We analyse the effect of chemical neuro-modulation on collective processes in Ising spin neural n... more We analyse the effect of chemical neuro-modulation on collective processes in Ising spin neural networks with separable Hebbian type synaptic interactions. Neuro-modulation is taken into account in the most simple way: a modulator-specific subset of neurons is prevented from transmitting signals. However, the presence of neuro-modulators is taken into account also during the learning stage, which leads to non-symmetric interaction matrices. We derive (in the limit of an infinite system size) the macroscopic laws that determine the system’s evolution in time on the level of order parameters. These laws are very transparant and show that, within the proposed framework, one can understand the functioning of neuro-modulators as follows: their role is to choose from the repertoire of learned behaviour a particular mode of operation. By considering specific examples of learning stages we indicate how neuro-modulation might be used by the brain as an extra degree of freedom for (a) performing selective pattern reconstruction, (b) controlling the reproduction speed of stored pattern sequences or (c) for choosing a particular path from a set of partially overlapping stored trajectories through state space (at points where the trajectories separate).
Early stages of visual motion processing are modelled by two networks, each consisting of velocit... more Early stages of visual motion processing are modelled by two networks, each consisting of velocity-tuned cells. One network extracts the (possibly multi-valued) velocity-field, while the other extracts the patterns which move. The ‘channel-coded’ output from the velocity-net selects those co-local units in the pattern-net which are tuned to the local velocity, so preventing motion-smear and allowing compensation of target positions for unavoidable processing delays. Under special conditions, however, the model creates some artefacts which correspond well with known visual illusions.
ABSTRACT Introduction. ‘Bistable perception is the phenomenon that when faced with ambiguous visu... more ABSTRACT Introduction. ‘Bistable perception is the phenomenon that when faced with ambiguous visual input, one experiences a percept alternating between two interpretations.’ Sentences like this form the starting point of myriad papers on the subject, but bistable perception as a two-state process is a simplification: between dominance phases of percept A and B there are phases during which neither prevails. This has been noted before1 but is routinely ignored. We try to clarify how this is possible and study the features of the transition stage in a binocular grating paradigm. Methods. Due to the transition stage’s short duration we cannot rely on subjects’ key presses to report it, but use a novel method based on synchronisation and cueing. Synchronisation means that we force the onset at t0 of a dominance phase, using the flash suppression effect. Then after a chosen time lag, by replacing the gratings by a mask, we cue subjects to report what they saw the instant before mask appearance. By repeating this at various lags, we get probability distributions of percepts as a function of time after t0. Subjects were tested twice: first we instructed them to report the percept to be either A or B; the second time we added the option of reporting a transition percept. Results. Subjects easily executed the two-alternative choice experiment. Still, with the option of reporting a third phase, its mean duration was 300-1100 ms: 0.2-0.7 times that of a dominance phase (the duration of an entire cycle was unaffected). Concluding, reports of putative bistable perception do not prove the absence of a transition percept; merely that subjects were not asked to report it. Instead, the transition stage is a substantial part of the alternation cycle, that models of bistable perception should incorporate. We are presently studying its characteristics, e.g. its duration distribution, its dependence on stimulus features and associated detection thresholds. 1: Mueller & Blake, Biol. Cybern. 61, 1989.
We sketch an integrated approach to achieve near-optimal target detection as well as resolution o... more We sketch an integrated approach to achieve near-optimal target detection as well as resolution of range/doppler ambiguities. The functional structure respects statistical principles, and allows a clear understanding and analysis of its performance.
Humans do not confound the motion of shadows cast upon a surface with the motion of the surface i... more Humans do not confound the motion of shadows cast upon a surface with the motion of the surface itself, although schemes that propose recombination of orientation-selective motion signals into a rigid motion percept of two-dimensional patterns would predict that they should do so. We propose a simple scheme that avoids recombination and instead attributes perception of two-dimensional pattern motion to the activation of orientation-selective end-stopped units that operate on the logarithm of the luminance. The proposed units respond to the change of contrast along a line, which typically occurs at an intersection. They are not active, however, when a shadow border intersects the edge of an object, because contrast does not change along either of these edges. Thus, end-stopped units signal the motion of transparent intersections weakly or not at all, and the independent motions of the shadow border and the object prevail. We tested two implications of this scheme, using plaids with variable intersection luminance. First, when the intersection luminance was such that it kept the contrast along the intersecting lines nearly constant, the sensitivity for the rigid plaid's direction of motion was minimal, and the sliding motion of the components prevailed. This occurred for light bars on dark backgrounds and for dark bars on light backgrounds. Thus, the effect of the intersection's luminance on the balance between the percepts of rigid-plaid motion and the motion of sliding components was independent of contrast inversion of bar and background. Secondly, when thin lines with the same luminance as the bars covered the borders of the intersection, the intersection's luminance did not affect the rigid-plaid motion percept very much, even when it corresponded to a transparent intersection. This indicates that, when the edges of the intersection and those of the bars were not collinear, the nulling of the end-stopped units did not occur. This result is in line with physiological studies, which showed that the response of an end-stopped cell to a line is only partially inhibited when a similar line is presented non-collinearly with the first in the inhibitory end-zone of its receptive field. Our results are consistent with a scheme in which a second stage of motion detectors combines signals of orientation-selective end-free and orientation-selective end-stopped units for perception of the rigid motion of two-dimensional patterns.
We analyse the effect of chemical neuro-modulation on collective processes in Ising spin neural n... more We analyse the effect of chemical neuro-modulation on collective processes in Ising spin neural networks with separable Hebbian type synaptic interactions. Neuro-modulation is taken into account in the most simple way: a modulator-specific subset of neurons is prevented from transmitting signals. However, the presence of neuro-modulators is taken into account also during the learning stage, which leads to non-symmetric interaction matrices. We derive (in the limit of an infinite system size) the macroscopic laws that determine the system’s evolution in time on the level of order parameters. These laws are very transparant and show that, within the proposed framework, one can understand the functioning of neuro-modulators as follows: their role is to choose from the repertoire of learned behaviour a particular mode of operation. By considering specific examples of learning stages we indicate how neuro-modulation might be used by the brain as an extra degree of freedom for (a) performing selective pattern reconstruction, (b) controlling the reproduction speed of stored pattern sequences or (c) for choosing a particular path from a set of partially overlapping stored trajectories through state space (at points where the trajectories separate).
Early stages of visual motion processing are modelled by two networks, each consisting of velocit... more Early stages of visual motion processing are modelled by two networks, each consisting of velocity-tuned cells. One network extracts the (possibly multi-valued) velocity-field, while the other extracts the patterns which move. The ‘channel-coded’ output from the velocity-net selects those co-local units in the pattern-net which are tuned to the local velocity, so preventing motion-smear and allowing compensation of target positions for unavoidable processing delays. Under special conditions, however, the model creates some artefacts which correspond well with known visual illusions.
ABSTRACT Introduction. ‘Bistable perception is the phenomenon that when faced with ambiguous visu... more ABSTRACT Introduction. ‘Bistable perception is the phenomenon that when faced with ambiguous visual input, one experiences a percept alternating between two interpretations.’ Sentences like this form the starting point of myriad papers on the subject, but bistable perception as a two-state process is a simplification: between dominance phases of percept A and B there are phases during which neither prevails. This has been noted before1 but is routinely ignored. We try to clarify how this is possible and study the features of the transition stage in a binocular grating paradigm. Methods. Due to the transition stage’s short duration we cannot rely on subjects’ key presses to report it, but use a novel method based on synchronisation and cueing. Synchronisation means that we force the onset at t0 of a dominance phase, using the flash suppression effect. Then after a chosen time lag, by replacing the gratings by a mask, we cue subjects to report what they saw the instant before mask appearance. By repeating this at various lags, we get probability distributions of percepts as a function of time after t0. Subjects were tested twice: first we instructed them to report the percept to be either A or B; the second time we added the option of reporting a transition percept. Results. Subjects easily executed the two-alternative choice experiment. Still, with the option of reporting a third phase, its mean duration was 300-1100 ms: 0.2-0.7 times that of a dominance phase (the duration of an entire cycle was unaffected). Concluding, reports of putative bistable perception do not prove the absence of a transition percept; merely that subjects were not asked to report it. Instead, the transition stage is a substantial part of the alternation cycle, that models of bistable perception should incorporate. We are presently studying its characteristics, e.g. its duration distribution, its dependence on stimulus features and associated detection thresholds. 1: Mueller & Blake, Biol. Cybern. 61, 1989.
We sketch an integrated approach to achieve near-optimal target detection as well as resolution o... more We sketch an integrated approach to achieve near-optimal target detection as well as resolution of range/doppler ambiguities. The functional structure respects statistical principles, and allows a clear understanding and analysis of its performance.
Humans do not confound the motion of shadows cast upon a surface with the motion of the surface i... more Humans do not confound the motion of shadows cast upon a surface with the motion of the surface itself, although schemes that propose recombination of orientation-selective motion signals into a rigid motion percept of two-dimensional patterns would predict that they should do so. We propose a simple scheme that avoids recombination and instead attributes perception of two-dimensional pattern motion to the activation of orientation-selective end-stopped units that operate on the logarithm of the luminance. The proposed units respond to the change of contrast along a line, which typically occurs at an intersection. They are not active, however, when a shadow border intersects the edge of an object, because contrast does not change along either of these edges. Thus, end-stopped units signal the motion of transparent intersections weakly or not at all, and the independent motions of the shadow border and the object prevail. We tested two implications of this scheme, using plaids with variable intersection luminance. First, when the intersection luminance was such that it kept the contrast along the intersecting lines nearly constant, the sensitivity for the rigid plaid's direction of motion was minimal, and the sliding motion of the components prevailed. This occurred for light bars on dark backgrounds and for dark bars on light backgrounds. Thus, the effect of the intersection's luminance on the balance between the percepts of rigid-plaid motion and the motion of sliding components was independent of contrast inversion of bar and background. Secondly, when thin lines with the same luminance as the bars covered the borders of the intersection, the intersection's luminance did not affect the rigid-plaid motion percept very much, even when it corresponded to a transparent intersection. This indicates that, when the edges of the intersection and those of the bars were not collinear, the nulling of the end-stopped units did not occur. This result is in line with physiological studies, which showed that the response of an end-stopped cell to a line is only partially inhibited when a similar line is presented non-collinearly with the first in the inhibitory end-zone of its receptive field. Our results are consistent with a scheme in which a second stage of motion detectors combines signals of orientation-selective end-free and orientation-selective end-stopped units for perception of the rigid motion of two-dimensional patterns.
Uploads
Papers by Andre J Noest