Howwhere is the stimuli processed

Thus, whereas phase entrainment might be an important and highly developed tool for the auditory system as outlined below , this might not be the case for the visual one. One way to disentangle entrainment from the other two variations would be a demonstration of the alignment of neural oscillations to, or a modulation of behavior by, the expected rhythm after stimulus offset. Indeed, some studies already provided promising results Gray et al.

However, it also needs to be shown that oscillatory signals or behavior measured after stimulus onset are not simply a reverberation introduced by a phase-reset of brain oscillations by the last stimulus: Indeed, in particular in the visual domain, periodic fluctuations of performance can already be observed in response to a single cue Landau and Fries, ; Song et al. Further studies are necessary that systematically test the impact on neural oscillations in the two systems when rhythmic stimuli evoking entrainment or non-rhythmic, but predictable stimuli evoking adjustment are presented, potentially combining electrophysiological and behavioral measurements.

It would also be interesting to see the outcome when visual and auditory stimuli are combined see next point. Although beyond the scope of this paper, auditory stimuli affect activity in the visual system, and vice versa Lakatos et al. Indeed, visual stimulation improves phase entrainment to speech sound Zion Golumbic et al. The oscillatory mechanisms involved in these cross-modal processes represent another exciting field of research—for instance, it needs to be determined whether stimuli of another modality can merely phase-reset i.

A recent suggestion emphasized the directionality between modalities, with preceding sound alerting the visual stimulation about subsequent input, and preceding visual stimulation preparing the auditory system about the exact timing of upcoming events Thorne and Debener, As described throughout this article, there is relatively clear evidence of a distinction between a faster occipital, and a slower frontal alpha.

However, both the functional roles and the origins of these two types of alpha oscillations are poorly understood. Experimental paradigms are needed in which subjects' attentional resources can be modulated in a controlled way: According to our hypothesis, occipital alpha would play a most pronounced role in regions or tasks in which external attention is weak, and frontal alpha would affect behavior most strongly in tasks in which visual attention is focused.

This non-trivial finding might indicate that occipital alpha can persist during an attentional state in certain cases: how the different factors occipital alpha, frontal alpha, and attention interact is an exciting topic for future research.

In contrast to the visual system, time is one of the most important features for the auditory system Kubovy, ; VanRullen et al. The need for the auditory system to adapt to the temporal structure of its input might thus be greater than for the visual one.

As shown in psychophysical experiments VanRullen et al. Moreover, auditory stimuli are often rhythmic, making neural oscillations a valuable and convenient tool for synchronization with the environment Schroeder and Lakatos, This notion might explain the variety of findings described in the previous section: In contrast to the visual system, the frequency of operation might strongly depend on the input to the system in the auditory case.

In a multi-speaker scenario or when speech is mixed with noise, the alignment between these oscillations and the envelope of speech is increased for attended speech, suggesting a mechanism of auditory stream selection Ding and Simon, ; Zion Golumbic et al.

Entrainment to speech persists even when slow spectral energy fluctuations have been removed, and this phenomenon can be observed in both humans and non-human primates Zoefel and VanRullen, a , b , c ; Zoefel et al.

Thus, as suggested before e. Nevertheless, evidence remains sparse and most paradigms have focused on multimodal or audio spatial attention reviewed in Foxe and Snyder, A single study Ten Oever et al.

Thus, further experimental evidence is needed to decide whether the auditory system adjusts its oscillations to expected input even if the latter is non-rhythmic—and, if yes, at what frequency this adjustment takes place.

Thus, the visual system might maintain its rhythm of stimulus processing even when it cannot be adjusted, such as during an unpredictable sequence of events. Interestingly, these two modes resemble two cortical states of primary auditory cortex that have recently been described Pachitariu et al. Recently, important experimental evidence for an internally oriented auditory mode of processing was reported by Lakatos et al. Bursts of gamma activity and multi-unit activity an index of neuronal firing were coupled to the dominant oscillation: To the entrained phase when phase entrainment was strong, and to the alpha phase when alpha power was high, but entrainment was weak.

Indeed, in contrast to the visual system, where target detection depends on the alpha phase Busch et al. Moreover, reduced alpha power in the auditory system has been linked with the perception of illusionary phenomena, such as the Zwicker tone, an illusionary tone that is perceived for several seconds after the offset of broadband noise with a spectral gap Leske et al. Finally, using intracranial recordings in human auditory cortex and an experimental protocol during which expectations had to be updated continuously, Sedley et al.

It is unclear how the dominant frequency of stimulus processing changes if no regular structure is present in the input but attention is focused on the auditory environment. Indeed, a recent study demonstrated a relationship between MEG alpha power and the detection of non-rhythmic i. Furthermore, EEG alpha power seems to be altered when presented speech is made less rhythmic i.

An alternative that needs to be tested is that the auditory system changes to a continuous processing mode in which sampling mechanisms of neural oscillations are suppressed.

This notion was described in detail in the opinion paper by Schroeder and Lakatos and based on studies reporting a suppression of low-frequency power and enhanced gamma-activity in experimental paradigms where continuous vigilance is required e. Nevertheless, these studies reported data from a specific part of the visual hierarchy V4 and it remained unclear how the auditory system operates when no rhythmic input is present.

An alternative would be a suppression of most oscillatory sampling mechanisms when auditory attention is focused on a non-rhythmic stimulation. Both speculations must be underlined with experimental evidence.

For instance, similar analyses as in Lakatos et al. Intracranial recordings might be appropriate in this case, as activity in auditory cortices is, due to their nestled structure in the lateral sulcus, difficult to measure using superficial methods, such as EEG. An increase in alpha or entrained activity for irregular vs. Another interesting approach would be the replication of previous experiments on the dependence of auditory stimulus detection in quiet on the phase of neural oscillations that so far resulted in negative results Zoefel and Heil, ; VanRullen et al.

The latter experimental manipulation would result in an absence of attention for the auditory stimulation. According to the hypothesis presented here, this lack of attention might provoke an increase of alpha activity in the auditory system. In the latter case, we would see an independence of auditory target detection from oscillatory activity as described previously Zoefel and Heil, It has been speculated that this switch might reflect a change from external to internal attention Lakatos et al.

It needs to be determined why this is the case, and what might be a trigger for this switch. Experimental paradigms requiring sustained attention i. In this case, it needs to be determined how the two co-existing modes can communicate, for instance when a salient stimulus reaches a certain threshold and triggers a switch back to a mode of external attention or high vigilance. Thus, in an unattended visual scene which leads to an increased alpha amplitude, as outlined above , occipital alpha might at the same time enable functional deactivation, but, given that an unattended stimulus is salient enough, also enable the system to switch attention to a potentially important event.

Similarly, VanRullen et al. Therefore, only stimuli that are located in the focus of visual attention seem to be sampled at a frequency of 7—8 Hz, and this sampling frequency is independent of stimulus input see Box 1 for further discussion. As developed above, this is in clear contrast to the auditory system where, in the presence of attention, the adaption i.

Some properties might be common across all systems: Neural oscillations can be used as a tool for attentional selection, and both oscillatory power and phase can be used to gate stimulus input.

Changes in power might reflect a tonic suppression of processing e. Indeed, a recent study by Haegens et al. Summary of mechanisms of stimulus selection and processing in the visual and auditory systems, including the hypotheses made in this article. If stimulation is rhythmic and attended: Frequency of stimulation, but bias for occipital alpha. However, there are differences between the visual and auditory systems: The oscillatory entrainment to rhythmic stimulation seems to be a fundamental feature of the auditory system, probably evolved due the rhythmic nature of the auditory environment.

Indeed, the tendency to synchronize with auditory rhythms is ubiquitous: We sing, we dance, we clap in response to music or even to a simple beat Nozaradan, Importantly, this phenomenon is much less pronounced for the visual system: For instance, the urge to dance is significantly lowered when watching someone dancing without the corresponding sound.

Thus, although in principle the visual system also seems to be able to entrain, the adjustment of power and phase might be more important in this system—visual stimuli are often predictable, but rarely rhythmic. Interestingly, and in line with this notion, it has been shown that the auditory system is superior to the visual one when movement has to be synchronized with a rhythmic sequence in either or both modalities Repp and Penel, ; Patel et al.

Task-irrelevant information in the auditory system impairs visual processing more strongly than vice versa if this information is of temporal nature Guttman et al. Thus, although visual stimuli can in principle influence auditory processing and perception potentially using alpha oscillations; Thorne et al. Finally, a simple cue without rhythmic component involved is sufficient to introduce the mentioned periodic fluctuations in visual performance Landau and Fries, ; Song et al.

The situation seems to be different for the auditory system, where similar periodic fluctuations in performance have been reported only after the offset of a rhythmic stimulus Hickok et al.

Speculatively, if stimulation is non-rhythmic, the auditory system might operate in the alpha rhythm as well see Box 2. However, whereas the visual alpha rhythm s might subsample sensory regions even when stimulus timing is unpredictable, it is possible that the auditory alpha is decoupled from sensory processes; in this way, the auditory system can avoid a loss of information that occurs when subsampling is applied to rapidly fluctuating auditory information with unknown timing.

Neural oscillations are a powerful tool of the brain to prioritize and select relevant information while ignoring distracting input. This article summarizes the current state-of-the-art and provides several proposals that can be systematically tested and extended. Future studies and theories are indispensable to advance this exciting field of research. All authors listed, have made substantial, direct and intellectual contribution to the work, and approved it for publication. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Parts of this article have been published in a modified form as part of the PhD thesis of the first author Zoefel, ; see References. Although some studies demonstrated an important role of frontal regions, such as the Frontal Eye Field FEF , for alpha oscillations Marshall et al. Further studies are necessary to answer this question. However, we emphasize that saccades are initiated by the brain: The timing of incoming information is thus known in advance—e.

Therefore, we argue that, in the visual system, it might not be necessary to adapt stimulus processing to the input per se , but rather to the rather irregular scanning of the environment introduced by eye movements: Indeed, there is evidence that the oscillatory phase and eye movements are linked Hogendoorn, ; McLelland et al.

National Center for Biotechnology Information , U. Journal List Front Neurosci v. Front Neurosci. Published online May Author information Article notes Copyright and License information Disclaimer. This article was submitted to Perception Science, a section of the journal Frontiers in Neuroscience. Received Mar 6; Accepted May The use, distribution or reproduction in other forums is permitted, provided the original author s or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice.

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Abstract All sensory systems need to continuously prioritize and select incoming stimuli in order to avoid overflow or interference, and provide a structure to the brain's input. Keywords: oscillation, attention, perception, alpha, entrainment.

Introduction Imagine looking for someone in a crowd, trying to keep the person's characteristics in mind while suppressing other, potentially distracting events: Constantly bombarded with a continuous stream of sensory information, our brain needs to select, filter and prioritize: the use of top-down processes for this task is indispensable.

Open in a separate window. Figure 1. Figure 2. Relation to the system's input Adjustment vs. Box 1 Speculations, open questions and how to test them. Box 2 CTD: Speculations, open questions and how to test them.

Table 1 Summary of mechanisms of stimulus selection and processing in the visual and auditory systems, including the hypotheses made in this article. Author contributions All authors listed, have made substantial, direct and intellectual contribution to the work, and approved it for publication. Conflict of interest statement The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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The first step in sensation is reception. The receptor can then respond to the stimuli. Think for a moment about the differences in receptive fields for the different senses. For the sense of touch, a stimulus must come into contact with body. For the sense of hearing, a stimulus can be a moderate distance away some baleen whale sounds can propagate for many kilometers.

For vision, a stimulus can be very far away; for example, the visual system perceives light from stars at enormous distances. The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. This takes place at the sensory receptor, and the change in electrical potential that is produced is called the receptor potential. How is sensory input, such as pressure on the skin, changed to a receptor potential?

In this example, a type of receptor called a mechanoreceptor as shown in. Figure Disturbance of these dendrites by compressing them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing its electrical potential. Receptor potentials are graded potentials: the magnitude of these graded receptor potentials varies with the strength of the stimulus.

If the magnitude of depolarization is sufficient that is, if membrane potential reaches a threshold , the neuron will fire an action potential. In most cases, the correct stimulus impinging on a sensory receptor will drive membrane potential in a positive direction, although for some receptors, such as those in the visual system, this is not always the case.

Sensory receptors for different senses are very different from each other, and they are specialized according to the type of stimulus they sense: they have receptor specificity. For example, touch receptors, light receptors, and sound receptors are each activated by different stimuli. Touch receptors are not sensitive to light or sound; they are sensitive only to touch or pressure.

However, stimuli may be combined at higher levels in the brain, as happens with olfaction, contributing to our sense of taste. Four aspects of sensory information are encoded by sensory systems: the type of stimulus, the location of the stimulus in the receptive field, the duration of the stimulus, and the relative intensity of the stimulus.

For example, auditory receptors transmit signals over their own dedicated system, and electrical activity in the axons of the auditory receptors will be interpreted by the brain as an auditory stimulus—a sound. The intensity of a stimulus is often encoded in the rate of action potentials produced by the sensory receptor. Thus, an intense stimulus will produce a more rapid train of action potentials, and reducing the stimulus will likewise slow the rate of production of action potentials.

A second way in which intensity is encoded is by the number of receptors activated. An intense stimulus might initiate action potentials in a large number of adjacent receptors, while a less intense stimulus might stimulate fewer receptors.

Integration of sensory information begins as soon as the information is received in the CNS, and the brain will further process incoming signals. Although perception relies on the activation of sensory receptors, perception happens not at the level of the sensory receptor, but at higher levels in the nervous system, in the brain.

The brain distinguishes sensory stimuli through a sensory pathway: action potentials from sensory receptors travel along neurons that are dedicated to a particular stimulus. These neurons are dedicated to that particular stimulus and synapse with particular neurons in the brain or spinal cord. All sensory signals, except those from the olfactory system, are transmitted though the central nervous system and are routed to the thalamus and to the appropriate region of the cortex.