A visuomotor disorder in the absence of movement: Does Optic Ataxia generalize to learned isometric hand action?

The Two Visual Systems Hypothesis: New Challenges and Insights from Visual form Agnosic Patient DF

“…Although there is clear evidence that optic ataxia can include visuomotor deficits in central vision [e.g., Ref. ( 13 , 14 , 39 , 40 )], it is well-known that optic ataxia more frequently manifests itself as misreaching to targets presented in the periphery ( 41 , 42 ). In fact, peripheral and centrally guided reaches might well rely on separate networks in the posterior parietal cortex ( 43 , 44 ).…”

Timing and communication of parietal cortex for visuomotor control

“…J Neurosci 2014, 34:11948-11958. This study shows anti-synchrony in partial spike-LFP correlation in the beta band (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) between parietal and dorsal premotor reach-related areas in monkeys, and hypothesizes that anti-synchrony can decrease the probability that actions potentials originated in parietal cortex activate premotor cortex during attentive fixation and reach planning. The results are interpreted within the frame of the literature suggesting that betaband activity can reduce the interaction between brain structures and can have an inhibitory role on movement initiation.…”

“…As an example, the effective connection strength between areas can be increased, if the timing of firing of cortical projecting neurons with small axon diameter, therefore slow conduction velocity, leads the activation of neurons with large axon diameter and faster conduction velocity. Studies of the temporal dynamics of parieto-frontal interactions indicates that while parietal and premotor cortex oscillate in-phase in the beta band (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30), partial spike-field coherence in parietal cortex is asynchronous with oscillations in premotor cortex during periods of attentive fixation and motor planning [8 ], probably down-modulating parieto-frontal communication. Using a population partial spike-field coherence approach to study temporal interactions between parietal areas 2/5 and motor cortex (MI), only a weak parietal influence was found on MI during reach planning, but a strong one during reach movement [9 ], with parietal cortex influencing encoding of movement direction in primary motor cortex.…”

“…In this context, the importance of the degree of eye-hand coupling is highlighted by a case report [24 ] ( Figure 2a) of a parietal patient showing a severe impairment in the specification of the direction of the hand force applied to an isometric manipulandum, necessary to move a cursor to visual targets. This impairment indicates that: first, OA generalizes to isometric conditions, that is to visuomotor transformations leading to action in absence of movement, regardless of the imposed physical relationship between the manipulandum and cursor; second, PPC is involved not only in the specification of kinematics [15,25], but also in that of higher-order dynamics underlying hand action.…”

“…The eye motor error (E Tg ), hand motor error (H Tg ), and their relative difference (HE Tg ) are shown for each task condition, together with a schematic general representation of their definition on the right of the bar graph. Modified from [24 ]. (c) Example of global tuning field (GTF) of a parietal neuron, where the preferred directions (PDs) during separated or coordinated eye and hand movements cluster within a restricted part of the work-space, thus allowing combination of eye and hand signals in a spatially congruent directional domain.…”