Faculty Bibliography

Solving the problem of consciousness remains one of the biggest challenges in modern science. One key step towards understanding consciousness is to empirically narrow down neural processes associated with the subjective experience of a particular content. To unravel these neural correlates of consciousness (NCC) a common scientific strategy is to compare perceptual conditions in which consciousness of a particular content is present with those in which it is absent, and to determine differences in measures of brain activity (the so called "contrastive analysis"). However, this comparison appears not to reveal exclusively the NCC, as the NCC proper can be confounded with prerequisites for and consequences of conscious processing of the particular content. This implies that previous results cannot be unequivocally interpreted as reflecting the neural correlates of conscious experience. Here we review evidence supporting this conjecture and suggest experimental strategies to untangle the NCC from the prerequisites and consequences of conscious experience in order to further develop the otherwise valid and valuable contrastive methodology.

During meditation, practitioners are required to center their attention on a specific object for extended periods of time. When their thoughts get diverted, they learn to quickly disengage from the distracter. We hypothesized that learning to respond to the dual demand of engaging attention on specific objects and disengaging quickly from distracters enhances the efficiency by which meditation practitioners can allocate attention. We tested this hypothesis in a global-to-local task while measuring electroencephalographic activity from a group of eight highly trained Buddhist monks and nuns and a group of eight age and education matched controls with no previous meditation experience. Specifically, we investigated the effect of attentional training on the global precedence effect, i.e., faster detection of targets on a global than on a local level. We expected to find a reduced global precedence effect in meditation practitioners but not in controls, reflecting that meditators can more quickly disengage their attention from the dominant global level. Analysis of reaction times confirmed this prediction. To investigate the underlying changes in brain activity and their time course, we analyzed event-related potentials. Meditators showed an enhanced ability to select the respective target level, as reflected by enhanced processing of target level information. In contrast with control group, which showed a local target selection effect only in the P1 and a global target selection effect in the P3 component, meditators showed effects of local information processing in the P1, N2, and P3 and of global processing for the N1, N2, and P3. Thus, meditators seem to display enhanced depth of processing. In addition, meditation altered the uptake of information such that meditators selected target level information earlier in the processing sequence than controls. In a longitudinal experiment, we could replicate the behavioral effects, suggesting that meditation modulates attention already after a 4-day meditation retreat. Together, these results suggest that practicing meditation enhances the speed with which attention can be allocated and relocated, thus increasing the depth of information processing and reducing response latency.

Perceptual learning not only improves sensitivity, but it also changes our subjective experience. However, the question of how these two learning effects relate is largely unexplored. Here we investigate how subjects learn to see initially indiscriminable metacontrast-masked shapes. We find that sensitivity and subjective awareness increase with training. However, sensitivity and subjective awareness dissociate in space: Learning effects on performance are lost when the task is performed at an untrained location in another quadrant, whereas learning effects on subjective awareness are maintained. This finding indicates that improvements in shape sensitivity involve visual areas up to V4, whereas changes in subjective awareness involve other brain regions. Furthermore, subjective awareness dissociates from sensitivity in time: In an early phase of perceptual learning, subjects perform above chance on trials that they rate as subjectively invisible. Later, this phenomenon disappears. Subjective awareness is thus neither necessary nor sufficient for achieving above-chance objective performance.

Previous experience allows the brain to predict what comes next. How these expectations affect conscious experience is poorly understood. In particular, it is unknown whether and when expectations interact with sensory evidence in granting access to conscious perception, and how this is reflected electrophysiologically. Here, we parametrically manipulate sensory evidence and expectations while measuring event-related potentials in human subjects to assess the time course of evoked responses that correlate with subjective visibility, the properties of the stimuli, and/or perceptual expectations. We found that expectations lower the threshold of conscious perception and reduce the latency of neuronal signatures differentiating seen and unseen stimuli. Without expectations, this differentiation occurs approximately 300 ms and with expectations approximately 200 ms after stimulus in occipitoparietal sensors. The amplitude of this differentiating response component (P2) decreases as visibility increases, regardless of whether this increase is attributable to enhanced sensory evidence and/or the gradual buildup of perceptual expectations. Importantly, at matched performance levels, responses to seen and unseen stimuli differed regardless of the physical stimulus properties. These findings indicate that the latency of the neuronal correlates of access to consciousness depend on whether access is driven by stimulus saliency or by a combination of expectations and sensory evidence.

Mechanisms of explicit object recognition are often difficult to investigate and require stimuli with controlled features whose expression can be manipulated in a precise quantitative fashion. Here, we developed a novel method (called "Dots"), for generating visual stimuli, which is based on the progressive deformation of a regular lattice of dots, driven by local contour information from images of objects. By applying progressively larger deformation to the lattice, the latter conveys progressively more information about the target object. Stimuli generated with the presented method enable a precise control of object-related information content while preserving low-level image statistics, globally, and affecting them only little, locally. We show that such stimuli are useful for investigating object recognition under a naturalistic setting--free visual exploration--enabling a clear dissociation between object detection and explicit recognition. Using the introduced stimuli, we show that top-down modulation induced by previous exposure to target objects can greatly influence perceptual decisions, lowering perceptual thresholds not only for object recognition but also for object detection (visual hysteresis). Visual hysteresis is target-specific, its expression and magnitude depending on the identity of individual objects. Relying on the particular features of dot stimuli and on eye-tracking measurements, we further demonstrate that top-down processes guide visual exploration, controlling how visual information is integrated by successive fixations. Prior knowledge about objects can guide saccades/fixations to sample locations that are supposed to be highly informative, even when the actual information is missing from those locations in the stimulus. The duration of individual fixations is modulated by the novelty and difficulty of the stimulus, likely reflecting cognitive demand.

Can practice effects on unconscious stimuli lead to awareness? Can we "learn to see"? Recent evidence suggests that blindsight patients trained for an extensive period of time can learn to discriminate and consciously perceive stimuli that they were previously unaware of. So far, it is unknown whether these effects generalize to normal observers. Here we investigated practice effects in metacontrast masking. Subjects were trained for five consecutive days on the stimulus onset asynchrony (SOA) that resulted in chance performance. Our results show a linear increase in sensitivity (d') but no change in bias (c) for the trained SOA. This practice effect on sensitivity spreads to all tested SOAs. Additionally, we show that subjects rate their perceptual awareness of the target stimuli differently before and after training, exhibiting not only an increase in sensitivity, but also in the subjective awareness of the percept. Thus, subjects can indeed "learn to see."

Here we explore whether mental training in the form of meditation can help to overcome age-related attentional decline. We compared performance on the attentional blink task between three populations: A group of long-term meditation practitioners within an older population, a control group of age-matched participants and a control group of young participants. Members of both control groups had never practiced meditation. Our results show that long-term meditation practice leads to a reduction of the attentional blink. Meditation practitioners taken from an older population showed a reduction in blink as compared to a control group taken from a younger population, whereas, the control group age-matched to the meditators' group revealed a blink that was comparatively larger and broader. Our results support the hypothesis that meditation practice can: (i) alter the efficiency with which attentional resources are distributed and (ii) help to overcome age-related attentional deficits in the temporal domain.

In natural vision, attention and eye movements are linked. Furthermore, eye movements structure the inflow of information into the visual system. Saccades, where little vision occurs, alternate with fixations, when most vision occurs. A mechanism must be in place to maximize information intake during fixations. Oscillatory synchrony has been proposed as a mechanism for rapid and reliable communication of signals, subserving cognitive functions such as attention and object identification. We propose that saccade-related corollary activity has a crucial role in anticipatory preparation of visual centers, which interacts with ongoing oscillation, favoring the processing of postfixational signals. During prolonged fixations, microsaccades could be generated to exploit this mechanism. Studying this interplay between the sensory and the motor system will provide novel insight into the dynamics of natural vision.

Following the discovery of context-dependent synchronization of oscillatory neuronal responses in the visual system, the role of neural synchrony in cortical networks has been expanded to provide a general mechanism for the coordination of distributed neural activity patterns. In the current paper, we present an update of the status of this hypothesis through summarizing recent results from our laboratory that suggest important new insights regarding the mechanisms, function and relevance of this phenomenon. In the first part, we present recent results derived from animal experiments and mathematical simulations that provide novel explanations and mechanisms for zero and nero-zero phase lag synchronization. In the second part, we shall discuss the role of neural synchrony for expectancy during perceptual organization and its role in conscious experience. This will be followed by evidence that indicates that in addition to supporting conscious cognition, neural synchrony is abnormal in major brain disorders, such as schizophrenia and autism spectrum disorders. We conclude this paper with suggestions for further research as well as with critical issues that need to be addressed in future studies.