Monday, April 28, 2008

Theta phase–specific codes for two-dimensional position, trajectory and heading in the hippocampus

John R Huxter, Timothy J Senior, Kevin Allen, Jozsef Csicsvari
Nature Neuroscience 11, 587 - 594 (2008)

Temporal coding is a means of representing information by the time, as opposed to the rate, at which neurons fire. Evidence of temporal coding in the hippocampus comes from place cells, whose spike times relative to theta oscillations reflect a rat's position while running along stereotyped trajectories. This arises from the backwards shift in cell firing relative to local theta oscillations (phase precession). Here we demonstrate phase precession during place-field crossings in an open-field foraging task. This produced spike sequences in each theta cycle that disambiguate the rat's trajectory through two-dimensional space and can be used to predict movement direction. Furthermore, position and movement direction were maximally predicted from firing in the early and late portions of the theta cycle, respectively. This represents the first direct evidence of a combined representation of position, trajectory and heading in the hippocampus, organized on a fine temporal scale by theta oscillations.


Unconscious determinants of free decisions in the human brain

Chun Siong Soon, Marcel Brass, Hans-Jochen Heinze, John-Dylan Haynes
Nature Neuroscience 11, 543 - 545 (2008)

There has been a long controversy as to whether subjectively 'free' decisions are determined by brain activity ahead of time. We found that the outcome of a decision can be encoded in brain activity of prefrontal and parietal cortex up to 10 s before it enters awareness. This delay presumably reflects the operation of a network of high-level control areas that begin to prepare an upcoming decision long before it enters awareness.


Reduction of stimulus visibility compresses apparent time intervals

Masahiko Terao, Junji Watanabe, Akihiro Yagi, Shin'ya Nishida
Nature Neuroscience 11, 541 - 542 (2008)

The neural mechanisms underlying visual estimation of subsecond durations remain unknown, but perisaccadic underestimation of interflash intervals may provide a clue as to the nature of these mechanisms. Here we found that simply reducing the flash visibility, particularly the visibility of transient signals, induced similar time underestimation by human observers. Our results suggest that weak transient responses fail to trigger the proper detection of temporal asynchrony, leading to increased perception of simultaneity and apparent time compression.


Instantaneous correlation of excitation and inhibition during ongoing and sensory-evoked activities

Michael Okun, Ilan Lampl
Nature Neuroscience 11, 535 - 537 (2008)

Temporal and quantitative relations between excitatory and inhibitory inputs in the cortex are central to its activity, yet they remain poorly understood. In particular, a controversy exists regarding the extent of correlation between cortical excitation and inhibition. Using simultaneous intracellular recordings in pairs of nearby neurons in vivo, we found that excitatory and inhibitory inputs are continuously synchronized and correlated in strength during spontaneous and sensory-evoked activities in the rat somatosensory cortex.


Saturday, April 19, 2008

Choices in neuroscience careers

Tamas Bartfai, Tom Insel, Gord Fishell & Nancy Rothwell
Nature Reviews Neuroscience 9, 401-405 (May 2008) | doi:10.1038/nrn2386

How do I choose a mentor?
How do I decide what field of neuroscience to work in?
Should I consider doing research in industry?
Most students and postdoctoral researchers aiming for a successful career in neuroscience ask themselves these questions.
In this article, Nature Reviews Neuroscience asks four successful neuroscientists for their thoughts on the factors one should consider when making these decisions.
We hope that this Viewpoint will serve as a useful resource for junior neuroscientists who have to make important and sometimes difficult decisions that might have long-lasting consequences for their careers.


Monday, April 14, 2008

Adaptation across the Cortical Hierarchy: Low-Level Curve Adaptation Affects High-Level Facial-Expression Judgments

Hong Xu,1 Peter Dayan,2 Richard M. Lipkin,1 and Ning Qian
The Journal of Neuroscience, March 26, 2008, 28(13):3374-3383; doi:10.1523/JNEUROSCI.0182-08.2008

Adaptation is ubiquitous in sensory processing. Although sensory processing is hierarchical, with neurons at higher levels exhibiting greater degrees of tuning complexity and invariance than those at lower levels, few experimental or theoretical studies address how adaptation at one hierarchical level affects processing at others. Nevertheless, this issue is critical for understanding cortical coding and computation. Therefore, we examined whether perception of high-level facial expressions can be affected by adaptation to low-level curves (i.e., the shape of a mouth). After adapting to a concave curve, subjects more frequently perceived faces as happy, and after adapting to a convex curve, subjects more frequently perceived faces as sad. We observed this multilevel aftereffect with both cartoon and real test faces when the adapting curve and the mouths of the test faces had the same location. However, when we placed the adapting curve 0.2° below the test faces, the effect disappeared. Surprisingly, this positional specificity held even when real faces, instead of curves, were the adapting stimuli, suggesting that it is a general property for facial-expression aftereffects. We also studied the converse question of whether face adaptation affects curvature judgments, and found such effects after adapting to a cartoon face, but not a real face. Our results suggest that there is a local component in facial-expression representation, in addition to holistic representations emphasized in previous studies. By showing that adaptation can propagate up the cortical hierarchy, our findings also challenge existing functional accounts of adaptation.


Sunday, April 13, 2008

One-dimensional dynamics of attention and decision making in LIP

Ganguli S, Bisley JW, Roitman JD, Shadlen MN, Goldberg ME, Miller KD.
Neuron. 2008 Apr 10;58(1):15-25

Where we allocate our visual spatial attention depends upon a continual competition between internally generated goals and external distractions. Recently it was shown that single neurons in the macaque lateral intraparietal area (LIP) can predict the amount of time a distractor can shift the locus of spatial attention away from a goal. We propose that this remarkable dynamical correspondence between single neurons and attention can be explained by a network model in which generically high-dimensional firing-rate vectors rapidly decay to a single mode. We find direct experimental evidence for this model, not only in the original attentional task, but also in a very different task involving perceptual decision making. These results confirm a theoretical prediction that slowly varying activity patterns are proportional to spontaneous activity, pose constraints on models of persistent activity, and suggest a network mechanism for the emergence of robust behavioral timing from heterogeneous neuronal populations.

PMID: 18400159


Prefrontal cortex function in the representation of temporally complex events

Browning PG, Gaffan D.
J Neurosci. 2008 Apr 9;28(15):3934-40

The frontal cortex and inferior temporal cortex are strongly functionally interconnected. Previous experiments on prefrontal function in monkeys have shown that a disconnection of prefrontal cortex from inferior temporal cortex impairs a variety of complex visual learning tasks but leaves simple concurrent object-reward association learning intact. We investigated the possibility that temporal components of visual learning tasks determine the sensitivity of those tasks to prefrontal-temporal disconnection by adding specific temporal components to the concurrent object-reward association learning task. Monkeys with crossed unilateral lesions of prefrontal cortex and inferior temporal cortex were impaired compared with unoperated controls at associating two-item sequences of visual objects with reward. The impairment was specific to the learning of visual sequences, because disconnection was without effect on object-reward association learning for an equivalent delayed reward. This result was replicated in monkeys with transection of the uncinate fascicle, thus determining the anatomical specificity of the dissociation. Previous behavioral results suggest that monkeys represent the two-item serial compound stimuli in a configural manner, similar to the way monkeys represent simultaneously presented compound stimuli. The representation of simultaneously presented configural stimuli depends on the perirhinal cortex. The present experiments show that the representation of serially presented compound stimuli depends on the interaction of prefrontal cortex and inferior temporal cortex. We suggest that prefrontal-temporal disconnection impairs a wide variety of learning tasks because in those tasks monkeys lay down similar temporally complex representations.

PMID: 18400892


Maps of visual space in human occipital cortex are retinotopic, not spatiotopic

Gardner JL, Merriam EP, Movshon JA, Heeger DJ.
J Neurosci. 2008 Apr 9;28(15):3988-99

We experience the visual world as phenomenally invariant to eye position, but almost all cortical maps of visual space in monkeys use a retinotopic reference frame, that is, the cortical representation of a point in the visual world is different across eye positions. It was recently reported that human cortical area MT (unlike monkey MT) represents stimuli in a reference frame linked to the position of stimuli in space, a "spatiotopic" reference frame. We used visuotopic mapping with blood oxygen level-dependent functional magnetic resonance imaging signals to define 12 human visual cortical areas, and then determined whether the reference frame in each area was spatiotopic or retinotopic. We found that all 12 areas, including MT, represented stimuli in a retinotopic reference frame. Although there were patches of cortex in and around these visual areas that were ostensibly spatiotopic, none of these patches exhibited reliable stimulus-evoked responses. We conclude that the early, visuotopically organized visual cortical areas in the human brain (like their counterparts in the monkey brain) represent stimuli in a retinotopic reference frame.

PMID: 18400898


Friday, April 11, 2008

Compartmentalized dendritic plasticity and input feature storage in neurons

Attila Losonczy, Judit K. Makara, Jeffrey C. Magee
Nature 452, 436-441 (27 March 2008) | doi:10.1038/nature06725

Although information storage in the central nervous system is thought to be primarily mediated by various forms of synaptic plasticity, other mechanisms, such as modifications in membrane excitability, are available. Local dendritic spikes are nonlinear voltage events that are initiated within dendritic branches by spatially clustered and temporally synchronous synaptic input. That local spikes selectively respond only to appropriately correlated input allows them to function as input feature detectors and potentially as powerful information storage mechanisms. However, it is currently unknown whether any effective form of local dendritic spike plasticity exists. Here we show that the coupling between local dendritic spikes and the soma of rat hippocampal CA1 pyramidal neurons can be modified in a branch-specific manner through an N-methyl-d-aspartate receptor (NMDAR)-dependent regulation of dendritic Kv4.2 potassium channels. These data suggest that compartmentalized changes in branch excitability could store multiple complex features of synaptic input, such as their spatio-temporal correlation. We propose that this 'branch strength potentiation' represents a previously unknown form of information storage that is distinct from that produced by changes in synaptic efficacy both at the mechanistic level and in the type of information stored.


Sunday, April 6, 2008

Dimensionality and Dynamics in the Behavior of C. elegans

Stephens GJ, Johnson-Kerner B, Bialek W, Ryu WS.
PLoS Comput Biol. 2008 Apr 4;4(4):e1000028

A major challenge in analyzing animal behavior is to discover some underlying simplicity in complex motor actions. Here, we show that the space of shapes adopted by the nematode Caenorhabditis elegans is low dimensional, with just four dimensions accounting for 95% of the shape variance. These dimensions provide a quantitative description of worm behavior, and we partially reconstruct "equations of motion" for the dynamics in this space. These dynamics have multiple attractors, and we find that the worm visits these in a rapid and almost completely deterministic response to weak thermal stimuli. Stimulus-dependent correlations among the different modes suggest that one can generate more reliable behaviors by synchronizing stimuli to the state of the worm in shape space. We confirm this prediction, effectively "steering" the worm in real time.

PMID: 18389066
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Saturday, April 5, 2008

Defining functional areas in individual human brains using resting functional connectivity MRI

Cohen AL, Fair DA, Dosenbach NU, Miezin FM, Dierker D, Van Essen DC, Schlaggar BL, Petersen SE.
Neuroimage. 2008 Mar 24

The cerebral cortex is anatomically organized at many physical scales starting at the level of single neurons and extending up to functional systems. Current functional magnetic resonance imaging (fMRI) studies often focus at the level of areas, networks, and systems. Except in restricted domains, (e.g., topographically-organized sensory regions), it is difficult to determine area boundaries in the human brain using fMRI. The ability to delineate functional areas non-invasively would enhance the quality of many experimental analyses allowing more accurate across-subject comparisons of independently identified functional areas. Correlations in spontaneous BOLD activity, often referred to as resting state functional connectivity (rs-fcMRI), are especially promising as a way to accurately localize differences in patterns of activity across large expanses of cortex. In the current report, we applied a novel set of image analysis tools to explore the utility of rs-fcMRI for defining wide-ranging functional area boundaries. We find that rs-fcMRI patterns show sharp transitions in correlation patterns and that these putative areal boundaries can be reliably detected in individual subjects as well as in group data. Additionally, combining surface-based analysis techniques with image processing algorithms allows automated mapping of putative areal boundaries across large expanses of cortex without the need for prior information about a region's function or topography. Our approach reliably produces maps of bounded regions appropriate in size and number for putative functional areas. These findings will hopefully stimulate further methodological refinements and validations.

PMID: 18367410
Fulltext: Science Direct

Mirror Neuron System Differentially Activated by Facial Expressions and Social Hand Gestures: A Functional Magnetic Resonance Imaging Study

Montgomery KJ, Haxby JV.
J Cogn Neurosci. 2008 Mar 27

Facial expressions and hand gestures are utilized in nonverbal communication to convey socially relevant information. One key process that mediates nonverbal communication is simulation. The mirror neuron system (MNS), which maps observed actions onto the motor representations used when producing those actions, likely plays a role in simulation. Previous neuroimaging experiments have identified a putative human MNS that includes the inferior parietal lobule (IPL) and the frontal operculum. Although understanding nonverbal communication presumably involves the MNS, it is unknown whether these two forms of nonverbal social communication have distinct representations within that system. Here we report the results of a functional magnetic resonance imaging experiment in which participants viewed, imitated, and produced facial expressions and social hand gestures. The observation and execution of facial expressions and social hand gestures activated the MNS, but the magnitude of response differed. Activation in the IPL was greater for social hand gestures, whereas activation in the frontal operculum was greater for viewing facial expressions. The locations of neural activity evoked by viewing facial expressions and social hand gestures in the frontal operculum were significantly different. These data argue that there are distinct representations of different types of social nonverbal communication in the MNS.

PMID: 18370602


It is time to take a stand for medical research and against terrorism targeting medical scientists

Krystal JH, Carter CS, Geschwind D, Manji HK, March JS, Nestler EJ, Zubieta JK, Charney DS, Goldman D, Gur RE, Lieberman JA, Roy-Byrne P, Rubinow DR, Anderson SA, Barondes S, Berman KF, Blair J, Braff DL, Brown ES, Calabrese JR, Carlezon WA Jr, Cook EH Jr, Davidson RJ, Davis M, Desimone R, Drevets WC, Duman RS, Essock SM, Faraone SV, Freedman R, Friston KJ, Gelernter J, Geller B, Gill M, Gould E, Grace AA, Grillon C, Gueorguieva R, Hariri AR, Innis RB, Jones EG, Kleinman JE, Koob GF, Krystal AD, Leibenluft E, Levinson DF, Levitt PR, Lewis DA, Liberzon I, Lipska BK, Marder SR, Markou A, Mason GF, McDougle CJ, McEwen BS, McMahon FJ, Meaney MJ, Meltzer HY, Merikangas KR, Meyer-Lindenberg A, Mirnics K, Monteggia LM, Neumeister A, O'Brien CP, Owen MJ, Pine DS, Rapoport JL, Rauch SL, Robbins TW, Rosenbaum JF, Rosenberg DR, Ross CA, Rush AJ, Sackeim HA, Sanacora G, Schatzberg AF, Shaham Y, Siever LJ, Sunderland T, Tecott LH, Thase ME, Todd RD, Weissman MM, Yehuda R, Yoshikawa T, Young EA, McCandless R.
Biol Psychiatry. 2008 Apr 15;63(8):725-7

Terrorists are attacking scientists who are attempting to alleviate human suffering. We need a concerted public effort to eliminate these acts, particularly the harassment of scientists studying nonhuman primates. This need is highlighted by the attacks upon the home of our friend and colleague, the noted medical scientist, Dr. Edythe London, professor of psychiatry and biobehavioral sciences and of molecular and medical pharmacology at the David Geffen School of Medicine at the University of California Los Angeles (UCLA). Her work exemplifies the unique role of research involving nonhuman primates in enabling the results of research in simple systems (oocytes, cell culture) and lower organisms to be applied to human diseases. The importance of Dr. London’s research was highlighted in a public letter issued on February 8, 2008 from the Director of the National Institutes of Health (NIH), Dr. Elias Zerhouni, who stated, “her work is a prime example of NIH’s efforts … to develop effective treatments for people suffering from addiction—a disease that devastates individuals, families, communities, and costs society more than half a trillion dollars annually in health and crime-related costs and losses in productivity.”

Dr. London suffered two attacks upon her home within 4 months that have escalated in their level of threat to her life and work. The first occurred on October 20, 2007, and it involved the flooding of her house with water. A press release from the Animal Liberation Front, a group that has publicized both attacks, noted that water was used for the initial act because “we don’t risk starting brush fires,” a serious public threat in Southern California. Nevertheless, in the second attack on February 5, 2008, a Molotov cocktail firebomb was ignited, setting fire to Dr. London’s home. These crimes mirror other recent attacks on scientists conducting medical research involving animals, only a few of which we will mention here (1). In June 2006, another incendiary device intended for UCLA neuroscientist, Dr. Lynn Fairbanks, was placed on the doorstep of her 70-year-old neighbor. In June 2007, a third incendiary device was found at the home of another UCLA neuroscientist, Dr. Arthur Rosenbaum, the chief of pediatric ophthalmology at that institution. Dr. Rosenbaum’s wife also received a letter that included death threats and that was accompanied by razor blades and animal hair. Problems also have been escalating at the University of California Santa Cruz (UCSC). Most recently, on February 25, 2008, after a series of other incidents, six people broke into the home of a UCSC faculty member, whose name has not been released, and attacked a member of that faculty person’s family. The Santa Cruz Sentinel, on February 26, implicated a group of six people and a corporation, Stop Huntingdon Animal Cruelty USA. Threatening acts have occurred at other American medical research institutions, including the Oregon Health Sciences University and the University of Utah (1). The attacks in the United States follow a more vigorous program of terrorism in the United Kingdom aimed at disrupting medical research, particularly research involving nonhuman primates (2).

The attacks are horribly misguided. It is impossible to reconcile the willingness of these terrorists to harm humans, particularly people who are working to alleviate human suffering, with their contention that they value life of all kinds. Scientists, like Dr. London, care about the primates that they study. Scientists are partners with other interested groups in the ongoing international effort to improve the principles and practices governing animal research (briefly reviewed at This peaceful and collaborative process is critical to preserve in the face of the recent violence.

We need to support our colleagues and to work to preserve the integrity of the mission of alleviating human suffering through biomedical research involving animals. In so doing, we might help to ensure that these attacks upon scientists do not discourage much-needed research by demoralizing scientists or by stimulating institutions to adopt overly burdensome administrative practices (2). The recent events at UCLA make clear that diligently improving the ethical standards for primate research procedures is not, by itself, sufficient to prevent attacks. It is encouraging, for example, that on February 22, 2008, a Los Angeles County Superior Court judge issued a restraining order against the Animal Liberation Brigade, the Animal Liberation Front, and UCLA Primate Freedom Project that created a protective buffer zone around the homes of UCLA research faculty members.

These terrorist acts might intimidate people and institutions that would otherwise speak out in support of nonhuman primate research and against terrorism. By failing to take public action, we contribute to the isolation of the scientists involved and the institutions in which they work. Frustration with the absence of a vigorous public response to recent terrorist attacks led Robert Palazzo, president of the Federation of American Societies for Experimental Biology in Bethesda, Maryland to ask “Where’s the noise on this?” (1). Several organizations, such as the Society for Neuroscience (, the National Association for Biomedical Research (, and the American College of Neuropsychopharmacology (, are helping to educate the public on these issues. There are growing opportunities for animal research advocacy. The failure to publicly address the crimes against its faculty was initially a problem at UCLA, but this institution now is at the vanguard of protecting its scientists and speaking out on behalf of medical research (1). In addition, the Society for Neuroscience has issued a report on “Best Practices for Protecting Researchers and Research” to assist investigators and institutions targeted by terrorists (

We seek a more vigorous investigation and prosecution of the criminals committing the crimes against these scientists, their staffs, their families, their neighbors, and the communities in which they live. We are heartened that stronger laws enacted in the United States and the United Kingdom provide enforcement agencies with legal tools needed to bring these offenders to justice (1). The United Kingdom is ahead of the United States in this regard. As reported in Science, the United Kingdom formed a National Extremism Tactical Coordination Unit in 2004. This unit helped to conduct a 2-year investigation involving more than 700 police, which resulted in raids in the United Kingdom, the Netherlands, and Belgium and the arrest of 30 suspected terrorists. There seem to be signs that the vigorous prosecution of terrorism in the United Kingdom is having a positive effect (3). However, the number of attacks on scientists conducting medical research in animals in the United States is increasing (1), and we need to mount an equally vigorous campaign in this country to prevent these heinous attacks.

Lastly, we wish to laud the dedication and courage shown by Dr. London and those like her that continue to strive to reduce suffering and advance science despite obvious personal cost. As the beneficiaries of progress in medical care, it is also our responsibility to join the struggle to preserve medical research.

This article was not prepared with support from any funding agency or within the context of any official capacity. The opinions expressed herein are solely the private personal opinions of the authors and do not indicate any official institutional (university, government agencies, private foundations) position. Financial disclosures for the contributing authors are presented online as supplementary material

Free Fulltext: Science Direct

Neural coding of natural stimuli: information at sub-millisecond resolution

Nemenman I, Lewen GD, Bialek W, de Ruyter van Steveninck RR.
PLoS Comput Biol. 2008 Mar 7;4(3):e1000025

Sensory information about the outside world is encoded by neurons in sequences of discrete, identical pulses termed action potentials or spikes. There is persistent controversy about the extent to which the precise timing of these spikes is relevant to the function of the brain. We revisit this issue, using the motion-sensitive neurons of the fly visual system as a test case. Our experimental methods allow us to deliver more nearly natural visual stimuli, comparable to those which flies encounter in free, acrobatic flight. New mathematical methods allow us to draw more reliable conclusions about the information content of neural responses even when the set of possible responses is very large. We find that significant amounts of visual information are represented by details of the spike train at millisecond and sub-millisecond precision, even though the sensory input has a correlation time of approximately 55 ms; different patterns of spike timing represent distinct motion trajectories, and the absolute timing of spikes points to particular features of these trajectories with high precision. Finally, the efficiency of our entropy estimator makes it possible to uncover features of neural coding relevant for natural visual stimuli: first, the system's information transmission rate varies with natural fluctuations in light intensity, resulting from varying cloud cover, such that marginal increases in information rate thus occur even when the individual photoreceptors are counting on the order of one million photons per second. Secondly, we see that the system exploits the relatively slow dynamics of the stimulus to remove coding redundancy and so generate a more efficient neural code.

PMID: 18369423
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Perception of emotional expressions is independent of face selectivity in monkey inferior temporal cortex

Hadj-Bouziane F, Bell AH, Knusten TA, Ungerleider LG, Tootell RB.
Proc Natl Acad Sci U S A. 2008 Mar 28

The ability to perceive and differentiate facial expressions is vital for social communication. Numerous functional MRI (fMRI) studies in humans have shown enhanced responses to faces with different emotional valence, in both the amygdala and the visual cortex. However, relatively few studies have examined how valence influences neural responses in monkeys, thereby limiting the ability to draw comparisons across species and thus understand the underlying neural mechanisms. Here we tested the effects of macaque facial expressions on neural activation within these two regions using fMRI in three awake, behaving monkeys. Monkeys maintained central fixation while blocks of different monkey facial expressions were presented. Four different facial expressions were tested: (i) neutral, (ii) aggressive (open-mouthed threat), (iii) fearful (fear grin), and (iv) submissive (lip smack). Our results confirmed that both the amygdala and the inferior temporal cortex in monkeys are modulated by facial expressions. As in human fMRI, fearful expressions evoked the greatest response in monkeys-even though fearful expressions are physically dissimilar in humans and macaques. Furthermore, we found that valence effects were not uniformly distributed over the inferior temporal cortex. Surprisingly, these valence maps were independent of two related functional maps: (i) the map of "face-selective" regions (faces versus non-face objects) and (ii) the map of "face-responsive" regions (faces versus scrambled images). Thus, the neural mechanisms underlying face perception and valence perception appear to be distinct.

PMID: 18375769
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High-resolution depth electrode localization and imaging in patients with pharmacologically intractable epilepsy

Ekstrom A, Suthana N, Behnke E, B S E , Salamon N, Bookheimer S, Fried I.
J Neurosurg. 2008 Apr;108(4):812-5

check markLocalization and targeting of depth electrodes in specific regions of the human brain is critical for accurate clinical diagnoses and treatment as well as for neuroscientific electrophysiological research. By using high-resolution magnetic resonance imaging combined with 2D computational unfolding, the authors present a method that improves electrode localization in the medial temporal lobe. This method permits visualization of electrode placements in subregions of the hippocampus and parahippocampal gyrus, allowing for greater specificity in relating electrophysiological and anatomical features in the human medial temporal lobe. Such methods may be extended to therapeutic procedures targeting specific neuronal circuitry in subfields of structures deep in the human brain.

PMID: 18377264

Quantitative multifocal fMRI shows active suppression in human V1

Pihlaja M, Henriksson L, James AC, Vanni S
Hum Brain Mapp. 2008 Apr 1

Multifocal functional magnetic resonance imaging has recently been introduced as an alternative method for retinotopic mapping, and it enables effective functional localization of multiple regions-of-interest in the visual cortex. In this study we characterized interactions in V1 with spatially and temporally identical stimuli presented alone, or as a part of a nine-region multifocal stimulus. We compared stimuli at different contrasts, collinear and orthogonal orientations and spatial frequencies one octave apart. Results show clear attenuation of BOLD signal from the central region in the multifocal condition. The observed modulation in BOLD signal could be produced either by neural suppression resulting from stimulation of adjacent regions of visual field, or alternatively by hemodynamic saturation or stealing effects in V1. However, we find that attenuation of the central response persists through a range of contrasts, and that its strength varies with relative orientation and spatial frequency of the central and surrounding stimulus regions, indicating active suppression mechanisms of neural origin. Our results also demonstrate that the extent of the signal spreading is commensurate with the extent of the horizontal connections in primate V1. Hum Brain Mapp, 2008. (c) 2008 Wiley-Liss, Inc.

PMID: 18381768

Addition of fornix transection to frontal-temporal disconnection increases the impairment in object-in-place memory in macaque monkeys

Wilson CR, Baxter MG, Easton A, Gaffan D.
Eur J Neurosci. 2008 Apr;27(7):1814-22

Both frontal-inferotemporal disconnection and fornix transection (Fx) in the monkey impair object-in-place scene learning, a model of human episodic memory. If the contribution of the fornix to scene learning is via interaction with or modulation of frontal-temporal interaction--that is, if they form a unitary system--then Fx should have no further effect when added to frontal-temporal disconnection. However, if the contribution of the fornix is to some extent distinct, then fornix lesions may produce an additional deficit in scene learning beyond that caused by frontal-temporal disconnection. To distinguish between these possibilities, we trained three male rhesus monkeys on the object-in-place scene-learning task. We tested their learning on the task following frontal-temporal disconnection, achieved by crossed unilateral aspiration of the frontal cortex in one hemisphere and the inferotemporal cortex in the other, and again following the addition of Fx. The monkeys were significantly impaired in scene learning following frontal-temporal disconnection, and furthermore showed a significant increase in this impairment following the addition of Fx, from 32.8% error to 40.5% error (chance = 50%). The increased impairment following the addition of Fx provides evidence that the fornix and frontal-inferotemporal interaction make distinct contributions to episodic memory.

PMID: 18380673