Effect Of Stimulus Probability Research Articles

P300 response modulation reflects breaches of non-probabilistic expectations

In oddball paradigms, infrequent stimuli elicit larger P300 event related potentials (ERPs) than frequent ones. One hypothesis is that P300 modulations reflect the degree of “surprise” associated with unexpected stimuli. That is the P300 represents how unlikely the stimulus is and this signal is then used to update the observer’s expectations. It could be hypothesized that P300 is modulated by any factor affecting an observer’s expectations, not only target probability. Alternatively, the P300 may reflect an evaluative process engaged whenever a discrepancy between task context and sensory inputs arises, irrespective of the latter probability. In previous ERP studies, stimulus probability was often the only determinant of task set confounding the effects of stimulus probability and set stimulus discrepancy. In this study, we used a speeded luminance detection task. The target was preceded by a central cue that predicted its location. The probability that the target was valid, i.e. would appear at the cued location was manipulated by varying the reliability of the cue. Reaction times were modulated by probabilistic expectations based on cue reliability and target validity while P300 was affected by target validity only. We conclude that increased P300 amplitude reflects primarily breaches of non-probabilistic expectations, rather than target probability.

Decision Making and Oddball Effects on Pupil Size: Evidence for a Sequential Process.

In our physical environment as well as in many experimental paradigms, we need to decide whether an occurring stimulus is relevant to us or not; further, stimuli have uneven probabilities to emerge. Both, decision making and the difference between rare and frequent stimuli (oddball effect) are described to affect pupil dilation. Surprisingly though, conjoint systematic pupillometric investigations into both factors are still rare. In two experiments, both factors as well as their interplay were investigated. Participants completed a sequential letter matching task. In this task, stimulus probability and letter matching (decision making) were manipulated independently. As dependent variables, pupil dilation and reaction time were assessed. Results suggest a clearly larger pupil dilation for target than for distractor letters, even when targets were frequent and distractors rare. When considering the data structure best, no main effect of stimulus probability was found, instead, oddball effects only emerged when stimuli were goal-relevant to participants. The results are discussed in the light of common theoretical concepts of decision making and stimulus probability. Finally, relating theories of each factor, we propose an integrated framework for effects of decision making and stimulus features on pupil dilation. We assume a sequential mechanism during which incoming stimuli are decided upon regarding their goal relevance and, about 200 ms later, relevant stimuli are appraised regarding their value.

Sequential processing in the classic oddball task: ERP components, probability, and behavior.

This study compared the ERP components and behavior associated with the auditory equiprobable and classic oddball tasks, to relate the cognitive processing stages in those paradigms and continue the development of the sequential processing schema. Target and nontarget ERP data were acquired from 66 healthy young adults (Mage = 20.1, SD = 2.4years, 14 male) who completed both equiprobable (target p=0.5) and oddball tasks (target p=0.3). Separate temporal PCAs were used to decompose the ERP data in each task and condition, and the similarity of the components identified in each condition was examined between tasks. Probability effects on component amplitudes and behavior were also analyzed to identify task differences in cognitive demands. A highly similar series of components was identified in each task, closely matching the schema: targets elicited N1-3, N1-1, PN, N2c, P3b, SW1, SW2; whereas nontargets elicited N1-3, N1-1, PN, N2b, P3a, SW1, SW2. N1-1 and PN amplitudes increased as stimulus probability decreased, irrespective of the condition. N2b, P3b, SW1, and SW2 amplitudes also varied between tasks, illustrating task-specific demands on those processing stages. These findings complemented the behavioral outcomes, which demonstrated greater accuracy and control in the classic oddball task. Overall, this study demonstrated comparable processing in the auditory equiprobable and classic oddball tasks, extending the generalizability of the schema and enabling further integration of the ERP theory associated with these tasks. This study also clarifies stimulus probability effects on the schema, providing important insight into the functionality of common ERP components.

Modeling Neural Adaptation in Auditory Cortex

Neural responses recorded from auditory cortex exhibit adaptation, a stimulus-specific decrease that occurs when the same sound is presented repeatedly. Stimulus-specific adaptation is thought to facilitate perception in noisy environments. Although adaptation is assumed to arise independently from cortex, this has been difficult to validate directly in vivo. In this study, we used a neural network model of auditory cortex with multicompartmental cell modeling to investigate cortical adaptation. We found that repetitive, non-adapted inputs to layer IV neurons in the model elicited frequency-specific decreases in simulated single neuron, population-level and local field potential (LFP) activity, consistent with stimulus-specific cortical adaptation. Simulated recordings of LFPs, generated solely by excitatory post-synaptic inputs and recorded from layers II/III in the model, showed similar waveform morphologies and stimulus probability effects as auditory evoked responses recorded from human cortex. We tested two proposed mechanisms of cortical adaptation, neural fatigue and neural sharpening, by varying the strength and type of inter- and intra-layer synaptic connections (excitatory, inhibitory). Model simulations showed that synaptic depression modeled in excitatory (AMPA) synapses was sufficient to elicit a reduction in neural firing rate, consistent with neural fatigue. However, introduction of lateral inhibition from local layer II/III interneurons resulted in a reduction in the number of responding neurons, but not their firing rates, consistent with neural sharpening. These modeling results demonstrate that adaptation can arise from multiple neural mechanisms in auditory cortex.

Interrelation of attention and prediction in visual processing: Effects of task-relevance and stimulus probability

The potentially interactive influence of attention and prediction was investigated by measuring event-related potentials (ERPs) in a spatial cueing task with attention (task-relevant) and prediction (probabilistic) cues. We identified distinct processing stages of this interactive influence. Firstly, in line with the attentional gain hypothesis, a larger amplitude response of the contralateral N1, and Nd1 for attended gratings was observed. Secondly, conforming to the attenuation-by-prediction hypothesis, a smaller negativity in the time window directly following the peak of the N1 component for predicted compared to unpredicted gratings was observed. In line with the hypothesis that attention and prediction interface, unpredicted/unattended stimuli elicited a larger negativity at central-parietal sites, presumably reflecting an increased prediction error signal. Thirdly, larger P3 responses to unpredicted stimuli pointed to the updating of an internal model. Attention and prediction can be considered as differentiated mechanisms that may interact at different processing stages to optimise perception.

Sequential Processing in the Traditional Go/NoGo Paradigm

Event Abstract Back to Event Sequential Processing in the Traditional Go/NoGo Paradigm Jack S. Fogarty1, 2*, Robert J. Barry1, 2 and Genevieve Z. Steiner1, 2, 3, 4 1 University of Wollongong, School of Psychology, Australia 2 Brain & Behaviour Research Institute, Australia 3 National Institute of Complimentary Medicine, Australia 4 Western Sydney University, School of Science and Health, Australia Aims: This study compares the series of ERP components associated with auditory equiprobable and traditional Go/NoGo tasks, to improve our conceptualisation of those components, and explore the generalisability of a sequential processing schema. Method: Continuous EEG data, recorded in-task from sixty healthy young adults (mean age = 20.4 years, S.D. = 3.1), were averaged to generate ERPs associated with each paradigm. Components were then extracted using separate temporal PCAs, to enhance the data-driven factorisation of Go and NoGo ERP components, for each task. Correlations were then conducted to examine the temporal and spatial similarity of corresponding components, before MANOVAs were applied to determine how component amplitudes differed between tasks. Results: A comparable series of Go and NoGo components was identified within each paradigm, with two additional long-latency components linked to non-target processing in the traditional Go/NoGo task. Four early components (i.e., P1, N1-3, N1-1, & PN) in each paradigm, were consistent across conditions, after which processing differed for Go and NoGo stimuli. For Go, PN was followed by P2, N2c, P3b, and two slow wave components (SW1 & SW2). In contrast, the NoGo PN was followed by N2b, P3a, a late P3 (LP3), and a non-target SW1. Component amplitudes also differed significantly between tasks, with the exception of N1-3, NoGo P1, and NoGo N1-1, reflecting shifts in cognitive demands. Conclusions: These findings illustrate consistent neurocognitive events associated with target and non-target processing in equiprobable and traditional Go/NoGo tasks, bridging the ERP literature between those paradigms. This supports theory development, by providing insight into the effects of stimulus probability on an orthogonal series of ERP components. These findings encourage the generalisation of our PCA-derived processing schema, increasing its utility and potential application in other two-choice paradigms. Keywords: Go/Nogo, PCA, ERP components, stimulus probability, sequential processing Conference: ASP2016 - The 26th Annual Meeting of the Australasian Society for Psychophysiology, Adelaide Australia, Adelaide,SA, Australia, 12 Dec - 14 Dec, 2016. Presentation Type: Oral Presentation Topic: Abstract (student award) Citation: Fogarty JS, Barry RJ and Steiner GZ (2016). Sequential Processing in the Traditional Go/NoGo Paradigm. Conference Abstract: ASP2016 - The 26th Annual Meeting of the Australasian Society for Psychophysiology, Adelaide Australia. doi: 10.3389/conf.fnhum.2016.221.00008 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 13 Nov 2016; Published Online: 05 Dec 2016. * Correspondence: Mr. Jack S Fogarty, University of Wollongong, School of Psychology, Wollongong, Australia, jf752@uowmail.edu.au Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Jack S Fogarty Robert J Barry Genevieve Z Steiner Google Jack S Fogarty Robert J Barry Genevieve Z Steiner Google Scholar Jack S Fogarty Robert J Barry Genevieve Z Steiner PubMed Jack S Fogarty Robert J Barry Genevieve Z Steiner Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Stimulus probability effects in absolute identification.

This study investigated the effect of stimulus presentation probability on accuracy and response times in an absolute identification task. Three schedules of presentation were used to investigate the interaction between presentation probability and stimulus position within the set. Data from individual participants indicated strong effects of presentation probability on both proportion correct and response times. The effects were moderated by the ubiquitous stimulus position effect. The accuracy and response time data were predicted by an exemplar-based model of perceptual cognition (Kent & Lamberts, 2005). The bow in discriminability was also attenuated when presentation probability for middle items was relatively high, an effect that will constrain future model development. The study provides evidence for item-specific learning in absolute identification. Implications for other theories of absolute identification are discussed. (PsycINFO Database Record

Stimulus probability effects on temporal bisection performance of mice (Mus musculus).

In the temporal bisection task, participants classify experienced stimulus durations as short or long based on their temporal similarity to previously learned reference durations. Temporal decision making in this task should be influenced by the experienced probabilities of the reference durations for adaptiveness. In this study, we tested the temporal bisection performance of mice (Mus musculus) under different short and long reference duration probability conditions implemented across two experimental phases. In Phase 1, the proportion of reference durations (compared to probe durations) was 0.5, whereas in Phase 2 it was increased to 0.8 to further examine the adjustment of choice behavior with more frequent reference duration presentations (under higher reinforcement rate). Our findings suggest that mice developed adaptive biases in their choice behaviors. These adjustments in choice behavior were nearly optimal as the mice maximized their gain to a great extent which required them to monitor stimulus probabilities as well as the level of variability in their temporal judgments. We further found that short but not long categorization response times were sensitive to stimulus probability manipulations, which in turn suggests an asymmetry between short and long categorizations. Finally, we investigated the latent decision processes underlying the bias manifested in subjects' choice behavior within the diffusion model framework. Our results revealed that probabilistic information influenced the starting point and the rate of evidence accumulation process. Overall, the stimulus probability effects on choice behavior were modulated by the reinforcement rate. Our findings illustrate that mice can adapt their temporal behaviors with respect to the probabilistic contingencies in the environment.

Prediction and Attention: an ERP Study on the Effects of Stimulus Probability and Task Relevance

Prediction and Attention: an ERP Study on the Effects of Stimulus Probability and Task Relevance

Dissociation between medial frontal negativity and cardiac responses in the ultimatum game: Effects of offer size and fairness.

In the present study, we examined the role of fairness and offer size on brain and cardiac responses in the ultimatum game (UG). Twenty healthy volunteers played the role of responder in a computerized version of the UG in which the fairness and size of the offers were systematically varied. Both fairness and size of the offer influenced the acceptance rates in a predictable way, leading to fewer accepted unfair and low offers. Only unfair high, but not unfair low offers were accompanied by a medial frontal negativity. An unexpected stronger cardiac deceleration to fairer offers was found, which was not affected by the size of the offers. Cardiac and electrocortical measures showed a different relation with performance, and both measures were correlated only modestly. This dissociation between cardiac responses and brain potentials is discussed in terms of a possible differential sensitivity to effects of stimulus probability and violation of the social rules.

Differential effects of frequency adaptation and sequential predictability on auditory evoked potentials

Event Abstract Back to Event Differential effects of frequency adaptation and sequential predictability on auditory evoked potentials Sabine Grimm1* and Carles Escera1 1 University of Barcelona, Spain Auditory processing can be modulated by the stimulation context, as for instance seen in a suppression of electrical brain activity in response to repeated and predictable sounds. To determine whether context effects relying on repetition-related frequency adaptation or on more complex forms of predictability share similar processing stages, we recorded human auditory evoked potentials (AEPs) in the range of the middle- and long-latency response (MLR, LLR) in a paradigm isolating the effects of stimulus probability and stimulus predictability. The same tone was presented in different contexts: either a) taking the role of a frequent tone (p = 0.8), or being presented in a block of five equi-probable tones of different frequencies, which occurred b) in a regular descending pattern, or c) in random order (serving as a control condition). Increased stimulus probability was associated with an attenuated Nb component (41-47ms) of the MLR and with a reduced early phase of N1 (80-100ms) of the LLR with the specific pattern of results indicating cross-frequency adaptation to be narrower at the level of the Nb compared to N1. Predictable, but rare, stimuli were associated with a slow wave starting before tone onset and an amplitude reduction of the Pa component (31-37ms) of the MLR and the later portion of N1 (120-150ms) of the LLR. The findings suggest that repeated input is filtered by probability-dependent frequency adaptation at different processing stages. Modulations of the AEP related to stimulus predictability suggest that the auditory system encodes regular sequential patterns and suppresses responses to predictable stimuli. Importantly, the neural processes showing probability-related adaptation can be dissociated from those sensitive to sequential predictability. Funding: Supported by the Spanish Ministry of Science and Innovation (PSI2009-08063, JCI-2009-04401) and the ERANET NEURON project PANS (EUI2009-04086). Keywords: AEP, Perception Conference: XI International Conference on Cognitive Neuroscience (ICON XI), Palma, Mallorca, Spain, 25 Sep - 29 Sep, 2011. Presentation Type: Poster Presentation Topic: Poster Sessions: Neurophysiology of Sensation and Perception Citation: Grimm S and Escera C (2011). Differential effects of frequency adaptation and sequential predictability on auditory evoked potentials. Conference Abstract: XI International Conference on Cognitive Neuroscience (ICON XI). doi: 10.3389/conf.fnhum.2011.207.00350 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 23 Nov 2011; Published Online: 28 Nov 2011. * Correspondence: Dr. Sabine Grimm, University of Barcelona, Barcelona, Spain, sabine.grimm@uni-leipzig.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Sabine Grimm Carles Escera Google Sabine Grimm Carles Escera Google Scholar Sabine Grimm Carles Escera PubMed Sabine Grimm Carles Escera Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Cerebral Responses to Change in Spatial Location of Unattended Sounds

The neural basis of spatial processing in the auditory cortex has been controversial. Human fMRI studies suggest that a part of the planum temporale (PT) is involved in auditory spatial processing, but it was recently argued that this region is active only when the task requires voluntary spatial localization. If this is the case, then this region cannot harbor an ongoing spatial representation of the acoustic environment. In contrast, we show in three fMRI experiments that a region in the human medial PT is sensitive to background auditory spatial changes, even when subjects are not engaged in a spatial localization task, and in fact attend the visual modality. During such times, this area responded to rare location shifts, and even more so when spatial variation increased, consistent with spatially selective adaptation. Thus, acoustic space is represented in the human PT even when sound processing is not required by the ongoing task.

The effect of stimulus probability on the somatosensory mismatch field

We investigated the effect of deviant stimulus probability on the somatosensory magnetic mismatch negativity (MMNm) using an electrical two-point stimulation. First, we determined the discrimination threshold (DT) of the two-point distance. We applied standard stimuli at a distance that subjects felt as one point and deviant stimuli at a distance that subjects definitely felt as two points. We used three deviant stimulus probabilities, 10, 30, and 50%. The components peaking around 30-70 ms (first component) and 150-250 ms (fourth component) following deviant stimuli were significantly larger than those following standard stimuli in 10% condition, but not in 30 or 50% condition. The equivalent current dipole (ECD) was located in the contralateral primary somatosensory cortex (cSI) for the first component, and in the cSI and in the contralateral secondary somatosensory cortex (cSII) for the fourth component. The peak amplitude of the MMNm decreased as the probability of the deviant stimulus increased. The Somatosensory MMNm was affected by deviant stimulus probability similar to an auditory mismatch negativity (MMN).

Effects of stimulus probability on pupillary dilation and reaction time in categorization

Stimuli presented with a low frequency elicit larger pupillary responses than those presented with a high frequency. Similar effects are known for motor responses observed in reaction time experiments. Utilizing this stimulus probability effect, we conducted a Go/NoGo reaction time experiment and measured pupillary dilation to evaluate categorization. We found no probability effect when participants used distinctive features to classify stimuli with different frequencies into the same category. If no distinctive features could be applied, however, the effect was observed in both pupillary and reaction time data. The stimulus probability effect can serve as a tool for the evaluation of categorization and is discussed in the context of orienting.

Cuing and stimulus probability effects on the P3 and the AB

Two experiments were conducted to investigate the relation between the attentional blink (AB), a deficit in reporting the second of two targets when it occurs 200–500 ms after the first, and the P3 component of the event-related potential. Consistent with the view that the AB reflects a limited ability to consolidate information in working memory and that the P3 reflects working memory updating, increasing the amplitude of the P3 elicited by a first target (T1) by varying T1 probability (Experiment 1) or T1 cue validity (Experiment 2) led to an increase of the AB. Overall, the P3 elicited by T1 was greater when T2 was not identified than when it was. However, the correlation between P3 and AB magnitude across participants was not significant, leaving open the question of how direct the relationship between the P3 and the AB is.

Effects of spatial separation and stimulus probability on the event-related potentials elicited by occasional changes in sound location

The ability to extract information about the spatial location of sounds plays an important role in auditory scene analysis. The present study examined the effects of spatial separation and stimulus probability on auditory event-related potentials (ERPs) to changes in sound location. In Experiment 1, we found that difference waves between ERPs elicited by standard and deviant stimuli showed a biphasic negative-positive response peaking around 126 and 226 ms after deviant onset. The amplitude of both responses increased with decreasing deviant stimulus probability, and increasing stimulus deviance. When the same stimuli were presented with equal probability for all locations (Experiment 2), there were no significant differences in the ERP amplitude and latency. These results suggest that the data reported in Experiment 1 are the result of contextual changes, rather than changes in simple acoustic features. Brain electrical source analyses are consistent with generators located in auditory cortices posterior to Heschel's gyrus. Although occasional changes in sound location elicit earlier peaks than the mismatch negativity (MMN) response reported for other types of deviation, their topographical distribution and behavior are consistent with MMN. The early latency of MMN for changes in sound location is interpreted in the context of an early-warning system to alert the organism to new sound sources in the environment.

Effects of ISI and stimulus probability on event-related go/nogo potentials after somatosensory stimulation

The present study investigated the characteristics of the middle-latency negative potential of event-related potentials (ERPs) using somatosensory go/nogo tasks. We manipulated interstimulus interval (ISI) in Experiment 1 and stimulus probability in Experiment 2 and analyzed the subtracted difference waveform resulting from subtraction of the ERP evoked by the go stimulation from that evoked by the nogo stimulation. In Experiment 1, the peak latency of negativity became significantly longer as the ISI increased, but the peak amplitude was unchanged. The reaction time (RT) was longer with increasing ISI. In Experiment 2, manipulation of the stimulus probability yielded an increase in peak amplitude with decreasing probability of the nogo stimulus, but did not affect the latency. The RT increased as the probability of a nogo stimulus rose. Because manipulation of the ISI and stimulus probability elicited different brain activities, we hypothesized that manipulation of the ISI elicited a delay of the stimulus evaluation process including response inhibition, and that stimulus probability significantly affected the strength of the response inhibition process.

The effect of numerical distance and stimulus probability on ERP components elicited by numerical incongruencies in mental addition

In two experiments, we investigated the effect of the numerical distance of incongruous results from correct results and stimulus probability on the N270/N400 event-related brain potential components. In Experiment 1, 12 subjects saw two one-digit addends and a possible solution and signaled if the proposed result (falling in the range of 3–17) was true or false. Incorrect results could deviate by ±2 or by ±9 from the correct answer. The probability of correct results was 50%. Twelve subjects carried out a similar task in Experiment 2 without giving behavioral responses. The probability of incorrect results was 20%, 50% or 80% in different conditions. Both raw potentials and incorrect minus correct difference potentials were analyzed. A fronto-central N3 and a centro-parietal dN3 (incorrect–correct difference) were present for incongruous results in both experiments. The amplitude of the dN3 was not sensitive to numerical distance, but the latency of the dN3 was longer when numerical distance was larger. The overall amplitude of the N3 and of the dN3 was not sensitive to the probability manipulation. However, there was a parietally localized effect of probability on N3 amplitude. The dN3 in mental addition is most probably identical to the arithmetic N400 effect reported earlier in mental multiplication. The distance effect in latency may be a correlate of the discrimination of correct vs. incorrect results. A parietally localized probability effect (right greater than left) was found in the N3 amplitude. The dN3 was insensitive to the probability manipulation. In accord with its insensitivity to stimulus probability, the dN3 seems to be more related to the N400 than to the N2b. Posterior attentional processes sensitive to the allocation of attentional resources may have contributed to the topography of the dN3. The N3 is more related to the detection of expectation violation, while the P3 reflects the ease of identifying stimulus categories.

Effect of stimulus probability on anti-saccade error rates

Subjects sometimes fail to suppress a reflexive saccade towards the flashed stimulus in an anti-saccade task. Here, we studied how error rates in the anti-saccade task varied as a function of saccadic probability. Ten subjects performed 200 anti-saccade trials for each of three saccade-direction probability conditions (20%, 50%, and 80%). We found that as the likelihood of a saccade in a given direction increased, the percentage of pro-saccade errors also increased for stimulus presentations in this direction. These results provide support for the hypothesis that errors in the anti-saccade task are the result of an increased level of motor preparation.

Evidence for the utilization of distinctive features in nonlinear discrimination problems

There is evidence that in human Pavlovian conditioning positive patterning (A−, B−, AB+) as well as negative patterning (A+, B+, AB−) are solved by applying abstract rules. In the present report we further investigated how humans solve patterning discriminations by conducting two Go/NoGo reaction time experiments that utilized the stimulus probability effect: stimuli presented with a low probability of occurrence elicit larger pupil dilations than those presented with a high frequency. In decreasing the ratio of compound and elemental trials from 1/1 (24 A, 24 B, 48 AB) in Experiment 1 to 1/2 (32 A, 32 B, 32 AB) in Experiment 2 we manipulated stimulus probability. The results of both experiments indicate that humans utilize distinctive features such as number information in order to classify the stimuli.