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Praxis in Left-Handers

100%

Króliczak G.

nr 6(99)

5-31

Neuropsychological and neuroimaging evidence convincingly implicates the left cerebral hemisphere in the representation of skilled movements (praxis) in right-handers. Compelling and consistent data on the organization of praxis in left-handed individuals has only recently started to emerge. This new evidence, again both from neuropsychology and neuroimaging, supports the notion that in left-handers the neural substrate of praxis skills is less asymmetric, i.e., it is more bilaterally organized. Up until recently, though, the neuropsychological literature on brain-damaged left-handers was often dominated by descriptions of more or less atypical cases and dissociations of functions observed in such individuals. Associations of deficits, linked to anatomic proximity rather than to a common cerebral specialization, were rarely found worth publishing and/or in-depth discussions. This paper first reviews some of the most relevant and/or well-known reports on representations of different categories of skilled manual gestures in right- and left-handers, with a view to support the idea that these skills are mediated by a common system. Then, based on neuroimaging evidence from healthy subjects, a few individuals with unusual organization of praxis are discussed. These disparate cases quite likely represent natural variation in functional asymmetries. It is yet to be determined whether the effect of a more bilateral organization of cognitive skills in this population is just due to a much higher incidence of atypical representations of functions or rather a general tendency for all left-handers to have their brains less asymmetrically organized.

Measuring unconscious cognition: Beyond the zero-awareness criterion

86%

Schmidt T.

tom 3

nr 1-2

275-287

Visual masking can be employed to manipulate observers' awareness of critical stimuli in studies of masked priming. This paper discusses two different lines of attack for establishing unconscious cognition in such experiments. Firstly,simple dissociationsbetweendirect measures (D)of visual awareness andindirect measures (I)of processing per se occur when I has some nonzero value whileDis at chance level; the traditional requirement of zero awareness is necessary for this criterion only. In contrast,double dissociationsoccur when some experimental manipulation has opposite effects on I andD, for instance, increasing priming effects despite decreasing prime identification performance (Schmidt & Vorberg, 2006). Double dissociations require much weaker measurement assumptions than other criteria. An attractive alternative to this dissociation approach would be to use tasks that are known to violatenecessary conditionsof visual awareness altogether. In particular, it is argued here that priming effects in speeded pointing movements (Schmidt, Niehaus, & Nagel, 2006) occur in the absence of the recurrent processing that is often assumed to be a necessary condition for awareness (for instance, DiLollo, Enns, & Rensink, 2000; Lamme & Roelfsema, 2000). Feedforward tasks such as this might thus be used to measure the time-course of unconscious processing directly, before intracortical feedback and awareness come into play.