Random Stimulus Sequences Research Articles

Tuning perception and decisions to temporal context

Recent work suggests that serial dependence, where perceptual decisions are biased toward previous stimuli, arises from the prior that sensory input is temporally correlated. However, existing studies have mostly used random stimulus sequences that do not involve such temporal consistencies. Here, we manipulated the temporal statistics of visual stimuli to examine the role of true temporal correlations in serial dependence. In two experiments, observers reproduced the orientation of the last stimulus in a sequence, while we varied temporal correlations in the stimulus features at two timescales: stimulus history within the trial and decision history across trials. We found a clear dissociation: increasing temporal correlation in the stimulus history led to adaptation-like repulsive biases, whereas increasing temporal correlation in the decision history reduced attractive biases. Thus, we suggest that temporal correlation enhances the discriminative ability of the visual system, revealing the fundamental role of the broader temporal context.

WildLab: A naturalistic free viewing experiment reveals previously unknown electroencephalography signatures of face processing.

Neural mechanisms of face perception are predominantly studied in well-controlled experimental settings that involve random stimulus sequences and fixed eye positions. Although powerful, the employed paradigms are far from what constitutes natural vision. Here, we demonstrate the feasibility of ecologically more valid experimental paradigms using natural viewing behaviour, by combining a free viewing paradigm on natural scenes, free of photographer bias, with advanced data processing techniques that correct for overlap effects and co-varying non-linear dependencies of multiple eye movement parameters. We validate this approach by replicating classic N170 effects in neural responses, triggered by fixation onsets (fixation event-related potentials [fERPs]). Importantly, besides finding a strong correlation between both experiments, our more natural stimulus paradigm yielded smaller variability between subjects than the classic setup. Moving beyond classic temporal and spatial effect locations, our experiment furthermore revealed previously unknown signatures of face processing: This includes category-specific modulation of the event-related potential (ERP)'s amplitude even before fixation onset, as well as adaptation effects across subsequent fixations depending on their history.

Spatial Separation between Two Sounds of an Oddball Paradigm Affects Responses of Neurons in the Rat’s Inferior Colliculus to the Sounds

The ability to sense occasionally occurring sounds in an environment is critical for animals. To understand this ability, we studied responses to acoustic oddball paradigms in the rat’s midbrain auditory neurons. An oddball paradigm is a random sequence of stimuli created using two tone bursts, with one presented at a high probability (standard stimulus) and the other at a low probability (oddball stimulus). The sounds were either colocalized at the ear contralateral to a neuron under investigation (c90° azimuth) or separated with one at c90° while the other at another azimuth. We found that most neurons generated stronger responses to a sound at c90° when it was presented as an oddball than as a standard stimulus. Relocating one sound from c90° to another azimuth changed both responses to the relocated sound and the sound that remained at c90°. Most notably, the response to an oddball stimulus at c90° was increased when a standard stimulus was relocated from c90° to a location that was in front of the animal or on the ipsilateral side of recording. The increase was particularly large in neurons that displayed transient firing under contralateral stimulation but no firing under ipsilateral stimulation. These neurons likely play a particularly important role in using spatial cues to detect occasionally occurring sounds. Results suggest that effects of spatial separation between two sounds of an oddball paradigm on responses to the sounds were dependent on changes in the level of adaptation and binaural inhibition.

Sequence Knowledge on When and What Supports Dual-Tasking.

The constraints in overlapping response selection have been established in dual-tasking studies with random sequence of stimuli and responses as well as random stimulus onset asynchrony (SOA). While this approach makes it possible to control for advance activation of upcoming stimuli or responses, it leaves open whether such preparatory processing can indeed influence dual-task performance. We investigated whether and how the sequence of stimuli and responses and the sequence of SOAs can be learned and used under dual-tasking. In each trial, participants (N = 28 in Experiment 1 and N = 30 in Experiment 2) were first presented with a random two-choice task followed by a four-choice Serial Reaction Time Task (SRTT), presented in a sequence of length four (position sequence). The SOA (timing) sequence also had length four. In test phases, one or both of the sequences were randomized. Results showed that both position and timing sequences were learned and supported dual-task performance, suggesting that predictive processing with respect to timing and identity of stimuli and responses can help to circumvent the response selection bottleneck constraints. Furthermore, in contrast to previous work on acquisition of interval sequences in single tasking, we found that the sequence of what (i.e. stimulus) and the sequence of when (i.e. interval between two tasks) contributed independently to performance.

Differential effects of cue-based and sequence knowledge-based predictability on multitasking performance

Everyday multitasking often is characterized by predictable sequences. While such sequential regularities are present in setups using the Serial Reaction Time Task (SRTT), many laboratory studies on dual-tasking performance use random sequences of stimuli in either of the two tasks. In the current study, following single-task training on the SRTT, participants completed trials where they were confronted with an additional visual-manual task with either a random (Experiment 1) or a partially predictable (Experiment 2) stimulus sequence. In the SRTT, we cued participants with respect to which of the four stimulus options were yet to occur (before a new round with all four options would start). We randomly mixed a sequence to be practiced with random sequences of the same length and with the same constraint. Thus, we were able to vary predictability of upcoming stimuli (from chance to 100%) as well as sequence knowledge (practiced vs. random sequence) in order to assess how cueing and sequence knowledge, as two potential bases of prediction, would affect performance in single- and dual-tasking. Results suggest that both cueing and sequence knowledge-based prediction can lead to shorter RTs in dual-tasking. In previous studies, the disruption of sequence learning by adding a task with a random stimulus sequence has been linked to the effects of automatic prediction between events in the two tasks. In line with these studies, dual-task performance did not impede usage of sequence knowledge when a task with a predictable (rather than random) sequence of stimuli was added to the SRTT.

Long-latency components of somatosensory evoked potentials during passive tactile perception of gratings.

Perception of tactile stimuli elicits Somatosensory Evoked Potentials (SEPs) that can be recorded via non-invasive electroencephalography (EEG). However, it is not yet clear how SEPs localization, shape and latency are modulated by different stimuli during mechanical tactile stimulation of fingertips. The aim of this work is thus to characterize SEPs generated by the tactile perception of gratings during dynamic passive stimulation of the dominant fingertip by means of a mechatronic platform. Results show that a random sequence of stimuli elicited SEPs with two long-latency components: (i) a negative deflection around 140 ms located in the frontal-central-parietal side in the contralateral hemisphere; (ii) a positive deflection around 250 ms located in the frontal-central midline. Time-frequency analysis revealed significant continuous bilateral desynchronization in the alpha band throughout the passive stimulation. These results are a fundamental step towards building a model of brain responses during perception of tactile stimuli for future benchmarking studies.

Response monitoring and expectancy in random serial RT tasks

Two different sequential patterns have been reported in serial reaction-time (RT) tasks with random stimulus sequences, depending on the response-to-stimulus interval (RSI). When RSI is short a first-order repetition effect and a higher order benefit-only pattern are found, and when RSI is long a cost-benefit pattern is observed. In a series of three experiments, we found that the patterns are not unique for spatial dimensions, and that the benefit-only pattern does not appear for irrelevant stimulus features. The cost-benefit pattern, on the other hand, can shift to irrelevant locations, but not to irrelevant colour. Distributional analyses of the sequential effects demonstrate that the first-order repetition effect can be dissociated from the higher order effects. The results are interpreted as support for the role of response monitoring in the development of the benefit-only pattern at short RSI, while the first-order repetition effect is explained as repetition priming. With a long RSI, the cost-benefit pattern is confirmed as a low-level subjective expectancy process, which is primarily controlled by stimulus location. The data clearly strengthen the support for three different processes generating sequential effects in random serial RT tasks.

The redundant target effect is affected by modality switch costs.

When participants have to respond to stimuli of two modalities, faster reaction times are observed for simultaneous, bimodal events than for unimodal events (the redundant target effect [RTE]). This finding has been interpreted as reflecting processing gains for bimodal relative to unimodal stimuli, possibly due to multisensory interactions. In random stimulus sequences, reaction times are slower when the stimulus is preceded by a stimulus of a different modality (modality switch effect [MSE]). Simple reaction time redundant target experiments with auditory-visual, visual-tactile, and auditory-tactile stimulus combinations were run to determine whether the RTE may be partly explained by MSEs because bimodal stimuli do not require a modality switch. In all three modality pairings, significant MSEs and RTEs were observed. However, the RTE was still significant after reaction times were corrected for the MSE, supporting the hypothesis that coactivation occurs independently of modality switch costs.

Learned material content and acquisition level modulate cerebral reactivation during posttraining rapid-eye-movements sleep

We have previously shown that several brain areas are activated both during sequence learning at wake and during subsequent rapid-eye-movements (REM) sleep (Nat. Neurosci. 3 (2000) 831–836), suggesting that REM sleep participates in the reprocessing of recent memory traces in humans. However, the nature of the reprocessed information remains open. Here, we show that regional cerebral reactivation during posttraining REM sleep is not merely related to the acquisition of basic visuomotor skills during prior practice of the serial reaction time task, but rather to the implicit acquisition of the probabilistic rules that defined stimulus sequences. Moreover, functional connections between the reactivated cuneus and the striatum—the latter being critical for implicit sequence learning—are reinforced during REM sleep after practice on a probabilistic rather than on a random sequence of stimuli. Our results therefore support the hypothesis that REM sleep is deeply involved in the reprocessing and optimization of the high-order information contained in the material to be learned. In addition, we show that the level of acquisition of probabilistic rules attained prior to sleep is correlated to the increase in regional cerebral blood flow during subsequent REM sleep. This suggests that posttraining cerebral reactivation is modulated by the strength of the memory traces developed during the learning episode. Our data provide the first experimental evidence for a link between behavioral performance and cerebral reactivation during REM sleep.

Preattentive processing of complex sounds in the human brain

Processing of complex sounds in the human brain was studied with event-related potentials (ERPs) recorded from the scalp. In random stimulus sequences, one serial tone pattern, consisting of nine consecutive tones of different frequencies, was repeatedly presented to the left ear and another pattern to the right ear. Subjects attended either to the left-ear or right-ear patterns in order to detect in the attended ear occasional deviant patterns differing from the repeating standard pattern in the 3rd or 7th tone. Even undetected deviant patterns of the attended ear and deviant patterns of the unattended ear elicited the mismatch negativity component of the ERP. These results indicate preattentive processing of complex sounds in the human brain.

Effects of Ethanol on Control of Attention

In order to investigate the effect of ethanol on visual attention, 18 subjects aged 20–50 years took part in a three-period crossover study in which they received placebo (PL) and two doses of ethanol in random order. The higher dose (E2: 0·88 g/kg, maximum 66 g for males, 55 g for females) was calculated to produce blood ethanol concentrations of 60–80 mg/100 ml. The lower dose (E1) was 75 per cent of E2. Subjects showed highly significant subjective drunkenness at both doses (p < 0·01) and on the higher dose were slowed by 6–11 per cent on most speeded measures, in agreement with previous results. In a Four-Choice Reaction-Time Task, subjects responded for part of the time to a fixed, repetitive sequence, and at other times to a random sequence of stimuli. At the transition from repetitive to random sequences, subjects on ethanol showed a disproportionate slowing (60 per cent on the higher dose). This slowing may be of particular relevance to driving, as the time taken to engage control processing after a period of relatively automatic activity may be important in dealing with unexpected events on the road. © 1997 John Wiley & Sons, Ltd.

Effect of subject's family name on visual event-related potential in schizophrenia

Visual event-related potentials (ERPs) were recorded from 15 schizophrenics and 15 age-matched and gender-matched controls, while they performed a modified version of the oddball paradigm. Each subject was required to detect target stimuli among a random sequence of stimuli under two conditions, name and color. In the name condition the stimulus sequence consisted of the subject's family name (deviant 18%), four other family names (standard 73%), and a city name (target 9%). In the color condition the respective stimuli were a pair of solid red circles, four white paired-arrows, and a pair of white plus and minus signs. ERPs elicited by stimuli contained triphasic potentials of P2, N2, and P3. In controls these waves were selectively enhanced for the subject's family name as compared with standards, whereas in schizophrenics no significant difference between the subject's family name and other names was observed. In contrast, selective enhancement for color deviants was observed in both subject groups. These results suggest that impairment of involuntary attention, especially for familiar and significant stimuli such as names in daily life, may underlie disturbances of attentionally controlled central processing in schizophrenia.

The pupil and stimulus probability.

ABSTRACTThree studies were conducted to examine the relationship of the task‐evoked pupillary response (TEPR) to the probability of the eliciting stimuli in Bernoulli sequences. Subjects counted target tones in sequences containing two types of tones differing in pitch. For a given probability level, both targets and non‐targets elicited pupillary dilations that were similar in amplitude. The dilation to both targets and non‐targets was inversely related to probability at low probability levels. This relationship appeared to break down at high probability levels, at least for non‐targets. When tone omissions were used as targets they elicited a larger dilation than tonal targets at very low target probability. The amplitude of dilations produced by omitted targets was also affected by probability. The data converge to indicate that the TEPR in random stimulus sequences is sensitive to the surprise value of events, whether involving a physical stimulus or not, and not to the target designation of these events.