Dual percept of movement and spatial periodicity in stroboscopically illuminated moving noise patterns

Defective Temporal Window of the Foveal Visual Processing in High Myopia

Purpose

To investigate the temporal characteristics of visual processing at the fovea and the periphery in high myopia.

Methods

Eighteen low (LM, ≤ -0.50 and > -6.00 D) and 18 high myopic (HM, ≤ -6.00 D) participants took part in this study. The contrast thresholds in an orientation discrimination task under various stimulus onset asynchrony (SOA) masking conditions were measured at the fovea and a more peripheral area (7°) for the two groups. An elaborated perceptual template model (ePTM) was fit to the behavioral data for each participant.

Results

An analysis of variance with three factors (SOA, degree of myopia and eccentricity) was performed on the threshold data. The interaction between SOA and degree of myopia in the fovea was significant (F (4, 128) = 2.66, P = 0.036), suggesting that the masking effect had different temporal patterns between the two groups. The temporal profiles for the two groups were derived based on the ePTM model. The peak and the spread of the temporal window in the fovea were much lower and wider, respectively, in the HM group than that in the LM group (both Ps < 0.05). There was no significant difference in the peripheral temporal window between the two groups.

Conclusions

High myopia is associated with defective temporal processing in the fovea, captured by a flattened temporal window.

Temporal tuning characteristics of the perceptual template and endogenous cuing of spatial attention

External noise presented in temporal contiguity with a target impairs perceptual performance, reflecting the temporal tuning of the perceptual template. Deriving the temporal characteristics of the perceptual template, however, requires an observer model that segregates the impact of non-linearities and intrinsic inefficiencies of the observer in order to account for the impact of external noise in various temporal configurations. In Experiment 1, we showed that the perceptual template model successfully accounts for temporal masking functions in foveal Gabor orientation identification masked by external noise with a wide range of temporal configurations, and estimates the temporal characteristics of the perceptual template. In Experiment 2, we extended the paradigm and the model to compare the temporal tuning characteristics of the perceptual template in central pre- and simultaneous cuing of endogenous spatial attention in a Gabor orientation identification task in visual periphery. We found that endogenous spatial attention excludes external noise by both sharpening the temporal window of the perceptual template and (mostly) reducing the impact of external noise uniformly across the entire temporal window.

The temporal integration and resolution of velocity signals

The temporal properties of human visual motion detection were explored. Experiment 1 measured thresholds for speed discrimination as a function of stimulus duration. Thresholds fell asymptotically to a Weber fraction around 0.06 over a period of approx. 100 msec, with faster speeds asymptoting at slightly shorter stimulus durations. A second experiment required subjects to discriminate a pattern that was modulated between two speeds from one which remained at a constant speed. The minimum depth of the modulation required to make this judgement was found to be equivalent to a Weber fraction of 0.3 at low modulation rates, around five times greater than when the velocities were presented in isolation (expt 1). At some higher modulation rate performance dramatically declined. The modulation rate at which this occurred decreased with stimulus speed, and increased with stimulus size. The results of expt 1 seem consistent with the known properties of primary motion sensors, while the results of the latter experiments may arise from a later stage integrating the output of these primary motion sensors.

Perception of movement and correlation in stroboscopically presented noise patterns

The detection of spatiotemporal correlation in visual displays has been studied with stroboscopically presented random-noise patterns and with a signal-to-noise ratio paradigm in which the moving pattern was masked with spatiotemporal white noise. These methods reveal the ability of the visual system to detect correlation of spatiotemporal structures, rather than luminance contrast. The effects of stroboscopic rate, exposure duration, target size, and the extent of discrete spatial shifts were studied in both the central and the peripheral visual field. Evidence for orientation-selective and speed-selective mechanisms was found, as well as for extensive spatiotemporal integration. Bounds on parameters of spatial and temporal correlation and integration were obtained. The results are similar to those reported earlier, and also extend them. Their relation to results obtained through other paradigms (eg the motion aftereffect) is explored.

Visual-vestibular interaction studied with stroboscopically illuminated visual patterns

Horizontal DC-electrooculograms were recorded in subjects rotating on a horizontal turntable sinusoidally at 0.1 Hz and 35 to 40 degrees amplitude. The subjects either fixated a stroboscopically illuminated vertically striped pattern (1.15 to 3.45 degrees period) rotating with the turntable or initiated Sigma-OKN before the rotation began and tried to maintain Sigma-OKN during rotation. In a third paradigm, interaction of vestibulo-ocular reflex (VOR) and Phi-OKN was studied. VOR-suppression by fixation was complete within the limits of EOG-recording precision (+/- 1 degree X s-1) for flash frequencies fs greater than 10 flashes X s-1. VOR-suppression decreased monotonically with fs between 10 and 1 flashes X s-1. A similar dependency on fs was found for VOR-suppression during Sigma- or Phi-OKN. Above 10 flashes X s-1 VOR-suppression remained incomplete; below 5 flashes X s-1 VOR-suppression was stronger with the Sigma-OKN paradigm than during fixation and depended on spatial frequency of the pattern. During sinewave rotation of the subject the perceived speed Vp of Sigma-movement correlated to the movement of gaze in space and not to the movement of the eye in head. In a control experiment with normal optokinetic stimulation, OKN-suppression by fixating a small flashing target was found to depend on fs in a similar way as VOR-suppression in the experiments described above.