Neural mechanisms of two different verbal working memory tasks: A VLSM study.

Currently, a distributed bilateral network of frontal-parietal areas is regarded as the neural substrate of working memory (WM), with the verbal WM network being more left-lateralized. This conclusion is based primarily on functional magnetic resonance imaging (fMRI) data that provides correlational evidence for brain regions involved in a task. However, fMRI cannot differentiate the areas that are fundamentally required for performing a task. These data can only come from brain-injured individuals who fail the task after the loss of specific brain areas. In addition to the lack of complimentary data, is the issue of the variety in the WM tasks used to assess verbal WM. When different tasks are assumed to measure the same behavior, this may mask the contributions of different brain regions. Here, we investigated the neural substrate of WM by using voxel-based lesion symptom mapping (VLSM) in 49 individuals with stroke-induced left hemisphere brain injuries. These participants completed two different verbal WM tasks: complex listening span and a word 2-back task. Behavioral results indicated that the two tasks were only slightly related, while the VLSM analysis revealed different critical regions associated with each task. Specifically, significant detriments in performance on the complex span task were found with lesions in the inferior frontal gyrus, while for the 2-back task, significant deficits were seen after injury to the superior and middle temporal gyri. Thus, the two tasks depend on the structural integrity of different, non-overlapping frontal and temporal brain regions, suggesting distinct neural and cognitive mechanisms triggered by the two tasks: rehearsal and cue-dependent selection in the complex span task, versus updating/auditory recognition in the 2-back task. These findings call into question the common practice of using these two tasks interchangeably in verbal WM research and undermine the legitimacy of aggregating data from studies with different WM tasks. Thus, the present study points out the importance of lesion studies in complementing functional neuroimaging findings and highlights the need to consider task demands in neuroimaging and neuropsychological investigations of WM.

[FMRI study of visual task switching in healthy individuals].

The present study was aimed, first, at developing a visual switching task for fMRI research and, second, at identification of the brain regions involved in task switching. Forty eight healthy individuals (from 21 to 56 years of age) participated in the study. The designed visual switching task was relatively short, it consisted of an easy stimulus set and involved a simple condition, in which participants had to shift their attention between two task conditions (classifying figures according to their form or number). Thus, this developed testing procedure can further be used to assess special populations, including patients with brain damage who cannot do monotonous tasks for a long period of time and have language impairments. The results of this testing technique revealed that task switching is carried out by an interconnected neuronal network, consisting of dorsolateral prefrontal cortex, inferior parietal area, secondary visual area, supplementary motor area and cerebellum cortex of both hemispheres.

[Sex Differences in Visual Task Switching (an fMRI Study)].

Executive functions are an important ability of human brain to program, regulate and control various cognitive processes; one of these processes is the voluntary task switching. However, the sex differences in this process are poorly studied. In our study, these differences were investigated in 70 healthy subjects (36 men and 34 women) aged 21-48 years by means of functional magnetic resonance imaging (fMRI) and neuropsychological examination. During the fM RI study, the subjects had to shift their attention between two tasks (classifying figures according to their form or number). During neuropsychological examination, the subjects completed a series of visual attention, task switching and memory tests. The results of fMRI study showed that visual task switching in women is carried out by a neuronal network, consisting of dorsolateral prefrontal cortex, inferior parietal areas, secondary visual areas of both hemispheres and cerebellum cortex. Task switching in men involved the same areas and, in addition, right supplementary motor area, right insula and left thalamus. As compared with women, the rate of activation of prefrontal cortex, left parietal lobe and right insula in men was significantly higher. The results of neuropsychological tests showed that men completed the tasks with attention switching, searching and arranging of stimuli significantly slower than women. The data demonstrate the differences in the organization of task switching processes in men and women.

[Brain mapping in verbal and spatial thinking].

The goal of this study was to describe the topography of the active cortical areas and subcortical structuresin verbal and spatial thinking. The method of functional magnetic resonance imaging (fMRI) was used. 18 right-handed subjects participated in the study. Four types of tasks were presented: two experimental tasks--verbal (anagram) and spatial (search for a piece to complement a square), and two types of control tasks (written words and a spatial task, where all the pieces are identical). In solving verbal tasks the greater volume of activation was observed in the left hemisphere involving Broca's area, while the right middle frontal gyrus was activated in solving the spatial tasks. For occipital region an activation of the visual field 18 was more explicitin solving spatial problems, while the solution of anagrams caused an activation of the field 19 associated with higher levels of visual processing. The cerebellum was active bilaterally in both tasks with predominance in the second. The obtained fMRI data indicate that the verbal and spatial types of thinking are provided by an activation of narrow specific sets of brain structures, while the previous electrophysiological studies indicate the distributed nature of the brain processes in thinking. Combining these two approaches, it can be concluded that cognitive functions are supported by the systemic brain processes with a distinct location of the particular salient structures.

[FMRI-study of speech perception impairment in post-stroke patients with sensory aphasia].

The aim of the study was to find neurophysiological correlates of the primary stage impairment of speech perception, namely phonemic discrimination, in patients with sensory aphasia after acute ischemic stroke in the left hemisphere by noninvasive method of fMRI. For this purpose we registered the fMRI-equivalent of mismatch negativity (MMN) in response to the speech phonemes--syllables "ba" and "pa" in odd-ball paradigm in 20 healthy subjects and 23 patients with post-stroke sensory aphasia. In healthy subjects active brain areas depending from the MMN contrast were observed in the superior temporal and inferior frontal gyri in the right and left hemispheres. In the group of patients there was a significant activation of the auditory cortex in the right hemisphere only, and this activation was less in a volume and intensity than in healthy subjects and correlated to the degree of preservation of speech. Thus, the method of recording fMRI equivalent of MMN is sensitive to study the speech perception impairment.