[Show abstract][Hide abstract] ABSTRACT: How visual object categories are represented in the brain is one of the key questions in neuroscience. Studies on low-level visual features have shown that relative timings or phases of neural activity between multiple brain locations encode information. However, whether such temporal patterns of neural activity are used in the representation of visual objects is unknown. Here, we examined whether and how visual object categories could be predicted (or decoded) from temporal patterns of electrocorticographic (ECoG) signals from the temporal cortex in five patients with epilepsy. We used temporal correlations between electrodes as input features, and compared the decoding performance with features defined by spectral power and phase from individual electrodes. While using power or phase alone, the decoding accuracy was significantly better than chance, correlations alone or those combined with power outperformed other features. Decoding performance with correlations was degraded by shuffling the order of trials of the same category in each electrode, indicating that the relative time series between electrodes in each trial is critical. Analysis using a sliding time window revealed that decoding performance with correlations began to rise earlier than that with power. This earlier rise time was replicated by a model using phase differences to encode categories. These results suggest that activity patterns arising from interactions between multiple neuronal units carry additional information on visual object categories.
[Show abstract][Hide abstract] ABSTRACT: Recognition of faces and written words is associated with category-specific brain activation in the ventral occipitotemporal cortex (vOT). However, topological and functional relationships between face-selective and word-selective vOT regions remain unclear. In this study, we collected data from patients with intractable epilepsy who underwent high-density recording of surface field potentials in the vOT. "Faces" and "letterstrings" induced outstanding category-selective responses among the 24 visual categories tested, particularly in high-γ band powers. Strikingly, within-hemispheric analysis revealed alternation of face-selective and letterstring-selective zones within the vOT. Two distinct face-selective zones located anterior and posterior portions of the mid-fusiform sulcus whereas letterstring-selective zones alternated between and outside of these 2 face-selective zones. Further, a classification analysis indicated that activity patterns of these zones mostly represent dedicated categories. Functional connectivity analysis using Granger causality indicated asymmetrically directed causal influences from face-selective to letterstring-selective regions. These results challenge the prevailing view that different categories are represented in distinct contiguous regions in the vOT.
[Show abstract][Hide abstract] ABSTRACT: Abstract Electrocorticogram (ECoG) is an electrophysiological brain activity recording technique that has been widely revisited in recent years, not only for clinical monitoring, but also for prosthetic applications. However, the extent and limitations of the technique are poorly understood. Higher areas of human and macaque ventral visual cortices are known to have functional domain structures that are selective to certain categories, and population vectors that have been derived from visually evoked single-unit activity (SUA) recording in this region have been shown to form category clusters. How can visually evoked potentials recorded with ECoG from the same region be exploited to extract category information? To answer this question, the development of a simultaneous ECoG and SUA recording device by the modification of a previously reported flexible mesh ECoG probe with a microelectromechanical system has been promising (Toda et al., 2011). Indeed, Toda et al. conducted simultaneous recordings and reported that mesh ECoG signals exhibited comparable or better signal variabilities compared to conventional methods in the rat visual cortex. With this approach, we conducted intensive simultaneous ECoG and SUA recordings from the macaque anterior inferior temporal (IT) cortex. We compared how basic visual category and fine information is decoded from different recording modalities. Our preliminary results indicated that ECoG signals from the IT cortex may be a useful source for reading out certain levels of category information from visual input.
Brain and nerve = Shinkei kenkyū no shinpo 06/2013; 65(6):643-50.
[Show abstract][Hide abstract] ABSTRACT: BACKGROUND:: There has been a growing interest in clinical single-neuron recording, to better understand epileptogenicity and brain function. It is crucial to compare this new information, single-neuronal activity, with that obtained from conventional intracranial electroencephalographyduring simultaneous recording. However, it is difficult to implant microwires and subdural electrodes during a single surgical operation, since the stereotactic frame hampers flexible craniotomy. OBJECTIVE:: We describenewly designed electrodes as well as surgical techniques to implant these with subdural electrodes that enable simultaneous recording from hippocampal neurons and broad areas of the cortical surface. METHODS:: We designed a depth electrode that does not protrude into the dura and pulsates naturally with the brain. The length and the tract of the depth electrode were determined preoperatively between the lateral subiculum and the lateral surface of the temporal lobe. A frameless navigation system was used to insert the depth electrode. Surface grids and ventral strips were placed before and after the insertion of the depth electrodes, respectively. Finally, a microwire bundle was inserted into the lumen of the depth electrode. We evaluated the precision of implantation, the recording stability, and therecording rate with microwire electrodes. RESULTS:: Depth-microwire electrodes were placed with a precision of 3.6mm. The mean successful recording rate of single- or multiple-unit activity was 14.8%, which was maintained throughout the entire recording period. CONCLUSION:: We achieved simultaneous implantation of microwires, depth electrodes and broad-area subdural electrodes. Our method enabled simultaneous and stable recording of hippocampal single-neuron activities and multi-channel intracranial electroencephalography.
[Show abstract][Hide abstract] ABSTRACT: A sensation of depth can arise from two-dimensional (2D) movies without any stereoscopic depth cue. Depth perception in three-dimensional (3D) space depends on the stability of stereoscopic gaze by vergence – coordinated movement of the two eyes in opposite directions – compensating the misalignment of the retinal images from the two eyes (i.e. binocular disparity) . On the other hand, the oculomotor mechanisms that stabilize stereoscopic gaze and depth perception in 2D movie space remain unclear . Here, we propose a hypothesis that vergence eye movements signifying 3D depth perception persist during prolonged 2D movie presentation without binocular disparity cues. By measuring eye positions while the subject viewed moving random-dot video stimuli, we show that sustained vergence is induced during 30-s exposure to radially expanding 2D optic flow. Moreover, a 2D video movie showing a passenger’s view of a roller coaster induces continuously changing vergence. In the absence of binocular disparity cues, the pictorial depth information within a 5° × 5° gaze window and optic flow in the movie simultaneously and independently influence vergence. The pictorial gaze-area depth information affects vergence mainly in the virtual far space, whereas optic flow robustly affects vergence irrespective of the nearness. These findings demonstrate that vergence serves as a reliable marker signifying 3D depth perception from 2D movies, imposing critical constraints on creation of an effective and safe virtual reality.
[Show abstract][Hide abstract] ABSTRACT: Electrocorticogram (ECoG) is a well-balanced methodology for stably mapping brain surface local field potentials (LFPs) over a wide cortical region with high signal fidelity and minimal invasiveness to the brain tissue. To directly compare surface ECoG signals with intracortical neuronal activity immediately underneath, we fabricated a flexible multichannel electrode array with mesh-form structure using micro-electro-mechanical systems. A Parylene-C-based "electrode-mesh" for rats contained a 6×6 gold electrode array with 1-mm interval. Specifically, the probe had 800×800 μm(2) fenestrae in interelectrode spaces, through which simultaneous penetration of microelectrode was capable. This electrode-mesh was placed acutely or chronically on the dural/pial surface of the visual cortex of Long-Evans rats for up to 2 weeks. We obtained reliable trial-wise profiles of visually evoked ECoG signals through individual eye stimulation. Visually evoked ECoG signals from the electrode-mesh exhibited as well or larger signal amplitudes as intracortical LFPs and less across-trial variability than conventional silver-ball ECoG. Ocular selectivity of ECoG responses was correlated with that of intracortical spike/LFP activities. Moreover, single-trial ECoG signals carried sufficient information for predicting the stimulated eye with a correct performance approaching 90%, and the decoding was significantly generalized across sessions over 6 hours. Electrode impedance or signal quality did not obviously deteriorate for 2 weeks following implantation. These findings open up a methodology to directly explore ECoG signals with reference to intracortical neuronal sources and would provide a key to developing minimally invasive next-generation brain-machine interfaces.
[Show abstract][Hide abstract] ABSTRACT: Electrocorticography (ECoG), multichannel brain-surface recording and stimulation with probe electrode arrays, has become a potent methodology not only for clinical neurosurgery but also for basic neuroscience using animal models. The highly evolved primate's brain has deep cerebral sulci, and both gyral and intrasulcal cortical regions have been implicated in important functional processes. However, direct experimental access is typically limited to gyral regions, since placing probes into sulci is difficult without damaging the surrounding tissues. Here we describe a novel methodology for intrasulcal ECoG in macaque monkeys. We designed and fabricated ultra-thin flexible probes for macaques with micro-electro-mechanical systems technology. We developed minimally invasive operative protocols to implant the probes by introducing cutting-edge devices for human neurosurgery. To evaluate the feasibility of intrasulcal ECoG, we conducted electrophysiological recording and stimulation experiments. First, we inserted parts of the Parylene-C-based probe into the superior temporal sulcus to compare visually evoked ECoG responses from the ventral bank of the sulcus with those from the surface of the inferior temporal cortex. Analyses of power spectral density and signal-to-noise ratio revealed that the quality of the ECoG signal was comparable inside and outside of the sulcus. Histological examination revealed no obvious physical damage in the implanted areas. Second, we placed a modified silicone ECoG probe into the central sulcus and also on the surface of the precentral gyrus for stimulation. Thresholds for muscle twitching were significantly lower during intrasulcal stimulation compared to gyral stimulation. These results demonstrate the feasibility of intrasulcal ECoG in macaques. The novel methodology proposed here opens up a new frontier in neuroscience research, enabling the direct measurement and manipulation of electrical activity in the whole brain.
[Show abstract][Hide abstract] ABSTRACT: The frontal and parietal eye fields serve as functional landmarks of the primate brain, although their correspondences between humans and macaque monkeys remain unclear. We conducted fMRI at 4.7 T in monkeys performing visually-guided saccade tasks and compared brain activations with those in humans using identical paradigms. Among multiple parietal activations, the dorsal lateral intraparietal area in monkeys and an area in the posterior superior parietal lobule in humans exhibited the highest selectivity to saccade directions. In the frontal cortex, the selectivity was highest at the junction of the precentral and superior frontal sulci in humans and in the frontal eye field (FEF) in monkeys. BOLD activation peaks were also found in premotor areas (BA6) in monkeys, which suggests that the apparent discrepancy in location between putative human FEF (BA6, suggested by imaging studies) and monkey FEF (BA8, identified by microstimulation studies) partly arose from methodological differences.
[Show abstract][Hide abstract] ABSTRACT: We examined prefrontal neuronal activity while monkeys performed a sequential target-shift task, in which, after a positional cue indicated the initial saccade target among 8 peripheral positions, the monkeys were required to internally shift the target by one position on every flash of a target-shift cue. The target-shift cue appeared in the center 0 to 3 times within a single trial and was always the same in shape, size, and color. We found selective neuronal activity related to the target position: when the target-shift cue implied the target shift to particular peripheral positions, neurons exhibited early-dominant and late-dominant activity during the following delay period. The early-dominant target-selective activity emerged early in the delay just after the presentation of the target-shift cue, whereas the late-dominant activity gradually built up toward the end of the delay. Because the target-shift cue was not related to any specific target location, the early-dominant target-selective activity could not be a mere visual response to the target-shift cue. We suggest that the early-dominant activity reflects the transitory representation for the saccade target that was triggered by the nonspatial target-shift cue, whereas the late-dominant activity reflects the target representation in the spatial working memory or the preparatory set for the possible impending saccade, being repeatedly updated during sequential target shifts.
Journal of Neurophysiology 04/2004; 91(3):1367-80. · 3.30 Impact Factor