Perceptual grouping produced by line figures

Two experiments, one with 2 and one with 3-line figures, studied the relative effectiveness of differences in orientation and shape in producing grouping by Similarity. The results showed that changes in shape or orientation which leave the component lines of the figures vertical and horizontal do not facilitate grouping as readily as changes which alter the direction of the component lines to 45° and 135°. These results corroborate and extend the findings of Beck (l966a, 1966b) and are discussed in relation to the problem of specifying the properties of line figures that produce grouping by similarity.     

Perceptual grouping in audition

The present experiments evaluated the effect of relative frequency as a determinant of the figure-ground organization of sequences of auditory tones. Observers counted sequences of 20 ms tones that were presented at the same frequency or that alternated between two different frequencies. The alternating tones differed in frequency by one whole tone, seven tones, or nineteen tones. Counting accuracy increased with increases in the silent interval between the tones. When the alternating tones differed by seven or nineteen tones, counting was disrupted at rates of presentation of eight tones per second or slower. In contrast to this decrement in the counting of tones that alternated by over an octave, very little decrement was observed when the tones alternated by just one whole tone. The best subjects counted these alternating tones more accurately than the tones presented at the same frequency. The poorest subjects showed a small decrement even when the tones alternated by just one whole tone. The results were discussed in terms of determinants of figure-ground organization in auditory information processing.     

Perceptual grouping in pigeons

Animal studies reveal that many species perceive partially occluded objects in the same way as do humans. Pigeons have been a notable exception. We re-investigated this anomaly of pigeon perception using a different approach from previous studies. With our method, we show that pigeons perceive occluded objects in the same manner as do other species. In addition, we report that pigeons can recognize perceptually transparent surfaces when the effect is induced by the same perceptual mechanisms as occlusion. These results give behavioral evidence that the perception of both occlusion and transparency is a common visual function shared by pigeons and humans, despite the structural differences between their visual systems.     

Perceptual grouping in change detection

Detection of an item's changing of its location from one instance to another is typically unaffected by changes in the shape or color of contextual items. However, we demonstrate here that such location change detection is severely impaired if the elongated axes of contextual items change orientation, even though individual locations remain constant and even though the orientation was irrelevant to the task. Changing the orientations of the elongated stimuli altered the perceptual organization of the display, which had an important influence on change detection. In detecting location changes, subjects were unable to ignore changes in orientation unless additional, invariant grouping cues were provided or unless the items changing orientation could be actively ignored using feature-based attention (color cues). Our results suggest that some relational grouping cues are represented in change detection even when they are task irrelevant.     

Perceptual grouping in shape from shading

We report on the reversal of asymmetry in visual-search tasks with shaded items. Previous studies have suggested that the target of a bottom-lit disk among distractors of top-lit disks is detected in a rapid and parallel manner, but not vice versa. However, in this study, we have shown that the compound items of top-lit disks were searched more quickly than those composed of bottom-lit disks where the items had to be segregated from their background. By modulating the inter-element distances, we confirmed that the reversal of search asymmetry cannot be due to the grouping of items. Further, we showed that the regions of the top-lit disks were perceived as figure more consistently than those of bottom-lit disks. The results indicate that the boundary assignment to the compound items of the top-lit disks enhances the segregation of search items from the background, and that the search mechanism may access the relatively higher representation that includes figure-ground relations.     

Perceptual grouping in visual word recognition

Four experiments are presented in which printed texts are read for their meaning. Some of the texts were mutilated by altering the size of selected letters. In Experiments 1, 2, and 3, the number of words mutilated per passage and the number of letters changed per word were both manipulated. In all three experiments, reading was slowed as a function of the number of words changed per passage, while the number of letters changed per word had a much smaller effect. The interaction between the number of words and number of letters changed was not significant in any of the experiments. It is difficult to explain these results merely in terms of changes in the discriminability of letters. In Experiment 2 all uppercase text was used, which argues against an explanation in terms of supraletter features such as word envelope. We propose an explanation in terms of visual attention and the perceptual grouping required prior to feature recognition. The last experiment supports this explanation through the counterintuitive finding that adding letters of intermediate size can improve legibility by allowing grouping processes to associate large and small letters as belonging to the same word object.     

Perceptual grouping and detection of trial-unique emergent structures by pigeons

Animal Cognition - Detecting global patterns in the environment is essential to object perception and recognition. Consistent with this, pigeons have been shown to readily detect and locate...     

Perceptual grouping and visual selective attention

The present study demonstrates that incongruent distractor letters at a constant distance from a target letter produce more response competition and negative priming when they share a target’s color than when they have a different color. Moreover, perceptual grouping by means of color, attenuated the effects of spatial proximity. For example, when all items were presented in the same color, near distractors produced more response competition and negative priming than far distractors (Experiment 3A). However, when near distractors were presented in a different color and far distractors were presented in the same color as the target, the response competition × distractor proximity interaction was eliminated and the proximity × negative priming interaction was reversed (Experiment 3B). A final experiment demonstrated that distractors appearing on the same object as a selected target produced comparable amounts of response competition and negative priming whether they were near or far from the target. This suggests that the inhibitory mechanisms of visual attention can be directed to perceptual groups/objects in the environment and not only to unsegmented regions of visual space.     

Perceptual grouping in Gabor lattices: proximity and alignment

We propose the Gabor lattice as a new stimulus designed to deal with multiple organizations in perceptual grouping, allowing both comparison between psychophysical data and neural findings and a systematic investigation of grouping based on several low-level characteristics and their interactions. A Gabor lattice is a geometric lattice with Gabor patches, evoking a multistable global orientation percept. Visual grouping in Gabor lattices with elements aligned in a global orientation was compared with grouping of nonaligned Gabor patches and of Gaussian blobs. The effect sizes of proximity and alignment were estimated in logistic regression analyses. The results confirmed the importance of proximity and local element alignment as factors in dynamic grouping. We also found a small but consistent enhancement of grouping along the global vector orthogonal to the local patch orientations. In light of these results, we further motivate the relevance of these stimuli and the associated experimental paradigm.     

Perceptual visual grouping under inattention: electrophysiological functional imaging

Two types of perceptual visual grouping, differing in complexity of shape formation, were examined under inattention. Fourteen participants performed a similarity judgment task concerning two successive briefly presented central targets surrounded by task-irrelevant simple and complex grouping patterns. Event-related potentials (ERPs) were recorded from 22 scalp electrodes and source current density estimations were conducted for the net response to the task-irrelevant background patterns, using low-resolution electromagnetic tomography (LORETA). Although participants’ subjective reports indicated that neither type of organization induced awareness, electrophysiological results showed they both evoked significant activation in occipital, parieto-temporal and frontal brain areas. Behavioral results demonstrated that only grouping of the simple pattern arose under inattention. In contrast to the complex pattern, the processing of the simple pattern was associated with an initially longer latency and higher activation beginning at 130 ms. These results support the distinction of grouping patterns differing in complexity of shape formation.     

Perceptual grouping and attention: Not all groupings are equal

We examined grouping under inattention using Driver, Davis, Russell, Turatto, & Freeman’s (2001) method. On each trial, two successive displays were briefly presented, each comprising a central target square surrounded by elements. The task was to judge whether the two targets were the same or different. The organization of the background elements stayed the same or changed, independently of the targets. In different conditions, background elements grouped into columns/rows by color similarity, a shape (a triangle/arrow, a square/cross, or a vertical/horizontal line) by color similarity, and a shape with no other elements in the background. We measured the influence of the background on the target same-different judgments. The results imply that background elements grouped into columns/rows by color similarity and into a shape when no segregation from other elements was involved and the shape was relatively “good.” In contrast, no background grouping was observed when resolving figure-ground relations for segregated units was required, as in grouping into a shape by color similarity. These results suggest that grouping is a multiplicity of processes that vary in their attentional demands. Regardless of attentional demands, the products of grouping are not available to awareness without attention.     

Perceptual grouping operates independently of attentional selection: Evidence from hemispatial neglect

To what extent can human observers process visual information that is not currently the focus of attention? We evaluated the extent to which unattended visual information (i.e., that which appears on the neglected side of space in individuals with hemispatial neglect) is perceptually organized and influences the perceptual processing of information on the attended side. To examine this, patients (and matched controls) judged whether successive, complex checkerboard stimuli (targets), presented entirely to their intact side of space, were the same or different. Concurrent with this demanding task, irrelevant distractor elements appeared on the unattended side and either changed or retained their perceptual grouping on successive displays, independently of changes in the ipsilesional task-relevant target. Changes in the grouping of the unattended task-irrelevant distractor elements produced congruency effects on the attended target-change judgment to the same extent in the neglect patients as in the control participants, and this was true even in those patients with severe attentional deficits. These results suggest that some perceptual processes, such as grouping, can operate in the absence of attention.     

Perceptual grouping affects pitch judgments across time and frequency

Pitch, the perceptual correlate of fundamental frequency (F0), plays an important role in speech, music, and animal vocalizations. Changes in F0 over time help define musical melodies and speech prosody, while comparisons of simultaneous F0 are important for musical harmony, and for segregating competing sound sources. This study compared listeners' ability to detect differences in F0 between pairs of sequential or simultaneous tones that were filtered into separate, nonoverlapping spectral regions. The timbre differences induced by filtering led to poor F0 discrimination in the sequential, but not the simultaneous, conditions. Temporal overlap of the two tones was not sufficient to produce good performance; instead performance appeared to depend on the two tones being integrated into the same perceptual object. The results confirm the difficulty of comparing the pitches of sequential sounds with different timbres and suggest that, for simultaneous sounds, pitch differences may be detected through a decrease in perceptual fusion rather than an explicit coding and comparison of the underlying F0s.     

Perceptual grouping in space and time: Evidence from the Ternus display

We report three experiments investigating the effect of perceptual grouping on the appearance of a bistable apparent-motion (Ternus) display. Subjects viewed a Ternus display embedded in an array of context elements that could potentially group with the Ternus elements. In contrast to several previous findings, we found that grouping influenced apparent motion perception. In Experiment 1, apparent motion perception was significantly affected via grouping by shape similarity, even when the visible persistence of the elements was controlled. In Experiment 2, elements perceived as moving without context were perceived as stationary when grouped with stationary context elements. In Experiment 3, elements perceived as stationary without context were perceived as moving when grouped with moving context elements. We argue that grouping in the spatial and temporal domains interact to yield perceptual experience of apparent-motion displays.     

Perceptual experience shapes our ability to categorize faces by national origin: A new other-race effect

People are better at recognizing own-race than other-race faces. This other-race effect has been argued to be the result of perceptual expertise, whereby face-specific perceptual mechanisms are tuned through experience. We designed new tasks to determine whether other-race effects extend to categorizing faces by national origin. We began by selecting sets of face stimuli for these tasks that are typical in appearance for each of six nations (three Caucasian, three Asian) according to people from those nations (Study 1). Caucasian and Asian participants then categorized these faces by national origin (Study 2). Own-race faces were categorized more accurately than other-race faces. In contrast, Asian American participants, with more extensive other-race experience than the first Asian group, categorized other-race faces better than own-race faces, demonstrating a reversal of the other-race effect. Therefore, other-race effects extend to the ability to categorize faces by national origin, but only if participants have greater perceptual experience with own-race, than other-race faces. Study 3 ruled out non-perceptual accounts by showing that Caucasian and Asian faces were sorted more accurately by own-race than other-race participants, even in a sorting task without any explicit labelling required. Together, our results demonstrate a new other-race effect in sensitivity to national origin of faces that is linked to perceptual expertise.     

Perceptual grouping based on temporal structure: Impact of subliminal flicker and visual transients

In two experiments we examined the perceived grouping of grids of equidistant dots, which are rapidly modulated over time so that alternate rows or columns are presented out of phase. In Experiment 1, we report that observers were able to group the grids consistent with the temporal modulation reliably, even at contrasts/frequencies for which flicker was not detectable. Moreover, flicker thresholds decreased with stimulus duration, whilst grouping thresholds did not change. In Experiment 2, we examined the impact of visual transients, by measuring performance when, either a mask or a contrast ramp was presented before and after the stimulus. Performance dropped substantially for both conditions, but remained significantly above chance. The results are discussed in relation to the role of temporal correlations in stimulus modulations and visual transients in grouping.     

Perceptual grouping of speech components differing in fundamental frequency and onset-time

Are there general auditory grouping principles that allow the sounds of a single speaker to be grouped together before phonetic categorisation? Four experiments are reported on the use made of a common fundamental frequency or a common starting time in grouping formants together to form phonetic categories. The first experiment shows that the perception of a vowel category is unaffected by formants being excited at different fundamentals or starting at 100-ms intervals. The second and third experiments show no effect of a different fundamental on the combination of the timbres of pairs of formants presented either binaurally or dichotically to form diphthongs. Onset-time also has no effect with binaural presentation. The fourth experiment finds both an effect of grouping formants by a common fundaental using formant trajectories that do not overlap in frequency, and also an effect of onset-time. Neither a common fundamental nor common onset-time is either a necessary or a sufficient condition for formants to be grouped into a common speech category, although they can be shown to exert an influence. Both these variables exert a considerable influence on the number of sounds that subjects report hearing, even under conditions where they do not influence the reported speech category, indicating a dissociation between mechanisms concerned with “how many” sound sources there are, and those concerned with “what” a source consists of.