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.

مدتی این مثنوی تاخیر شد ...

مدتی این مثنوی تاخیر شد
مهلتی بایست تا ...

Relationship between color discrimination and neural responses in the inferior temporal cortex of the monkey

Matsumora T, Koida K, Komatsu H.
J Neurophysiol. 2008 Oct 15.

Earlier studies suggest that the inferior temporal (IT) cortex of the monkey plays a key role in color discrimination. Here, we examined the quantitative relationship between color judgment in monkeys and the responses of color-selective neurons in the anterior part of the IT cortex (area TE) by comparing neuronal activity and behavior recorded simultaneously while the monkeys performed a color judgment task. We first compared the abilities of single neurons and monkeys to discriminate color. To calculate a neuron's ability to discriminate color, we computed a neurometric function using receiver-operating-characteristics analysis. We then compared the neural and behavioral thresholds for color discrimination and found that, in general, the neural threshold was higher than the behavioral threshold, though occasionally the reverse was true. Variation in the neural threshold across the color space corresponded well with that of the behavioral threshold. We then calculated the Choice Probability (CP), which is a measure of the correlation between the trial-to-trial fluctuations in neuronal responses and the monkeys' color judgment. On average, CPs were slightly but significantly larger than 0.5, indicating the activities of these TE neurons correlate positively with the monkeys' color judgment. This suggests that individual color-selective TE neurons only weakly contribute to color discrimination and that a large population of color-selective TE neurons contribute to the performance of color discrimination.

PMID: 18922950

Full text: http://jn.physiology.org/cgi/reprint/90551.2008v1

Stimulus similarity-contingent neural adaptation can be time and cortical area dependent.

Verhoef BE, Kayaert G, Franko E, Vangeneugden J, Vogels R.
J Neurosci. 2008 Oct 15;28(42):10631-40.

Repetition of a stimulus results in decreased responses in many cortical areas. This so-called adaptation or repetition suppression has been used in several human functional magnetic resonance imaging studies to deduce the stimulus selectivity of neuronal populations. We tested in macaque monkeys whether the degree of neural adaptation depends on the similarity between the adapter and test stimulus. To manipulate similarity, we varied stimulus size. We recorded the responses of single neurons to different-sized shapes in inferior temporal (IT) and prefrontal cortical (PFC) areas while the animals were engaged in a size or shape discrimination task. The degree of response adaptation in IT decreased with increasing size differences between the adapter and the test stimuli in both tasks, but the dependence of adaptation on the degree of similarity between the adapter and test stimuli was limited mainly to the early phase of the neural response in IT. PFC neurons showed only weak size-contingent repetition effects, despite strong size selectivity observed with the same stimuli. Thus, based on the repetition effects in PFC, one would have erroneously concluded that PFC shows weak or no size selectivity in such tasks. These findings are relevant for the interpretation of functional magnetic resonance adaptation data: they support the conjecture that the degree of adaptation scales with the similarity between adapter and test stimuli. However, they also show that the temporal evolution of adaptation during the course of the response, and differences in the way individual regions react to stimulus repetition, may complicate the inference of neuronal tuning from functional magnetic resonance adaptation.

PMID: 18923039

Full text: http://www.jneurosci.org/chttp://www.jneurosci.org/cgi/content/full/28/42/10631gi/content/full/28/42/10631

Spatial updating: how the brain keeps track of changing object locations during observer motion

Thomas Wolbers, Mary Hegarty, Christian Büchel & Jack M Loomis
Nature Neuroscience 11, 1223 - 1230 (2008)

As you move through an environment, the positions of surrounding objects relative to your body constantly change. Updating these locations is a central feature of situational awareness and readiness to act. Here, we used functional magnetic resonance imaging and a virtual environment to test how the human brain uses optic flow to monitor changing object coordinates. Only activation profiles in the precuneus and the dorsal premotor cortex (PMd) were indicative of an updating process operating on a memorized egocentric map of space. A subsequent eye movement study argued against the alternative explanation that activation in PMd could be driven by oculomotor signals. Finally, introducing a verbal response mode revealed a dissociation between the two regions, with the PMd only showing updating-related responses when participants responded by pointing. We conclude that visual spatial updating relies on the construction of updated representations in the precuneus and the context-dependent planning of motor actions in PMd.

Fulltext:http://www.nature.com.proxy.kib.ki.se/neuro/journal/v11/n10/full/nn.2189.html

Improved visual sensitivity during smooth pursuit eye movements

Alexander C Schütz, Doris I Braun, Dirk Kerzel & Karl R Gegenfurtner1
Nature Neuroscience 11, 1211 - 1216 (2008)

When we view the world around us, we constantly move our eyes. This brings objects of interest into the fovea and keeps them there, but visual sensitivity has been shown to deteriorate while the eyes are moving. Here we show that human sensitivity for some visual stimuli is improved during smooth pursuit eye movements. Detection thresholds for briefly flashed, colored stimuli were 16% lower during pursuit than during fixation. Similarly, detection thresholds for luminance-defined stimuli of high spatial frequency were lowered. These findings suggest that the pursuit-induced sensitivity increase may have its neuronal origin in the parvocellular retino-thalamic system. This implies that the visual system not only uses feedback connections to improve processing for locations and objects being attended to, but that a whole processing subsystem can be boosted. During pursuit, facilitation of the parvocellular system may reduce motion blur for stationary objects and increase sensitivity to speed changes of the tracked object.

Fulltext:http://www.nature.com.proxy.kib.ki.se/neuro/journal/v11/n10/full/nn.2194.html

Unsupervised Natural Experience Rapidly Alters Invariant Object Representation in Visual Cortex.

Li N, Dicarlo JJ.
Science. 2008 Sep 12;321(5895):1502-1507

Object recognition is challenging because each object produces myriad retinal images. Responses of neurons from the inferior temporal cortex (IT) are selective to different objects, yet tolerant ("invariant") to changes in object position, scale, and pose. How does the brain construct this neuronal tolerance? We report a form of neuronal learning that suggests the underlying solution. Targeted alteration of the natural temporal contiguity of visual experience caused specific changes in IT position tolerance. This unsupervised temporal slowness learning (UTL) was substantial, increased with experience, and was significant in single IT neurons after just 1 hour. Together with previous theoretical work and human object perception experiments, we speculate that UTL may reflect the mechanism by which the visual stream builds and maintains tolerant object representations.

PMID: 18787171

Full text: http://www.sciencemag.org/cgi/reprint/321/5895/1502.pdf

Effects of category learning on the stimulus selectivity of macaque inferior temporal neurons

De Baene W, Ons B, Wagemans J, Vogels R.
Learn Mem. 2008 Aug 26;15(9):717-27.

Primates can learn to categorize complex shapes, but as yet it is unclear how this categorization learning affects the representation of shape in visual cortex. Previous studies that have examined the effect of categorization learning on shape representation in the macaque inferior temporal (IT) cortex have produced diverse and conflicting results that are difficult to interpret owing to inadequacies in design. The present study overcomes these issues by recording IT responses before and after categorization learning. We used parameterized shapes that varied along two shape dimensions. Monkeys were extensively trained to categorize the shapes along one of the two dimensions. Unlike previous studies, our paradigm counterbalanced the relevant categorization dimension across animals. We found that categorization learning increased selectivity specifically for the category-relevant stimulus dimension (i.e., an expanded representation of the trained dimension), and that the ratio of within-category response similarities to between-category response similarities increased for the relevant dimension (i.e., category tuning). These small effects were only evident when the learned category-related effects were disentangled from the prelearned stimulus selectivity. These results suggest that shape-categorization learning can induce minor category-related changes in the shape tuning of IT neurons in adults, suggesting that learned, category-related changes in neuronal response mainly occur downstream from IT.

PMID: 18772261

Full text: http://learnmem.cshlp.org/cgi/content/full/15/9/717