Vision Lunch 2007


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For the current Vision Lunch schedule see: Vision Lunch

Other years: Previous Vision Lunches


[edit] 2007 Vision Lunches

[edit] January

10 Discussion: EEG, MRI and Figure-Ground Segmentation

Appelbaum et al, 2006: Cue-invariant networks for figure and background processing in human visual cortex.

18 (THU 10am) Takeo Watanabe: What gates visual plasticity?

How can the brain adapt to new environments while protecting its architecture from modification from the continual bombardment of undesirable information? The goal of the present talk is to clarify how low-level sensory systems solve this stability-plasticity dilemma. Until recently, it was thought that low-level sensory systems were rigid and, therefore, resistant to environmental changes, and that only with persistent and focused attention on primitive features can these systems be modified. Recently it has been found that low-level visual stages are more plastic than previously thought, since PL can occur without attention to, and even awareness of, a stimulus. The sensitivity to motion direction increased as a result of exposure to a direction which was both sub-threshold and irrelevant to a given task (Watanabe et al, 2001). A recent fMRI study (Tsushima et al, 2006) has confirmed that such a task-irrelevant subthreshold stimulus is not subject to attentional control. Further evidence suggests that this type of PL occurs only at a low-level stage of visual processing (Watanabe et al, 2002). We call this new type of learning task-irrelevant PL. If attention does not prevent the low-level visual architecture from undesirable plasticity, what mechanism protects the architecture while allowing adaptation to useful information that is too weak or noisy to perceive? Seitz & Watanabe (2003) have found that the occurrence of task -irrelevant learning is restricted to the case in which an exposed, irrelevant, and sub-threshold feature was paired with a task-target. This indicates that plasticity is gated by a spatially diffusive learning signal that is triggered by presentation of a task target and enhances sensitivity to the paired irrelevant feature.

Seitz AR, Watanabe T. Nature. 2003 422:36.

Tsushima Y, Sasaki Y, Watanabe T. Science. 2006 314:1786-8.

Watanabe T, Nanez JE, Sasaki Y. Nature. 2001 413:844-8.

Watanabe, T. et al. (2002). Nature Neuroscience 5: 1002-9.

24 Discussion: Object concepts and representation (from KGS's holiday reading list)

Martin, 2007: The representation of object concepts in the brain.

31 Data discussion: 2D vs 3D spiral acquisitions at 1.5mm (8-channel coil) - Yanle Hu, David Remus, Nick Davidenko

We will discuss Yanle / Gary's new 3D sequence, and compare data from 3 subjects.

[edit] February

NOTE: There are several psychology faculty candidates visiting in February. To accommodate their presentations, Vision Lunch will be shifted to THURSDAY afternoon for most of the month.

8 SPECIAL TIME: Thursday, 11:30 am (room 417):

Dr. Yaoda Xu, Yale University: Object individuation and identification in the brain: Future directions

Dr. Xu is a job candidate in the cog/neuro division of Psychology, and will be giving this informal workshop in addition to her colloquium (Wed. 3:45m room 041) presentation entitled: "Dissociable parietal mechanisms supporting visual object individuation and identification".

When we are confronted with a large number of objects competing for limited processing resources, I found that the parietal cortex plays an important role in object individuation and object identification. In this talk, I will describe some exciting directions that this research can take, and how it may bridge a number of different areas of research. In particular, I will show that object individuation and identification in the parietal cortex may be linked to recently discovered parietal retinotopic maps. I will also highlight the connection between this work and the development of object concepts in infants, which, in turn, will inform us about the maturation of this system during development. Finally, combining fMRI adaptation and visual short-term memory paradigms, I will present some preliminary data showing that the superior intra-parietal sulcus represents different features together as a bound object, addressing a central topic in vision science regarding feature binding. The outcome of this research program should advance our understanding of how the mind and brain perceives and maintains coherent visual objects from complex input.

15 SPECIAL TIME: Thursday, 11:30 am (Building 60, Room 61-G):

Dr. Charles Kemp, MIT: "Knowledge-rich models of inductive learning and reasoning"

Dr. Kemp is a job candidate in the cog/neuro division of Psychology, and will be giving an informal workshop in addition to his colloquium (Wed. 3:45m room 041) presentation entitled: "The structure and acquisition of semantic knowledge".

Human inferences are often guided by sophisticated knowledge, but theories of human learning have achieved their greatest successes when applied to problems that are relatively knowledge-free. I will present models and experiments that help us better understand how rich systems of knowledge are acquired and used. Many studies focus on a single kind of knowledge, but humans are often required to combine several kinds of knowledge. I will discuss how inductive inferences can simultaneously draw on causal knowledge and knowledge about similarity. Elegant accounts of learning often work with simple representations, such as similarity metrics or sets of features, but more expressive representations can capture a greater proportion of human knowledge. I will discuss the acquisition and use of simple theories: systems of related concepts, where each concept is defined in part by its relationships to other concepts.

21 Visit from Tony Morland and Andre Gouws on reorganization (MEG and MR) This will be an informal meeting, at 10:30 AM in room 417.

22 SPECIAL TIME: Thursday, 11:30 am (room 102):

Dr. Roshan Cools, University of Cambridge: "Serotonergic modulation of learning and behavior"

Dr. Cools is a job candidate in the cog/neuro division of Psychology, and will be giving an informal workshop in addition to her colloquium (Wed. 3:45m room 041) presentation entitled: "Dopaminergic modulation of the adaptive mind". Workshop abstract TBA.

28 Discussion: DTI and white matter reorganization in blind people.

Shimony et al 2007: Diffusion tensor imaging reveals white matter reorganization in early blind humans.

[edit] March

7 Discussion: Microstimulation of caudate

Williams and Eskandar, 2006 (Selective enhancement of associative learning by microstimulation of the anterior caudate.)

15 SPECIAL TIME: Thurdsay, 11 AM. Josh Wallman, City College of New York. Josh will discuss his recent work on visual attention and saccades

28 Discussion: Is face recognition special? (from KGS's holiday reading list)

McKone et. al. 2007: Can generic expertise explain special processing for faces?

[edit] April

4 Rory Sayres, Grill-Spector lab: recent results on object invariance

This was a warm-up for parts of my thesis defense. Slides

25 David Andresen, Grill-Spector Lab: Further adventures measuring object rotation invariance

[edit] May

2 DISCUSSION: Correlations in neuronal coding of contrast/direction in V1

Montani F, Kohn A, Smith MA, Schultz SR. The role of correlations in direction and contrast coding in the primary visual cortex. J. Neurosci 2007

full text article at

The spiking activity of nearby cortical neurons is not independent. Numerous studies have explored the importance of this correlated responsivity for visual coding and perception, often by comparing the information conveyed by pairs of simultaneously recorded neurons with the sum of information provided by the respective individual cells. Pairwise responses typically provide slightly more information so that encoding is weakly synergistic. The simple comparison between pairwise and summed individual responses conflates several forms of correlation, however, making it impossible to judge the relative importance of synchronous spiking, basic tuning properties, and stimulus-independent and stimulus-dependent correlation. We have applied an information theoretic approach to this question, using the responses of pairs of neurons to drifting sinusoidal gratings of different directions and contrasts that have been recorded in the primary visual cortex of anesthetized macaque monkeys. Our approach allows us to break down the information provided by pairs of neurons into a number of components. This analysis reveals that, although synchrony is prevalent and informative, the additional information it provides frequently is offset by the redundancy arising from the similar tuning properties of the two cells. Thus coding is approximately independent with weak synergy or redundancy arising, depending on the similarity in tuning and the temporal precision of the analysis. We suggest that this would allow cortical circuits to enjoy the stability provided by having similarly tuned neurons without suffering the penalty of redundancy, because the associated information transmission deficit is compensated for by stimulus-dependent synchrony.

9 VSS Practice Talks

Please contact Rory or David if you'll be presenting at the VSS meeting and would like to do a run-through.


23 VSS wrap-up

30 Uri Hasson, NYU: A hierarchy of temporal receptive windows in human cortex

Real-world events unfold at different time scales, and therefore cognitive and neuronal processes must likewise occur at different time scales. In the talk I will present a novel procedure that identifies brain regions responsive to the preceding sequence of events (past time) over different time scales. The fMRI activity was measured while observers viewed silent films presented forward, backward, or piecewise-scrambled in time. The results demonstrate that responses in different brain areas are affected by information that has been accumulated over different time scales, with a hierarchy of temporal receptive windows spanning from short (~4 s) to intermediate (~12 s) and long (~ 36 s). Thus, although we adopted an open-ended experimental protocol (free viewing of complex stimuli), we found that parametric manipulation of the temporal structure of a complex movie sequence produced lawful changes in cortical activity across different brain regions. In addition to the reliable cortical response patterns, I will also show that films exerted considerable control over the subjects' behavior (i.e., eye movements or galvanic skin responses), and memory performance. Finally, I will present few applications of this method for studying the neuronal correlates of complex human behaviors under more natural settings.

[edit] June

6 Nicolas Davidenko and Dave Remus: results of 2D/3D pulse sequence and coil comparisons

We ruled!

13 No Vision Lunch

26 SPECIAL TIME: Tuesday, 6/26, 1:30 pm; Rainer Goebel, University of Maastricht / Real-time fMRI, BOLD communications, and neurofeedback

28 SPECIAL TIME: Thursday, 6/28, 4:00 pm; Rainer Goebel, University of Maastricht / Multi-modal spatio-temporal neuroimaging: New approaches

29 11:15 AM: Michael Lustig, EE Department: Sparse MRI: The Application of Compressed Sensing for Rapid MRI

Magnetic Resonance Imaging (MRI) is a non-invasive non-toxic imaging modality. The vast number of control parameters in MRI provides flexibility in image contrast. For example, MRI can distinguish between different type of soft tissues, it can image functional activity in the brain, measure flow velocities, monitor temperature changes and more. So far, MRI has been very successful in imaging parts of the body that are easily immobilized, such as the brain and joints. Its success has been much more limited for rapidly changing settings, as in imaging the heart and dynamic imaging. MRI requires a relatively long scan time compared to other imaging modalities, a requirement that limits and sometimes prevents its use in important applications. Recently, the theory of Compressed Sensing (CS) was introduced. According to CS, compressible images can be recovered from a significantly small number of measurements, well beyond the Nyquist rate. The recovery is possible as long as the measurements are random projections and the recovery is a special non-linear procedure. One of the most promising applications of compressed sensing (CS) is to MRI. MRI images in general, and dynamic MRI images in particular are often highly compressible. The MRI data are collected in the spatial frequency domain (k-space) and in most imaging scenarios, scan time is directly related to the number of data samples needed for proper reconstruction. Therefore, MRI scans can be significantly accelerated by obtaining fewer samples. All of these facts as well as that the data sampling of frequency is controllable (For example, it is possible with some restrictions to obtain random Fourier samples that can serve as approximately random projections) makes MRI a natural compressed sensing imaging system. This talk aims to review the current development and applications of compressed sensing in MRI. The talk starts with a short overview of the MR system as well as some of the physics that is essential to understanding the imaging mechanism in MRI. This is followed by a brief review of the CS theory that is directly related to MRI, e.g., transform sparsity of MRI images and dynamic sequences, incoherent sampling and the non-linear reconstruction. Finally examples are given from a variety of applications in which scan time can be significantly reduced such as: angiography, coronary artery imaging, dynamic heart imaging and brain imaging. Joint work with David L. Donoho, Statistics Department, Stanford University, Juan M Santos and John M Pauly, EE Department, Stanford.

[edit] July

2 SPECIAL TIME: Monday, 7/2, 12:00 pm; Rainer Goebel, University of Maastricht / From “where” and “when” to “what”: Simulating large-scale neural network models “in the brain”.

This will be the third of three talks Rainer will give while he is visiting this summer.


11 No Vision Lunch'

18 Discussion: What do we perceive in a glance of a real-world scene? Fei-Fei et al 2007

What do we see when we glance at a natural scene and how does it change as the glance becomes longer? We asked naive subjects to report in a free-form format what they saw when looking at briefly presented real-life photographs. Our subjects received no specific information as to the content of each stimulus. Thus, our paradigm differs from previous studies where subjects were cued before a picture was presented and/or were probed with multiple-choice questions. In the first stage, 90 novel grayscale photographs were foveally shown to a group of 22 native-English-speaking subjects. The presentation time was chosen at random from a set of seven possible times (from 27 to 500 ms). A perceptual mask followed each photograph immediately. After each presentation, subjects reported what they had just seen as completely and truthfully as possible. In the second stage, another group of naive individuals was instructed to score each of the descriptions produced by the subjects in the first stage. Individual scores were assigned to more than a hundred different attributes. We show that within a single glance, much object- and scene-level information is perceived by human subjects. The richness of our perception, though, seems asymmetrical. Subjects tend to have a propensity toward perceiving natural scenes as being outdoor rather than indoor. The reporting of sensory- or feature-level information of a scene (such as shading and shape) consistently precedes the reporting of the semantic-level information. But once subjects recognize more semantic-level components of a scene, there is little evidence suggesting any bias toward either scene-level or object-level recognition.

25 No Vision Lunch

[edit] August

1 Discussion: New optical imaging and manipulation methods

This will be feature 2 recent papers featuring methods which are generating a lot of buzz in the neuroscience community:

(1) Restoring sight in retinal degeneration mice

Bi et al., Neuron 2006

Ectopic expression of a microbial-type rhodopsin restores visual responses in mice with photoreceptor degeneration.

The death of photoreceptor cells caused by retinal degenerative diseases often results in a complete loss of retinal responses to light. We explore the feasibility of converting inner retinal neurons to photosensitive cells as a possible strategy for imparting light sensitivity to retinas lacking rods and cones. Using delivery by an adeno-associated viral vector, here, we show that long-term expression of a microbial-type rhodopsin, channelrhodopsin-2 (ChR2), can be achieved in rodent inner retinal neurons in vivo. Furthermore, we demonstrate that expression of ChR2 in surviving inner retinal neurons of a mouse with photoreceptor degeneration can restore the ability of the retina to encode light signals and transmit the light signals to the visual cortex. Thus, expression of microbial-type channelrhodopsins, such as ChR2, in surviving inner retinal neurons is a potential strategy for the restoration of vision after rod and cone degeneration.

(2) Multimodal fast optical interrogation of neural circuitry

Zhang et al., Nature 2007

Our understanding of the cellular implementation of systems-level neural processes like action, thought and emotion has been limited by the availability of tools to interrogate specific classes of neural cells within intact, living brain tissue. Here we identify and develop an archaeal light-driven chloride pump (NpHR) from Natronomonas pharaonis for temporally precise optical inhibition of neural activity. NpHR allows either knockout of single action potentials, or sustained blockade of spiking. NpHR is compatible with ChR2, the previous optical excitation technology we have described, in that the two opposing probes operate at similar light powers but with well-separated action spectra. NpHR, like ChR2, functions in mammals without exogenous cofactors, and the two probes can be integrated with calcium imaging in mammalian brain tissue for bidirectional optical modulation and readout of neural activity. Likewise, NpHR and ChR2 can be targeted together to Caenorhabditis elegans muscle and cholinergic motor neurons to control locomotion bidirectionally. NpHR and ChR2 form a complete system for multimodal, high-speed, genetically targeted, all-optical interrogation of living neural circuits.

[edit] August

8 Discussion: Natural grouping of objects by electrophysiology data

Kiani et al 2007 Object Category Structure in Response Patterns of Neuronal Population in Monkey Inferior Temporal Cortex.

Our mental representation of object categories is hierarchically organized, and our rapid and seemingly effortless categorization ability is crucial for our daily behavior. Here, we examine responses of a large number (>600) of neurons in monkey inferior temporal (IT) cortex with a large number (>1000) of natural and artificial object images. During the recordings the monkeys performed a passive fixation task. We found that the categorical structure of objects is represented by the pattern of activity distributed over the cell population. Animate and inanimate objects created distinguishable clusters in the population code. The global category of animate objects was divided into bodies, hands and faces. Faces were divided into primate and non-primate faces, and the primate-face group was divided into human and monkey faces. Bodies of human, birds, and four-limb animals clustered together, while lower animals such as fish, reptile and insects made another cluster. Thus, the cluster analysis showed that IT population responses reconstruct a large part of our intuitive category structure, including the global division into animate and inanimate objects, and further hierarchical subdivisions of animate objects. The representation of categories was distributed in several respects, e.g., the similarity of response patterns to stimuli within a category was maintained by both the cells that maximally responded to the category and the cells that responded weakly to the category. These results advance our understanding of the nature of the IT neural code, suggesting an inherently categorical representation that comprises a range of categories including the amply investigated face category.

22 Discussion: Retina- and Head-Centered coordinate systems in LOC

Beyond Retinotopic Mapping: The Spatial Representation of Objects in the Human Lateral Occipital Complex

MacKyton and Zohary, 2007

The spatial representation in the human ventral object-related areas (i.e., the lateral occipital complex [LOC]) is currently unknown. It seems plausible, however, that it would diverge from the strict retinotopic mapping (characteristic of V1) to a more invariant coordinate frame, thereby allowing for reliable object recognition in the face of eye, head, or body movement. To study this, we compared the fMRI activation in LOC when object displacement was limited to either the retina or the screen by manipulating eye position and object locations. We found clear adaptation in LOC when the object's screen position was fixed, regardless of the object's retinal position. Furthermore, we found significantly greater activation in LOC in the hemisphere contralateral to the object's screen position, although the visual task was constructed in a way that the objects were present equally often on each of the 2 retinal hemifields. Together, these results indicate that a sizeable fraction of the neurons in LOC may have head-based receptive fields. Such an extraretinal representation may be useful for maintenance of object coherence across saccadic eye movements, which are an integral part of natural vision.

29 Josef Parvizi, Department of Neurology and Neurological Science.

Dr. Parvizi is a new professor who works with epileptics. This will be an opportunity to get to know him, and may be particularly useful for students interested in behavioral projects.

[edit] September

5 No Vision Lunch

12 No Vision Lunch

19 No Vision Lunch

26 Welcome meeting. No talk scheduled.

[edit] October

3 Michael Silver, Helen Wills Neuroscience Institute and School of Optometry, University of California, Berkeley

"Human Visual Spatial Attention: Topographic Maps, Top-down Processing, and Cholinergic Modulation"

In my talk, I will describe functional magnetic resonance (fMRI) experiments that demonstrate the existence of attention signals in human early visual cortex. These signals occur in the absence of visual stimulation, and they are sustained for as long as subjects maintain attention at a particular spatial location. Further experiments designed to identify possible sources of these top-down attention signals revealed two novel cortical areas in the human intraparietal sulcus. These areas each contain a topographic map of visual attention, and they respond poorly to passive viewing of visual stimuli.

Finally, I will present results from pharmacology studies in which the cholinesterase inhibitor donepezil was used to increase synaptic levels of acetylcholine during visual stimulus presentation or performance of a visual spatial attention task. This cholinergic enhancement increases top-down spatial attention as measured behaviorally, and it also augments neural correlates of top-down spatial attention in the superior parietal lobule as measured with fMRI. In addition, cholinergic enhancement with donepezil changes responses to visual stimuli in early visual cortex, resulting in decreased spatial spread of the cortical response, consistent with increased spatial precision of the neural representation of the stimulus. Together, these studies establish a foundation for functional and pharmacological characterization of top-down visual attention pathways in the human brain.

10 Jon Winawer: Layered image representations and the computation of surface lightness

I will present a paper I am currently working on with Bart Anderson (UNSW).

A fundamental goal of research in the perception of surface lightness is to understand the nature of the computations and representations underlying lightness perception. A significant challenge posed to the visual system is recovering surface lightness from the multiple physical causes that contribute to image luminance. One view asserts that the visual system decomposes the image into estimates of illumination, lightness, and transparency, generating layered image representations. An alternative view is that the image regions are separated into 2D regions, and principles of anchoring and/or statistical estimation are used to compute surface lightness. Here, a number of experiments and demonstrations are presented that reveal the role layered image representations play in the computation of surface lightness. We provide new evidence demonstrating that the contrast relationships along boundaries can play a decisive role in determining whether images are decomposed into multiple layers, and that the constraints that regulate how this decomposition occurs can have a dramatic influence on perceived lightness.

17 Two for the price of one: Cris Neill and Sunil Gandhi, UCSF

1: Cris Neill: Highly selective receptive fields in mouse visual cortex

Abstract : Genetic methods available in mice are likely to be powerful tools in dissecting cortical circuits. However the visual cortex, where sensory coding has been most thoroughly studied in other species, has largely been neglected in mice perhaps due to their poor spatial acuity, and the lack of large-scale organization such as orientation maps. We therefore have applied recent methods for quantitative description of visual receptive fields to mouse V1, by performing extracellular recordings with silicon electrode arrays in anesthetized mice. In addition, we were able to use current source density analysis to determine laminar location, and spike waveforms to discriminate putative excitatory and inhibitory units. We find that although the typical size of receptive fields is an order of magnitude larger than other visual model species, neurons can be very highly selective for stimulus parameters such as orientation and spatial frequency. Furthermore, most response properties that have been studied in other species are present in mouse V1, and are correlated with laminar position and cell type. Thus, the mouse visual cortex may provide a system to apply genetic tools to longstanding questions of visual neuroscience and cortical processing. In a separate line of experiments, we are using this knowledge of single-unit responsiveness to develop novel stimuli, based on periodically modulated stochastic noise, that will facilitate rapid measurement of cortical response properties such as ocular dominance and orientation selectivity. We hope these stimuli will be generally applicable across various measurement modalities, such as intrinsic signal imaging, two-photon microscopy, and electrophysiology, which operate at different temporal and spatial scales.

2: Sunil Gandhi: Distinct in vivo patterns of experience-dependent plasticity in the inhibitory and excitatory cells of developing visual cortex

Abstract: During a brief period in childhood, diverging visual experience through the eyes can rapidly and permanently tilt the balance of eye-specific responses in primary visual cortex. The maturation of local cortical inhibitory circuits permits the expression of this form of experience-dependent plasticity. We set out to examine the eye-specific visual responses of inhibitory and excitatory cells when plasticity was stimulated through the reversible deprivation of vision through one eye. We measured visual responses from populations of cells in mouse visual cortex via 2-photon time-lapse imaging of a calcium indicator bulk loaded into many cells at once. By simultaneously imaging a genetically encoded marker of inhibitory cells, we were able to directly visualize the inhibitory subpopulation. In unmanipulated animals throughout the critical period we find that inhibitory and excitatory cells share a similar response bias for visual stimulation through the contralateral eye. During periods of monocular deprivation, however, eye specific visual activity in the two populations temporarily diverge; at 2 days of deprivation, excitatory cells newly respond in favor of non-deprived eye vision whereas inhibitory cell responses retain their initial bias to the deprived eye. The temporary divergence is the result of a rapid weakening of deprived-eye responses in excitatory cells that is not seen in inhibitory cells. The mismatch is most pronounced among cells in a narrow domain at the top of layer II/III. After 4 days, the eye-specific response bias of the two populations become matched through the delayed strengthening of responses to the non-deprived eye in all cells. These results suggest that 1) inhibitory neurons in superficial cortex are driven predominantly by selective inputs from non-local sources, either from other cortical layers, or else longer range connections, and 2) inhibitory cells lack the rapid plasticity of deprived eye responses seen in excitatory cells.


email Jon or David if you'd like to do a practice talk / poster

31 Rubi Hammer, and PRACTICE FOR SFN The Role of Exemplars Comparison in Category Learning: The Emergence of Expertise

Recent studies stressed the importance of objects comparison for category learning. Recently we compared adults' capacity to learn new categorization principle in a condition in which few exemplars were identified as belong to the same category, versus a condition in which exemplars were identified as belong to different categories. We found that almost all participants learned the categorization rule quite efficiently in the Same-Class Exemplars condition. In contrast, when not provided with explicit directions, many adults failed to learn the categorization rule in the Different-Class Exemplars condition. This was the case even when insuring that the objective information provided in the Different-Class Exemplars condition was sufficient for inferring the rule, and was objectively equally informative as the information provided in the Same-Class Exemplars condition. When we compared adult performance to that of children we found that when presented only with Same- Class Exemplars, young children (6-10 YO) learn the novel categories just as well as older children and adults. However, when presented only with Different-Class Exemplars, unlike older children and adults, young children failed to learn the novel categories. These findings may explain known phenomena in cognitive development, such as the common difficulty of young children in learning subordinate-level categories, and the late emergence of expertise. We suggest that such difficulties results from a cognitive, not a perceptual, immaturity. Interestingly, preliminary findings from fMRI study (with adults) shows that the right-FFA – a brain area associated with a domain-specific function of face recognition – is highly involved in category learning by comparison of non-face stimuli. This imaging study suggests that the right-FFA (also?) performs, or is partially involved in, "domain-general" functions.

[edit] November


12 SPECIAL TIME: Monday 12pm (419) Yuval Nir, Weizmann Institute of Science:

The relation between neuronal firing rates, gamma LFP, and BOLD fMRI in human sensory cortex during stimulation and rest

To what extent is activity of individual neurons coupled to the local field potential (LFP) and to blood-oxygenation-level dependent (BOLD) functional magnetic resonance imaging (fMRI)? This issue is of high significance for understanding brain function and for relating animal studies to fMRI, yet it is still under debate.

In this talk, I will present data from simultaneous recordings of isolated unit activity and LFP in the human auditory cortex of patients undergoing monitoring with intracranial depth electrodes for potential surgical treatment. We found a wide range of coupling levels between the activity of individual neurons and gamma LFP. However, this large variability could be predominantly explained (r = 0.66) by the degree of firing-rate correlations between neighboring neurons. Importantly, this phenomenon occurred during both sensory stimulation and spontaneous activity.

Concerning the coupling of neuronal activity to BOLD fMRI, we found that gamma LFP was well coupled to BOLD measured across different individuals (r = 0.62) under identical stimulation conditions. By contrast, the coupling of single units to BOLD was highly variable and, again, tightly related to inter-neuronal firing-rate correlations (r = 0.70).

These results may offer a resolution to a central controversy regarding the coupling between neurons, LFP, and BOLD signals by demonstrating that the coupling of single units to the other measures is variable yet it is tightly related to the degree of inter-neuronal correlations.

Finally, examining slow fluctuations in LFP power and firing rate modulations during rest reveals possible electrophysiological underpinnings of resting state fMRI, which is the focus of much recent research.

14 SFN Recap

We'll discuss our favorite SFN presentations.

21 NO MEETING: Thanksgiving holiday

28 CANCELED: Mark Schira, University of New South Wales: Adventures in High-resolution Retinotopies

[edit] December

5 Discussion: High-resolution fMRI and optical imaging in monkeys

Chen et al, 2007

High-resolution maps of real and illusory tactile activation in primary somatosensory cortex in individual monkeys with functional magnetic resonance imaging and optical imaging.

This study imaged primary somatosensory cortex in monkeys using both high-resolution fMRI (.15 x .15 x 2 mm) and optical intrisic signal imaging, and found very comparable results. One of the cool things is, they show reliable activity shifts of under 1mm using BOLD, indicating the point-spread function may not be as large as some estimates.

12 Discussion: Temporal frequency in V1 & tissue segmentation

Sun et al, 2007

A temporal frequency-dependent functional architecture in human V1 revealed by high-resolution fMRI.

Although cortical neurons with similar functional properties often cluster together in a columnar organization, only ocular dominance columns, the columnar structure representing segregated anatomical input (from one of the two eyes), have been found in human primary visual cortex (V1). It has yet to be shown whether other columnar organizations that arise only from differential responses to stimulus properties also exist in human V1. Using high-resolution functional magnetic resonance imaging, we have found such a functional architecture containing domains that respond preferentially to either low or high temporal frequency.

Also, Bob will introduce some new tissue segmentation tools



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