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Title: Pattern adaptation and its interocular transfer in the primary visual cortex
Author: Howarth, Christopher
ISNI:       0000 0004 2751 4730
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2008
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Adaptation to a high contrast grating temporarily reduces the contrast sensitivity of neurons in the primary visual cortex (VI). If this adaptation is induced in one eye and the contrast tested with the other a partial transfer of the after-effect is produced, known as interocular transfer (IOT). Intrinsic hyperpolarisation of a cells membrane explains most of this effect, but not the orientation selective nature of adaptation. Optical imaging of intrinsic signals in anaesthetised cats and tree shrews was used to visualise orientation selective responses in VI before and after brief and chronic adaptation. Short term adaptation was achieved with drifting gratings of 12.5 or 50% contrast and fixed orientation (0). Three 1-sec flashes of a 100% contrast grating were used as test stimuli. 8 orientation domains were created according to orientation preference, determined on the basis of pre-adaptation orientation maps. 8 oriented test stimulus responses for each domain were obtained from the absorption signal time course averaged over all pixels. Orientation tuning curves comparable to those in single-cell experiments were produced for the orientation selective pixel populations. A region specific reduction in response was seen in the tuning curves such that responses to 0 were reduced most strongly in regions responding best to 0. An additional stimulus specific reduction was observed in responses to 6, even if 0 wasn't the optimal orientation for a domain. Chronic adaptation was induced with 1 hour of drifting sinusoidal grating in tree shrews. In contrast to a similar experiment in the cat, no alteration in the functional layout of the orientation map was observed. Extracellular recording of IOT in the cat primary visual cortex was performed to elucidate its physiological substrate. Orientation tuning curves were recorded before and after left or right eye adaptation with a 25% or 50% contrast drifting grating with the cells preferred orientation and spatial frequency. Cells were a priori categorised according to the binocularity of their control responses. Surprisingly, significant levels of IOT were observed in virtually all monocular cells. Only a weak link was found between ocular dominance and IOT in the full cell population. However, a moderate link between OD and IOT was seen in simple cells. An increase in the response to orthogonal stimuli was also seen in both monocular and binocular cells after adaptation with the non-dominant eye. A subset of complex cells did not display any IOT when adapting with the non-dominant eye and testing with the dominant eye.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available