Spatial Integration in Glass Patterns

Perception of complex Glass patterns through spatial summation across unique frames

When processing visual information from the surroundings, human vision depends on the constant integration of form and motion cues. Dynamic Glass patterns (GPs) may be used to study how such visual integration occurs in the human visual system. Dynamic GPs are visual stimuli composed of two or more unique frames consisting of different configurations of dot pairs, called dipoles, presented in rapid succession. Previous psychophysical studies showed that the discrimination of translational and circular dynamic GPs is influenced by both the number of unique frames and the pattern update rate. In this study, we manipulated these two variables to assess their influence on the discrimination threshold of circular, radial, and spiral GPs, partially replicating previous findings on circular GPs. Our results indicate that circular GPs are more easily perceived than radial and spiral GPs, showing lower discrimination thresholds. Furthermore, we found that discrimination thresholds vary as a function of the number of unique frames but not as a function of the pattern update rate. Specifically, coherence thresholds decreased with increasing the number of unique frames. In conclusion, our findings support the existence of spatial summation of form signals coming from the unique frames that generate complex GPs. On the other hand, they do not support temporal integration of local form-motion signals based on the pattern update rate.

Lack of orientation specific adaptation to vertically oriented Glass patterns in human visual cortex: an fMRI adaptation investigation

The perception of coherent form configurations in natural scenes relies on the activity of early visual areas that respond to local orientation cues. Subsequently, high-level visual areas pool these local signals to construct a global representation of the initial visual input. However, it is still debated whether neurons in the early visual cortex respond also to global form features. Glass patterns (GPs) are visual stimuli employed to investigate local and global form processing and consist of randomly distributed dots pairs called dipoles arranged to form specific global configurations. In the current study, we used GPs and functional magnetic resonance imaging (fMRI) adaptation to reveal the visual areas that subserve the processing of oriented GPs. Specifically, we adapted participants to vertically oriented GP, then we presented test GPs having either the same or different orientations with respect to the adapting GP. We hypothesized that if local form features are processed exclusively by early visual areas and global form by higher-order visual areas, then the effect of visual adaptation should be more pronounced in higher tier visual areas as it requires global processing of the pattern. Contrary to this expectation, our results revealed that adaptation to GPs is robust in early visual areas (V1, V2, and V3), but not in higher tier visual areas (V3AB and V4v), suggesting that form cues in oriented GPs are primarily derived from local-processing mechanisms that originate in V1. Finally, adaptation to vertically oriented GPs causes a modification in the BOLD response within early visual areas, regardless of the relative orientations of the adapting and test stimuli, indicating a lack of orientation selectivity.

Global dot integration in typically developing children and in Williams syndrome

Williams Syndrome (WS) is a neurodevelopmental disorder that results in deficits in visuospatial perception and cognition. The dorsal stream vulnerability hypothesis in WS predicts that visual motion processes are more susceptible to damage than visual form processes. We asked WS participants and typically developing children to detect the global structure Glass patterns, under "static" and "dynamic" conditions in order to evaluate this hypothesis. Sequentially presented Glass patterns are coined as dynamic because they induce illusory motion, which is modeled after the interaction between orientation (form) and direction (motion) mechanisms. If the dorsal stream vulnerability holds in WS participants, then they should process real and illusory motion atypically. However, results are consistent with the idea that form and motion integration mechanisms are functionally delayed or attenuated in WS. Form coherence thresholds for both static and dynamic Glass patterns in WS were similar to those of 4-5year old children, younger than what is predicted by mental age. Dynamic presentation of Glass patterns improved thresholds to the same degree as typical participants. Motion coherence thresholds in WS were similar to those of mental age matches. These data pose constraints on the dorsal vulnerability hypothesis, and refine our understanding of the relationship between form and motion processing in development.

NMDA receptor antagonists distort visual grouping in rats performing a modified two-choice visual discrimination task

RATIONALE

Visual perception is impaired during pathological psychosis, which can be mimicked by NMDA receptor antagonists. However, the underlying mechanisms are poorly understood, partly due to limits of current rodent models for visual integration.

OBJECTIVES

The objectives of the study are (1) to develop a rodent task that can differentiate between effects on perception and nonspecific effects on task performance and (2) to test whether NMDA receptor antagonists affect visual perception in rats.

METHODS

We used an adaptation of Glass patterns to assess visual grouping in rats using a two-choice visual discrimination task in an infrared touch screen conditioning chamber. After rats learned to discriminate between a radial and a concentric bipole pattern, the ability to discriminate between these patterns was tested at various levels of distortion and a psychometric function was fit to obtain the maximum task performance and signal level needed for half-maximum performance.

RESULTS

NMDA receptor antagonists ketamine and phencyclidine at low doses increased the signal quality needed to discriminate between the visual patterns, without affecting the ability to discriminate between undistorted images. At higher doses, the ability to perform the task even with undistorted images was impaired, which was associated with stereotypic behaviour and increased impulsivity.

CONCLUSIONS

The Glass pattern-based visual grouping task is able to differentiate the effect of psychotomimetic NMDA receptor antagonists on visual perception from the effects on motor and memory functions. The half-maximum performance signal level allows quantification of cognitive psychosis in rodents, which can be translated to human psychometric functions and can be used in the development of more effective treatments.

Learning to see, but not discriminate, visual forms is impaired in aging

Despite the central role of learning in visual recognition, it is largely unknown whether visual form learning is maintained in older age. We examined whether training improved performance in both young and older adults at two key stages of visual recognition: integration of local elements and global form discrimination. We used a shape-discrimination task (concentric vs. radial patterns) in which young and older adults showed similar performance before training. Using a parametric stimulus space that allowed us to manipulate global features and background noise, we were able to distinguish integration and discrimination processes. We found that training improves global form discrimination in both young and older adults. However, learning to integrate local elements is impaired in older age, possibly because of reduced tolerance to external noise. These findings suggest that visual selection processes, rather than global feature representations, provide a fundamental limit for learning-dependent plasticity in the aging brain.

Scale-dependent loss of global form perception in strabismic amblyopia

Amblyopic humans are known to have a range of spatial vision abnormalities. Prior studies have documented amblyopic deficits in global form perception but have typically used only one set of stimulus parameters. Our aim in this study was to examine the extent and nature of global form perception deficits in strabismic amblyopia using a range of spatial scales and pattern types. Glass patterns are random dot stimuli in which the local orientations of paired dots must be integrated over space to yield a global form percept. We measured coherence thresholds for discrimination of pattern structure in translational (linear) and concentric Glass patterns at three spatial scales in two control and six amblyopic observers. We found that sensitivity to Glass patterns depended on both spatial scale and pattern type in all observers. Participants with a history of abnormal early visual experience showed greater interocular threshold difference when the discrimination was based on translational patterns than when it was based on concentric patterns, and the degree of amblyopic loss was greatest at fine spatial scale. Our results show that the nature and extent of global form vision deficits vary substantially with stimulus parameters and are greatest at fine spatial scales.

Connecting the dots: how local structure affects global integration in infants

Glass patterns are moirés created from a sparse random-dot field paired with its spatially shifted copy. Because discrimination of these patterns is not based on local features, they have been used extensively to study global integration processes. Here, we investigated whether 4- to 5.5-month-old infants are sensitive to the global structure of Glass patterns by measuring visual-evoked potentials. Although we found strong responses to the appearance of the constituent dots, we found sensitivity to the global structure of the Glass patterns in the infants only over a very limited range of spatial separation. In contrast, we observed robust responses in the infants when we connected the dot pairs of the Glass pattern with lines. Moreover, both infants and adults showed differential responses to exchanges between line patterns portraying different global structures. A control study varying luminance contrast in adults suggests that infant sensitivity to global structure is not primarily limited by reduced element visibility. Together our results suggest that the insensitivity to structure in conventional Glass patterns is due to inefficiencies in extracting the local orientation cues generated by the dot pairs. Once the local orientations are made unambiguous or when the interpolation span is small, infants can integrate these signals over the image.

The role of luminance contrast in the detection of global structure in static and dynamic, same- and opposite-polarity, Glass patterns

Perception of global structure conveyed in static Glass patterns is difficult, though not impossible, when the constituent dipoles are formed by partnering opposite polarity dots. We investigate whether the addition of motion signals to opposite-polarity Glass patterns can act to restore the perception of global structure. The stimuli were concentric Glass patterns consisting of 200 dipoles concentrically orientated, or oriented at random orientations, placed on a grey background. For each dipole, one luminance-increment dot (Weber contrast of 1) was paired with another dot set to a contrast ranging between luminance increment and luminance decrement (i.e., a Weber contrast range of approximately -1 to 1). Dipoles were either stationary (Experiment 1), or randomly re-positioned at 17Hz (Experiment 2), on each frame transition. A two-interval forced-choice paradigm, in conjunction with an adaptive staircase, was used to obtain Glass-pattern detection thresholds. The task required observers to identify the interval that contained concentric Glass structure; the other interval contained randomly orientated dipoles. Generally, lower global form thresholds were observed for dynamic and same-polarity Glass patterns than for static and opposite-polarity Glass patterns. In particular, for dynamic presentations improvement in sensitivity was more evident for opposite-polarity than for same-polarity Glass patterns. These findings suggest that motion plays an important role in the detection of global structure in dynamic Glass patterns.

Shape perception in human vision: specialized detectors for concentric spatial structures?

It has been suggested that second-order shape integration mechanisms in human spatial vision (e.g. the neurons at V2 and V4 cortical areas) may contain specialized detectors for concentric shapes [Vision Res. 37 (1997) 2325; Vision Res. 38 (1998) 2933]. We tested this notion using psychophysical techniques in which the observers' task was to detect orientation noise (randomly oriented dot pairs) in concentric and linear Glass patterns. Our results showed that the detection of orientation noise was easier in concentric Glass patterns, thus supporting the notion of specialized detectors.