Separate adaptive mechanisms for the control of reactive and volitional saccadic eye movements

Adaptive reduction of the gain of the saccadic system was induced by means of two basically different paradigms. In the first approach the subjects had to follow a step-wise moving target. During each follow-up saccade the target was systematically displaced by 25% of the initial step, into the opposite direction of the saccade. In the second approach the subjects scanned a display of six small items. During each scanning saccade the whole display was displaced by 25% into the opposite direction of the saccade. Both conditions lead to fast and consistent saccadic gain reductions. However, adaptation with the stepping target did not transfer to the saccades in the scanning situation, nor to delayed saccades in an overlap paradigm, nor to memory-guided saccades. Conversely, when saccades were adapted in the scanning situation, induced gain changes transferred to overlap and memory-guided saccades, but not to saccades following steps of a single target. The results suggest that two separate and largely independent mechanisms are involved in the generation of reactive, stimulus-triggered and volitional, internally generated saccades, respectively. Both types of responses can be selectively adapted.     

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.     

Contrast sensitivity during saccadic eye movements

The experiment measured contrast sensitivity of three human observers to sinusoidal gratings presented in 10 msec exposures. The gratings were presented to the steadily fixating eye and during 6° horizontal saccades. Experimental conditions of viewing in a Ganzfeld reduced possible effects of contour masking. The use of horizontal gratings minimized retinal smear.Results showed a significant suppression of sensitivity (more than 0.6 log unit of contrast) to low spatial frequency gratings presented during saccades. The magnitude of saccadic suppression decreased as spatial frequency of the gratings increased.We conclude that optical and neural effects combine in normal viewing to produce saccadic suppression. Minimizing the optically originating effects of contour masking and retinal image smear failed to eliminate the considerable impairment of vision that occurs during a saccadic eye movement.     

Rapid gain adaptation affects the dynamics of saccadic eye movements in humans

The effect of rapid gain adaptation on the dynamics of visually guided saccades was investigated in six human subjects by using a search coil system. Saccadic adaptation was induced artificially by dislocating the target (by about 30% of the initial step) either forward (gain increase) or backward (gain decrease) during the primary saccade (“double-step paradigm”). Duration, peak velocity and peak acceleration and deceleration of a “standard 12 deg saccade” were computed from the data and were compared for the conditions of gain decrease, gain increase and the control without gain adaptation. The gain as well as the peak velocity and duration of the saccades showed an increased variability during the adaptation. In general, the abducting saccades had a higher peak acceleration than the adducting saccades, and all subjects showed an idiosyncratic pattern of the acceleration and deceleration. In the gain increase paradigm the subjects showed an increase in the duration and a decrease in the peak velocity. In the gain decrease paradigm there was a significant smaller ratio of peak acceleration/peak deceleration compared to the gain increase and the control condition. The findings demonstrate that rapid gain adaptation influences the dynamics of saccades in a specific way: peak saccadic velocity decreases and duration increases in the gain increase paradigm and peak acceleration/peak deceleration decreases in the gain decrease paradigm. Moreover, these results also suggest that the deceleration is neuronally controlled and not merely a result of mechanical constraints.     

Spatial frequency tuning in the visual evoked potential elicited by sine-wave gratings

This investigation examined the onset response of the transient visually evoked potential elicited by the appearance-disappearance of sine-wave gratings at various levels of spatial frequency, suprathreshold contrast, and stimulus duration. The response was most easily characterized by two negative-positive complexes that were differentially tuned to spatial frequency. The earlier complex peaked at high spatial frequencies while the later complex peaked at low spatial frequencies. For both complexes, amplitude showed only slight variations across three-octave ranges of contrast and duration. Latency was curvilinearly related to spatial frequency, decreased with increasing contrast, and showed no apparent change as a function of duration.     

Spatial working memory load impairs manual but not saccadic inhibition of return

Although spatial working memory has been shown to play a central role in manual IOR (Castel, Pratt, & Craik, 2003), it is so far unclear whether spatial working memory is involved in saccadic IOR. The present study sought to address this question by using a dual task paradigm, in which the participants performed an IOR task while keeping a set of locations in spatial working memory. While manual IOR was eliminated, saccadic IOR was not affected by spatial working memory load. These findings suggest that saccadic IOR does not rely on spatial working memory to process inhibitory tagging.     

The role of presaccadic compression of visual space in spatial remapping across saccadic eye movements

When multiple bars are briefly flashed near the saccadic goal on a visual reference just before a saccade, the total width of the multiple bars appears to be compressed toward the saccadic goal. We show that presaccadic compression of visual space is related to the attribution of the displacement of a visual stimulus to the displacement of another stimulus appearing after the saccade. Subjects observed a bar and a ruler. The bar was displaced during a saccade and the ruler disappeared briefly at the same time, and then the ruler reappeared at the same location after the saccade. The subjects had the impression that the bar appeared to remain stationary and the ruler appeared to be displaced after the saccade. This impression occurs strongly when the amount of the compression of visual space reaches the maximum at the saccade onset. Also, it occurs only at the saccadic goal in the same way as presaccadic compression of visual space. Saccadic suppression of displacement was equivalent at the saccadic goal and in the location opposite to the saccadic goal, indicating that the attribution of the bar displacement to the displacement of the ruler appearing after the saccade is not a consequence of saccadic suppression of displacement. Furthermore, performing a direction discrimination task showed that the bar appears stationary at the saccadic goal during compression of visual space even when the bar was actually displaced. We interpret these results as showing that presaccadic compression of visual space establishes the location of the saccadic goal (the bar) as a reference and then the location of the ruler is remapped relative to the reference location after the saccade, resulting in the illusory displacement of the ruler.     

The relative contributions of luminance contrast and task demands on saccade target selection

Luminance contrast and spatial frequency have a strong effect on when saccades are initiated. In this study, we ask to what extent the internal contrast response determines where saccades are directed to. Observers signalled, with a manual button press, which of two patterns was of higher (Experiment 1) or lower (Experiment 2) contrast. Even though the visual stimuli were identical in both experiments, the pattern of first fixated items was very different. Saccade target selection largely reflected the task instructions, suggesting that luminance contrast can be used to rapidly and effectively guide the eyes to task-relevant information.     

Some characteristics of saccadic eye movements in children of primary school age

The characteristics of saccadic eye movements have been extensively studied in adults; researches have also been devoted to the saccades of preschool age children. On the contrary, for primary school-age children no data exist; we investigate the eye movements (recorded utilizing an infrared technique) of six children 7 to 11 years old. The main results indicate that the values of some parameters (for example the saccadic latency and duration) are in the same range as the values of the correspondent parameters in adults, while the values of other parameters (in particular peak velocity and mean velocity/peak velocity ratio) are distinctly different from the ones measured in adult subjects.     

Contrast sensitivity and magnocellular functioning in schizophrenia

It has been suggested that schizophrenia is associated with a magnocellular deficit. This would predict a loss of contrast sensitivity at low spatial and/or at high temporal frequencies. We here review research that tested contrast sensitivity in individuals with schizophrenia. We find that the results of this research tend to show uniform reductions in contrast sensitivity that are generally not consistent with a magnocellular deficit. While much of this data may be consistent with an attentional deficiency on the part of the schizophrenic individuals, it is difficult to link such an attentional deficiency specifically to the magnocellular system. The conclusion of the present review is that contrast sensitivity data do not indicate the existence of an association between magnocellular deficits and schizophrenia.     

The effect of contrast on sustained detection

During a l min observation, the percentage of time a high spatial frequency grating can be detected is influenced by the contrast and spatial frequency of a second, superimposed and orthogonally-oriented sine wave grating. Increasing contrast of the second pattern aids detection of the first by providing a more effective accommodative stimulus. The function relating spatial frequency and the minimum contrast needed to activate accommodation is similar in shape to the classical contrast sensitivity function. However, an order of magnitude more contrast is required to stabilize accommodation than is required to simply detect a pattern. These results suggest that performance on visual tasks requiring sustained rather than brief detection may be markedly impaired under low contrast conditions.     

基于空间频率通道的弱视眼与正常眼对比敏感度的差异分析

背景 对比敏感度(CS)作为弱视诊疗手段之一在临床中已广泛应用,但其与空间频率通道之间的相互影响还有待研究. 目的 比较儿童弱视眼与正常眼的CS,通过空间频率通道探讨弱视眼CS缺损的原因.方法 采用OPTEC 6500型视功能测试仪对166眼正常眼和143眼弱视眼进行CS测量,通过主成分分析法和非正交旋转法推导空间频率通道的调谐曲线,由半高宽法计算通道带宽,比较通道数目和通道带宽,对弱视眼和正常眼的CS进行差异分析.空间频率通道的有效性采用43眼弱视眼CS数据进行交叉验证.结果 在空间频率为1.5、3.0、6.0、12.0、18.0cpd时,弱视眼CS分别为:36.35±21.40、50.33±33.46、46.88±41.72、16.24±17.26、4.67±5.79;正常眼CS分别为:49.49±24.69、87.23±40.87、93.18±51.99、36.63±24.72、15.70±13.87(H=27.83、66.61、68.34、78.23、89.88,P＜0.05).弱视眼和正常眼的CS存在三个空间频率通道;在空间频率峰值为3.0、6.0、12.0 cpd时,正常眼的通道带宽分别为1.03、1.02、0.99 octaves,弱视眼的通道带宽分别为1.04、1.01、0.73 octaves. 结论 弱视眼CS的缺损可能由对应空间频率通道带宽的缩小引起.     

Effects of stimulus contrast and temporal delays in saccadic distraction

In a recent study, we observed that saccadic distraction (i.e., the remote distractor effect, RDE) was reduced when target and distractor were displayed at unequal contrast [Born, S., & Kerzel, D. (2008). Influence of target and distractor contrast on the remote distractor effect. Vision Research, 48(28), 2805-2816]. We hypothesized that arrival times explain the RDE modulation: With equal contrast, target and distractor signals arrive simultaneously in the oculomotor system so that mutual inhibition (and therefore saccadic distraction) is largest. With unequal contrast, high-contrast signals arrive earlier than low-contrast signals, resulting in less mutual inhibition and little saccadic distraction. In the current contribution, we presented target and distractor at different stimulus onset asynchronies (SOA) to re-align arrival times with unequal contrast. Results confirmed that unequal contrast of target and distractor reduced saccadic distraction with simultaneous presentation and that strong distraction could be reestablished by introducing a SOA. However, maximal saccadic distraction also varied strongly with the specific combination of target and distractor contrast. Thus, contrast may not only modulate arrival times of target and distractor signals, but also their strength in the mutual inhibition process. Finally, we found more saccadic distraction when the distractor was presented slightly after the target. A second experiment suggests that alerting effects superimposed on the distraction contribute to this effect, but may not fully explain it. We suggest that distraction may be strongest when the rise-to-threshold of the target-related signal has already advanced.     

Spatial consequences of bridging the saccadic gap

We report six experiments suggesting that conscious perception is actively redrafted to take account of events both before and after the event that is reported. When observers saccade to a stationary object they overestimate its duration, as if the brain were filling in the saccadic gap with the post-saccadic image. We first demonstrate that this illusion holds for moving objects, implying that the perception of time, velocity, and distance traveled become discrepant. We then show that this discrepancy is partially resolved up to 500 ms after a saccade: the perceived offset position of a post-saccadic moving stimulus shows a greater forward mislocalization when pursued after a saccade than during pursuit alone. These data are consistent with the idea that the temporal bias is resolved by the subsequent spatial adjustment to provide a percept that is coherent in its gist but inconsistent in its detail.     

The influence of attending to multiple locations on eye movements

The present paper reports results of a dual task study in which two locations were endogenously cued as possible target locations, while only one eye movement had to be executed. During the cue period, letters were briefly presented at the saccade goals and at no-saccade goals. Results show that performance was better for letters presented at any of the saccade goals than for letters presented at the no-saccade locations. Furthermore saccades deviated away from the non-saccaded target location, suggesting inhibition of the location to which the eyes should not go. The results indicate that the premotor theory also holds for conditions in which attention is allocated to multiple locations.     

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.     

Spatial frequency selectivity of visual suppression during convergence eye movements

Visual suppression of low-spatial frequency information during eye movements is believed to contribute to a stable perception of our visual environment. While visual perception has been studied extensively during saccades, vergence has been somewhat neglected. Here, we show that convergence eye movements reduce contrast sensitivity to low spatial frequency information around the onset of the eye movements, but do not affect sensitivity to higher spatial frequencies. This suggests that visual suppression elicited by convergence eye movements may have the same temporal and spatial characteristics as saccadic suppression.     

The influence of spatial frequency and contrast on saccade latencies

We characterised the impact of spatial frequency and contrast on saccade latencies to single Gabor patches. Saccade latencies decreased as a function of contrast, and increased with spatial frequency. The observed latency variations are qualitatively similar to those observed for manual reaction times. For single target detection, our findings highlight the similarity in the visual processes that support both saccadic and manual responses.     

The vergence eye movements induced by radial optic flow: some fundamental properties of the underlying local-motion detectors

Radial optic flow applied to large random dot patterns is known to elicit horizontal vergence eye movements at short latency, expansion causing convergence and contraction causing divergence: the Radial Flow Vergence Response (RFVR). We elicited RFVRs in human subjects by applying radial motion to concentric circular patterns whose radial luminance modulation was that of a square wave lacking the fundamental: the missing fundamental (mf) stimulus. The radial motion consisted of successive 1/4-wavelength steps, so that the overall pattern and the 4n+1 harmonics (where n=integer) underwent radial expansion (or contraction), whereas the 4n-1 harmonics--including the strongest Fourier component (the 3rd harmonic)--underwent the opposite radial motion. Radial motion commenced only after the subject had fixated the center of the pattern. The initial RFVRs were always in the direction of the 3rd harmonic, e.g., expansion of the mf pattern causing divergence. Thus, the earliest RFVRs were strongly dependent on the motion of the major Fourier component, consistent with early spatio-temporal filtering prior to motion detection, as in the well-known energy model of motion analysis. If the radial mf stimulus was reduced to just two competing harmonics--the 3rd and 5th--the initial RFVRs showed a nonlinear dependence on their relative contrasts: when the two harmonics differed in contrast by more than about an octave then the one with the higher contrast completely dominated the RFVRs and the one with lower contrast lost its influence: winner-take-all. We suggest that these nonlinear interactions result from mutual inhibition between the mechanisms sensing the motion of the different competing harmonics. If single radial-flow steps were used, a brief inter-stimulus interval resulted in reversed RFVRs, consistent with the idea that the motion detectors mediating these responses receive a visual input whose temporal impulse response function is strongly biphasic. Lastly, all of these characteristics of the RFVR, which we attribute to the early cortical processing of visual motion, are known to be shared by the Ocular Following Response (OFR)--a conjugate tracking (version) response elicited at short-latency by linear motion-and even the quantitative details are generally very similar. Thus, although the RFVR and OFR respond to very different patterns of global motion-radial vs. linear-they have very similar local spatiotemporal properties as though mediated by the same low-level, local-motion detectors, which we suggest are in the striate cortex.     

The remote distractor effect in saccade programming: channel interactions and lateral inhibition

We explored the dependency of the saccadic remote distractor effect (RDE) on the spatial frequency content of target and distractor Gabor patches. A robust RDE was obtained with low-medium spatial frequency distractors, regardless of the spatial frequency of the target. High spatial frequency distractors interfered to a similar extent when the target was of the same spatial frequency. We developed a quantitative model based on lateral inhibition within an oculomotor decision unit. This lateral inhibition mechanism cannot account for the interaction observed between target and distractor spatial frequency, pointing to the existence of channel interactions at an earlier level.