Biomedical subjects

V Dragoi

Publications and source records attributed to V Dragoi.

6 recordsLinked to original sources

PubMed2001

Stability of cortical responses and the statistics of natural scenes.

The primary visual cortex (V1) of higher mammals contains maps of stimulus features; how these maps influence vision remains unknown. We have examined the functional significance of an asymmetry in the orientation map in cat V1, i.e., the fact that a larger area of V1 is preferentially activated by vertical and horizontal contours than by contours at oblique orientations. Despite the fact that neurons tuned to cardinal and oblique orientations have indistinguishable tuning characteristics, cardinal neurons remain more stable in their response properties after selective perturbation induced by adaptation. Similarly, human observers report different adaptation-induced changes in orientation tuning between cardinal and oblique axes. We suggest that the larger cortical area devoted to cardinal orientations imposes stability on the processing of cardinal contours during visual perception, by retaining invariant cortical responses along cardinal axes.

Adaptation, Physiological↗

PubMed2001

Foci of orientation plasticity in visual cortex.

Cortical areas are generally assumed to be uniform in their capacity for adaptive changes or plasticity. Here we demonstrate, however, that neurons in the cat striate cortex (V1) show pronounced adaptation-induced short-term plasticity of orientation tuning primarily at specific foci. V1 neurons are clustered according to their orientation preference in iso-orientation domains that converge at singularities or pinwheel centres. Although neurons in pinwheel centres have similar orientation tuning and responses to those in iso-orientation domains, we find that they differ markedly in their capacity for adaptive changes. Adaptation with an oriented drifting grating stimulus alters responses of neurons located at and near pinwheel centres to a broad range of orientations, causing repulsive shifts in orientation preference and changes in response magnitude. In contrast, neurons located in iso-orientation domains show minimal changes in their tuning properties after adaptation. The anisotropy of adaptation-induced orientation plasticity is probably mediated by inhomogeneities in local intracortical interactions that are overlaid on the map of orientation preference in V1.

Animals↗

PubMed2000

Adaptation-induced plasticity of orientation tuning in adult visual cortex.

A key emergent property of the primary visual cortex (V1) is the orientation selectivity of its neurons. The extent to which adult visual cortical neurons can exhibit changes in orientation selectivity is unknown. Here we use single-unit recording and intrinsic signal imaging in V1 of adult cats to demonstrate systematic repulsive shifts in orientation preference following short-term exposure (adaptation) to one stimulus orientation. In contrast to the common view of adaptation as a passive process by which responses around the adapting orientation are reduced, we show that changes in orientation tuning also occur due to response increases at orientations away from the adapting stimulus. Adaptation-induced orientation plasticity is thus an active time-dependent process that involves network interactions and includes both response depression and enhancement.

Adaptation, Physiological↗

PubMed2000

Dynamic properties of recurrent inhibition in primary visual cortex: contrast and orientation dependence of contextual effects.

A fundamental feature of neural circuitry in the primary visual cortex (V1) is the existence of recurrent excitatory connections between spiny neurons, recurrent inhibitory connections between smooth neurons, and local connections between excitatory and inhibitory neurons. We modeled the dynamic behavior of intermixed excitatory and inhibitory populations of cells in V1 that receive input from the classical receptive field (the receptive field center) through feedforward thalamocortical afferents, as well as input from outside the classical receptive field (the receptive field surround) via long-range intracortical connections. A counterintuitive result is that the response of oriented cells can be facilitated beyond optimal levels when the surround stimulus is cross-oriented with respect to the center and suppressed when the surround stimulus is iso-oriented. This effect is primarily due to changes in recurrent inhibition within a local circuit. Cross-oriented surround stimulation leads to a reduction of presynaptic inhibition and a supraoptimal response, whereas iso-oriented surround stimulation has the opposite effect. This mechanism is used to explain the orientation and contrast dependence of contextual interactions in primary visual cortex: responses to a center stimulus can be both strongly suppressed and supraoptimally facilitated as a function of surround orientation, and these effects diminish as stimulus contrast decreases.

Animals↗

PubMed1999

Context-dependent changes in visual sensitivity induced by Müller-Lyer stimuli.

We measured the detectability of a single line (target) flanked by high-contrast inward- or outward-pointing arrowheads (context). We show that as a function of target contrast, context angle, and context position there is a continuum of contextual modulations of target detectability that vary from strong inhibition (target detection is impaired) to strong excitation (target detection is facilitated), but target detection is not affected when the context is presented at low contrasts. The results show striking correlations with the perceived length distortions in the Müller-Lyer illusion, i.e. an inward-pointing arrowhead results in improved target detectability and increased perceived length of the bar, whereas an outward-pointing arrowhead results in diminished target detectability and decreased perceived length of the bar. Both suppressive and facilitatory effects diminish as target contrast, arrowhead angle, and line-arrowhead spatial disparity are increased. At larger distances between line and arrowhead the suppressive effects become facilitatory (the Müller-Lyer illusion reverses). When concurrent Müller-Lyer extent experiments are run, we found that the perceived length of the target stimulus is overestimated or underestimated as it is flanked by high-contrast inward or outward-pointing arrowheads, the magnitude of the length distortion effects diminishing as target contrast increases. To explain the nature of both context-induced suppression and facilitation in contrast detection we present a population model of orientation detectors in visual cortex that relies on short and long-range horizontal cortical connections, and suggest that that the same type of mechanism that accounts for contrast detection may account for perceived extent.

Adult↗

PubMed1999

The dynamics of operant conditioning.

Existing models of operant learning are relatively insensitive to historical properties of behavior and applicable to only limited data sets. This article proposes a minimal set of principles based on short-term and long-term memory mechanisms that can explain the major static and dynamic properties of operant behavior in both single-choice and multiresponse situations. The critical features of the theory are as follows: (a) The key property of conditioning is assessment of the degree of association between responses and reinforcement and between stimuli and reinforcement; (b) the contingent reinforcement is represented by learning expectancy, which is the combined prediction of response-reinforcement and stimulus-reinforcement associations; (c) the operant response is controlled by the interplay between facilitatory and suppressive variables that integrate differences between expected (long-term) and experienced (short-term) events; and (d) very-long-term effects are encoded by a consolidated memory that is sensitive to the entire reinforcement history. The model predicts the major qualitative features of operant phenomena and then suggests an experimental test of theoretical predictions about the joint effects of reinforcement probability and amount of training on operant choice. We hypothesize that the set of elementary principles that we propose may help resolve the long-standing debate about the fundamental variables controlling operant conditioning.

Animals↗