Change blindness: eradication of gestalt strategies

Arrays of eight, texture-defined rectangles were used as stimuli in a one-shot change blindness (CB) task where there was a 50% chance that one rectangle would change orientation between two successive presentations separated by an interval. CB was eliminated by cueing the target rectangle in the first stimulus, reduced by cueing in the interval and unaffected by cueing in the second presentation. This supports the idea that a representation was formed that persisted through the interval before being 'overwritten' by the second presentation (Landman et al, 2003 Vision Research 43149–164]. Another possibility is that participants used some kind of grouping or Gestalt strategy. To test this we changed the spatial position of the rectangles in the second presentation by shifting them along imaginary spokes (by ±1 degree) emanating from the central fixation point. There was no significant difference seen in performance between this and the standard task [F(1,4)=2.565, p=0.185]. This may suggest two things: (i) Gestalt grouping is not used as a strategy in these tasks, and (ii) it gives further weight to the argument that objects may be stored and retrieved from a pre-attentional store during this task

Perception of patterned vibratory stimulation: An evaluation of the tactile vision substitution system

Sensory substitution--The replacing of an impaired sensory channel by a properly functioning one--is possibly best manifested today in attempts to provide visual aids for the blind. The tactile vision substitution system (T.V.S.S.) is an example of one such visual aid. The system presents patterned tactile stimulation to the skin of the observer provided by the output of a closed-circuit television system. Research conducted with congenitally blind Ss in evaluation of the T.V.S.S. has provided useful information concerning the potentialities and limitations of the prototype systems, similarities and differences between tactile and visual perception, and the development of visual perception in the congenitally blind Investigation demonstrated that the congenitally blind Ss can learn to make valid judgements of three-dimensional displays with the T.V.S.S. Such judgements are made on the basis of properties contained in the proximal stimulation properties analogous to the monocular clues of depth presence in vision, such as linear-perspective, apparent elevation in the visual field, size change as a function of distance, occlusion, and texural gradients. Similarities have been noted between judgements made by sighted Ss using vision and by blind Ss using the T.V.S.S. on comparable tasks. A display consisting of two slightly displaced alternating lights is perceived in both situations as a single spot of light moving back-and-forth between two display boundaries. A rotating drum made up of alternate black and white stripes is, when stopped, perceived as briefly moving in the opposite direction. External localization of the source of stimulation also occurs with both sensory inputs. The major differences between the visual and tactile inputs that have been noted have occurred in form recognition tacks. Although blind Ss using the patterned tactile stimulation are able to identify both geometric forms and abstract patterns, accuracy is consistently lower than that of sighted Ss using vision, and the latencies for the blind Ss are significantly longer. It is hypothesized that the longer latencies for the blind Ss using the T.V.S.S. can be accounted for primarily by the need to hand-position the television camera during scanning. A major factor in the lower accuracy for the tactile group is the noted difficulty in detecting and identifying display features located within a mass of stimulation. This difficulty with internal display detail may be a function of sensory inhibition and/or masking. The research findings support a concept of sensory substitution as well as a theory of perception which stresses the modality of many qualities contained in visible displays. Further research is needed to determine the significance of sensor movement--either eye movements or camera manipulation--in the perceptual process

Head-mounted displays and dynamic text presentation to aid reading in macular disease

The majority of individuals living with significant sight loss have residual vision which can be enhanced using low vision aids. Smart glasses and smartphone-based headsets, both increasing in prevalence, are proposed as a low vision aid platform. Three novel tests for measuring the visibility of displays to partially sighted users are described, along with a questionnaire for assessing subjective preference. Most individuals tested, save those with the weakest vision, were able to see and read from both a smart glasses screen and a smartphone screen mounted in a headset. The scheme for biomimetic scrolling, a text presentation strategy which translates natural eye movement into text movement, is described. It is found to enable the normally sighted to read at a rate five times that of continuous scrolling and is faster than rapid serial visual presentation for individuals with macular disease. With text presentation on the smart glasses optimised to the user, individuals with macular disease read on average 65% faster than when using their habitual optical aid. It is concluded that this aid demonstrates clear benefit over the commonly used devices and is thus recommended for further development towards widespread availability

Binocular interactions in human vision

Early visual processing is subject to binocular interactions because cells in striate cortex show binocular responses and ocular dominance (Hubel & Weisel, 1968). The work presented in this thesis suggests that these physiological interactions can be revealed in psychophysical experiments using normal human observers. In the region corresponding to the blind spot, where binocular interactions differ from areas of the visual field which are represented by two eyes, monocular contrast sensitivity is increased. This finding can be partially explained by an absence of normal binocular interactions in this location (Chapter 2). A hemianopic patient was studied in an attempt to discover whether the effect in normal observers was mediated by either a mechanism in striate cortex or via a subcortical pathway. However, the results were unable to distinguish between these two explanations (Chapter 3).In a visual search task, no difference in reaction time was observed for targets presented to the region corresponding to the blind spot compared with targets presented to adjacent binocularly represented areas of the visual field. Since performance was unaffected by the monocularity of the region corresponding to the blind, pop-out for orientation may be mediated beyond striate cortex where cells are binocularly balanced (Chapter 5). Further support for this contention was provided by studies of orientation pop-out in central vision which found that dichoptic presentation of stimuli did not affect the degree of pop-out obtained and that in general, visual search for a target based solely on eye of origin is impossible (Chapter 6). However, a task that measured orientation difference sensitivity more directly than the search experiments, found that thresholds were higher for dichoptically presented stimuli. This suggests the involvement of neurons that receive a weighted input from each eye. A model of orientation difference coding can account for the results by assuming that the range of inhibition across which orientation differences are coded is narrower for dichoptic stimuli leading to a greater resolvable orientation difference (Chapter 7)

Color vision screening using eye movements

Most of color vision tests require that the participant is aware of and actively takes part in the test. While this might seem like a minor requirement, it presents the possibility that, when motivated, participants can engage in subterfuge in order to pass the test. This study introduces the use of eye movements as a covert test to detect the presence of a color deficiency without requiring explicit input. An experiment was conducted in which the results suggest that the observed differences in eye movements alone can be used as an efficient metric for detecting colorblindness using the Ishihara images. To reduce the possibility that participants may realize they are being tested, a method to select new complex scenes for screening color vision status was developed. Another eye movement experiment was performed using selected scenes to validate the usefulness of the proposed method. The results show that this approach can be used to screen colorblind individuals without their knowledge or active participation, making the test less sensitive to subterfuge

U.S. Law of the Sea Cruise to Map and Sample the US Arctic Ocean Margin

U.S. Law of the Sea cruise to map and sample the US Arctic Ocean margin CRUISE HEALY 1202 August 25 to September 27, 2012 Barrow, AK to Dutch Harbor, A

An investigation of the role of the scanning laser ophthalmoscope in the assessment of patients with macular disease

Diseases of the macula are the primary cause of blindness in the western world. Given the current lack of effective medical treatment, there is a requirement for investigation of alternative therapeutic modalities. Traditionally, low vision services provide magnifying devices and advice on illumination. However, there have been claims that training is essential for successful rehabilitation. Using the scanning laser ophthalmoscope (SLO), novel techniques have been developed for the investigation of visual function. Validation was undertaken on subjects with normal vision and patients with a variety of central field defects. Microperimetry in patients demonstrated that visual function did not always correlate with fundus appearance. In normals, small inaccuracies of fixation were found to have no measurable influence on the reproducibility of scotoma maps. Measurements of fixation indicated that the ability to maintain a steady eye position varied significantly between observers both with normal and low vision. Some patients had exceptionally poor fixation whilst in others it was normal. To minimise the problems of target acquisition due to eye movements, a scrolling text system was developed. Assessment of reading performance at specific retinal locations demonstrated that the ability to recognise letters declined with increasing retinal eccentricity and decreasing text contrast. Patients were notably worse at reading tasks than normal observers. Longitudinal evaluation of visual function was undertaken on patients entered into a low vision training programme. Most patients self-selected a single eccentric retinal location for viewing and no suggested alternative locus could be identified that provided superior performance. Although training improved visual performance, it probably resulted from enhancement of patients' psychological status and skill acquisition. This study has demonstrated the usefulness of the SLO in the functional evaluation of vision and its potential for optimising the use of residual vision in patients with macular disease

The conservation biology of tropical inshore dolphins in north-western Australian waters

Concerns exist over the vulnerability of tropical inshore dolphin populations in waters off northern Australia to anthropogenic impacts, yet a lack of data precludes assessment of their conservation status and the management of threats. Three species occur in shallow, nearshore waters: the Australian snubfin (Orcaella heinsohni), Australian humpback (Sousa sahulensis) and Indo-Pacific bottlenose dolphin (Tursiops aduncus). In this thesis, I provide: i) quantitative data on the abundance and site fidelity of all three species at five sites in north-western Australia; (ii) an examination of population genetic structure in snubfin and humpback dolphins; (iii) a sex-specific investigation of the social structure of one population of snubfin dolphins; and, (iv) an analysis of sex- and geographic-differences in dorsal fin features of humpback dolphins. The abundance of each species was highly variable across the five c. 130 km2 study sites surveyed. While the estimated abundance of most species was ≤ 60 individuals, and fewer than 20 humpback dolphins were identified at each site in any one sampling period, larger estimates of c. 130 snubfin and c. 160 bottlenose dolphins were obtained at two different sites. Several local populations showed evidence of site fidelity, particularly snubfin dolphins. Mitochondrial and microsatellite data revealed significant genetic differentiation of local populations separated by geographic distances of >200 km, suggesting that snubfin and humpback dolphins may exist as metapopulations of small, predominantly isolated population fragments, and should be managed accordingly. Additionally, genetic data revealed the first documented case of hybridisation between a snubfin and a humpback dolphin. I documented pronounced sex-differences in individual sociability within a small population of snubfin dolphins: males formed stronger, longer-lasting associations and were far more gregarious than females. Associations were not correlated to genetic relatedness for either sex. Based on a quantitative analysis of dorsal fin images of a sample of humpback dolphins of known sex from north-western and north-eastern Australia, I revealed that the sex of adult individuals could be distinguished with a high level of accuracy (97%) based on dorsal fin features. Additionally, significant differences in dorsal fin colouration between the two regions suggested some level of population structure. Overall, these results extend the geographic scope of quantitative population data on Australia’s tropical inshore dolphins into the western third of their distribution, and provide valuable data to inform their conservation and management both within this region and throughout northern Australia

Abstracts from CIP 2007: Segundo Congreso Ibérico de Percepción

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