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Google Tech Talks April 9, 2007
The speaker, Craig Mello, was awarded the Nobel Prize in Physiology or Medicine in 2006.

Context-dependent perceptual modulation of single neurons in primate visual cortex

Maier A, Logothetis NK, Leopold DA
Proc Natl Acad Sci U S A. 2007 Mar 27;104(13):5620-5

Some neurons in the visual cortex alter their spiking rate according to the perceptual interpretation of an observed stimulus, rather than its physical structure alone. Experiments in monkeys have suggested that, although the proportion of neurons showing this effect differs greatly between cortical areas, this proportion remains similar across different stimuli. These findings have raised the intriguing questions of whether the same neurons always participate in the disambiguation of sensory patterns and whether such neurons might represent a special class of cortical cells that relay perceptual signals to higher cortical areas. Here we explore this question by measuring activity in the middle temporal cortex of monkeys and asking to what degree the percept-related responses of individual neurons depend upon the specific sensory input. In contrast to our expectations, we found that even small differences in the stimuli led to significant changes in the signaling of the perceptual state by single neurons. We conclude that nearly all feature-responsive neurons in this area, rather than a select subset, can contribute to the resolution of sensory conflict, and that the role of individual cells in signaling the perceptual outcome is tightly linked to the fine details of the stimuli involved.

PMID: 17369363

Free Fulltext: http://www.pnas.org/cgi/reprint/104/13/5620

Effects of Long-Term Object Familiarity on Event-Related Potentials in the Monkey

Jessie J. Peissig, Jedediah Singer, Keisuke Kawasaki, David L. Sheinberg
Cerebral Cortex June 2007;17:1323--1334 doi:10.1093/cercor/bhl043

Although some change in the neural representation of an object
must occur as it becomes familiar, the nature of this change is not
fully understood. In humans, it has been shown that the N170—an
evoked visual potential—is enhanced for classes of objects for
which people have visual expertise. In this study, we explored
whether monkeys show a similar modulation in event-related
potential (ERP) amplitude as a result of long-term familiarity by
recording ERPs with chronically implanted electrodes over extended
training periods spanning many sessions. In each of 3
experiments, we found larger amplitude visual evoked responses to
highly familiar images for the time period of 120--250 ms after
stimulus onset. This difference was found when the monkeys were
trained in an individual-level discrimination task, in a task that
required only color discrimination, and even following a viewingonly
task. We thus observed this familiarity effect across several
tasks and different object categories and further found that the
difference between ‘‘familiar’’ and ‘‘novel’’ became smaller as the
animals gained experience with the previously unfamiliar objects
across multiple test sessions. These data suggest that changes in
visual responses associated with familiarity are evident early in the
evoked visual response, are robust, and may be automatic, driven at
least in part by repeated object exposure.

Duration-Dependent fMRI Adaptation and Distributed Viewer-Centered Face Representation in Human Visual Cortex

Fang Fang, Scott O. Murray, Sheng He
Cerebral Cortex June 2007;17:1402--1411 doi:10.1093/cercor/bhl053

Two functional magnetic resonance imaging (fMRI) face viewpoint
adaptation experiments were conducted to investigate whether
fMRI adaptation in high-level visual cortex depends on the duration
of adaptation and how different views of a face are represented in
the human visual system. We found adaptation effects in multiple
face-selective areas, which suggest a distributed, viewer-centered
representation of faces in the human visual system. However, the
nature of the adaptation effects was dependent on the length of adaptation.
With long adaptation durations, face-selective areas along
the hierarchy of the visual system gradually exhibited viewpointtuned
adaptation. As the angular difference between the adapter
and test stimulus increased, the blood oxygen level--dependent
(BOLD) signal evoked by the test stimulus gradually increased as
a function of the amount of 3-dimensional (3D) rotation. With short
adaptation durations, however, face-selective areas in the ventral
pathway, including the lateral occipital cortex and right fusiform
area, exhibited viewpoint-sensitive adaptation. These areas showed
an increase in the BOLD signal with a 3D rotation, but this signal
increase was independent of the amount of rotation. Further, the
right superior temporal sulcus showed little or very weak viewpoint
adaptation with short adaptation durations. Our findings suggest
that long- and short-term fMRI adaptations may reflect selective
properties of different neuronal mechanisms.

Behavioral Detection of Electrical Microstimulation in Different Cortical Visual Areas

Dona K. Murphey, John H.R. Maunsell
Current Biology 17, 862–867, May 15, 2007

The extent to which areas in the visual cerebral cortex
differ in their ability to support perceptions has been
the subject of considerable speculation. Experiments
examining the activity of individual neurons have suggested
that activity in later stages of the visual cortex
is more closely linked to perception than that in earlier
stages [1–9]. In contrast, results from functional imaging,
transcranial magnetic stimulation, and lesion
studies have been interpreted as showing that earlier
stages are more closely coupled to perception [10–
15]. We examined whether neuronal activity in early
and later stages differs in its ability to support detectable
signals by measuring behavioral thresholds for
detecting electrical microstimulation in different cortical
areas in two monkeys. By training the animals to
perform a two-alternative temporal forced-choice
task, we obtained criterion-free thresholds from five
visual areas—V1, V2, V3A, MT, and the inferotemporal
cortex. Every site tested yielded a reliable threshold.
Thresholds varied little within and between visual
areas, rising gradually from early to later stages. We
similarly found no systematic differences in the slopes
of the psychometric detection functions from different
areas. These results suggest that neuronal signals of
similar magnitude evoked in any part of visual cortex
can generate percepts.

Human Computation

An Interesting Site

Take a look at this website: Classics in the History of Psychology

Temporal limitations in object processing across the human ventral visual pathway

Thomas James McKeeff, David A. Remus, and Frank Tong
J Neurophysiol (May 9, 2007). doi:10.1152/jn.00568.2006

Behavioral studies have shown that object recognition becomes severely impaired at fast presentation rates, indicating a limitation in temporal processing capacity. Here, we investigated whether this behavioral limit in object recognition reflects limitations in the temporal processing capacity of early visual areas tuned to basic features or high-level areas tuned to complex objects. We used functional MRI (fMRI) to measure the temporal processing capacity of multiple areas along the ventral visual pathway progressing from the primary visual cortex (V1) to high-level object-selective regions, specifically the fusiform face area (FFA) and parahippocampal place area (PPA). Subjects viewed successive images of faces or houses at presentation rates varying from 2.3-37.5 items/second while performing an object discrimination task. Measures of the temporal frequency response profile of each visual area revealed a systematic decline in peak tuning across the visual hierarchy. Areas V1-V3 showed peak activity at rapid presentation rates of 18-25 items/sec, area V4v peaked at intermediate rates (9 items/sec), and the FFA and PPA peaked at the slowest temporal rates (4-5 items/sec). Our results reveal a progressive loss in the temporal processing capacity of the human visual system as information is transferred from early visual areas to higher areas. These data suggest that temporal limitations in object recognition likely result from the limited processing capacity of high-level object-selective areas rather than that of earlier stages of visual processing.

Population Activity in the Human Dorsal Pathway Predicts the Accuracy of Visual Motion Detection

Donner TH, Siegel M, Oostenveld R, Fries P, Bauer M, Engel AK.
J Neurophysiol. 2007 May 9;


A person's ability to detect a weak visual target stimulus varies from one viewing to the next. We tested if the trial-to-trial fluctuations of neural population activity in the human brain are related to the fluctuations of behavioral performance in a 'yes-no' visual motion detection task. We recorded neural population activity with whole-head magnetoencephalography (MEG) while subjects searched for a weak coherent motion signal embedded in spatiotemporal noise. We found that, during motion viewing, MEG activity in the 12-24 Hz ('beta') frequency range is higher, on average, before correct behavioral choices than before errors, and that it predicts correct choices on a trial-by-trial basis. This performance-predictive activity is not evident in the pre-stimulus baseline and builds up slowly after stimulus onset. Source reconstruction revealed that the performance-predictive activity is expressed in the posterior parietal and dorsolateral prefrontal cortices, and, less strongly, in the visual motion-sensitive area MT+. 12-24 Hz activity in these key stages of the human dorsal visual pathway is correlated with behavioral choice in both, target present and absent conditions. Importantly, in the absence of the target, 12-24 Hz activity tends to be higher before 'no' choices ('correct rejects') than before 'yes' choices ('false alarms'). It thus predicts the accuracy, and not the content, of subjects' upcoming perceptual reports. We conclude that beta band activity in the human dorsal visual pathway indexes, and potentially controls, the efficiency of neural computations underlying simple perceptual decisions.

PMID: 17493916

Intrinsic functional architecture in the anaesthetized

J. L. Vincent, G. H. Patel1, M. D. Fox, A. Z. Snyder, J. T. Baker, D. C. Van Essen, J. M. Zempel, L. H. Snyder, M. Corbetta, M. E. Raichle
Nature Vol 447| 3 May 2007| doi:10.1038/nature05758

The traditional approach to studying brain function is to measure
physiological responses to controlled sensory, motor and cognitive
paradigms. However, most of the brain’s energy consumption
is devoted to ongoing metabolic activity not clearly associated with
any particular stimulus or behaviour1. Functional magnetic resonance
imaging studies in humans aimed at understanding this
ongoing activity have shown that spontaneous fluctuations of the
blood-oxygen-level-dependent signal occur continuously in the
resting state. In humans, these fluctuations are temporally coherent
within widely distributed cortical systems that recapitulate the
functional architecture of responses evoked by experimentally
administered tasks2–6. Here, we show that the same phenomenon
is present in anaesthetized monkeys even at anaesthetic levels
known to induce profound loss of consciousness. We specifically
demonstrate coherent spontaneous fluctuations within three well
known systems (oculomotor, somatomotor and visual) and the
‘default’ system, a set of brain regions thought by some to support
uniquely human capabilities. Our results indicate that coherent
system fluctuations probably reflect an evolutionarily conserved
aspect of brain functional organization that transcends levels of
consciousness.