Investigation of Long-Range Interactions in the Human Visual System

  • Petar Mihaylov

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

The receptive field theory of visual neurones is one of the most influential achievements in vision science as it has helped us to gain a deeper understanding of visual information processing. However, research has shown that cortical neurones can be influenced by stimuli positioned well outside their classical localised receptive field. The aim of the present study was to explore the long-range interactions within the human visual system. To achieve this, we used methods of psychophysics and visual evoked potentials in conjunction with the artificial scotoma paradigm.

Previous studies have shown that exposure to a homogeneous gray patch surrounded by a dynamic texture causes filling-in of the artificial scotoma by the surround texture. When the background is switched off, observers report the perception of a prolonged patch of twinkling noise in the unstimulated area. We found that a central artificial scotoma, surrounded by dynamic visual noise, induced internal neural noise with a variance proportional to the strength of the surround noise. These findings suggest that the artificial scotoma phenomenon may reflect an increased magnitude of neural activity resulting from long-range effects induced by the surround noise.

Using steady-state visual evoked potentials, an electrophysiological correlation of the twinkling noise afterimage was found. Following prolonged exposure to dynamic visual noise surrounding a gray patch, the amplitude of the VEPs elicit by a localised test pattern was significantly reduced. These findings suggest the existence of neural activity within the cortical retinotopic projections corresponding to the artificial scotoma which could be induced by long-range interactions activated by the surround noise.

We found evidence for active long-range processes of neural excitation and inhibition within cortical projections of the unstimulated region of artificial scotomas. The long-range excitatory effects produced changes in the phase and amplitude of VEPs elicit by the surround pattern which could be associated with different levels of perceptual filling-in of the artificial scotoma. The long-range suppressive effects were produced by a surround grating with orientations either parallel or perpendicular to the orientation of the test stimulus suggesting that these interactions involve spatial channels selective to a wide range of onentations.

The results showed that the strength of filling-in of peripheral artificial scotomas, surrounded by a stationary grating embedded in dynamic noise, was inversely proportional to the magnitude of the surround texture. We propose a coherent representation of data for filling-in of artificial scotoma at various experimental conditions, by a single power function, using the quantities of speed of filling-in and signal-to-noise ratio of the surround texture.

We also found a new type of artificial scotoma, produced by a negative pattern afterimage of a filled-in artificial scotoma stimulus. Subjects are able to fill-in such scotomas which suggests that the filling-in phenomenon occurs at cortical levels, rather than at retinal levels.
Date of Award2011
Original languageEnglish
Awarding Institution
  • Glasgow Caledonian University
SupervisorVelitchko Manahilov (Supervisor)

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