New objective and psychophysical techniques to study the processing of visual signals with emphasis on chromatic afterimages

• Main Author: Bi, Wei
• Published: City University London 2012
• Subjects: 152.148; RE Ophthalmology
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.590035

The research work described in this thesis embodies a number of studies designed to investigate human vision with emphasis on aspects of the pupil response and chromatic mechanisms in relation to the perceived chromatic afterimages. The aim of the first study was to establish the relationship between the perception of chromatic afterimages and the corresponding involuntary pupil responses. We started by designing and developing a new, computer-based, psychophysics program and employed it to measure the strength and duration of perceived chromatic afterimages in normal trichromats and in colour deficient observers. The dynamic luminance noise technique was used to isolate colour signals and to elicit pupil responses to coloured stimuli of known photoreceptor contrast. A model was developed to explain the afterimage results obtained in the normal trichromats and in colour deficient subjects. The model and the pupil colour responses provided an understanding of luminance and colour processing in dichromats that also helped to explain previously reported pupil colour responses. The model also predicts the colour confusion lines and the characteristics of pupil colour responses in dichromats at any given background chromaticity. In the second study, we investigated and compared pupil responses to visual stimuli that isolate photopic luminance and colour in both the sighted and blind region of the visual fields on subjects with either acquired or congenital homonymous hemianopia. The measured pupil responses in the blind hemifield of patients with acquired cortical damage are either absent or of reduced amplitude when compared to those measured in the corresponding regions of the sighted field, whereas the patients with congenital loss of visual field show similar and even enhanced pupil responses when compared to their sighted hemifield. These results suggest that in the absence of normal functioning of the direct geniculostriate projection, other projections to midbrain nuclei or to extrastriate regions can be enhanced and these include the pupillary pathways. These findings suggest that early damage to the brain might be partly compensated for by reorganising the strength of neural projections to the remaining, non-compromised visual areas. The purpose of the last study was to examine whether melanopsin contributes to the dynamic pupil light reflex responses in humans. A light source containing of four primary components was employed to generate pupillary stimuli that isolate luminance, colour or combined rod and melanopsin. Normal trichormats, rod deficient subjects, one subject with retinitis pigmentosa, one rod monochromat, three subjects with Leber’s Hereditary Optic Neuropathy (LHON) and one subject with Optic Neuritis were investigated using this approach. The results from the LHON subjects suggest not all classes of ganglion cells are affected uniformly in LHON, and that the pupil light reflex responses mediated through rod photoreceptors were affected the least. The characteristics of the pupil responses to the rod/melanopsin stimulus from the rod monochromat and the retinis pigmentosa subjects suggest that melanopsin does not contribute to dynamic pupil light reflex response in humans.