Idai Uchida

Transient activation of inferior prefrontal cortex during cognitive set shifting

The Wisconsin Card Sorting Test, which probes the ability to shift attention from one category of stimulus attributes to another (shifting cognitive sets), is the most common paradigm used to detect human frontal lobe pathology. However, the exact relationship of this card test to prefrontal function and the precise anatomical localization of the cognitive shifts involved are controversial. By isolating shift-related signals using the temporal resolution of functional magnetic resonance imaging, we reproducibly found transient activation of the posterior part of the bilateral inferior frontal sulci. This activation was larger as the number of dimensions (relevant stimulus attributes that had to be recognized) were increased. These results suggest that the inferior frontal areas play an essential role in the flexible shifting of cognitive sets.

No-go dominant brain activity in human inferior prefrontal cortex revealed by functional magnetic resonance imaging

We investigated the response inhibition function of the prefrontal cortex associated with the go/no‐go task using functional magnetic resonance imaging in five human subjects. The go/no‐go task consisted of go and no‐go trials given randomly with roughly equal probability. In go trials a green square was presented and the subjects had to respond by promptly pushing a button using their right or left thumbs, but in no‐go trials a red square was presented and subjects were instructed not to respond. When brain activity in no‐go trials is dominant over that in go trials in areas in the prefrontal cortex, this no‐go dominant brain activity would reflect the neural processes for inhibiting inherent response tendency. We used a new strategy of image data analysis by which transient brain activity in go or no‐go trials can be analysed separately, and looked for the prefrontal areas in which the brain activity in no‐go trials is dominant over that in go trials. We found the no‐go dominant foci in the posterior part of the right inferior frontal sulcus reproducibly among the subjects. This was true whether the right or left hand was used. These results suggest that this region in the prefrontal cortex is related to the neural mechanisms underlying the response inhibition function.

Hemispheric asymmetry in human lateral prefrontal cortex during cognitive set shifting

Functional organization of human cerebral hemispheres is asymmetrically specialized, most typically along a verbal/nonverbal axis. In this event-related functional MRI study, we report another example of the asymmetrical specialization. Set-shifting paradigms derived from the Wisconsin card sorting test were used, where subjects update one behavior to another on the basis of environmental feedback. The cognitive requirements constituting the paradigms were decomposed into two components according to temporal stages of task events. Double dissociation of the component brain activity was found in the three bilateral pairs of regions in the lateral frontal cortex, the right regions being activated during exposure to negative feedback and the corresponding left regions being activated during updating of behavior, to suggest that both hemispheres contribute to cognitive set shifting but in different ways. The asymmetrical hemispheric specialization within the same paradigms further implies an interhemispheric interaction of these task components that achieve a common goal.

Functional mapping of the human colour centre with echo-planar magnetic resonance imaging

Clinical studies of cerebral achromatopsia have suggested a colour centre in the human fusiform gyrus. By using functional magnetic resonance imaging, we examined whether the fusiform gyrus shows activity correlated with the perception of colour. We tested three stimulus conditions in which the subject maintained fixation: (i) a circular array of six coloured circles; (ii) the same as (i) except that each circle is equiluminant grey with its colour counterpart; and (iii) the same as (i) plus a clockwise shift of circles to neighbouring positions every 1 s. After termination of the stimulus, the subject perceived an after-image of circles with complementary colours in (i), but not in (iii). In condition (i), we found a focal signal increase in the posterior part of the fusiform gyrus. In condition (ii), the activation in the same locus during the stimulation period was weaker than that in (i). In condition (iii), the signal intensity after termination of the stimulus was weaker than that in (i). The colour effect and after-effect on activation of the fusiform gyrus observed here suggest its critical role in human colour perception.

Transient brain activity used in magnetic resonance imaging to detect functional areas.

FUNCTIONAL areas were detected with short stimuli eliciting transient brain activity using the method of ‘transient’ regions of interest (ROIs) and functional magnetic resonance imaging (fMRI). This method was validated by comparing the results with sustainedly activated areas identified conventionally. Eighty-eight and 89% of the total areas of transient ROIs derived from 0.2 and 2 s stimulation, respectively, were identified at 5–7 s and 5–9 s, respectively, after stimulus onset. Eighty-eight and 76%, respectively, of these areas overlapped ‘conventional’ ROIs derived from 20 s stimulation. These results suggest that the delineation of transient ROIs, by targeting a period ∼7 s after transient neural activity, can be useful for fMRI studies of cognitive functions.

2602 Mapping of response inhibition function of the prefrontal cortex using functional magnetic resonance imaging

ATSUSHI IRIKI’32. MICHIO TANAKA’, YOSHIAKI IWAMURA’ Monkeys were trained to retrieve food by watching their hand movement through a real-time video monitor instead of seeing it directly. Single unit activities of bimodal neurons, which integrate somatosensory and visual information to code the schema of the hand (Iriki et al., 1996), were recorded form the anterior bank of the intraparietal sulcus of the contralateral cerebral cortex. After training, additional visual receptive fields were formed around the image of the hand in the video screen. Their size and position were modified according to expansion, compression, or change of the position of the visual image in the video screen, although the posture or the position of the hand was not actually altered. This phenomenon was observed only after the monkeys learned to retrieve food through the video screen, suggesting that the self image, if exists in monkeys, was projected to the video screen as the result of learning.

A functional MRI study of orthographic processing of Kanji

Optical imaging of the intrinsic signal has been used to visualize functional structures. This signal induced by neural activation is related to concentration changes of deoxy-hemoglobin (deoxy-Hb), oxy-hemoglobin (oxy-Hb), blood volume (BV), and change in light scattering (LS). In this study we investigated spatial patterns of orientation columns derived from these different components in anesthetized and awake cat cortices. We recorded the intrinsic signal at 540, 570, and 620 nm simultaneously by using three CCD cameras and decomposed it into the above-mentioned components, A flashed grating or moving grating was used for visual stimulation. The average response evoked by two orthogonal gratings indicated the global change (GC) for each component and a difference image obtained from the two gratings indicated the local difference (LD) of each component. The LD of BV showed an identical columnar pattern with that of deoxy-Hb. These small laocal changes were superimposed on a larger global changes across the cortex, with increasing GC of BV, and deoxy-Hb. These results suggest that, as well as deoxy-Hb, neural activation induces a large increase in BV both in active and less active columns, but the amount is larger in active columns. We hypothesize that there is a fine control mechanism of blood flow at the columnar level. Analysis of oxy-Hb, LS and differences in awake and anesthetized condition will be also discussed.