Michael Smith PhD

Michael Thomas Smith

  The Postsubiculum and Spatial Learning
Bett, D, Stevenson, CH, Shires, KL, Smith, MT, Martin, SJ, Dudchenko, PA & Wood, ER 2013, 'The Postsubiculum and Spatial Learning: The Role of Postsubicular Synaptic Activity and Synaptic Plasticity in Hippocampal Place Cell, Object, and Object-Location Memory' Journal of Neuroscience, vol 33, no. 16, pp. 6928-6943. DOI: 10.1523/JNEUROSCI.5476-12.2013

Visual landmarks exert stimulus control over spatial behavior and the spatially tuned firing of place, head-direction, and grid cells in the rodent. However, the neural site of convergence for representations of landmarks and representations of space has yet to be identified. A potential site of plasticity underlying associations with landmarks is the postsubiculum. To test this, we blocked glutamatergic transmission in the rat postsubiculum with CNQX, or NMDA receptor-dependent plasticity with D-AP5. These infusions were sufficient to block evoked potentials from the lateral dorsal thalamus and long-term depression following tetanization of this input to the postsubiculum, respectively. In a second experiment, CNQX disrupted the stability of rat hippocampal place cell fields in a familiar environment. In a novel environment, blockade of plasticity with D-AP5 in the postsubiculum did not block the formation of a stable place field map following a 6 h delay. In a final behavioral experiment, postsubicular infusions of both compounds blocked object-location memory in the rat, but did not affect object recognition memory. These results suggest that the postsubiculum is necessary for the recognition of familiar environments, and that NMDA receptor-dependent plasticity in the postsubiculum is required for the formation of new object-place associations that support recognition memory. However, plasticity in the postsubiculum is not necessary for the formation of new spatial maps.

General Information
Organisations: Centre for Cognitive and Neural Systems.
Authors: Bett, David, Stevenson, Cassie H., Shires, Kate L., Smith, Michael T., Martin, Stephen J., Dudchenko, Paul A. & Wood, Emma R..
Number of pages: 16
Pages: 6928-6943
Publication Date: 17 Apr 2013
Publication Information
Category: Article
Journal: Journal of Neuroscience
Volume: 33
Issue number: 16
ISSN: 0270-6474
Original Language: English
DOIs: https://doi.org/10.1523/JNEUROSCI.5476-12.2013
  Fluctuations in the open time of synaptic channels: An application to noise analysis based on charge
Feldwisch-Drentrup, H, Barrett, A, Smith, MT & Rossum, MV 2012, 'Fluctuations in the open time of synaptic channels: An application to noise analysis based on charge' Journal of Neuroscience Methods, vol 210, no. 1, pp. 15-21. DOI: 10.1016/j.jneumeth.2011.11.004
Synaptic channels are stochastic devices. Even recording from large ensembles of channels, the fluctuations, described by Markov transition matrices, can be used to extract single channel properties. Here we study fluctuations in the open time of channels, which is proportional to the charge flowing through the channel. We use the results to implement a novel type of noise analysis that uses the charge rather than the current to extract fundamental channel parameters. We show in simulations that this charge based noise analysis is more robust if the synapse is located on the dendrites and thus subject to cable filtering. However, we also demonstrate that when multiple synapses are distributed on the dendrites, noise analysis breaks down. We finally discuss applications of our results to other biological processes.
General Information
Organisations: Institute for Adaptive and Neural Computation .
Authors: Feldwisch-Drentrup, Hinnerk, Barrett, Adam, Smith, Michael T. & Rossum, Mark van.
Keywords: (, , . )
Number of pages: 7
Pages: 15-21
Publication Date: 2012
Publication Information
Category: Article
Journal: Journal of Neuroscience Methods
Volume: 210
Issue number: 1
ISSN: 0165-0270
Original Language: English
DOIs: https://doi.org/10.1016/j.jneumeth.2011.11.004
  Directional bias in cortical motion processing: A combined fMRI and computational modelling study
Smith, MT, Wutte, M, Flanagin, V & Wolbers, T 2011, 'Directional bias in cortical motion processing: A combined fMRI and computational modelling study' The British Neuroscience Association Meeting 2011, Harrogate, United Kingdom, 17/04/11 - 20/04/11, .
The columnar organisation of the human motion complex hMT+ allows fMRI studies to decode the direction of visual motion from the multivoxel pattern responses in that region. While previous studies have shown that directional information is present in hMT+, a more precise characterisation of its functional properties is needed if we are to understand the computational mechanisms underlying cortical motion processing. In the present study, we sought to address this problem by characterising the response profiles of individual voxels to a visual stimulus, moving in four possible directions. In addition to more voxels preferring vertical over horizontal dot motion, we observed a correlation of directional preferences: a voxel which prefers dot motion in one direction will be more likely to prefer dot motion in the opposing direction, too. Several hypotheses exist which could explain this result. The first is that any particular column preferring a particular direction functions through the interaction of opposing inhibitory interneurons (1). The second possible cause is due to the organisation of columns found in electrophysiological experiments in primates, in which columns of opposing direction lie next to each other in stripes (2). The final possible hypothesis is based on the finding that a small proportion of neurons in MT (again in primates) were found to prefer two, opposing, directions (2). To test which of the above hypotheses was correct, we built a computational model of part of hMT+, in which neurons were given specific 3d locations and orientation preferences, based on electrophysiological data. Each voxel within the model had its own particular preference correlation, and these were compared with the results from the fMRI study. The model suggests that the correlations seen in the fMRI data appear to be due to both the columnar organisation and the effect of bidirectional neurons.
General Information
Organisations: Deanery of Biomedical Sciences.
Authors: Smith, Michael T., Wutte, Magdalena, Flanagin, Virginia & Wolbers, Thomas.
Publication Date: 2011
Publication Information
Category: Poster
Original Language: English
  Physiological Signal Variability in hMT+ Reflects Performance on a Direction Discrimination Task
Wutte, MG, Smith, MIT, Flanagin, VL & Wolbers, T 2011, 'Physiological Signal Variability in hMT+ Reflects Performance on a Direction Discrimination Task' Frontiers in Psychology, vol 2, pp. 185. DOI: 10.3389/fpsyg.2011.00185
Our ability to perceive visual motion is critically dependent on the human motion complex (hMT+) in the dorsal visual stream. Extensive electrophysiological research in the monkey equivalent of this region has demonstrated how neuronal populations code for properties such as speed and direction, and that neurometric functions relate to psychometric functions within the individual monkey. In humans, the physiological correlates of inter-individual perceptual differences are still largely unknown. To address this question, we used functional magnetic resonance imaging (fMRI) while participants viewed translational motion in different directions, and we measured thresholds for direction discrimination of moving stimuli in a separate psychophysics experiment. After determining hMT+ in each participant with a functional localizer, we were able to decode the different directions of visual motion from it using pattern classification (PC). We also characterized the variability of fMRI signal in hMT+ during stimulus and rest periods with a generative model. Relating perceptual performance to physiology, individual direction discrimination thresholds were significantly correlated with the variability measure in hMT+, but not with PC accuracies. Individual differences in PC accuracy were driven by non-physiological sources of noise, such as head-movement, which makes this method a poor tool to investigate inter-individual differences. In contrast, variability analysis of the fMRI signal was robust to non-physiological noise, and variability characteristics in hMT+ correlated with psychophysical thresholds in the individual participants. Higher levels of fMRI signal variability compared to rest correlated with lower discrimination thresholds. This result is in line with theories on stochastic resonance in the context of neuronal populations, which suggest that endogenous or exogenous noise can increase the sensitivity of neuronal populations to incoming signals.
General Information
Organisations: Deanery of Biomedical Sciences.
Authors: Wutte, Magdalena G, Smith, MIchael T., Flanagin, Virginia L & Wolbers, Thomas.
Pages: 185
Publication Date: 2011
Publication Information
Category: Article
Journal: Frontiers in Psychology
Volume: 2
Original Language: English
DOIs: https://doi.org/10.3389/fpsyg.2011.00185

Head direction cells in the hippocampal formation (PhD)