Changes in visual motion perception before saccadic eye movements

Execution of a saccadic eye movement influences subsequent motion perception [Park, J., Lee, J., & Lee, C. (2001). Non-veridical visual motion perception immediately after saccades. Vision Research, 41, 3751-3761]. In the current study, we determined the pattern of perceptual changes for visual motion presented before saccades. The accuracy of judging the direction of a moving target was variable depending on the direction of target motion. Based on the pattern of judgment errors, the direction associated with no error, or DNE, could be defined. When a moving target was seen by stationary eyes, the DNE was roughly vertical, and the perceptual judgment for adjacent directions was biased away from the vertical direction. When the same visual motion was seen before horizontal saccades, the DNE shifted in the direction of the impending saccade, and the perceptual judgment of adjacent directions was shifted away from the new DNE, thus, shifting the perceived direction of the vertical in the direction opposite to the saccade. These changes improved the accuracy of direction judgment for visual motion in the visual field ipsiversive to impending saccades. In addition to shift of the DNE, perceptual judgment for oblique directions became near veridical before saccades, which we call the anti-oblique effect. These results suggest that motion perception is dynamically and anisotropically modulated at the time of saccades, and the DNE shift may be a part of processes dynamically reallocating computational resources, improving perceptual performance in advance for sensory events to be acquired by impending saccades.     

The use of optimal object information in fronto-parallel orientation discrimination

When determining an object's orientation an implicit object axis is formed, based on local contour information. Due to the oblique effect (i.e., the more precise perception of horizontal/vertical orientations than oblique orientations), an object's orientation will be perceived more precise if the axis is either horizontal or vertical than when the axis is oblique. In this study we investigated which object axis is used to determine orientation for objects containing multiple axes. We tested human subjects in a series of experiments using the method of adjustment. We found that observers always use object axes allowing for the highest object orientation discrimination, namely the axes lying closest to the horizontal/vertical. This implies that the weight the visual system attaches to axial object information is in accordance with the precision with which this information is perceived.     

Anisotropies in global stereoscopic orientation discrimination

Human orientation discrimination of long rectangular targets formed from dynamic random-element stereograms was assessed at four orientations (horizontal, vertical, left, and right oblique) as a function of width, disparity direction and magnitude using a temporal two-alternative, forced-choice paradigm. The results revealed the presence of an oblique effect in cyclopean orientation discrimination. In addition, observers discriminated targets with crossed disparity better than targets with uncrossed disparity, and had lower discrimination thresholds when the targets were oriented horizontally than vertically. These results demonstrate the existence of the oblique effect in the hypercyclopean domain comparable in magnitude to that present in the luminance domain.     

Accuracy of spatial localizations near the time of saccadic eye movements

Two-dimensional eye movements were recorded while subjects used a hammer to strike targets that were flashed-on briefly before, during or up to 750 msec after a horizontal saccade. Mean position of hammer blows was 20 min arc (SD = 67 min arc) from the target when the only cue to target location was eye position. Position of responses varied slightly with time of target exposure relative to the saccade. These results show that observers can closely monitor small changes in eye position during and near the time of saccadic eye movements.     

The effect of practice on the oblique effect in line orientation judgments

Line orientation discrimination improves with selective practice for oblique orientations and not for principal orientations. This training effect was observed with an identification task as well as with two alternative forced choice tasks. Despite the improvement for oblique orientations, just noticeable differences in orientation are still larger for the practised oblique orientation than for the principal orientations after 5000 practice trials. These findings suggest that the oblique effect in line orientation has at least two sensorial components, one of which is attributed to the meridional variations in the preferred orientation of area 17 S-cells.     

Attribute-invariant orientation discrimination at an early stage of processing in the human visual system

This study investigated event-related brain potentials (ERPs) during selective attention to the orientation of a bar comprised of two squares, which were defined by only color or motion (intra-attribute conditions) or both (interattribute condition). An early positive potential in association with orientation selection was elicited for all conditions in similar latency ranges but with different scalp distributions. These results suggest that attribute-invariant orientations can be discriminated at an early stage of processing in the human brain, which fills a gap between monkey electrophysiology and human psychophysics, while attribute-specific orientations are also available in a given context.     

The time course of saccadic eye movements in goldfish

Spontaneous saccadic eye movements (2–25°) made by goldfish have an initial brief acceleration, followed by a more prolonged deceleration, and occasionally a terminal phase during which the velocity reverses sign following overshoot of the final position. Overshoots by the individual eyes are independent events. The duration and the maximum velocity increase monotonically with the size of the saccade. Goldfish saccades have greater durations and slightly smaller maximum velocities than mammalian saccades of similar size.     

Decoupling location specificity from perceptual learning of orientation discrimination

Perceptual learning of orientation discrimination is reported to be precisely specific to the trained retinal location. This specificity is often taken as evidence for localizing the site of orientation learning to retinotopic cortical areas V1/V2. However, the extant physiological evidence for training improved orientation turning in V1/V2 neurons is controversial and weak. Here we demonstrate substantial transfer of orientation learning across retinal locations, either from the fovea to the periphery or amongst peripheral locations. Most importantly, we found that a brief pretest at a peripheral location before foveal training enabled complete transfer of learning, so that additional practice at that peripheral location resulted in no further improvement. These results indicate that location specificity in orientation learning depends on the particular training procedures, and is not necessarily a genuine property of orientation learning. We suggest that non-retinotopic high brain areas may be responsible for orientation learning, consistent with the extant neurophysiological data.     

The effects of aging on orientation discrimination

The current experiments measured orientation discrimination thresholds in younger (mean age approximately 23 years) and older (mean age approximately 66 years) subjects. In Experiment 1, the contrast needed to discriminate Gabor patterns (0.75, 1.5, and 3c/deg) that differed in orientation by 12deg was measured for different levels of external noise. At all three spatial frequencies, discrimination thresholds were significantly higher in older than younger subjects when external noise was low, but not when external noise was high. In Experiment 2, discrimination thresholds were measured as a function of stimulus contrast by varying orientation while contrast was fixed. The resulting threshold-vs-contrast curves had very similar shapes in the two age groups, although the curve obtained from older subjects was shifted to slightly higher contrasts. At contrasts greater than 0.05, thresholds in both older and younger subjects were approximately constant at 0.5deg. The results from Experiments 1 and 2 suggest that age differences in orientation discrimination are due solely to differences in equivalent input noise. Using the same methods as Experiment 1, Experiment 3 measured thresholds in 6 younger observers as a function of external noise and retinal illuminance. Although reducing retinal illumination increased equivalent input noise, the effect was much smaller than the age difference found in Experiment 1. Therefore, it is unlikely that differences in orientation discrimination were due solely to differences in retinal illumination. Our findings are consistent with recent physiological experiments that have found elevated spontaneous activity and reduced orientation tuning on visual cortical neurons in senescent cats (Hua, T., Li, X., He, L., Zhou, Y., Wang, Y., Leventhal, A. G. (206). Functional degradation of visual cortical cells in old cats. Neurobiology Aging, 27(1), 155-162) and monkeys (Yu, S., Wang, Y., Li, X., Zhou, Y. & Leventhal, A. G. (2006). Functional degradation of visual cortex in senescent rhesus monkeys. Neuroscience, 140(3), 1023-1029; Leventhal, A. G., Wang, Y., Pu, M., Zhou, Y. & Ma. Y. (2003). GABA and its agonists improved visual cortical function in senescent monkeys. Science,300 (5620), 812-815).     

Changes of saccadic eye movements in thyroid‐associated ophthalmopathy

Purpose: To establish whether or not the dynamics of saccadic eye movements are significantly changed in patients with different stages of thyroid‐associated ophthalmopathy (TAO) and, subsequently, if analysis of saccades could serve as an additional diagnostic tool for early detection of inflammatory activity in TAO. Methods: Thirty‐seven patients with TAO and 10 age‐ and gender‐matched control subjects were investigated. The patients were divided into four groups according to the stage of the disease: (i) early mild disease (n = 10), (ii) early severe disease (n = 11), (iii) long‐standing restrictive disease (n = 10) and (iv) proven auto‐immune hyperthyroidism without any signs of TAO (n = 6). Horizontal and vertical saccades with amplitudes of 10°, 20°, 30° and 40° were recorded binocularly using the induction scleral search coil technique. The two main sequence constants V_max and C were calculated for each eye. Repeated measurement analysis of variance was carried out to test for differences between different gaze directions, eyes and groups. Results: In horizontal saccades, significant differences were found between groups but not between abduction and adduction. In vertical saccades, differences between groups and the interaction between groups and up‐ and down‐gaze saccades were significant. Compared with the control group, analysis of the main sequence curves revealed larger differences in patients of group 3 and 4 than in those of group 1 and 2. Whereas in the control group down‐gaze saccades were faster than up‐gaze saccades, the opposite behaviour was found in all patient groups. The largest differences were detected among those patients who had no signs of TAO (group 4). Conclusions: In this study, significant saccade differences were detected in all patients with TAO. In contrast to our earlier studies where evaluation of multiple individual saccade parameters did not reveal significant differences, analysis of the main sequence constants and mathematical reconstruction of the main sequence curves turned out to be a sensitive technique for reliable detection of subtle ocular motility changes. Significant differences were detected even in patients with auto‐immune thyroiditis where no clinical signs of TAO were apparent.     

GABAergic inhibition and orientation selectivity of neurons in the kitten visual cortex at the time of eye opening

Effects of iontophoretic application of gamma-aminobutyric acid (GABA) and its antagonist, N-methyl-bicuculline (BIC), on visual responses of striate cortical neurons were studied in kittens 6-13 days old. Visually responsive cells were classified into three groups, i.e. orientation-selective, orientation-bias and nonoriented cells. In almost all of the orientation-selective cells, their responses were completely suppressed by GABA while the majority of the others were not significantly or only weakly suppressed. An application of BIC abolished or reduced the selectivity of all the orientation-selective cells but did not affect any of the nonoriented cells tested. These results suggest that GABAergic inhibition already operates on a group of cortical neurons to make them orientation-selective at the time of eye opening, but such an action of GABA on other groups of neurons develops later.     

Precision of speed discrimination and smooth pursuit eye movements

Several studies have shown that the precision of smooth pursuit eye speed can match perceptual speed discrimination thresholds during the steady-state phase of pursuit [Kowler, E., & McKee, S. (1987). Sensitivity of smooth eye movement to small differences in target velocity. Vision Research, 27, 993-1015; Gegenfurtner, K., Xing, D., Scott, B., & Hawken, M. (2003). A comparison of pursuit eye movement and perceptual performance in speed discrimination. Journal of Vision, 3, 865-876]. Recently, Osborne et al. [Osborne, L. C., Lisberger, S. G., & Bialek, W. (2005). A sensory source for motor variation. Nature, 437, 412-416; Osborne, L. C., Hohl, S. S., Bialek, W., & Lisberger S. G. (2007). Time course of precision in smooth-pursuit eye movements of monkeys. Journal of Neuroscience, 27, 2987-2998] claimed that pursuit precision during the initiation phase of pursuit also matches the sensory variability, implying that there is no motor noise added during pursuit initiation. However, these results were derived from a comparison of monkey pursuit data to human perceptual data from the literature, which were obtained with different stimuli. To directly compare precision for perception and pursuit, we measured pursuit and perceptual variability in the same human observers using the same stimuli. Subjects had to pursue a Gaussian blob in a step-ramp paradigm and give speed judgments on the same or in different trials. Speed discrimination thresholds were determined for different presentation durations. The analysis of pursuit precision was performed for short intervals containing the initiation period only and also for longer intervals including steady-state pursuit. In agreement with published studies, we found that the Weber fractions for psychophysical speed discrimination were fairly constant for different presentation durations, even for the shortest presentation duration of 150 ms. Pursuit variability was 3-4 times as high for the analysis interval (300 ms) containing the open-loop phase only. For pursuit analysis intervals of 400-500 ms, pursuit variability approached perceptual variability. Our results show that, for the stimuli we used, the motor system contributes at least 50% to the total variability of smooth pursuit eye movements during the initiation phase.     

The Simon effect of spatial words in eye movements: Comparison of vertical and horizontal effects and of eye and finger responses

Spatial stimulus location information impacts on saccades: Pro-saccades (saccades towards a stimulus location) are faster than anti-saccades (saccades away from the stimulus). This is true even when the spatial location is irrelevant for the choice of the correct response (Simon effect). The results are usually ascribed to spatial sensorimotor coupling. However, with finger responses Simon effects can be observed with irrelevant spatial word meaning, too. Here we tested whether a Simon effect of spatial word meaning in saccades could be observed for words with vertical (“above” or “below”) and horizontal (“left” or “right”) meanings. We asked our participants to make saccades towards one of the two saccade targets depending on the color of the centrally presented spatial word, while ignoring their spatial meaning (Experiment 1 and 2a). Results are compared to a condition in which finger responses instead of saccades were required (Experiment 2b). In addition to response latency we compared the time course of vertical and horizontal effects. We found the Simon effects due to irrelevant spatial meaning of the words in both saccades and finger responses. The time course investigations revealed different patterns for vertical and horizontal effects in saccades, indicating that distinct processes may be involved in the two types of Simon effects.     

Foveal blur discrimination of the human eye

Although the effect of retinal defocus on the foveal blur detection threshold has been well investigated, knowledge regarding the foveal blur discrimination threshold is limited. In the present study, both thresholds were assessed psychophysically using the ascending method of limits at the fovea with accommodation paralyzed. The unidirectional blur detection threshold was 0.87+/-0.18 D (+/-1 S.E.M.). The subsequent blur discrimination thresholds were relatively constant and significantly smaller than the blur detection threshold, with an average value of 0.48+/-0.006 D (+/-1 S.E.M.). We speculate that the difference in magnitude between these two thresholds may be attributed to the defocus-related change in the ocular modulation transfer function (MTF) and its interaction with contrast discrimination ability, as well as to the presence of a neuroperceptual blur buffering mechanism.     

Attentional release in the saccadic gap effect

Can a release of attention from fixation help explain the saccadic ‘gap effect’, the shortening of saccadic latency (SL) when the fixation spot is extinguished just before saccade target onset? Practiced observers generated SLs and button-presses to one of four 10° eccentric targets in overlap (fixation spot stays on), gap0 (fixation offsets at target onset), and gap200 conditions; in gap200, the fixation spot was removed, dimmed, expanded, or brightened 200 ms before target onset. Our data excluded speed-accuracy trade-offs, express saccades, stimulus salience, and oculomotor readiness, while fixation offset and general warning had minor effects, leaving attention release as the default explanation. Supporting this notion, finger-press reactions to foveal probe dots presented after the fixation spot was brightened (to hold attention) were faster than those made after the spot was removed (to release attention). Varying the time from gap onset to the probe dot mapped out the time-course of the putative attentional release, which takes ∼140 ms.     

A metanalysis of the effect of the Müller-Lyer illusion on saccadic eye movements: No general support for a dissociation of perception and oculomotor action

Milner and Goodale’s (1995) proposal of a functional division of labor between vision-for-perception and vision-for-action is supported by neuropsychological, brain-imaging, and psychophysical evidence. However, there remains considerable debate as to whether, as their proposal would predict, the effect of contextual illusions on vision-for-action can be dissociated from that on vision-for-perception. Meta-analytical efforts examining the effect of the Müller-Lyer (ML) illusion on pointing (Bruno, Bernardis, & Gentilucci, 2008) or grasping (Bruno & Franz, 2009) have been conducted to resolve the controversy. To complement this work, here we re-analyzed 17 papers detailing 21 independent studies investigating primary saccades to target locations that were perceptually biased by the ML illusion. Using a corrected percent illusion effect measure to compare across different studies and across experimental conditions within studies, we find that saccadic eye movements are always strongly biased by the illusion although the size of this effect can be reduced by factors such as display duration and between-trials variability in display length and orientation, possibly due to a process of saccadic adaptation. In contrast to some reports, we find no general support for differences between voluntary and reflexive saccades or between saccades performed in conjunction with a pointing movement and saccades performed without pointing. We conclude that studies on the effect of the Müller-Lyer illusion do not provide evidence for a functional dissociation between primary saccades and perception.     

Binocular recognition summation in the peripheral visual field: contrast and orientation dependence

Spatial frequency thresholds for recognition were measured for binocular and monocular viewing conditions at two contrast levels (95% and 7%). Measurements were obtained at the fovea and at four different eccentric retinal locations. Each eccentric retinal location was 8 degrees from the fovea, one on the horizontal axis (180 degrees ), and the other three in the superior field on retinal axes of 90 degrees, 45 degrees and 135 degrees. For the superior and horizontal retinal locations, the orientations of the gratings tested were horizontal (180 degrees ) and vertical (90 degrees ). For the retinal points on the oblique axes, the orientations of the gratings were 45 degrees and 135 degrees. Measurements were also obtained at the fovea for all four different grating orientations at both levels of contrast. Recognition threshold was defined as the highest spatial frequency at which luminance gratings were perceived vertically. At the fovea, binocular summation ratios (binocular spatial frequency/monocular spatial frequency) showed no significant differences for gratings of either contrast levels or for any orientation (p>0.05). In the superior periphery, significantly higher summation ratios were shown by low contrast vertical gratings (p<0.05), and in the horizontal periphery by low contrast horizontal gratings (p<0.05). On the oblique axis, low contrast gratings that were parallel to the oblique meridian showed higher summation ratios compared to those at right angles to it. High contrast gratings, at least at 8 degrees eccentricity, did not show this effect. Data suggest that meridional organisation of the retina (e.g. vertical gratings seen maximally in the superior field) occurs for resolution and that it is evidenced closer to the fovea than previously shown.     

Influence of foveal distractors on saccadic eye movements: a dead zone for the global effect

Three experiments investigated the global effect with foveal distractors displayed in the same hemifield as more eccentric saccade targets. Distractors were x-letter strings of variable length and targets corresponded to the central letter of letter strings (e.g., 'xxxkxxx'). Results showed that only foveal distractors longer than four letters (about 1 degree) deviated the eyes in a center-of-gravity manner thus suggesting a dead zone for the global effect. Short distractors influenced the likelihood of small-amplitude saccades (less than about 1 degree) and the latency of longer saccades. The findings were interpreted based on the dissociation between fixation and saccadic neurons. Implications for eye movements in reading were discussed.