Ionization of counting gases and ionizable gaseous additives in proportional chambers by UV lasers

The ionization induced in argon-methane proportional counters by lasers emitting at 337 nm and 266 nm has been investigated. The ionization is due to impurities in the gas which can be removed by appropriate filters. Observations are consistent with a two-photon absorption mechanism. Two gaseous additives (trimethylamine and tetramethylphenylenediamine) are studied which are easily ionized at λ = 266 nm by a two-photon process. They can be used with concentrations at the ppm level to simulate minimum ionizing charged particle tracks. The ionization density is proportional to the concentration of the additive trimethylamine and to the square of the energy density of the laser beam.     

Dissociation of memory retrieval and search processes: an event-related fMRI study

A cognitive task can often be subdivided into several subprocesses, which follow a specific temporal order. Here, we report an event-related functional magnetic resonance imaging (fMRI) experiment on memory search, in which the temporal onset of search in primary memory was varied relative to retrieval from secondary memory. Furthermore, previous behavioral studies demonstrated that search times in primary memory depend on the number of items in a memory set, whereas retrieval from secondary memory is a set-size independent process. We analyzed the dependency of the blood oxygenation level dependent (BOLD)-response on the temporal onset of memory search on the one hand and on memory set size on the other hand to differentiate the contribution of retrieval from secondary memory, maintenance in primary memory, item search in primary memory, and response-related processes. The timing of activation followed cue presentation bilaterally in the middle frontal gyri (Brodmann area (BA) 9,46) and the inferior parts of the precentral gyri (BA6). In all other regions of interest (ROI), supplementary motor area (SMA), posterior parietal cortex, antero-superior insula, and primary motor cortex, the onset of activation was delayed with delayed probe presentation, ruling out participation in retrieval from secondary memory. The amplitude of the BOLD-response increased with increasing memory set size in all ROI except primary motor cortex and left posterior parietal cortex. All areas with cue-associated BOLD onset, suggesting involvement in retrieval, showed prolonged BOLD activation, suggesting that they also support maintenance of the retrieved information.     

Redundancy gains for visual search after complete commissurotomy.

Redundant-targets effects (RTE) for visual search were investigated in 2 commissurotomy patients (L.B., N.G.). L.B., who showed no evidence of visual interhemispheric transfer, exhibited a paradoxical enhancement of the redundancy gain in the bilateral compared with the within-hemifield redundant-targets conditions, whereas N.G., who showed evidence of interhemispheric transfer of visual information, exhibited no enhancement of the bilateral redundancy gain. When only uncrossed responses were considered, both bilateral and within-field RTE were evident only when attentional demands were high. Bilateral redundant targets led to stronger gains, some indicative of coactivation, in the slower response hand. The authors suggest that the enhancement of the bilateral RTE comes about by neural coactivation, which is especially pronounced when the slower hemisphere elicits the response. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dichotic listening in patients with splenial and nonsplenial callosal lesions.

The authors found splenial lesions to be associated with left ear suppression in dichotic listening of consonant-vowel syllables. This was found in both a rapid presentation dichotic monitoring task and a standard dichotic listening task, ruling out attentional limitations in the processing of high stimulus loads as a confounding factor. Moreover, directed attention to the left ear did not improve left ear target detection in the patients, independent of callosal lesion location. The authors' data may indicate that auditory callosal fibers pass through the splenium more posterior than previously thought. However, further studies should investigate whether callosal fibers between primary and secondary auditory cortices, or between higher level multimodal cortices, are vital for the detection of left ear targets in dichotic listening. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Cortical areas and the control of self-determined finger movements: an fMRI study

We investigated cortical areas involved in the control of self-determined finger movements. In a tapping task, subjects tapped with different movement frequencies in two different movement conditions (predetermined vs self-determined). fMRI provided evidence for the involvement of the horizontal and ascending parts of the intraparietal sulcus (IPS), the left superior frontal gyrus and the posterior cingulate gyrus in the control of self-determined finger movements. Higher movement frequency increased the extent of activated area only in the horizontal part of IPS. The results suggest a major role of the IPS in controlling sequences of finger movements. This area probably serves as a region for integration of motor, sensory and sensorimotor feedback information used for movement control.