Random Visual Noise Research Articles

What surprises the Mona Lisa? The relative importance of the eyes and eyebrows for detecting surprise in briefly presented face stimuli

The classification image (CI) technique has been used to derive templates for judgements of facial emotion and reveal which facial features inform specific emotional judgements. For example, this method has been used to show that detecting an up- or down-turned mouth is a primary strategy for discriminating happy versus sad expressions. We explored the detection of surprise using CIs, expecting widened eyes, raised eyebrows, and open mouths to be dominant features. We briefly presented a photograph of a female face with a neutral expression embedded in random visual noise, which modulated the appearance of the face on a trial-by-trial basis. In separate sessions, we showed this face with or without eyebrows to test the importance of the raised eyebrow element of surprise. Noise samples were aggregated into CIs based on participant responses. Results show that the eye-region was most informative for detecting surprise. Unless attention was specifically directed to the mouth, we found no effects in the mouth region. The eye effect was stronger when the eyebrows were absent, but the eyebrow region was not itself informative and people did not infer eyebrows when they were missing. A follow-up study was conducted in which participants rated the emotional valence of the neutral images combined with their associated CIs. This verified that CIs for ‘surprise’ convey surprised expressions, while also showing that CIs for ‘not surprise’ convey disgust. We conclude that the eye-region is important for the detection of surprise.

You Don’t See What I See: Individual Differences in the Perception of Meaning from Visual Stimuli

Everyone has their own unique version of the visual world and there has been growing interest in understanding the way that personality shapes one’s perception. Here, we investigated meaningful visual experiences in relation to the personality dimension of schizotypy. In a novel approach to this issue, a non-clinical sample of subjects (total n = 197) were presented with calibrated images of scenes, cartoons and faces of varying visibility embedded in noise; the spatial properties of the images were constructed to mimic the natural statistics of the environment. In two experiments, subjects were required to indicate what they saw in a large number of unique images, both with and without actual meaningful structure. The first experiment employed an open-ended response paradigm and used a variety of different images in noise; the second experiment only presented a series of faces embedded in noise, and required a forced-choice response from the subjects. The results in all conditions indicated that a high positive schizotypy score was associated with an increased tendency to perceive complex meaning in images comprised purely of random visual noise. Individuals high in positive schizotypy seemed to be employing a looser criterion (response bias) to determine what constituted a ‘meaningful’ image, while also being significantly less sensitive at the task than those low in positive schizotypy. Our results suggest that differences in perceptual performance for individuals high in positive schizotypy are not related to increased suggestibility or susceptibility to instruction, as had previously been suggested. Instead, the observed reductions in sensitivity along with increased response bias toward seeing something that is not there, indirectly implicated subtle neurophysiological differences associated with the personality dimension of schizotypy, that are theoretically pertinent to the continuum of schizophrenia and hallucination-proneness.

Impaired visual processing preceding image recognition in Parkinson's disease patients with visual hallucinations

Impaired visual processing may play a role in the pathophysiology of visual hallucinations in Parkinson's disease. In order to study involved neuronal circuitry, we assessed cerebral activation patterns both before and during recognition of gradually revealed images in Parkinson's disease patients with visual hallucinations (PDwithVHs), Parkinson's disease patients without visual hallucinations (PDnonVHs) and healthy controls. We hypothesized that, before image recognition, PDwithVHs would show reduced bottom-up visual activation in occipital-temporal areas and increased (pre)frontal activation, reflecting increased top-down demand. Overshoot of the latter has been proposed to play a role in generating visual hallucinations. Nine non-demented PDwithVHs, 14 PDnonVHs and 13 healthy controls were scanned on a 3 Tesla magnetic resonance imaging scanner. Static images of animals and objects gradually appearing out of random visual noise were used in an event-related design paradigm. Analyses were time-locked on the moment of image recognition, indicated by the subjects' button-press. Subjects were asked to press an additional button on a colour-changing fixation dot, to keep attention and motor action constant and to assess reaction times. Data pre-processing and statistical analysis were performed with statistical parametric mapping-5 software. Bilateral activation of the fusiform and lingual gyri was seen during image recognition in all groups (P < 0.001). Several seconds before image recognition, PDwithVHs showed reduced activation of the lateral occipital cortex, compared with both PDnonVHs and healthy controls. In addition, reduced activation of extrastriate temporal visual cortices was seen just before image recognition in PDwithVHs. The association between increased vulnerability for visual hallucinations in Parkinson's disease and impaired visual object processing in occipital and temporal extrastriate visual cortices supported the hypothesis of impaired bottom-up visual processing in PDwithVHs. Support for the hypothesized increased top-down frontal activation was not obtained. The finding of activation reductions in ventral/lateral visual association cortices in PDwithVHs before image recognition further helps to explain functional mechanisms underlying visual hallucinations in Parkinson's disease.

Detecting fearful and neutral faces: BOLD latency differences in amygdala–hippocampal junction

Evolutionary survival and procreation are augmented if an individual organism quickly detects environmental threats and rapidly initiates defensive behavioral reactions. Thus, facial emotions signaling a potential threat, e.g., fear or anger, should be perceived rapidly and automatically, possibly through a subcortical processing route which includes the amygdala. Using event-related functional magnetic resonance imaging (fMRI), we investigated the time course of the response in the amygdala to neutral and fearful faces, which appear from dynamically decreasing random visual noise. We aimed to detect differences of the amygdala response between fearful and neutral faces by estimating the latency of the blood oxygenation level-dependent (BOLD) response. We found that bilateral amygdala–hippocampal junction activation occurred earlier for fearful than for neutral faces. Our findings support the theory of a dual route architecture in which the subcortical thalamic–hippocampal–amygdala route serves fast preconscious threat perception.

Do Humans and Baboons Use the Same Information When Categorizing Human and Baboon Faces?

What information is used for sorting pictures of complex stimuli into categories? We applied a reverse correlation method to reveal the visual features mediating categorization in humans and baboons. Two baboons and 6 humans were trained to sort, by species, pictures of human and baboon faces on which random visual noise was superimposed. On ambiguous probe trials, a human-baboon morph was presented, eliciting "human" responses on some trials and "baboon" responses on others. The difference between the noise patterns that induced the two responses made explicit the information mediating the classification. Unlike the humans, the baboons based their categorization on information that closely matched that used by a theoretical observer responding solely on the basis of the pixel similarities between the probe and training images. We show that the classification-image technique and principal components analysis provide a method to make explicit the differences in the information mediating categorization in humans and animals.

The robustness of perception

The natural environment around us, which is often crowded, cluttered or even foggy, is subject to a dynamically changing composition of objects and events. The human brain is continuously perceiving, recognizing and evaluating this dynamic scene composition. If the perception of degraded visual objects is important, e.g. in the case of potential threat stimuli, the brain needs to be more sensitive in detecting these objects from the natural environment. It is therefore hypothesized that reacting to the dynamically changing environment involves a robust and quick processing of salient information, which can be either with or without conscious awareness. We investigated the dynamics and robustness of perception using pictures of three salience levels, i.e. fearful faces (most salient), neutral faces (salient) and houses (nonsalient), which appear from dynamically decreasing random visual noise. Stimuli were matched for luminance, contrast, brightness and spatial frequency information. Reaction times show a significantly earlier response for faces than for houses. Fearful faces were significantly more quickly detected than neutral faces. The neural correlates sustaining robust perception were investigated with event-related functional magnetic resonance imaging (fMRI). The amygdala showed a significant perception-related response for faces, as compared to houses, that was further enhanced for fearful faces as compared to neutral faces. Our data indicate that emotionally salient information processing is (i) mediated by the amygdala and (ii) more robust than for nonsalient stimuli as it shows a significantly lower perceptual threshold.

Random visual noise impairs object-based attention.

Object-based visual attention is observed when the benefit of attending to one element in a display extends to other elements that are part of the same perceptual object. Apperceptive agnosia is an object identification deficit in which spatial attention is preserved but object-based attention is impaired. Some debate exists regarding the extent to which the object-based impairment can be attributed to perceptual mechanisms that are specifically involved in grouping and segmentation of a scene, as opposed to early sensory processes. In the present paper we show that random visual noise is sufficient to eliminate the object benefit, a result inconsistent with the view that grouping mechanisms are responsible for the effect. The results have implications for an understanding of apperceptive agnosia, and for an understanding of object-based attention more generally.

Parallel search for conjunctions with stimuli in apparent motion.

A series of experiments was conducted to determine whether apparent motion tends to follow the similarity rule (i.e. is attribute-specific) and to investigate the underlying mechanism. Stimulus duration thresholds were measured during a two-alternative forced-choice task in which observers detected either the location or the motion direction of target groups defined by the conjunction of size and orientation. Target element positions were randomly chosen within a nominally defined rectangular subregion of the display (target region). The target region was presented either statically (followed by a 250 ms duration mask) or dynamically, displaced by a small distance (18 min of arc) from frame to frame. In the motion display, the position of both target and background elements was changed randomly from frame to frame within the respective areas to abolish spatial correspondence over time. Stimulus duration thresholds were lower in the motion than in the static task, indicating that target detection in the dynamic condition does not rely on the explicit identification of target elements in each static frame. Increasing the distractor-to-target ratio was found to reduce detectability in the static, but not in the motion task. This indicates that the perceptual segregation of the target is effortless and parallel with motion but not with static displays. The pattern of results holds regardless of the task or search paradigm employed. The detectability in the motion condition can be improved by increasing the number of frames and/or by reducing the width of the target area. Furthermore, parallel search in the dynamic condition can be conducted with both short-range and long-range motion stimuli. Finally, apparent motion of conjunctions is insufficient on its own to support location decision and is disrupted by random visual noise. Overall, these findings show that (i) the mechanism underlying apparent motion is attribute-specific; (ii) the motion system mediates temporal integration of feature conjunctions before they are identified by the static system; and (iii) target detectability in these stimuli relies upon a nonattentive, cooperative, directionally selective motion mechanism that responds to high-level attributes (conjunction of size and orientation).

Segregation of Motion Structure from Random Visual Noise

A potential source of information about spatial layout, surface slant, and self-motion is provided by transformations of the optic flow field. Theoretical analyses have demonstrated that such flow fields can be decomposed into translation, rotation, expansion, and deformation components. The objective in this study was to investigate the effectiveness of these motions as visual primitives for preattentive scene segregation. The stimuli consisted of two-frame patterns containing 998 dots. A variable number of these dots (5 to 50) were defined as target dots, representing a specific motion structure, with the residual dots defined as a noise field. The observers' task was to identify the location and type of motion structure. On the basis of the number of target dots needed to detect and correctly identify the target structure, it was determined that translation was the most easily detected structure, followed by rotation, expansion, and compression. Deformation was found to have a detection threshold as high as compression. The results are related to other studies of these motion components.

Modelling visual detection: luminance response non-linearity and internal noise.

Two experiments that investigate the effect of various display factors on the detectability of a thin line signal in random visual noise are described. Three statistical decision models are described, together with their ability to account for the results. The first is an "ideal detector" model, the second an "energy integrator" model, and the third a model based upon the operation of retinal ganglion cells which incorporates a gain control mechanism. The ideal detector model fails to give a good account of human performance, whereas the other two models provide a good fit to the data. The digital Laplacian with gain control model has the slight advantage over the energy integrating model in being able to account for a small superiority in the detection of dark as opposed to bright signals. Finally, both models require the inclusion of an estimate of the internal noise of the human visual system to account for the pattern of performance observed under changing conditions of display contrast.

A note on the masking of pictures.

An experiment is described in which photographs of everyday objects were masked by frequency-filtered random visual noise. The masking functions obtained were of the same type as those obtained by masking letters and words with random noise or a pattern mask. That is, the high-frequency mask produced a type A function while the low-frequency mask produced a type B function. This result is discussed in terms of the general applicability of models of visual information processing constructed on the basis of experiments with letter or word stimuli. It is suggested that spatial-frequency concepts may usefully be employed to describe the relevant features of different types of mask.

On the locus of age differences in visual information processing.

Age differences in visual information processing were investigated in a study of monoptic backward masking. Young (18 to 22) and old (58 to 73) adults were tested in three experiments utilizing both a random visual noise and a pattern (fragments) mask, with two-letter combinations serving as target stimuli. Critical target duration needed to escape masking was measured at several mask durations in Exps. 1 and 2; critical interstimulus interval was assessed at several target durations in Exp. 3. With random noise as the mask, a small but reliable age difference was seen across the range of mask or target durations, supporting previous reports of age difference in peripheral processing. With the pattern mask as the masking stimulus, there was mixed evidence for age differences in central processing. Such age effects were clearest in Exp. 3 in which the role of luminance summation had been reduced. Control procedures in Exps. 1 and 2 tended to eliminate evidence for central masking. Results are discussed in terms of the magnitude of age differences in peripheral and central processing and in terms of procedural artifacts of previous research.

Age-related Differences in Binocular Backward Masking With Visual Noise

Peripheral processing, as defined by Turvey (1973), was investigated in a study of binocular backward masking involving random visual noise. For young (mean age 20.3 years) and old (mean age 55.4 years) adults, processing time (represented by stimulus onset asynchrony, SOA) was characterized as a power function of target energy (TE) (i.e., TEb x SOA = K). Over a wide range of target energy levels, processing time showed a similar rate of decline (similar exponent, b) for both age groups with increasing target energy. However, old subjects processed targets more slowly at all target energy levels (different constant, K), and this age difference was constant across all target energy levels. Results were independent of variables such as sex, education, response criterion, unmasked threshold, and method of determining stimulus onset asynchrony.

Perceptual effect of pursuit eye movements in the absence of a target.

THE traditional explanation1 of visual tracking has been that smooth tracking by the eyes depends on the presence of a moving retinal image at or near the fovea. However, evidence is steadily accumulating suggesting that this explanation is incomplete2. The presence of an extrafoveal afterimage or stabilised retinal image in certain circumstances suffice to initiate smooth tracking movements3–5; observers are able to track, in the dark, the position of their own hand6 or of a moving sound source7 and may, after considerable practice, be able to generate sinusoidal tracking movements purely voluntarily4,8. We describe here a perceptual effect apparently created by the generation of smooth horizontal tracking eye movements, in the absence of a target, by observers regarding a display of dynamic visual noise. We interpret this effect in terms of the presence within random visual noise of features which lead to the neuronal representation of movement at some unspecified level of organisation of the visual system.

Does prior knowledge facilitate the detection of visual targets in random noise?

The detectability of forms embedded within random visual noise has been found to be predictable from the autocorrelation transform of the stimulus pattern (Uttal, 1975). A basic assumption in the autocorrelation theory of form detection is that detectability is determined by the organization of the stimulus pattern, irrespective of the observer’s prior knowledge or expectations about the characteristics of the form. This assumption was tested by determining the effect of the size of the set of alternative target forms on performance of a forced-choice detection task. The targets were composed of dots in a straight line, appearing in one of a specified set of 2, 4, or 8 alternative positions in a pattern of randomly distributed masking dots. Detection accuracy was found to decrease as set size increased, but this decrease was close to what was predicted on the assumption that the random background was independently confusable with the target at each of the alternative positions. Thus, prior knowledge of the set of alternative targets appeared to have no effect on thevisual process, but only on thedecision process by virtue of the features that were relevant criteria for deciding which of the two patterns on each trial was most likely to contain the target. This result is consistent with the autocorrelation theory. This experiment may illustrate how the decision process has influenced the performance in many other experiments that have been assumed to demonstrate an effect of prior knowledge on perception.

Observations on Binocular Spatial Frequency Reduction in Random Noise

Binocular spatial frequency reduction for random visual noise stimuli is contrasted with MacKay's (1964) homogenisation effect. Several observations lead to suggestions about the anatomical sites of these two perceptual effects.