Fine-scale measurement of the blind spot borders.

The blind spot is both a necessity and a nuisance for seeing. It is the portion of the visual field projecting to where the optic nerve crosses the retina, a region devoid of photoreceptors and hence visual input. The precise way in which vision transitions into blindness at the blind spot border is to date unknown. A chief challenge to map this transition is the incessant movement of the eye, which unavoidably smears measurements across space. In this study, we used high-resolution eye-tracking and state-of-the-art retinal stabilization to finely map the blind spot borders. Participants reported the onset of tiny high-contrast probes that were briefly flashed at precise positions around the blind spot. This method has sufficient resolution to enable mapping of blood vessels from psychophysical measurements. Our data show that, even after accounting for eye movements, the transition zones at the edges of the blind spot are considerable. On the horizontal meridian, the regions with detection rates between 80% and 20% span approximately 25% of the overall width of the blind spot. These borders also vary considerably in size across different axes. These data show that the transition from full visibility to blindness at the blind spot border is not abrupt but occurs over a broad area.

Volitional control of saccadic adaptation.

Saccadic adaptation is assumed to be driven by an unconscious and automatic mechanism. We wondered if the adaptation process is accessible to volitional control, specifically whether any change in saccade gain can be inhibited. Participants were exposed to post-saccadic error by using the double-step paradigm in which a target is presented in a peripheral location and then stepped during the saccade to another location. In one condition, participants were instructed to follow the target step and look at the final target location. In the other condition they were instructed to inhibit the adjustment of saccade amplitude and look at the initial target location. We conducted two experiments, which differed in the size of the intra-saccadic target step. We found that when told to inhibit amplitude adjustment, gain change was close to zero for outward steps, but some adaptation remained for inward steps. Saccadic latency was not affected by the instruction type for inward steps, but when the target was stepped outward, latencies were longer in the inhibition than in the adaptation condition. The results show that volitional control can be exerted on saccadic adaptation. We suggest that volitional control affects the remapping of the target, thus having a larger impact on outward adaptation.

The reward of seeing: Different types of visual reward and their ability to modify oculomotor learning.

Saccadic adaptation is an oculomotor learning process that maintains the accuracy of eye movements to ensure effective perception of the environment. Although saccadic adaptation is commonly considered an automatic and low-level motor calibration in the cerebellum, we recently found that strength of adaptation is influenced by the visual content of the target: pictures of humans produced stronger adaptation than noise stimuli. This suggests that meaningful images may be considered rewarding or valuable in oculomotor learning. Here we report three experiments that establish the boundaries of this effect. In the first, we tested whether stimuli that were associated with high and low value following long term self-administered reinforcement learning produce stronger adaptation. Twenty-eight expert gamers participated in two sessions of adaptation to game-related high- and low-reward stimuli, but revealed no difference in saccadic adaptation (Bayes Factor01 = 5.49). In the second experiment, we tested whether cognitive (literate) meaning could induce stronger adaptation by comparing targets consisting of words and nonwords. The results of twenty subjects revealed no difference in adaptation strength (Bayes Factor01 = 3.21). The third experiment compared images of human figures to noise patterns for reactive saccades. Twenty-two subjects adapted significantly more toward images of human figures in comparison to noise (p < 0.001). We conclude that only primary (human vs. noise), but not secondary, reinforcement affects saccadic adaptation (words vs. nonwords, high- vs. low-value video game images).

New is always better: Novelty modulates oculomotor learning.

Saccadic adaptation aims at keeping saccades accurate to enable precise foveation of objects. It has been believed to be a rather low-level adjustment, responding chiefly to direction and magnitude of postsaccadic position error. However, recent studies have shown that image content can modify saccadic adaptation. Adaptation is more complete for saccades toward socially relevant human figures in comparison to noise when time constraints exist. In the present experiment, we show that saccadic adaptation is also susceptible to the novelty of a stimulus. In a scanning adaptation paradigm, 20 subjects participated in two sessions of forward adaptation to one position at which the same human picture was always displayed versus a position at which a new human figure was presented in every trial. Saccadic adaptation was more complete to the novel-target position. This suggests that novelty can increase oculomotor learning and corroborates the claim that saccadic adaptation includes influences that reflect the target's visual properties.

Instability of visual error processing for sensorimotor adaptation in schizophrenia.

Saccadic adaptation can be used to study disturbances of sensory processing and motor learning. We investigated whether patients with schizophrenia can adjust saccadic amplitudes to account for an increase in visual error while the saccade is in flight, and whether they transfer this change to a visuo-manual localization task. Fourteen patients (mean 37.1 years) and 14 healthy controls (mean 35.1 years) performed 200 adaptation trials of 10° with target shifts of 4° in the outward direction. We determined the percent amplitude change during adaptation and adaptation speed. In addition, subjects localized a stimulus that was flashed 50 ms after saccade target onset to measure the transfer of change in visual space perception to visuo-manual coordination. Eye movements were recorded at 1000 Hz. Saccade amplitudes increased over adaptation trials by 11 % (p < 0.001) similarly in both groups. Amplitude variability during adaptation was higher in patients (1.06° ± 0.32°) than in controls (0.71° ± 0.14°; p = 0.001), while adaptation speed was slower in patients (0.02 ± 0.03) than in controls (0.11 ± 0.11; p = 0.01). Other pre- and post-adaptation saccade metrics did not differ between groups. The adaptation process shifted localization of the flashed target in the adaptation direction similarly in both groups. The use of error signals for the internal recalibration of sensorimotor systems and the transfer of this recalibration to visual space perception appear basically unimpaired in schizophrenia. Higher amplitude variability in patients suggests a certain instability of saccadic control in cerebellar systems. Patients seem to rely on visual error processing in frontal circuitry, resulting in slower adaptation speeds, despite unimpaired adaptation strength.

The influence of image content on oculomotor plasticity.

When we observe a scene, we shift our gaze to different points of interest via saccadic eye movements. Saccades provide high resolution views of objects and are essential for vision. The successful view of an interesting target might constitute a rewarding experience to the oculomotor system. We measured the influence of image content on learning efficiency in saccade control. We compared meaningful pictures to luminance and spatial frequency-matched random noise images in a saccadic adaptation paradigm. In this paradigm a shift of the target during the saccades results in a gradual increase of saccade amplitude. Stimuli were masked at different times after saccade onset. For immediate masking of the stimuli, as well as for their permanent visibility, saccadic adaptation was similar for both types of targets. However, when stimuli were masked 200 ms after saccade onset, adaptation of saccades directed toward the meaningful target stimuli was significantly greater than that of saccades directed toward noise targets. Thus, the percept of a meaningful image at the saccade landing position facilitates learning of the appropriate parameters for saccadic motor control when time constraints exist. We conclude that oculomotor learning, which is traditionally considered a low-level and highly automatized process, is modulated by the visual content of the image.