Biomedical subjects

Kenichi Ohki

Publications and source records attributed to Kenichi Ohki.

6 recordsLinked to original sources

PubMed2006

Diffusion tensor studies dissociated two fronto-temporal pathways in the human memory system.

Recent functional neuroimaging studies have shown that multiple cortical areas are involved in memory encoding and retrieval. However, the underlying anatomical connections among these memory-related areas in humans remain elusive due to methodological limitations. Diffusion tensor imaging (DTI) is a technique based on detecting the diffusion of water molecules from magnetic resonance images. DTI allows non-invasive mapping of anatomical connections and gives a comprehensive picture of connectivity throughout the entire brain. By combining functional magnetic resonance imaging (fMRI) and DTI, we show that memory-related areas in the left dorsolateral prefrontal cortex (DLPFC) and the left ventrolateral prefrontal cortex (VLPFC) each connect with memory-related areas in the left temporal cortex. This result suggests there are two pathways between prefrontal cortex and temporal cortex related to the human memory system.

Adult↗

PubMed2006

Highly ordered arrangement of single neurons in orientation pinwheels.

In the visual cortex of higher mammals, neurons are arranged across the cortical surface in an orderly map of preferred stimulus orientations. This map contains 'orientation pinwheels', structures that are arranged like the spokes of a wheel such that orientation changes continuously around a centre. Conventional optical imaging first demonstrated these pinwheels, but the technique lacked the spatial resolution to determine the response properties and arrangement of cells near pinwheel centres. Electrophysiological recordings later demonstrated sharply selective neurons near pinwheel centres, but it remained unclear whether they were arranged randomly or in an orderly fashion. Here we use two-photon calcium imaging in vivo to determine the microstructure of pinwheel centres in cat visual cortex with single-cell resolution. We find that pinwheel centres are highly ordered: neurons selective to different orientations are clearly segregated even in the very centre. Thus, pinwheel centres truly represent singularities in the cortical map. This highly ordered arrangement at the level of single cells suggests great precision in the development of cortical circuits underlying orientation selectivity.

Animals↗

PubMed2005

Functional imaging with cellular resolution reveals precise micro-architecture in visual cortex.

Neurons in the cerebral cortex are organized into anatomical columns, with ensembles of cells arranged from the surface to the white matter. Within a column, neurons often share functional properties, such as selectivity for stimulus orientation; columns with distinct properties, such as different preferred orientations, tile the cortical surface in orderly patterns. This functional architecture was discovered with the relatively sparse sampling of microelectrode recordings. Optical imaging of membrane voltage or metabolic activity elucidated the overall geometry of functional maps, but is averaged over many cells (resolution >100 microm). Consequently, the purity of functional domains and the precision of the borders between them could not be resolved. Here, we labelled thousands of neurons of the visual cortex with a calcium-sensitive indicator in vivo. We then imaged the activity of neuronal populations at single-cell resolution with two-photon microscopy up to a depth of 400 microm. In rat primary visual cortex, neurons had robust orientation selectivity but there was no discernible local structure; neighbouring neurons often responded to different orientations. In area 18 of cat visual cortex, functional maps were organized at a fine scale. Neurons with opposite preferences for stimulus direction were segregated with extraordinary spatial precision in three dimensions, with columnar borders one to two cells wide. These results indicate that cortical maps can be built with single-cell precision.

Analysis of Variance↗

PubMed2003

Conversion of working memory to motor sequence in the monkey premotor cortex.

To plan a serial order behavior, we hold serial sensory information in our minds and convert it to a movement program. We trained monkeys to memorize a sequence of positional cues and to reproduce it by making saccades in either the original or reverse order. The order was determined in the middle of a trial on the basis of an instruction stimulus. Triggered by the instruction stimulus, single neurons in the dorsal premotor cortex became transiently active only when the order needed to be determined. These transient neurons, together with nearby sustained neurons that hold information on cue or movement sequences, appear to mediate the generation of a motor program from the maintained information.

Animals↗

PubMed2002

The role of the parahippocampal gyrus in source memory for external and internal events.

We can discriminate between the memories of real and imagined events. In this study, the traces of the perceived external events and the imagined internal events were investigated in the established paradigm of reality monitoring using event-related fMRI. In the retrieval phase, we found that the left parahippocampal gyrus represented the traces of visually encoded memory. The right inferior parietal cortex was activated when subjects judged that the original event was imagined. We suggest that these traces are used to distinguish what is seen from what is thought during reality monitoring. Furthermore, we found that the incorrect judgments were associated with signal increases in the left frontal operculum, suggesting that this area is a candidate for the monitoring system of contextual information or failure in the retrieval phase.

Adult↗

PubMed2002

Neural correlates for feeling-of-knowing: an fMRI parametric analysis.

The "feeling-of-knowing" (FOK) is a subjective sense of knowing a word before recalling it, and the FOK provides us clues to understanding the mechanisms of human metamemory systems. We investigated neural correlates for the FOK based on the recall-judgment-recognition paradigm. Event-related functional magnetic resonance imaging with a parametric analysis was used. We found activations in left dorsolateral, left anterior, bilateral inferior, and medial prefrontal cortices that significantly increased as the FOK became greater, and the activations remained significant even when the potentially confounding factor of the response latency was removed. Furthermore, we demonstrated that the FOK region in the right inferior frontal gyrus and a subset of the FOK region in the left inferior frontal gyrus are not recruited for successful recall processes, suggesting their particular role in metamemory processing.

Adult↗