Daigo Takeuchi

Reversal of Interlaminar Signal Between Sensory and Memory Processing in Monkey Temporal Cortex

Feed-forward signals flowing across cortical layers during sensory coding reverse direction during memory retrieval. The primate temporal cortex implements visual long-term memory. However, how its interlaminar circuitry executes cognitive computations is poorly understood. Using linear-array multicontact electrodes, we simultaneously recorded unit activities across cortical layers in the perirhinal cortex of macaques performing a pair-association memory task. Cortical layers were estimated on the basis of current source density profiles with histological verifications, and the interlaminar signal flow was determined with cross-correlation analysis between spike trains. During the cue period, canonical “feed-forward” signals flowed from granular to supragranular layers and from supragranular to infragranular layers. During the delay period, however, the signal flow reversed to the “feed-back” direction: from infragranular to supragranular layers. This reversal of signal flow highlights how the temporal cortex differentially recruits its laminar circuits for sensory and mnemonic processing.

Active Maintenance of Associative Mnemonic Signal in Monkey Inferior Temporal Cortex

We investigated the contribution of the inferior temporal (IT) cortical neurons to the active maintenance of internal representations. The activity of single neurons in the IT cortex was recorded while the monkeys performed a sequential-type associative memory task in which distractor stimuli interrupted the delay epoch between the cue and target (paired-associate) stimuli. For each neuron, information about each stimulus conveyed by the delay activity was estimated as a coefficient of multiple regression analysis. We found that target information derived from long-term memory (LTM) persisted despite the distractors. By contrast, cue information derived from the visual system was attenuated and frequently replaced by distractor information. These results suggest that LTM-derived information required for upcoming behavior is actively maintained in the IT neurons, whereas visually derived information tends to be updated irrespective of behavioral relevance.

Direct Comparison of Spontaneous Functional Connectivity and Effective Connectivity Measured by Intracortical Microstimulation: An fMRI Study in Macaque Monkeys

Correlated spontaneous activity in the resting brain is increasingly recognized as a useful index for inferring underlying functional-anatomic architecture. However, despite efforts for comparison with anatomical connectivity, neuronal origin of intrinsic functional connectivity (inFC) remains unclear. Conceptually, the source of inFC could be decomposed into causal components that reflect the efficacy of synaptic interactions and other components mediated by collective network dynamics (e.g., synchronization). To dissociate these components, it is useful to introduce another connectivity measure such as effective connectivity, which is a quantitative measure of causal interactions. Here, we present a direct comparison of inFC against emEC (effective connectivity probed with electrical microstimulation [EM]) in the somatosensory system of macaque monkeys. Simultaneous EM and functional magnetic resonance imaging revealed strong emEC in several brain regions in a manner consistent with the anatomy of somatosensory system. Direct comparison of inFC and emEC revealed colocalization and overall positive correlation within the stimulated hemisphere. Interestingly, we found characteristic differences between inFC and emEC in their interhemispheric patterns. Our results suggest that intrahemispheric inFC reflects the efficacy of causal interactions, whereas interhemispheric inFC may arise from interactions akin to network-level synchronization that is not captured by emEC.

Unitized representation of paired objects in area 35 of the macaque perirhinal cortex

The perirhinal cortex, which is critical for long‐term stimulus–stimulus associative memory, consists of two cytoarchitectonically distinct subdivisions: area 35 (A35) and area 36 (A36). Previous electrophysiological studies suggested that macaque A36 is involved in both association and retrieval processes during a visual pair‐association task. However, the neuronal properties of macaque A35 have never been examined because A35 is located in a very narrow region, which makes it difficult to systematically record single‐unit activity from there. In the present study, we overcame this technical difficulty for targeting A35 by combining magnetic resonance imaging‐guided in‐vivo localization with postmortem histological localization. This two‐track approach enabled us to record from 181 A35 neurons in two macaque monkeys while they performed a pair‐association task. Among these neurons, 64 showed stimulus‐selective responses during the cue period (cue‐selective neurons), whereas 18 did during the delay period (delay‐selective neurons). As in A36, the responses of cue‐selective neurons in A35 to paired associates were correlated. In both areas, these correlations were stronger in neurons showing delay selectivity than in those without delay selectivity. Notably, delay‐selective neurons in A35 responded similarly to the optimal stimulus and its paired associate, whereas delay‐selective neurons in A36 discriminated between them. However, these neurons in both areas discriminated the primary pair, consisting of the optimal stimulus and its paired associate, from other pairs, indicating that selectivity across pairs was maintained between the two areas. These results suggest that delay‐selective neurons in A35 represent these paired stimuli as a single unitized item rather than two associated items.

Microcircuits for hierarchical elaboration of object coding across primate temporal areas.

Hierarchy and Representation Neuronal representations of objects are elaborated through the hierarchy of occipitotemporal cortical areas, and the recognition of a feature as “novel” is commonly thought to emerge and become prevalent at a cortical area because of local signal processing. Hirabayashi et al. (p. 191) tested another possibility—that a feature representation becomes prevalent in a given area because a microcircuit creates a small number of precursor representations in a prior area in the cortical hierarchy, and the representations then become prevalent through proliferation in the subsequent area. In support of this notion, critical microcircuits for object association were observed using multiple single-unit recordings in two areas of the macaque temporal cortex. Neuronal activity representing novel features emerges in hierarchically lower brain areas earlier than previously thought. In primates, neuronal representations of objects are processed hierarchically in occipitotemporal cortices. A “novel” feature of objects is thought to emerge and become prevalent at a cortical area because of processing in this area. We tested the possibility that a feature representation prevalent in a given area emerges in the microcircuit of a hierarchically prior area as a small number of prototypes and then becomes prevalent in the subsequent area. We recorded multiple single units in each of hierarchically sequential areas TE and 36 of macaque temporal cortex and found the predicted convergent microcircuit for object-object association in area TE. Associative codes were then built up over time in the microcircuit of area 36. These results suggest a computational principle underlying sequentially elaborated object representations.

Triphasic Dynamics of Stimulus-Dependent Information Flow between Single Neurons in Macaque Inferior Temporal Cortex

The functional connectivity between cortical neurons is not static and is known to exhibit contextual modulations in terms of the coupling strength. Here we hypothesized that the information flow in a cortical local circuit exhibits complex forward-and-back dynamics, and conducted Granger causality analysis between the neuronal spike trains that were simultaneously recorded from macaque inferior temporal (IT) cortex while the animals performed a visual object discrimination task. Spikes from neuron pairs with a displaced peak on the cross-correlogram (CCG) showed Granger causality in the gamma-frequency range (30–80 Hz) with the dominance in the direction consistent with the CCG peak (forward direction). Although, in a classical view, the displaced CCG peak has been interpreted as an indicative of a pauci-synaptic serial linkage, temporal dynamics of the gamma Granger causality after stimulus onset exhibited a more complex triphasic pattern, with a transient forward component followed by a slowly developing backward component and subsequent reappearance of the forward component. These triphasic dynamics of causality were not explained by the firing rate dynamics and were not observed for cell pairs that exhibited a center peak on the CCG. Furthermore, temporal dynamics of Granger causality depended on the feature configuration within the presented object. Together, these results demonstrate that the classical view of functional connectivity could be expanded to incorporate more complex forward-and-back dynamics and also imply that multistage processing in the recognition of visual objects might be implemented by multiphasic dynamics of directional information flow between single neurons in a local circuit in the IT cortex.

Distinct Neuronal Interactions in Anterior Inferotemporal Areas of Macaque Monkeys during Retrieval of Object Association Memory

In macaque monkeys, the anterior inferotemporal cortex, a region crucial for object memory processing, is composed of two adjacent, hierarchically distinct areas, TE and 36, for which different functional roles and neuronal responses in object memory tasks have been characterized. However, it remains unknown how the neuronal interactions differ between these areas during memory retrieval. Here, we conducted simultaneous recordings from multiple single-units in each of these areas while monkeys performed an object association memory task and examined the inter-area differences in neuronal interactions during the delay period. Although memory neurons showing sustained activity for the presented cue stimulus, cue-holding (CH) neurons, interacted with each other in both areas, only those neurons in area 36 interacted with another type of memory neurons coding for the to-be-recalled paired associate (pair-recall neurons) during memory retrieval. Furthermore, pairs of CH neurons in area TE showed functional coupling in response to each individual object during memory retention, whereas the same class of neuron pairs in area 36 exhibited a comparable strength of coupling in response to both associated objects. These results suggest predominant neuronal interactions in area 36 during the mnemonic processing, which may underlie the pivotal role of this brain area in both storage and retrieval of object association memory.

Submodality-dependent spatial organization of neurons coding for visual long-term memory in macaque inferior temporal cortex

The inferior temporal (IT) cortex has been shown to serve as a storehouse of visual long-term memory for object shapes. However, it is currently unclear how information regarding multiple visual attributes of objects, including shape and color, is stored and retrieved in an organized way. Specifically, the question of whether information regarding different visual attributes is encoded by different neurons, and the spatial organization of neurons that encode visual attribute-dependent object information remain to be elucidated. In the present study, we trained monkeys to perform a pair-association task with two stimulus sets, in which individual stimuli were either visually discernible by shape or by color. We examined both the responses of single neurons and their spatial distributions in area 36 of the IT cortex. We found that a significant majority of visually responsive neurons showed stimulus selectivity for only one of the two visual attributes. Moreover, neuronal activity encoding the learned pair-associations was observed only in neurons that exhibited stimulus selectivity for one of the two visual attributes. A spatial distribution analysis demonstrated that the neurons coding for each stimulus set were not randomly distributed, but were localized in two separate clusters, each corresponding to a different visual attribute. Together, these results suggest that pair-association memory for different visual attributes is distinctly stored in the IT cortex both in terms of neuronal responses and the spatial organization of neurons coding for each visual attribute.

Sensory and mnemonic demands flexibly recruit interlaminar microcircuits in macaque temporal cortex

orientation map obtained by OISI. As a result, fOCT revealed that optimal orientation was roughly the same along the depth axis, and the orientation shifted gradually along the axis parallel to the cortical surface consistent with the classical view of columnar organization. Integrated orientation selectivity from depth showed high correlation with OISI result (R > 0.7). However, sudden change in optimal orientation selectivity was also observed locally in depth axis unlike previous studies.

A new glass-coated tungsten optrode enclosing multiple optic fibers for fluorescence measurement, optogenetic photo-stimulation, and single-unit recording in deep brain regions

increasing the local electrical field of the observed region by increasing their resistance higher than around. The Ca imaging result indicated that the neurons not only on the electrode but ones around it responds. We found that the stimulating voltage where first Ca spikes occur has threshold. Also, we analyzed the Ca response by the raster plot of spikes, and confirmed specific Ca propagation between neurons. The trigger of the response is the Ca spike of on-electrode neurons. In conclusion, this device is the best tool to observe the change of the plasticity of neuronal synaptic connection by controlling stimulus timing of multiple electrode lines. Research fund: Strategic Research Foundation Grant-aided Project for Private Universities from Ministry of Education, Culture, Sport, Science, and Technology, Japan (MEXT), 2008–2012 (S0801008).