Saturday, October 27, 2007

Correlation between neural spike trains increases with firing rate

de la Rocha J, Doiron B, Shea-Brown E, Josić K, Reyes A.
Nature. 2007 Aug 16;448(7155):802-6

Populations of neurons in the retina, olfactory system, visual and somatosensory thalamus, and several cortical regions show temporal correlation between the discharge times of their action potentials (spike trains). Correlated firing has been linked to stimulus encoding, attention, stimulus discrimination, and motor behaviour. Nevertheless, the mechanisms underlying correlated spiking are poorly understood, and its coding implications are still debated. It is not clear, for instance, whether correlations between the discharges of two neurons are determined solely by the correlation between their afferent currents, or whether they also depend on the mean and variance of the input. We addressed this question by computing the spike train correlation coefficient of unconnected pairs of in vitro cortical neurons receiving correlated inputs. Notably, even when the input correlation remained fixed, the spike train output correlation increased with the firing rate, but was largely independent of spike train variability. With a combination of analytical techniques and numerical simulations using 'integrate-and-fire' neuron models we show that this relationship between output correlation and firing rate is robust to input heterogeneities. Finally, this overlooked relationship is replicated by a standard threshold-linear model, demonstrating the universality of the result. This connection between the rate and correlation of spiking activity links two fundamental features of the neural code.

PMID: 17700699

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

Sun P, Ueno K, Waggoner RA, Gardner JL, Tanaka K, Cheng K.
Nat Neurosci. 2007 Oct 14;

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.

PMID: 17934459

Shape Selectivity for Camouflage-Breaking Dynamic Stimuli in Dorsal V4 Neurons

Mysore SG, Vogels R, Raiguel SE, Orban GA.
Cereb Cortex. 2007 Oct 12;

Motion is a potent cue for breaking camouflage in the natural world. To understand the neural basis of this phenomenon, one must utilize moving shapes defined by coherent motion of random texture elements against a similar, but stationary texture. To investigate how well neurons in area V4 process this novel, ecologically relevant stimulus and to compare shape selectivity for these shapes with static and other moving shapes, we tested V4 neurons with 5 static or moving shapes defined either by luminance or kinetic cues. The kinetic cues included a temporal frequency cue due to the difference in temporal frequencies of the moving dots inside the shape boundary and stationary dots outside the boundary. Therefore, static opponent motion-defined shapes without this cue were tested as an additional control. Approximately 44% (95/216) of V4 neurons showed shape selectivity. Analyses of these selective neurons both at single-neuron and population levels revealed that the shape-selective V4 neurons responded selectively to the moving kinetic shapes and that these neurons demonstrated robust invariance for shape preference across different shape conditions. Cue-invariant shape selectivity was more pronounced when kinetic shapes included the temporal frequency cue. This invariance may be rooted in nonlinearities occurring early in the visual pathway.

PMID: 17934186

TMS evidence for the involvement of the right occipital face area in early face processing

Pitcher D, Walsh V, Yovel G, Duchaine B.
Curr Biol. 2007 Sep 18;17(18):1568-73. Epub 2007 Aug 30.

Extensive research has demonstrated that several specialized cortical regions respond preferentially to faces [1-6]. One such region, located in the inferior occipital gyrus, has been dubbed the occipital face area (OFA) [7]. The OFA is the first stage in two influential face-processing models [8, 9], both of which suggest that it constructs an initial representation of a face, but how and when it does so remains unclear. The present study revealed that repetitive transcranial magnetic stimulation (rTMS) targeted at the right OFA (rOFA) disrupted accurate discrimination of face parts but had no effect on the discrimination of spacing between these parts. rTMS to left OFA had no effect. A matched part and spacing discrimination task that used house stimuli showed no impairment. In a second experiment, rTMS to rOFA replicated the face-part impairment but did not produce the same effect in an adjacent area, the lateral occipital cortex. A third experiment delivered double pulses of TMS separated by 40 ms at six periods after stimulus presentation during face-part discrimination. Accuracy dropped when pulses were delivered at 60 and 100 ms only. These findings indicate that the rOFA processes face-part information at an early stage in the face-processing stream.

PMID: 17764942

The timecourse of higher-level face aftereffects.

Rhodes G, Jeffery L, Clifford CW, Leopold DA.
Vision Res. 2007 Aug;47(17):2291-6. Epub 2007 Jul 9

Perceptual aftereffects for simple visual attributes processed early in the cortical hierarchy increase logarithmically with adapting duration and decay exponentially with test duration. This classic timecourse has been reported recently for a face identity aftereffect [Leopold, D. A., Rhodes, G., Müller, K.-M., & Jeffery, L. (2005). The dynamics of visual adaptation to faces. Proceedings of the Royal Society of London, Series B, 272, 897-904], suggesting that the dynamics of visual adaptation may be similar throughout the visual system. An alternative interpretation, however, is that the classic timecourse is a flow-on effect of adaptation of a low-level, retinotopic component of the face identity aftereffect. Here, we examined the timecourse of the higher-level (size-invariant) components of two face aftereffects, the face identity aftereffect and the figural face aftereffect. Both showed the classic pattern of logarithmic build-up and exponential decay. These results indicate that the classic timecourse of face aftereffects is not a flow-on effect of low-level retinotopic adaptation, and support the hypothesis that dynamics of visual adaptation are similar at higher and lower levels of the cortical visual hierarchy. They also reinforce the perceptual nature of face aftereffects, ruling out demand characteristics and other post-perceptual factors as plausible accounts.

PMID: 17619045

Saturday, October 13, 2007

Thursday, October 11, 2007

Recent History of Stimulus Speeds Affects the Speed Tuning of Neurons in Area MT

Anja Schlack, Bart Krekelberg, Thomas D. Albright
The Journal of Neuroscience, October 10, 2007, 27(41):11009-11018

Visual motion processing plays a key role in enabling primates' successful interaction with their dynamic environments. Although in natural environments the speed of visual stimuli continuously varies, speed tuning of neurons in the prototypical motion area MT has traditionally been assessed with stimuli that moved at constant speeds. We investigated whether the representation of speed in a continuously varying stimulus context differs from the representation of constant speeds. We recorded from individual MT neurons of fixating macaques while stimuli moved either at a constant speed or in a linearly accelerating or decelerating manner. We found clear speed tuning even when the stimulus consisted of visual motion with gradual speed changes. There were, however, important differences with the speed tuning as measured with constant stimuli: the stimulus context affected neuronal preferred speed as well as the associated tuning width of the speed tuning curves. These acceleration-dependent changes in response lead to an accurate representation of the acceleration of these stimuli in the MT cells. To elucidate the mechanistic basis of this signal, we constructed a stochastic firing rate model based on the constant speed response profiles. This model incorporated each cell's speed tuning and response adaptation dynamics and accurately predicted the response to constant speeds as well as accelerating and decelerating stimuli. Because the response of the model neurons had no explicit acceleration dependence, we conclude that speed-dependent adaptation creates a strong influence of temporal context on the MT response and thereby results in the representation of acceleration signals.

Perseverance pays off

Chris Rowan
POSTDOC JOURNAL, NATURE|Vol 449|11 October 2007

At long last, I've published the final part of my PhD research. Now what?

I've just heard that the final bit of research from my PhD has been accepted for publication. I'm obviously pleased, but I feel more like sighing in relief than punching the air. It's taken me two-and-a-half years, and two major rewrites, to get to this point — and there's only so many times I can read and recraft the same sentences before I start to get jaded....

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Tuesday, October 9, 2007

Mutating Pictures Gallery

A population of 1,000 random pictures each, created in October 2007. You allow the fittest pictures to survive. The higher your rating for a pic the more mutated offspring it produces.
Read more:

What would you do if you could publish only 20 papers throughout your career?


There are researchers who think that something is seriously
wrong with the way science is published.
Some complain about power: “Editors of ‘high-profile’
journals have too much power over scientists, because their
decisions profoundly affect one’s chances to get a grant/job/
tenure.” However, with a number of excellent journals to
choose from, an important paper should find a good home
in one of them, despite any poor editorial decisions from the
others. Besides, high-profile status is ultimately conferred to
a journal by the community, and publications that don’t do
a good job of filtering scientific information (an important
raison d’etre for professional editors) do pay the price.
Others complain about money: “Publishing firms make a
fortune selling back to universities the papers that scientists
produce.” The open-access movement stems at least in part
from this complaint. So much ink has been spilled over this
argument that it is hard to say anything new. Instead, let’s
acknowledge that if there are so many journals on the market,
it’s because of the tendency of authors to submit very incremental
papers—what one could call ‘minimal publishable
units’. There are as many scientists who ‘slice the salami’ as
thin as possible as there are specialized journals willing to
publish their work.
It’s certainly possible to argue that the proliferation of papers
also results from an explosion of research disciplines, each of
which needs its own set of journals, and from the need the community
has for the independent replication of published results.
One could also say that trainees need papers to advance to the
next stage of their careers and that researchers need papers to
show funding agencies where their money has been going.
For the sake of argument, though, let’s agree that there are
problems about cost and power in scientific publishing that
need to be fixed. So, here’s a bold proposal to reduce both the
number of publications that your library has to pay for and
the influence of editors on what is granted visibility in highprofile
journals—let’s set a limit on the number of papers that
scientists can publish during their careers.
These are the basic rules: whenever you get your first academic
job (that is, the first lab of your own), you get 20 tickets.
Ticket scalpers
What would you do if you could publish only 20 papers throughout your career?
Every time you publish a paper, you hand over one of them.
Once you run out of tickets, your publishing days are over. As
simple as that.
If we adopted this model, many articles reporting incremental
advances would no longer be written, and many specialized
journals would disappear. And with far fewer papers to read,
each one reporting a much more complete piece of research,
search committees or funding bodies could directly evaluate
the work of a given scientist, instead of (as is often the case)
leaning on surrogate indicators such as a journal’s impact factor
or number of citations.
At the extreme, we might not even need journals (and editors)
anymore; everything would be published in preprint
servers like those used by physicists, and the community
would simply evaluate and rank the different contributions
as they become available. This way, the whole community could
act as reviewers, doing away with the existing peer-review process.
This is somewhat reminiscent of what some websites are
already trying to do, so far with limited success. But if everybody
agreed to publish just 20 papers to keep the size of the
literature manageable, then the journal of the future might
conceivably be a preprint server.
As this model is adopted, it will certainly need some tweaking.
Maybe a particularly seminal paper would be exempt from
ticket usage. Would review articles require a ticket? Maybe a
much coveted award could come with half an extra ticket,
and a very competitive postdoctoral fellowship could get you
one-fourth of a ticket. If you collaborate with another lab, you
could be sole senior author of a paper at the expense of one
ticket or, alternatively, ‘split the bill’ with your collaborator.
Authors of fraudulent papers could be penalized by taking away
from them, say, three tickets. Maybe we would even see the birth
of a new class of ticket scalper and the production of counterfeit
tickets. And if you are one of those extraordinarily persuasive
scientists, you can always try to convince another researcher to
give you one of their tickets so you can publish another influential
piece of work.
The key question is: if you are unhappy with scientific publishing,
would you agree to the 20-paper limit?