Probabilistic Model of Onset Detection Explains Paradoxes in Human Time Perception

A very basic computational model is proposed to explain two puzzling findings in the time perception literature. First, spontaneous motor actions are preceded by up to 1–2 s of preparatory activity (Kornhuber and Deecke, 1965). Yet, subjects are only consciously aware of about a quarter of a second of motor preparation (Libet et al., 1983). Why are they not aware of the early part of preparation? Second, psychophysical findings (Spence et al., 2001) support the principle of attention prior entry (Titchener, 1908), which states that attended stimuli are perceived faster than unattended stimuli. However, electrophysiological studies reported no or little corresponding temporal difference between the neural signals for attended and unattended stimuli (McDonald et al., 2005; Vibell et al., 2007). We suggest that the key to understanding these puzzling findings is to think of onset detection in probabilistic terms. The two apparently paradoxical phenomena are naturally predicted by our signal detection theoretic model

Law, responsibility, and the brain.

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Subliminal stimuli in the near absence of attention influence top-down cognitive control

Recent research has shown that visual stimuli can influence cognitive control functions, even if subjects are unaware of the identity of the stimuli. However, in those previous studies, subjects actively attended to the location of the subliminal stimuli. Here we assessed the role of endogenous spatial attention in such paradigms. We required subjects to quickly prepare for one of two numerical judgment tasks on the basis of the direction of motion in patches of moving dots presented in cued spatial locations. We found that irrelevant motion patches presented in the uncued spatial locations also influenced task performance. Motion in the uncued patches was weak and did not affect the perception of the cued patches. Further analyses suggested that the effect of priming by the uncued stimuli was present even for subjects who could only discriminate such stimuli at chance level. Three additional experiments confirmed that subjects paid minimal attention to the uncued locations, in that the subjects could not perform simple discriminations of conjunctions of features in those locations

Illusory temporal binding in meditators

We investigate conditions in which more accurate metacognition may lead to greater susceptibility to illusion; and thus conditions under which mindfulness meditation may lead to less accurate perceptions. Specifically, greater awareness of intentions may lead to an illusory compression of time between a voluntary action and its outcome (“intentional binding”). Here we report that experienced Buddhist mindfulness meditators rather than non-meditators display a greater illusory shift of the timing of an outcome towards an intentional action. Mindfulness meditation involves awareness of causal connections between different mental states, including intentions. We argue that this supports improvements in metacognition targeted at motor intentions. Changes in metacognitive ability may result in an earlier and less veridical experience of the timing of action outcomes either through increased access to sensorimotor pre-representations of an action outcome or by affording greater precision to action timing judgements. Furthermore, as intentional binding is an implicit measure of the sense of agency, these results also provide evidence that mindfulness meditators experience a stronger sense of agency

Should Scientists Think?: Comment on Machado and Silva (2007)

The author has previously claimed that neural activity in a medial frontal brain area represents conscious motor intentions. A. Machado and F. J. Silva (2007) attempted to challenge this claim by arguing that such intentions are not necessarily causally effective and must be caused by something else, so that they therefore cannot be the unmoved first movers in action. The author's reply is that he made no such claims about the causal status and origin of conscious intentions. In fact, he has elsewhere presented empirical evidence in support of the idea that intentions are not necessarily causal. But this does not stop researchers from studying the neural basis for the conscious impression that one intends and wills one's actions. One can feel and attend to such intentions, be they causal or not. The author's claim is that there is neural activity in the medial frontal wall that reflects such conscious feelings. Other recent empirical evidence that supports this view is described. © 2007 American Psychological Association.Link_to_subscribed_fulltex

A higher order Bayesian decision theory of consciousness

It is usually taken as given that consciousness involves superior or more elaborate forms of information processing. Contemporary models equate consciousness with global processing, system complexity, or depth or stability of computation. This is in stark contrast with the powerful philosophical intuition that being conscious is more than just having the ability to compute. I argue that it is also incompatible with current empirical findings. I present a model that is free from the strong assumption that consciousness predicts superior performance. The model is based on Bayesian decision theory, of which signal detection theory is a special case. It reflects the fact that the capacity for perceptual decisions is fundamentally limited by the presence and amount of noise in the system. To optimize performance, one therefore needs to set decision criteria that are based on the behaviour, i.e. the probability distributions, of the internal signals. One important realization is that the knowledge of how our internal signals behave statistically has to be learned over time. Essentially, we are doing statistics on our own brain. This 'higher-order' learning, however, may err, and this impairs our ability to set and maintain optimal criteria for perceptual decisions, which I argue is central to perception consciousness. I outline three possibilities of how conscious perception might be affected by failures of 'higher-order' representation. These all imply that one can have a dissociation between consciousness and performance. This model readily explains blindsight and hallucinations in formal terms, and is beginning to receive direct empirical support. I end by discussing some philosophical implications of the model. © 2008 Elsevier B.V. All rights reserved.Link_to_subscribed_fulltex

Are we studying consciousness yet?

© Oxford University Press 2008. All rights reserved. This chapter is concerned with the behavioural paradigms used in the search for the neural correlate of consciousness (NCC). Many researchers have questioned whether looking for the neural correlates would eventually lead to an explanatory theory of consciousness, while the proponents of NCC research maintain that focusing on correlates is a strategically sensible first step, given the complexity of the problem. The aim of this chapter is to question whether researchers are really studying the NCC at all. It argues that in hoping to sidestep the difficult conceptual issues, the phenomenon of perceptual consciousness itself is also missed.Link_to_subscribed_fulltex

Relative blindsight in normal observers and the neural correlate of visual consciousness

By using a paradigm based on metacontrast masking, we created experimental conditions in which the subjective report of consciousness differs but the objectively measured ability to discriminate visual targets does not. This approach allowed us to study the neural correlate of consciousness while having performance levels carefully matched in healthy human subjects. A comparison of the neural activity associated with these conditions as measured by functional MRI showed that conscious perception is associated with spatially specific activity in the mid-dorsolateral prefrontal cortex (area 46). Further analysis confirms that this activation is not only free from any performance confound, but is also not driven by differences in the timing of the physical stimuli. Our results suggest that the prefrontal cortex is important for the essentially subjective aspects of conscious perception

Manipulating the experienced onset of intention after action execution

& Using transcranial magnetic stimulation (TMS), we have tested the time needed for the perceived onset of spontaneous motor intention to be fully determined. We found that TMS applied over the presupplementary motor area after the execution of a simple spontaneous action shifted the perceived onset of the motor intention backward in time, and shifted the perceived time of action execution forward in time. The size of the effect was similar regardless of whether TMS was applied immediately after the action or 200 msec after. The results of three control studies suggest that this effect is timelimited, specific to modality, and also specific to the anatomical site of stimulation. We conclude that the perceived onset of intention depends, at least in part, on neural activity that takes place after the execution of action. A model, which is based on the mechanism of cue integration under the presence of noise, is offered to explain the results. The implications for the conscious control of spontaneous actions are discussed. &amp