A pure case of Gerstmann syndrome with a subangular lesion

The four symptoms composing Gerstmann's syndrome were postulated to result from a common cognitive denominator (Grundstörung) by Gerstmann himself. He suggested that it is a disorder of the body schema restricted to the hand and fingers. The existence of a Grundstörung has since been contested. Here we suggest that a common psychoneurological factor does exist, but should be related to transformations of mental images rather than to the body schema. A patient (H.P.) was studied, who presented the four symptoms of Gerstmann's syndrome in the absence of any other neuropsychological disorders. MRI showed a focal ischaemic lesion, situated subcortically in the inferior part of the left angular gyrus and reaching the superior posterior region of T1. The cortical layers were spared and the lesion was seen to extend to the callosal fibres. On the basis of an extensive cognitive investigation, language, praxis, memory and intelligence disorders were excluded. The four remaining symptoms (finger agnosia, agraphia, right-left disorientation and dyscalculia) were investigated thoroughly with the aim of determining any characteristics that they might share. Detailed analyses of the tetrad showed that the impairment was consistently attributable to disorders of a spatial nature. Furthermore, cognitive tests necessitating mental rotation were equally shown to be impaired, confirming the essentially visuospatial origin of the disturbance. In the light of this report, the common cognitive denominator is hypothesized to be an impairment in mental manipulation of images and not in body schem

Hearing of a presence

Here we describe a patient with epilepsy (secondary to left parieto-temporal brain damage) suffering from the paroxysmal unilateral experience of hearing a person in her near extrapersonal space. The paroxysmal auditory experience was associated with a deficit in spatial auditory perception and other paroxysmal disorders of somatognosia. Based on these findings, it is suggested that the paroxysmal hearing of a person nearby corresponds to an auditory disorder of somatognosia

Letters lost in space: hemispace dependent handwriting errors

Although handwriting is a daily life activity commonly attributed to the left hemisphere in the majority of right-handers, it is also known to require attentional and spatial mechanisms that rely on right hemispheric processing. The underlying spatial organization of handwriting in patients with right brain damage remains unresolved. Here we show in a patient with circumscribed right superior parietal damage that handwriting systematically depends on the hand's position in space with respect to her body-midline. Most importantly, handwriting in contralesional space not only leads to spatial but also to language errors. This suggests that the right hemisphere's role in handwriting may surpass its generally assumed purely spatial contribution. We discuss our results in term of co-registration between both cerebral hemispheres in language processing

Touch perception reveals the dominance of spatial over digital representation of numbers

We learn counting on our fingers, and the digital representation of numbers we develop is still present in adulthood [Andres M, et a. (2007) J Cognit Neurosci 19:563-576]. Such an anatomy-magnitude association establishes tight functional correspondences between fingers and numbers [Di Luca S, et al. (2006) Q J Exp Psychol 59:16481663]. However, it has long been known that small-to-large magnitude information is arranged left-to-right along a mental number line [Dehaene S, et A (1993) J Exp Psychol Genet 122:371-396]. Here, we investigated touch perception to disambiguate whether number representation is embodied on the hand ("1" = thumb; "5" = little finger) or disembodied in the extrapersonal space ("1" = left, "5" = right). We directly contrasted these number representations in two experiments using a single centrally located effector (the foot) and a simple postural manipulation of the hand (palm-up vs. palm-down). We show that visual presentation of a number ("1" or "5") shifts attention cross-modally, modulating the detection of tactile stimuli delivered on the little finger or thumb. With the hand resting palm-down, subjects perform better when reporting tactile stimuli delivered to the little finger after presentation of number "5" than number "1." Crucially, this pattern reverses (better performance after number "1" than "5") when the hand is in a palm-up posture, in which the position of the fingers in external space, but not their relative anatomical position, is reversed. The human brain can thus use either space- or body-based representation of numbers, but in case of competition, the former dominates the latter, showing the stronger role played by the mental number line organization