Kyoichi Nakajima

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.

From Perception to Sentence Comprehension: The Convergence of Auditory and Visual Information of Language in the Left Inferior Frontal Cortex

We used functional magnetic resonance imaging (fMRI) to characterize cortical activation associated with sentence processing, thereby elucidating where in the brain auditory and visual inputs of words converge during sentence comprehension. Within one scanning session, subjects performed three types of tasks with different linguistic components from perception to sentence comprehension: nonword (N(AV); auditory and visual), phrase (P; either auditory or visual), and sentence (S; either auditory or visual) tasks. In a comparison of the P and N(AV) tasks, the angular and supramarginal gyri showed bilateral activation, whereas the inferior and middle frontal gyri showed left-lateralized activation. A comparison of the S and P tasks, together with a conjunction analysis, revealed a ventral region of the left inferior frontal gyrus (F3t/F3O), which was sentence-processing selective and modality-independent. These results unequivocally demonstrated that the left F3t/F3O is involved in the selection and integration of semantic information that are separable from lexico-semantic processing.

Functional differentiation in the human auditory and language areas revealed by a dichotic listening task.

The human auditory cortex plays a special role in speech recognition. It is therefore necessary to clarify the functional roles of individual auditory areas. We applied functional magnetic resonance imaging (fMRI) to examine cortical responses to speech sounds, which were presented under the dichotic and diotic (binaural) listening conditions. We found two different response patterns in multiple auditory areas and language-related areas. In the auditory cortex, the medial portion of the secondary auditory area (A2), as well as a part of the planum temporale (PT) and the superior temporal gyrus and sulcus (ST), showed greater responses under the dichotic condition than under the diotic condition. This dichotic selectivity may reflect acoustic differences and attention-related factors such as spatial attention and selective attention to targets. In contrast, other parts of the auditory cortex showed comparable responses to the dichotic and diotic conditions. We found similar functional differentiation in the inferior frontal (IF) cortex. These results suggest that multiple auditory and language areas may play a pivotal role in integrating the functional differentiation for speech recognition.

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.

Feedback signal from medial temporal lobe mediates visual associative mnemonic codes of inferotemporal neurons.

Functional roles of the cortical backward signal in long-term memory formation were studied in monkeys performing a visual pair-association task. Before learning of the task, the anterior commissure of the monkeys was transected, disconnecting the anterior temporal cortex of each hemisphere. After training with 12 pairs of pictures, we injected a grid of ibotenic acid unilaterally into the entorhinal and perirhinal cortex that provide massive backward projections ipsilaterally to the inferotemporal cortex. According to the histological examination, the lesions covered medial and lateral banks of the rhinal sulcus completely and most the entorhinal and perirhinal cortex. After the injections, the monkeys fixated the cue stimulus normally, relearned the preoperatively learned set (set-A) and learned a new set (set-B) of paired associates. Then single units were recorded from the same area as that for the prelesion control. We found that (i) in spite of the lesion, the sampled neurons responded strongly and selectively to both the set-A and set-B patterns, and that (ii) the paired associates elicited significantly correlated responses in the control neurons but not in the cells tested after the lesion either for set-A or set-B stimuli. We conclude that the ability of inferotemporal neurons to represent association between picture pairs was lost after disruption of backward neural signals from the limbic cortex to the inferotemporal neurons, while the ability of the neurons to respond to a particular visual stimulus was left intact.

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.