Temporal Dynamics of Neural Responses in Human Visual Cortex.

Neural responses to visual stimuli exhibit complex temporal dynamics, including subadditive temporal summation, response reduction with repeated or sustained stimuli (adaptation), and slower dynamics at low contrast. These phenomena are often studied independently. Here, we demonstrate these phenomena within the same experiment and model the underlying neural computations with a single computational model. We extracted time-varying responses from electrocorticographic recordings from patients presented with stimuli that varied in duration, interstimulus interval (ISI) and contrast. Aggregating data across patients from both sexes yielded 98 electrodes with robust visual responses, covering both earlier (V1-V3) and higher-order (V3a/b, LO, TO, IPS) retinotopic maps. In all regions, the temporal dynamics of neural responses exhibit several nonlinear features. Peak response amplitude saturates with high contrast and longer stimulus durations, the response to a second stimulus is suppressed for short ISIs and recovers for longer ISIs, and response latency decreases with increasing contrast. These features are accurately captured by a computational model composed of a small set of canonical neuronal operations, that is, linear filtering, rectification, exponentiation, and a delayed divisive normalization. We find that an increased normalization term captures both contrast- and adaptation-related response reductions, suggesting potentially shared underlying mechanisms. We additionally demonstrate both changes and invariance in temporal response dynamics between earlier and higher-order visual areas. Together, our results reveal the presence of a wide range of temporal and contrast-dependent neuronal dynamics in the human visual cortex and demonstrate that a simple model captures these dynamics at millisecond resolution.SIGNIFICANCE STATEMENT Sensory inputs and neural responses change continuously over time. It is especially challenging to understand a system that has both dynamic inputs and outputs. Here, we use a computational modeling approach that specifies computations to convert a time-varying input stimulus to a neural response time course, and we use this to predict neural activity measured in the human visual cortex. We show that this computational model predicts a wide variety of complex neural response shapes, which we induced experimentally by manipulating the duration, repetition, and contrast of visual stimuli. By comparing data and model predictions, we uncover systematic properties of temporal dynamics of neural signals, allowing us to better understand how the brain processes dynamic sensory information.

Mechanisms of speed encoding in the human middle temporal cortex measured by 7T fMRI.

Perception of dynamic scenes in our environment results from the evaluation of visual features such as the fundamental spatial and temporal frequency components of a moving object. The ratio between these two components represents the object's speed of motion. The human middle temporal cortex hMT+ has a crucial biological role in the direct encoding of object speed. However, the link between hMT+ speed encoding and the spatiotemporal frequency components of a moving object is still under explored. Here, we recorded high resolution 7T blood oxygen level-dependent BOLD responses to different visual motion stimuli as a function of their fundamental spatial and temporal frequency components. We fitted each hMT+ BOLD response with a 2D Gaussian model allowing for two different speed encoding mechanisms: (1) distinct and independent selectivity for the spatial and temporal frequencies of the visual motion stimuli; (2) pure tuning for the speed of motion. We show that both mechanisms occur but in different neuronal groups within hMT+, with the largest subregion of the complex showing separable tuning for the spatial and temporal frequency of the visual stimuli. Both mechanisms were highly reproducible within participants, reconciling single cell recordings from MT in animals that have showed both encoding mechanisms. Our findings confirm that a more complex process is involved in the perception of speed than initially thought and suggest that hMT+ plays a primary role in the evaluation of the spatial features of the moving visual input.

A mechanistic computational framework to investigate the hemodynamic fingerprint of the blood oxygenation level-dependent signal.

Blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is one of the most used imaging techniques to map brain activity or to obtain clinical information about human cortical vasculature, in both healthy and disease conditions. Nevertheless, BOLD fMRI is an indirect measurement of brain functioning triggered by neurovascular coupling. The origin of the BOLD signal is quite complex, and the signal formation thus depends, among other factors, on the topology of the cortical vasculature and the associated hemodynamic changes. To understand the hemodynamic evolution of the BOLD signal response in humans, it is beneficial to have a computational framework available that virtually resembles the human cortical vasculature, and simulates hemodynamic changes and corresponding MRI signal changes via interactions of intrinsic biophysical and magnetic properties of the tissues. To this end, we have developed a mechanistic computational framework that simulates the hemodynamic fingerprint of the BOLD signal based on a statistically defined, three-dimensional, vascular model that approaches the human cortical vascular architecture. The microvasculature is approximated through a Voronoi tessellation method and the macrovasculature is adapted from two-photon microscopy mice data. Using this computational framework, we simulated hemodynamic changes-cerebral blood flow, cerebral blood volume, and blood oxygen saturation-induced by virtual arterial dilation. Then we computed local magnetic field disturbances generated by the vascular topology and the corresponding blood oxygen saturation changes. This mechanistic computational framework also considers the intrinsic biophysical and magnetic properties of nearby tissue, such as water diffusion and relaxation properties, resulting in a dynamic BOLD signal response. The proposed mechanistic computational framework provides an integrated biophysical model that can offer better insights regarding the spatial and temporal properties of the BOLD signal changes.

Moving in on human motor cortex. Characterizing the relationship between body parts with non-rigid population response fields.

For cortical motor activity, the relationships between different body part representations is unknown. Through reciprocal body part relationships, functionality of cortical motor areas with respect to whole body motor control can be characterized. In the current study, we investigate the relationship between body part representations within individual neuronal populations in motor cortices, following a 7 Tesla fMRI 18-body-part motor experiment in combination with our newly developed non-rigid population Response Field (pRF) model and graph theory. The non-rigid pRF metrics reveal somatotopic structures in all included motor cortices covering frontal, parietal, medial and insular cortices and that neuronal populations in primary sensorimotor cortex respond to fewer body parts than secondary motor cortices. Reciprocal body part relationships are estimated in terms of uniqueness, clique-formation, and influence. We report unique response profiles for the knee, a clique of body parts surrounding the ring finger, and a central role for the shoulder and wrist. These results reveal associations among body parts from the perspective of the central nervous system, while being in agreement with intuitive notions of body part usage.

A vision of 14 T MR for fundamental and clinical science.

OBJECTIVE: We outline our vision for a 14 Tesla MR system. This comprises a novel whole-body magnet design utilizing high temperature superconductor; a console and associated electronic equipment; an optimized radiofrequency coil setup for proton measurement in the brain, which also has a local shim capability; and a high-performance gradient set. RESEARCH FIELDS: The 14 Tesla system can be considered a 'mesocope': a device capable of measuring on biologically relevant scales. In neuroscience the increased spatial resolution will anatomically resolve all layers of the cortex, cerebellum, subcortical structures, and inner nuclei. Spectroscopic imaging will simultaneously measure excitatory and inhibitory activity, characterizing the excitation/inhibition balance of neural circuits. In medical research (including brain disorders) we will visualize fine-grained patterns of structural abnormalities and relate these changes to functional and molecular changes. The significantly increased spectral resolution will make it possible to detect (dynamic changes in) individual metabolites associated with pathological pathways including molecular interactions and dynamic disease processes. CONCLUSIONS: The 14 Tesla system will offer new perspectives in neuroscience and fundamental research. We anticipate that this initiative will usher in a new era of ultra-high-field MR.

Size of the spatial correlation between ECoG and fMRI activity.

Electrocorticography (ECoG) is typically employed to accurately identify the seizure focus as well as the location of brain functions to be spared during surgical resection in participants with drug-resistant epilepsy. Increasingly, this technique has become a powerful tool to map cognitive functions onto brain regions. Cortical mapping is more commonly investigated with functional MRI (fMRI), which measures blood-oxygen level dependent (BOLD) changes induced by neuronal activity. The multimodal integration between typical 3T fMRI activity maps and ECoG measurements can provide unique insight into the spatiotemporal aspects of cognition. However, the optimal integration of fMRI and ECoG requires fundamental insight into the spatial smoothness of the BOLD signal under each electrode. Here we use ECoG as ground truth for the extent of activity, as each electrode is thought to record from the cortical tissue directly underneath the contact, to estimate the spatial smoothness of the associated BOLD response at 3T fMRI. We compared the high-frequency broadband (HFB) activity recorded with ECoG while participants performed a motor task. Activity maps were obtained with fMRI at 3T for the same task in the same participant prior to surgery. We then correlated HFB power with the fMRI BOLD signal change in the area around each electrode. This latter measure was quantified by applying a 3D Gaussian kernel of varying width (sigma between 1 mm and 20 mm) to the fMRI maps including only gray-matter. We found that the correlation between HFB and BOLD activity increased sharply up to the point when the kernel width was set to 4 mm, which we defined as the kernel width of maximal spatial specificity. After this point, as the kernel width increased, the highest level of explained variance was reached at a kernel width of 9 mm for most participants. Intriguingly, maximal specificity was also limited to 4 mm for low-frequency bands, such as alpha and beta, but the kernel width with the highest explained variance was less spatially limited than the HFB. In summary, spatial specificity is limited to a kernel width of 4 mm but explained variance keeps on increasing as you average over more and more voxels containing the relatively noisy BOLD signal. Future multimodal studies should choose the kernel width based on their research goal. For maximal spatial specificity, ECoG electrodes are best compared to 3T fMRI with a kernel width of 4 mm. When optimizing the correlation between modalities, highest explained variance can be obtained at larger kernel widths of 9 mm, at the expense of spatial specificity. Finally, we release the complete pipeline so that researchers can estimate the most appropriate kernel width from their multimodal datasets.

Validating Linear Systems Analysis for Laminar fMRI: Temporal Additivity for Stimulus Duration Manipulations.

Advancements in ultra-high field (7 T and higher) magnetic resonance imaging (MRI) scanners have made it possible to investigate both the structure and function of the human brain at a sub-millimeter scale. As neuronal feedforward and feedback information arrives in different layers, sub-millimeter functional MRI has the potential to uncover information processing between cortical micro-circuits across cortical depth, i.e. laminar fMRI. For nearly all conventional fMRI analyses, the main assumption is that the relationship between local neuronal activity and the blood oxygenation level dependent (BOLD) signal adheres to the principles of linear systems theory. For laminar fMRI, however, directional blood pooling across cortical depth stemming from the anatomy of the cortical vasculature, potentially violates these linear system assumptions, thereby complicating analysis and interpretation. Here we assess whether the temporal additivity requirement of linear systems theory holds for laminar fMRI. We measured responses elicited by viewing stimuli presented for different durations and evaluated how well the responses to shorter durations predicted those elicited by longer durations. We find that BOLD response predictions are consistently good predictors for observed responses, across all cortical depths, and in all measured visual field maps (V1, V2, and V3). Our results suggest that the temporal additivity assumption for linear systems theory holds for laminar fMRI. We thus show that the temporal additivity assumption holds across cortical depth for sub-millimeter gradient-echo BOLD fMRI in early visual cortex.

Sub-millimeter fMRI reveals multiple topographical digit representations that form action maps in human motor cortex.

The human brain coordinates a wide variety of motor activities. On a large scale, the cortical motor system is topographically organized such that neighboring body parts are represented by neighboring brain areas. This homunculus-like somatotopic organization along the central sulcus has been observed using neuroimaging for large body parts such as the face, hands and feet. However, on a finer scale, invasive electrical stimulation studies show deviations from this somatotopic organization that suggest an organizing principle based on motor actions rather than body part moved. It has not been clear how the action-map organization principle of the motor cortex in the mesoscopic (sub-millimeter) regime integrates into a body map organization principle on a macroscopic scale (cm). Here we developed and applied advanced mesoscopic (sub-millimeter) fMRI and analysis methodology to non-invasively investigate the functional organization topography across columnar and laminar structures in humans. Compared to previous methods, in this study, we could capture locally specific blood volume changes across entire brain regions along the cortical curvature. We find that individual fingers have multiple mirrored representations in the primary motor cortex depending on the movements they are involved in. We find that individual digits have cortical representations up to 3 â€‹mm apart from each other arranged in a column-like fashion. These representations are differentially engaged depending on whether the digits' muscles are used for different motor actions such as flexion movements, like grasping a ball or retraction movements like releasing a ball. This research provides a starting point for non-invasive investigation of mesoscale topography across layers and columns of the human cortex and bridges the gap between invasive electrophysiological investigations and large coverage non-invasive neuroimaging.

Pushing functional MRI spatial and temporal resolution further: High-density receive arrays combined with shot-selective 2D CAIPIRINHA for 3D echo-planar imaging at 7 T.

To be able to examine dynamic and detailed brain functions, the spatial and temporal resolution of 7 T MRI needs to improve. In this study, it was investigated whether submillimeter multishot 3D EPI fMRI scans, acquired with high-density receive arrays, can benefit from a 2D CAIPIRINHA sampling pattern, in terms of noise amplification (g-factor), temporal SNR and fMRI sensitivity. High-density receive arrays were combined with a shot-selective 2D CAIPIRINHA implementation for multishot 3D EPI sequences at 7 T. In this implementation, in contrast to conventional inclusion of extra kz gradient blips, specific EPI shots are left out to create a CAIPIRINHA shift and reduction of scan time. First, the implementation of the CAIPIRINHA sequence was evaluated with a standard receive setup by acquiring submillimeter whole brain T2 *-weighted anatomy images. Second, the CAIPIRINHA sequence was combined with high-density receive arrays to push the temporal resolution of submillimeter 3D EPI fMRI scans of the visual cortex. Results show that the shot-selective 2D CAIPIRINHA sequence enables a reduction in scan time for 0.5 mm isotropic 3D EPI T2 *-weighted anatomy scans by a factor of 4 compared with earlier reports. The use of the 2D CAIPIRINHA implementation in combination with high-density receive arrays, enhances the image quality of submillimeter 3D EPI scans of the visual cortex at high acceleration as compared to conventional SENSE. Both the g-factor and temporal SNR improved, resulting in a method that is more sensitive to the fMRI signal. Using this method, it is possible to acquire submillimeter single volume 3D EPI scans of the visual cortex in a subsecond timeframe. Overall, high-density receive arrays in combination with shot-selective 2D CAIPIRINHA for 3D EPI scans prove to be valuable for reducing the scan time of submillimeter MRI acquisitions.

Electrocorticography Evidence of Tactile Responses in Visual Cortices.

There is ongoing debate regarding the extent to which human cortices are specialized for processing a given sensory input versus a given type of information, independently of the sensory source. Many neuroimaging and electrophysiological studies have reported that primary and extrastriate visual cortices respond to tactile and auditory stimulation, in addition to visual inputs, suggesting these cortices are intrinsically multisensory. In particular for tactile responses, few studies have proven neuronal processes in visual cortex in humans. Here, we assessed tactile responses in both low-level and extrastriate visual cortices using electrocorticography recordings in a human participant. Specifically, we observed significant spectral power increases in the high frequency band (30-100 Hz) in response to tactile stimuli, reportedly associated with spiking neuronal activity, in both low-level visual cortex (i.e. V2) and in the anterior part of the lateral occipital-temporal cortex. These sites were both involved in processing tactile information and responsive to visual stimulation. More generally, the present results add to a mounting literature in support of task-sensitive and sensory-independent mechanisms underlying functions like spatial, motion, and self-processing in the brain and extending from higher-level as well as to low-level cortices.

Laminar fMRI: What can the time domain tell us?

The rapid developments in functional MRI (fMRI) acquisition methods and hardware technologies in recent years, particularly at high field (≥7 T), have enabled unparalleled visualization of functional detail at a laminar or columnar level, bringing fMRI close to the intrinsic resolution of brain function. These advances highlight the potential of high resolution fMRI to be a valuable tool to study the fundamental processing performed in cortical micro-circuits, and their interactions such as feedforward and feedback processes. Notably, because fMRI measures neuronal activity via hemodynamics, the ultimate resolution it affords depends on the spatial specificity of hemodynamics to neuronal activity at a detailed spatial scale, and by the evolution of this specificity over time. Several laminar (≤1 mm spatial resolution) fMRI studies have examined spatial characteristics of the measured hemodynamic signals across cortical depth, in light of understanding or improving the spatial specificity of laminar fMRI. Few studies have examined temporal features of the hemodynamic response across cortical depth. Temporal features of the hemodynamic response offer an additional means to improve the specificity of fMRI, and could help target neuronal processes and neurovascular coupling relationships across laminae, for example by differences in the onset times of the response across cortical depth. In this review, we discuss factors that affect the timing of neuronal and hemodynamic responses across laminae, touching on the neuronal laminar organization, and focusing on the laminar vascular organization. We provide an overview of hemodynamics across the cortical vascular tree based on optical imaging studies, and review temporal aspects of hemodynamics that have been examined across cortical depth in high spatiotemporal resolution fMRI studies. Last, we discuss the limits and potential of high spatiotemporal resolution fMRI to study laminar neurovascular coupling and neuronal processes.

Potential acceleration performance of a 256-channel whole-brain receive array at 7 T.

PURPOSE: Assess the potential gain in acceleration performance of a 256-channel versus 32-channel receive coil array at 7 T in combination with a 2D CAIPIRINHA sequence for 3D data sets. METHODS: A 256-channel receive setup was simulated by placing 2 small 16-channel high-density receive arrays at 2 × 8 different locations on the head of healthy participants. Multiple consecutive measurements were performed and coil sensitivity maps were combined to form a complete 256-channel data set. This setup was compared with a standard 32-channel head coil, in terms of SNR, noise correlation, and acceleration performance (g-factor). RESULTS: In the periphery of the brain, the receive SNR was on average a factor 1.5 higher (ranging up to a factor 2.7 higher) than the 32-channel coil; in the center of the brain the SNR was comparable or lower, depending on the size of the region of interest, with a factor 1.0 on average (ranging from 0.7 up to a factor of 1.6). The average noise correlation between coil elements was 3% for the 256-channel coil, and 5% for the 32-channel coil. At acceptable g-factors (< 2), the achievable acceleration factor using SENSE and 2D CAIPIRINHA was 24 and 28, respectively, versus 9 and 12 for the 32-channel coil. CONCLUSION: The receive performance of the simulated 256 channel array was better than the 32-channel reference. Combined with 2D CAIPIRINHA, a peak acceleration factor of 28 was assessed, showing great potential for high-density receive arrays.

SNR optimized 31 P functional MRS to detect mitochondrial and extracellular pH change during visual stimulation.

Energy metabolism of the human visual cortex was investigated by performing 31 P functional MRS. INTRODUCTION: The human brain is known to be the main glucose demanding organ of the human body and neuronal activity can increase this energy demand. In this study we investigate whether alterations in pH during activation of the brain can be observed with MRS, focusing on the mitochondrial inorganic phosphate (Pi) pool as potential marker of energy demand. METHODS: Six participants were scanned with 16 consecutive 31 P-MRSI scans, which were divided in 4 blocks of 8:36 minutes of either rest or visual stimulation. Since the signals from the mitochondrial compartments of Pi are low, multiple approaches to achieve high SNR 31 P measurements were combined. This included: a close fitting 31 P RF coil, a 7 T-field strength, Ernst angle acquisitions and a stimulus with a large visual angle allowing large spectroscopy volumes containing activated tissue. RESULTS: The targeted resonance downfield of the main Pi peak could be distinguished, indicating the high SNR of the 31 P spectra. The peak downfield of the main Pi peak is believed to be connected to mitochondrial performance. In addition, a BOLD effect in the PCr signal was observed as a signal increase of 2-3% during visual stimulation as compared to rest. When averaging data over multiple volunteers, a small subtle shift of about 0.1 ppm of the downfield Pi peak towards the main Pi peak could be observed in the first 4 minutes of visual stimulation, but no longer in the 4 to 8 minute scan window. Indications of a subtle shift during visual stimulation were found, but this effect remains small and should be further validated. CONCLUSION: Overall, the downfield peak of Pi could be observed, revealing opportunities and considerations to measure specific acidity (pH) effects in the human visual cortex.

Detailed somatotopy in primary motor and somatosensory cortex revealed by Gaussian population receptive fields.

The relevance of human primary motor cortex (M1) for motor actions has long been established. However, it is still unknown how motor actions are represented, and whether M1 contains an ordered somatotopy at the mesoscopic level. In the current study we show that a detailed within-limb somatotopy can be obtained in M1 during finger movements using Gaussian population Receptive Field (pRF) models. Similar organizations were also obtained for primary somatosensory cortex (S1), showing that individual finger representations are interconnected throughout sensorimotor cortex. The current study additionally estimates receptive field sizes of neuronal populations, showing differences between finger digit representations, between M1 and S1, and additionally between finger digit flexion and extension. Using the Gaussian pRF approach, the detailed somatotopic organization of M1 can be obtained including underlying characteristics, allowing for the in-depth investigation of cortical motor representation and sensorimotor integration.

Laminar imaging of positive and negative BOLD in human visual cortex at 7T.

Deciphering the direction of information flow is critical to understand the brain. Data from non-human primate histology shows that connections between lower to higher areas (e.g. retina→V1), and between higher to lower areas (e.g. V1←V2) can be dissociated based upon the distribution of afferent synapses at the laminar level. Ultra-high field scanners opened up the possibility to image brain structure and function at an unprecedented level of detail. Taking advantage of the increased spatial resolution available, it could theoretically be possible to disentangle activity from different cortical depths from human cerebral cortex, separately studying different compartments across depth. Here we use half-millimeter human functional and structural magnetic resonance imaging (fMRI, MRI) to derive laminar profiles in early visual cortex using a paradigm known to elicit two separate responses originating from an excitatory and a suppressive source, avoiding any contamination due to blood-stealing. We report the shape of laminar blood level oxygenation level dependent (BOLD) profiles from the excitatory and suppressive conditions. We analyse positive and negative %BOLD laminar profiles with respect to the dominating linear trend towards the pial surface, a confounding feature of gradient echo BOLD fMRI, and examine the correspondence with the anatomical landmark of input-related signals in primary visual cortex, the stria of Gennari.

Ultra-high field MRI: Advancing systems neuroscience towards mesoscopic human brain function.

Human MRI scanners at ultra-high magnetic field strengths of 7 T and higher are increasingly available to the neuroscience community. A key advantage brought by ultra-high field MRI is the possibility to increase the spatial resolution at which data is acquired, with little reduction in image quality. This opens a new set of opportunities for neuroscience, allowing investigators to map the human cortex at an unprecedented level of detail. In this review, we present recent work that capitalizes on the increased signal-to-noise ratio available at ultra-high field and discuss the theoretical advances with a focus on sensory and motor systems neuroscience. Further, we review research performed at sub-millimeter spatial resolution and discuss the limits and the potential of ultra-high field imaging for structural and functional imaging in human cortex. The increased spatial resolution achievable at ultra-high field has the potential to unveil the fundamental computations performed within a given cortical area, ultimately allowing the visualization of the mesoscopic organization of human cortex at the functional and structural level.

Maximizing sensitivity for fast GABA edited spectroscopy in the visual cortex at 7 T.

The combination of functional MRI (fMRI) and MRS is a promising approach to relate BOLD imaging to neuronal metabolism, especially at high field strength. However, typical scan times for GABA edited spectroscopy are of the order of 6-30 min, which is long compared with functional changes observed with fMRI. The aim of this study is to reduce scan time and increase GABA sensitivity for edited spectroscopy in the human visual cortex, by enlarging the volume of activated tissue in the primary visual cortex. A dedicated setup at 7 T for combined fMRI and GABA MRS is developed. This setup consists of a half volume multi-transmit coil with a large screen for visual cortex activation, two high density receive arrays and an optimized single-voxel MEGA-sLASER sequence with macromolecular suppression for signal acquisition. The coil setup performance as well as the GABA measurement speed, SNR, and stability were evaluated. A 2.2-fold gain of the average SNR for GABA detection was obtained, as compared with a conventional 7 T setup. This was achieved by increasing the viewing angle of the participant with respect to the visual stimulus, thereby activating almost the entire primary visual cortex, allowing larger spectroscopy measurement volumes and resulting in an improved GABA SNR. Fewer than 16 signal averages, lasting 1 min 23 s in total, were needed for the GABA fit method to become stable, as demonstrated in three participants. The stability of the measurement setup was sufficient to detect GABA with an accuracy of 5%, as determined with a GABA phantom. In vivo, larger variations in GABA concentration are found: 14-25%. Overall, the results bring functional GABA detections at a temporal resolution closer to the physiological time scale of BOLD cortex activation.

Topographic representations of object size and relationships with numerosity reveal generalized quantity processing in human parietal cortex.

Humans and many animals analyze sensory information to estimate quantities that guide behavior and decisions. These quantities include numerosity (object number) and object size. Having recently demonstrated topographic maps of numerosity, we ask whether the brain also contains maps of object size. Using ultra-high-field (7T) functional MRI and population receptive field modeling, we describe tuned responses to visual object size in bilateral human posterior parietal cortex. Tuning follows linear Gaussian functions and shows surround suppression, and tuning width narrows with increasing preferred object size. Object size-tuned responses are organized in bilateral topographic maps, with similar cortical extents responding to large and small objects. These properties of object size tuning and map organization all differ from the numerosity representation, suggesting that object size and numerosity tuning result from distinct mechanisms. However, their maps largely overlap and object size preferences correlate with numerosity preferences, suggesting associated representations of these two quantities. Object size preferences here show no discernable relation to visual position preferences found in visuospatial receptive fields. As such, object size maps (much like numerosity maps) do not reflect sensory organ structure but instead emerge within the brain. We speculate that, as in sensory processing, optimization of cognitive processing using topographic maps may be a common organizing principle in association cortex. Interactions between object size and numerosity maps may associate cognitive representations of these related features, potentially allowing consideration of both quantities together when making decisions.

Correspondence between fMRI and electrophysiology during visual motion processing in human MT.

Changes in brain neuronal activity are reflected by hemodynamic responses mapped through Blood Oxygenation Level Dependent (BOLD) functional magnetic resonance imaging (fMRI), a primary tool to measure brain functioning non-invasively. However, the exact relationship between hemodynamics and neuronal function is still a matter of debate. Here, we combine 3T BOLD fMRI and High Frequency Band (HFB) electrocorticography (ECoG) signals to investigate the relationship between neuronal activity and hemodynamic responses in the human Middle Temporal complex (hMT+), a higher order brain area involved in visual motion processing. We modulated the ECoG HFB and fMRI BOLD responses with a visual stimulus moving at different temporal frequencies, and compared measured BOLD responses to estimated BOLD responses that were predicted from the temporal profile of the HFB power change. We show that BOLD responses under an electrode over hMT+ can be well predicted not only be the strength of the neuronal response but also by the temporal profile of the HFB responses recorded by this electrode. Our results point to a linear relationship between BOLD and neuronal activity in hMT+, extending previous findings on primary cortex to higher order cortex.

Separate spatial and temporal frequency tuning to visual motion in human MT+ measured with ECoG.

The human middle temporal complex (hMT+) has a crucial biological relevance for the processing and detection of direction and speed of motion in visual stimuli. Here, we characterized how neuronal populations in hMT+ encode the speed of moving visual stimuli. We evaluated human intracranial electrocorticography (ECoG) responses elicited by square-wave dartboard moving stimuli with different spatial and temporal frequency to investigate whether hMT+ neuronal populations encode the stimulus speed directly, or whether they separate motion into its spatial and temporal components. We extracted two components from the ECoG responses: (1) the power in the high-frequency band (HFB: 65-95 Hz) as a measure of the neuronal population spiking activity and (2) a specific spectral component that followed the frequency of the stimulus's contrast reversals (SCR responses). Our results revealed that HFB neuronal population responses to visual motion stimuli exhibit distinct and independent selectivity for spatial and temporal frequencies of the visual stimuli rather than direct speed tuning. The SCR responses did not encode the speed or the spatiotemporal frequency of the visual stimuli. We conclude that the neuronal populations measured in hMT+ are not directly tuned to stimulus speed, but instead encode speed through separate and independent spatial and temporal frequency tuning. Hum Brain Mapp 38:293-307, 2017. © 2016 Wiley Periodicals, Inc.