Toni P. Saarela

Global stimulus configuration modulates crowding

In crowding, neighboring elements impair the perception of a peripherally presented target. Crowding is often regarded to be a consequence of spatial pooling of information that leads to the perception of textural wholes. We studied the effects of stimulus configuration on crowding using Gabor stimuli. In accordance with previous studies, contrast and orientation discrimination of a Gabor target were impaired in the presence of flanking Gabors of equal length. The stimulus configuration was then changed (1) by making the flankers either shorter or longer than the target or (2) by constructing each flanker from two or three small Gabors. These simple configural changes greatly reduced or even abolished crowding, even though the orientation, spatial frequency, and phase of the stimuli were unchanged. The results challenge simple pooling explanations for crowding. We propose that crowding is weak whenever the target stands out from the stimulus array and strong when the target groups with the flanking elements to form a coherent texture.

The effect of spacing regularity on visual crowding

Crowding limits peripheral visual discrimination and recognition: a target easily identified in isolation becomes impossible to recognize when surrounded by other stimuli, often called flankers. Most accounts of crowding predict less crowding when the target-flanker distance increases. On the other hand, the importance of perceptual organization and target-flanker coherence in crowding has recently received more attention. We investigated the effect of target-flanker spacing on crowding in multi-element stimulus arrays. We show that increasing the average distance between the target and the flankers does not always decrease the amount of crowding but can even sometimes increase it. We suggest that the regularity of inter-element spacing plays an important role in determining the strength of crowding: regular spacing leads to the perception of a single, coherent, texture-like stimulus, making judgments about the individual elements difficult.

Time-course and surround modulation of contrast masking in human vision

Neural and perceptual responses to a visual stimulus can be suppressed by the addition of both spatially overlapping and spatially adjacent contextual stimuli. We investigated the temporal characteristics of these suppressive interactions in psychophysical contrast masking experiments using Gabor and grating stimuli with a spatial frequency of 4 cycles per degree. We found that the time course of masking strongly depended on mask orientation. Most interestingly, masking by a spatially overlaid, iso-oriented mask was strongest when the target was presented immediately before or immediately after the mask. This masking was transient, presumably caused by the neural responses to mask onset and offset. Adding a surround to the mask modulated the backward masking effect, but only when the target and the central mask were iso-oriented. Our results provide evidence for a surround suppression mechanism that affected the transient responses to the mask onset, but not the responses to the mask offset. Together, these results demonstrate how the effects of spatial context in visual processing critically depend on stimulus timing.

Combination of texture and color cues in visual segmentation

The visual system can use various cues to segment the visual scene into figure and background. We studied how human observers combine two of these cues, texture and color, in visual segmentation. In our task, the observers identified the orientation of an edge that was defined by a texture difference, a color difference, or both (cue combination). In a fourth condition, both texture and color information were available, but the texture and color edges were not spatially aligned (cue conflict). Performance markedly improved when the edges were defined by two cues, compared to the single-cue conditions. Observers only benefited from the two cues, however, when they were spatially aligned. A simple signal-detection model that incorporates interactions between texture and color processing accounts for the performance in all conditions. In a second experiment, we studied whether the observers are able to ignore a task-irrelevant cue in the segmentation task or whether it interferes with performance. Observers identified the orientation of an edge defined by one cue and were instructed to ignore the other cue. Three types of trial were intermixed: neutral trials, in which the second cue was absent; congruent trials, in which the second cue signaled the same edge as the target cue; and conflict trials, in which the second cue signaled an edge orthogonal to the target cue. Performance improved when the second cue was congruent with the target cue. Performance was impaired when the second cue was in conflict with the target cue, indicating that observers could not discount the second cue. We conclude that texture and color are not processed independently in visual segmentation.

Size tuning and contextual modulation of backward contrast masking

The strength of contrast masking depends not only on spatial but also on temporal parameters. In a previous study (T. P. Saarela & M. H. Herzog, 2008), we showed that the detection of a briefly presented Gabor patch is most strongly impaired when an iso-oriented grating mask immediately follows the Gabor and that this masking effect is relieved when a surround is added to the mask. Here, we studied the spatial characteristics of this backward masking effect. Gradually changing the size of the iso-oriented masking grating changes contrast detection thresholds in a non-monotonic way that can be explained in terms of contrast-dependent spatial summation and inhibition. However, these spatial interactions seem only to take place when the mask is a uniform grating. When the mask is divided into a small center and a larger surround by changing the surround parameters or by adding a small gap, masking is as strong as with the small center mask only. We suggest that spatial interactions are weaker or even absent when the stimulus elements are perceptually segregated.

Perception-memory interactions reveal a computational strategy for perceptual constancy

A key challenge for the visual system is to extract constant object properties from incoming sensory information. This information is ambiguous because the same sensory signal can arise from many combinations of object properties and viewing conditions and noisy because of the variability in sensory encoding. The competing accounts for perceptual constancy of surface lightness fall into two classes of model: One derives lightness estimates from border contrasts, and another explicitly infers surface reflectance. To test these accounts, we combined a novel psychophysical task with probabilistic implementations of both models. Observers compared the lightness of two stimuli under a memory demand (a delay between the stimuli), a context change (different surround luminance), or both. Memory biased perceived lightness toward the mean of the whole stimulus ensemble. Context change caused the classical simultaneous lightness contrast effect, in which a target appears lighter against a dark surround and darker against a light surround. These effects were not independent: Combined memory load and context change elicited a bias smaller than predicted assuming an independent combination of biases. Both models explain the memory bias as an effect of prior expectations on perception. Both models also produce a context effect, but only the reflectance model correctly describes the magnitude. The reflectance model, finally, captures the memory-context interaction better than the contrast model, both qualitatively and quantitatively. We conclude that (a) lightness perception is more consistent with reflectance inference than contrast coding and (b) adding a memory demand to a perceptual task both renders it more ecologically valid and helps adjudicate between competing models.