Programming Of Eye Movements Research Articles

Frontal and parietal transcranial magnetic stimulation (TMS) disturbs programming of saccadic eye movements

Transcranial magnetic stimulation (TMS) of human motor cortex typically evokes motor responses. TMS has failed to elicit eye movements in humans, whereas prolongations of saccadic latency have been reported with TMS. In previous studies we demonstrated that saccades can be abolished or saccadic trajectories can be changed through TMS in the 100 msec before saccade onset. This effect was especially marked when TMS was applied parietally. TMS never influenced a saccade in flight. Simulations of predictive experimental saccades that were impaired through TMS of the frontal or parietal cortex demonstrated especially that the dynamics of small saccades were markedly influenced, resulting in a significant decrease in acceleration and amplitude, or an almost complete inhibition. The impact of inhibition through TMS was critically dependent on timing: early TMS (−70 msec) had a much larger inhibitory effect than late TMS (−20 msec) on experimental saccades. Differential timing of TMS in influencing the cortical control signal is demonstrated through simulations of a reciprocally innervated eye movement model that paralleled empirically determined changes in eye movement dynamics after real TMS. There is a reasonable match between the model and the experimental data. We conclude that the inhibitory action of a presaccadic disturbance, such as a TMS pulse, on saccadic programming is inversely related to timing and amplitude of the predicted saccade.

Selective neuropsychological dysfunction in schizophrenia syndromes

Previous ultra-rapid go/no-go categorization studies with manual responses have demonstrated the remarkable speed and efficiency with which humans process natural scenes. Using a forced-choice saccade task we show here that when two scenes are simultaneously flashed in the left and right hemifields, human participants can reliably make saccades to the side containing an animal in as little as 120 ms. Low level differences between target and distractor images were unable to account for these exceptionally fast responses. The results suggest a very fast and unexpected route linking visual processing in the ventral stream with the programming of saccadic eye movements.

The role of visual attention in saccadic eye movements.

The relationship between saccadic eye movements and covert orienting or visual spatial attention was investigated in two experiments. In the first experiment, subjects were required to make a saccade to a specified location while also detecting a visual target presented just prior to the eye movement. Detection accuracy was highest when the location of the target coincided with the location of the saccade, suggesting that subjects use spatial attention in the programming and/or execution of saccadic eye movements. In the second experiment, subjects were explicitly directed to attend to a particular location and to make a saccade to the same location or to a different one. Superior target detection occurred at the saccade location regardless of attention instructions. This finding shows that subjects cannot move their eyes to one location and attend to a different one. The result of these experiments suggest that visuospatial attention is an important mechanism in generating voluntary saccadic eye movements.

Neural basis of saccade target selection.

Saccade target selection must be understood in relation to the obvious fact that vision naturally occurs in a continuous cycle of fixations interrupted by gaze shifts. The guidance of eye movements requires information about what is where in the visual field. The identities of objects are derived from their visible features. Single neurons in the visual system represent the presence of specific features by the level of activation; the reliability of the discriminating signal from single neurons varies over time. Each point in the visual field is represented by many populations of neurons activated by all types of features. Topographic representations are found throughout the visual and oculomotor systems; neighboring neurons tend to represent similar visual field locations or saccades. Selecting one out of many stimuli to which to direct gaze requires comparing stimulus attributes across the visual field. The existence of retinotopic maps of the visual field makes possible local interactions to implement such comparisons /41/. For example, a lateral inhibition network can extract the location of the most conspicuous stimulus in the visual field /30,40,81/. Coordinated with this parallel visual processing is activation in structures responsible for producing the movement such as FEF and the superior colliculus. A saccade is produced when the neurons at one location within the motor maps become sufficiently active. One job of visual processing, then, is to ensure that only one site within a movement map becomes activated. This is done when the neurons signalling the location of the desired target develop enhanced activation while the neurons responding to other locations are attenuated. Saccade target selection often converts an initially ambiguous pattern of neural activation into a pattern that reliably signals one target location. The ambiguity may be reduced through prior knowledge of the likely target location or identity, and extraretinal signals reflecting such expectations can modulate the responsiveness of afferent visual neurons. Specifying the metrics of a saccade and triggering the movement are coordinated but dissociable processes. Speed-accuracy trade-offs can thereby be produced allowing the visuomotor system to produce a saccade that is inaccurate because it is premature relative to the target selection process. While there are many gaps in our knowledge, the questions to ask seem reasonably clear. Because saccade target selection involves visual processing and eye movement programming combined with mnemonic influences, only continued experimental ingenuity will disentangle the various and variable contributions of individual neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Eye movement programming and its EEG mapping in humans

Error scores on the Farnsworth-Munsell 100-hue test were partitioned into those representing red-green and those representing blue-yellow losses. Data from two groups of normal observers were used. One group showed results characteristic of published norms; one group showed superior performance. Both observers showed a correlation between red-green and blue-yellow scores indicative of a strong performance factor in this test. The difference between blue-yellow and red-green scores eliminates their correlated variance and allows evaluation of the axis. Both groups showed an increase in difference scores, with age indicating development of a blue-yellow axis. This increase was significant for the observers characteristic of the norms. We suggest cutoff scores to allow a decision as to whether a given patient shows a blue-yellow or red-green axis.

Scrutinization, spatial attention, and the spatial programming of saccadic eye movements.

Results are presented from an experiment in which subjects' eye movements were recorded while they carried out two visual tasks with similar material. One task was chosen to require close visual scrutiny; the second was less visually demanding. The oculomotor behaviour in the two tasks differed in three ways. (1) When scrutinizing, there was a reduction in the area of visual space over which stimulation influences saccadic eye movements. (2) When moving their eyes to targets requiring scrutiny, subjects were more likely to make a corrective saccade. (3) The duration of fixations on targets requiring scrutiny was increased. The results are discussed in relation to current theories of visual attention and the control of saccadic eye movements.

Reading without saccadic eye movements

To assess the limitation on reading speed imposed by saccadic eye movements, we measured reading speed in 13 normally-sighted observers using two modes of text presentations: PAGE text which presents an entire passage conventionally in static, paragraph format, and rapid serial visual presentation (RSVP) which presents text sequentially, one word at a time at the same location in the visual field. In Expt 1, subjects read PAGE and RSVP text orally across a wide range of letter sizes (2X to 32X single-letter acuity) and reading speed was computed from the number of correct words read per minute. Reading speeds were consistently faster for RSVP compared to PAGE text at all letter sizes tested. The average speeds for text of an intermediate letter size (8X acuity) were 1171 words/min for RSVP and 303 words/min for PAGE text. In Expt 2 subjects read PAGE and RSVP text silently and a multiple-choice comprehension test was administered after each passage. All subjects continued to read RSVP text faster, and 6 subjects read at the maximum testable rate (1652 words/min) with at least 75% correct on the comprehension tests. Experiment 3 assessed the minimum word exposure time required for decoding text using RSVP to minimize potential delays due to saccadic eye movement control. Successive words were presented for a fixed duration (word duration) with a blank interval (ISI) between words. The minimum word duration required for accurate oral reading averaged 69.4 msec and was not reduced by increasing ISI. We interpret these results as an indication that the programming and execution of saccadic eye movements impose an upper limit on conventional reading speed.

Types of eye movements evoked by thalamic microstimulation in the alert cat

We characterized the type of eye movements evoked by electrical stimulation of the thalamic internal medullary lamina in alert cats. For comparison, other thalamic nuclei were studied: the lateral posterior, lateral dorsal, pulvinar complex, and ventral lateral nuclei. Three types of eye movements were distinguished: (i) Direction and amplitude-specific saccades were the most common type of eye movements evoked by electrical stimulation of the majority of the explored nuclei with the exception of the ventral lateral nucleus. Mean latency and threshold were 45 ms and 50 to 100 μA, compared with 60 ms and 200 to 300 μA for the same type of saccades evoked by electrical stimulation of the posterior thalamus. (ii) Saccades which appeared goal directed were evoked in 25% of the stimulations in the region of the internal medullary lamina. Their latency and threshold depended on the initial position of the eyes: The more deviated the eyes were from the “goal area,” the shorter the latency (minimum, 35 ms) and the lower the threshold (minimum, 16 μA). (iii) Centering saccades were evoked by stimulation of the ventral lateral nucleus. They were always accompanied by other movements. The minimum latency and threshold were, respectively, 90 ms and 200 μA. The finding that goal-directed saccades were selectively produced from certain internal medullary lamina sites is consistent with the view that a translation of spatial coordinates occurs at this level in the programming of eye movements.

The Programming of Fast Eye Movements During Natural Vestibular Stimulation - Two Types of Interaction

summary Quick phases of vestibular nystagmus and saccades coinciding with smooth movements of vestibular origin were investigated. Two types of interaction were found: 1) Quick and slow phases act alternatively and not simultaneously. 2)Saccades and vestibular smooth movements were summated to the extent of about 75%. In addition, the velocity of saccades is increased by up to 100°/sec in both directions with respect to the smooth movement. We conclude that the systems generating saccades and quick phases are different at least in part.

Dysmetria of fast phase of optokinetic nystagmus in cerebellectomized rabbits

The whole cerebellum except for the lobulus petrosus was removed in young rabbits, which were kept alive for 3–4 months before studying eye movements. The general motor disturbances were similar to those described in the literature for other species. Recovery was slow and very incomplete. Eye movements elicited by moving a striped drum around the animal were precisely recorded. Sinusoidal movements of the drum were followed in the same way as by normal animals, however, with fewer saccades. Steady drum rotation produced an extreme eye deviation from the midposition (up to 40°) with insufficient correction by saccades. A similar reaction was elicited by darkness. Once the eye reached this extreme position, it continued spontaneously with a very small nystagmoid movement (slow drift to the periphery, saccade to the middle). This condition, sometimes lasting more than an hour, was often uninfluenced by any drum movement. The amplitude-maximal speed relationship for saccades was similar to that in normal animals. I concluded that the cerebellum is indispensable for the correct programming of fast eye movements.