Narumi Katsuyama

Mechanisms underlying orientation selectivity of neurons in the primary visual cortex of the macaque.

1. Effects of blocking intracortical inhibition by microiontophoretic administration of bicuculline methiodide (BMI), a selective antagonist for GABAA receptors, on orientation selectivity of 109 neurones were studied in the primary visual cortex (V1) of anaesthetized and paralysed monkeys. 2. The averaged orientation tuning of visual responses of cells was poor in cytochrome oxidaserich blobs of layer II/III and in layer IVc beta, moderate in layers IVb, IVc alpha and V, and sharp in the interblob region of layer II/III and in layers IVa and VI. 3. Iontophoretic administration of BMI reduced the sharpness of orientation tuning of cells to a varying extent in each layer. In most cells, furthermore, the originally ineffective stimuli induced visual responses during the BMI administration, suggesting that excitatory inputs evoked by the non‐optimally oriented stimuli were masked by GABAergic inhibition. Nevertheless, the maximal facilitation was observed in the response to the optimally or near‐optimally oriented stimuli. 4. There was a difference in such an effect of BMI among layers. Orientation selectivity of cells in interblobs in layer II/III and in layer IVb was sensitive to BMI whereas that of cells in layer VI was relatively insensitive to BMI, suggesting a larger contribution of excitatory mechanisms to the orientation selectivity in this layer. 5. In the orientation‐selective cells, an analysis of the magnitude of excitation and inhibition evoked by stimuli at various orientations suggests that both inputs tune around the optimal orientation and their magnitudes are almost proportional to each other except at the optimal orientation. This analysis also indicates that the orientation tuning of inhibition had a less prominent peak around the optimal orientation than that of excitation. This dominance of excitation over inhibition around the optimal orientation may function to accentuate the response to the optimally oriented stimulus. 6. These results suggest that, in the monkey V1, the orientation selectivity of cells is largely dependent on the orientation‐biased excitatory and inhibitory inputs which have a broader tuning profile, covering from the optimal to null‐orientation, than that observed in extracellularly recorded responses at the control level.

Less Segregated Processing of Visual Information in V2 than in V1 of the Monkey Visual Cortex

To test the possibility of cross‐talk between parallel pathways dealing with different aspects of visual information, such as orientation, direction of motion and colour in cortical area V2, we quantitatively analysed visual responses of 121 V2 cells recorded from anaesthetized and paralysed macaques and compared them with those of 147 V1 cells. A selectivity index of visual responses was calculated for each neuron, which was then classified as selective or not to a particular attribute of visual stimuli. Twenty‐one percent of the V2 neurons had dual selectivity to both colour and direction of stimulus motion (C&D cells). In V1, only 5% of the cells were C&D cells. Thus, the proportion of C&D cells significantly increased from V1 to V2. We also carried out cross‐correlation analysis of spike trains recorded simultaneously from pairs of V2 neurons or pairs of V1 neurons. In V2, correlated firings could be observed between cells with completely different optimal orientation, such as orthogonal, while it was never observed in V1. The cross‐correlation analysis further indicated that functional interactions in V2 were more widespread than those in V1. These results suggest that neurons which have different functional properties become less segregated, and that functional interactions become more widespread in V2 than in V1.

Cortical areas related to performance of WAIS Digit Symbol Test: a functional imaging study.

Many neuropsychological studies have shown that the Digit Symbol Test (DST) of the Wechsler Adult Intelligence Scale (WAIS) is useful for screening for dysfunctions of the brain. However, it remains unclear which brain areas are actually involved in the performance of DST and what brain functions are used for executing this test. In this study, we examined the cortical areas related to cognitive aspects of DST using functional magnetic resonance imaging (fMRI) and determined executive brain functions involved in this test on the basis of fMRI results. Eleven healthy young adults (mean=21.6 years) performed a modified DST (mDST) task and its control task, which required a simple graphomotor response during fMRI data acquisition. The direct comparison of brain activations between the mDST task and the control task revealed greater activations in a fronto-parietal cortical network, including the bilateral inferior frontal sulci, left middle frontal gyrus (close to the frontal eye field) and left posterior parietal cortex. These activations are interpreted as reflecting the visual search process and/or the updating process of working memory during the mDST task execution. Furthermore, we found a positive correlation between the number of correct responses and activations in the bilateral inferior frontal regions, suggesting that these prefrontal areas have a crucial role in the performance of DST in a healthy young adult population.

Functional and histological properties of caudal intraparietal area of macaque monkey

In our previous studies, we found that cells in the caudal intraparietal (CIP) area of the macaque monkey selectively responded to three-dimensional (3D) features, such as the axis and surface orientations, and we suggested that this area played a crucial role in 3D vision. In this study, we investigated (1) whether cells in CIP respond to other 3D features, such as curvature, and (2) whether CIP has any histological property to distinguish it from neighboring areas. Curvatures defined by a random-dot stereogram were presented on a display while the monkey performed a fixation task. The shape and amount of curvature were manipulated by two independent variables, shape index and curvedness, respectively. Two-way ANOVA showed that 19 out of 56 visually responsive cells (34.0%) showed the main effect of shape index. We tentatively designated these cells as 3D curvature-selective (3DCS). Of these, six 3DCS cells showed the main effects of shape index and curvedness, whereas 13 showed the main effect of shape index only. In both types of 3DCS cells, preferred shape indices calculated from tuning curves at two levels of curvedness matched well. These results indicate that the majority of 3DCS cells responded equally to a particular shape of curvatures with different curvedness levels. An immunohistochemical study showed that the recording sites of 3DCS cells were in a cortical region characterized by a dense SMI-32 immunoreactivity in the caudal portion of the lateral intraparietal sulcus (IPS), which suggests that this region is comparable to the lateral occipital parietal (LOP) designated in the caudal IPS previously. Further investigations showed that this region was separated from LIPv, the ventral subdivision of lateral intraparietal (LIP) located rostral to CIP/LOP. These results suggest that CIP is a cortical area distinct from LIP histologically as well as functionally.

Information processing of geometrical features of a surface based on binocular disparity cues: an fMRI study

Cortical areas related to the information processing of binocular disparity-defined geometrical features of a surface, such as depth, orientation and shape are examined by functional magnetic resonance imaging while subjects discriminated these three types of geometrical feature in random dot stereograms. Results indicate that disparity-defined information of depth and that of orientation are processed in the parietal area. Furthermore, the visual system for 3D vision in the parietal area may be organized in a hierarchical manner and the posterior part of the right intraparietal sulcus may be involved in cognitive process of 3D vision. On the other hand, disparity-defined shape information seems to be processed in the occipital visual areas and the crucial involvement of human LOS for this process is suggested.

Design of a head fixation device for experiments in behaving monkeys.

We have designed a new device for head fixation of behaving monkeys. The fixation device consists of a duralumin head ring mounted with four screw holders. It is firmly fixed to the animals skull with four stainless steel screw pins. The head ring is then attached to a primate chair in any desirable position and angle using a set of adjustable plates. The device has been used for behavioral training that requires accurate gaze monitoring and for recording single-unit activity over a several-month period. The advantage of our device is that it is simple to use; it can be attached readily without major surgical procedures and it can be quickly removed when experiments are not running. This head fixation system is suitable for behavioral experiments and single-unit recording studies. It may also be applicable for studies on functional imaging of the macaque brain, by constructing it of non-magnetic materials.

Lateral suprasylvian visual cortex is activated earlier than or synchronously with primary visual cortex in the cat

To explore functional connectivity between area 17 and posteromedial lateral suprasylvian (PMLS) area of the cat cerebral cortex, we carried out cross-correlation analysis of spike trains of neurons recorded simultaneously from both areas of anesthetized, paralyzed cat. Most of correlated pairs had peaks which indicate that PMLS cells were activated earlier than or simultaneously with their partners in area 17, suggesting that backward as well as synchronous activation operates between these cortical areas.

Stress-related activities induced by predator odor may become indistinguishable by hinokitiol odor.

Predator odors, such as 2,5-dihydro-2,4,5-trimethylthiazoline (TMT), induce a stress-like behavior in some rodents, and there is activation of a complex mix of brain regions including the anterior piriform cortex (APC) and the bed nucleus of stria terminalis (BST). In contrast, rose odor can counteract TMT-induced activation of the ventrorostral part of APC and the medial part of BST. In the present study, two novel odors, woody (hinokitiol) and caraway [S(+)-carvone] odors, were evaluated to determine whether they have an antistress effect. Plasma adrenocorticotropic hormone levels, a marker of stress, and the number of c-Fos-immunopositive cells were determined in APC and BST. Plasma adrenocorticotropic hormone levels were increased by TMT alone and in combination with S(+)-carvone; however, hinokitiol with or without TMT did not have an effect. The number of activated cells in the medial part of BST was increased by TMT alone and in combination with S(+)-carvone or hinokitiol. Although TMT alone activated the medial part of BST, a mixture of TMT and hinokitiol activated both the medial and the lateral part of BST. These data suggest that the selective responses to TMT in the medial part of BST were obscured by activation of more odor-related regions by hinokitiol with TMT. In addition, the ratio of medial to lateral BST activation may be critical in stress-related behavior. In conclusion, hinokitiol can alleviate TMT-induced stress; however, the underlying mechanism appears to be different from that of the rose odor, as found in our previous study.

Coding of 3D curvature in the parietal cortex (area CIP) of macaque monkey

We investigated responses of area CIP of macaque monkey to a set of 3D curvatures defined by random-dot stereogram (RDS). In 82 neurons recorded, 75 (91.5 %) had significant response to one of curvatures tested. Most of visually responsive neurons showed preference to curvature with convex or concave. To evaluate the strength of tuning to the shape, modulation index (MI) was computed for the responses of each neuron to the curvatures, and neuron with MI more than 0.6 was defined as ‘curvature selective neuron’. Of 75 visually responsive neurons, 20 (26.7 %) met the criterion. Five neurons had highly selective response to a particular curvature (MI >= 0.8). Then we plotted the preferred shape index of the curvature selective neurons computed from Gaussian function fit to the responses of neurons. The distribution of the preferred shape index was biased toward concave and convex ellipsoid, although it distributed over the range of shape index (−1.0 − 1.0). These results suggest that 3D curvatures might be represented in area CIP and this area plays a critical role in 3D vision.

Perception of object motion in three-dimensional space induced by cast shadows.

Cast shadows can be salient depth cues in three-dimensional (3D) vision. Using a motion illusion in which a ball is perceived to roll in depth on the bottom or to flow in the front plane depending on the slope of the trajectory of its cast shadow, we investigated cortical mechanisms underlying 3D vision based on cast shadows using fMRI techniques. When modified versions of the original illusion, in which the slope of the shadow trajectory (shadow slope) was changed in 5 steps from the same one as the ball trajectory to the horizontal, were presented to participants, their perceived ball trajectory shifted gradually from rolling on the bottom to floating in the front plane as the change of the shadow slope. This observation suggests that the perception of the ball trajectory in this illusion is strongly affected by the motion of the cast shadow. In the fMRI study, cortical activity during observation of the movies of the illusion was investigated. We found that the bilateral posterior-occipital sulcus (POS) and right ventral precuneus showed activation related to the perception of the ball trajectory induced by the cast shadows in the illusion. Of these areas, it was suggested that the right POS may be involved in the inferring of the ball trajectory by the given spatial relation between the ball and the shadow. Our present results suggest that the posterior portion of the medial parietal cortex may be involved in 3D vision by cast shadows.