Measuring image distortions arising from age-related macular degeneration: An Iterative Amsler Grid (IAG).

Metamorphopsia, perceived as distortion of a shape, is experienced in age-related macular degeneration (AMD): straight lines appear to be curved and wavy to AMD patients and some other retinal pathologies. Conventional clinical assessment largely relies on asking patients to identify irregularities in Amsler Grids - a standardized set of equally spaced vertical and horizontal lines. Perceived distortions or gaps in the grid are a sign of macular pathology. Here, we developed an iterative Amsler Grid (IAG) procedure to obtain a quantifiable map of visual deformations. Horizontal and vertical line segments representing metamorphopsia are displayed on a computer screen. Line segments appearing distorted are adjusted by participants using the computer mouse to change their orientation in several iteratively such that they appear straight. Control participants are able to reliably correct deformations that simulate metamorphopsia while maintaining fixation in the center. In a pilot experiment, we attempted to obtain deformation maps from a small number of AMD patients. Whereas some patients with extensive scotomas found this procedure challenging, others were comfortable using the IAG and generating deformation maps corresponding to their subjective reports. This procedure may potentially be used to quantify local distortions and map them reliably in patients with early AMD.

The effect of action on perceptual feature binding.

Color-motion asynchrony (CMA) refers to an apparent lag of direction of motion when a dynamic stimulus changes both color and direction at the same time. The subjective order of simultaneous events, however, is not only perceptual but also subject to illusions during voluntary actions. Self-initiated actions, for example, seem to precede their sensory outcomes following an adaptation to a delay between the action and the sensory feedback. Here, we demonstrate that the extent of the apparent asynchrony can be substantially reduced when direction change is induced by a voluntary key press following a delay adaptation regime. We also show that the reduced color-motion asynchrony effect size following a motor-sensory recalibration is not a result of a change in the onset of perceived direction change relative to that of the color. This is particularly important as it implies, for the first time in the literature, that voluntary action is not only important in forming action-sensory outcome associations but may also act as a binding factor between the two perceptual features of a sensory event.

Action-induced changes in the perceived temporal features of visual events.

Perceived duration can be subject to deviations around the time of a voluntary action. Whether the mechanisms underlying action-induced visual duration effects are effector-specific or require a more generalized action-linked multimodal calibration with the transient visual system, however, is a question yet to be answered. Here, we investigate this using dynamic visual stimuli presented as contingent upon the execution of an arbitrarily associated voluntary manual response. Our results demonstrate that the duration of intervals with arbitrarily associated keypress-visual event pair is perceived as shorter than the duration in a pure visual condition, where the same stimuli are rather passively observed without the execution of a concurrent action. Whereas the control experiments show that motor memory and attention cannot explain the action-induced changes in perceived temporal features, action-induced changes in perceived speed are dissociated from those in perceived duration, and that the duration compression disappears using isoluminant or static stimuli, which together provide evidence that these two effects can be modulated in the motion-processing units, although via separate neural mechanisms.

Short-term global motion adaptation induces a compression in the subjective duration of dynamic visual events.

Apparent duration can be manipulated in a local region of visual field by long-term adaptation to motion or flicker (Johnston, Arnold, & Nishida, 2006). These effects show narrow spatial tuning (Ayhan, Bruno, Nishida, & Johnston, 2009), as well as retinotopic position dependency (Bruno, Ayhan, & Johnston, 2010), supporting early locus in the visual pathway. Here, we introduce a new effect using RDK as a short-term visual adaptor and demonstrate that a brief, subsecond range adaptation induces a significant subjective duration compression (∼10%) on a subsequently presented test stimulus (RDK pattern) only for global motion patterns drifting at 50% motion coherence but not for those drifting at 0% coherence, suggesting a higher level area as a source of origin. In another set of experiments using a plaid stimulus as the adaptor and gratings as the tests, we report again a significant duration compression following a brief motion adaptation, although the effect does not seem to be consistently selective for a particular direction of the standard test relative to that of the plaid adaptor (two-dimensional motion) or its components (one-dimensional motion). Finally, we conduct an experiment using shutter glasses and find that the effects of a short-term adaptor presented monocularly to one eye transfer to the nonadapted eye, providing evidence for the interocular transfer. In a series of control experiments, we also show that the duration effects cannot be explained by adaptation-induced changes in perceived speed, perceived onset-and-offset, and attentional resource allocation. Overall, the duration compression effect requiring motion coherence in RDK, persisting in plaid stimulus, and showing interocular transfer imply explicit genuine mechanisms mediating duration effects in the higher level motion areas.

Changes in apparent duration follow shifts in perceptual timing.

It is well established that the apparent duration of moving visual objects is greater at higher as compared to slower speeds. Here we report the effects of acceleration and deceleration on the perceived duration of a drifting grating with average speed kept constant (10°/s).For acceleration, increasing the speed range progressively reduced perceived duration. The magnitude of apparent duration compression was determined by speed rather than temporal frequency and was proportional to speed range (independent of standard duration) rather than acceleration. The perceived duration reduction was also proportional to the standard length. The effects of increases and decreases in speed were highly asymmetric. Reducing speed through the interval induced a moderate increase in perceived duration. These results could not be explained by changes in apparent onset or offset or differences in perceived average speed between intervals containing increasing speed and intervals containing decreasing speed. Paradoxically, for intervals combining increasing speed and decreasing speed, compression only occurred when increasing speed occurred in the second half of the interval. We show that this pattern of results in the duration domain was concomitant with changes in the reported direction of apparent motion of Gaussian blobs, embedded in intervals of increasing or decreasing speed, that could be predicted from adaptive changes in the temporal impulse response function. We detected similar changes after flicker adaptation, suggesting that the two effects might be linked through changes in the temporal tuning of visual filters.

Duration judgments over multiple elements.

We investigated the limits of the number of events observers can simultaneously time. For single targets occurring in one of eight positions sensitivity to duration was improved for spatially pre-cued items as compared to post-cued items indicating that exogenous driven attention can improve duration discrimination. Sensitivity to duration for pre-cued items was also marginally better for single items as compared to eight items indicating that even after the allocation of focal attention, distractor items can interfere with the encoding of duration. For an eight item array discrimination was worse for post-cued locations as compared to pre-cued locations indicating both that attention can improve duration discrimination performance and that it was not possible to access a perfect memory trace of the duration of eight elements. The interference from the distractors in the pre-cued eight item array may reflect some mandatory averaging of target and distractor events. To further explore duration averaging we asked subjects to explicitly compare average durations of multiple item arrays against a single item standard duration. Duration discrimination thresholds were significantly lower for single elements as compared to multiple elements, showing that averaging, either automatically or intentionally, impairs duration discrimination. There was no set size effect. Performance was the same for averages of two and eight items, but performance with even an average of two items was worse than for one item. This was also true for sequential presentation indicating poor performance was not due to limits on the division of attention across items. Rather performance appears to be limited by an inability to remember or aggregate duration information from two or more items. Although it is possible to manipulate perceived duration locally, there appears to be no perceptual mechanisms for aggregating local durations across space.

Effects of Temporal Features and Order on the Apparent duration of a Visual Stimulus.

The apparent duration of a visual stimulus has been shown to be influenced by its speed. For low speeds, apparent duration increases linearly with stimulus speed. This effect has been ascribed to the number of changes that occur within a visual interval. Accordingly, a higher number of changes should produce an increase in apparent duration. In order to test this prediction, we asked subjects to compare the relative duration of a 10-Hz drifting comparison stimulus with a standard stimulus that contained a different number of changes in different conditions. The standard could be static, drifting at 10 Hz, or mixed (a combination of variable duration static and drifting intervals). In this last condition the number of changes was intermediate between the static and the continuously drifting stimulus. For all standard durations, the mixed stimulus looked significantly compressed (∼20% reduction) relative to the drifting stimulus. However, no difference emerged between the static (that contained no changes) and the mixed stimuli (which contained an intermediate number of changes). We also observed that when the standard was displayed first, it appeared compressed relative to when it was displayed second with a magnitude that depended on standard duration. These results are at odds with a model of time perception that simply reflects the number of temporal features within an interval in determining the perceived passing of time.

Duration expansion at low luminance levels.

Duration distortions have been shown to occur at the time of saccades and following high temporal frequency or contrast adaptation. Under all these conditions, changes in the temporal tuning of M neurons also occur, suggesting that there might be a link between the two phenomena. In order to explore this relationship further, we measured the apparent duration of visual stimuli in the dark, where the temporal impulse response has been reported to lengthen. We first measured a progressive shift and reduction of the occurrence of an apparent motion reversal as we decreased the luminance level, indicating a lengthening of the temporal impulse response. We then measured perceived duration at these luminance levels (0.75, 3, and 50 cd/m(2)) after matching for apparent contrast and temporal frequency. While perceived temporal frequency did not substantially differ across luminance levels, duration appeared expanded at the lowest luminance level relative to the highest by approximately 60 ms. Thus, we have shown that reduced luminance is associated with both a lengthening of the temporal impulse response and a duration expansion, linking the two and providing further evidence for a relationship between changes in the neuronal tuning in the early stages of the visual system and time perception.

Effect of the luminance signal on adaptation-based time compression.

Traditionally, time perception has been considered the product of a central, generic, cognitive mechanism. Recent evidence, however, has shown that high temporal frequency adaptation induces local reductions in the apparent duration of brief intervals suggesting a distributive system with modality-specific sensory components. Here, we examine the effect of the luminance signal on these adaptation-based temporal distortions. Our results show that the luminance signal is crucial to generate duration compression as the effect disappears at isoluminance and that low visibility and task difficulty at isoluminance cannot explain the discrepancy. We also demonstrate that the effects of adaptation on perceived duration are dissociable from those on apparent temporal frequency. These results provide further evidence for the involvement of the magnocellular system in the neural encoding and representation of visual time.

Retinotopic adaptation-based visual duration compression.

Eye movements present the visual system with the challenge of providing the experience of a stable world. This appears to require the location of objects to be mapped from retinal to head and body referenced coordinates. Following D. Burr, A. Tozzi, and M. C. Morrone (2007), here we address the issue of whether adaptation-based duration compression (A. Johnston, D. H. Arnold, & S. Nishida, 2006) takes place in a retinocentric or head-centric frame of reference. Duration compression may be associated with shifts in apparent temporal frequency. However, using an adaptation schedule that minimizes any effect of adaptation on apparent temporal frequency, we still find substantial apparent duration compression. Duration compression remains when the adaptor continuously translates in head-centered coordinates but is fixed on the retina, isolating retinal adaptation. Apparent duration was also measured after a change in gaze direction-a strategy which allows eye-centered and head-centered components of adaptation-induced duration compression to be distinguished. In two different paradigms, we found significant compression was elicited by retinotopic adaptation, with no significant change in apparent duration following spatiotopic adaptation. We also observed no interocular transfer of adaptation. These findings point to an early locus for the adaptation-based duration compression effect.

The spatial tuning of adaptation-based time compression.

Temporal processing is traditionally dissociated from spatial vision. Recent evidence, however, has shown that adaptation to high temporal frequency (D. Burr, A.Tozzi, & M. C. Morrone, 2007; A. Johnston, D. H. Arnold, & S. Nishida, 2006; A. Johnston et al., 2008) induces spatially specific reductions in the apparent duration of subsecond intervals containing medium frequency drift or flicker. Here we examine the spatial tuning of these temporal adaptation effects. Our results show that duration compression is tightly tuned to the spatial location of the adaptor and can be induced by very narrow adaptors. We also demonstrate that the effects of adaptation on perceived duration are dissociable from those on apparent temporal frequency, which suggests early but separate influences of temporal frequency adaptation on time and speed perception.