Vergence eye movements and visual suppression

Visual sensitivity was measured during vergence eye movements in order to determine whether a suppression of vision similar to that associated with saccades is also present during vergence. Suppression was evaluated psychophysically by determining sensitivity to briefly presented, full-field decrements of light in a Ganzfeld. Subjects were rougly 0.5 log unit less sensitive when stimuli were presented at the beginning of a 2-3 deg convergent or divergent eye movement, than during steady fixation. Thus, the concept of saccadic suppression must be broadened to include visual suppression that also accompanies nonsaccadic eye movements. These results support the hypothesis that vision is affected by signals that accompany initiation of oculomotor activity.     

The reentry hypothesis: linking eye movements to visual perception

Cortical organization of vision appears to be divided into perception and action. Models of vision have generally assumed that eye movements serve to select a scene for perception, so action and perception are sequential processes. We suggest a less distinct separation. According to our model, occulomotor areas responsible for planning an eye movement, such as the frontal eye field, influence perception prior to the eye movement. The activity reflecting the planning of an eye movement reenters the ventral pathway and sensitizes all cells within the movement field so the planned action determines perception. We demonstrate the performance of the computational model in a visual search task that demands an eye movement toward a target.     

The influence of geometric visual illusions on saccadic eye movements

BACKGROUND: The aim of this study was to investigate whether visually perceived length differences of Mueller-Lyer figures result in amplitude differences of saccadic eye movements along the figures. METHODS: Video recordings of eye movements were obtained from 35 healthy volunteers (aged 18 - 30 years) when scanning the endpoints of Mueller-Lyer targets. RESULTS: Saccades between the ends of lines with inward-pointing arrowheads had significantly greater amplitudes than saccades between the ends of lines with outward-pointing arrowheads. This bias was observed for both horizontal and vertical target orientations, and was maintained with a smaller amplitude, after fixation periods were extended from 1 s to 4 s. CONCLUSIONS: Changes in length perception by visual illusions correspond with changes in the amplitude of saccadic eye movements. Therefore, recordings of saccadic eye movements offer the opportunity to assess changes in visual object perception based on changes of central image processing at the cortical level.     

A novel cone visual cycle in the cone-dominated retina

The visual processing of humans is primarily reliant upon the sensitivity of cone photoreceptors to light during daylight conditions. This underscores the importance of understanding how cone photoreceptors maintain the ability to detect light. The vertebrate retina consists of a combination of both rod and cone photoreceptors. Subsequent to light exposure, both rod and cone photoreceptors are dependent upon the recycling of vitamin A to regenerate photopigments, the proteins responsible for detecting light. Metabolic processing of vitamin A in support of rod photopigment renewal, the so-called "rod visual cycle", is well established. However, the metabolic processing of vitamin A in support of cone photopigment renewal remains a challenge for characterization in the recently discovered "cone visual cycle". In this review we summarize the research that has defined the rod visual cycle and our current concept of the novel cone visual cycle. Here, we highlight the research that supports the existence of a functional cone-specific visual cycle: the identification of novel enzymatic activities that contribute to retinoid recycling, the observation of vitamin A recycling in cone-dominated retinas, and the localization of some of these activities to the Müller cell. In the opinions of the authors, additional research on the possible interactions between these two visual cycles in the duplex retina is needed to understand visual detection in the human retina.     

Torsional eye movements and constancy of the visual field

The eye movements during lateral head tilting were investigated. For recording the eye movements we used Yarbus' method of the suction device. It is found that the smooth tilts of the head to the shoulder cause two kinds of the eye movements. The first is a slow rotation of the eye in the direction opposite to one of the head. During this phase the attitude of the eye with reference to the gravity vertical is not altering. The second kind of the eye movement is a saccadic rotation about the visual axis in the direction of the head tilt. The speed of these saccades is about 200°/sec. The sizes of the saccades are in the 0.5–8° range.The two described types of eye rotations correspond to the mechanisms of the orientation constancy in the visual system. In the slow phase the constancy is provided by keeping the eye attitude unchanged. But during the fast phase the saccades change the eye orientation in space and the action of a neuronal mechanism is supposed. This mechanism is to transform the retinae afferentation some proper way.From this point of view the results of the near electrophisiological studies of the constant elements in the visual cortex of unaesthetized cats are cleared.     

The ganglion cell and cone distributions in the monkey''s retina: Implications for central magnification factors

The distribution of cones and ganglion cells was determined in whole-mounted monkey retinae. Ganglion cell density along the horizontal meridian was asymmetric, being up to three times greater in nasal retina. A similar but smaller asymmetry occurred with cones. The total number of ganglion cells varied from 1.4 to 1.8 X 10(6), agreeing well with counts of optic nerve axons. The variation of ganglion cell density with eccentricity indicates the magnification factor (MF) of the retina. This was compared with MF at the dorsal lateral geniculate nucleus and at striate cortex, revealing that the relative representation of the fovea increases substantially in both thalamus and cortex.     

The eye movements of dyslexic children during reading and visual search: impact of the visual attention span

The eye movements of 14 French dyslexic children having a VA span reduction and 14 normal readers were compared in two tasks of visual search and text reading. The dyslexic participants made a higher number of rightward fixations in reading only. They simultaneously processed the same low number of letters in both tasks whereas normal readers processed far more letters in reading. Importantly, the children's VA span abilities related to the number of letters simultaneously processed in reading. The atypical eye movements of some dyslexic readers in reading thus appear to reflect difficulties to increase their VA span according to the task request.     

The influence of artificial scotomas on eye movements during visual search.

PURPOSE: Fixation durations are normally adapted to the difficulty of the foveal analysis task. We examine to what extent artificial central and peripheral visual field defects interfere with this adaptation process. METHODS: Subjects performed a visual search task while their eye movements were registered. The latter were used to drive a real-time gaze-dependent display that was used to create artificial central and peripheral visual field defects. Recorded eye movements were used to determine saccadic amplitude, number of fixations, fixation durations, return saccades, and changes in saccade direction. RESULTS: For central defects, although fixation duration increased with the size of the absolute central scotoma, this increase was too small to keep recognition performance optimal, evident from an associated increase in the rate of return saccades. Providing a relatively small amount of visual information in the central scotoma did substantially reduce subjects' search times but not their fixation durations. Surprisingly, reducing the size of the tunnel also prolonged fixation duration for peripheral defects. This manipulation also decreased the rate of return saccades, suggesting that the fixations were prolonged beyond the duration required by the foveal task. CONCLUSIONS: Although we find that adaptation of fixation duration to task difficulty clearly occurs in the presence of artificial scotomas, we also find that such field defects may render the adaptation suboptimal for the task at hand. Thus, visual field defects may not only hinder vision by limiting what the subject sees of the environment but also by limiting the visual system's ability to program efficient eye movements. We speculate this is because of how visual field defects bias the balance between saccade generation and fixation stabilization.     

The distribution and kinetics of visual pigments in the cat retina

An imaging fundus reflectometer has been used to study the distribution and regeneration of visual pigments in the retina of the adult living cat. Measurements were made over an area of tapetal retina extending 25 degrees nasal and 40 degrees temporal to the area centralis on the horizontal meridian and 5 degrees inferior to 40 degrees superior on the vertical meridian. The measured density differences show large variations with retinal location, with values in the central retina up to 60% higher than those in the superior region. The area of high density differences forms a horizontal streak. There are two peaks of density difference values, one centered on the area centralis and the second in the nasal section of the streak. Spectral measurements indicate that the contribution of cone pigments is negligible everywhere except at the area centralis, where it is about 5%. The distribution of density differences is shown to correlate well with anatomical data if the effects of fundal stray light are taken into account and if it is assumed that the cross section of light capture by the rods is determined by the dimensions of the inner segments. The time required for essentially complete regeneration of rhodopsin depends on retinal location, varying from about 80 min in the superior retina to more than 90 min in the regions of peak densities.     

Differential distributions of red-green and blue-yellow cone opponency across the visual field

The color vision of Old World primates and humans uses two cone-opponent systems; one differences the outputs of L and M cones forming a red-green (RG) system, and the other differences S cones with a combination of L and M cones forming a blue-yellow (BY) system. In this paper, we show that in human vision these two systems have a differential distribution across the visual field. Cone contrast sensitivities for sine-wave grating stimuli (smoothly enveloped in space and time) were measured for the two color systems (RG & BY) and the achromatic (Ach) system at a range of eccentricities in the nasal field (0-25 deg). We spatially scaled our stimuli independently for each system (RG, BY, & Ach) in order to activate that system optimally at each eccentricity. This controlled for any differential variations in spatial scale with eccentricity and provided a comparison between the three systems under equivalent conditions. We find that while red-green cone opponency has a steep decline away from the fovea, the loss in blue-yellow cone opponency is more gradual, showing a similar loss to that found for achromatic vision. Thus only red-green opponency, and not blue-yellow opponency, can be considered a foveal specialization of primate vision with an overrepresentation at the fovea. In addition, statistical calculations of the level of chance cone opponency in the two systems indicate that selective S cone connections to postreceptoral neurons are essential to maintain peripheral blue-yellow sensitivity in human vision. In the red-green system, an assumption of cone selectivity is not required to account for losses in peripheral sensitivity. Overall, these results provide behavioral evidence for functionally distinct neuro-architectural origins of the two color systems in human vision, supporting recent physiological results in primates.     

Expansion of visual space after saccadic eye movements

Human subjects reported the perceived two-dimensional location of a visual target that was briefly presented after a saccade in the absence of visual references. Consistent with previous studies, immediately after horizontal saccades, there was a salient horizontal component in mislocalization in the direction opposite to the saccade. However, the horizontal component in mislocalization was not constant and was larger for targets presented further into the visual field contraversive to the saccade. For the same horizontal saccades, the vertical component in mislocalization was also obvious, and it was larger for targets located further away from the saccade trajectory. The saccadic effects resulted in an overall pattern of mislocalization that could be best described as a two-dimensional expansion of visual space. The point of expansive origin was not associated with the saccade goal, but was shifted from the saccade goal in the direction of the saccade. These results suggest that spatial information processing at the time of saccades reflects topographic interactions between neural activations from saccade execution and the visual target. The configuration of mislocalized positions of single point stimuli along a line was not comparable to the pattern of non-veridical motion perception described by Park, Lee & Lee (2001), indicating that spatial mislocalization and non-veridical motion perception after saccades are independent phenomena.     

Binocular vision: its influence on the development of visual and postural reflex eye movements

The time course for the development of binocular vision is similar to the time course for the development of mature, symmetrical, monocular optokinetic nystagmus in response to movements of the visual field in the nasalward or temporalward direction. If binocular vision does not develop normally, then the monocular optokinetic response remains immature, or asymmetric. This paper examines the connection between the development of symmetrical OKN and binocular vision, and relates how disturbances in visual development affect the ability of postural reflex eye movements to adapt to changes in the visual environment.     

Suppression of visual phosphenes during saccadic eye movements

A loss of visual sensitivity is shown to accompany a saccadic eye movement even under conditions of total darkness. Optical factors are eliminated by the use of electrically produced phosphenes, rather than flashes of light, to test the changes in visual threshold. Separate experiments matching the brightness of electrical phosphenes to real lights allow the threshold changes to be expressed in terms of real light. The results point to the conclusion that a substantial portion of saccadic suppression is neural, rather than optical in its origin. The time course of the suppression is consistent with electrophysiological studies in cat and monkey in which inhibition of nerve impulses occurs over a period that includes, but is considerably longer than the duration of the eye movement. No correlation has yet been shown, however, between the occurrence of saccadic suppression and identifiable features of the human visually evoked occipital responses. Saccades of a given amplitude are found to be slower in total darkness.     

Localization of visual targets during optokinetic eye movements

We investigated localization of brief visual targets during reflexive eye movements (optokinetic nystagmus). Subjects mislocalized these targets in the direction of the slow eye movement. This error decreased shortly before a saccade and temporarily increased afterwards. The pattern of mislocalization differs markedly from mislocalization during voluntary eye movements in the presence of visual references, but (spatially) resembles mislocalization during voluntary eye movements in darkness. Because neither reflexive eye movements nor voluntary eye movements in darkness have explicit (visual) goals, these data support the view that visual goals support perceptual stability as an important link between pre- and post-saccadic scenes.     

The effect of visual field defects on eye movements and practical fitness to drive

Eye movements of subjects with visual field defects due to ocular pathology were monitored while performing a dot counting task and a visual search task. Subjects with peripheral field defects required more fixations, longer search times, made more errors, and had shorter fixation durations than control subjects. Subjects with central field defects performed less well than control subjects although no specific impairment could be pinpointed. In both groups a monotonous relationship was observed between the visual field impairment and eye movement parameters. The use of eye movement parameters to predict viewing behavior in a complex task (e.g. driving) was limited.     

Effect of a scotoma on eye movements during visual search

Eye movements have been recorded during a free visual search task both with and without an artificial foveal scotoma. Results of the eye movement measurements show that neither fixation durations nor mean saccade amplitudes are significantly modified by the loss of foveal vision.