Fluctuations in instantaneous frequency predict alpha amplitude during visual perception.

Rhythmic neural activity in the alpha band (8-13 Hz) is thought to have an important role in the selective processing of visual information. Typically, modulations in alpha amplitude and instantaneous frequency are thought to reflect independent mechanisms impacting dissociable aspects of visual information processing. However, in complex systems with interacting oscillators such as the brain, amplitude and frequency are mathematically dependent. Here, we record electroencephalography in human subjects and show that both alpha amplitude and instantaneous frequency predict behavioral performance in the same visual discrimination task. Consistent with a model of coupled oscillators, we show that fluctuations in instantaneous frequency predict alpha amplitude on a single trial basis, empirically demonstrating that these metrics are not independent. This interdependence suggests that changes in amplitude and instantaneous frequency reflect a common change in the excitatory and inhibitory neural activity that regulates alpha oscillations and visual information processing

Selective attention and its roles in enhancing sensory information processing and perceptual performance

In a complex visual environment—such as a crowed street in Bangkok— driving would be impossible if drivers do not have an intact attentional system. They have to monitor cars and trucks surrounding their vehicles, and at the same time they have to also attend to traffic lights and street signs and watch out for pedestrians and motorbikes that could cross the street unexpectedly at any given time. In this type of scenarios, drivers need to divide their attention into multiple spotlights and flexibly change the size of their attention field (zoom-in and zoom-out) such that relevant information is most efficiently encoded at the expense of irrelevant information. The first two experiments in my dissertation examined neural mechanisms underlying these two natures of our attentonal system, and the last experiment officially evaluated the relative contributions of alternative neural mechanisms that may account for attention-related improvement in perceptual performance. In the first experiment (Chapter 2), we provided neural evidence showing that attention could be divided into multiple spotlights across non-contiguous locations in the visual scene, even when visual objects were in close proximity (i.e., in a single quadrant). Using a stimulus- frequency-tagging technique where we flashed two visual targets and a distractor at the intermediate location at different frequencies, we were able to monitor changes of steady state visually evoked potentials (SSVEPs) that oscillated at the same frequencies as the target and distractor stimuli. We found the significant divergence of the target-related and distractor-related SSVEPs ~150-350ms before human participants correctly discriminated the orientations of the two targets. In the second experiment (Chapter 3), we examined the neural basis underlying changes of the spatial scope of attention and studied how such changes may alter the way sensory information is encoded in the visual cortex. By manipulating the spatial extent of visual target in a stream of flickering non-target stimuli, we observed changes in the spread of cortical activity in the contralateral visual cortex measured using functional magnetic resonance imaging (fMRI). As attention became more distributed due to the uncertainty of target locations, we observed a larger spread of cortical activity compared to when attention was more focused to a single target location. Importantly, we found that this spread of the spatial attention modulated the magnitude of attentional modulations of sensory signals measured via SSVEPs in the way that was consistent with predictions from computational models based on divisive normalization. Lastly, in the third experiment (Chapter 4), we made a further step to formally examine the quantitative relationship between attentional gain modulations of neural signals and attention-related improvement in behavioral performance, and evaluated the relative contributions of attentional gain mechanisms and other alternative mechanisms, including noise reduction and efficient read-out mechanisms. We found that in a relatively simple attention task, attentional gain modulations of early visually evoked responses measured via electroencephalography (EEG) could sufficiently predict attention-related improvement in perceptual performance, without the need to invoke the other alternative mechanisms. Taken together, the results from these three experiments suggest that selective attention enhances sensory information processing via changes in gain modulations of early sensory signals and these attentional gain modulations play a critical role in supporting attention-related improvement in perceptual performance

Functional MRI and EEG Index Complementary Attentional Modulations

Functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) are two noninvasive methods commonly used to study neural mechanisms supporting visual attention in humans. Studies using these tools, which have complementary spatial and temporal resolutions, implicitly assume they index similar underlying neural modulations related to external stimulus and internal attentional manipulations. Accordingly, they are often used interchangeably for constraining understanding about the impact of bottom-up and top-down factors on neural modulations. To test this core assumption, we simultaneously manipulated bottom-up sensory inputs by varying stimulus contrast and top-down cognitive modulations by changing the focus of spatial attention. Each of the male and female subjects participated in both fMRI and EEG sessions performing the same experimental paradigm. We found categorically different patterns of attentional modulation on fMRI activity in early visual cortex and early stimulus-evoked potentials measured via EEG (e.g., the P1 component and steady-state visually-evoked potentials): fMRI activation scaled additively with attention, whereas evoked EEG components scaled multiplicatively with attention. However, across longer time scales, a contralateral negative-going potential and oscillatory EEG signals in the alpha band revealed additive attentional modulation patterns like those observed with fMRI. These results challenge prior assumptions that fMRI and early stimulus-evoked potentials measured with EEG can be interchangeably used to index the same neural mechanisms of attentional modulations at different spatiotemporal scales. Instead, fMRI measures of attentional modulations are more closely linked with later EEG components and alpha-band oscillations. Considered together, hemodynamic and electrophysiological signals can jointly constrain understanding of the neural mechanisms supporting cognition.SIGNIFICANCE STATEMENT fMRI and EEG have been used as tools to measure the location and timing of attentional modulations in visual cortex and are often used interchangeably for constraining computational models under the assumption that they index similar underlying neural processes. However, by varying attentional and stimulus parameters, we found differential patterns of attentional modulations of fMRI activity in early visual cortex and commonly used stimulus-evoked potentials measured via EEG. Instead, across longer time scales, a contralateral negative-going potential and EEG oscillations in the alpha band exhibited attentional modulations similar to those observed with fMRI. Together, these results suggest that different physiological processes assayed by these complementary techniques must be jointly considered when making inferences about the neural underpinnings of cognitive operations

Expectations Do Not Alter Early Sensory Processing during Perceptual Decision-Making

Two factors play important roles in shaping perception: the allocation of selective attention to behaviorally relevant sensory features, and prior expectations about regularities in the environment. Signal detection theory proposes distinct roles of attention and expectation on decision-making such that attention modulates early sensory processing, whereas expectation influences the selection and execution of motor responses. Challenging this classic framework, recent studies suggest that expectations about sensory regularities enhance the encoding and accumulation of sensory evidence during decision-making. However, it is possible, that these findings reflect well documented attentional modulations in visual cortex. Here, we tested this framework in a group of male and female human participants by examining how expectations about stimulus features (orientation and color) and expectations about motor responses impacted electroencephalography (EEG) markers of early sensory processing and the accumulation of sensory evidence during decision-making (the early visual negative potential and the centro-parietal positive potential, respectively). We first demonstrate that these markers are sensitive to changes in the amount of sensory evidence in the display. Then we show, counter to recent findings, that neither marker is modulated by either feature or motor expectations, despite a robust effect of expectations on behavior. Instead, violating expectations about likely sensory features and motor responses impacts posterior alpha and frontal theta oscillations, signals thought to index overall processing time and cognitive conflict. These findings are inconsistent with recent theoretical accounts and suggest instead that expectations primarily influence decisions by modulating post-perceptual stages of information processing.SIGNIFICANCE STATEMENT Expectations about likely features or motor responses play an important role in shaping behavior. Classic theoretical frameworks posit that expectations modulate decision-making by biasing late stages of decision-making including the selection and execution of motor responses. In contrast, recent accounts suggest that expectations also modulate decisions by improving the quality of early sensory processing. However, these effects could instead reflect the influence of selective attention. Here we examine the effect of expectations about sensory features and motor responses on a set of electroencephalography (EEG) markers that index early sensory processing and later post-perceptual processing. Counter to recent empirical results, expectations have little effect on early sensory processing but instead modulate EEG markers of time-on-task and cognitive conflict