Neural correlates of visual extinction or awareness in a series of patients with right temporoparietal damage

Patients with visual extinction following right-hemisphere damage can typically detect left visual field stimulation when it is presented in isolation, but tend to miss this when it is paired with competing concurrent right visual stimulation. Some single-case studies have provided preliminary evidence that right visual cortex may show residual activation for contralesional, extinguished visual stimuli. Here we go beyond prior work by using individual retinotopic mapping and online eye-tracking during fMRI to study activity in stimulus-responsive retinotopic visual cortex for a case series of four extinction patients. We found consistent activation of retinotopic right visual cortex for bilateral visual stimulation that resulted in left extinction. This residual unconscious activation included areas V1 to V3 and was not due to inadvertent eye movements. We also provide further evidence for the emerging view that awareness may require activity of frontal and parietal regions well beyond visual cortex.

The role of prestimulus activity in visual extinction

Patients with visual extinction following right-hemisphere damage sometimes see and sometimes miss stimuli in the left visual field, particularly when stimuli are presented simultaneously to both visual fields. Awareness of left visual field stimuli is associated with increased activity in bilateral parietal and frontal cortex. However, it is unknown why patients see or miss these stimuli. Previous neuroimaging studies in healthy adults show that prestimulus activity biases perceptual decisions, and biases in visual perception can be attributed to fluctuations in prestimulus activity in task relevant brain regions. Here, we used functional MRI to investigate whether prestimulus activity affected perception in the context of visual extinction following stroke. We measured prestimulus activity in stimulus-responsive cortical areas during an extinction paradigm in a patient with unilateral right parietal damage and visual extinction. This allowed us to compare prestimulus activity on physically identical bilateral trials that either did or did not lead to visual extinction. We found significantly increased activity prior to stimulus presentation in two areas that were also activated by visual stimulation: the left calcarine sulcus and right occipital inferior cortex. Using dynamic causal modelling (DCM) we found that both these differences in prestimulus activity and stimulus evoked responses could be explained by enhanced effective connectivity within and between visual areas, prior to stimulus presentation. Thus, we provide evidence for the idea that differences in ongoing neural activity in visually responsive areas prior to stimulus onset affect awareness in visual extinction, and that these differences are mediated by fluctuations in extrinsic and intrinsic connectivity.

•

Prestimulus activity in visual extinction affects perception.

•

The BOLD signal in two visual areas is indicative of perception in bilateral trials.

•

Intrinsic and extrinsic connectivity is elevated prior to perceived trials.

A mutation in the thyroid hormone receptor alpha gene

Thyroid hormones exert their effects through alpha (TRα1) and beta (TRβ1 and TRβ2) receptors. Here we describe a child with classic features of hypothyroidism (growth retardation, developmental retardation, skeletal dysplasia, and severe constipation) but only borderline-abnormal thyroid hormone levels. Using whole-exome sequencing, we identified a de novo heterozygous nonsense mutation in a gene encoding thyroid hormone receptor alpha (THRA) and generating a mutant protein that inhibits wild-type receptor action in a dominant negative manner. Our observations are consistent with defective human TRα-mediated thyroid hormone resistance and substantiate the concept of hormone action through distinct receptor subtypes in different target tissues.

Prism adaptation does not change the rightward spatial preference bias found with ambiguous stimuli in unilateral neglect

Previous research has shown that prism adaptation (prism adaptation) can ameliorate several symptoms of spatial neglect after right-hemisphere damage. But the mechanisms behind this remain unclear. Recently we reported that prisms may increase leftward awareness for neglect in a task using chimeric visual objects, despite apparently not affecting awareness in a task using chimeric emotional faces (Sarri et al., 2006). Here we explored potential reasons for this apparent discrepancy in outcome, by testing further whether the lack of a prism effect on the chimeric face task task could be explained by: i) the specific category of stimuli used (faces as opposed to objects); ii) the affective nature of the stimuli; and/or iii) the particular task implemented, with the chimeric face task requiring forced-choice judgements of lateral 'preference' between pairs of identical, but left/right mirror-reversed chimeric face tasks (as opposed to identification for the chimeric object task). We replicated our previous pattern of no impact of prisms on the emotional chimeric face task here in a new series of patients, while also similarly finding no beneficial impact on another lateral 'preference' measure that used non-face non-emotional stimuli, namely greyscale gradients. By contrast, we found the usual beneficial impact of prism adaptation (prism adaptation) on some conventional measures of neglect, and improvements for at least some patients in a different face task, requiring explicit discrimination of the chimeric or non-chimeric nature of face stimuli. The new findings indicate that prism therapy does not alter spatial biases in neglect as revealed by 'lateral preference tasks' that have no right or wrong answer (requiring forced-choice judgements on left/right mirror-reversed stimuli), regardless of whether these employ face or non-face stimuli. But our data also show that prism therapy can beneficially modulate some aspects of visual awareness in spatial neglect not only for objects, but also for face stimuli, in some cases.

Task-related modulation of visual neglect in cancellation tasks

Unilateral neglect involves deficits of spatial exploration and awareness that do not always affect a fixed portion of extrapersonal space, but may vary with current stimulation and possibly with task demands. Here, we assessed any 'top-down', task-related influences on visual neglect, with novel experimental variants of the cancellation test. Many different versions of the cancellation test are used clinically, and can differ in the extent of neglect revealed, though the exact factors determining this are not fully understood. Few cancellation studies have isolated the influence of top-down factors, as typically the stimuli are changed also when comparing different tests. Within each of three cancellation studies here, we manipulated task factors, while keeping visual displays identical across conditions to equate purely bottom-up factors. Our results show that top-down task demands can significantly modulate neglect as revealed by cancellation on the same displays. Varying the target/non-target discrimination required for identical displays has a significant impact. Varying the judgement required can also have an impact on neglect even when all items are targets, so that non-targets no longer need filtering out. Requiring local versus global aspects of shape to be judged for the same displays also has a substantial impact, but the nature of discrimination required by the task still matters even when local/global level is held constant (e.g. for different colour discriminations on the same stimuli). Finally, an exploratory analysis of lesions among our neglect patients suggested that top-down task-related influences on neglect, as revealed by the new cancellation experiments here, might potentially depend on right superior temporal gyrus surviving the lesion.

Prism adaptation aftereffects in stroke patients with spatial neglect: pathological effects on subjective straight ahead but not visual open-loop pointing

Prism adaptation to rightward optical shifts during visually guided pointing is considered a promising intervention in right-hemisphere stroke patients with left spatial neglect. Conventionally, prism adaptation is assessed via aftereffects, on subjective straight ahead (SSA) pointing with eyes closed; or by visual open-loop pointing (VOL), i.e. pointing to a visual target without seeing the hand. Previous data suggest indirectly that prism aftereffects in neglect patients may be larger (pathologically so) when assessed by SSA than by VOL. But these measures have never been directly compared within the same patients after identical prism exposure. Accordingly we implemented both measures here within the same group of 13 neglect patients and 13 controls. Prism aftereffects were much larger for SSA than VOL in neglect patients, falling outside the normative range only for SSA. This may arise because the SSA task can itself involve aspects of neglect that may be ameliorated by the prism intervention, hence showing abnormal changes after prisms. The extent of SSA change after prisms varied between patients, and correlated with improvements on a standard cancellation measure for neglect. The lesions of patients who did versus did not show neglect improvement immediately after prisms provide an initial indication that lack of improvement may potentially relate to cortical damage in right intraparietal sulcus and white matter damage in inferior parietal lobe and middle frontal gyrus. Future studies of possible rehabilitative impact from prisms upon neglect may need to consider carefully how to measure prism adaptation per se, separately from any impact of such adaptation upon manifestations of neglect.

Neural correlates of crossmodal visual-tactile extinction and of tactile awareness revealed by fMRI in a right-hemisphere stroke patient

We used fMRI to study the neural correlates of crossmodal, visual-tactile extinction in a single case (patient GK). GK has chronic extinction after a lesion centred on right inferior parietal cortex, and has previously been investigated extensively in purely visual fMRI studies [e.g. Rees, G., Wojciulik, E., Clarke, K., Husain, M., Frith, C., & Driver, J. (2000). Unconscious activation of visual cortex in the damaged right hemisphere of a parietal patient with extinction. Brain, 123(Pt 8), 1624-1633; Rees, G., Kreiman, G., & Koch, C. (2002). Neural correlates of consciousness in humans. Nature Reviews Neuroscience, 3(4), 261-270]. With concurrent stimulation of the right visual field plus left index finger, GK showed crossmodal extinction of left touch on approximately half of such trials here (reflecting impaired sensitivity, i.e. lowered d-prime), albeit becoming aware of left touch on the other half. fMRI revealed activation of contralateral primary somatosensory cortex on crossmodal trials when touch was extinguished from awareness, suggesting unconscious residual processing there. When GK became aware of the left touch, additional activation was found in surviving right parietal cortex, and in frontal regions; moreover, functional coupling was enhanced with a region of frontal cortex implicated in awareness by previous work. Finally, on trials where crossmodal extinction arose, surviving right parietal cortex showed stronger functional coupling with the left visual and right somatosensory regions driven by the competing stimuli, indicating that crossmodal extinction arises when inputs to separate modalities interact competitively via multimodal cortex.

Prism adaptation changes perceptual awareness for chimeric visual objects but not for chimeric faces in spatial neglect after right-hemisphere stroke

Prism adaptation can ameliorate some symptoms of left spatial neglect after right-hemisphere stroke. The mechanisms behind this remain unclear. Prism therapy may increase exploration towards the contralesional side, yet without improving perceptual awareness, as apparently for the left side of chimeric face stimuli (Ferber et al. 2003). However, other prism studies suggest that perceptual awareness might be improved (e.g., Maravita et al., 2003). We tested the impact of prism therapy on visual awareness for the left side of chimeric objects as well as chimeric faces, in three neglect patients. Prism therapy dramatically improved awareness for the identity of the left side of chimeric non-face objects, but had no effect on judging expressions for chimeric faces. The latter may thus be unique in showing no prism benefit.