Bottom-up retinotopic organization supports top-down mental imagery

Finding a path between locations is a routine task in daily life. Mental navigation is often used to plan a route to a destination that is not visible from the current location. We first used functional magnetic resonance imaging (fMRI) and surface-based averaging methods to find high-level brain regions involved in imagined navigation between locations in a building very familiar to each participant. This revealed a mental navigation network that includes the precuneus, retrosplenial cortex (RSC), parahippocampal place area (PPA), occipital place area (OPA), supplementary motor area (SMA), premotor cortex, and areas along the medial and anterior intraparietal sulcus. We then visualized retinotopic maps in the entire cortex using wide-field, natural scene stimuli in a separate set of fMRI experiments. This revealed five distinct visual streams or ‘fingers’ that extend anteriorly into middle temporal, superior parietal, medial parietal, retrosplenial and ventral occipitotemporal cortex. By using spherical morphing to overlap these two data sets, we showed that the mental navigation network primarily occupies areas that also contain retinotopic maps. Specifically, scene-selective regions RSC, PPA and OPA have a common emphasis on the far periphery of the upper visual field. These results suggest that bottom-up retinotopic organization may help to efficiently encode scene and location information in an eye-centered reference frame for top-down, internally generated mental navigation. This study pushes the border of visual cortex further anterior than was initially expected

Changing Human Visual Field Organization from Early Visual to Extra-Occipital Cortex

BACKGROUND: The early visual areas have a clear topographic organization, such that adjacent parts of the cortical surface represent distinct yet adjacent parts of the contralateral visual field. We examined whether cortical regions outside occipital cortex show a similar organization. METHODOLOGY/PRINCIPAL FINDINGS: The BOLD responses to discrete visual field locations that varied in both polar angle and eccentricity were measured using two different tasks. As described previously, numerous occipital regions are both selective for the contralateral visual field and show topographic organization within that field. Extra-occipital regions are also selective for the contralateral visual field, but possess little (or no) topographic organization. A regional analysis demonstrates that this weak topography is not due to increased receptive field size in extra-occipital areas. CONCLUSIONS/SIGNIFICANCE: A number of extra-occipital areas are identified that are sensitive to visual field location. Neurons in these areas corresponding to different locations in the contralateral visual field do not demonstrate any regular or robust topographic organization, but appear instead to be intermixed on the cortical surface. This suggests a shift from processing that is predominately local in visual space, in occipital areas, to global, in extra-occipital areas. Global processing fits with a role for these extra-occipital areas in selecting a spatial locus for attention and/or eye-movements

Areas activated during naturalistic reading comprehension overlap topological visual, auditory, and somatotomotor maps

Cortical mapping techniques using fMRI have been instrumental in identifying the boundaries of topological (neighbor-preserving) maps in early sensory areas. The presence of topological maps beyond early sensory areas raises the possibility that they might play a significant role in other cognitive systems, and that topological mapping might help to delineate areas involved in higher cognitive processes. In this study, we combine surface-based visual, auditory, and somatomotor mapping methods with a naturalistic reading comprehension task in the same group of subjects to provide a qualitative and quantitative assessment of the cortical overlap between sensory-motor maps in all major sensory modalities, and reading processing regions. Our results suggest that cortical activation during naturalistic reading comprehension overlaps more extensively with topological sensory-motor maps than has been heretofore appreciated. Reading activation in regions adjacent to occipital lobe and inferior parietal lobe almost completely overlaps visual maps, whereas a significant portion of frontal activation for reading in dorsolateral and ventral prefrontal cortex overlaps both visual and auditory maps. Even classical language regions in superior temporal cortex are partially overlapped by topological visual and auditory maps. By contrast, the main overlap with somatomotor maps is restricted to a small region on the anterior bank of the central sulcus near the border between the face and hand representations of M-I

Mapping the human cortical surface by combining quantitative T(1) with retinotopy

We combined quantitative relaxation rate (R1= 1/T1) mapping-to measure local myelination-with fMRI-based retinotopy. Gray-white and pial surfaces were reconstructed and used to sample R1 at different cortical depths. Like myelination, R1 decreased from deeper to superficial layers. R1 decreased passing from V1 and MT, to immediately surrounding areas, then to the angular gyrus. High R1 was correlated across the cortex with convex local curvature so the data was first "de-curved". By overlaying R1 and retinotopic maps, we found that many visual area borders were associated with significant R1 increases including V1, V3A, MT, V6, V6A, V8/VO1, FST, and VIP. Surprisingly, retinotopic MT occupied only the posterior portion of an oval-shaped lateral occipital R1 maximum. R1 maps were reproducible within individuals and comparable between subjects without intensity normalization, enabling multi-center studies of development, aging, and disease progression, and structure/function mapping in other modalities

The topographic connectome

Central to macro-connectomics and much of systems neuroscience is the idea that we can summarise macroscopic brain connectivity using a network of ‘nodes’ and ‘edges’ — functionally distinct brain regions and the connections between them. This is an approach that allows a deep understanding of brain dynamics and how they relate to brain circuitry. This approach, however, ignores key features of anatomical connections, such as spatial arrangement and topographic mappings. In this article, we suggest an alternative to this paradigm. We propose that connection topographies can inform us about brain networks in ways that are complementary to the concepts of ‘nodes’ and ‘edges’. We also show that current neuroimaging technology is capable of revealing details of connection topographies in vivo. These advances, we hope, will allow us to explore brain connectivity in novel ways in the immediate future

Tremotopic mapping of the human thalamic reticular nucleus

The thalamic reticular nucleus is an important structure in the mammalian brain, participating in the coordination of large-scale processes such as sleep and attention. To date, this structure has not been investigated in the human brain. I developed a series of methods for anatomically and functionally localizing the visual regions of the thalamic reticular nucleus in the human brain using magnetic resonance imaging and the presentation of various flicker frequencies. First, I describe the results obtained from a modified retinotopy analysis. I next apply network theory to the data in an attempt to localize the TRN is a data-driven way. Third, I describe a lateral-inhibitory network the TRN participates in. I conclude the TRN plays a role in regulating interhemispheric activity in the brain, and that flicker can be used to probe the resonance properties of neural populations with magnetic resonance imaging

Measuring visual stimulation and attention signals in human superior colliculus using high-resolution fMRI

textThe superior colliculus (SC) is a laminated oculomotor structure in the midbrain. In non-human primates SC has long been known to contain a retinotopically-organized map of visual stimulation in its superficial layers, which is aligned to a map of saccadic eye movements in the deeper layers. Microstimulation and electrophysiology experiments have shown that SC also plays a key role in covert visuospatial attention and suggest that attentional modulation also occurs in a retinotopic manner. Retinotopic organization of the visual field can be non-invasively mapped in humans using functional MRI with a technique called phase-encoded retinotopy. In this technique, rotating wedges and expanding rings of visual stimuli are used to map the polar angle and eccentricity dimensions of a polar coordinates system, respectively. A similar technique can also be used to map spatial attention by keeping the visual stimulus constant and cueing subjects to attend to apertures of rotating wedges and expanding rings within the stimulus. A previous study using fMRI has shown the polar angle representation of visual stimulation in human SC but was unable to find a representation of eccentricity. This work uses high-resolution fMRI along with special surface analysis techniques developed in our lab to demonstrate maps of both polar angle and eccentricity for visual stimulation. Moreover, visual attention is also shown to be topographically organized within SC and in registration with visual stimulation. Finally, in human visual cortex, fMRI is known to show activity for sustained spatial attention even in the absence of a significant visual stimulus, an attentional "base response". In this work, SC is shown to exhibit a similar sustained attention base response using a threshold-contrast detection paradigm. This base response was compared with a response for attention with visual stimulation. The peak amplitude of the base response occurred more deeply within SC tissue than the peak for attention with stimulation. It is proposed that this reflects the specific attentional enhancement of the deeper visuomotor neurons, which are hypothesized to be a direct neuronal correlate of the oculomotor theory of attention.Psycholog