Shifting attention in viewer- and object-based reference frames after unilateral brain injury

The aims of the present study were to investigate the respective roles that object- and viewer-based reference frames play in reorienting visual attention, and to assess their influence after unilateral brain injury. To do so, we studied 16 right hemisphere injured (RHI) and 13 left hemisphere injured (LHI) patients. We used a cueing design that manipulates the location of cues and targets relative to a display comprised of two rectangles (i.e., objects). Unlike previous studies with patients, we presented all cues at midline rather than in the left or right visual fields. Thus, in the critical conditions in which targets were presented laterally, reorienting of attention was always from a midline cue. Performance was measured for lateralized target detection as a function of viewer-based (contra- and ipsilesional sides) and object-based (requiring reorienting within or between objects) reference frames. As expected, contralesional detection was slower than ipsilesional detection for the patients. More importantly, objects influenced target detection differently in the contralesional and ipsilesional fields. Contralesionally, reorienting to a target within the cued object took longer than reorienting to a target in the same location but in the uncued object. This finding is consistent with object-based neglect. Ipsilesionally, the means were in the opposite direction. Furthermore, no significant difference was found in object-based influences between the patient groups (RHI vs. LHI). These findings are discussed in the context of reference frames used in reorienting attention for target detection

Disentangling neural processing of masked and masking stimulus by means of event-related contralateral – ipsilateral differences of EEG potentials

In spite of the excellent temporal resolution of event-related EEG potentials (ERPs), the overlapping potentials evoked by masked and masking stimuli are hard to disentangle. However, when both masked and masking stimuli consist of pairs of relevant and irrelevant stimuli, one left and one right from fixation, with the side of the relevant element varying between pairs, effects of masked and masking stimuli can be distinguished by means of the contralateral preponderance of the potentials evoked by the relevant elements, because the relevant elements may independently change sides in masked and masking stimuli. Based on a reanalysis of data from which only selected contralateral-ipsilateral effects had been previously published, the present contribution will provide a more complete picture of the ERP effects in a masked-priming task. Indeed, effects evoked by masked primes and masking targets heavily overlapped in conventional ERPs and could be disentangled to a certain degree by contralateral-ipsilateral differences. Their major component, the N2pc, is interpreted as indicating preferential processing of stimuli matching the target template, which process can neither be identified with conscious perception nor with shifts of spatial attention. The measurements showed that the triggering of response preparation by the masked stimuli did not depend on their discriminability, and their priming effects on the processing of the following target stimuli were qualitatively different for stimulus identification and for response preparation. These results provide another piece of evidence for the independence of motor-related and perception-related effects of masked stimuli

Fast Detection of Unexpected Sound Intensity Decrements as Revealed by Human Evoked Potentials

The detection of deviant sounds is a crucial function of the auditory system and is reflected by the automatically elicited mismatch negativity (MMN), an auditory evoked potential at 100 to 250 ms from stimulus onset. It has recently been shown that rarely occurring frequency and location deviants in an oddball paradigm trigger a more negative response than standard sounds at very early latencies in the middle latency response of the human auditory evoked potential. This fast and early ability of the auditory system is corroborated by the finding of neurons in the animal auditory cortex and subcortical structures, which restore their adapted responsiveness to standard sounds, when a rare change in a sound feature occurs. In this study, we investigated whether the detection of intensity deviants is also reflected at shorter latencies than those of the MMN. Auditory evoked potentials in response to click sounds were analyzed regarding the auditory brain stem response, the middle latency response (MLR) and the MMN. Rare stimuli with a lower intensity level than standard stimuli elicited (in addition to an MMN) a more negative potential in the MLR at the transition from the Na to the Pa component at circa 24 ms from stimulus onset. This finding, together with the studies about frequency and location changes, suggests that the early automatic detection of deviant sounds in an oddball paradigm is a general property of the auditory system

Processing of Abstract Rule Violations in Audition

The ability to encode rules and to detect rule-violating events outside the focus of attention is vital for adaptive behavior. Our brain recordings reveal that violations of abstract auditory rules are processed even when the sounds are unattended. When subjects performed a task related to the sounds but not to the rule, rule violations impaired task performance and activated a network involving supratemporal, parietal and frontal areas although none of the subjects acquired explicit knowledge of the rule or became aware of rule violations. When subjects tried to behaviorally detect rule violations, the brain's automatic violation detection facilitated intentional detection. This shows the brain's capacity for abstraction – an important cognitive function necessary to model the world. Our study provides the first evidence for the task-independence (i.e. automaticity) of this ability to encode abstract rules and for its immediate consequences for subsequent mental processes

Auditory event-related potentials

Auditory event related potentials are electric potentials (AERP, AEP) and magnetic fields (AEF) generated by the synchronous activity of large neural populations in the brain, which are time-locked to some actual or expected sound event

Behavioral and neural dissociations of rhythmic temporal expectations from memory-based expectations

Background Sensory input with predictable dynamics can be used to create temporal expectations. It has been suggested that when the input is rhythmic, temporal expectations are created to in-phase time points due to oscillatory synchronization with the rhythm period. However, they can also be created by intentionally memorizing a fixed interval and applying it recursively. Using performance measures and EEG in 20 subjects, we dissociated these two processes. In one condition, targets appeared in-phase with a rhythmic sequence of stimuli in 80% of the trials. In another condition, they appeared in 80% of the trials at an interval that was recursively presented to allow memorization, but without creating a rhythmic sequence. In both conditions, a cue indicated explicitly that the next stimulus is the target. Results The behavioral validity effect of rhythmic stimulation was stronger than that of interval memorization, suggesting that the former is more temporally accurate. The CNV, an ERP which reflects temporal anticipation to the target, was more negative in the rhythmic condition, suggesting increased expectation. Furthermore, the effect of cue invalidity on the terminal CNV and on the latency of the P3, an ERP reflecting target evaluation, was more evident in the rhythmic condition, suggesting faster evaluation. Finally, in expected target times there was increased alpha desynchronization, previously shown to occur when directing attention. Conclusion Although the amount of temporal information was matched, the expectation created by rhythmic stimulation was superior over interval-based temporal expectation, both in behavior and in expectation-related neural activity. These findings are inconsistent with the idea that rhythmic expectation is no more than recursive applying a memorized interval, and instead suggest involvement of either an additional mechanism or a different one

Overcoming distracting temporal regularities: preparatory and stimulus-evoked mechanisms of attentional shifting in time away from rhythmic input

Background: To survive in a dynamically changing world, our brain constantly predicts the timing of future events. Rhythmic temporal structure is a potent cue for temporal predictions, such that processing of events is facilitated when they coincide with rhythmic input (i.e. appear on-beat), even for task-irrelevant rhythms. Mechanistically, the CNV, a slow premotor preparatory brain potential, is incidentally driven by rhythms such that it peaks at on-beat times. We tested the potency of rhythms as temporal cues by presenting targets with high probability between visual rhythmic stimuli (i.e. off-beat). This made off-beat moments task-relevant and encouraged observers to intentionally shift attention to them, while ignoring on-beat moments. In another condition, on-beat moments were task-relevant and off-beat moments were irrelevant. Results: When the time of the off-beat targets was jittered, the CNV peaked at on-beat times, which also showed behavioral benefit. However, when the time of the off-beat targets was fixed relative to rhythmic stimuli, responses were faster to off-beat targets compared to on-beat. Crucially, this was accompanied by modulation of the CNV trajectory such that it peaked at the task-relevant times and not at on-beat times. Finally, post-target brain activity demonstrated a sustained positivity for on-beat relative to off-beat targets, starting from 100 ms after target onset, while task-relevance only affected the later N2 ERP. Conclusions: Our findings imply that what seems to be an automatic effect of rhythms can be overcome given sufficient temporal information. This stands in contrast to prevalent entrainment models of temporal predictions, which explain rhythmic temporal predictions by synchronization of slow brain activity to the rhythmicity. Further, the dissociation we find in post-target responses extends the understanding of the unintentional and intentional components of temporal expectation formation in rhythmic environments

Native and learned sources for expectations: exogenous and endogenous shifting of temporal attention

Predicting the timing of upcoming events is crucial for both perception and action, as it has been shown that temporal preparation facilitates response time, accuracy and perception thresholds to a stimulus. The prevalent explanation for this effect is that it reflects increased allocation of attentional resources at the expected moment in time. There is evidence that this shifting of attention can be based on two levels of information. One level, 'low-level' expectation, is based on some regular temporal pattern preceding the target (e.g. stimuli at a fixed rhythm). Another, 'high-level' expectation, is based on abstract knowledge regarding target timing (e.g. association with a previous stimulus). In the current work, we examined the interactions of high-level and low-level temporal expectation on performance in a visual detection task. Subjects responded to a target that appeared in either short or long SOA after a sequence of flickering stimuli. In alternate blocks, flicker frequency or sequence color were predictive of the ISI between the sequence and the target. Subjects were instructed to attend the predictive cue. In some trials (incongruent trials) the target appeared in the SOA which was unpredicted by the attended cue. Orthogonally, the unattended cue could be congruent with the target SOA. We found an effect of congruity of the attended cue, which indicates facilitation by temporal expectation. This effect was modulated by the congruity of the unattended cue, with better performance in incongruent trials when the target SOA was congruent rather than incongruent with the unattended cue. Further, our results demonstrate the superiority of low-level expectations in these effects. Stronger automatic expectation by the low-level cue may result from it being more native to the observer, and not dependant on task-specific associations

Integration without awareness: expanding the limits of unconscious processing

Human conscious awareness is commonly seen as the climax of evolution. However, what function-if any-it serves in human behavior is still debated. One of the leading suggestions is that the cardinal function of conscious awareness is to integrate numerous inputs-including the multitude of features and objects in a complex scene-across different levels of analysis into a unified, coherent, and meaningful perceptual experience. Here we demonstrate, however, that integration of objects with their background scenes can be achieved without awareness of either. We used a binocular rivalry technique known as continuous flash suppression to induce perceptual suppression in a group of human observers. Complex scenes that included incongruent objects escaped perceptual suppression faster than normal scenes did. We conclude that visual awareness is not needed for object-background integration or for processing the likelihood of an object to appear within a given semantic context, but may be needed for dealing with novel situations