Biomedical subjects

Valentin Dragoi

Publications and source records attributed to Valentin Dragoi.

8 recordsLinked to original sources

PubMed2006

Image structure at the center of gaze during free viewing.

It is generally believed that the visual system is adapted to the statistics of the visual world. Measuring and understanding these statistics require precise knowledge of the structure of the signals reaching fovea during image scanning. However, despite the fact that eye movements cause retinal stimulation to change several times in a second, it is implicitly assumed that images are sampled uniformly during natural viewing. By analyzing the eye movements of three rhesus monkeys freely viewing natural scenes, we report here significant anisotropy in stimulus statistics at the center of gaze. We find that fixation on an image patch is more likely to be followed by a saccade to a nearby patch of similar orientation structure or by a saccade to a more distant patch of largely dissimilar orientation structure. Furthermore, we show that orientation-selective neurons in the primary visual cortex (V1) can take advantage of eye movement statistics to selectively improve their discrimination performance.

Action Potentials↗

PubMed2005

Tilt aftereffect and adaptation-induced changes in orientation tuning in visual cortex.

The tilt aftereffect (TAE) is a visual illusion in which prolonged adaptation to an oriented stimulus causes shifts in subsequent perceived orientations. Historically, neural models of the TAE have explained it as the outcome of response suppression of neurons tuned to the adapting orientation. Recent physiological studies of neurons in primary visual cortex (V1) have confirmed that such response suppression exists. However, it was also found that the preferred orientations of neurons shift away from the adapting orientation. Here we show that adding this second factor to a population coding model of V1 improves the correspondence between neurophysiological data and TAE measurements. According to our model, the shifts in preferred orientation have the opposite effect as response suppression, reducing the magnitude of the TAE.

Adaptation, Physiological↗

PubMed2003

V1 neurons signal acquisition of an internal representation of stimulus location.

A fundamental aspect of visuomotor behavior is deciding where to look or move next. Under certain conditions, the brain constructs an internal representation of stimulus location on the basis of previous knowledge and uses it to move the eyes or to make other movements. Neuronal responses in primary visual cortex were modulated when such an internal representation was acquired: Responses to a stimulus were affected progressively by sequential presentation of the stimulus at one location but not when the location was varied randomly. Responses of individual neurons were spatially tuned for gaze direction and tracked the Bayesian probability of stimulus appearance. We propose that the representation arises in a distributed cortical network and is associated with systematic changes in response selectivity and dynamics at the earliest stages of cortical visual processing.

Analysis of Variance↗

PubMed2003

Interval timing as an emergent learning property.

Interval timing in operant conditioning is the learned covariation of a temporal dependent measure such as wait time with a temporal independent variable such as fixed-interval duration. The dominant theories of interval timing all incorporate an explicit internal clock, or "pacemaker," despite its lack of independent evidence. The authors propose an alternative, pacemaker-free view that demonstrates that temporal discrimination can be explained by using only 2 assumptions: (a) variation and selection of responses through competition between reinforced behavior and all other, elicited, behaviors and (b) modulation of the strength of response competition by the memory for recent reinforcement. The model departs radically from existing timing models: It shows that temporal learning can emerge from a simple dynamic process that lacks a periodic time reference such as a pacemaker.

Conditioning, Operant↗

PubMed2002

Cortical plasticity: time for a change.

Two recent studies have tested whether synaptic learning rules, inferred earlier from work on cell cultures and brain slices, apply in intact brains. The evidence indicates that they do, and reveals interesting implications for brain development and perceptual learning.

Animals↗

PubMed2002

A feedforward model of suppressive and facilitatory habituation effects.

Simple exposure to repetitive stimulation is known to induce short-term learning effects across a wide range of species. These effects can be both suppressive and facilitatory depending on stimulus conditions: repetitive presentation of a weak stimulus decreases the strength of the response (habituation), whereas presentation of a tonic stimulus following a series of weak stimuli transiently increases the response strength (dishabituation). Although these phenomena have been comprehensively characterized at both behavioral and cellular levels, most existing models of nonassociative learning focus exclusively on the suppressive or facilitatory changes in response, and do not attempt to relate cellular events to behavior. I propose here a feedforward model of habituation effects that explains both suppressive and facilitatory changes in response relying on the interaction between excitatory and inhibitory processes that develop in parallel on two different timescales. The model's properties are used to explain the rate sensitivity property of habituation and recovery and stimulus dishabituation.

Animals↗

PubMed2002

Dynamics of neuronal sensitivity in visual cortex and local feature discrimination.

A striking aspect of natural scenes is that image features such as line orientation are strongly correlated at neighboring spatial locations but not at distant locations. Thus, during the viewing of a scene, eye movements are often accompanied by a change in the orientation structure of the image. How does this behavior influence the discrimination of local features and their encoding by visual cortical neurons? Here we examined the perceived changes in orientation induced by brief exposure to oriented image patterns in monkeys and humans, and then used reverse correlation to investigate dynamic changes in neuronal sensitivity in the primary visual cortex (V1) of behaving monkeys. Whereas brief adaptation to an oriented grating impaired identification of nearby orientations by broadening orientation selectivity and changing the preferred orientation of individual V1 neurons, it actually enhanced the identification of orthogonal orientations by sharpening neuronal selectivity. Hence, successive exposure to image patches of dissimilar spatial structure enhances both the ability to discriminate local features and the encoding of these features by V1 neurons.

Action Potentials↗

PubMed1997

A Dynamic Theory of Acquisition and Extinction in Operant Learning.

This article offers a new neural network framework for understanding both the transients and the asymptotes of operant (instrumental) learning. The theory shows that interplay between simple short and long-term memory mechanisms is sufficient to explain a large number of operant phenomena. It describes short- and long-term effects of reinforcement and how these effects modulate the operant response, how novel events are detected and processed, and how their consequences also modulate the operant response. The critical features of the present theory are: (1) reinforcement expectancy is defined as the aggregate prediction of response-reinforcement and stimulus-reinforcement associations; (2) reinforcement expectancy controls the rate of increase of the operant response; (3) the response is controlled by a behavioral inhibition unit which integrates the mismatch between expected (long-term) and experienced (short-term) events. The model predicts the general features of several operant phenomena such as response selection, development of preference under different manipulations of reinforcement probabilities, negative contrast, partial reinforcement extinction effect, and spontaneous recovery. Implications of the present theory for other operant conditioning phenomena, classical conditioning, and avoidance behavior are suggested. Copyright 1997 Elsevier Science Ltd. All Rights Reserved.

Journal Article↗