Speed discrimination under stabilized and normal viewing conditions

Three-Dimensional Motion Perception: Comparing Speed and Speed Change Discrimination for Looming Stimuli

Judging the speed of objects moving in three dimensions is important in our everyday lives because we interact with objects in a three-dimensional world. However, speed perception has been seldom studied for motion in depth, particularly when using monocular cues such as looming. Here, we compared speed discrimination, and speed change discrimination, for looming stimuli, in order to better understand what visual information is used for these tasks. For the speed discrimination task, we manipulated the distance and duration information available, in order to investigate if participants were specifically using speed information. For speed change discrimination, total distance and duration were held constant; hence, they could not be used to successfully perform that task. For the speed change discrimination task, our data were consistent with observers not responding specifically to speed changes within an interval. Instead, they may have used alternative, arguably less optimal, strategies to complete the task. Evidence suggested that participants used a variety of cues to complete the speed discrimination task, not always solely relying on speed. Further, our data suggested that participants may have switched between cues on a trial to trial basis. We conclude that speed changes in looming stimuli were not used in a speed change discrimination task, and that naïve participants may not always exclusively use speed for speed discrimination.

Speed change discrimination for motion in depth using constant world and retinal speeds

Motion at constant speed in the world maps into retinal motion very differently for lateral motion and motion in depth. The former is close to linear, for the latter, constant speed objects accelerate on the retina as they approach. Motion in depth is frequently studied using speeds that are constant on the retina, and are thus not consistent with real-world constant motion. Our aim here was to test whether this matters: are we more sensitive to real-world motion? We measured speed change discrimination for objects undergoing accelerating retinal motion in depth (consistent with constant real-world speed), and constant retinal motion in depth (consistent with real-world deceleration). Our stimuli contained both looming and binocular disparity cues to motion in depth. We used a speed change discrimination task to obtain thresholds for conditions with and without binocular and looming motion in depth cues. We found that speed change discrimination thresholds were similar for accelerating retinal speed and constant retinal speed and were notably poor compared to classic speed discrimination thresholds. We conclude that the ecologically valid retinal acceleration in our stimuli neither helps, nor hinders, our ability to make judgements in a speed change discrimination task.

Precision and accuracy of ocular following: influence of age and type of eye movement

Previous work on ocular-following emphasises the accuracy of tracking eye movements. However, a more complete understanding of oculomotor control should account for variable error as well. We identify two forms of precision: 'shake', occurring over shorter timescales; 'drift', occurring over longer timescales. We show how these can be computed across a series of eye movements (e.g. a sequence of slow-phases or collection of pursuit trials) and then measure accuracy and precision for younger and older observers executing different types of eye movement. Overall, we found older observers were less accurate over a range of stimulus speeds and less precise at faster eye speeds. Accuracy declined more steeply for reflexive eye movements and shake was independent of speed. In all other instances, the two measures of precision expanded non-linearly with mean eye speed. We also found that shake during fixation was similar to shake for reflexive eye movement. The results suggest that deliberate and reflexive eye movement do not share a common non-linearity or a common noise source. The relationship of our data to previous studies is discussed, as are the consequences of imprecise eye movement for models of oculomotor control and perception during eye movement.

Refining the predictive pursuit endophenotype in schizophrenia

BACKGROUND

To utilize fully a schizophrenia endophenotype in gene search and subsequent neurobiological studies, it is critical that the precise underlying physiologic deficit is identified. Abnormality in smooth pursuit eye movements is one of the endophenotypes of schizophrenia. The precise nature of the abnormality is unknown. Previous work has shown a reduced predictive pursuit response to a briefly masked (i.e., invisible) moving object in schizophrenia. However, the overt awareness of target removal can confound the measurement.

METHODS

This study employed a novel method that covertly stabilized the moving target image onto the fovea. The foveal stabilization was implemented after the target on a monitor had oscillated at least for one cycle and near the change of direction when the eye velocity momentarily reached zero. Thus, the subsequent pursuit eye movements were completely predictive and internally driven. Eye velocity during this foveally stabilized smooth pursuit was compared among schizophrenia patients (n = 45), their unaffected first-degree relatives (n = 42), and healthy comparison subjects (n = 22).

RESULTS

Schizophrenia patients and their unaffected relatives performed similarly and both had substantially reduced predictive pursuit acceleration and velocity under the foveally stabilized condition.

CONCLUSIONS

These findings show that inability to maintain internal representation of the target motion or integration of such information into a predictive response may be the specific brain deficit indexed by the smooth pursuit endophenotype in schizophrenia. Similar performance between patients and unaffected relatives suggests that the refined predictive pursuit measure may index a less complex genetic origin of the eye-tracking deficits in schizophrenia families.

The Role of Explicit and Implicit Standards in Visual Speed Discrimination

Five experiments were designed to investigate visual speed discrimination. Variations of the method of constant stimuli were used to obtain speed discrimination thresholds in experiments 1, 2, 4, and 5, while the method of single stimuli was used in experiment 3. The observers' thresholds were significantly influenced by the choice of psychophysical method and by changes in the standard speed. The observers' judgments were unaffected, however, by changes in the magnitude of random variations in stimulus duration, reinforcing the conclusions of Lappin et al (1975 Journal of Experimental Psychology: Human Perception and Performance1 383–394). When an implicit standard was used, the observers produced relatively low discrimination thresholds (7.0% of the standard speed), verifying the results of McKee (1981 Vision Research21 491–500). When an explicit standard was used in a 2AFC variant of the method of constant stimuli, however, the observers' discrimination thresholds increased by 74% (to 12.2%), resembling the high thresholds obtained by Mandriota et al (1962 Science138 437–438). A subsequent signal-detection analysis revealed that the observers' actual sensitivities to differences in speed were in fact equivalent for both psychophysical methods. The formation of an implicit standard in the method of single stimuli allows human observers to make judgments of speed that are as precise as those obtained when explicit standards are available.

Psychophysical estimation of speed discrimination. II. Aging effects

We studied the effects of aging on a speed discrimination task using a pair of first-order drifting luminance gratings. Two reference speeds of 2 and 8 deg/s were presented at stimulus durations of 500 ms and 1000 ms. The choice of stimulus parameters, etc., was determined in preliminary experiments and described in Part I. Thresholds were estimated using a two-alternative-forced-choice staircase methodology. Data were collected from 16 younger subjects (mean age 24 years) and 17 older subjects (mean age 71 years). Results showed that thresholds for speed discrimination were higher for the older age group. This was especially true at stimulus duration of 500 ms for both slower and faster speeds. This could be attributed to differences in temporal integration of speed with age. Visual acuity and contrast sensitivity were not statistically observed to mediate age differences in the speed discrimination thresholds. Gender differences were observed in the older age group, with older women having higher thresholds.

Visual short-term memory during smooth pursuit eye movements

Visual short-term memory (VSTM) was probed while observers performed smooth pursuit eye movements. Smooth pursuit keeps a moving object stabilized in the fovea. VSTM capacity for position was reduced during smooth pursuit compared with a condition with eye fixation. There was no difference between a condition in which the items were approximately stabilized on the retina because they moved with the pursuit target and a condition in which the items moved across the retina because they were stationary in space. The reduction of capacity for position was eliminated when miniature items were presented on the pursuit target. Similarly, VSTM capacity for color did not differ between smooth pursuit and fixation. The results suggest that visuospatial attention is tied to the target during smooth pursuit, which impairs VSTM for the position of peripheral objects. Sensory memory during smooth pursuit was only slightly impaired.

Illusory jitter in a static stimulus surrounded by a synchronously flickering pattern

The eyes are always moving even during fixation, making the retinal image move concomitantly. While these motions activate early visual stages, they are excluded from one's perception. A striking illusion reported here renders them visible: a static pattern surrounded by a synchronously flickering pattern appears to move coherently in random directions. There was a positive correlation between the illusion and fixational eye movements. A simulation revealed that motion computation artificially creates a motion difference between center and surround, which is usually a cue to object motion but now a wrong cue to seeing eye movements of oneself on-line. Therefore, this novel illusion indicates that the visual system normally counteracts shaky visual inputs due to small eye movements by using retinal, as opposed to extraretinal, motion signals. As long as they comprise common image motions over space, they are interpreted as coming from a static outer world viewed through moving eyes. Such visual stability fails in the condition of artificial flicker, because common image motions due to eye movements are registered differently between flickering and non-flickering regions.

Apparent depth with retinal image motion of expansion and contraction yoked to head movement

The two main questions addressed in this study were (a) what effect does yoking the relative expansion and contraction (EC) of retinal images to forward and backward head movements have on the resultant magnitude and stability of perceived depth, and (b) how does this relative EC image motion interact with the depth cues of motion parallax? Relative EC image motion was produced by moving a small CCD camera toward and away from the stimulus, two random-dot surfaces separated in depth, in synchrony with the observers' forward and backward head movements. Observers viewed the stimuli monocularly, on a helmet-mounted display, while moving their heads at various velocities, including zero velocity. The results showed that (a) the magnitude of perceived depth was smaller with smaller head velocities (< 10 cm s-1), including the zero-head-velocity condition, than with a larger velocity (10 cm s-1), and (b) perceived depth, when motion parallax and the EC image motion cues were simultaneously presented, is equal to the greater of the two possible perceived depths produced from either of these two cues alone. The results suggested the role of nonvisual information of self-motion on perceiving depth.

OKN, perceptual and VEP direction biases in strabismus

The present study quantified nasalward/temporalward biases in monocular optokinetic nystagmus (MOKN) and perceived velocity in patients with either early onset esotropia, late onset esotropia and in normals. MOKN was measured with low spatial frequency, small-field gratings drifting at 9.4 degrees/s. MOKN bias was quantified as the ratio of nasalward slow-phase velocity divided by the sum of temporalward and nasalward slow-phase velocities (N/(N + T)). Observers also rated the perceived velocity of gratings moving in nasalward and temporalward directions (3 or 9.4 degrees/s) using a two interval forced choice task. MOKN and perceived velocity biases were correlated negatively in both early onset and late onset groups in the perceptual task--nasalward moving targets were rated as slower than temporalward targets, but in the MOKN task, slow-phase gain was higher for nasalward than for temporalward targets. Oscillatory-motion, visual evoked potentials (VEPs), were recorded in response to 1 c/deg gratings undergoing apparent motion at 10 Hz in a subset of the observers. VEP direction biases were quantified by calculating the ratio of first harmonic response amplitudes to the sum of first and second harmonic amplitudes. Significant correlations were found between the direction biases obtained on all three measures. Perceived velocity and MOKN bias measures were also correlated negatively. Patients with early onset esotropia (infantile esotropia) had larger biases than late onset esotropes or normals on each measure and the biases were more frequently bilateral in the early onset patients. The pattern of result is consistent with early critical periods for the mechanism(s) underlying MOKN, perceived velocity and cortical responsiveness. A single site model for all three asymmetries is unlikely, at least in simple form, because of the negative correlation between MOKN and perceived velocity biases and because of the differences in relative magnitude between the perceptual and MOKN biases.

Forced-choice staircases with fixed step sizes: asymptotic and small-sample properties

Visual detection and discrimination thresholds are often measured using adaptive staircases, and most studies use transformed (or weighted) up/down methods with fixed step sizes--in the spirit of Wetherill and Levitt (Br J Mathemat Statist Psychol 1965;18:1-10) or Kaernbach (Percept Psychophys 1991;49:227-229)--instead of changing step size at each trial in accordance with best-placement rules--in the spirit of Watson and Pelli (Percept Psychophys 1983;47:87-91). It is generally assumed that a fixed-step-size (FSS) staircase converges on the stimulus level at which a correct response occurs with the probabilities derived by Wetherill and Levitt or Kaernbach, but this has never been proved rigorously. This work used simulation techniques to determine the asymptotic and small-sample convergence of FSS staircases as a function of such parameters as the up/down rule, the size of the steps up or down, the starting stimulus level, or the spread of the psychometric function. The results showed that the asymptotic convergence of FSS staircases depends much more on the sizes of the steps than it does on the up/down rule. Yet, if the size delta+ of a step up differs from the size delta- of a step down in a way that the ratio delta-/delta+ is constant at a specific value that changes with up/down rule, then convergence percent-correct is unaffected by the absolute sizes of the steps. For use with the popular one-, two-, three- and four-down/one-up rules, these ratios must respectively be set at 0.2845, 0.5488, 0.7393 and 0.8415, rendering staircases that converge on the 77.85%-, 80.35%-, 83.15%- and 85.84%-correct points. Wetherill and Levitt's transformed up/down rules--which require delta-/delta+ = 1--and the general version of Kaernbach's weighted up/down rule--which allows any delta-/delta+ ratio--fail to reach their presumed targets. The small-sample study showed that, even with the optimal settings, short FSS staircases (up to 20 reversals in length) are subject to some bias, and their precision is less than reasonable, but their characteristics improve when the size delta+ of a step up is larger than half the spread of the psychometric function. Practical recommendations are given for the design of efficient and trustworthy FSS staircases.

Speed discrimination of distal stimuli during smooth pursuit eye motion

We evaluated the hypothesis that smooth pursuit eye movements affect speed discrimination thresholds of distal stimuli because they alter the retinal image speed. Subjects judged speed differences of sine-wave gratings while they simultaneously pursued a superimposed moving bar. Speed discrimination thresholds were measured, under conditions of controlled eye movements, for grating speeds of 0.5 and 2.0 deg/sec across a range of eye velocities. Thresholds were stimulated using a Monte Carlo method based on the retinal speed hypothesis, and the simulation predictions were compared to the psychophysically determined thresholds. The simulation results provided a good match to the psychophysical data for conditions where the eye moved at a slower speed than the grating, regardless of whether the eye moved in the same or opposite direction. However, when the eye moved at a faster speed than the grating in the same direction, the psychophysical thresholds were significantly higher than predicted by the simulation. Control experiments and analyses rule out explanations based on relative motion cues, saccadic involvement, and attentional demands.