Tuesday, June 30, 2009

Color vision, cones, and color-coding in the cortex.

Conway BR.
Neuroscientist. 2009 Jun;15(3):274-90.

Color processing begins with the absorption of light by cone photoreceptors, and progresses through a series of hierarchical stages: Retinal signals carrying color information are transmitted through the lateral geniculate nucleus of the thalamus (LGN) up to the primary visual cortex (V1). From V1, the signals are processed by the second visual area (V2); then by cells located in subcompartments ("globs") within the posterior inferior temporal (PIT) cortex, a brain region that encompasses area V4 and brain regions immediately anterior to V4. Color signals are then processed by regions deep within the inferior temporal (IT) cortex including area TE. As a heuristic, one can consider each of these stages to be involved in constructing a distinct aspect of the color percept. The three cone types are the basis for trichromacy; retinal ganglion cells that respond in an opponent fashion to activation of different cone classes are the basis for color opponency (these "cone-opponent" cells increase their firing rate above baseline to activation of one cone class and decrease their firing rate below baseline to activation of a different cone class); double-opponent neurons in the V1 generate local color contrast and are the building blocks for color constancy; glob cells elaborate the perception of hue; and IT integrates color perception in the context of behavior. Finally, though nothing is known, these signals presumably interface with motor programs and emotional centers of the brain to mediate the widely acknowledged emotional salience of color.

PMID: 19436076

Acute Treatment of Intractable Migraine With Sphenopalatine Ganglion Electrical Stimulation.

Tepper SJ, Rezai A, Narouze S, Steiner C, Mohajer P, Ansarinia M.
Headache. 2009 May 26.

Background.- We report preliminary results of a novel acute treatment for intractable migraine. The sphenopalatine ganglion (SPG) has sensorimotor and autonomic components and is involved in migraine pathophysiology. Methods.- In 11 patients with medically refractory migraine, the sphenopalatine fossa was accessed with a 20-gauge needle using the standard infrazygomatic transcoronoid approach under fluoroscopy. Patients underwent temporary unilateral electric stimulation of the SPG with a Medtronic 3057 test stimulation lead after induction of full-blown migraine. Both sham and active stimulations with different settings were carried out for
PMID: 19486173

Directed Interactions Between Auditory and Superior Temporal Cortices and their Role in Sensory Integration.

Kayser C, Logothetis NK.
Front Integr Neurosci. 2009;3:7. Epub 2009 May 4

Recent studies using functional imaging and electrophysiology demonstrate that processes related to sensory integration are not restricted to higher association cortices but already occur in early sensory cortices, such as primary auditory cortex. While anatomical studies suggest the superior temporal sulcus (STS) as likely source of visual input to auditory cortex, little evidence exists to support this notion at the functional level. Here we tested this hypothesis by simultaneously recording from sites in auditory cortex and STS in alert animals stimulated with dynamic naturalistic audio-visual scenes. Using Granger causality and directed transfer functions we first quantified causal interactions at the level of field potentials, and subsequently determined those frequency bands that show effective interactions, i.e. interactions that are relevant for influencing neuronal firing at the target site. We found that effective interactions from auditory cortex to STS prevail below 20 Hz, while interactions from STS to auditory cortex prevail above 20 Hz. In addition, we found that directed interactions from STS to auditory cortex make a significant contribution to multisensory influences in auditory cortex: Sites in auditory cortex showing multisensory enhancement received stronger feed-back from STS during audio-visual than during auditory stimulation, while sites with multisensory suppression received weaker feed-back. These findings suggest that beta frequencies might be important for inter-areal coupling in the temporal lobe and demonstrate that superior temporal regions indeed provide one major source of visual influences to auditory cortex.

PMID: 19503750

Where Are the Human Speech and Voice Regions, and Do Other Animals Have Anything Like Them?

Petkov CI, Logothetis NK, Obleser J.
Neuroscientist. 2009 Jun 10.

Modern lesion and imaging work in humans has been clarifying which brain regions are involved in the processing of speech and language. Concurrently, some of this work has aimed to bridge the gap to the seemingly incompatible evidence for multiple brain-processing pathways that first accumulated in nonhuman primates. For instance, the idea of a posterior temporal-parietal "Wernicke's" territory, which is thought to be instrumental for speech comprehension, conflicts with this region of the brain belonging to a spatial "where" pathway. At the same time a posterior speech-comprehension region ignores the anterior temporal lobe and its "what" pathway for evaluating the complex features of sensory input. Recent language models confirm that the posterior or dorsal stream has an important role in human communication, by a reconceptualization of the "where" into a "how-to" pathway with a connection to the motor system for speech comprehension. Others have tried to directly implicate the "what" pathway for speech comprehension, relying on the growing evidence in humans for anterior-temporal involvement in speech and voice processing. Coming full circle, we find that the recent imaging of vocalization and voice preferring regions in nonhuman primates allows us to make direct links to the human imaging data involving the anterior-temporal regions. The authors describe how comparison of the structure and function of the vocal communication system of humans and other animals is clarifying evolutionary relationships and the extent to which different species can model human brain function.

V4 activity predicts the strength of visual short-term memory representations.

Sligte IG, Scholte HS, Lamme VA.
J Neurosci. 2009 Jun 10;29(23):7432-8

Recent studies have shown the existence of a form of visual memory that lies intermediate of iconic memory and visual short-term memory (VSTM), in terms of both capacity (up to 15 items) and the duration of the memory trace (up to 4 s). Because new visual objects readily overwrite this intermediate visual store, we believe that it reflects a weak form of VSTM with high capacity that exists alongside a strong but capacity-limited form of VSTM. In the present study, we isolated brain activity related to weak and strong VSTM representations using functional magnetic resonance imaging. We found that activity in visual cortical area V4 predicted the strength of VSTM representations; activity was low when there was no VSTM, medium when there was a weak VSTM representation regardless of whether this weak representation was available for report or not, and high when there was a strong VSTM representation. Altogether, this study suggests that the high capacity yet weak VSTM store is represented in visual parts of the brain. Allegedly, only some of these VSTM traces are amplified by parietal and frontal regions and as a consequence reside in traditional or strong VSTM. The additional weak VSTM representations remain available for conscious access and report when attention is redirected to them yet are overwritten as soon as new visual stimuli hit the eyes.

Representing the forest before the trees: a global advantage effect in monkey inferotemporal cortex.

Sripati AP, Olson CR.
J Neurosci. 2009 Jun 17;29(24):7788-96
Fulltext: http://www.jneurosci.org/cgi/content/full/29/24/7788

Hierarchical stimuli (large shapes composed of small shapes) have long been used to study how humans perceive the global and the local content of a scene--the forest and the trees. Studies using these stimuli have revealed a global advantage effect: humans consistently report global shape faster than local shape. The neuronal underpinnings of this effect remain unclear. Here we demonstrate a correlate and possible mechanism in monkey inferotemporal cortex (IT). Inferotemporal neurons signal the global content of a hierarchical display approximately 30 ms before they signal its local content. This is a specific expression of a general principle, related to spatial scale or spatial frequency rather than to hierarchical level, whereby the representation of a large shape develops in IT before that of a small shape. These findings provide support for a coarse-to-fine model of visual scene representation.

Seeing the future: Natural image sequences produce "anticipatory" neuronal activity and bias perceptual report.

Perrett DI, Xiao D, Barraclough NE, Keysers C, Oram MW.
Q J Exp Psychol (Colchester). 2009 Jun 23:1-24.
Fulltext: http://www.informaworld.com/smpp/content~db=all?content=10.1080/17470210902959279

This paper relates human perception to the functioning of cells in the temporal cortex that are engaged in high-level pattern processing. We review historical developments concerning (a) the functional organization of cells processing faces and (b) the selectivity for faces in cell responses. We then focus on (c) the comparison of perception and cell responses to images of faces presented in sequences of unrelated images. Specifically the paper concerns the cell function and perception in circumstances where meaningful patterns occur momentarily in the context of a naturally or unnaturally changing visual environment. Experience of visual sequences allows anticipation, yet one sensory stimulus also "masks" perception and neural processing of subsequent stimuli. To understand this paradox we compared cell responses in monkey temporal cortex to body images presented individually, in pairs and in action sequences. Responses to one image suppressed responses to similar images for approximately 500 ms. This suppression led to responses peaking 100 ms earlier to image sequences than to isolated images (e.g., during head rotation, face-selective activity peaks before the face confronts the observer). Thus forward masking has unrecognized benefits for perception because it can transform neuronal activity to make it predictive during natural change.

Traditional waveform based spike sorting yields biased rate code estimates

Valérie Ventura
PNAS April 28, 2009 vol. 106 no. 17 6921–6926
Free Fulltext: http://www.pnas.org/cgi/doi/10.1073/pnas.0901771106

Much of neuroscience has to do with relating neural activity and
behavior or environment. One common measure of this relationship
is the firing rates of neurons as functions of behavioral or environmental
parameters, often called tuning functions and receptive
fields. Firing rates are estimated from the spike trains of neurons
recorded by electrodes implanted in the brain. Individual neurons’
spike trains are not typically readily available, because the signal
collected at an electrode is often a mixture of activities from different
neurons and noise. Extracting individual neurons’ spike trains
from voltage signals, which is known as spike sorting, is one of the
most important data analysis problems in neuroscience, because it
has to be undertaken prior to any analysis of neurophysiological
data in which more than one neuron is believed to be recorded
on a single electrode. All current spike-sorting methods consist
of clustering the characteristic spike waveforms of neurons. The
sequence of first spike sorting based on waveforms, then estimating
tuning functions, has long been the accepted way to proceed.
Here, we argue that the covariates that modulate tuning functions
also contain information about spike identities, and that if tuning
information is ignored for spike sorting, the resulting tuning function
estimates are biased and inconsistent, unless spikes can be
classified with perfect accuracy. This means, for example, that the
commonly used peristimulus time histogram is a biased estimate of
the firing rate of a neuron that is not perfectly isolated.We further
argue that the correct conceptual way to view the problem out is to
note that spike sorting provides information about rate estimation
and vice versa, so that the two relationships should be considered
simultaneously rather than sequentially. Indeed we show that
when spike sorting and tuning-curve estimation are performed in
parallel, unbiased estimates of tuning curves can be recovered even
from imperfectly sorted neurons.

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