Sunday, July 29, 2007

Abnormal FMRI adaptation to unfamiliar faces in a case of developmental prosopamnesia

Williams MA, Berberovic N, Mattingley JB
Curr Biol. 2007 Jul 17;17(14):1259-64

In rare cases, damage to the temporal lobe causes a selective impairment in the ability to learn new faces, a condition known as prosopamnesia [1]. Here we present the case of an individual with prosopamnesia in the absence of any acquired structural lesion. "C" shows intact processing of simple and complex nonface objects, but her ability to learn new faces is severely impaired. We used a neural marker of perceptual learning known as repetition suppression to examine functioning within C's fusiform face area (FFA), a region of cortex involved in face perception [2]. For comparison, we examined repetition suppression in the scene-selective parahippocampal place area (PPA) [3]. As expected, normal controls showed significant region-specific attenuation of neural activity across repetitions of each stimulus class. C also showed normal attenuation within the PPA to familiar and unfamiliar scenes, and within the FFA to familiar faces. Critically, however, she failed to show any adaptive change within the FFA for repeated unfamiliar faces, despite a face-specific blood-oxygen-dependent response (BOLD) response in her FFA during viewing of face stimuli. Our findings suggest that in developmental prosopamnesia, the FFA cannot maintain stable representations of new faces for subsequent recall or recognition.

PMID: 17614283


Statistics of Midbrain Dopamine Neuron Spike Trains in the Awake Primate

Bayer HM, Lau B, Glimcher PW
J Neurophysiol. 2007 Jul 5;

Work in behaving primates indicates that midbrain dopamine neurons encode a prediction error, the difference between an obtained reward and the reward expected. Studies of dopamine action potential timing in the alert and anaesthetized rat indicate that dopamine neurons respond in tonic and phasic modes, a distinction that has been less well characterized in the primates. We used spike train models to examine the relationship between the tonic and burst modes of activity in dopamine neurons while monkeys were performing a reinforced visuo-saccadic movement task. We studied spiking activity during four task-related intervals; two of these were intervals during which no task-related events occurred, while two were periods marked by task-related phasic activity. We found that dopamine neuron spike trains during the intervals when no events occurred were well described as tonic. Action potentials appeared to be independent, to occur at low frequency, and to be almost equally well described by Gaussian and Poisson-like (Gamma) processes. Unlike in the rat, interspike intervals as low as 20 ms were often observed during these presumptively tonic epochs. Having identified these periods of presumptively tonic activity we were able to quantitatively define phasic modulations (both increases and decreases in activity) during the intervals in which task-related events occurred. This analysis revealed that the phasic modulations of these neurons include both bursting, as has been described previously, and pausing. Together bursts and pauses seemed to provide a continuous, although non-linear, representation of the theoretically defined reward prediction error of reinforcement learning.

PMID: 17615124


Individuation and holistic processing of faces in rhesus monkeys

Dahl CD, Logothetis NK, Hoffman KL
Proc Biol Sci. 2007 Sep 7;274(1622):2069-76

Despite considerable evidence that neural activity in monkeys reflects various aspects of face perception, relatively little is known about monkeys' face processing abilities. Two characteristics of face processing observed in humans are a subordinate-level entry point, here, the default recognition of faces at the subordinate, rather than basic, level of categorization, and holistic effects, i.e. perception of facial displays as an integrated whole. The present study used an adaptation paradigm to test whether untrained rhesus macaques (Macaca mulatta) display these hallmarks of face processing. In experiments 1 and 2, macaques showed greater rebound from adaptation to conspecific faces than to other animals at the individual or subordinate level. In experiment 3, exchanging only the bottom half of a monkey face produced greater rebound in aligned than in misaligned composites, indicating that for normal, aligned faces, the new bottom half may have influenced the perception of the whole face. Scan path analysis supported this assertion: during rebound, fixation to the unchanged eye region was renewed, but only for aligned stimuli. These experiments show that macaques naturally display the distinguishing characteristics of face processing seen in humans and provide the first clear demonstration that holistic information guides scan paths for conspecific faces.

PMID: 17609192

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The Neural Basis of Decision Making

Gold JI, Shadlen MN
Annu Rev Neurosci. 2007 Jul 21;30:535-574

The study of decision making spans such varied fields as neuroscience, psychology, economics, statistics, political science, and computer science. Despite this diversity of applications, most decisions share common elements including deliberation and commitment. Here we evaluate recent progress in understanding how these basic elements of decision formation are implemented in the brain. We focus on simple decisions that can be studied in the laboratory but emphasize general principles likely to extend to other settings.

PMID: 17600525


Multiple dopamine functions at different time courses

Schultz W
Annu Rev Neurosci. 2007;30:259-88

Many lesion studies report an amazing variety of deficits in behavioral functions that cannot possibly be encoded in great detail by the relatively small number of midbrain dopamine neurons. Although hoping to unravel a single dopamine function underlying these phenomena, electrophysiological and neurochemical studies still give a confusing, mutually exclusive, and partly contradictory account of dopamine's role in behavior. However, the speed of observed phasic dopamine changes varies several thousand fold, which offers a means to differentiate the behavioral relationships according to their time courses. Thus dopamine is involved in mediating the reactivity of the organism to the environment at different time scales, from fast impulse responses related to reward via slower changes with uncertainty, punishment, and possibly movement to the tonic enabling of postsynaptic motor, cognitive, and motivational systems deficient in Parkinson's disease.

PMID: 17600522


A Model of V4 Shape Selectivity and Invariance

Cadieu C, Kouh M, Pasupathy A, Connor C, Riesenhuber M, Poggio TA
J Neurophysiol. 2007 Jun 27;

Object recognition in primates is mediated by the ventral visual pathway and is classically described as a feedforward hierarchy of increasingly sophisticated representations. Neurons in macaque monkey area V4, an intermediate stage along the ventral pathway, have been shown to exhibit selectivity to complex boundary conformation and invariance to spatial translation. How could such a representation be derived from the signals in lower visual areas such as V1? We show that a quantitative model of hierarchical processing, which is part of a larger model of object recognition in the ventral pathway, provides a plausible mechanism for the translation-invariant shape representation observed in area V4. Simulated model neurons successfully reproduce V4 selectivity and invariance through a nonlinear, translation-invariant combination of locally selective subunits, suggesting that a similar transformation may occur or culminate in area V4. Specifically, this mechanism models the selectivity of individual V4 neurons to boundary conformation stimuli, exhibits the same degree of translation invariance observed in V4, and produces observed V4 population responses to bars and non-Cartesian gratings. This work provides a quantitative model of the widely described shape selectivity and invariance properties of area V4 and points toward a possible canonical mechanism operating throughout the ventral pathway.

PMID: 17596412


Quantitative comparison between neural response in macaque inferotemporal cortex and behavioral discrimination of photographic images

Allred SR, Jagadeesh B
J Neurophysiol. 2007 Jun 27;

Inferotemporal (IT) cortex plays a critical role in the primate ability to perceive and discriminate between images, but the relationship between responses of single neurons and behavioral capacities is poorly understood. We studied this relationship by recording from IT neurons while monkeys performed a delayed-match-to-sample task with two images. On each day, two sample images were chosen to maximize the selectivity of the neuron, and task difficulty was manipulated by varying sample duration and by masking the sample. On each trial, monkeys reported which of the two sample images was presented. Neural performance was described using an ideal observer analysis. Across the population, neural and behavioral sensitivity to changes in sample duration were indistinguishable. Neural sensitivity was dependent on epoch used to analyze neural response; maximal neural sensitivity was achieved in the 128 ms epoch that began 85 ms after sample onset. At most sample durations, the epoch that yielded optimal neural performance was longer than the sample duration, suggesting that neural selectivity persisted after the presentation of the mask during performance of the task. A control experiment showed that neural and behavioral performance improved in the absence of the mask. These observations suggest that the responses of individual IT neurons contain sufficient information to allow behavioral discrimination of images in a demanding task.

PMID: 17596424

Modulation of neuronal interactions through neuronal synchronization

Womelsdorf T, Schoffelen JM, Oostenveld R, Singer W, Desimone R, Engel AK, Fries P
Science. 2007 Jun 15;316(5831):1609-12

Brain processing depends on the interactions between neuronal groups. Those interactions are governed by the pattern of anatomical connections and by yet unknown mechanisms that modulate the effective strength of a given connection. We found that the mutual influence among neuronal groups depends on the phase relation between rhythmic activities within the groups. Phase relations supporting interactions between the groups preceded those interactions by a few milliseconds, consistent with a mechanistic role. These effects were specific in time, frequency, and space, and we therefore propose that the pattern of synchronization flexibly determines the pattern of neuronal interactions.

PMID: 17569862