Shani Shalgi

On the positive side of error processing: error-awareness positivity revisited

Performance errors are indexed in the brain even if they are not consciously registered, as demonstrated by the error‐related negativity (ERN or Ne) event‐related potential. It has recently been shown that another response‐locked potential, the error positivity (Pe), follows the Ne, but only in those trials in which the participants consciously detect making the error (‘Aware Errors’). In the present study we generalize these findings to an auditory task and investigate possible caveats in the interpretation of the Pe as an index of error awareness. In an auditory Go/No‐Go error‐awareness task (auditory EAT) participants pressed an additional ‘fix error’ button after noticing that they had made an error. As in visual tasks, the Ne was similar for aware (‘fixed’) and unaware (‘unfixed’) errors, while the Pe was enhanced only for Aware Errors. Within subjects, the Ne and Pe behaved in similar fashions for auditory and visual errors. A control condition confirmed that the awareness effect was not due to the requirement to report error awareness. These results reinforce the evidence in favor of the Pe as a correlate of conscious error processing, and imply that this process is not modality‐specific. Nevertheless, single‐trial analysis suggested that the Pe may be a delayed P3b related to stimulus processing rather than to response monitoring.

Direct Evidence for Differential Roles of Temporal and Frontal Components of Auditory Change Detection.

Automatic change detection is a fundamental capacity of the human brain. In audition, this capacity is indexed by the mismatch negativity (MMN) event-related potential, which is putatively supported by a network consisting of superior temporal and frontal nodes. The aim of this study was to elucidate the roles of these nodes within the neural network of change detection. We used a dichotic paradigm in which subjects (N=14) attended targets defined by either pitch or spatial location in one auditory stream while the MMN was measured in response to unattended deviants of pitch and spatial location in the other stream. The frontal and temporal components of the MMN were examined using current source density (CSD) measurements. Competition for processing resources nearly eliminated the temporal CSD mismatch response, in a highly feature-specific manner: the response to spatial location deviants was reduced when the target dimension was spatial location but not when it was pitch, whereas the reverse occurred for pitch deviants. In contrast, the frontal CSD mismatch response was neither affected by competition nor by general attention demands. Thus, within the network of change detection, the temporal generators are specifically associated with processing feature-specific information, whereas the role of the frontal generators remains unclear. Moreover, the results are inconsistent with a serial model in which the frontal generator is contingent on activation of the temporal generator.

Left visual-field advantage in the dual-stream RSVP task and reading-direction: A study in three nations

In the dual-stream Rapid Serial Visual Presentation task, a stream of stimuli containing two target stimuli is rapidly presented left and right. In previous studies, the second target was better identified in the left than in the right hemifield. In all those studies, alphanumeric stimuli were used both as targets and distracters. We examined to what extent this left visual-field advantage is dependent on reading-direction. The task was performed by Germans (with Latin characters), Israelis (with Latin and Hebrew characters) and Taiwanese (with Latin and Chinese characters). If caused by overlearnt associative links between Latin characters and left-to-right reading, the prominent left visual-field bias should be reversed in Hebrew and disappear in Chinese. Furthermore, if caused by direction of reading in the participants native language, the left visual-field advantage in Latin conditions should be larger in Germans than in Israelis and Taiwanese. A left visual-field advantage was always observed, though slightly smaller in Hebrew and in Chinese, and there was no difference in the Latin conditions between the three nations. Therefore, it seems that the left visual-field advantage in speeded target identification is not primarily caused by the left-to-right reading-direction, but may be a combined effect resulting from the asymmetric organization of general mechanisms of visual processing and from stimulus-induced preferences.

Absent minded but accurate: delaying responses increases accuracy but decreases error awareness

Previous work has suggested that conscious error awareness may fluctuate with levels of attention. Here, we explore this relationship by showing that error awareness can be impaired when exogenous support to attentional systems is reduced by decreasing task demands. Twenty participants performed a manual Go/No-Go response-inhibition task optimized to examine error awareness. In one condition (Immediate), participants were asked to respond as quickly and as accurately as possible to each Go stimulus, and in the other condition (Delayed) they were asked to time their responses to the offset of the stimulus, thereby decreasing task difficulty and imposing a more automated response set. As expected, speeding increased the error rate. However, contrary to the expectation (and to participants’ subjective reports) that speeding would impair awareness of performance, we found the opposite to be true: errors were more likely to be unnoticed when the task was easier. We suggest that this tradeoff reflects two qualitatively different types of errors arising from the different cognitive demands of the Immediate and Delayed conditions. We propose that unaware errors reflect pure lapses of sustained attention and are therefore more susceptible to changes in task demands, while aware errors mostly reflect failures to inhibit responses, and are therefore most susceptible to increased response speed.

Is any awareness necessary for an Ne

The Error-Related Negativity (Ne or ERN) is a reliable electrophysiological index of error processing, which has been found to be independent of whether a subject is aware of an error or not. A large Ne was equally seen after errors that were consciously detected (Aware errors) and those that were not (Unaware errors), compared to a small negativity for correct responses (CRN). This suggests a dissociation between an automatic, preconscious error processing mechanism and subjective evaluation. A common concern regarding this finding is that subjects could have been somewhat aware of their errors, but did not report them due to lack of confidence. Here we tested this possibility directly using a betting paradigm which allowed us to separate occasions in which the subjects were confident of their response and trials in which they were unsure. In a choice reaction time task, subjects directly judged the accuracy of each response (correct or error) and then bet on this judgment using a high, medium, or low amount of money. The bets were used to determine the level of confidence the subjects had of their response. The average across all subjects regardless of confidence (betting) measure replicated the reported finding of an equal Ne for Aware and Unaware errors which was larger than the CRN. However, when Ne measurement was confined to high confidence (high bet) trials in confident subjects, a prominent Ne was seen only for Aware errors, while confident Unaware errors (i.e., error trials on which subjects made high bets that they were correct) elicited a response that did not differ from the CRN elicited by truly correct answers. In contrast, for low confidence trials in unconfident subjects, an intermediate and equal Ne/CRN was elicited by Correct responses, Aware and Unaware errors. These results provide direct evidence that the Ne is related to error awareness, and suggest the amplitude of the Ne/CRN depends on individual differences in error reporting and confidence.

Synchronous contextual irregularities affect early scene processing: replication and extension.

Whether contextual regularities facilitate perceptual stages of scene processing is widely debated, and empirical evidence is still inconclusive. Specifically, it was recently suggested that contextual violations affect early processing of a scene only when the incongruent object and the scene are presented a-synchronously, creating expectations. We compared event-related potentials (ERPs) evoked by scenes that depicted a person performing an action using either a congruent or an incongruent object (e.g., a man shaving with a razor or with a fork) when scene and object were presented simultaneously. We also explored the role of attention in contextual processing by using a pre-cue to direct subjects׳ attention towards or away from the congruent/incongruent object. Subjects׳ task was to determine how many hands the person in the picture used in order to perform the action. We replicated our previous findings of frontocentral negativity for incongruent scenes that started ~ 210 ms post stimulus presentation, even earlier than previously found. Surprisingly, this incongruency ERP effect was negatively correlated with the reaction times cost on incongruent scenes. The results did not allow us to draw conclusions about the role of attention in detecting the regularity, due to a weak attention manipulation. By replicating the 200-300 ms incongruity effect with a new group of subjects at even earlier latencies than previously reported, the results strengthen the evidence for contextual processing during this time window even when simultaneous presentation of the scene and object prevent the formation of prior expectations. We discuss possible methodological limitations that may account for previous failures to find this an effect, and conclude that contextual information affects object model selection processes prior to full object identification, with semantic knowledge activation stages unfolding only later on.

The neurophysiology of bi-directional synesthesia (Commentary on Gebuis et al.).

Synesthesia research, which just a decade ago was considered a decidedly esoteric, and perhaps even a rather dubious area of study for a ‘serious’ neuroscientist, has since emerged as an area of vigorous interest. On the one hand, there is simply no denying the fascination that this unusual condition engenders. More important though, are the potential implications of synesthesia for theories of functional brain connectivity, multisensory integration and perceptual binding. One of the most common and therefore widely studied forms of synesthesia is the graphemecolor (GC) variety, where standard monochromatic letters and digits elicit idiosyncratic color percepts. That is, what typical observers see as black letters on a white page appear as a multicolored array to these synesthetes. Recently, it has been shown that GC synesthesia can be bidirectional (Knoch et al., 2005; Cohen Kadosh & Henik, 2006; Cohen Kadosh et al., 2007), with visualized colors eliciting number or letter percepts. This finding adds a new dimension to synesthesia, previously thought to be a strictly one-way phenomenon. However, before this finding can be incorporated into models of synesthesia, it must first be established whether similar neuronal mechanisms underlie both directions of synesthesia. In this issue of EJN, Gebuis et al. (2009) investigated this question by inducing bi-directional synesthesia whilst recording event related potentials (ERPs), and comparing the responses to both directions. GC synesthetes had longer reaction times for incongruent compared to congruent trials in both number fi color and color fi number priming tasks compared to controls. That is, synesthetes are negatively impacted (have longer reaction times) when the number is primed by a color, or color is primed by a number, other than the one that they have a specific color-number mapping for and vice-versa, whereas control subjects are oblivious to this distinction. Turning to the electrophysiological responses, incongruent trials in both tasks elicited larger frontal P3a amplitudes and longer parietal P3b latencies for the synesthetes. These effects did not differ between tasks, implying that the neurophysiological correlates underlying synesthesia are similar in both directions. However, the effects did differ between two types of synesthetes: those who had a small behavioral effect showed only the frontal modulation, while those who had a large behavioral effect showed both the frontal and the parietal modulations. Gebuis et al. (2009) interpret these findings as support for a distinction between ‘higher’ and ‘lower’ synesthetes (Hubbard et al., 2005). ‘Higher’ synesthetes’ experiences are elicited by the concept of the trigger (top-down attentional ⁄ inhibitory processes) whereas the experience of ‘lower’ synesthetes is elicited by the percept of the trigger (bottom-up perceptual processes). Based on their ERP results, Gebuis et al. (2009) hypothesize that synesthetic experiences require top-down processes, but for some synesthetes (the ‘lower’ group) bottom-up processes are also required. Presumably, these synesthetes also have a stronger behavioral effect. Some of our own recent work using ERPs also concerns this issue of feedforward vs. feedback mechanisms (Barnett et al., 2008). We took a rather different approach in that we decided not to look at orthographic stimuli at all in our GC synesthetes. Instead, using ERP recordings, we simply asked if there were basic sensory processing differences early in the visual system in a large cohort of synesthetes in response to simple visual stimuli that did not induce synesthetic experiences. We found clear evidence of very early differences in visual sensory processing that began just 70–100 ms after stimulus presentation, which is only 25–30 ms after initial afference in the primary visual cortex. These data suggest a basic difference in the wiring of the synesthetic brain, a difference that impacts very early sensory processing and implicates bottom-up processes in GC synesthesia. While these results and those of Gebuis et al. (2009) add to the ongoing debate regarding whether synesthetic experience arises from early or later processing stages, they do not resolve this issue, but they stress how synesthesia is far from a unitary phenomenon, and the importance of taking into account individual differences among synesthetes in future research. Interestingly, Gebuis et al. (2009) did not find any behavioral or electrophysiological differences between ‘projector’ and ‘associator’ synesthetes. The former are those who experience the grapheme ‘physically’ colored, the latter are those who experience the color in the ‘mind’s eye’. When Gebuis et al. (2009) divided their synesthetes into two groups based on their behavioral results, both groups had the same number of associators and projectors. Thus, contrary to the suggestion made by Dixon & Smilek (2005), these results imply that that these two dimensions of individual differences (high vs. low, projector vs. associator) are dissociable rather than correlated. While many theories have been posited to explain the synesthetic experience, the Gebuis et al. (2009) study leaves open the interpretation of the electrophysiological effects and does not distinguish between (for example) the disinhibited feedback model (Grossenbacher & Lovelace, 2001) or the cross-activation model (Ramachandran & Hubbard, 2001). However, these models will have to incorporate the bi-directional feature of synesthesia and the results of this provocative study by Gebuis and colleagues, which suggests that similar neural mechanisms subserve both directions. European Journal of Neuroscience, Vol. 29, pp. 1701–1702, 2009 doi:10.1111/j.1460-9568.2009.06763.x