Nicholas Hon

Frontoparietal Activity with Minimal Decision and Control

In the human brain, a well known frontoparietal circuit, including lateral prefrontal cortex (LPFC), presupplementary motor area/anterior cingulate cortex (pre-SMA/ACC), and both the superior and inferior parietal cortex, is involved in cognitive control. One proposal is that the frontoparietal cortex holds a flexible description of attended or task-relevant information, biasing processing in favor of this information in many different parts of the brain. Here, we separate frontoparietal coding of attended information from its active use in behavior. In two experiments, subjects watch a stream of visual stimuli in a fixed location. In the first experiment, there is no task to perform; in the second, decisions are orthogonal to the occurrence of new stimulus events. Even in these simple circumstances, we find that attended stimulus changes give extensive activation of LPFC, pre-SMA/ACC and parietal cortex, whereas unattended changes do not. Even without behavior to control, these classical “control” regions are active in simple update of attended information.

Dysconnectivity in the frontoparietal attention network in schizophrenia.

Cognitive impairment is common in patients with schizophrenia, and even those with relatively preserved function perform worse than healthy volunteers (HVs) on attentional tasks. This is consistent with the hypothesis that connectivity – in the frontoparietal network (FPN) activated during attention – is disrupted in schizophrenia. We examined attentional effects on connectivity in the FPN, in schizophrenia, using magnetoencephalography (MEG). Twenty-three HVs and 19 first-episode schizophrenia patients were scanned during a simple visual change test, known to activate the FPN, in which attention was monitored and directed with an orthogonal flicker-detection task. Dynamic causal modeling (DCM) of evoked responses was used to assess effective connectivity – and its modulation by changes in the attended stimulus dimension – in the following network: higher visual area; temporoparietal junction (TPJ); intraparietal sulcus (IPS); dorsal anterior cingulate cortex; and ventrolateral prefrontal cortex. The final MEG analysis included 18 HVs and 14 schizophrenia patients. While all participants were able to maintain attention, HVs responded slightly, but non-significantly, more accurately than schizophrenia patients. HVs, but not schizophrenia patients, exhibited greater cortical responses to attended visual changes. Bayesian model comparison revealed that a DCM with attention dependent changes in both top-down and bottom-up connections best explained responses by patients with schizophrenia, while in HVs the best model required only bottom-up changes. Quantitative comparison of connectivity estimates revealed a significant group difference in changes in the right IPS-TPJ connection: schizophrenia patients showed relative reductions in connectivity during attended stimulus changes. Crucially, this reduction predicted lower intelligence. These data are consistent with the hypothesis that functional dysconnections in the FPN contribute to cognitive impairment in schizophrenia.

Evidence for long-range feedback in target detection: Detection of semantic targets modulates activity in early visual areas

In a variety of attention and search tasks, single-cell recordings of the primate brain have frequently shown an enhancement of responses in early visual areas to selected target stimuli. This enhancement is observed only at longer latencies, suggesting the possibility that it reflects the action of feedback or return signals from upstream processing areas. However, in typical studies, targets are specified on the basis of elementary visual features; as these are coded at multiple levels of the visual system, it is impossible to determine where enhanced target processing begins. Using human functional magnetic resonance imaging (fMRI), we demonstrate enhancement of activity in early visual areas even when low-level visual information is insufficient for target detection to occur. We found enhanced activity in early visual areas to targets defined purely by semantic category, suggesting that feedback signals returning from at least as far forward as temporal lobe semantic processing can influence visual responses. These findings also suggest feedback signaling as a mechanism by which early and late brain systems coding for different properties of a target object can integrate their activity, allowing for the target object to dominate overall processing.

Preoccupied minds feel less control: Sense of agency is modulated by cognitive load

People have little difficulty distinguishing effects they cause and those they do not. An important question is what underlies this sense of agency. A prevailing idea is that the sense of agency arises from a comparison between a predictive representation of the effect (of a given action) and the actual effect that occurs, with a clear match between the two producing a strong sense of agency. Although there is general agreement on this comparison process, one important theoretical issue that has yet to be fully determined is whether these computations are consciously performed. Here, we studied this issue by requiring participants to perform a simple judgment of agency task under conditions of different concurrent working memory load. Working memory operations are known to tax conscious cognitive resources. We found that agency judgments were moderated by working memory load, with lower agency ratings being observed in the high load condition, suggesting that the sense of agency is dependent on the availability of conscious cognitive resources. An examination of the time-course of this load effect suggests that it is the construction of the mental representation of the predicted effect which is particularly dependent on said resources.

Why rare targets are slow: Evidence that the target probability effect has an attentional locus

Target probability has a well-known effect on detection times: Targets that occur with lower probability are detected more slowly than their higher-probability counterparts. A long-standing issue of interest is what causes this effect. In the two experiments of this study, we examined the possibility that the target probability effect has an attentional locus. We report two key findings that are consistent with this hypothesis. First, we observed a magnification of the effect when the attentional resources available for target detection were limited. Second, we also observed the complementary pattern: an attenuation of the effect when more attentional resources were available for detection. We propose that the target probability effect is caused by an asymmetry in the attentional demands made by targets that occur with different probabilities, with low-probability targets being more attentionally demanding than high-probability ones.

The trajectory of the target probability effect

The effect of target probability on detection times is well-established: Even when detection accuracy is high, lower probability targets are detected more slowly than higher probability ones. Although this target probability effect on detection times has been well-studied, one aspect of it has remained largely unexamined: How the effect develops over the span of an experiment. Here, we investigated this issue with two detection experiments that assessed different target probability ratios. Conventional block segment analysis and linear mixed-effects modeling converged on two key findings. First, we found that the magnitude of the target probability effect increases as one progresses through a block of trials. Second, we found, by examining the trajectories of the low- and high-probability targets, that this increase in effect magnitude was driven by the low-probability targets. Specifically, we found that low-probability targets were detected more slowly as a block of trials progressed. Performance to high-probability targets, on the other hand, was largely invariant across the block. The latter finding is of particular interest because it cannot be reconciled with accounts that propose that the target probability effect is driven by the high-probability targets.

Different types of target probability have different prefrontal consequences

One of the factors known to affect target detection is target probability. It is clear, though, that target probability can be manipulated in different ways. Here, in order to more accurately characterize the effects of target probability on frontal engagement, we examined the effects of two commonly-used but different target probability manipulations on neural activity. We found that manipulations that affected global stimulus class probability had a pronounced effect on ventrolateral prefrontal cortex and the insula, an effect which was absent with manipulations that only affected the likelihood of specific target stimuli occurring. This latter manipulation only modulated activity in dorsolateral prefrontal cortex and the precentral sulcus. Our data suggest two key points. First, different types of target probability have different neural consequences, and may therefore be very different in nature. Second, the data indicate that ventral and dorsal portions of prefrontal cortex respond to different types of task-relevant information.

Sleep deprivation produces feelings of vicarious agency.

A variety of self-related psychological constructs are supported by the fundamental ability to accurately sense either self-agency or lack of agency in some action or outcome. Agency judgments are typically studied in individuals who are well-rested and mentally-fresh; however, in our increasingly fast-paced world, such judgments often need to be made while in less optimal states. Here, we studied the effect of being in one such non-optimal state - when sleep-deprived - on judgments of agency. We found that 24h of total sleep deprivation elevated agency ratings on trials designed to produce a strong sense of non-agency. These data provide the first evidence that physiological state variables can affect agency processing in the normal population.

Attention and the sense of agency: A review and some thoughts on the matter

Much is now known about the sense of agency and how it is produced. What is lacking, though, is an understanding of how it relates to other cognitive domains and operations. Here, the relationship between the sense of agency and attention is explored. A review of the literature suggests that attention is involved in the sense of agency in (at least) two key ways. First, agency processing is dependent on attentional resources. Second, attentional enhancement is necessary for extraneous information to have an influence over the sense of agency. These are discussed and some suggestions for future research are offered.

Rare targets are less susceptible to attention capture once detection has begun

Rare or low probability targets are detected more slowly and/ or less accurately than higher probability counterparts. Various proposals have implicated perceptual and response-based processes in this deficit. Recent evidence, however, suggests that it is attentional in nature, with low probability targets requiring more attentional resources than high probability ones to detect. This difference in attentional requirements, in turn, suggests the possibility that low and high probability targets may have different susceptibilities to attention capture, which is also known to be resource-dependent. Supporting this hypothesis, we found that, once attentional resources have begun to be engaged by detection processes, low, but not high, probability targets have a reduced susceptibility to capture. Our findings speak to several issues. First, they indicate that the likelihood of attention capture occurring when a given task-relevant stimulus is being processed is dependent, to some extent, on how said stimulus is represented within mental task sets. Second, they provide added support for the idea that the behavioural deficit associated with low probability targets is attention-based. Finally, the current data point to reduced top-down biasing of target templates as a likely mechanism underlying the attentional locus of the deficit in question.