Temporal characteristics of neural activity related to target detection during visual search.

A previous MEG study on neural activities during the orientation singleton search showed that both efficient and inefficient searches shared a common neural network and the search efficiency was determined by a neural process executed in the temporal and parietal areas. The target segmentation stage, however, remains to be elucidated. In the present study, MEG and fMRI experiments were conducted, and moment-magnitudes of equivalent current dipoles were estimated with an fMRI-constrained MEG multi-dipole method to obtain differences between target-present and -absent conditions in each brain region for the whole time course. The dipole moments around the calcarine sulcus (CaS) and posterior fusiform gyrus (pFuG) increased at latencies around 70-350 ms. Activity around the CaS consisted of a prominent and a subsequent smaller but still obvious peak (117, 215 ms); the first peak showed no difference between conditions, while the second peak was significantly larger in the target-present condition. Activity around the pFuG had a prominent peak (125 ms) and subsequent small activity (237 ms), whereas the target's presence or not had no influence on either activity. The activity of the right intraparietal sulcus (IPS) was significantly larger than that for the left IPS at latencies around 196 ms irrespective of the target's presence or not. The activity of the other brain regions such as the posterior superior temporal sulcus, cingulate sulcus and central sulcus showed no difference between target conditions. The results demonstrate that neural activities of multiple regions had different temporal characteristics, and the later activity around the CaS was related to the target segregation from its surroundings during the orientation contrast search.

Maintaining coherence of dynamic objects requires coordination of neural systems extended from anterior frontal to posterior parietal brain cortices.

Object representation in visual working memory enables humans to perceive a consistent visual world and must satisfy two attributes: coherence and dynamic updating. The present study measured brain activity using functional magnetic resonance imaging (fMRI) during the multiple object permanence tracking (MOPT) task, which requires observers to process simultaneously both coherence maintenance and dynamic updating of objects. Whole brain analysis revealed anterior and ventral parts of frontal area and dorsal frontoparietal activation during both object-moving and object-stationary conditions. Subsequent region-of-interest analyses in the anterior/ventral frontal and the dorsal frontoparietal regions revealed that these two systems engage the two different cognitive processes involved in the MOPT task, with coherency maintenance processed in the anterior/ventral frontal areas and spatial processing in the dorsal frontoparietal network. These results suggest that cooperation between these two systems underpins object representations in visual working memory.

Adaptation to left-right reversed vision rapidly activates ipsilateral visual cortex in humans.

The brain mechanisms of adaptation to visual transposition are of increasing interest, not only for research on sensory-motor coordination, but also for neuropsychological rehabilitation. Sugita [Nature 380 (1996) 523] found that after adaptation to left-right reversed vision for one and a half months, monkey V1 neurons responded to stimuli presented not only in the contralateral visual field, but also in the ipsilateral visual field. To identify the underlying neuronal mechanisms of adaptation to visual transposition, we conducted fMRI and behavioral experiments for which four adult human subjects wore left-right reversing goggles for 35/39 days, and investigated: (1) whether ipsilateral V1 activation can be induced in human adult subjects; (2) if yes, when the ipsilateral activity starts, and what kind of behavioral/psychological changes occur accompanying the ipsilateral activity; (3) whether other visual cortices also show an ipsilateral activity change. The results of behavioral experiments showed that visuomotor coordinative function and internal representation of peripersonal space rapidly adapted to the left-right reversed vision within the first or second week. Accompanying these behavioral changes, we found that both primary (V1) and extrastriate (MT/MST) visual cortex in human adults responded to visual stimuli presented in the ipsilateral visual field. In addition, the ipsilateral activity started much sooner than the one and a half months, which had been expected from the monkey neurophysiological study. The results of the present study serve as physiological evidence of large-scale, cross-hemisphere, cerebral plasticity that exists even in adult human brain.

Circulatory basis of fMRI signals: relationship between changes in the hemodynamic parameters and BOLD signal intensity.

Blood oxygenation level-dependent functional magnetic resonance imaging (BOLD-fMRI) is widely used as a tool for functional brain mapping. During brain activation, increases in the regional blood flow lead to an increase in blood oxygenation and a decrease in paramagnetic deoxygenated hemoglobin (deoxy-Hb), causing an increase in the MR signal intensity at the site of brain activation. However, not a few studies using fMRI have failed to detect activation of areas that ought to have been activated. We assigned BOLD-positive (an increase in the signal intensity), BOLD-negative (a decrease in the signal intensity), and BOLD-silent (no change) brain activation to respective circulatory conditions through a description of fMRI signals as a function of the concentration of oxygenated Hb (oxy-Hb) and deoxy-Hb obtained with near-infrared optical imaging (NIOI). Using this model, we explain the sensory motor paradox in terms of BOLD-positive, BOLD-negative, and BOLD-silent brain activation.

Attentional set for external information activates the right intraparietal area.

Visual attention can be allocated to a location or an object by using two different types of information: internal information and external information. The results of recent psychological studies [Bagon and Egeth, Percept. Psychophys. 55 (1994) 485] suggest that an observer's attentional set determines how these two kinds of information are used in visual tasks. In this study, we measured brain activities during two modes of visual search; one is the feature search mode, in which an attentional set for knowledge of a target item (internal information) is used, and the other is the singleton detection mode, in which an attentional set for oddness in the visual scene (external information) is used. We found extended activation in the frontal and parietal areas for both search modes. In addition, a direct comparison of brain activity during the singleton detection mode and the feature search mode revealed that the areas around the right intraparietal sulcus were more involved in the attentional set for oddness. These results suggest that the human right intraparietal cortex is related to the attentional set for external information.