Tanja C.W. Nijboer

Attended but unseen: Visual attention is not sufficient for visual awareness

Does any one psychological process give rise to visual awareness? One candidate is selective attention-when we attend to something it seems we always see it. But if attention can selectively enhance our response to an unseen stimulus then attention cannot be a sufficient precondition for awareness. Kentridge, Heywood & Weiskrantz [Kentridge, R. W., Heywood, C. A., & Weiskrantz, L. (1999). Attention without awareness in blindsight. Proceedings of the Royal Society of London, Series B, 266, 1805-1811; Kentridge, R. W., Heywood, C. A., & Weiskrantz, L. (2004). Spatial attention speeds discrimination without awareness in blindsight. Neuropsychologia, 42, 831-835.] demonstrated just such a dissociation in the blindsight subject GY. Here, we test whether the dissociation generalizes to the normal population. We presented observers with pairs of coloured discs, each masked by the subsequent presentation of a coloured annulus. The discs acted as primes, speeding discrimination of the colour of the annulus when they matched in colour and slowing it when they differed. We show that the location of attention modulated the size of this priming effect. However, the primes were rendered invisible by metacontrast-masking and remained unseen despite being attended. Visual attention could therefore facilitate processing of an invisible target and cannot, therefore, be a sufficient precondition for visual awareness.

Predicting Functional Outcome after Stroke: The Influence of Neglect on Basic Activities in Daily Living

One prominent deficit resulting from stroke is visuo-spatial neglect, which has been associated with slower and more attenuated recovery patterns of sensory-motor impairment as well as limitations in activities of daily living (ADL). The aim of the current study was to further specify the relationship between neglect and recovery of different domains of ADL. One hundred eighty four patients were assessed with the Functional Independence Measure in the first week of inpatient rehabilitation, and again at 6, 12, and 36 months post-stroke. On average, neglect patients scored significantly lower on Self-care, Transfers, and Locomotion compared to non-neglect patients, but these differences became smaller with progress of time. Overall, no differences between groups were found for Sphincter control and Cognition. Patients with more severe neglect scored significantly lower on Self-care and Transfers compared to patients with mild neglect. During rehabilitation, it would be of importance to test for independence in ADL domains in neglect in order to define realistic treatment goals. The current findings could be taken into account in early multidisciplinary intervention planning in the sub-acute phase, to optimize regaining ADL.

The Impact of Recovery of Visuo-Spatial Neglect on Motor Recovery of the Upper Paretic Limb after Stroke

The aim of the current study was to investigate the longitudinal relationship between improvements of synergism and strength of the upper paretic limb and severity of visuo-spatial neglect during the first 52 weeks post-stroke. The longitudinal association between severity of VSN and motor impairment using Fugl Meyer motor score and Motricity Index of the arm was measured in an intensive repeated measurement design including 18 measurement sessions for each subject. Neglect was assessed using the letter cancellation test applied in a prospective cohort of 101 ischemic, first-ever, hemispheric stroke patients. All time-dependent measures were taken weekly, starting within 14 days post-stroke. From week 10 to 20 biweekly measurements are obtained. The longitudinal relationship of (bi)weekly time on improvement of motor functions and severity of neglect was investigated using random coefficient analysis and trend analyses. Fifty-one of the 101 stroke patients showed neglect at stroke onset. Less improvement of synergism and strength of the upper paretic limb was associated with more severe neglect. This association was most pronounced in the first 10 weeks post-stroke. The seemingly suppressive effect of neglect on upper-limb motor recovery appears to take place mainly during spontaneous neurological recovery of first 10 weeks post-stroke. This finding suggests that damage to large-scale white matter tracts of especially the perceptual-attention networks suppress recovery of other networks at distance in the brain suggesting a common underlying mechanism.

Prism adaptation improves voluntary but not automatic orienting in neglect

Prism adaptation has been shown to temporarily ameliorate the symptoms of unilateral neglect. The underlying mechanisms of change are not yet fully understood. In this study, we investigate the influence of this treatment on attentional orienting under conditions of exogenous (peripheral onset) and endogenous (central symbolic) cueing. In one patient with left visual extinction and recovered neglect, and another patient with left visual neglect, visuo-motor adaptation to a rightward prismatic shift of 10° improved leftward orienting of attention following an endogenous but not an exogenous cue; leftward re-orienting of attention was also improved in the endogenous task for the second patient. We suggest that prism adaptation may ameliorate neglect by improving compensatory processes of leftward voluntary orienting, rather than by a fundamental change in attentional bias.

Repetitive long-term prism adaptation permanently improves the detection of contralesional visual stimuli in a patient with chronic neglect.

The aim of the current study was to investigate long-term effects in spatial awareness after daily exposure to prism adaptation during three months in a patient with hemispatial neglect. Results showed improvement in the detection of stimuli in the contralesional visual field, as measured with perimetry, in the contralesional visual field up to 24 months after ending prism adaptation. These perimetrical results suggest that compensatory eye movements are an unlikely candidate for an underlying mechanism.

Multiple dimensions in bi-directional synesthesia.

Grapheme‐color synesthetes report seeing a specific color when a number is perceived. The reverse, the synesthetic experience of a specific grapheme after the percept of a color is extremely rare. However, recent studies have revealed these interactions at both behavioral and neurophysiological levels. We investigated whether similar neuronal processes (i.e. perceptual and/or attentional) may underlie this bi‐directional interaction by measuring event‐related potentials (ERPs) during both a number‐color and color‐number priming task. In addition, we investigated the unitarity of synesthesia by comparing two distinct subtypes of synesthetes, projectors and associators, and assessed whether consistencies between measures (i.e. behavioral and electrophysiological) were present across synesthetes. Our results show longer reaction times for incongruent compared with congruent trials in both tasks. This priming effect is also present in the P3b latency (parietal electrode site) and P3a amplitude (frontal electrode site) of the ERP data. Interestingly, projector and associator synesthetes did not reveal distinct behavioral or electrophysiological patterns. Instead, a dissociation was found when synesthetes were divided in two groups on the basis of their behavioral data. Synesthetes with a large behavioral priming effect revealed ERP modulation at the frontal and parietal electrode sites, whereas synesthetes with a small priming effect revealed a frontal effect only. Together, these results show, for the first time, that similar neural mechanisms underlie bi‐directional synesthesia in synesthetes that do not report a synesthetic experience of a grapheme when a color is perceived. In addition, they add support for the notion of the existence of both ‘lower’ and ‘higher’ synesthetes.

Multisensory interactions in the depth plane in front and rear space: a review.

In this review, we evaluate the neurophysiological, neuropsychological, and psychophysical evidence relevant to the claim that multisensory information is processed differently depending on the region of space in which it happens to be presented. We discuss how the majority of studies of multisensory interactions in the depth plane that have been conducted to date have focused on visuotactile and audiotactile interactions in frontal peripersonal space and underline the importance of such multisensory interactions in defining peripersonal space. Based on our review of studies of multisensory interactions in depth, we question the extent to which peri- and extra-personal space (both frontal and rear) are characterized by differences in multisensory interactions (as evidenced by multisensory stimuli producing a different behavioral outcome as compared to unisensory stimulation). In addition to providing an overview of studies of multisensory interactions in different regions of space, our goal in writing this review has been to demonstrate that the various kinds of multisensory interactions that have been documented may follow very similar organizing principles. Multisensory interactions in depth that involve tactile stimuli are constrained by the fact that such stimuli typically need to contact the skin surface. Therefore, depth-related preferences of multisensory interactions involving touch can largely be explained in terms of their spatial alignment in depth and their alignment with the body. As yet, no such depth-related asymmetry has been observed in the case of audiovisual interactions. We therefore suggest that the spatial boundary of peripersonal space and the enhanced audiotactile and visuotactile interactions that occur in peripersonal space can be explained in terms of the particular spatial alignment of stimuli from different modalities with the body and that they likely reflect the result of prior multisensory experience.

Exploring near and far regions of space: Distance- specific visuospatial neglect after stroke

Visuospatial neglect has been observed in the horizontal (left/right) and vertical (up/down) dimension and depends on the distance at which a task is presented (near/far). Previously, studies have mainly focused on investigating the overall severity of neglect in near and far space in a group of neglect patients instead of examining subgroups of neglect patients with different types of distance-specific neglect. We investigated the spatial specificity (near vs. far space), frequency, and severity of neglect in the horizontal and vertical dimensions in a large group of stroke patients. We used three tasks to assess neglect in near (30 cm) and far (120 cm) space: a shape cancellation, letter cancellation, and a line bisection task. Patients were divided into four groups based on their performance: a group without neglect (N–F–), a near only neglect (N+F–), a far only neglect (N–F+), and a near and far neglect group (N+F+). About 40% of our sample showed neglect. Depending on the task, N+F– was observed in 8 to 22% of the sample, whereas N–F+ varied between 8% and 11%, and N+F+ varied between 11% to 14% of the sample. The current findings indicate that horizontal and vertical biases in performance can be confined to one region of space and are task dependent. We recommend testing for far space neglect during neuropsychological assessments in clinical practice, because this cannot be diagnosed using standard paper-and-pencil tasks.

Prism adaptation alters spatial remapping in healthy individuals: evidence from double-step saccades.

The visual system is able to represent and integrate large amounts of information as we move our gaze across a scene. This process, called spatial remapping, enables the construction of a stable representation of our visual environment despite constantly changing retinal images. Converging evidence implicates the parietal lobes in this process, with the right hemisphere having a dominant role. Indeed, lesions to the right parietal lobe (e.g., leading to hemispatial neglect) frequently result in deficits in spatial remapping. Research has demonstrated that recalibrating visual, proprioceptive and motor reference frames using prism adaptation ameliorates neglect symptoms and induces neglect-like performance in healthy people - one example of the capacity for rapid neural plasticity in response to new sensory demands. Because of the influence of prism adaptation on parietal functions, the present research investigates whether prism adaptation alters spatial remapping in healthy individuals. To this end twenty-eight undergraduates completed blocks of a double-step saccade (DSS) task after sham adaptation and adaptation to leftward- or rightward-shifting prisms. The results were consistent with an impairment in spatial remapping for left visual field targets following adaptation to leftward-shifting prisms. These results suggest that temporarily realigning spatial representations using sensory-motor adaptation alters right-hemisphere remapping processes in healthy individuals. The implications for the possible mechanisms of the amelioration of hemispatial neglect after prism adaptation are discussed.

DEVELOPMENTAL COLOUR AGNOSIA

Colour agnosia concerns the inability to recognise colours despite intact colour perception, semantic memory for colour information, and colour naming. Patients with selective colour agnosia have been described and the deficit is associated with left hemisphere damage. Here we report a case study of a 43-year-old man who was referred to us with a stroke in his right cerebellar hemisphere. During the standard assessment it transpired that he was unable to name coloured patches. Detailed assessment of his colour processing showed that he suffers from a selective colour agnosia. As he claimed to have had this problem all his life, and the fact that the infratentorial infarct that he had incurred was in an area far away from the brain structures that are known to be involved in colour processing, we suggest that he is the first reported case of developmental colour agnosia.