Near-threshold Detection Research Articles

The effect of pupil size on near-threshold detection is not mediated by alpha, beta, or theta power

The effect of pupil size on near-threshold detection is not mediated by alpha, beta, or theta power

Dissociating the Neural Correlates of Subjective Visibility from Those of Decision Confidence.

A key goal of consciousness science is identifying neural signatures of being aware versus unaware of simple stimuli. This is often investigated in the context of near-threshold detection, with reports of stimulus awareness being linked to heightened activation in a frontoparietal network. However, because of reports of stimulus presence typically being associated with higher confidence than reports of stimulus absence, these results could be explained by frontoparietal regions encoding stimulus visibility, decision confidence, or both. In an exploratory analysis, we leverage fMRI data from 35 human participants (20 females) to disentangle these possibilities. We first show that, whereas stimulus identity was best decoded from the visual cortex, stimulus visibility (presence vs absence) was best decoded from prefrontal regions. To control for effects of confidence, we then selectively sampled trials before decoding to equalize confidence distributions between absence and presence responses. This analysis revealed striking differences in the neural correlates of subjective visibility in PFC ROIs, depending on whether or not differences in confidence were controlled for. We interpret our findings as highlighting the importance of controlling for metacognitive aspects of the decision process in the search for neural correlates of visual awareness.SIGNIFICANCE STATEMENT While much has been learned over the past two decades about the neural basis of visual awareness, the role of the PFC remains a topic of debate. By applying decoding analyses to functional brain imaging data, we show that prefrontal representations of subjective visibility are contaminated by neural correlates of decision confidence. We propose a new analysis method to control for these metacognitive aspects of awareness reports, and use it to reveal confidence-independent correlates of perceptual judgments in a subset of prefrontal areas.

Visual detection of time-varying signals: Opposing biases and their timescales.

Human visual perception is a complex, dynamic and fluctuating process. In addition to the incoming visual stimulus, it is affected by many other factors including temporal context, both external and internal to the observer. In this study we investigate the dynamic properties of psychophysical responses to a continuous stream of visual near-threshold detection tasks. We manipulate the incoming signals to have temporal structures with various characteristic timescales. Responses of human observers to these signals are analyzed using tools that highlight their dynamical features as well. Our experiments show two opposing biases that shape perceptual decision making simultaneously: positive recency, biasing towards repeated response; and adaptation, entailing an increased probability of changed response. While both these effects have been reported in previous work, our results shed new light on the timescales involved in these effects, and on their interplay with varying inputs. We find that positive recency is a short-term bias, inversely correlated with response time, suggesting it can be compensated by afterthought. Adaptation, in contrast, reflects trends over longer times possibly including multiple previous trials. Our entire dataset, which includes different input signal temporal structures, is consistent with a simple model with the two biases characterized by a fixed parameter set. These results suggest that perceptual biases are inherent features which are not flexible to tune to input signals.

Recent Applications of Synchrotron VUV Photoionization Mass Spectrometry: Insight into Combustion Chemistry

Combustion is one of the earliest developed human technologies and remains our primary source of energy, yet it embodies a complex suite of physical and chemical processes that are inadequately understood. Combustion chemistry involves both chemical thermodynamics and chemical kinetics, and experimental advances mostly depend on the development of combustion diagnostics, which effectively serve as the foundation of theoretical progress. The major objective of combustion diagnostics is to provide comprehensive product identification and concentration information of a flame species, which can be used to develop kinetic models for the simulation of practical combustion. However, conventional combustion diagnostic methods face difficult challenges in distinguishing isomeric species, detecting reactive radicals, obtaining real-time measurements, and so forth. Therefore, for deeper insight into combustion chemistry, a diagnostic method with high detection sensitivity, isomeric selectivity, and radical detectability is required. In this Account, we report recent applications of synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) in various areas of combustion chemistry research. The wide tunability of synchrotron photon energy can facilitate the selective identification of isomeric intermediates and the near-threshold detection of radicals (thus avoiding fragmentation interference). Moreover, the convenient combination of SVUV-PIMS with various laboratory-based combustion approaches demonstrates its universality in combustion studies. Recent experimental achievements have demonstrated the successful applications of this technique in premixed flames, pyrolysis in flow reactors, coflow diffusion flames, catalytic oxidation, plasma diagnostics, and analysis of polycyclic aromatic hydrocarbons (PAHs) and soot. More applications of SVUV-PIMS are expected in the near future, not only in combustion studies, but also in other research topics of chemistry such as analytical chemistry, photochemistry, biochemistry, and the like. In all applications, combustion intermediates, including isomers and radicals, can be distinguished unambiguously, extending our knowledge of intermediate pools and providing more precise targets for quantum chemical calculations of significant reaction channels. The observed mass range covers both small and large combustion products, such as PAHs with two to five carbonic rings. Such analyses present clues toward understanding the molecular growth process from fuel to PAHs and, consequently, soot in fuel-rich hydrocarbon flames. Furthermore, quantitative analyses of chemical structure are available in most applications. For example, one can acquire concentration profiles of flame species versus position in premixed and diffusion flames or versus temperature in pyrolysis and catalytic oxidation. The objectives of validating current kinetic models and developing new kinetic models are thus well served with SVUV-PIMS as an analytical tool in combustion research.

Temporal constraints on conscious vision: On the ubiquitous nature of the attentional blink

The attentional blink (AB) refers to the finding that observers often miss the second of two masked visual targets (T1 and T2, e.g., letters) appearing within 200-500 ms. Although the presence of a T1 mask is thought to be required for this effect, we recently found that an AB deficit can be observed even in the absence of a T1 mask if T2 is shown very briefly and followed by a pattern mask (M. R. Nieuwenstein, M. C. Potter, & J. Theeuwes, 2009). Using such a sensitive T2 task, the present study sought to determine the minimum requirements for eliciting an AB deficit. To this end, we examined if the occurrence of an AB depends on T1 exposure duration, the requirement to perform a task for T1, and awareness of T1. The results showed that an AB deficit occurs regardless of the presentation duration of T1, and regardless of whether there is a T1 task. A boundary condition for the occurrence of an AB was found in conscious awareness of T1. With a near-threshold detection task for T1, attention blinked when T1 was seen, but not when T1 was missed. Accordingly, we conclude that the minimum requirement for an AB deficit is T1 awareness.

Response-dependent effects on near-threshold detection performance: saccades versus manual responses.

The inverse relationship between flash intensity and response times was studied under identical conditions using two different response measures, saccadic eye movements and manual responses. It was found that saccade latencies were less dependent on flash intensity than were manual responses. In addition, detection failures to near-threshold flashes were significantly less frequent when subjects signaled detection of the flash by making a saccade. These differences in detection performance may reflect a greater capacity of the oculomotor system to decode near-threshold ganglion cell responses.