On the visual tracking of continuous and apparent motion stimuli : saccadic and smooth pursuit tracking components and their relationship to motion sensitivity

The present thesis uses continuous and apparent motion stimuli to investigate saccadic and smooth pursuit visual tracking eye movements and their relationship to visual motion processing. The aims were as follows: first, to examine the limits and coordination of saccadic and smooth pursuit components of visual tracking in response to apparent motion stimuli in comparison with traditional continuous motion over a wide range of target velocities. A second aim was to investigate the relationship between visual tracking and visual motion perception. In the present thesis visual tracking of continuous motion was compared with two different types of apparent motion stimuli, which systematically fractured the motion stimulus by manipulating the spatial and temporal components of the target signal in order to investigate their effect on smooth pursuit and saccadic eye movements. In addition, evidence for shared inputs to motion perception and smooth pursuit eye movements was investigated by comparing their performance in response to both continuous and apparent motion stimuli under very similar experimental conditions. Three experiments were conducted. Experiment 1 investigated visual tracking of a single dot in continuous motion at nine target speeds (ranging from 2.5 ‚ÄövÑvÆ 40.0 deg/s) in males and females of different age groups (ranging from 18-39 years). This experiment provided a baseline for subsequent studies of apparent motion generated visual tracking and also evaluated different methodologies used to describe and quantify the saccadic component of visual tracking. The findings of Experiment 1 showed no significant age and gender differences. It revealed a significant saccadic contribution to visual tracking at all target velocities and identified the measure of the ratio of distance covered by saccadic versus pursuit eye movements (Ross et al., 1999) as the preferred measure of saccadic tracking. Experiment 2 used two kinds of apparent motion stimuli to elicit visual tracking, an intermittently presented stationary target (jumping-dot motion) and an intermittently presented moving target (slashed motion). These were presented at three stimulus durations (20, 60, 100ms) and five spatial separations (0.5, 1.0, 2.0, 4.0, 5.0 deg) and compared with the tracking of a continuously moving target at nine target velocities (ranging from 2.5- 35.0 deg/s). This study aimed to investigate the interplay of saccadic and smooth pursuit eye movements during visual tracking as a function of stimulus velocity and the spatio-temporal stimulus parameters that result in the two kinds of eye movements. The results demonstrated that single-mode pursuit gain elicited by continuous motion decreased linearly with increasing target speed, in conjunction with a parallel increase in saccadic tracking. In contrast, single-mode pursuit gain of apparent motion displayed an inverted U-shape function with increasing target velocity and the optimal velocity resulting in peak smooth pursuit gain depended on the spatial separation, stimulus duration and type of apparent motion stimulus. At target velocities below optimal velocity for peak smooth pursuit gain, a large number of small saccadic eye movements were generated, and at target speeds above the optimum, a small number of larger, faster, and temporally longer saccades were produced. In order to further investigate the lower limits of the saccadic and smooth pursuit components in visual tracking a final experiment was conducted, which extended the velocity range downwards to 1.0-24.0 deg/s for the visual tracking of continuous and apparent motion stimuli at 2.0 deg spatial separation and 60ms stimulus duration. This was compared with motion sensitivity in order to investigate a link between motion perception and smooth pursuit eye movements. The results of Experiment 3 replicated the inverted U-shaped (band-pass) function with increasing target velocity for single-mode pursuit gain elicited by apparent motion stimuli and revealed a similar band-pass function for single-mode pursuit gain in response to continuous motion, which peaked at 2.0 deg/s target velocity. These findings demonstrated that smooth pursuit eye movements reach peak single-mode gain at an optimal velocity that depends on the spatio-temporal characteristics of the target stimulus. Single-mode pursuit gain decreases when target velocity is above or below a given optimal velocity. The contribution of saccadic eye movements increases when single-mode pursuit gain decreases, but the nature of this contribution is different above and below optimal velocity for peak smooth pursuit gain. When target speed decreases below a given optimal target velocity for smooth pursuit eye movements a large number of smaller, slower, and briefer saccades are generated. In contrast, a smaller number of saccades of larger amplitude, peak velocity and duration are produced when target speed increases beyond optimal pursuit velocity. This demonstrates a more complex and extensive contribution of saccades to visual tracking rather than the one-dimensional process of an increase in saccade frequency and amplitude with increasing target velocity that has traditionally been suggested in the literature. The findings are more consistent with recent models of the coordination of saccadic and pursuit eye movements in visual tracking proposed by Krauzlis (2005) or Orban de Xifry and Lefevre (2007), which suggest a much closer link between saccadic and smooth pursuit eye movements that are based on shared visual processing and shared target selection. In addition, the profile of the functions for motion sensitivity and single-mode pursuit gain for continuous and apparent motion stimuli displayed many similarities, which provides strong support for the view that motion perception and smooth pursuit have shared inputs. It is concluded that the current findings demonstrate the conceptual and methodological usefulness of employing continuous and apparent motion stimuli in investigating visual tracking because continuous, slashed, and jumping-dot motion stimuli can be seen to exist on a continuum that allows fracturing the motion signal in a systematic way, providing graded levels of visual motion energy. Thus, these stimuli have great potential to further investigate shared inputs into saccadic and smooth pursuit eye movements and its underlying mechanisms. In addition, they have a particular application to the study of disordered visual tracking, particularly in schizophrenia, in which the origin of the deficit has not yet been identified in relation to saccades, smooth pursuit, or motion perception.