Dissociation of neglect subtypes with transcranial magnetic stimulation.

OBJECTIVE: To identify two forms of hemispatial neglect, attentional and intentional, in healthy volunteers using frontal and parietal repetitive transcranial magnetic stimulation (rTMS). METHODS: Ten healthy volunteers performed line bisection tasks while viewing stimuli on closed circuit TV. Direct view of the exploring hand and the target was precluded and the TV monitor guided performance. In the normal condition, the direction of hand movement on the table (workspace) was congruent with that on the monitor, and in the reversed condition, the lateral movement in the workspace occurred in the opposite direction on the monitor. The line bisections were performed in three conditions: without rTMS (control), with right frontal rTMS, and with right parietal rTMS. RESULTS: In the normal condition, both right frontal and right parietal rTMS caused a significant rightward deviation (left hemispatial neglect). In the reversed condition, right frontal rTMS continued to cause rightward deviation in the workspace corresponding to leftward bias on the monitor, whereas right parietal rTMS caused leftward deviation in the workspace, corresponding to rightward bias on the monitor. CONCLUSIONS: Right frontal repetitive transcranial magnetic stimulation causes motor-intentional neglect, whereas right parietal repetitive transcranial magnetic stimulation causes sensory-attentional neglect. Alternatively, these findings can also be explained based on a dichotomy related to an egocentric-oriented system.

On-line anosognosia: unawareness for chorea in real time but not on videotape delay.

In hemiplegics, anosognosia (unawareness of deficit) rests on a mismatch between expected and actual movement: a feedback hypothesis emphasizes sensory deficits or neglect, a feedforward hypothesis postulates impaired intention to move. Anosognosia for other problems is less studied. The authors report a man without sensory deficits who was unaware of choreiform movements, except on videotape delay. The authors believe that a feed-forward mechanism underlies his "on-line" unawareness.

Mamillothalamic tract transection blocks anterior thalamic training-induced neuronal plasticity and impairs discriminative offidance behavior in rabbits.

Rabbits with bilateral transecting lesions of the mamillothalamic tract, control (tract-sparing and sham) lesions, or no lesions, and chronic, fixed-position anterior ventral (AV) and medial dorsal (MD) thalamic and posterodorsal subicular complex unit recording electrodes were trained to step in an activity wheel in response to a 0.5 sec tone (CS+) in order to avoid a brief foot shock. The rabbits also learned to ignore a different tone (CS-) not predictive of shock. Behavioral acquisition was significantly retarded in rabbits with mamillothalamic tract transection compared to controls. When trained, transected rabbits failed to avoid the shock more often than controls. Mamillothalamic tract transection abolished and control lesions attenuated AV thalamic discriminative training-induced activity (i.e., development with training of greater discharges in response to the CS+ than to the CS-). Transection and control lesions attenuated AV thalamic excitatory training-induced activity (greater elicited activity during training than during unpaired tone-shock presentations before training) as well as AV thalamic "spontaneous" baseline unit activity. CS-elicited discharge magnitude was reduced by control lesions and it was further reduced by tract transecting lesions. Significant lesion-related changes were not found in the subicular or MD thalamic neuronal receptor. Mamillothalamic tract afferent information flow is thus essential for AV thalamic discriminative training-induced activity, excitatory training-induced activity, tone-elicited discharges and maintenance of conditioned avoidance responses. The effects of the control lesions suggested that afferents which course in parallel with and near the mamillothalamic tract may contribute to AV thalamic spontaneous activity and excitatory training-induced activity.

Dissociations in memory for item identity and item frequency: evidence from hemispheric interactions.

Evidence for a dissociation between memory for the identity of an item and memory for its frequency of occurrence was provided by two tachistoscopic studies employing lateralized abstract designs as stimuli. In both experiments, subjects viewed a random series of presentation trials, in which certain designs appeared only once, others 5 times, and still others 9 times. Then in a series of probe trials, subjects decided, on each trial, if a design was familiar or novel, either by an old/new decision (Experiment 1) or a forced-choice decision (Experiment 2), after which they estimated the frequency of item occurrence. Presentation and probe trials were either displayed in the same visual field (within-hemisphere trials) or opposite visual fields (across-hemisphere trials). Both experiments revealed that the pattern of interhemispheric processing as well as lateralized processing differed for item identity as compared to item frequency, providing further evidence for a dissociation between these two memory processes.