No description available.

No description available.

No description available.

This study was designed to test the claim that a deficit in low-level visual processing is a major factor in the etiology of developmental dyslexia. The transient and sustained pathways are neuro-anatomical pathways which underlie low level visual processing. Dyslexics are hypothesized to suffer from a transient pathway deficit which manifests itself in reading difficulties. Normal and disabled adult readers were compared on two visual processing tasks. One task measured the contrast threshold of subjects for flickering sinewave gratings; normal and disabled readers did not differ in contrast sensitivity. On the second task--a visual search task--disabled readers were consistently slower than normal readers, rather than showing the pattern of performance predicted by the transient deficit model; the results provide little evidence for a transient pathway deficit. The results of this and related studies are discussed; it is concluded that empirical evidence for a transient pathway deficit in dyslexia is equivocal.

Undergraduate honors paper--Mount Holyoke College, 2006. Program in Neuroscience and Behavior. / Includes bibliographical references (leaves 58-60).

Thesis (Ph. D.)--University of Chicago, Dept. of Ophthalmology and Visual Science, March 2001. / Includes bibliographical references. Also available on the Internet.

Animals in general, but fish in particular, inhabit environments characterized by dynamic photic conditions that are influenced by cyclical events such as the night-day cycle, or by spatial heterogeneity in the distribution of light. Effects of these dynamic properties on the visual system are compounded in salmonid fishes by migrations that expose individuals to various types of habitats, at various stages of their ontogeny. This dissertation examines some of the adaptations that enable the retina of salmonid fishes to cope with their changes of “visual environment” caused by migration and by the night-day cycle. In the first part of this dissertation, I used a combination of optic nerve response (ONR) recordings and conventional histology of the retina to investigate the ontogeny of sensitivity to ultraviolet (UV) light in salmonid fishes. I found that the UV cone mechanism contributed mostly to the ON response of retinal ganglion cells in rainbow trout (Oncorhpichus mykiss). Furthermore, the presence of UV sensitivity in rainbow trout was associated with the presence of accessory corner cones in the retinal cone mosaic, as both UV sensitivity and these cones were absent in larger (59.5-83 5g) juveniles. These results suggest that corner cones in the salmonid retina are sensitive to UV light, and that their ontogenetic disappearance leads to the loss of UV sensitivity. The changes in the photic environment that occur when mature salmonid fishes return to their natal stream to reproduce mirror those undergone during the first migration. To determine if the accessory comer cones, lost during this first migration, reappear at the time of the return migration, I studied the structure of the photoreceptor layer in sexually mature Pacific salmonids from four species: chinook (O. tschawytscha), chum (O. keta) and coho (O. kisutch) salmon, and rainbow trout. I found accessory comer cones over a large area of the dorso-temporal retina in all four species examined, which provides support for the contention that these cones are the product of late cellular addition. I investigated possible pathways for visual information to various brain centers in rainbow trout by labelling retinal projections and torus semicircularis connections in the same individuals. Double-labelling of neuronal tracts revealed two possible indirect pathways between the retina and the torus semicircularis, through the accessory optic center of the diencephalon and the optic tectum respectively. In the second part of this dissertation, I qualitatively and quantitatively examined the effects of various levels and spectral types of ambient lighting conditions on the sensitivity and time course of multi-unit responses recorded from the optic nerve of juvenile rainbow trout. Change in threshold from the dark-adapted state to progressively brighter ambient light conditions was examined at four wavelengths (380, 430, 540 and 620 nm) and found to be linear over most of the scotopic range, with a slope around 0.8. The results also suggested that, under mesopic conditions, rods and the long-wavelength cone mechanism were active simultaneously, in their respective parts of the spectrum. Implicit time, or time-to-peak of the scotopic responses decreased with stimulus intensity following a logarithmic relationship with a slope of -0.10, suggesting that the scotopic system of trout acts as an 11-stage low-pass filter, a number similar to that inferred in cat and rat, but different from other non-mammalian vertebrates. Similarly, implicit time at threshold decreased logarithmically with background intensity for the scotopic system, with a slope of -0.09 Varying the spectral content of ambient light led to differences in sensitivity and time course of ONRs across the spectrum, suggesting physiological differences between cone mechanisms. Possible implications for the coding of visual information are briefly discussed. In conclusion I provide a qualitative model of light adaptation in the trout visual system. / Graduate

The visual biology and its relevance to orientation was examined in fishes in the subfamily Salmoninae (salmon, trout, and char) by characterising the spectral and polarisation sensitivity of their visual systems. Integrated spike activity of axons from the optic nerve was used as a measure to determine the polarised-light sensitive mechanism underlying the ability of rainbow trout (Oncorhynchus mykiss) to orient in down-welling, linearly-polarised light. Relative sensitivity curves were then obtained for the five types of photoreceptor cells in this trout's retina: rods, ultraviolet sensitive cones (UV), short wavelength cones (blue-sensitive) (S), medium wavelength cones (green-sensitive) (M), and long wavelength cones (red-sensitive) (L). Under scotopic conditions (dark adapted), no sensitivity to e-vector was apparent. Under photopic conditions (light adapted), trout parr exhibited on-responses with e-vector sensitivity in two orthogonal channels. No evidence of polarisation sensitivity was observed in the on-responses of larger fish (50–78 g smolts) under UV-isolating background conditions, whereas the off-responses were unchanged.M and L cones, in contrast, retained their unimodal response. The decrease in UV-polarisation sensitivity in larger fish was found not to be attributable to size-related changes in the ability of the ocular media of the eye to transmit polarised light. Rainbow trout was used as a model species and spectral and polarisation sensitivity were compared with steelhead (an anadromous form of O. mykiss), cutthroat trout (O. clarki), kokanee (land-locked form of O.nerka), and brook char (Salvelinus fontinalis ). Visual pigment templates from rainbow trout were corrected for ocular media absorbance and overlaid on the spectral sensitivity curves for the purposes of comparison. Some differences in sensitivity were observed among species. In particular, on-responses were dominated by L-cones in most species.However, in kokanee, the M- cone was dominant.These differences may be related to the photic environments from which these fish originate. The first physiological evidence of near-UV sensitivity in the genus Salvelinus is also provided. It was therefore concluded that UV sensitivity is ubiquitous in the subfamily Salmoninae. Polarisation sensitivities in the above species were modelled using periodic regression analysis. Comparisons suggest that all of these species have a similar dual-channel polarisation detection system. The effect of the tuberculostatic drug Ethambutol on the visual physiology of rainbow trout was also examined. This drug appears to cause a decrease in sensitivity analogous to chromatic adaptation. Fish were fed daily for one month and then spectral sensitivity and polarisation sensitivity of Ethambutol-treated fish were compared to control fish. Relative to controls the visual systems of treated fish were dominated by the M-cone mechanism. Spectral and polarised light sensitivity of UV and L-cones were reduced. Finally, orientation responses of juvenile rainbow trout, steelhead, and brook char to a polarised-light stimulus were examined under controlled laboratory and semi-natural field conditions. Trained fish of all species oriented to the plane of polarised light, whereas untrained fish could not. Fish trained in the lab were tested at twilight and were found to be able to orient under natural skylight. Under semi-natural conditions, fish in floating net-pens in a lake were provided food rewards at a specific compass bearing. Their orientation responses were assessed under various natural and artificial conditions. When the sun was visible the fish typically had a unimodal distribution. However, they were typically not oriented when presented with only brightness and spectral cues at Zenith or on cloudy days. In contrast, both steelhead and sockeye were oriented correctly at civil twilight if the horizon was not obscured by heavy cloud or if blue sky was visible at Zenith. / Graduate

We investigated the spatio-temporal frequency selectivity properties of 248 neurons in VI and VII of the macaque monkey, and the receptive field properties of 43 inferotemporal neurons of the owl monkey. The study of VI and VII neurons was carried out using sine wave gratings. Our results show that VI and VII neurons were selective to different but partially overlapping spatial frequency ranges (at retinal eccentricities of 2°-5°, VII neurons are tuned approximately two octaves lower than VI neurons). The spatial frequency bandwidths for the two visual areas are similar. The temporal frequency selectivity of VI neurons was mostly lowpass (70%), or wide bandpass (30%, mean bandwidth 3 octaves); 62% of VII neurons had bandpass temporal frequency tuning curves (mean bandwidth 2 octaves). The range of preferred temporal frequencies and the bandwidths of VII neurons suggest the existence of at least two non-overlapping temporal frequency filters at each spatial frequency. Results also indicate that spatial and temporal selectivities are independent fundamental properties. Our results may provide physiological support for psychophysical findings in man, suggesting several independent detection mechanisms centered at different temporal frequencies, especially at low spatial frequencies. In a different study, a possible substructure of the large inferotemporal receptive fields was investigated in the owl monkey. A novel kind of stimulus derived from the Gabor elementary functions was used as test signal. A first set of stimuli with a constant relative spatial frequency bandwidth was used to probe the existence of either one or more than one spatial frequency band within one receptive field. A second set of constant spatial width was used to test the neruon's spatial frequency selectivity. Results suggest that only one orientation and spatial frequency band project onto each neuron. The preferred spatial frequencies (0.2 - 0.6 c/deg) were in the very low spatial frequency range for this animal. The spatial frequency bandwidth comparable to that of striate cells, the inclusion of many cycles of the preferred spatial frequency within the receptive field, and the generally reduced response to constant aperture test signals, suggest non-linear processing during summation of information from preceding visual areas.

The goal of this thesis was to investigate the role of colour in visual selective attention. Previous experiments exploring this topic in tasks where location varied led to mixed results. Some studies only found evidence of colour as a guide to a specific location where selection then takes place (e.g., Nissen, 1985). Others reported an effect, but could not decide clearly if the benefit was due to direct selection of colour in perception (e.g., Humphreys, 1981). One major contributor to the inconsistencies of findings seems to be the confounding of colour and location in these tasks. For that reason the initial paradigm used here was a rapid serial visual presentation (RSVP) task. Previous studies using similar paradigms have found no evidence for direct selection by colour (Poder, 2001; Shih & Sperling, 1996), but in these studies advance colour information was of limited usefulness because it only reduced the set of candidate stimuli by half. To assess an effect of colour in selection similar to the one associated with location, in all experiments reported here valid colour information led to only one item, as is typical in location cuing tasks. The first RSVP experiment explored whether colour certainty improved performance over a colour uncertainty condition. Colour was the defining feature of the target participants had to discriminate. In one condition the target colour was certain; in the other it could be one of two colours. Performance was improved when participants could focus on one colour. Further experiments used colour not as a defining feature of the target but as additional information presented in the form of cues, similar to the typical use of location cues. The participants� task was to discriminate whether a target letter within the RSVP sequence appeared in its upper or lower case version, and an advance cue indicated the colour in which the target letter was most likely to occur. An accuracy benefit of valid colour information was found, supporting the hypothesis that colour cuing allows the direct selection of objects for further perceptual processing. In addition, an effect of invalid colour cues was also observed. Subsequent experiments investigated possible factors influencing the colour cuing effect. Together, task requirements and properties of the stimulus set were shown to have an influence on the effect size, whereas an increase in perceptual load had no impact. Furthermore, the colour cuing effect seems to be due partially to both automatic and strategic processes. In all these experimental variations, benefits of colour cuing remained, indicating that the effect is very robust. Colour cuing effects were also found in a design where location could vary, extending the previous findings from selection in the time domain to selection in space. The two last experiments investigated whether advance colour knowledge would also lead to a performance benefit in single item tasks. No effect of colour cuing was found, indicating that colour information is only helpful in multiple item displays when a selection of one target stimulus among distractor items is necessary.