Visual search—looking for a target object in the presence of a number of distractor items—is an everyday activity for humans (for example, finding the car in a busy car park) and animals (for ...example, foraging for food). Our understanding of visual search has been enriched by an interdisciplinary effort using a wide range of research techniques including behavioural studies in humans 1, single-cell electrophysiology 2, transcranial magnetic stimulation 3, event-related potentials 4 and studies of patients with focal brain injury 5. A central question is what kind of information controls the search process. Visual search is typically accompanied by a series of eye movements, and investigating the nature and location of fixations helps to identify the kind of information that might control the search process. It has already been demonstrated that objects are fixated if they are visually similar to the target 6. Also, if an item has been fixated, it is less likely to be returned to on the subsequent saccade. This automatic process is referred to as inhibition of return (IOR 7,8). Here, we investigated the role of memory for which items had been fixated previously. We found that, during search, subjects often refixated items that had been previously fixated. Although there were fewer return saccades than would be expected by chance, the number of refixations indicated limited functional memory, indeed the memory effects that were present may primarily be a result of IOR.
► Patients with left visual neglect can correct their reaches to left jumps, even when instructed to stop their movements. ► Such corrections are slowed in neglect patients when the target jumps to ...the left. ► The ‘automatic pilot’ system is functional in neglect, but its processing is slowed on the left. ► Neglect patients are impaired at stopping their movements in response to both left and right target jumps.
It is widely accepted that the posterior parietal cortex is critical for the on-line control of action and optic ataxia patients are unable to correct their movements in-flight to changes in target position. The current study investigated on-line correction in patients with left visual neglect, right brain damaged patients without neglect and healthy controls. Participants were asked to reach towards a central target that could jump unexpectedly, at movement onset, to the right or left sides of space. In response to the jump, participants were asked either to follow the target or to stop their movement. Neglect patients were able to correct their ongoing movements smoothly and accurately towards right and left target jumps. They did so even when told to stop their movement, indicating that these corrections occurred automatically (i.e., without instruction). However, the timing of corrections to the left was delayed in neglect patients and this produced a drastic increase in movement time. To our surprise, we also found that neglect patients were impaired at stopping their ongoing reaches, when compared to the control groups, in response to either left or right jump trials. We suggest that the ‘automatic pilot’ system for the hand is spared in neglect, but its processing speed is unilaterally slowed due to a deficit in orienting of attention to the contralesional side. We relate these findings to the breakdown of a system that combines information for attention, perception and action. Damage to this system may not only slow corrective movements to the contralesional side, but also produce non-lateralized deficits in interrupting an ongoing reach.
Experiments using chimeric faces, where the left and the right hand side of the face are different, have shown that observers tend to bias their responses toward the information on the left. Here we ...investigate the effects of aging as well as exposure duration on this leftward bias. Forty female and male blended as well as chimeric faces were presented to 24 young and 23 elderly adults in either sub-saccadic 100
msec, 300
msec or free view conditions. We found firstly that an increase in exposure duration resulted in an increase in the degree of leftward perceptual biases, irrespective of age, in line with hypotheses stressing the contribution of scanning to chimeric face processing. Secondly, fundamental differences in the perceptual biases between the groups were found in so far that the younger subjects demonstrated significant perceptual biases to chimeric face stimuli even at sub-saccadic exposure durations, whilst for older adults this was the case for the 300
msec and free view conditions only. This differential perceptual activity can be viewed in terms of either reduced right hemispheric function, or increased bilateral function as a possible consequence of elderly adults experiencing the task as more effortful.
Pre‐stimulus oscillatory neural activity has been linked to the level of awareness of sensory stimuli. More specifically, the power of low‐frequency oscillations (primarily in the alpha‐band, i.e., ...8–14 Hz) prior to stimulus onset is inversely related to measures of subjective performance in visual tasks, such as confidence and visual awareness. Intriguingly, the same EEG signature does not seem to influence objective measures of task performance (i.e., accuracy). We here examined whether this dissociation holds when stringent accuracy measures are used. Previous EEG‐studies have employed 2‐alternative forced choice (2‐AFC) discrimination tasks to link pre‐stimulus oscillatory activity to correct/incorrect responses as an index of accuracy/objective performance at the single‐trial level. However, 2‐AFC tasks do not provide a good estimate of single‐trial accuracy, as many of the responses classified as correct will be contaminated by guesses (with the chance correct response rate being 50%). Here instead, we employed a 19‐AFC letter identification task to measure accuracy and the subjectively reported level of perceptual awareness on each trial. As the correct guess rate is negligible (~5%), this task provides a purer measure of accuracy. Our results replicate the inverse relationship between pre‐stimulus alpha/beta‐band power and perceptual awareness ratings in the absence of a link to discrimination accuracy. Pre‐stimulus oscillatory phase did not predict either subjective awareness or accuracy. Our results hence confirm a dissociation of the pre‐stimulus EEG power–task performance link for subjective versus objective measures of performance, and further substantiate pre‐stimulus alpha power as a neural predictor of visual awareness.
We employed a multiple‐alternative forced‐choice (m‐AFC) discrimination task to study the single‐trial EEG predictors of subjective reports of level of visual awareness, as well as accuracy. Our results replicate findings from a 2‐AFC task, showing an inverse relationship between pre‐stimulus alpha‐power and visual awareness, but no link to discrimination accuracy. This confirms a dissociation between subjective and objective measures of performance, with pre‐stimulus EEG power predicting the former.
Young adults typically display a processing advantage towards the left side of space (“pseudoneglect”), possibly as a result of right parietal dominance for spatial attention. This bias is ...ameliorated with age, with older adults displaying either no strongly lateralised bias, or a slight bias towards the right. This may represent an age-related reduction of right hemispheric dominance and/or increased left hemispheric involvement. Here, we applied anodal transcranial direct current stimulation (atDCS) to the right posterior parietal cortex (PPC; R-atDCS), the left PPC (L-atDCS) and a Sham protocol in young and older adults during a titrated lateralised visual detection task. We aimed to facilitate visual detection sensitivity in the contralateral visual field with both R-atDCS and L-atDCS relative to Sham. We found no differences in the effects of stimulation between young and older adults. Instead the effects of atDCS were state-dependent (i.e. related to task performance at baseline). Relative to Sham, poor task performers were impaired in both visual fields by anodal stimulation of the left posterior parietal cortex (PPC). Conversely, good performers maintained sensitivity in both visual fields in response to R-atDCS, although this effect was small. We highlight the importance of considering baseline task ability when designing tDCS experiments, particularly in older adults.
•P3, P4 and Sham atDCS applied during target detection in young and older adults.•Effects of atDCS are related to baseline performance rather than age.•Poor performers are impaired in both VFs with left anodal tDCS vs Sham.•Good performers maintain task sensitivity in both VFs with R-atDCS vs Sham.•This highlights the importance of considering baseline task performance in tDCS.
It is clear already that in current and future years more people will suffer from stroke, whether related to COVID-19 or not, and given its prevalence, many more people's lives will be affected by ...neglect. Here we hope to have contributed to its possible amelioration with highlights of the latest thinking on neglect diagnosis, prevalence and treatment.
► Patients with hemispatial neglect present with severe visuospatial impairments. ► Some investigations have found that perceptual difficulties of neglect patients are mirrored in their actions. ► ...Other studies have shown neglect patients to perform relatively better in action tasks. ► We reconcile these diverging findings, addressing differences in the type of visuomotor actions required in particular. ► We also highlight the diverging neuroanatomy that seems to be driving these differences in performance.
It is well established that patients with hemispatial neglect present with severe visuospatial impairments, but studies that have investigated visuomotor control directly have revealed diverging results, with some investigations finding impairments mirroring the perceptual difficulties of these patients, while others have shown that such neglect patients perform relatively better in action tasks.
In this review we attempt to reconcile these diverging findings, addressing differences in the type of visuomotor tasks studied but also highlighting the diverging neuroanatomy that seems to be driving the differences in performance.
We argue that there are different types of actions and that these in turn depend on different cortical networks (Goodale, Westwood, & Milner, 2004; Milner & Goodale, 2006). Patients with visuospatial neglect, in contrast to patients with optic ataxia, are relatively unimpaired at performing target-directed tasks even towards stimuli located in their ‘neglected’ field. We relate these findings to the view that for the on-line guidance of action, spatial information is coded in egocentric coordinates and depends on the visuomotor networks of the visual dorsal stream. Furthermore, based on recent lesion-symptom mapping studies, we postulate that deficits in on-line actions that are observed after right-brain damage are associated with damage to the visuomotor control network, in particular with damage to the basal ganglia, frontal and parieto-occipital regions. On the other hand, clear neglect-specific deficits emerge when the action is off-line and not directly target-driven, thus requiring relational metrics or scene-based coordinates (as is the case for example in delayed and mirrored (anti-pointing) reaches). We review recent studies that support our argument that such deficits in off-line actions are associated with damage to occipito-temporal and parahippocampal cortex, perhaps as part of the ventral visual stream or areas where information from the two visual streams is combined.
Human perception of perithreshold stimuli critically depends on oscillatory EEG activity prior to stimulus onset. However, it remains unclear exactly which aspects of perception are shaped by this ...pre‐stimulus activity and what role stochastic (trial‐by‐trial) variability plays in driving these relationships. We employed a novel jackknife approach to link single‐trial variability in oscillatory activity to psychometric measures from a task that requires judgement of the relative length of two line segments (the landmark task). The results provide evidence that pre‐stimulus alpha fluctuations influence perceptual bias. Importantly, a mediation analysis showed that this relationship is partially driven by long‐term (deterministic) alpha changes over time, highlighting the need to account for sources of trial‐by‐trial variability when interpreting EEG predictors of perception. These results provide fundamental insight into the nature of the effects of ongoing oscillatory activity on perception. The jackknife approach we implemented may serve to identify and investigate neural signatures of perceptual relevance in more detail.
We employed a jackknife method to link pre‐stimulus neural oscillations to psychometric measures during line bisection performance. The analysis identified both spontaneous trial‐by‐trial (stochastic) and systematic long‐term (deterministic) predictors of visuospatial bias, but no predictors of visual sensitivity. The approach we implemented helps to further characterize the influence of ongoing neural activity on perception and cognition.
PDF
