Biomedical subjects

K N Gurney

Publications and source records attributed to K N Gurney.

11 recordsLinked to original sources

PubMed2002

The role of intra-thalamic and thalamocortical circuits in action selection.

We previously proposed that the basal ganglia (BG) play a crucial role in action selection. Quantitative analysis and simulation of a computational model of the intrinsic BG demonstrated that its output was consistent with this proposition. Here we build on that model by embedding it into a wider circuit containing the motor thalamocortical loop and thalamic reticular nucleus (TRN). Simulation of this extended model showed that the additions gave five main results which are desirable in a selection/switching mechanism. First, low salience actions (i.e. those with low urgency) could be selected. Second, the range of salience values over which actions could be switched between was increased. Third, the contrast between the selected and non-selected actions was enhanced via improved differentiation of outputs from the BG. Fourth, transient increases in the salience of a non-selected action were prevented from interrupting the ongoing action, unless the transient was of sufficient magnitude. Finally, the selection of the ongoing action persisted when a new closely matched salience action became active. The first result was facilitated by the thalamocortical loop; the rest were dependent on the presence of the TRN. Thus, we conclude that the results are consistent with these structures having clearly defined functions in action selection.

Algorithms↗

PubMed2001

Information processing in dendrites I. Input pattern generalisation.

In this paper and its companion, we address the question as to whether there are any general principles underlying information processing in the dendritic trees of biological neurons. In order to address this question, we make two assumptions. First, the key architectural feature of dendrites responsible for many of their information processing abilities is the existence of independent sub-units performing local non-linear processing. Second, any general functional principles operate at a level of abstraction in which neurons are modelled by Boolean functions. To accommodate these assumptions, we therefore define a Boolean model neuron-the multi-cube unit (MCU)-which instantiates the notion of the discrete functional sub-unit. We then use this model unit to explore two aspects of neural functionality: generalisation (in this paper) and processing complexity (in its companion). Generalisation is dealt with from a geometric viewpoint and is quantified using a new metric-the set of order parameters. These parameters are computed for threshold logic units (TLUs), a class of random Boolean functions, and MCUs. Our interpretation of the order parameters is consistent with our knowledge of generalisation in TLUs and with the lack of generalisation in randomly chosen functions. Crucially, the order parameters for MCUs imply that these functions possess a range of generalisation behaviour. We argue that this supports the general thesis that dendrites facilitate input pattern generalisation despite any local non-linear processing within functionally isolated sub-units.

Action Potentials↗

PubMed2001

Information processing in dendrites II. Information theoretic complexity.

In the companion paper, we established a rationale for exploring the general principles of dendritic processing using a class of Boolean functions-the Multi-Cube Units (MCUs). Here, we use this approach to further characterise dendritic processing using ideas from information theory and studies in complexity. The starting point is a novel decomposition of a Boolean function's total mutual information (between input variables and the output). Each component of the decomposition is a mutual information measure with respect to a single input, conditioned on a subset of the remaining inputs. We call this decomposition the information spectrum and conceive of it as a re-representation of the function in the information domain. Furthermore, the information spectrum of a Boolean function may be assigned a complexity value using the approximate entropy introduced by Pincus (Pincus, S. M. (1991). Approximate entropy as a measure of system complexity. Proc. Natl. Acad. Sci. USA, 88, 2297-2301). Using Monte Carlo methods, we provide evidence that the information spectral complexity of MCUs is larger than that of any other class of Boolean function. We explain this phenomenon in terms of information flow through the 2-stage MCU architecture. Under our modelling assumptions, the implication for biological neural processing is that dendrites implement functions that have maximal information spectral complexity with respect to the class of multivariate functions from which they are drawn.

Animals↗

PubMed1999

Visual discrimination of direction changes based upon two types of angular motion.

We address the question of how the visual system analyses changes in direction. Using plaid stimuli, we define type O direction changes which entail a change in the orientations of the plaid components, and type V direction changes in which the orientations of the components remain constant, relative to the observer but their relative speeds change. Lower thresholds for discriminating type O and type V direction changes were compared. Type O thresholds for clockwise/anticlockwise direction change were very low (0.2-0.5 degree), were resistant to directional noise, and showed a low-pass relationship with drift velocity. Type V thresholds on the other hand were higher (1-5 degrees), and exhibited a bandpass relationship with drift velocity. Type O direction changes gave low thresholds at short inter-stimulus intervals (ISI) (< 160 ms) and higher thresholds (successive orientation discrimination) at long ISI (240 ms-12.8 s). Type V thresholds, on the other hand, exhibited no short-range process and performance at short ISI, was no better than for successive direction discrimination at long ISI. A two-stage rotary motion model is sufficient to explain the discrimination of type O direction changes and results rule out a model based on velocity discrimination. For type V direction changes, a two-stage mechanism is insufficient and results are consistent with a minimum of three computational stages.

Humans↗

PubMed1997

Coherence and motion transparency in rigid and nonrigid plaids.

Under a wide range of conditions, stimuli composed of two superimposed grating components with unequal rotation velocities (differing in sign and/or magnitude) gave a striking percept of a single, coherent, nonrigidly deforming plaid surface. Conversely, component angular velocities of the same sign and magnitude yielded rigidly rotating plaids. Rigidity and motion coherence were shown to be independent percepts, and coherent plaids were categorised unambiguously as rigid or nonrigid. Coherence and motion transparency were found to depend upon the relative orientation of components, and polar plots showed two lobes of high coherence for narrow intercomponent angles. There was a slight tendency for plaids with unequal component rotations to appear less coherent, but this was nonsignificant, once the effect of intercomponent angle was taken into account. Changes in the relative spatial frequency of components, relative contrast of components, and repeated presentation produced equivalent effects on coherence for rigid and nonrigid types of plaid motion. Manipulation of the terminators in the display by making the aperture diameters for the two component gratings unequal reduced coherence and increased transparency. The effect was the same for rigid and nonrigid plaids. Coherence in rigid and nonrigid plaids thus depends primarily on local processes and there is no strong interaction between rigidity and coherence.

Humans↗

PubMed1996

A model for the spatial integration and differentiation of velocity signals.

We present a model of optic flow processing which is able to reconcile the integrative, cooperative phenomena of motion capture and coherence with the differentiation of velocity signals in motion segmentation and transparency. The model uses a Markov random field to compute the behaviour of coextensive topographic neural maps of retinotopy and velocity. We have used the model to simulate the psychophysics of motion coherence, motion capture and transparency. Further, it exhibits motion segmentation without extra postulates. The model is robust and able to display all types of motion percept with the same parameter set.

Humans↗

PubMed1995

The discrimination of dynamic orientation changes in gratings.

Thresholds were measured for discrimination of direction of a step angular rotation of gratings. The addition of simultaneous phase displacements (translation) had little effect on rotation thresholds for gratings over a considerable range; discrimination of rotation is unaffected by random directional translations an order of magnitude larger. Angular rotation discrimination thresholds increased with interstimulus interval (ISI). Thus discrimination is based at short ISIs (180 ms or less) on a percept of rotary motion, but at ISIs of several seconds by a spatial strategy (comparing static component orientations) relying on visual memory. Data points for the short-ISI region fell below the best-fitting straight line, and the slope of the short-ISI region of the curve was steeper than that of the long-ISI region. However, when either compound or simple gratings with uncorrelated spatial frequencies were used in the two stimulus frames, there was no evidence for a separate function at short ISIs. Orientation-change thresholds were measured for simple gratings as a function of contrast and spatial frequency. The contrast function showed saturation and the spatial frequency function was U-shaped. Rotation sensitivity for gratings is thus similar in its spatiotemporal properties to translation sensitivity. The findings support the proposal that rotation discrimination (at short ISIs) is achieved by a template mechanism combining signals from different directional detectors, rather than by congnitive comparison of the outputs of the directional mechanisms themselves.

Attention↗

PubMed1992

A self-organising neural network model of image velocity encoding.

A self-organising neural network has been developed which maps the image velocities of rigid objects, moving in the fronto-parallel plane, topologically over a neural layer. The input is information in the Fourier domain about the spatial components of the image. The computation performed by the network may be viewed as a neural instantiation of the Intersection of Constraints solution to the aperture problem. The model has biological plausibility in that the connectivity develops simply as a result of exposure to inputs derived from rigid translation of textures and its overall organisation is consistent with psychophysical evidence.

Animals↗

PubMed1992

Lower threshold of motion for one and two dimensional patterns in central and peripheral vision.

Lower motion thresholds for discriminating opposing motion directions were compared for one dimensional (grating) and two dimensional (plaid) stimuli in central and peripheral vision. The results were consistent with a two-stage model of motion sensitivity in which threshold-limiting noise occurs at both stages, and the speed as well as the direction of the resultant motion is determined by intersection-of-constraints (IOC) from the component motions. The results do not support a purely geometric interpretation of the IOC model, in which thresholds for plaid stimuli are related to thresholds of component gratings by a geometric factor. Neither do the data favour explanations in which local luminance features (i.e. blobs) are detected and their velocity determined. Monte-Carlo simulations of the two-stage process predict thresholds across variations in component direction, contrast, and visual field eccentricity. Lower motion thresholds for gratings and plaids both follow a saturating function of contrast; the fit between grating and plaid data is improved when the plaid contrast is expressed in terms of the contrast of its components. Although less contrast saturation was found in the periphery, in relative terms, plaid and grating motion thresholds were similar in central and peripheral vision, implying cortical magnifications are similar for mechanisms which process grating and plaid motion.

Contrast Sensitivity↗

PubMed1992

Dependence of stereomotion on the orientation of spatial-frequency components.

It is known that sensitivity to stereoscopic motion in depth is not based upon the fine analysis of static disparities but instead is based on the binocular combination of motion-sensitive mechanisms. We show in this paper that an 'aperture problem' arises for the analysis of motion in depth, just as it does for monocular motion sensitivity. We extend Adelson and Movshon's solution to the aperture problem by intersection of perpendicular constraints to the three-dimensional case, and show that it predicts velocity matches for oblique gratings moving in depth, for orientations close to vertical. We show that binocular plaids give rise to motion in depth when the component orientations match in each eye, and the monocular motions are horizontal. The match velocities are consistent with intersection of perpendicular constraints. In three dimensions intersection of perpendicular constraints may be necessary, but is not a sufficient condition for the perception of coherent stereoscopic motion in depth.

Contrast Sensitivity↗

PubMedDate not supplied

A pulsed neural network model of bursting in the basal ganglia.

We present new techniques for extending the functionality of spiking neurons which allow the incorporation of several aspects of neuron function previously confined to the domain of low level ion-channel based models. These aspects include spontaneous (or endogenous) firing, the complex interaction of multiple ion-species and the spatial distribution of synaptic contacts over the cell membrane. These ideas are applied to a neural circuit consisting of the cortex and a subset of the nuclei in the basal ganglia-the subthalamic nucleus (STN) and the external segment of the globus pallidus (GPe). This circuit has been studied extensively in vitro by Plenz and Kitai [Plenz, D., & Kitai, S. T. (1999). A basal ganglia pacemaker formed by the subthalamic nucleus and external globus pallidus. Nature, 400 677-682] whose data we use to constrain our model. With respect to this circuit, we have obtained three main results. First, that its characteristic burst firing is due to a Ca2+ current mediated mechanism. Second, that noise can assist in the generation of bursting and, paradoxically, stabilise the network behaviour under synaptic weight variations. Third, that a variety of dendritic processing is necessary in order to obtain the full range of bursting behaviour.

Action Potentials↗