What is the difference between a saccade and smooth pursuit

Green intrusions are regions where a saccade was removed and velocity interpolated. Figure 2 Typical smooth pursuit trial, participant P1. Figure 3. Eye position for individual smooth pursuit trials relative to the center of a small, 0. Dot cluster of each color represents an individual smooth pursuit trial, and each dot is a single frame from that trial. Target is centered at 0, 0. Crosses are per-trial average eye positions.

Inset: Criteria for eye—position location. Solid dots represent eye positions on the target center, that fall within the BCEA , whereas open circles are outside the target center, even when fixation stability BCEA is taken into account. The dashed oval marks the locus of fixations within BCEA of the target perimeter and includes eye positions on any part of the target see relative increase in the number of solid dots.

Figure 3 Eye position for individual smooth pursuit trials relative to the center of a small, 0. Figure 2 illustrates a typical smooth pursuit trial. First, we looked at the position of the fovea the center of the loci of fixations as defined by the BCEA relative to the center of mass of the target during pursuit, across all participants Figure 3. For each participant and each trial, we calculated the ratio of frames that the fovea eye position defined by the BCEA landed on the target center [0,0] in Figure 3 for small 0.

Across participants, the fixational BCEA rarely fell on the target center, for either small ratio: 0. However, when we increased the area of interest to include the entire target dashed oval in Figure 3 , instances where the BCEA fell within this target area were substantially more common, as shown in Figure 4B small: 0.

To ensure that these results were not an artifact of the fixation data being collected in a different SLO over a longer time interval, we also used the pretrial fixation periods to estimate BCEA parameters in the Rodenstock SLO. Figure 4. Blue bars are data for small 0. Error bars are one standard deviation of the mean. Group means are shown as the rightmost bars for each plot. Because pursuit gains showed some variation across trials, we considered the possibility that retinal slip could account for the participants' lack of foveation on some trials.

The results did not change significantly small: 0. To further confirm that the trend was not due to a possible increase in BCEA size during pursuit, we also analyzed the average eye position during continuous smooth pursuit for each trial, relative to the center of the target, and computed the ratio of trials where mean eye position fell within the BCEA relative to target center and within the BCEA of target perimeter, on average small: 0.

Two observations from the data provide evidence against a centering strategy of the eye on target. First, the eye was often placed on some noncentral portion of the target compare Figures 4A vs. Summarized another way, when we looked at the average eye position placement relative to the center of the target, participants tended to place their eyes further away from center for the large target than for the small mean eye distance to center: 0.

At an individual level, all but one participant P6 showed an increase in eye distance to center with increased target size. We also examined if there was a preferred placement location of the eye relative to target across trials for a given participant, or across the entire group. We rotated all trials so that the target was at the origin moving leftwards and found a broad distribution of eye placements for both target sizes Figure 5B , C.

In particular, there is no systematic trend for the eyes to lag the target to be on the right of the target. However, eye position is further from target center for the larger target Figure 5C.

Figure 5. Mean eye distance relative to the center of the target during smooth pursuit A. Asterisk indicates a significant difference in eye—target distance for small 0. Group means are the rightmost set of bars. Distribution of average eye positions relative to the target crosses in Figure 3 during smooth pursuit for 0. For all trials, eye positions have been rotated such that they lie along the horizontal axis, with target center at the origin, and the direction of motion leftward.

Figure 5 Mean eye distance relative to the center of the target during smooth pursuit A. A sample of a saccade trial is illustrated in Figure S2 in Supplementary Materials. Fixation stability was measured at the initial and post-saccade positions for both target sizes using the BCEA approach described above 30 central fixation and 30 postsaccade eccentric fixation frames were used to compute the respective BCEAs.

To confirm previous findings that eyes tend toward the center of mass of the target after a saccade, we compared BCEAs for large versus small targets at central fixation and postsaccade eccentric locations. Consistent with previous observations, BCEA values were not significantly different for 1. Although this measure addresses the effects of target size on fixational stability, it does not directly address the eye centering characteristics.

Therefore, we also compared the target center and BCEA center locations at the eccentric location. Table 1 View Table. Mean BCEAs for central and eccentric gaze fixations units: deg 2 of visual angle. Figure 6. Distribution of postsaccade eye positions relative to the target for 0. All target and eye positions have been rotated to lie on the horizontal axis, with target center at the origin.

Negative eye positions relative to the target indicate hypometric saccades. To determine whether eye position was placed on the center of the target, we looked at the eye positions relative to the target in the direction of target motion saccade.

First, we examined the absolute distance between mean eye position and target center across trials, for each participant. To determine whether the offset was in a particular direction relative to the target, all targets and eye positions were rotated to lie on the horizontal axis with the saccade target at the origin Figure 6B , C.

On the whole, saccades were hypometric but there was no significant difference between eye placements for 1. To test whether the annular shape of the target was responsible for the results reported previously, we repeated the smooth pursuit and saccade experiments in two participants with a 1.

Specifically, we wanted to determine whether the higher fraction of eye positions on the edge of the annular target was because participants were tracking a visible part of the annulus, rather than its virtual center.

We calculated eye—target distance as described for Figures 5 and 6A for smooth pursuit and saccades, respectively. These results suggest that the presentation of a solid versus annular target shape does not alter the distribution of eye positions around the target. Table 2 View Table. Mean eye—target distances for 1. Table 2 Mean eye—target distances for 1. Experiment 4: Relationship of eye position and target size. Our data from Experiments 1 and 2 suggest that target size affects eye placement during smooth pursuit, but not saccades.

To determine whether the pattern of results we obtained in Experiments 1 and 2 would hold for a larger range of target sizes, we repeated the saccade and smooth pursuit experiments using three target sizes 0. Target directions were randomized within blocks. We plot the leftward and rightward data separately in the Supplementary Materials Figure S3 , but show the combined data across rightward and leftward trials in Figure 7.

Figure 7. Each symbol represents an individual trial. Solid black lines are the linear regression fit. For smooth pursuit, there was a strong relationship between eye distance from target center and target size.

For saccades, the relationship was present, but substantially weaker Table 3. We repeated the analysis for P1's smooth pursuit trials, separating the two target directions and found a significant positive slope for both directions Table 3. Table 3 View Table. Linear regression fit parameters for eye—target distance as a function of target size. Table 3 Linear regression fit parameters for eye—target distance as a function of target size.

This paper examines foveation during smooth pursuit and static pre- and postsaccade viewing in untrained observers. Our results suggest that unlike for saccades, the fovea is not centered on the target during smooth pursuit.

The current study addresses the complementary question of whether foveation necessarily occurs during pursuit. The results suggest that the fovea might not track the center of the target, as is illustrated in Figure 3 e. Interestingly, these trials are not an indication of worse pursuit gain. Our data also show an increase in the average distance of eye position from target center for bigger 1.

When we repeated the pursuit experiment with two of the participants using a larger number of trials per condition and three target sizes, we confirmed a significant increase in the distance of eye placements relative to target center with increasing target size Figure 7C , D. Our results for pursuit are consistent with previous work that argues against a center-of-mass strategy for smooth pursuit when a static target is flashed in the vicinity of a pursuit target Blohm et al.

The variation between participants in their eye placement strategies is interesting to note, with some virtually never tracking the target center and others more frequently foveating the center e. Although all subjects had little to no prior experience with smooth pursuit tracking, other experiences may have played a role in this variability. For example, when we examined eye placement strategies in terms of average eye position, participants who had extensive video game or psychophysical experience P3, P4, P5, and P8 seemed to exhibit, on average, a more central eye placement strategy for big targets.

To compare this result to the center-of mass strategy previously reported for saccades, we modified the experiment so that observers made a single saccade to an eccentric target. Consistent with previous literature, we found similar eye placement relative to target center for large 1. When we examined the target size—eye placement relationship with three target sizes, we did find a small, but significant, increase in distance of eye placement from target, across the three sizes Figure 7A , B.

Although this outcome is similar to that reported for one of the participants in the Kowler and Blaser study, the size effect in that work was most evident for largest targets, whereas in our data, the greatest increase in eye—target distance occurs between the smallest 0. Figure 6 demonstrates a similar variation between participants in our data, although to a lesser extent. Eye placement accuracy may decrease simply due to the eccentricity of the target in the visual field.

Our results confirm that although BCEA may increase when viewing eccentric targets, its placement is consistent with a centering strategy for saccades.

Our findings provide another interesting insight. When we looked at the average distance of eye relative to target center, we found that for both saccades and smooth pursuit there was an offset in the absolute distance between eye position and target center. You just did what we call a pursuit. Saccades are rapid eye jumps, bringing our focus from one object to another.

Pursuits are smooth eye movements that involve following or tracking a moving target. This is especially important for people such as athletes who need to keep their eyes on a moving ball. We do these two eye movements on a regular basis without much thought. However, there is an intricate system involving the brain and our eyes working together to help us execute these movements. A corpus of 84 short texts taken from articles in the science section of the German online magazine Spiegel Online was used in both experiments.

The texts had to be meaningful when read in isolation usually we chose the opening sentences of magazine articles. For the pretest session, observers read one full article with words from the same magazine section.

To avoid mindless reading, observers had to answer four questions about the full article afterwards. This pretest session was used to measure the natural reading behavior in a fairly standard presentation condition.

The sentences were displayed on a in. The text was displayed in black on a white background. We used the Arial font, at size 20, corresponding to a character height of Stimulus presentation was controlled using Matlab MathWorks, Inc.

The position of the right eye was sampled at Hz. The spatial resolution was 0. The average velocity in the last 40 ms is added to the velocity threshold, which allows saccade detection even during ongoing smooth-pursuit eye movements. Two experiments were conducted, involving either horizontally or vertically drifting text. In both cases, we also tested a condition where static text was read in a comparable display and with a comparable presentation rate.

In general, a trial involved the presentation of one text, followed by the question to which the observers responded by pressing one of two keys. At the beginning of each trial, the observers fixated on a dot in the center of the screen Figure 1A.

Figure 1. View Original Download Slide. Display and procedure. A Horizontally drifting text. B Vertically drifting text. For both drift axes, observers fixated on the red dot and pressed a key to start reading.

The red arrows and the red dashed rectangles not present in the experimental display indicate the direction of text motion and the window where the text was visible, respectively.

Figure 1 Display and procedure. In the static-text condition, the text was segmented in lines which were as long as possible while still fitting in the viewing window. Observers fixated on the left of the text window and the lines were presented sequentially, aligned on the left side of the viewing window.

The presentation time of each line was scaled to the line length 44 ms per character. The three text speeds 0, At the beginning of each block of trials, the observer was informed whether the text would move slowly, move fast, or be static.

The first trial of each block was considered practice, and the corresponding data were not analyzed. At the beginning of the trial, the observers fixated on a dot located After a key press, the first line of the text appeared next to the fixation point and drifted at a speed of 3. The subsequent lines appeared in the same position, one after the other, at a distance of 5. The lines were visible within a window whose vertical size was In order to approximately equate reading-speed demands when the text speed increased, we imposed a maximum number of characters per line of 38 when the text speed was 3.

In the case of the static text, four lines were present on the screen at the same time. We tested two conditions with static text, where the maximum line length was matched to the one we used in the two text speeds 38 and 18 characters. In order to make clear that our experimental design is fully factorial, in the remainder of this article we refer to the static-text conditions with short and long lines by the labels which define the slow and fast speeds of the drifting text associated with the same line length.

Again, the four lines remained on the screen for a time equal to 44 ms times the total number of characters. Each of the six combinations of text speed 3. At the beginning of each block of trials, the observer was informed about the speed and direction of the test motion. In the case of the pretest, the text was presented on two pages, the first containing 18 lines and the second containing 16 lines.

Reading was self-paced: Observers pressed the space bar on the keyboard in order to switch from the first page to the second and in order to signal that they had finished reading. In both the horizontal- and vertical-reading experiments, we tried to isolate the steady-state reading saccades. To this end, we discarded the data from both the first and last 2 s of recording from each trial.

Second, in order to identify perisaccadic pursuit tracks not contaminated by multiple saccadic events, we discarded all saccadic events where another saccade was observed in the 50 ms preceding the start of the event and in the ms following the start of the event.

Third, in the case of the horizontally drifting text, we decided to discard the saccadic events whose horizontal starting and landing positions were within 3. These borders constituted a screen-fixed landmark even though the text was drifting and in these regions only parts of the words would be visible. In a similar approach, when the text drifted vertically we discarded the saccadic events whose vertical starting and landing position were within 1.

Finally, we only selected saccades going from left to right, thus removing saccades leading to refixations and line jumps in the vertically drifting text experiment. In the case of the pretest data, we only considered the fixation and saccadic events whose positions were to the right of the immediately preceding one and left of the next one, thus excluding events directly following or preceding regressions or line jumps. After selecting the saccadic events, we isolated the eye tracks of the relevant coordinate x for the horizontally drifting text and y for the vertically drifting text , ranging from 50 ms before to ms after the beginning of each saccade.

Each track was baseline-corrected by subtracting the average position in the first 50 ms. For each observer and experimental-design cell, we computed the mean and standard deviation of the baseline-corrected position tracks in 16 equally spaced samples i. The tracks whose position value differed from the mean by more than two standard deviations in any of the 16 samples were discarded from the analysis together with the corresponding saccade events. The percentages of epochs which were discarded based on the different criteria are indicated in Table 1.

Table 1 View Table. Percentages of saccadic epochs discarded from the analysis based on different selection criteria. Table 1 Percentages of saccadic epochs discarded from the analysis based on different selection criteria. We analyzed the extracted eye-movement parameters with repeated-measure ANOVAs with factors for text motion and text speed.

Differences between individual factor levels were assessed with t tests, and p values were adjusted with the Bonferroni correction for multiple comparisons.

To avoid mindless reading, participants had to answer one question after each text, with four questions after the self-paced reading in the pretest. The participants who read the text drifting horizontally answered Their accuracy did not change between the three text speeds static, In the case of the vertically drifting text, the answers were correct in The observers read the drifting text through episodes of smooth-pursuit tracking.

This behavior largely compensates for text drift, and the resulting text-referenced eye movements are similar to those observed during the reading of static text see Figure 2 for an example from the vertically drifting text experiment.

Figure 2. Exemplary eye-movement traces. A Raw eye-position measurements from one observer reading vertically drifting text direction upward, speed 3. B Same eye-movement data as in A , but this time referenced to the position of the drifting text.

C Example of eye movements while the same observer read a static text. Red identifies the smooth-pursuit episodes in A and B and fixation episodes in C , whereas saccadic eye movements are marked in black.

Notice that smooth-pursuit episodes appear as elongated lines along the vertical axis i. The gray rectangles mark the vertical position of the text lines.

The origin of the reference frame is the lower left corner of the screen in A and C , whereas the vertical position in B is referenced to the initial position of the text line which was read first. Figure 2 Exemplary eye-movement traces. The duration of those smooth-pursuit episodes, along with the durations of the fixations during the reading of static text in the horizontally drifting text experiment, are shown in Figure 3A.

Furthermore, post hoc comparisons indicate that fixations executed during the reading of the static text were shorter than both the smooth-pursuit episodes when the text moved at Figure 3. Durations of fixations in the static-text condition and of smooth-pursuit episodes in drifting-text conditions. The fixations in the static-text condition were shorter than the smooth-pursuit episodes in reading drifting text.

The durations were reduced in the condition with a text speed of 7. Error bars in both parts of the figure are standard errors of the mean. Figure 3 Durations of fixations in the static-text condition and of smooth-pursuit episodes in drifting-text conditions. The duration of smooth-pursuit episodes and fixations in the vertically drifting text experiment are shown in Figure 3B. The condition with the higher text speed was associated with a shorter duration of the fixations when the text moved upward and with shorter fixations when the text was static.

The fact that fixations were shorter in the 7. The duration of the fixations and smooth-pursuit episodes in the horizontally drifting text Table 2 and the vertically drifting text Table 3 was strongly positively correlated to the duration of the fixations that each observer produced in the self-paced reading pretest experiment.

This indicates that individual differences in reading style were not completely erased by the reading pace imposed by our experimental manipulations. Table 2 View Table. Correlation of episode fixation and smooth-pursuit durations in the pretest self-paced reading and in each condition of the horizontally drifting text experiment.

All three correlations are positive and significant. Table 2 Correlation of episode fixation and smooth-pursuit durations in the pretest self-paced reading and in each condition of the horizontally drifting text experiment. Comparison condition r p value Bonferroni corrected Static 0. Table 3 View Table. Correlation of episode fixation and smooth-pursuit durations in the pretest self-paced reading and in each condition of the vertically drifting text experiment.

All six correlations are positive and significant. Table 3 Correlation of episode fixation and smooth-pursuit durations in the pretest self-paced reading and in each condition of the vertically drifting text experiment. Comparison condition r p value Bonferroni corrected Upward, 3. In Figure 4A and B is presented the evolution of smooth-pursuit gain through the time course of a selected subset of smooth-pursuit episodes.

In order to construct the plots, we selected episodes from the horizontally and vertically drifting text experiments whose durations ranged between and ms and realigned all the corresponding pursuit gain tracks to a fixed duration of ms. A further Gaussian filtering with 4. For each observer, we pooled episodes from all text speeds, and for the vertically drifting text experiment we pooled episodes also from the upward and downward directions of text motion, and subsequently we averaged the tracks across observers.

In the case of the horizontally drifting text data, a postsaccadic enhancement of pursuit gain is evident as soon as the gaze reaccelerates at the end of the saccade saccade and pursuit are in opposite directions , and after around 50 ms the pursuit gain decreases below 1.

A similar pattern of decreasing pursuit gain along the episode time course is evident in the case of the vertically drifting text, although, given that the saccade and smooth-pursuit directions are substantially orthogonal, the pursuit gain is already near 1 at saccade onset.

Figure 4. Evolution of smooth-pursuit gain throughout the fixation time course A and B and across saccades C through H. A and B Tracks with slightly different durations have been remapped to a fixed duration of ms. Data have been pooled over all text speeds and text-motion directions. C through H Perisaccadic modulation of pursuit gain in the horizontally C and D and vertically E through H drifting text experiments.

Presaccadic gain tracks were aligned on saccade onset, whereas postsaccadic tracks were aligned on saccade offset. Light-gray areas delimit the time intervals where pre- and postsaccadic pursuit gain was sampled in order to construct Figure 5. Figure 4 Evolution of smooth-pursuit gain throughout the fixation time course A and B and across saccades C through H.

Figure 5. Pursuit gain. Pursuit gain was computed in two time windows before: 50—30 ms before saccade onset; after: 30—50 ms after saccade onset. Pursuit gain increased after saccade execution and was higher for the lower speed. B Upward-drifting text.

C Downward-drifting text. B and C Similar to horizontally drifting text, the gain of the smooth pursuit increased after saccade execution and was larger for the lower text speed. All error bars are standard errors of the mean. Figure 5 Pursuit gain. In order to investigate the modulation of smooth-pursuit gain in a larger number of episodes without having to deal with largely different durations, we also investigated the evolution of smooth pursuit, aligning the tracks separately on saccade onset and offset Figure 4C through H.

The postsaccadic enhancement of smooth-pursuit gain 30—50 ms after saccade offset is still present when all episode durations are considered, and in all cases the gain of the smooth pursuit seems to increase across saccades. In order to test the effect of the execution of a saccade on the ongoing smooth pursuit, we compared the gain of the smooth pursuit i. In the case of the horizontally drifting text Figure 5A , results were analyzed by means of a two-way repeated-measure ANOVA with time window before vs.

In the case of the vertically drifting text, results were analyzed by means of a three-way repeated-measure ANOVA with time window before vs. The pattern of results was partially similar to the one observed in the case of the horizontally drifting text Figure 5B and C. In the case when the text was moving vertically, the question arises as to whether the programming and execution of the mainly horizontally oriented reading saccades took into account the concomitant orthogonal drifting of the text.

The average vertical and horizontal components of saccades executed while reading vertically drifting text are represented in Figure 6. The data seem to indicate that saccades target the position where the text arrives at the end of the saccade, rather than being horizontal.

Figure 6. Average vertical and horizontal components of saccadic eye movements executed while observers read text drifting vertically at the two speeds A and B, respectively. Each dot represents an observer's average; the crosses represent the mean and standard error of the mean of the components. The vertical component of saccades executed while reading static text was subtracted from the component observed while observers read drifting text.

The vertical component of the saccades largely overlaps with the text motion, indicating that the tracking of the text is integrated in the programming of saccades. Figure 6 Average vertical and horizontal components of saccadic eye movements executed while observers read text drifting vertically at the two speeds A and B, respectively. In order to construct the plot in Figure 6 , we first isolated forward reading saccades i.

We noticed that for most observers, the saccades executed while reading static text had a quite sizable vertical component, which we attribute to a slight calibration misalignment. Thus, as a second step in the analysis, for each observer and text speed we linearly regressed the vertical amplitude of the saccades executed while reading static text on their horizontal amplitude.

The regression parameters were then used to predict the amount of artifactual vertical component for the saccades executed while reading vertically drifting text, as a function of their horizontal component. Furthermore, for each saccade executed while observers read the drifting text, we computed the amount of vertical drift that the text covered between the start and the end of the saccade, that is, the vertical component that the saccade should have in order to perfectly target the future position of the text.

In order to evaluate the results statistically, we performed a three-way repeated-measure ANOVA on the vertical components taking as positive a displacement coherent with the direction of text motion with prediction observed vs.

The ANOVA results confirm that the vertical component of saccades scales with text-motion speed, so as to cover the vertical displacement of the text which occurs while the saccade is executed. Average saccadic amplitude in the horizontally drifting text experiment is shown in Figure 7A.

Notice that if saccadic programming fully took into account the motion of the text, thus simply summing to smooth pursuit, the horizontal saccades should be shorter by the space which is covered by the text during the saccade. This does not seem to be the case, and indeed the data in Figure 11A seem to indicate that saccades land farther to the right within a word when the text drifts towards the left as compared to when it is static.

Figure 7. Saccade amplitudes. Saccade amplitude did not differ between different text-motion speeds. Saccade amplitude was reduced in the condition with a text speed of 7.

Figure 7 Saccade amplitudes. Average saccadic amplitude in the vertically drifting text experiment is shown in Figure 7B.

Notice that the fact that text speed had an effect on saccade amplitude when the text was static strongly suggests that this effect can be interpreted as our observers executing shorter saccades when the text lines contained fewer characters rather than as an effect of text motion per se.

Tables 4 and 5 represent the coefficients and associated p values for the correlations between saccadic-amplitude averages in the pretest, where reading was paced, and in the different conditions of the two experiments. Similar to the case of the fixation duration, all correlations were significantly positive, indicating that the individual differences in reading style were preserved and suggesting that the reading behavior was not completely altered by the display properties.

Table 4 View Table. Correlation of observers' saccadic-amplitude averages in the pretest self-paced reading and in each condition of the horizontally drifting text experiment. Table 4 Correlation of observers' saccadic-amplitude averages in the pretest self-paced reading and in each condition of the horizontally drifting text experiment. Table 5 View Table. Correlation of observers' saccadic-amplitude averages in the pretest self-paced reading and in each condition of the vertically drifting text experiment.

Table 5 Correlation of observers' saccadic-amplitude averages in the pretest self-paced reading and in each condition of the vertically drifting text experiment. We investigated both the correlation between the amplitude of a saccade and the duration of the next fixation and the correlation of the duration of a fixation with the amplitude of the next saccade. After removing outlier events saccades smaller than 0.

The average parameters of the linear regression for the relationship between fixation duration and the amplitude of the previous saccade in the horizontally drifting text experiment are represented in Figure 8A and C. Post hoc comparisons indicate that the intercept values when the text was drifting at Figure 8. A and C Horizontally drifting text. All slope values were positive and did not differ between experimental conditions.

Intercept values were higher when the text speed was B and D Vertically drifting text. Intercept values were higher when the text speed was 7. The average parameters of the linear regression for the vertically drifting text experiment are represented in Figure 8B and D. The average parameters of the linear regression for the relationship between fixation duration and the amplitude of the next saccade in the horizontally drifting text experiment are represented in Figure 9A and C.

Contrary to what we observed in the case of the previous saccade, the relationship between the duration of a fixation and the amplitude of the next saccade is not straightforward. The slope values when the text drifted at Figure 9.

Slope values were higher when the text drifted at Slope values were higher when the text speed was 3. Intercept values were lower when text speed was 7.

The average parameters of the linear regression for the vertically drifting text experiment are represented in Figure 9B and D.