Methods: We have reviewed gastrointestinal features of, and faecal test results in, COVID-19 from case reports and retrospective clinical studies relating to the digestive system published since the outbreak.
Feature Possibility
When making decisions, humans can maximize the positive outcome of their actions by choosing the option associated with the highest reward. We have recently shown that choices modulate effects of reward via a bias in spatial attention: Locations associated with a lower reward are anticipatorily suppressed, as indicated by delayed responses to low-reward targets and increased parieto-occipital alpha power. Here, we investigated whether this inhibition also occurs when reward is not coupled to location but to a nonspatial feature (color). We analyzed reaction times to single targets associated with a low or high reward as a function of whether a second trial type, choice-trials, were interleaved. In choice-trials, participants could choose either one of two targets to obtain the associated reward. Indeed, responses to low-reward targets were slower when choice-trials were present, magnifying the influence of reward, and this delay was more pronounced in trials immediately following a choice. No corresponding changes in parieto-occipital alpha power were observed, but the behavioral findings suggest that choices modulate a reward-related bias in feature-based attention in a similar manner as for spatial attention, and support the idea that reward primarily affects behaviour when it is of immediate relevance.
In our previous studies11,12, items were associated with different magnitudes of reward based on their spatial location. However, the value of visual objects cannot only be determined by their location, but also by non-spatial characteristics, which can be simple visual features such as color or size, or more complex stimulus attributes such as object category. This might, in fact, be the more common case in everyday life. In the current study, we examined whether the possibility to make choices in order to maximize positive outcome modulates the effects of reward when reward magnitude is coupled to a non-spatial feature in a similar manner as it does when reward is determined by spatial location11,12.
In analogy to our findings with respect to effects of spatially defined reward12, we assumed that choices would modulate the effects of featurally defined reward via a reward-related bias in feature-based attention. Feature-based attention selectively increases sensitivity to specific features across the visual field and thereby prioritizes the visual processing of behaviorally relevant stimuli22,23. Feature-based and spatial attention thus serve the same function (i.e., to tune visual perception to what is important) and they have similar effects on neuronal responses in visual cortex24,25, activating a largely overlapping frontoparietal network26,27,28. In spite of these commonalities, however, feature-based and spatial attention appear to be distinct attentional mechanisms that independently enhance relevant visual signals with dissociable behavioral signatures29,30,31,32, and that are supported by specialized regions within the common network26,27,28,33,34. Thus, what has been established for spatial attention does not necessarily apply to feature-based attention.
To determine whether the possibility to make choices between options of different value modulates feature-based effects of reward as it does for spatially indicated reward, we modified the task used in our previous studies11,12 and coupled reward to color. Delayed responses to low-reward single targets in blocks with choice-trials would indicate that choices do not only induce a reward-related bias in visuospatial attention11,12 but also in feature-based attention.
Our prior work identified a spatially specific increase in alpha power, anticipatorily inhibiting regions associated with a low reward, as the neural mechanism that mediated the choice-induced modulation of reward effects12. Therefore, we additionally analyzed parieto-occipital alpha power during a preparatory period preceding target presentation to examine whether a reward-related bias in feature-based attention is supported by the same oscillatory mechanism. However, whereas the involvement of alpha oscillations in spatial attention has been well established14,20, only few studies have investigated whether preparatory feature-based attention is similarly reflected in alpha power changes, and these studies produced somewhat mixed evidence35,36,37. Inducing the expectation of an upcoming target feature (left- or rightward motion) has been found to increase overall alpha power over occipital cortex as compared to when participants had no expectation35. A more specific pattern of alpha-band increases has been observed for different feature dimensions, consistent with the hypothesized role of alpha as a suppression mechanism in spatial attention: When participants were cued to attend to either the color or the motion of an upcoming dot array, alpha power in the cortical areas processing the irrelevant feature dimension increased, indicating their functional inhibition36. A recent study directly comparing spatial, feature-based and combined spatial and feature-based cues, by contrast, failed to observe any alpha power modulations specifically related to feature-based attention, even though the feature-based cues were behaviorally effective37. Thus, we did not expect that a choice-induced modulation of feature-based reward effects, analogous to our previous findings for spatially indicated reward, would necessarily be reflected in a corresponding modulation of parieto-occipital alpha power. But we tentatively hypothesized that an anticipatory suppression of low-value target features might result in an alpha power increase in neural subpopulations coding for the low-value target feature, reflected in overall higher parieto-occipital alpha power in blocks with choice-trials compared to block without choice trials (see also de Lange et al.35).
In the present study, we investigated whether responses to single targets associated with different levels of reward are modulated by interleaved choices when reward is coupled to a nonspatial feature (i.e., color) as is the case when reward is coupled to spatial location11,12. Indeed, responses to single targets associated with a low reward were found to be delayed when choice-trials were present, magnifying the effect of reward on reaction times, while responses to high-reward single targets were not influenced by the manipulation of choice-trial proportion. The analysis of intertrial effects revealed that this selective slowing of responses to low-reward targets was particularly pronounced for single targets directly following a choice-trial, whereas responses to low-reward single-trials following a high-reward single-trial were not similarly delayed. This confirms that the increased reaction times to low-reward targets after a choice-trial were due to the previously made choice rather than due to a change in the color participants responded to from one trial to the next. Notably, the intertrial effects could not fully account for the response delay induced by the presence of choices (see also Wolf et al.11): Irrespective of the preceding trial type, responses to low-reward targets were slower with 33% choice-trials in the same block than without interleaved choice-trials. Overall, these findings mirror the pattern of results we obtained in our previous studies11,12, in which the level of reward was coupled to spatial location, and thus strongly suggest that choices modulate a reward-related bias in feature-based attention in a similar way as they do for spatial attention.
To examine whether the same oscillatory mechanism that we have previously identified for spatially indicated reward12 also supported the reward-related bias in feature-based attention observed in the present study, we additionally analyzed alpha-oscillations over parieto-occipital cortex during the preparatory period preceding target presentation. There was an overall decrease in alpha power during this period in all conditions, indicating that neural excitability in visual cortex was increased in order to facilitate processing of the upcoming target. However, even though the behavioral results clearly indicated that the feature associated with a low reward was effectively and more strongly suppressed when choices were present and especially so immediately following a choice-trial, this pattern was not reflected in posterior alpha power.
It might be tempting to assume that this lack of any reward-related modulation of posterior alpha oscillations shows that the suppression of low-value features was supported by a different neural mechanism than the suppression of low-value regions of space. This could be regarded as in line with previous findings. Of particular interest in this context is the study by Wildegger et al.37, who cued either the location, orientation, both or neither of an upcoming target stimulus and examined preparatory alpha modulations over visual cortex. While the anticipation of target location with spatial and combined spatial and feature cues was reflected in robust alpha lateralizations, preparing for a target feature (i.e., orientation) modulated neither lateralized nor global alpha power, even though the feature cues yielded clear performance benefits. The authors proposed that preparatory alpha modulations reflect a spatial gating mechanism that is involved in the gating of information processed by nonoverlapping sensory areas. This is for instance the case for different retinotopical locations17,18 but also for different feature dimensions36, which are processed in dedicated, separate areas. By contrast, alpha modulations would not operate at the level of specificity that is required when overlapping and interdigitating populations process the attended information37. This account accordingly predicts that no alpha modulations would be observed when feature values within the same dimension are attended, for example different orientations as in Wildegger et al.37 or different colors as in the present study. Along these lines, our results could be seen as extending empirical support for this idea put forward by Wildegger et al.37 to another feature dimension (i.e., color). 2ff7e9595c
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