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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - Human Decisions (The Neural Determinants of Perceptual Decision Making in the Human Brain)

Teaser

Summary

The term â€˜decision makingâ€™ often calls to mind scenarios such as how to vote in an election or which course to take in college, yet even our perception of our sensory environment relies on a continuous stream of elementary judgments that can be equally life altering (e.g. is the traffic light red or green?). In the highly complex and dynamic environment that we inhabit, making accurate and timely decisions is a considerable challenge for the brain since the information it receives is almost always to some degree unreliable. Understanding how the brain overcomes this problem stands to illuminate principles of computation that extend to a wide range of cognitive operations (Shadlen & Kiani 2013), as well as holding the key to improving the diagnosis and treatment of the many brain disorders that impact on decision making abilities. This ERC project builds on the recent discovery of human brain signals that finely trace decision formation in real-time. These signals provide a direct view on the brain mechanisms that enable us to make our decisions and thus offers new opportunities to answer a number of major questions that were previously impossible to fully address. These include establishing the neural principles and processes that allow us to achieve the best balance between speed and accuracy in our decisions, to account for unreliable evidence and to represent our confidence in our decisions. An additional goal of this project is also to leverage these methodological and theoretical developments in order to gain deeper insights into the impact of natural cognitive aging on our decision making abilities.

Work performed

æ€€æ€€ate research on the three themes of this action has resulted in 8 peer-reviewed publications at leading journals, 3 peer-review publications in review, 11 conference presentations, 9 invited talks and 3 manuscripts currently at an advanced stage of perparation. Key achievements associated with each Theme are detailed below.

Theme 1
Task 1.1.
The goal of this task is to determine how the reliability of sensory evidence is factored in to the decision making process. In an initial study, we first examined the influence of temporal and spatial uncertainty on decision formation i.e. uncertainty regarding the time and/or location at which evidence relevant to the decision will appear. This fundamental question has yet to be considered in the literature with the most popular paradigms providing the participant with complete certainty regarding where and when the evidence informing a particular decision will be presented. We conducted a series of experiments in which we varied the spatial and temporal predictability of sensory targets using specially tailored contrast and motion discrimination tasks. The key insight garnered from this work has been that, under such uncertain circumstances, the brain utilizes early target selection signals to trigger evidence accumulation. We found that this reliance on target selection signals was evident even when targets appeared at an already fixated and attended location, suggesting that they play a fare more general role in perceptual decision making than previously thought. Thus we have identified a novel mechanistic component of the decision making process that has yet to be considered by the field. This work has resulted in two publications in leading international journals (Loughnane et al 2016, Current Biology; Newman et al, 2017, Journal of Neuroscience).
In a second experiment we tested the degree to which the invocation of target selection signals is dependent on the presence of a robust sensory transient at evidence onset. In two distinct experiments the onset of â€œevidenceâ€ was rendered very subtle (individually titrated to ensure 80% detection) and its onset unpredictable (occurring with equal likelihood 800 ms, 1200 ms or 1600 ms after trial start). We have uncovered exquisite behavioral and neural evidence that subjects deal with this uncertainty by initiating evidence accumulation at the outset of each trial. This means that on trials with the longest delay before evidence onset, subjects engage evidence accumulation processes to a strong degree well before any actual evidence has been physically presented. In accordance with this, their neural decision signals build significantly before evidence, presumably favoring a randomly selected alternative that has not yet physically diverged from the other grating, and they make a high number of very early (post-evidence) responses that are very low-accuracy. This builds on the first experiment of Task 1.1 by showing that the mechanisms employed for initiating evidence accumulation are highly context-dependent. A paper reporting these findings is in preparation (Devine et al, in preparation).
In analyzing the data of Task 1.1, it became clear that the occurrence of microsaccades was a major mediating factor in the relationship between contrast perception, evidence accumulation and behavior on the contrast discrimination task. Despite their small size, microsaccades have a profound impact on vision, supporting perceptual stability but also impeding stimulus detections if executed at inopportune times. Neurophysiological studies have thus far demonstrated that microsaccades evoke a combination of inhibitory and excitatory responses in a variety of visual regions. However, the impact that microsaccades have on the higher-level neural decision processes that bridge sensory responses to action selection has yet to be examined. Further analysis of the data collected for Task 1.1 revealed that when human observers monitor stimuli fo

Final results

The above research achievements have offered a number of fundamental new insights into the neural mechanisms governing decision making while also developing new methodologies for finely tracing information processing at distinct levels along the sensorimotor hierarchy. In particular, this work has highlighted several additional mechanistic components of the brain\'s decision making machinery that have not previously been considered by the field. Most prominently, we have demonstrated that target selection signals play a fundamental role in triggering evidence accumulation when evidence onsets are unpredictable (Loughnane et al 2016, Current Biology). We have also highlighted that prefrontal metacognitive signals play an important role in determining the extent to which evidence accumulation persists following choice commitment and thus facilitates error detection.

An overarching goal of the action has been to use neurophysiological human brain recordings to empirically test and refine the predictions of computational models. Until recently, work of this kind has been the sole preserve of invasive brain recording studies but owing to methodological advances achieved by the PI we do now have the ability to isolate non-invasive brain signals that can be directly related to specific neural computations underpinning decision formation. This paves the way for a powerful new neurally-informed modelling approach in humans, in which neural signal observations can mediate between alternative model variants otherwise difficult to discern through behavioural fits alone, and where necessary, guide the construction of new models that capture key realities of neural implementation that are overlooked in existing models.

The isolation of non-invasive target selection signals lso highlighted a potentially powerful new diagnostic metric for clinical disorders impacting on visuospatial orienting. An important and unique feature of the target selection signals identified (Loughnane et al 2016, Current Biology; Newman et al 2017, J Neurosci) is that they can be measured independently over each hemisphere, a capability which has potentially significant benefits for research on clinical brain disorders associated with selective attention impairments (e.g. unilateral neglect following stroke).