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Purpose: Chick choroidal lymphatics swell rapidly during recovery from form-deprivation occlusion, leading to the question of functional significance. To date, analysis of lymph in initial lymphatics has been problematic because of the difficulties of access. However, elemental composition can be determined using scanning electron microscope (SEM) X-ray microanalysis. This study investigated whether cryo-fixation would permit vascular fluids to be analysed in situ., Methods: Two chicks were raised normally and seven were raised with monocular occlusion for 12 days before varying periods of normal visual experience. The eyes were rapidly frozen, fractured and X-ray spectra obtained from the lumina of lymphatic and blood vessels in a Cambridge S360 SEM., Results: The elemental signatures of Na/Cl/K/P/S distinguished the two vessel types., Conclusions: These results suggest this bulk-frozen tissue technique can quantify relative changes in elemental species present in various ocular compartments in response to light-induced changes in the retina.

Purpose: The refractory period during which detection or identification of a probe is degraded, following the successful visual identification of a target, is referred to as the attentional blink (AB). Previous work in this laboratory using global/local letter forms has shown that the degree of complexity of information influences the length of time between successful visual identifications, as does the attentional modality required of the subject. In the standard rapid serial visual presentation (RSVP) task, a gap following the first target is reported to destroy the AB. The current study examines the effect on the AB duration of inserting a gap in the stream of local orthographic information while not interrupting the stream of global information. It was hypothesized that masking of the local elements of the item immediately following the target (the +1 item) would lead to a decreased AB duration for local identification., Methods: Twenty-one adult participants, experienced in the experimental paradigm, viewed sequences of compound letter stimuli and were required to identify a global or local red letter (target) and detect the presence of a global or local X (probe) in the sequence following., Results: In one-half of the trials the local letter identity of the +1 global/local form was obscured, leaving only a global form. Neither the attentional condition nor local +1 masking significantly influenced the slope of recovery from the AB. Furthermore, no difference was observed in the AB duration for global or local probe detection between control and crystallized trials., Conclusions: The results suggest that the visual image of the item is accepted nto VSTM and processed for target candidature.

The aim of the current study was to develop a method for simultaneously assessing central and peripheral photoreceptor alignment in vivo in animal models. The stimulus apparatus consisted of nine light-emitting diodes (LED) positioned 7.5 degrees apart around an arc. The stimulus was viewed through a pinhole imaged into the entrance pupil of the eye using a telecentric lens system. Photodiodes placed over an array of the VERIS imaging system stimulated the electroretinogram. Data were obtained by positioning the pinhole at 0.25-mm intervals across the pupil and recording (Volk Optical, Mentor, OH, USA) at each location. Orientation assessed in normal chickens demonstrates that photoreceptors orientate towards a locus near the centre of the pupil and that there is a systematic change in peak location with eccentricity. This technique provides a valuable method for determining photoreceptor orientation properties in vivo and can be applied to animal models of pathology.

Purpose: The present study compares morphological changes associated with optical defocus-induced myopia (ODM) with those from form deprivation myopia (FDM), as it is yet to be determined whether the latter is a consequence of visual deprivation or active myopic eye growth., Methods: Hatchling chicks were reared for 2 weeks wearing a monocular translucent occluder or -10.00 D lens, with the other eye used as a control. After enucleation, eyes were processed for scanning electron microscopy., Results: Anatomical profiles of control eyes revealed significant variations in tissue thickness as a function of nasotemporal location. Both FDM and ODM eyes demonstrated similar morphological profiles of retinal thinning, cartilaginous scleral thickening, enlarged photoreceptor inner segments and decreased photoreceptor cell density towards the ora serrata. A graded differential orientation of photoreceptors existed in all eyes., Conclusion: The comparable morphological changes in FDM and ODM suggest that the response to altered visual experience is similar in both paradigms.

Purpose: Brain imaging studies have shown that global and local attention activate different areas of the brain, with implication for dependence of perception on attentional state. The aim of the present experiment was to investigate the duration of the attentional blink in global versus local attention., Methods: Rapid Serial Visual Presentation (RSVP) sequences of global/local stimuli were presented to 34 adult subjects who had to identify a target red figure followed by a determination of whether a certain probe letter, 'X' (specified to be either the global or local form), was present in the subsequent string of letters., Results: Attentional blinks were longer than any previously reported (1.67 s global; 2.97 s local) and were significantly different.Thus, the length of the attentional blink is dependent on the attentional state of the subject., Conclusion: This pattern of results is consistent with the hypothesis that global attention mechanisms receive predominantly M-pathway input.

Purpose: Global and local attention are two forms of selective visual attention which activate different areas of the cortex. The purpose of this experiment was to test subjects' motion coherence thresholds under conditions of global or local attention. It was hypothesized that thresholds in global attention would be lower than in local attention., Methods: Eleven adult subjects participated in this study. Subjects were required to identify direction of motion at variable coherence levels, while simultaneously identifying either the global or local letter. Three velocities were used for coherent motion (3, 6 and 18 degrees/s)., Results: The results showed that letter identification (global or local) did not significantly affect motion coherence thresholds; however, thresholds were significantly higher at 18 degrees/s than in the lower velocities., Conclusions: These results highlight the attentional limitations of visual information shown by increased motion coherence thresholds when two objects must be identified simultaneously in a brief display.

Purpose: The effects of metahue and metacontrast masking on the responses of the M- and P-pathways of the human visual system were investigated using temporal analysis of the multifocal flash visually evoked potential (VEP) (VERIS system)., Methods: The experiments were carried out at three achromatic luminance contrasts, 24, 70 and 96%, with three peripheral masks: a steady 65% grey mask, a grey flicker mask and an isoluminant red/green flicker mask. The stimuli were viewed monocularly and the mask and VERIS stimuli superposed with a mirror-aperture system., Results/conclusions: The masks had no significant effect on the behaviour of the M-pathway as stimulus contrast increased. The amplitude of the P-pathway was found to be approximately level at all contrasts of the stimulus and with all three masks rather than showing the expected steady increase in activity with increasing stimulus contrast.

Purpose: Although it is generally thought that lymphatics do not exist in the choroid, there is mounting evidence that lymphatic-like structures do exist in primates and fowl., Methods/results: Our comparison of the ultrastructure of chick and rabbit outer choroid indicates that major differences exist in the non-vascular areas of the choroid: notably, the chick has large lymphatic sinusoids, whereas the rabbit only has a system of large vacuoles contained within the processes of individual fibroblasts., Conclusions: These results raise questions regarding the similarities or otherwise between species of the management of retinal metabolites, excess tissue fluid and choroidal homeostasis, especially during conditions of overload.

Purpose: It was investigated whether alterations in neuronal structure and function occasioned by strabismic amblyopia also may be reflected in alterations in the expression on Y type neurons of a Cat-301 antibody sensitive antigen in the lateral geniculate nucleus (LGN) and cortex of our cat model of strabismic amblyopia., Methods/results: The percentage of positively labelled cells was reduced in LGN laminae that received input from the deviated eye in strabismic amblyopic cats compared with normal cats. In the strabismic cortex, the density of immunopositive neurons was significantly reduced compared with normal the effect being most pronounced in layer IV., Conclusions: Despite previous physiological recordings indicating a decrease in X-cell associated acuity in strabismic amblyopia, the present findings imply that the changes in the early visual experience occasioned by strabismus also produce specific molecular changes in the Y neuronal class.

Both behavioural and neurophysiological changes can be observed in cats that have experienced interference with their normal visual environment. This visual deprivation may result from alterations to the path of light forming the normal image on the retina, and includes changes that cause the image to fall on an inappropriate part of the retina so that normal binocular interactions are affected. While some neurophysiological changes can be observed at the level of the lateral geniculate nucleus they become more prominent as information reaches the visual cortex, where cells commonly receive neural excitation from both eyes and require the information to come from corresponding parts of the two retinas and that the stimulus should have appropriate orientation and direction of movement. Many of the observations of deprivation in animals have clear parallels in the human environment.