Pre-stimulus alpha activity modulates long-lasting unconscious feature integration

Pre-stimulus activities affect subsequent visual processing: Empirical evidence and potential neural mechanisms

PURPOSE

Humans obtain most of their information from visual stimuli. The perception of these stimuli may be modulated by the ongoing pre-stimulus brain activities. Depending on the task design, the processing of different cognitive functions such as spatial attention, feature-based attention, temporal attention, arousal, and mental imagery may start prior to the stimulus onset.

METHOD

This process is typically accompanied by changes in pre-stimulus oscillatory activities including power, phase, or connectivity in different frequency bands. To explain the effect of these changes, several mechanisms have been proposed. In this article, we review these changes and the potential mechanisms in the context of the pre-stimulus enabled cognitive functions. We provide evidence both in favor of and against the most documented mechanisms and conclude that no single mechanism can solely delineate the effects of pre-stimulus brain activities on later processing. Instead, multiple mechanisms may work in tandem to guide pre-stimulus brain activities.

FINDING

Additionally, our findings indicate that in many studies a combination of these cognitive functions begins prior to stimulus onset.

CONCLUSION

Thus, dissociating these cognitive functions is challenging based on the current literature, and the need for precise task designs in later studies to differentiate between them is crucial.

Continuous temporal integration in the human visual system

The human visual system is continuously processing visual information to maintain a coherent perception of the environment. Temporal integration, a critical aspect of this process, allows for the combination of visual inputs over time, enhancing the signal-to-noise ratio and supporting high-level cognitive functions. Traditional methods for measuring temporal integration often require a large number of trials made up of a fixation period, stimuli separated by a blank interval, a single forced choice, and then a pause before the next trial. This trial structure potentially introduces fatigue and biases. Here, we introduce a novel continuous temporal integration (CTI) task designed to overcome these limitations by allowing free visual exploration and continuous mouse responses to dynamic stimuli. Fifty participants performed the CTI, which involved adjusting a red bar to indicate the point where a flickering sine wave grating became indistinguishable from noise. Our results, modeled by an exponential function, indicate a reliable temporal integration window of ∼100 ms. The CTI's design facilitates rapid and reliable measurement of temporal integration, demonstrating potential for broader applications across different populations and experimental settings. This task provides a more naturalistic and efficient approach to understanding this fundamental aspect of visual perception.

Spectral tuning and after-effects in neural entrainment

Neural entrainment has become a popular technique to non-invasively manipulate brain rhythms via external, periodic stimulation. However, there is still debate regarding its underlying mechanisms and effects on brain activity. Here, we used EEG recordings during a visual entrainment paradigm to assess characteristic changes in the spectral content of EEG signals due to entrainment. Our results demonstrate that entrainment not only increases synchrony between neural oscillations and the entraining stimulus but also elicits previously unreported spectral tuning effects and long-lasting after-effects. These findings offer compelling evidence for the presence of dedicated, flexible, and adaptive mechanisms for neural entrainment, which may have key roles in adjusting the sensitivity and dynamic range of brain oscillators in response to environmental temporal structures.

Investigating the relationship between subjective perception and unconscious feature integration

Visual features need to be temporally integrated to detect motion signals and solve the many ill-posed problems of vision. It has previously been shown that such integration occurs in windows of unconscious processing of up to 450 milliseconds. However, whether features are integrated should be governed by perceptually meaningful mechanisms. Here, we expand on previous findings suggesting that subjective perception and integration may be linked. Specifically, different observers were found to group elements differently and to exhibit corresponding feature integration behavior. If the former were to influence the latter, perception would appear to not only be the outcome of integration but to potentially also be part of it. To test any such linkages more systematically, we here examined the role of one of the key perceptual grouping cues, color similarity, in the Sequential Metacontrast Paradigm (SQM). In the SQM, participants are presented with two streams of lines that are expanding from the center outwards. If several lines in the attended motion stream are offset, offsets integrate unconsciously and mandatorily for periods of up to 450 milliseconds. Across three experiments, we presented lines of varied colors. Our results reveal that individuals who perceive differently colored lines as "popping out" from the motion stream do not exhibit mandatory integration but that individuals who perceive such lines as part of an integrated motion stream do show offset integration behavior across the entire stream. These results attest to the proposed linkage between subjective perception and integration behavior in the SQM.

Temporal windows of unconscious processing cannot easily be disrupted

Conscious perception is preceded by long periods of unconscious processing. These periods are crucial for analyzing temporal information and for solving the many ill-posed problems of vision. An important question is what starts and ends these windows and how they may be interrupted. Most experimental paradigms do not offer the methodology required for such investigation. Here, we used the sequential metacontrast paradigm, in which two streams of lines, expanding from the center to the periphery, are presented, and participants are asked to attend to one of the motion streams. If several lines in the attended motion stream are offset, the offsets are known to integrate mandatorily and unconsciously, even if separated by up to 450 ms. Using this paradigm, we here found that external visual objects, such as an annulus, presented during the motion stream, do not disrupt mandatory temporal integration. Thus, if a window is started once, it appears to remain open even in the presence of disruptions that are known to interrupt visual processes normally. Further, we found that interrupting the motion stream with a gap disrupts temporal integration but does not terminate the overall unconscious processing window. Thus, while temporal integration is key to unconscious processing, not all stimuli in the same processing window are integrated together. These results strengthen the case for unconscious processing taking place in windows of sensemaking, during which temporal integration occurs in a flexible and perceptually meaningful manner.

Alpha Oscillations and Temporal Binding Windows in Perception-A Critical Review and Best Practice Guidelines

An intriguing question in cognitive neuroscience is whether alpha oscillations shape how the brain transforms the continuous sensory inputs into distinct percepts. According to the alpha temporal resolution hypothesis, sensory signals arriving within a single alpha cycle are integrated, whereas those in separate cycles are segregated. Consequently, shorter alpha cycles should be associated with smaller temporal binding windows and higher temporal resolution. However, the evidence supporting this hypothesis is contentious, and the neural mechanisms remain unclear. In this review, we first elucidate the alpha temporal resolution hypothesis and the neural circuitries that generate alpha oscillations. We then critically evaluate study designs, experimental paradigms, psychophysics, and neurophysiological analyses that have been employed to investigate the role of alpha frequency in temporal binding. Through the lens of this methodological framework, we then review evidence from between-subject, within-subject, and causal perturbation studies. Our review highlights the inherent interpretational ambiguities posed by previous study designs and experimental paradigms and the extensive variability in analysis choices across studies. We also suggest best practice recommendations that may help to guide future research. To establish a mechanistic role of alpha frequency in temporal parsing, future research is needed that demonstrates its causal effects on the temporal binding window with consistent, experimenter-independent methods.