definition of LIP from the posterior third of the lateral intraparietal sulcus to most of the middle and posterior thirds. Detailed histological analyses of the recorded hemispheres suggest that there exists, in this newly defined unitary functional cortical area, a coarse but systematic topographical organization in area LIP that supports the distinction between its dorsal and ventral regions, LIPd and LIPv, respectively. Paralleling the physiological data, the central visual field is mostly represented in the middle dorsal region and the visual periphery more ventral and posterior. An anteroposterior gradient from the lower to the upper visual field representations can also be identified. In conclusion, this study provides the basis for a reliable mapping method in awake monkeys and a reference for the organization of the properties of the visual space representation in an area LIP extended with respect to the previously described LIP and showing a relative emphasis of central visual field.

Control of eye movement reflexes.

We investigated the effect of strategic suppression of reflexive eye movements on external control over fixation using a fixation offset paradigm. A visual signal at fixation facilitates the fixation reflex and inhibits eye movements. Certain preparatory states render the fixation reflex less reactive to visual stimulation at fixation, as evidenced by a reduction in the fixation offset effect (FOE). For example, past studies have suggested that the reduced FOE during anti-saccade tasks results from the requirement to inhibit reflexive eye movements. We tested whether suppressing reflexive saccades reduces external control over ocular fixation using a go-nogo saccade paradigm. During each trial, one of two targets appeared in the periphery. Participants were instructed to saccade to one target (go), but when the other target appeared they either had to maintain fixation (nogo) or move their eyes in the direction opposite the target (anti). When nogo trials were admixed with target-directed saccades a large FOE was observed compared to when target-directed saccades occurred alone (experiment 1); however, when anti-saccades were mixed with target-directed saccades, a small FOE was observed for both types of eye movements (experiment 2). We conclude that suppressing reflexive eye movements does not reduce external control over fixation. Further research is necessary to elucidate which other component of preparing to make an anti-saccade diminishes the FOE.

Strategic control over saccadic eye movements: studies of the fixation offset effect.

We studied the strategic (presumably cortical) control of ocular fixation in experiments that measured the fixation offset effect (FOE) while manipulating readiness to make reflexive or voluntary eye movements. The visual grasp reflex, which generates reflexive saccades to peripheral visual signals, reflects an opponent process in the superior colliculus (SC) between fixation cells at the rostral pole, whose activity helps maintain ocular position and increases when a stimulus is present at fixation, and movement cells, which generate saccades and are inhibited by rostral fixation neurons. Voluntary eye movements are controlled by movement and fixation cells in the frontal eye field (FEF). The FOE--a decrease in saccade latency when the fixation stimulus is extinguished--has been shown to reflect activity in the collicular eye movement circuitry and also to have an activity correlate in the FEF. Our manipulation of preparatory set to make reflexive or voluntary eye movements showed that when reflexive saccades were frequent and voluntary saccades were infrequent, the FOE was attenuated only for reflexive saccades. When voluntary saccades were frequent and reflexive saccades were infrequent, the FOE was attenuated only for voluntary saccades. We conclude that cortical processes related to task strategy are able to decrease fixation neuron activity even in the presence of a fixation stimulus, resulting in a smaller FOE. The dissociation in the effects of a fixation stimulus on reflexive and voluntary saccade latencies under the same strategic set suggests that the FOEs for these two types of eye movements may reflect a change in cellular activity in different neural structures, perhaps in the SC for reflexive saccades and in the FEF for voluntary saccades.

Cortical synchronization suggests neural principles of visual feature grouping

Compositions of visual scenes are related here to neural signals in visual cortex and to cortical circuit models to understand neural mechanisms of perceptual feature grouping. Starting from the hypothesis that synchronization and decoupling of cortical γ-activities (35-90 Hz) define the relations among visual objects, we concentrate on synchronization related to (1) static retinal stimulation during ocular fixation, and (2) transient stimulation by sudden shifts in object position. The synchronization hypothesis has been tested by analyzing signal correlations in visual cortex of monkeys with the following results: Static retinal stimuli induce loosely phase-coupled γ-activities among neurons of an object's cortical representation. Patches of γ-synchronization become decoupled across the representation of an object's contour, and therby can code figure-ground segregation. Transient stimuli evoke synchronized volleys of stimulus-locked activities that are typically non-rhythmic and include low frequency components in addition to those in the γ-range. It is argued that stimulus-induced and stimulus-locked synchronizations may play different roles in perceptual feature grouping.

Ocular gaze is anchored to the target of an ongoing pointing movement.

It is well known that, typically, saccadic eye movements precede goal-directed hand movements to a visual target stimulus. Also pointing in general is more accurate when the pointing target is gazed at. In this study, it is hypothesized that saccades are not only preceding pointing but that gaze also is stabilized during pointing in humans. Subjects, whose eye and pointing movements were recorded, had to make a hand movement and a saccade to a first target. At arm movement peak velocity, when the eyes are usually already fixating the first target, a new target appeared, and subjects had to make a saccade toward it (dynamical trial type). In the statical trial type, a new target was offered when pointing was just completed. In a control experiment, a sequence of two saccades had to be made, with two different interstimulus intervals (ISI), comparable with the ISIs found in the first experiment for dynamic and static trial types. In a third experiment, ocular fixation position and pointing target were dissociated, subjects pointed at not fixated targets. The results showed that latencies of saccades toward the second target were on average 155 ms longer in the dynamic trial types, compared with the static trial types. Saccades evoked during pointing appeared to be delayed with approximately the remaining deceleration time of the pointing movement, resulting in "normal" residual saccadic reaction times (RTs), measured from pointing movement offset to saccade movement onset. In the control experiment, the latency of the second saccade was on average only 29 ms larger when the two targets appeared with a short ISI compared with trials with long ISIs. Therefore the saccadic refractory period cannot be responsible for the substantially bigger delays that were found in the first experiment. The observed saccadic delay during pointing is modulated by the distance between ocular fixation position and pointing target. The largest delays were found when the targets coincided, the smallest delays when they were dissociated. In sum, our results provide evidence for an active saccadic inhibition process, presumably to keep steady ocular fixation at a pointing target and its surroundings. Possible neurophysiological substrates that might underlie the reported phenomena are discussed.

Divided visuo-spatial attention systems with total and anterior callosotomy

The role of the corpus callosum in the inter-hemispheric integration of the visuo-spatial attention system, was investigated in patients with a total callosotomy or with an anterior callosal section. Subjects produced simple reaction times (RTs) to visual targets shown to the left or right visual hemifield. Preceding the target by an interval of 500 ms, arrow cues predicting the target location were shown left and right of the point of ocular fixation. For a majority of total and anterior callosotomy patients, results with valid focused cues (both arrows pointing to the target location) and with divided-attention cues (arrows pointing away from fixation) did not differ and both conditions produced shorter RTs than with neutral cues (equal signs). In contrast, neurologically intact subjects showed equal RTs with divided-attention and neutral cues, whereas valid focused cues produced reduced RTs relative to neutral cues. These results indicate that most split-brains, in contrast to normal observers, are capable of directing their attention to left and right visual field locations simultaneously, and therefore that each cerebral hemisphere controls its own visuo-spatial attention mechanism.

Metabolic Mapping of Suppression Scotomas in Striate Cortex of Macaques with Experimental Strabismus

Misalignment of the ocular axes induces double vision and rivalry. To prevent these unpleasant sensations, most subjects fixate preferentially with one eye and suppress entirely the deviating eye or else suppress portions of the visual field of either eye. To explore the mechanism of visual suppression, a divergent strabismus (exotropia) was induced in six normal, adult Macaca fascicularis by disinserting the medial rectus muscles. After 4-8 weeks, each animal was chaired to measure its exotropia and to determine its ocular fixation preference. Five of the monkeys developed a clearly dominant eye. It was injected with [(3)H]proline. Alternate sections from flat-mounts of striate cortex were then processed either for autoradiography to label the ocular dominance columns or for cytochrome oxidase (CO) to assess local metabolic activity. Two CO patterns were seen, often in the same cortex. The first consisted of thin dark columns alternating with wide pale columns. This pattern arose from reduced CO activity in the suppressed eye's monocular core zones and both eyes' binocular border strips. The second pattern consisted of thin pale bands from reduced metabolic activity in both eyes' border strips. The thin dark-wide pale CO pattern was more widespread in the three animals with a strong fixation preference. The dark CO columns usually fit in register with the ocular dominance columns of the fixating eye, suggesting that perception was suppressed in the deviating eye. In most animals, however, the correlation switched in peripheral cortex contralateral to the deviating eye, implying local suppression of the fixating eye's temporal retina (beyond 10 degrees), as reported in humans with divergent strabismus. In the two animals with a weak fixation preference, pale border strips were found within the central visual field representation in both hemispheres. This CO pattern was consistent with alternating visual suppression. These experiments provide the first anatomical evidence for changes in cortical metabolism that can be correlated with suppression scotomas in subjects with strabismus.

Long-term reversal of hemispheric specialization for visual memory in a split-brain macaque

Accuracy of response and pattern of ocular fixations in three split-brain macaques were used to evaluate performance of each hemisphere in a continuous visual recognition task. The animal indicated by ocular fixation upon response points whether a displayed image or face was ‘NEW’ or ‘OLD’. An inadvertent lesion of cingulate gyrus severely reduced contralateral fixations and impaired performance of the affected hemisphere in one animal, confirming the inferred relation between hemisphere and laterality of fixations. The hemispheres in the other two animals were initially remarkably similar in accuracy with human faces and with images; but the right hemisphere was significantly superior to the left for macaque faces. Parallel to this, in the one animal tested while simultaneously using both eyes/hemispheres, fixations were made primarily on the left half of human and macaque faces (right hemispheric control), whereas for images the ocular fixations were predominantly focussed on the right half. However, after further, extensive training the left hemisphere performed with significantly greater accuracy than the right on all material and this shift was accompanied and further corroborated by a reversal of the fixational pattern to favor the right half of faces, as continued to be the case with images. Thus, over the long term both the pattern of ocular fixations and the accuracy of performance demonstrate a migration from right to left hemispheric dominance as familiarity with the task increased. Performance of the initially superior hemisphere actually diminished with this shift, presenting a uniquely puzzling question of hemispheric balance in the absence of the forebrain commissures.

Localization of static positional nystagmus with the ocular fixation test.

Characteristics of static positional nystagmus (SPN) (i.e., persistency, direction fixed, direction changing) are observed in both peripheral and central disturbances and possess no localizing value for vestibular lesions. Our objective was to investigate whether the ocular fixation test as applied to SPN could assist in localizing vestibular lesions. A 3-year prospective study that included 43 patients with SPN. All patients underwent a standard vestibular test battery and cerebral imaging (7, computed tomography scan; 36, magnetic resonance imaging). The ocular fixation index (OFI) was calculated by the ratio of the mean slow component velocity of SPN (measured with red light-emitting diode fixation) to that measured in darkness, multiplied by 100. An OFI less than 50 was considered normal. In 33 of 35 patients whose OFI was less than 50, the cerebral imaging was normal and a peripheral vestibular lesion was diagnosed (two benign tumors of the fourth ventricle were missed). In all eight patients whose OFI was greater than 50, the cerebral imaging was abnormal and a central vestibular lesion was noted. These results indicate that the visual suppression of SPN does, indeed, permit the localization of vestibular lesions. The predictive value of the ocular fixation test on the origin of SPN is greater than 94% for peripheral lesions and 100% for central disorders.

Effect of infantile strabismus on visuomotor development in the squirrel monkey (Saimiri sciureus): Optokinetic nystagmus, motion VEP and spatial sweep VEP

PURPOSE. To determine whether squirrel monkeys made artificially strabismic in infancy had ocular fixation abnormalities, directional asymmetries of horizontal optokinetic nystagmus (OKN) and asymmetries of motion visually evoked potentials (MVEPs) similar to those of humans with infantile strabismus. METHODS. Esotropia was produced in a newborn squirrel monkey by surgical tenotomy of both lateral rectus muscles. The alignment and eye rotations of the monkey were examined longitudinally and VEP testing was performed at the age of one year. Visual acuity was measured using spatial frequency sweep VEPs (SSVEP) in response to grating stimulation. OKN was tested under conditions of monocular viewing using full-visual-field, vertically oriented, moving stripes. MVEPs in response to horizontal motion were recorded with the animal sedated to reduce the possibility of eye movement artifact. RESULTS. The artificially strabismic squirrel monkey displayed a constant, comitant esotropic strabismus accompanied by latent nystagmus. Monocular SSVEP acuity was subnormal in one eye, consistent with mild monocular strabismic amblyopia. The monkey demonstrated asymmetric OKN favoring nasally-directed stimulus motion when viewing with either eye. Monocular MVEPs were also characterized by a horizontal asymmetry with a directional bias inverted 180 degrees between the right and the left eyes. The eye movements and MVEP asymmetries were similar to those observed in strabismic macaque monkeys and humans with early-onset strabismus. Neither the OKN asymmetry nor the MVEP asymmetry was evident in a normal squirrel or normal macaque monkey. CONCLUSION. The artificially strabismic squirrel monkey is an appropriate eye movement and VEP model for the study of neural mechanisms in human infantile strabismus.

Fluctuations in tremor at rest and eye movements during ocular fixation in subjects with Parkinson's disease

This exploratory study investigates possible relationships between fluctuations in tremor at rest (TR) and fluctuations in eye movement amplitude during ocular fixation. TR of the hand and eye movements were analyzed in five subjects with Parkinson's disease (PD) and five age-matched controls. TR was recorded using a position laser system and eye movements were recorded using an infrared reflectometry technique (Ober2). TR amplitudes were significantly larger in the group of subjects with PD than in the control group (p</=0.01). In addition, subjects with PD showed more fluctuations in their TR (p</=0.01). Eye movements did not present more fluctuations in amplitude during ocular fixation (p>/=0.05). Changes in TR amplitude were not systematically correlated with modifications of eye movement amplitude in either group. However, occasional but distinct monocular oscillations were found in subjects with PD. Coherence values between frequencies of TR from the tested hand (3.5-7Hz) and frequencies of oscillatory eye movements (within the same frequency band) were clearly higher for the eye ipsilateral to the side of the body most affected by the disease in three subjects with PD. It is believed that these monocular oscillations may be a consequence of PD. Results from two previously published exploratory studies are integrated with the present results and new avenues of research are proposed.

Neuronal Activity in Monkey Superior Colliculus Related to the Initiation of Saccadic Eye Movements

The introduction of a temporal gap between the disappearance of an initially fixated target and the appearance of an eccentric saccadic target results in a general reduction of saccadic reaction times (SRTs)-the gap effect-and often in the production of express saccades, the latencies of which approach the conduction time of the shortest neural pathways from the retina to the eye muscles. We investigated saccade initiation by recording neuronal activity in the superior colliculus in monkeys performing the gap paradigm. Fixation-related neurons reduced their discharge rate during the gap period, regardless of the SRT. This reduction in activity is consistent with the hypothesized release of ocular fixation that facilitates premotor processes and may contribute to the gap effect. In addition to saccade-related discharges, many saccade-related neurons displayed phasic target-related responses and/or low-frequency preparatory activity during the gap period. The level of this preparatory activity correlated with both SRT and express saccade occurrence when the saccade was made into the response field of the neuron. Evidence indicates that advanced motor preparation is required for express saccade generation, which may be subserved by specific increases in the preparatory activity of saccade-related neurons. Increased preparatory activity may allow the target-related responses to trigger short-latency express saccades directly. This study provides insights into the functional mechanism of saccade initiation and may be relevant to the generation of all voluntary motor responses.

Ophthalmoscopic evaluation of ocular fixation in children up to 2 years old

The examination of fixation on the fundus oculi is one of the main points in the diagnosis of amblyopia. Therefore it is necessary to describe the fixation ability and to clarify at what age children can fixate foveally and at what age this examination can be carried out, considering not only physiological aspects, but also co-operation reasons. Fixation on the fundus oculi was examined in 111 children aged between 1 and 25 months with the ophthalmoscop-star. This examination was carried out in 64 cases by a second and in 15 cases by a third examiner on the same day. During the first 3 months of age children cannot fixate foveally, one can observe instead, a kind of scatter at the posterior pole in a diameter of about 5 degrees. In the course of the 4, and 5, month of age foveal fixation develops. Repeated examinations of the same child on the same day by a 2, and 3, examiner reveal a very high rate of accordance (78%) of their results. The examination of fixation on the fundus can be carried out at 4 to 5 months of age. At that age one can differentiate quite reliable between a foveal and eccentric fixation as well as scatter at the posterior pole. The rate of accordance 78% with this fixation examination by several examiners shows that same reliability of this examination carried out in school children and adults. Comparing this result with fixation examination carried out with the Haidinger brushes then one must calculate with a fault rate of about 20% when examining with the ophthalmoscope, considering that the examination with Haidinger brushes as a reference examination.

Lexical Constraints on Reading Accuracy in Neglect Dyslexia

To elucidate the contribution of attention to reading, a detailed analysis of the performance of a right-brain-damaged patient with neglect dyslexia was conducted. Four-letter words and legal nonwords were presented to the right of ocular fixation for overt naming. More neglect errors occurred with nonwords 60 than with words 15. A significant majority of words produced on neglect errors to lexical targets had a higher frequency than the target. Lexical properties of the stimulus affecting neglect error rates were also studied. Examination of a particular source of effect was performed by isolating it from the possible effects of covariates through multidimensional matching. Word frequency had no effecton the rate of neglect errors. However, an increased number of orthographic neighbours that are of a higher frequency than the target and overlap iton their lastthree letters markedly increased neglecterrorrates. Neglect errors to words also increased if the first letter of these orthographic neighbours was visually similar to the first letter of the target. Neighbours with a lower frequency than the target had no effect. These results suggest that attention may act on low-level visual mechanisms involved in the encoding of letter shapes. Partial encoding of the letters constituting the target may lead to the activation of lexical representations. This lexical activation tends to be captured by high frequency words compatible with the correctly encoded portion of the stimulus.

A correlation procedure for the single-sweep analysis of event-related potentials

There has been considerable focus placed on how individuals with autism spectrum disorder (ASD) visually perceive and attend to social information, such as facial expressions or social gaze. The role of eye movements is inextricable from visual perception, however this aspect is often overlooked. We performed a series of meta-analyses based on data from 28 studies of eye movements in ASD to determine whether there is evidence for ocular motor dysfunction in ASD. Tasks assessed included visually-guided saccade tasks, gap/overlap, anti-saccade, pursuit tasks and ocular fixation. These analyses revealed evidence for ocular motor dysfunction in ASD, specifically relating to saccade dysmetria, difficulty inhibiting saccades and impaired tracking of moving targets. However there was no evidence for deficits relating to initiating eye movements, or engaging and disengaging from simple visual targets. Characterizing ocular motor abnormalities in ASD may provide insight into the functional integrity of brain networks in ASD across development, and assist our understanding of visual and social attention in ASD.

Habituation of ocular following reflex requires corpus callosum for interhemispheric transfer

In cats, unanesthetized following transection of the brainstem at a level precluding painful sensation, and limiting ocular motility to a vertically oriented course (the pretrigeminal preparation), habituation of the orienting reflex, consisting of ocular fixation and smooth pursuit, readily transferred between moving visual stimuli directed first at one and then the other cerebral hemisphere. Under the same conditions, when the corpus callosum had been transected 2 weeks prior to the habituation, interhemispheric transfer was absent. Thus, despite substantial brainstem involvement and bilateral coordination of ocular motility the neocortex plays an essential role in this habituation, just as it does in the interhemispheric transfer of visual discrimination learning. This suggests that habituation is a fundamental form of learning in the mammalian forebrain.