Human thalamus contributes to perceptual stability across eye movements.

We continuously move our eyes when we inspect a visual scene. Although this leads to a rapid succession of discontinuous and fragmented retinal snapshots, we perceive the world as stable and coherent. Neural mechanisms underlying visual stability may depend on internal monitoring of planned or ongoing eye movements. In the macaque brain, a pathway for the transmission of such signals has been identified that is relayed by central thalamic nuclei. Here, we studied a possible role of this pathway for perceptual stability in a patient with a selective lesion affecting homologous regions of the human thalamus. Compared with controls, the patient exhibited a unilateral deficit in monitoring his eye movements. This deficit was manifest by a systematic inaccuracy both in successive eye movements and in judging the locations of visual stimuli. In addition, perceptual consequences of oculomotor targeting errors were erroneously attributed to external stimulus changes. These findings show that the human brain draws on transthalamic monitoring signals to bridge the perceptual discontinuities generated by our eye movements.

Dissociable spatial and non-spatial attentional deficits after circumscribed thalamic stroke.

Thalamic nuclei act as sensory, motor and cognitive relays between multiple subcortical areas and the cerebral cortex. They play a crucial role in cognitive functions such as executive functioning, memory and attention. In the acute period after thalamic stroke attentional deficits are common. The precise functional relevance of specific nuclei or vascular sub regions of the thalamus for attentional sub functions is still unclear. The theory of visual attention (TVA) allows the measurement of four independent attentional parameters (visual short term memory storage capacity (VSTM), visual perceptual processing speed, selective control and spatial weighting). We combined parameter-based assessment based on TVA with lesion symptom mapping in standard stereotactic space in sixteen patients (mean age 41.2 ± 11.0 SD, 6 females), with focal thalamic lesions in the medial (N = 9), lateral (N = 5), anterior (N = 1) or posterior (N = 1) vascular territories of the thalamus. Compared with an age-matched control group of 52 subjects (mean age 40.1 ± 6.4, 35 females), the patients with thalamic lesions were, on the group level, mildly impaired in visual processing speed and VSTM. Patients with lateral thalamic lesions showed a deficit in processing speed while all other TVA parameters were within the normal range. Medial thalamic lesions can be associated with a spatial bias and extinction of targets either in the ipsilesional or the contralesional field. A posterior case with a thalamic lesion of the pulvinar replicated a finding of Habekost and Rostrup (2006), demonstrating a spatial bias to the ipsilesional field, as suggested by the neural theory of visual attention (NTVA) (Bundesen, Habekost, & Kyllingsbæk, 2011). A case with an anterior-medial thalamic lesion showed reduced selective attentional control. We conclude that lesions in distinct vascular sub regions of the thalamus are associated with distinct attentional syndromes (medial = spatial bias, lateral = processing speed).

A role of the human thalamus in predicting the perceptual consequences of eye movements.

Internal monitoring of oculomotor commands may help to anticipate and keep track of changes in perceptual input imposed by our eye movements. Neurophysiological studies in non-human primates identified corollary discharge (CD) signals of oculomotor commands that are conveyed via thalamus to frontal cortices. We tested whether disruption of these monitoring pathways on the thalamic level impairs the perceptual matching of visual input before and after an eye movement in human subjects. Fourteen patients with focal thalamic stroke and 20 healthy control subjects performed a task requiring a perceptual judgment across eye movements. Subjects reported the apparent displacement of a target cue that jumped unpredictably in sync with a saccadic eye movement. In a critical condition of this task, six patients exhibited clearly asymmetric perceptual performance for rightward vs. leftward saccade direction. Furthermore, perceptual judgments in seven patients systematically depended on oculomotor targeting errors, with self-generated targeting errors erroneously attributed to external stimulus jumps. Voxel-based lesion-symptom mapping identified an area in right central thalamus as critical for the perceptual matching of visual space across eye movements. Our findings suggest that trans-thalamic CD transmission decisively contributes to a correct prediction of the perceptual consequences of oculomotor actions.

A dysexecutive syndrome of the medial thalamus.

Thalamic stroke is associated with neurological and cognitive sequelae. Resulting neuropsychological deficits vary with the vascular territory involved. Whereas sensory, motor and memory deficits following thalamic stroke are comparatively well characterized, the exact relationship between executive dysfunction and thalamic damage remains more ambiguous. To assess the pattern of executive-cognitive deficits following thalamic stroke and its possible association with distinct thalamic nuclei, 19 patients with focal thalamic lesions were examined with high-resolution structural imaging and neuropsychological testing. Twenty healthy individuals served as controls. Patient MRIs were co-registered to an atlas of the human thalamus. Lesion overlap and subtraction analyses were used for lesion-to-symptom mapping. In eight patients (42.1%), neuropsychological assessment demonstrated a disproportionate deficit in the Wisconsin Card Sorting Test (WCST), while other executive and memory functions were much less affected. Subtraction analysis revealed an area in the left medial thalamus, mainly consisting of the centromedian and parafascicular nuclei (CM-Pf complex) that was damaged in these patients and spared in patients with normal WCST performance. Thus, damage to the CM-Pf complex may yield a distinct dysexecutive syndrome in which deficient maintenance and shifting between cognitive sets predominates. We hypothesize that the CM-Pf complex may contribute to maintenance and shifting of cognitive sets by virtue of its dense connections with the striatum. The pattern of executive dysfunction following thalamic stroke may vary considerably with lesion location.

Subjective cognitive-affective status following thalamic stroke.

Previous patient studies suggest that thalamic stroke may yield persistent deficits in several cognitive domains. At present, the subjective dimension and everyday relevance of these impairments is unclear, since many patients with thalamic stroke only show minor changes on physical examination. Here, we have studied subjective consequences of focal thalamic lesions. A sample of 68 patients with a history of ischemic thalamic stroke was examined by using established clinical self-report questionnaires assessing memory, attention, executive functions, emotional status and health-related quality of life. In order to control for general factors related to cerebrovascular disease, self-reports were compared to an age-matched group of 34 patients with a history of transient ischemic attack. Thalamic lesions were co-registered to an atlas of the human thalamus. Lesion overlap and subtraction analyses were used for lesion-to-symptom mapping. When both patient groups were compared, no significant differences were found for either questionnaire. However, when subgroups were compared, patients with infarctions involving the posterior thalamus showed significant emotional disturbances and elevated anxiety levels compared to patients with more anterior lesions. Our findings thus point to the existence of a persistent affective impairment associated with chronic lesions of the posterior thalamus. This syndrome may result from damage to connections between medial pulvinar and extra-thalamic regions involved in affective processing. Our findings suggest that the posterior thalamus may contribute significantly to the regulation of mood.