Perceptual History Biases Are Predicted by Early Visual-Evoked Activity.

What we see in the present is affected by what we saw in the recent past. Serial dependence, a bias making a current stimulus appear more similar to a previous one, has been indeed shown to be ubiquitous in vision. At the neural level, serial dependence is accompanied by a signature of stimulus history (i.e., past stimulus information) emerging from early visual-evoked activity. However, whether this neural signature effectively reflects the behavioral bias is unclear. Here we address this question by assessing the neural (electrophysiological) and behavioral signature of stimulus history in human subjects (both male and female), in the context of numerosity, duration, and size perception. First, our results show that while the behavioral effect is task-dependent, its neural signature also reflects task-irrelevant dimensions of a past stimulus, suggesting a partial dissociation between the mechanisms mediating the encoding of stimulus history and the behavioral bias itself. Second, we show that performing a task is not a necessary condition to observe the neural signature of stimulus history, but that in the presence of an active task such a signature is significantly amplified. Finally, and more importantly, we show that the pattern of brain activity in a relatively early latency window (starting at ∼35-65 ms after stimulus onset) significantly predicts the behavioral effect. Overall, our results thus demonstrate that the encoding of past stimulus information in neural signals does indeed reflect serial dependence, and that serial dependence occurs at a relatively early level of visual processing.SIGNIFICANCE STATEMENT What we perceive is determined not only by the information reaching our sensory organs, but also by the context in which the information is embedded. What we saw in the recent past (perceptual history) can indeed modulate the perception of a current stimulus in an attractive way, a bias that is ubiquitous in vision. Here we show that this bias can be predicted by the pattern of brain activity reflecting the encoding of past stimulus information, very early after the onset of a stimulus. This in turn suggests that the integration of past and present sensory information mediating the attractive bias occurs early in the visual processing stream, and likely involves early visual cortices.

Subjective time is predicted by local and early visual processing.

Time is as pervasive as it is elusive to study, and how the brain keeps track of millisecond time is still unclear. Here we addressed the mechanisms underlying duration perception by looking for a neural signature of subjective time distortion induced by motion adaptation. We recorded electroencephalographic signals in human participants while they were asked to discriminate the duration of visual stimuli after different types of translational motion adaptation. Our results show that perceived duration can be predicted by the amplitude of the N200 event-related potential evoked by the adapted stimulus. Moreover, we show that the distortion of subjective time can be predicted by the activity in the Beta band frequency spectrum, at the offset of the adaptor and during the presentation of the subsequent adapted stimulus. Both effects were observed from posterior electrodes contralateral to the adapted stimulus. Overall, our findings suggest that local and low-level perceptual processes are involved in generating a subjective sense of time.

The nature of magnitude integration: Contextual interference versus active magnitude binding.

Magnitude dimensions such as duration and numerosity have been shown to systematically interact, biasing each other in a congruent fashion: the more numerous a set of items is, the longer it is perceived to last in time. This integration between dimensions plays an important role in defining how we perceive magnitude. So far, however, the nature of magnitude integration remains unclear. Is magnitude integration a contextual interference, occurring whenever different types of information are concurrently available in the visual field, or does it involve an active "binding" of the different dimensions of the same object? To address these possibilities, we measured the integration bias induced by numerosity on perceived duration, in two cases: with duration and numerosity conveyed by distinct stimuli, or by the same stimulus. We show that a congruent integration effect can be observed only when the two magnitudes belong to the same stimulus. Instead, when the two magnitudes are conveyed by distinct stimuli, we observed an opposite effect. These findings demonstrate for the first time that a congruent integration occurs only between the dimensions of the same stimulus, suggesting the involvement of an active mechanism integrating the different dimensions of the same object in a unified percept.

Decoding of Electroencephalogram Signals Shows No Evidence of a Neural Signature for Subitizing in Sequential Numerosity.

Numerosity perception is largely governed by two mechanisms. The first so-called subitizing system allows one to enumerate a small number of items (up to three or four) without error. The second system allows only an approximate estimation of larger numerosities. Here, we investigate the neural bases of the two systems using sequentially presented numerosity. Sequential numerosity (i.e., the number of events presented over time) starts as a subitizable set but may eventually transition into a larger numerosity in the approximate estimation range, thus offering a unique opportunity to investigate the neural signature of that transition point, or subitizing boundary. If sequential numerosity is encoded by two distinct perceptual mechanisms (i.e., for subitizing and approximate estimation), neural representations of the sequentially presented items crossing the subitizing boundary should be sharply distinguishable. In contrast, if sequential numerosity is encoded by a single perceptual mechanism for all numerosities and subitizing is achieved through an external postperceptual mechanism, no such differences in the neural representations should indicate the subitizing boundary. Using the high temporal resolution of the EEG technique incorporating a multivariate decoding analysis, we found results consistent with the latter hypothesis: No sharp representational distinctions were observed between items across the subitizing boundary, which is in contrast with the behavioral pattern of subitizing. The results support a single perceptual mechanism encoding sequential numerosities, whereas subitizing may be supported by a postperceptual attentional mechanism operating at a later processing stage.

The specious interaction of time and numerosity perception.

Magnitude information is essential to create a representation of the external environment and successfully interact with it. Duration and numerosity, for example, can shape our predictions and bias each other (i.e. the greater the number of people queuing, the longer we expect to wait). While these biases suggest the existence of a generalized magnitude system, asymmetric effects (i.e. numerosity affecting duration but not vice versa) challenged this idea. Here, we propose that such asymmetric integration depends on the stimuli used and the neural processing dynamics they entail. Across multiple behavioural experiments employing different stimulus presentation displays (static versus dynamic) and experimental manipulations known to bias numerosity and duration perceptions (i.e. connectedness and multisensory integration), we show that the integration between numerosity and time can be symmetrical if the stimuli entail a similar neural time-course and numerosity unfolds over time. Overall, these findings support the idea of a generalized magnitude system, but also highlight the role of early sensory processing in magnitude representation and integration.

Serial dependence in time and numerosity perception is dimension-specific.

The perception of a visual event (e.g., a flock of birds) at the present moment can be biased by a previous perceptual experience (e.g., the perception of an earlier flock). Serial dependence is a perceptual bias whereby a current stimulus appears more similar to a previous one than it actually is. Whereas serial dependence emerges within several visual stimulus dimensions, whether it could simultaneously operate across different dimensions of the same stimulus (e.g., the numerosity and the duration of a visual pattern) remains unclear. Here we address this question by assessing the presence of serial dependence across duration and numerosity, two stimulus dimensions that are often associated and can bias each other. Participants performed either a duration or a numerosity discrimination task, in which they compared a constant reference with a variable test stimulus, varying along the task-relevant dimension (either duration or numerosity). Serial dependence was induced by a task-irrelevant inducer, that is, a stimulus presented before the reference and always varying in both duration and numerosity. The results show systematic serial dependencies only within the task-relevant stimulus dimension, that is, stimulus numerosity affects numerosity perception only, and duration affects duration perception only. Additionally, at least in the numerosity condition, the task-irrelevant dimension of the inducer (duration) had an opposite, repulsive effect. These findings thus show that attractive serial dependence operates in a highly specific fashion and does not transfer across different stimulus dimensions. Instead, the repulsive influence, possibly reflecting perceptual adaptation, can transfer from one dimension to another.

Neural Dynamics of Serial Dependence in Numerosity Perception.

Serial dependence-an attractive perceptual bias whereby a current stimulus is perceived to be similar to previously seen ones-is thought to represent the process that facilitates the stability and continuity of visual perception. Recent results demonstrate a neural signature of serial dependence in numerosity perception, emerging very early in the time course during perceptual processing. However, whether such a perceptual signature is retained after the initial processing remains unknown. Here, we address this question by investigating the neural dynamics of serial dependence using a recently developed technique that allowed a reactivation of hidden memory states. Participants performed a numerosity discrimination task during EEG recording, with task-relevant dot array stimuli preceded by a task-irrelevant stimulus inducing serial dependence. Importantly, the neural network storing the representation of the numerosity stimulus was perturbed (or pinged) so that the hidden states of that representation can be explicitly quantified. The results first show that a neural signature of serial dependence emerges early in the brain signals, starting soon after stimulus onset. Critical to the central question, the pings at a later latency could successfully reactivate the biased representation of the initial stimulus carrying the signature of serial dependence. These results provide one of the first pieces of empirical evidence that the biased neural representation of a stimulus initially induced by serial dependence is preserved throughout a relatively long period.

Spontaneous repulsive adaptation in the absence of attractive serial dependence.

Despite noisy and discontinuous input, vision is remarkably stable and continuous. Recent work suggests that such a remarkable feat is enabled by an active stabilization process integrating information over time, resulting in attractive serial dependence. However, precise mechanisms underlying serial dependence are still unknown. Across three psychophysical experiments, we demonstrate that suppressing high-level modulatory signals on early cortical activity via visual backward masking completely abolishes the serial dependence effect, indicating the critical role of cortical feedback processing on serial dependence. Moreover, we show that the absence of modulatory feedback results in a robust repulsive aftereffect, as in perceptual adaptation, after only 50 ms of stimulation, indicating the presence of a local neurocomputational process for an automatic and spontaneous recalibration of the stimulus representation. These findings collectively illustrate the interplay between two contrasting cortical mechanisms at short timescales that serve as a basis for our perceptual experience.

Early Numerosity Encoding in Visual Cortex Is Not Sufficient for the Representation of Numerical Magnitude.

Recent studies have demonstrated that the numerosity of visually presented dot arrays is represented in low-level visual cortex extremely early in latency. However, whether or not such an early neural signature reflects the perceptual representation of numerosity remains unknown. Alternatively, such a signature may indicate the raw sensory representation of the dot-array stimulus before becoming the perceived representation of numerosity. Here, we addressed this question by using the connectedness illusion, whereby arrays with pairwise connected dots are perceived to be less numerous compared with arrays containing isolated dots. Using EEG and fMRI in two independent experiments, we measured neural responses to dot-array stimuli comprising 16 or 32 dots, either isolated or pairwise connected. The effect of connectedness, which reflects the segmentation of the visual stimulus into perceptual units, was observed in the neural activity after 150 msec post stimulus onset in the EEG experiment and in area V3 in the fMRI experiment using a multivariate pattern analysis. In contrast, earlier neural activity before 100 msec and in area V2 was strictly modulated by numerosity regardless of connectedness, suggesting that this early activity reflects the sensory representation of a dot array before perceptual segmentation. Our findings thus demonstrate that the neural representation for numerosity in early visual cortex is not sufficient for visual number perception and suggest that the perceptual encoding of numerosity occurs at or after the segmentation process that takes place later in area V3.

Attractive Serial Dependence in the Absence of an Explicit Task.

Attractive serial dependence refers to an adaptive change in the representation of sensory information, whereby a current stimulus appears to be similar to a previous one. The nature of this phenomenon is controversial, however, as serial dependence could arise from biased perceptual representations or from biased traces of working memory representation at a decisional stage. Here, we demonstrated a neural signature of serial dependence in numerosity perception emerging early in the visual processing stream even in the absence of an explicit task. Furthermore, a psychophysical experiment revealed that numerosity perception is biased by a previously presented stimulus in an attractive way, not by repulsive adaptation. These results suggest that serial dependence is a perceptual phenomenon starting from early levels of visual processing and occurring independently from a decision process, which is consistent with the view that these biases smooth out noise from neural signals to establish perceptual continuity.

Serial dependence in numerosity perception.

Our conscious experience of the external world is remarkably stable and seamless, despite the intrinsically discontinuous and noisy nature of sensory information. Serial dependencies in visual perception-reflecting attractive biases making a current stimulus to appear more similar to previous ones-have been recently hypothesized to be involved in perceptual continuity. However, while these effects have been observed across a variety of visual features and at the neural level, several aspects of serial dependence and how it generalizes across visual dimensions is still unknown. Here we explore the behavioral signature of serial dependence in numerosity perception by assessing how the perceived numerosity of dot-array stimuli is biased by a task-irrelevant "inducer" stimulus presented before task-relevant stimuli. First, although prior work suggests that numerosity perception starts in the subcortex, the current study rules out a possible involvement of subcortical processing in serial dependence, confirming that the effect likely starts in the visual cortex. Second, we show that the effect is coarsely spatially localized to the position of the inducer stimulus. Third, we demonstrate that the effect is present even with a stimulus presentation procedure minimizing the involvement of post-perceptual processes, but only when participants actively pay attention to the inducer stimulus. Overall, these results provide a comprehensive characterization of serial dependencies in numerosity perception, demonstrating that attractive biases occur by means of spatially localized attentional modulations of early sensory activity.

Trans-saccadic integration of orientation information.

Does visual processing start anew after each eye movement, or is information integrated across saccades? Here we test a strong prediction of the integration hypothesis: that information acquired after a saccade interferes with the perception of images acquired before the saccade. We investigate perception of a basic visual feature, grating orientation, and we take advantage of a delayed interference phenomenon-in human participants, the reported orientation of a target grating, briefly presented at an eccentric location, is strongly biased toward the orientation of flanker gratings that are flashed shortly after the target. Crucially, we find that the effect is the same whether or not a saccade is made during the delay interval even though the eye movement produces a large retinotopic separation between target and flankers. However, the trans-saccadic effect nearly vanishes when flankers are displaced to a different screen location even when this location matches the retinotopic coordinates of the target. We conclude that information about grating orientation is integrated across saccades within a spatial region that is defined in external coordinates and thereby is stable in spite of the movement of the eyes.

Numerosity processing in early visual cortex.

While parietal cortex is thought to be critical for representing numerical magnitudes, we recently reported an event-related potential (ERP) study demonstrating selective neural sensitivity to numerosity over midline occipital sites very early in the time course, suggesting the involvement of early visual cortex in numerosity processing. However, which specific brain area underlies such early activation is not known. Here, we tested whether numerosity-sensitive neural signatures arise specifically from the initial stages of visual cortex, aiming to localize the generator of these signals by taking advantage of the distinctive folding pattern of early occipital cortices around the calcarine sulcus, which predicts an inversion of polarity of ERPs arising from these areas when stimuli are presented in the upper versus lower visual field. Dot arrays, including 8-32dots constructed systematically across various numerical and non-numerical visual attributes, were presented randomly in either the upper or lower visual hemifields. Our results show that neural responses at about 90ms post-stimulus were robustly sensitive to numerosity. Moreover, the peculiar pattern of polarity inversion of numerosity-sensitive activity at this stage suggested its generation primarily in V2 and V3. In contrast, numerosity-sensitive ERP activity at occipito-parietal channels later in the time course (210-230ms) did not show polarity inversion, indicating a subsequent processing stage in the dorsal stream. Overall, these results demonstrate that numerosity processing begins in one of the earliest stages of the cortical visual stream.

Spatiotemporal feature integration shapes approximate numerical processing.

Numerosity perception involves a complex cascade of processing stages comprising an early sensory representation stage followed by a later stage providing a conceptual representation of numerical magnitude. While much recent work has focused on understanding how nonnumerical spatial features (e.g., density, area) influence numerosity perception in this processing cascade, little is known about how the spatiotemporal properties of the stimuli affect numerosity processing. Whether numerosity information is integrated over space and time in the processing cascade is an important question as it can provide insights into how the system dedicated for numerosity interacts with other perceptual systems. To address these issues, in four independent experiments, we asked participants to judge the numerosities of various different kinds of dynamically presented dot arrays, such as dots randomly changing in their locations, moving in smooth trajectories, or flickering on and off. The results revealed a systematic overestimation of dynamically presented dot arrays, which implicates the existence of spatiotemporal integration mechanisms, both at the early sensory representation stage and the later conceptual representation stage. The results also revealed the influence of motion and color processing areas on numerosity processing. The findings thus provide empirical evidence that numerosity perception arises from a complex interaction between multiple perceptual mechanisms in the visual stream, and that it is shaped by the integration of spatiotemporal properties of visual stimuli.

The effect of abstract representation and response feedback on serial dependence in numerosity perception.

Serial dependence entails an attractive bias based on the recent history of stimulation, making the current stimulus appear more similar to the preceding one. Although serial dependence is ubiquitous in perception, its nature and mechanisms remain unclear. Here, in two independent experiments, we test the hypothesis that this bias originates from high-level processing stages at the level of abstract information processing (Exp. 1) or at the level of judgment (Exp. 2). In Experiment 1, serial dependence was induced by a task-irrelevant "inducer" stimulus in a numerosity discrimination task, similarly to previous studies. Importantly, in this experiment, the inducers were either arrays of dots similar to the task-relevant stimuli (e.g., 12 dots), or symbolic numbers (e.g., the numeral "12"). Both dots and symbol inducers successfully yielded attractive serial dependence biases, suggesting that abstract information about an image is sufficient to bias the perception of the current stimulus. In Experiment 2, participants received feedback about their responses in each trial of a numerosity estimation task, which was designed to assess whether providing external information about the accuracy of judgments would modulate serial dependence. Providing feedback significantly increased the attractive serial dependence effect, suggesting that external information at the level of judgment may modulate the weight of past perceptual information during the processing of the current image. Overall, our results support the idea that, although serial dependence may operate at a perceptual level, it originates from high-level processing stages at the level of abstract information processing and at the level of judgment.

Disentangling feedforward versus feedback processing in numerosity representation.

Numerosity is a fundamental aspect of the external environment, needed to guide our behavior in an effective manner. Previous studies show that numerosity processing involves at least two temporal stages (~100 and ~150 msec after stimulus onset) in early visual cortex. One possibility is that the two stages reflect an initial feedforward processing followed by feedback signals from higher-order cortical areas that underlie segmentation of visual inputs into perceptual units that define numerosity. Alternatively, multiple stages of feedforward processing might progressively refine the input leading to the segmented representation. Here, we distinguish these two hypotheses by exploiting the connectedness illusion (i.e., the systematic underestimation of pairwise-connected dots), backward masking (to suppress feedback signals), and serial dependence (i.e., a perceptual bias making a stimulus appear to be more similar to its preceding one). Our results show that a connected dot array biases the numerosity representation of the subsequent dot array based on its illusory perception, irrespective of whether it is visible or suppressed by masking. These findings demonstrate that feedback processing is not strictly necessary for the perceptual segmentation that gives rise to perceived numerosity, and instead suggest that different stages of feedforward activity presumably carrying low and high spatial frequency information are sufficient to create a numerosity representation in early visual areas.

Causality shifts the perceived temporal order of audiovisual events.

Causality poses explicit constraints to the timing of sensory signals produced by events, as sound travels slower than light, making auditory stimulation to lag visual stimulation. Previous studies show that implied causality between unrelated events can change the tolerance of simultaneity judgments for audiovisual asynchronies. Here, we tested whether apparent causality between audiovisual events may also affect their perceived temporal order. To this aim, we used a disambiguated stream-bounce display, with stimuli either bouncing or streaming upon each other. These two possibilities were accompanied by a sound played around the time of contact between the objects, which could be perceived as causally related to the visual event according to the condition. Participants reported whether the visual contact occurred before or after the sound. Our results show that when the audiovisual stimuli are consistent with a causal interpretation (i.e., the bounce caused the sound), their perceived temporal order is systematically biased. Namely, a stimulus dynamic consistent with a causal relation induces a perceptual delay in the audio component, even if the sound was presented first. We thus conclude that causality can systematically bias the perceived temporal order of events, possibly due to expectations based on the dynamics of events in the real world. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Attractive serial dependence between memorized stimuli.

Attractive serial dependence - a bias whereby the current stimulus appears more similar to the previous ones - is thought to reflect a stability mechanism integrating past and current visual signals. Prior work suggests that serial dependence originates from both perceptual and cognitive mechanisms, but the conditions under which this attractive bias occurs remain to be studied. In particular, whether serial dependence can occur solely from memory interference remains unclear. Here, we address this question by testing the hypothesis that if memory interference is sufficient to generate serial dependence, it should occur within memorized stimuli irrespective of the order of stimulus presentation. In Exp. 1, we used a numerosity estimation task in which participants estimated the number of dots of a briefly flashing dot-array comprising 8 to 32 dots. The pattern of serial dependence was found in that numerical estimates of a dot array were biased towards the numerosity of the preceding dot array. In Exp. 2, we presented a series of three such dot arrays, and cued the one to be estimated only after the whole series was presented, making the participants first form a memorized representation of the three dot arrays. The results show a pattern of attractive biases both in the forward (the stimulus presented before biases the one presented after) and the backward (the stimulus presented after biases the one presented before) directions. Overall, our results demonstrate that serial dependence can be induced solely from memory interference and that this interference can operate irrespective of the chronological order of the stimulus presentation.

Serial dependence generalizes across different stimulus formats, but not different sensory modalities.

Visual perception is thought to be supported by a stabilization mechanism integrating information over time, resulting in a systematic attractive bias in experimental contexts. Previous studies show that this effect, whereby a current stimulus appears more similar to the one previous to it, depends on attention, suggesting an active high-level mechanism that modulates perception. Here, we test the hypothesis that such a mechanism generalizes across different stimulus formats or sensory modalities, effectively abstracting from the low-level properties of the stimuli. Participants performed a numerosity discrimination task, with task-relevant dot-array stimuli preceded by a sequence of visual (flashes) or auditory (tones) stimuli encompassing different numerosities. Our results show a clear attractive bias induced by visual sequential numerosity affecting an array of simultaneously presented dots, thus operating across different stimulus formats. Conversely, auditory sequences did not affect the judgment on visual numerosities. Overall, our results demonstrate that serial dependence in numerosity perception operates according to the abstract representation of numerical magnitude of visual stimuli irrespective of their format. These results thus support the idea that a high-level mechanism mediates visual stability and continuity, which integrates relevant information over time irrespective of the low-level sensory properties of the stimuli.

Looking for more food or more people? Task context influences basic numerosity perception.

Approximate numerical magnitude (or numerosity) is thought to represent one of the fundamental sensory properties driving perceptual choices. Recent studies indicate that numerosity judgment on a dot array is primarily driven by its numerical magnitude, largely independent from its other non-numerical visual dimensions. Nevertheless, these findings do not preclude the possibility that non-numerical cues such as size or spacing of a dot array influence numerosity judgment. Here, we test the hypothesis that numerosity judgment is influenced by non-numerical dimensions of a dot array depending on the context to which those non-numerical cues could be useful. Participants were asked to choose the more numerous of two dot arrays in two different contexts that differed only in one aspect. In one condition, the task was framed as choosing a set with more fruits to consume. In the other condition, the task was framed as choosing a group with more people to join. The results demonstrate that the influence of non-numerical cues - and particularly of the dimension of size - was significantly smaller when participants made quantitative choices about people than when they made choices about food, illustrating that the representation of discrete magnitude is more pronounced in the former case. These findings suggest that the information pooled to reach a decision about numerosity is flexibly determined according to the context and the goals of such judgment.