Low-level Stimulus Properties Research Articles

Perceived duration of visual stimuli contracts due to crowding.

Recent research on duration perception suggests that duration encoding is not a single general process but involves several separate processes, some of which are specific to visual modality. Moreover, different functional aspects of visual processing can influence duration perception in distinct ways. One of the most important functions of the visual system is to identify and recognize features, shapes, and objects. However, it is still unclear whether and how computations related to these processes affect duration perception. To clarify this issue, we used a spatial crowding phenomenon, which allows the dissociation of low-level feature extraction from high-level processes such as object recognition. We created letter and vernier stimuli matched for their low-level properties but different in their discriminability due to spatial crowding. Here, we show that stimuli that became more difficult to discriminate appeared shorter in duration (data collected in 2019-2023). This difference in perceived duration could not be explained by low-level stimulus properties, cognitive bias due to discriminability, or perceived stimulus onsets or offsets. These results suggest the existence of time-sensitive structures specific to visual processing of features, shapes, and objects that is affected by crowding. These findings support the notion of distributed timing mechanisms in the visual system. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Brain representations of motion and position in the double-drift illusion.

In the 'double-drift' illusion, local motion within a window moving in the periphery of the visual field alters the window's perceived path. The illusion is strong even when the eyes track a target whose motion matches the window so that the stimulus remains stable on the retina. This implies that the illusion involves the integration of retinal signals with non-retinal eye-movement signals. To identify where in the brain this integration occurs, we measured BOLD fMRI responses in visual cortex while subjects experienced the double-drift illusion. We then used a combination of univariate and multivariate decoding analyses to identify (1) which brain areas were sensitive to the illusion and (2) whether these brain areas contained information about the illusory stimulus trajectory. We identified a number of cortical areas that responded more strongly during the illusion than a control condition that was matched for low-level stimulus properties. Only in area hMT+ was it possible to decode the illusory trajectory. We additionally performed a number of important controls that rule out possible low-level confounds. Concurrent eye tracking confirmed that subjects accurately tracked the moving target; we were unable to decode the illusion trajectory using eye position measurements recorded during fMRI scanning, ruling out explanations based on differences in oculomotor behavior. Our results provide evidence for a perceptual representation in human visual cortex that incorporates extraretinal information.

The Emergence of Tuning to Global Shape Properties of Radial Frequency Patterns in the Ventral Visual Pathway.

Radial frequency (RF) patterns, created by sinusoidal modulations of a circle's radius, are processed globally when RF is low. These closed shapes therefore offer a useful way to interrogate the human visual system for global processing of curvature. RF patterns elicit greater responses than those to radial gratings in V4 and more anterior face-selective regions of the ventral visual pathway. This is largely consistent with work on nonhuman primates showing curvature processing emerges in V4, but is evident also higher up the ventral visual stream. Rather than contrasting RF patterns with other stimuli, we presented them at varied frequencies in a regimen that allowed tunings to RF to be derived from 8 human participants (3 female). We found tuning to low RF in lateral occipital areas and to some extent in V4. In a control experiment, we added a high-frequency ripple to the stimuli disrupting the local contour. Low-frequency tuning to these stimuli remained in the ventral visual stream, underscoring its role in global processing of shape curvature. We then used representational similarity analysis to show that, in lateral occipital areas, the neural representation was related to stimulus similarity, when it was computed with a model that captured how stimuli are perceived. We therefore show that global processing of shape curvature emerges in the ventral visual stream as early as V4, but is found more strongly in lateral occipital regions, which exhibit responses and representations that relate well to perception.SIGNIFICANCE STATEMENT We show that tuning to low radial frequencies, known to engage global shape processing mechanisms, was localized to lateral occipital regions. When low-level stimulus properties were accounted for such tuning emerged in V4 and LO2 in addition to the object-selective region LO. We also documented representations of global shape properties in lateral occipital regions, and these representations were predicted well by a proxy of the perceptual difference between the stimuli.

Effects of Emotional Expression on Face Recognition May Be Accounted for by Image Similarity

We examined the degree to which differences in face recognition rates across emotional expression conditions varied concomitantly with differences in mean objective image similarity. Effects of emotional expression on face recognition performance were measured via an old/new recognition paradigm in which stimuli at both learning and testing had happy, neutral, and angry expressions. Results showed an advantage for faces learned with neutral expressions, as well as for angry faces at testing. Performance data was compared to three quantitative image-similarity indices. Findings showed that mean human performance was strongly correlated with mean image similarity, suggesting that the former may be at least partly explained by the latter. Our findings sound a cautionary note regarding the necessity of considering low-level stimulus properties as explanations for findings that otherwise may be prematurely attributed to higher order phenomena such as attention or emotional arousal.

No effect of value learning on awareness and attention for faces: Evidence from continuous flash suppression and the attentional blink.

It is widely believed that the emotional and movtivational value of social signals, such as faces, influences perception and attention. However, effects reported for stimuli with intrinsic affective value, such as emotional facial expressions, can often be explained by differences in low-level stimulus properties. To rule out such low-level effects, here we used a value-learning procedure, in which faces were associated with different probabilities of monetary gain and loss in a choice game. In three experiments involving 149 participants, we tested the influence of affective valence (win- vs. loss-associated faces) and motivational salience (probability of monetary gain or loss) on visual awareness, attention, and memory. Using continuous flash suppression and rapid serial visual presentation, we found no effects of affective valence or motivational salience on visual awareness of faces. Furthermore, in two experiments, there was no evidence for a modulation of the attentional blink, indicating that acquired emotional and motivational value does not influence attentional priority of faces. However, we found that motivational salience boosted recognition memory, and this effect was particularly pronounced for win-associated faces. These results indicate that acquired affective valence and motivational salience affect only later processing of faces related to memory but do not directly affect visual awareness and attention. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Regression-based analysis of combined EEG and eye-tracking data: Theory and applications.

Fixation-related potentials (FRPs), neural responses aligned to the end of saccades, are a promising tool for studying the dynamics of attention and cognition under natural viewing conditions. In the past, four methodological problems have complicated the analysis of such combined eye-tracking/electroencephalogram experiments: (1) the synchronization of data streams, (2) the removal of ocular artifacts, (3) the condition-specific temporal overlap between the brain responses evoked by consecutive fixations, and (4) the fact that numerous low-level stimulus and saccade properties also influence the postsaccadic neural responses. Although effective solutions exist for the first two problems, the latter two are only beginning to be addressed. In the current paper, we present and review a unified regression-based framework for FRP analysis that allows us to deconvolve overlapping potentials while also controlling for both linear and nonlinear confounds on the FRP waveform. An open software implementation is provided for all procedures. We then demonstrate the advantages of this proposed (non)linear deconvolution modeling approach for data from three commonly studied paradigms: face perception, scene viewing, and reading. First, for a traditional event-related potential (ERP) face recognition experiment, we show how this technique can separate stimulus ERPs from overlapping muscle and brain potentials produced by small (micro)saccades on the face. Second, in natural scene viewing, we model and isolate multiple nonlinear effects of saccade parameters on the FRP. Finally, for a natural sentence reading experiment using the boundary paradigm, we show how it is possible to study the neural correlates of parafoveal preview after removing spurious overlap effects caused by the associated difference in average fixation time. Our results suggest a principal way of measuring reliable eye movement-related brain activity during natural vision.

Decoding Neural Responses to Motion-in-Depth Using EEG.

Two stereoscopic cues that underlie the perception of motion-in-depth (MID) are changes in retinal disparity over time (CD) and interocular velocity differences (IOVD). These cues have independent spatiotemporal sensitivity profiles, depend upon different low-level stimulus properties, and are potentially processed along separate cortical pathways. Here, we ask whether these MID cues code for different motion directions: do they give rise to discriminable patterns of neural signals, and is there evidence for their convergence onto a single “motion-in-depth” pathway? To answer this, we use a decoding algorithm to test whether, and when, patterns of electroencephalogram (EEG) signals measured from across the full scalp, generated in response to CD- and IOVD-isolating stimuli moving toward or away in depth can be distinguished. We find that both MID cue type and 3D-motion direction can be decoded at different points in the EEG timecourse and that direction decoding cannot be accounted for by static disparity information. Remarkably, we find evidence for late processing convergence: IOVD motion direction can be decoded relatively late in the timecourse based on a decoder trained on CD stimuli, and vice versa. We conclude that early CD and IOVD direction decoding performance is dependent upon fundamentally different low-level stimulus features, but that later stages of decoding performance may be driven by a central, shared pathway that is agnostic to these features. Overall, these data are the first to show that neural responses to CD and IOVD cues that move toward and away in depth can be decoded from EEG signals, and that different aspects of MID-cues contribute to decoding performance at different points along the EEG timecourse.

Pupil diameter encodes the idiosyncratic, cognitive complexity of belief updating.

Pupils tend to dilate in response to surprising events, but it is not known whether these responses are primarily stimulus driven or instead reflect a more nuanced relationship between pupil-linked arousal systems and cognitive expectations. Using an auditory adaptive decision-making task, we show that evoked pupil diameter is more parsimoniously described as signaling violations of learned, top-down expectations than changes in low-level stimulus properties. We further show that both baseline and evoked pupil diameter is modulated by the degree to which individual subjects use these violations to update their subsequent expectations, as reflected in the complexity of their updating strategy. Together these results demonstrate a central role for idiosyncratic cognitive processing in how arousal systems respond to new inputs and, via our complexity-based analyses, offer a potential framework for understanding these effects in terms of both inference processes aimed to reduce belief uncertainty and more traditional notions of mental effort.

Artificially-generated scenes demonstrate the importance of global scene properties for scene perception

Recent electrophysiological research highlights the significance of global scene properties (GSPs) for scene perception. However, since real-world scenes span a range of low-level stimulus properties and high-level contextual semantics, GSP effects may also reflect additional processing of such non-global factors. We examined this question by asking whether Event-Related Potentials (ERPs) to GSPs will still be observed when specific low- and high-level scene properties are absent from the scene. We presented participants with computer-based artificially-manipulated scenes varying in two GSPs (spatial expanse and naturalness) which minimized other sources of scene information (color and semantic object detail). We found that the peak amplitude of the P2 component was sensitive to the spatial expanse and naturalness of the artificially-generated scenes: P2 amplitude was higher to closed than open scenes, and in response to manmade than natural scenes. A control experiment showed that the effect of Naturalness on the P2 is not driven by local texture information, while earlier effects of naturalness, expressed as a modulation of the P1 and N1 amplitudes, are sensitive to texture information. Our results demonstrate that GSPs are processed robustly around 220 ms and that P2 can be used as an index of global scene perception.

Examining Whether Semantic Cues Can Affect Felt Heaviness When Lifting Novel Objects.

It is well established that manipulations of low-level stimulus properties unrelated to mass can impact perception of heaviness, the most famous example being the size-weight illusion whereby small objects feel heavier than equally-weighted larger objects. Interestingly, manipulations of high-level cues such as material have also induced weight illusions, highlighting that cognitive expectations alone are enough to create illusory weight differences. Less is known, however, about what type of cognitive expectations can influence perception of heaviness. As labels are often used to signify the heaviness of objects, this study examined whether semantic cues could induce a novel weight illusion. Participants lifted equally-sized and equally-weighted sets of objects labelled as ‘light’ and ‘heavy’ and reported their perceived heaviness both prior to and after lifting. Fingertip forces were also measured to understand how semantic cues may influence sensorimotor prediction. The labels clearly affected pre-lift-off expectations of heaviness. By contrast, we found no effect of these labels on the perceived heaviness of objects, nor on the forces used to grip and lift them on early trials. In other words, we find no evidence that semantic cues affect perception or action enough to induce a novel weight illusion. These findings suggest that the explicit expectations created by the labels did not dominate the implicit expectations created by the equal sizes of the objects, highlighting the segregated nature of cognitive expectations and their variable influences on perception and action.

Local context effects in the magnitude-duration illusion: Size but not numerical value sequentially alters perceived duration

Many aspects of an event can change perceived duration. A common example of this is the magnitude-duration illusion, in which a high magnitude (e.g. large or high value) stimulus will be perceived to last longer than a low magnitude stimulus. The effects of magnitude on perceived duration are normally considered in terms of global context effects; what is large depends on the stimuli used throughout the experiment. In the current article, we examine local context effects in the magnitude-duration illusion, how trial-by-trial changes in magnitude affect the subjective duration of an event. We performed two experiments in which numerical magnitude and stimulus size were varied within either the example phase or reproduction phase of a temporal reproduction task. We showed that in the current trial the combined value-size magnitude presented in the example phase affected subsequent reproductions, while the magnitude presented in the reproduction phase did not. The size magnitude presented in the reproduction phase also affected the reproduction in the following trial, such that a larger stimulus in the current reproduction phase resulted in shorter reproductions in the next reproduction phase. This indicates that low level stimulus properties (i.e. size) can act to contextualize subsequent stimulus properties, which in turn affect perceived duration. The findings of our experiments add local, low-level, context effects to the known modifiers of perceived duration, as well as provide evidence with regards to the role of magnitude in interval timing.

Do emotional faces capture attention, and does this depend on awareness? Evidence from the visual probe paradigm.

The visual probe (VP) paradigm provides evidence that emotional stimuli attract attention. Such effects have been reported even when stimuli are presented outside of awareness. These findings have shaped the idea that humans possess a processing pathway that detects evolutionarily significant signals independently of awareness. Here, we addressed 2 unresolved questions: First, if emotional stimuli attract attention, is this driven by their affective content, or by low-level image properties (e.g., luminance contrast)? Second, does attentional capture occur under conditions of genuine unawareness? We found that observers preferentially allocated attention to emotional faces under aware viewing conditions. However, this effect was best explained by low-level stimulus properties, rather than emotional content. When stimuli were presented outside of awareness (via continuous flash suppression or masking), we found no evidence that attention was directed toward emotional face stimuli. Finally, observer's awareness of the stimuli (assessed by d') predicted attentional cuing. Our data challenge existing literature: First, we cast doubt on the notion of preferential attention to emotional stimuli in the absence of awareness. Second, we question whether effects revealed by the VP paradigm genuinely reflect emotion-sensitive processes, instead suggesting they can be more parsimoniously explained by low-level variability between stimuli. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Neural entrainment is associated with subjective groove and complexity for performed but not mechanical musical rhythms

Both movement and neural activity in humans can be entrained by the regularities of an external stimulus, such as the beat of musical rhythms. Neural entrainment to auditory rhythms supports temporal perception, and is enhanced by selective attention and by hierarchical temporal structure imposed on rhythms. However, it is not known how neural entrainment to rhythms is related to the subjective experience of groove (the desire to move along with music or rhythm), the perception of a regular beat, the perception of complexity, and the experience of pleasure. In two experiments, we used musical rhythms (from Steve Reich’s Clapping Music) to investigate whether rhythms that are performed by humans (with naturally variable timing) and rhythms that are mechanical (with precise timing), elicit differences in (1) neural entrainment, as measured by inter-trial phase coherence, and (2) subjective ratings of the complexity, preference, groove, and beat strength of rhythms. We also combined results from the two experiments to investigate relationships between neural entrainment and subjective perception of musical rhythms. We found that mechanical rhythms elicited a greater degree of neural entrainment than performed rhythms, likely due to the greater temporal precision in the stimulus, and the two types only elicited different ratings for some individual rhythms. Neural entrainment to performed rhythms, but not to mechanical ones, correlated with subjective desire to move and subjective complexity. These data, therefore, suggest multiple interacting influences on neural entrainment to rhythms, from low-level stimulus properties to high-level cognition and perception.

Unconscious processing of facial dominance: The role of low-level factors in access to awareness.

Visual stimuli with social-emotional relevance have been claimed to gain preferential access to awareness. For example, recent studies used the breaking continuous flash suppression paradigm (b-CFS) to show that faces that are perceived as less dominant and more trustworthy are prioritized for awareness. Here we asked whether these effects truly reflect differences in social-emotional meaning or whether they can be equally explained by differences in low-level stimulus properties. In Experiment 1, we successfully replicated dominance- and untrustworthiness-related slowing for upright faces. However, these effects were equally strong for inverted faces, even though it was more difficult to perceive social characteristics in inverted faces. The previously reported correlation between dominance- and untrustworthiness-related slowing in b-CFS and self-reported propensity to trust did not replicate. Experiment 2 showed that dominance-related slowing in b-CFS can also be observed when only presenting the eye region of faces, and even when the eye region was presented inverted and/or with reversed contrast polarity, in which case personality traits were no longer discernible. These results were replicated in Experiment 3 following a preregistration protocol. Altogether, our findings link dominance-related slowing in b-CFS to physical differences in the eye region that are-when presented in isolation-unrelated to the perception of dominance. We conclude that low-level physical stimulus differences provide a parsimonious explanation for the effect of social facial characteristics on access to awareness. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Probing the temporal dynamics of the exploration-exploitation dilemma of eye movements.

When scanning a visual scene, we are in a constant decision process regarding whether to further exploit the information content at the current fixation or to go on and explore the scene. The balance of these two processes determines the distribution of fixation durations. Using a gaze-contingent paradigm, we experimentally interrupt this process to probe its state. Here, we developed a guided-viewing task where only a single 3° aperture of an image ("bubble") is displayed. Subjects had to fixate the bubble for an experimentally controlled time (forced fixation time). Then, the previously fixated bubble disappeared, and one to five bubbles emerged at different locations. The subjects freely selected one of these by performing a saccade toward it. By repeating this procedure, the subjects explored the image. We modeled the resulting saccadic reaction times (choice times) from bubble offset to saccade onset using a Bayesian linear mixed model. We observed an exponential decay between the forced fixation time and the choice time: Short fixation durations elicited longer choice times. In trials with multiple bubbles, the choice time increased monotonically with the number of possible future targets. Additionally, we found only weak influences of the saccade amplitude, low-level stimulus properties, and saccade angle on the choice times. The exponential decay of the choice times suggests that the sampling and processing of the current stimulus were exhausted for long fixation durations, biasing toward faster exploration. This observation also shows that the decision process took into account processing demands at the current fixation location.

The impact of semantically congruent and incongruent visual information on auditory object recognition across development

The ability to use different sensory signals in conjunction confers numerous advantages on perception. Multisensory perception in adults is influenced by factors beyond low-level stimulus properties such as semantic congruency. Sensitivity to semantic relations has been shown to emerge early in development; however, less is known about whether implementation of these associations changes with development or whether development in the representations themselves might modulate their influence. Here, we used a Stroop-like paradigm that requires participants to identify an auditory stimulus while ignoring a visual stimulus. Prior research shows that in adults visual distractors have more impact on processing of auditory objects than vice versa; however, this pattern appears to be inverted early in development. We found that children from 8years of age (and adults) gain a speed advantage from semantically congruent visual information and are disadvantaged by semantically incongruent visual information. At 6years of age, children gain a speed advantage for semantically congruent visual information but are not disadvantaged by semantically incongruent visual information (as compared with semantically unrelated visual information). Both children and adults were influenced by associations between auditory and visual stimuli, which they had been exposed to on only 12 occasions during the learning phase of the study. Adults showed a significant speed advantage over children for well-established associations but showed no such advantage for newly acquired pairings. This suggests that the influence of semantic associations on multisensory processing does not change with age but rather these associations become more robust and, in turn, more influential.

The Face-Race Lightness Illusion Is Not Driven by Low-level Stimulus Properties: An Empirical Reply to Firestone and Scholl (2014).

Levin and Banaji (Journal of Experimental Psychology: General, 135, 501-512, 2006) reported a lightness illusion in which participants appeared to perceive Black faces to be darker than White faces, even though the faces were matched for overall brightness and contrast. Recently, this finding was challenged by Firestone and Scholl (Psychonomic Bulletin and Review, 2014), who argued that the nominal illusion remained even when the faces were blurred so as to make their race undetectable, and concluded that uncontrolled perceptual differences between the stimulus faces drove at least some observations of the original distortion effect. In this paper we report that measures of race perception used by Firestone and Scholl were insufficiently sensitive. We demonstrate that a forced choice race-identification task not only reveals that participants could detect the race of the blurred faces but also that participants' lightness judgments often aligned with their assignment of race.

Contour integration, attentional cuing, and conscious awareness: An investigation on the processing of collinear and orthogonal contours.

Previous literature suggests that low-level stimulus properties determine the detection performance of contours and are used to define different contour types. Here we investigated the processing of different types of contours under conscious and unconscious conditions. In Experiment 1, we adopted an inattentional blindness paradigm and showed that collinear contours (i.e., a contour type that is frequently observed in natural images) induced a positive cuing effect in both the conscious and unconscious conditions, whereas orthogonal contours (which are less prevalent in the natural environment) attracted attention only when consciously perceived. In Experiment 2, we showed that collinear contours rendered invisible by continuous flash suppression emerged from suppression more rapidly than a random field, whereas orthogonal contours had no such breaking superiority. These results suggest that collinear but not orthogonal contours can be processed and serve as attentional cues without conscious awareness. Our findings provide further evidence that the relevance of the contours to natural statistics could be a key evolutionary factor that decides whether a contour can be unconsciously processed to increase its detectability in a clutter environment.

Segmentation precedes face categorization under suboptimal conditions

Both categorization and segmentation processes play a crucial role in face perception. However, the functional relation between these subprocesses is currently unclear. The present study investigates the temporal relation between segmentation-related and category-selective responses in the brain, using electroencephalography (EEG). Surface segmentation and category content were both manipulated using texture-defined objects, including faces. This allowed us to study brain activity related to segmentation and to categorization. In the main experiment, participants viewed texture-defined objects for a duration of 800 ms. EEG results revealed that segmentation-related responses precede category-selective responses. Three additional experiments revealed that the presence and timing of categorization depends on stimulus properties and presentation duration. Photographic objects were presented for a long and short (92 ms) duration and evoked fast category-selective responses in both cases. On the other hand, presentation of texture-defined objects for a short duration only evoked segmentation-related but no category-selective responses. Category-selective responses were much slower when evoked by texture-defined than by photographic objects. We suggest that in case of categorization of objects under suboptimal conditions, such as when low-level stimulus properties are not sufficient for fast object categorization, segmentation facilitates the slower categorization process.

Photographic but not line-drawn faces show early perceptual neural sensitivity to eye gaze direction.

Our brains readily decode facial movements and changes in social attention, reflected in earlier and larger N170 event-related potentials (ERPs) to viewing gaze aversions vs. direct gaze in real faces (Puce et al., 2000). In contrast, gaze aversions in line-drawn faces do not produce these N170 differences (Rossi et al., 2014), suggesting that physical stimulus properties or experimental context may drive these effects. Here we investigated the role of stimulus-induced context on neurophysiological responses to dynamic gaze. Sixteen healthy adults viewed line-drawn and real faces, with dynamic eye aversion and direct gaze transitions, and control stimuli (scrambled arrays and checkerboards) while continuous electroencephalographic (EEG) activity was recorded. EEG data from 2 temporo-occipital clusters of 9 electrodes in each hemisphere where N170 activity is known to be maximal were selected for analysis. N170 peak amplitude and latency, and temporal dynamics from Event-Related Spectral Perturbations (ERSPs) were measured in 16 healthy subjects. Real faces generated larger N170s for averted vs. direct gaze motion, however, N170s to real and direct gaze were as large as those to respective controls. N170 amplitude did not differ across line-drawn gaze changes. Overall, bilateral mean gamma power changes for faces relative to control stimuli occurred between 150–350 ms, potentially reflecting signal detection of facial motion. Our data indicate that experimental context does not drive N170 differences to viewed gaze changes. Low-level stimulus properties, such as the high sclera/iris contrast change in real eyes likely drive the N170 changes to viewed aversive movements.