Search Results

This chapter is concerned with orienting the eye to visual targets. Research has suggested some direct links between studies of human behaviour and the underlying neurophysiology. There are two key questions. Firstly, what determines the latency of the saccade? That is the time to initiate an eye movement after the onset of the saccade target. Secondly, what determines where the saccade lands? The chapter describes the experimental paradigms that have been used to investigate these two processes as well as the underlying neurophysiological mechanisms. The chapter also includes a detailed discussion of the Findlay and Walker model of visual orienting, and illustrates how this model accounts for the behavioural evidence in a way that is consistent with findings in neurophysiology. The chapter concludes with a discussion of the development and plasticity of the oculomotor system.

In this chapter, a variety of saccadic eye-head movements evoked by visual and auditory stimuli are reviewed. Variation in head movement strategies resulting from methodology as well as the subject's own biases are considered alongside factors already known to affect eye-head movements. Search strategies are compared in different tasks. First, in the relatively simple situation in which the head is immobilized; second, in the more complicated situation when the head is free to move. The variables of movement amplitude and sensory modality are compared at the same time. In the studies of Guitton and Volle as well as Bizzi et al., saccadic latency was majorly affected by the predictability of the fixation-saccade interval and the saccade amplitude and direction.

This chapter reviews recent findings that link eye orienting(saccade)mechanism to conscious preference judgment. A systematic gaze bias was observed preceding a cognitive preference decision(the gaze cascade effect), but such bias was very specific to preference(i.e., relative attractiveness)judgment. The effect turned out to be very robust across a variety of conditions, including when the preference task was easy, when gaze direction in the face stimulus was manipulated, and when the preference was on objects other than faces(such as geometric figures or commercial products).

The chapter deals with two visual (dynamic vision and subitizing) and two optomotor aspects (saccade control and fixation) of dyslexia. In each domain variables and the methods for their assessments are defined. The following studies are described: 1. Development: Assessment of normal (control) subjects from 7 to 17 years old. 2. Diagnosis: Assessment of dyslexic subjects and comparison with age-matched controls. The percentages of subjects failing to reach the 16^th percentile was determined. 3. Training: Affected subjects were given specific, adaptive, and controlled daily training at home. Pre and post training data were compared and the percentage of subjects who reached the control range determined. 4. The transfer of the optomotor training to reading skills was investigated by comparing a trained group with an untrained waiting group. 5. The percentages of dyslexics suffering from auditory and/or visual or optomotor deficits were estimated. In conclusion, there was a very small pure “visual” subgroup of dyslexics — below 7%. On the other hand between 80% and 92% of dyslexics (depending on age) suffered from both visual/optomotor and auditory deficits. This means that one has to identify the deficits in each individual dyslexic subject and try to reduce his identified problems by training. Further remediation may be required to further improve reading and spelling skills.

Anatomically, the retina consists of primary photoreceptors occupying the outer layers, which relay visual information to second-order neurons (bipolar neurons) in the middle retinal layers. These synapse with ganglion cells (in the inner retinal layers), whose axons travel in the optic nerve. The photoreceptors are of two kinds, rods and cones. Rods primarily function in dim light; cones mediate color vision, and operate in bright light. Retinal function can be studied using flash electroretinography (ERG) and pattern electroretinography (P-ERG). Flash ERG is a means to evaluate photoreceptor and middle retinal layer function. It also permits distinguishing rod from cone abnormalities. The ganglion cell layer plays no role in the flash ERG response. P-ERG, however, permits ganglion cell evaluation. Function of the retinal pigment epithelium cells is difficult to study directly by ERG; instead, a variation of the saccade test can be used.

This chapter reviews some behavioural aspects of saccades recorded in alert, head-fixed, trained cats. The first part deals with characteristics of oblique saccades and their trajectory. The stretching of the smaller component of oblique saccades is mainly achieved by a prolongation of the deceleratory phase. The decelerator phase of centrifugal saccades can be controlled much more accurately in direction than the acceleratory period, and the generation of straight trajectories is more frequent when the eye returns to the primary eye position. Direction of the movement is thus a major factor that accounts for the trajectory of cats' saccades. Another factor is the initial position of the eye. These factors modify the dynamics of the components of oblique saccade and the co-ordination between them. The second part of the chapter is devoted to a behavioral analysis of slow corrective movements that follow the visually guided eye saccades. In the cat, the end of the saccadic eye displacement, or the intersaccadic period between a principal saccade and the following corrective saccade, often shows some long-lasting slow movements. Most of the saccadic eye shifts of cats end abruptly, but some of them are followed by a large displacement different from the preceding high-velocity movement. These drifts are target directed, and have been observed in trained and untrained cats. Some of these movements are obviously driven by a system reducing the residual error at the end of the saccade, referred to as “slow correcting movements.”

The vertical gaze displacement has seldom been studied systematically. Also, gaze-orienting movement in the vertical plane to a continuously visible target offset has not been investigated. The main sequence of eye saccade and head movement was studied in three subjects in order to determine the eye and head contributions to gaze in the midsagittal plane under visually guided conditions. The methods conducted were the one with a bite board in a head-fixed condition and the one with a head-free condition. The main finding in these experiments was that the saccade main sequence in the vertical midsagittal plane is asymmetrical for upward and downward saccades, particularly for those larger than 20°.

It has been shown from recent studies that concurrent head velocity affects gaze shift kinematics due to an inhibition of the VOR during the saccade in man. The goal of this study is to quantify the VOR-saccade interactions during small gaze shifts. A procedure of passive head rotation is thus designed to be able to quantify VOR gain associate with gaze shifts of even smaller amplitude. The main findings are summarized in the gaze velocity profiles. It was found that when saccadic response is executed when the head is moving in the same direction, gaze velocity is faster than when the head is fixed. Another finding is that the gaze shift duration is inversely related to gaze shift velocity.

When skilled readers read English texts, about one-third of the words are skipped. This chapter reviews the different explanations that have been proposed to explain this. It takes an in-depth look at the variables that influence word skipping. These are errors in the programming and execution of a saccade, the length of the upcoming word n+1 in parafoveal vision, the distance from word n+1 relative to the current fixation location, and the difficulty of word n+1 within the sentence. This chapter provides evidence that the effects of word length and distance cannot be explained by assuming that word n+1 is skipped only when it has been identified in parafoveal vision. Rather, readers often seem to make an educated guess about where to send the next forward saccade on the basis of incomplete information.

Saccadic eye movements make details clearer and more accurate, working around the limitations of human vision. This chapter takes into account saccadic programming costs in the evaluation of overall rationality of a specific saccadic plan. This institutes knowing saccades that are crucial in performing a task and those that are just insignificant glances.

It is possible to measure human gaze control, especially under restricted conditions. This chapter provides a PowerPoint presentation and propriety eye movement visualization to elaborate on human gaze control.

This chapter examines the relation between chronostasis illusion and spatial attention. It discusses saccadic chronostasis which describes the temporal overestimation of a stimulus seen immediately following a saccade. The findings indicate that prior entry and attention-based temporal dilation are not particularly convincing explanations for the chronostasis illusion. However, there is some controversy regarding the spatial extent of chronostasis and its relationship with the shift of attention that precedes a saccade.

This chapter examines the role of neural activity in the lateral intraparietal area (LIP) on intention to make saccadic eye movement and on attention in the lateral intraparietal area in monkeys. It describes the results of experiments which reveal that LIP predicts a monkey's attention on a millisecond-by-millisecond basis and is independent from saccade planning. It shows that the activity of LIP predicts the goal and latency of saccades in a free-viewing visual search task and suggests that it provides a salience map which is interpreted by the oculomotor system as a saccade goal when a saccade is appropriate.

The fifth chapter begins the support for the empirical premise of the main argument by focusing on results from perceptual psychology. It includes a discussion of a range of evidence that supports the empirical premise and it also critically discusses common views that are at odds with the empirical premise. The chapter includes a discussion of the way in which visual attention can be understood with regard to the empirical premise.

This chapter discusses the costs and reward discounts of motor commands. It suggests that certain manipulations (food, repetition, etc.) change the implicit value that the brain assigns to the target of the saccade, and that in turn affect the motor commands that move the eyes. This chapter shows that movement trajectories are a result of motor commands that produce changes in state that attempt to meet task goals, while minimizing some measure of effort.

This chapter explains feedback-dependent motor control. It presents examples that support the idea that the motor commands that move the body rely on internal predictions regarding the state of the body/environment, and sensory observations. It shows that during a movement, the motor commands depend on the state of the body part that is being controlled, as well as the overall goal of the task. This chapter suggests that the motor commands to control eye and head movements during head-free gaze changes respond to sensory feedback, and the conditions are simulated in which the head is perturbed, demonstrating how it affects the ongoing saccade of the eye.

Animals with limited spatial vision use a variety of strategies (kineses and taxes) to navigate towards appropriate light environments. Nearly all animals with good vision have a repertoire of eye movements. The majority show a pattern of stable fixations with fast saccades that shift the direction of gaze. These movements may be made by the eyes themselves, or the head, or in some insects the whole body. The main reason for keeping gaze still during fixations is the need to avoid the blur that results from the long response time of the photoreceptors. Blur begins to degrade the image at a retinal velocity of about one receptor acceptance angle per response time. Some insects (e.g., hoverflies) stabilize their gaze much more rigidly than this rule implies, and it is suggested that the need to see the motion of small objects against a background imposes even more stringent conditions on image motion. A third reason for not allowing rotational image motion is to prevent contamination of the translational flow-field, by which a moving animal can judge its heading and the distances of objects. Head-bobs in birds can be thought of as of translational for stabilising the lateral image while walking. Some animals do let their eyes rotate smoothly, and these include some heteropod molluscs, copepods, mantis shrimps, jumping spiders, and water beetle larvae, all of which have narrow linear retinas that scan across the surroundings.

Model systems have been critical to developing our understanding of cerebellar function. The vestibulo-ocular reflex stabilizes the eyes during head movement and depends on the cerebellum to maintain accurate function. Classical conditioning of the eye blink reflex is an example of predictive motor learning where the role of the cerebellum is to appropriately time the conditioned response. Voluntary goal-directed behaviour, such as target-directed eye movements, harnesses the cerebellar circuitry to maintain accuracy and compensates for self-induced perturbations that occur during the movement such as an eye blink. In the general context of everyday movement, the role of the cerebellum in the actions and reactions that underlie animal athleticism is likely to be pervasive, but also inextricably intertwined with the wider motor control networks.