Browsing by Subject "Vision"

Understanding how animals see the world and how visual systems have evolved to meet the needs of particular animals are major goals of visual ecology research. The Poeciliidae are a diverse family of Neotropical freshwater fishes and are excellent models for visual ecology research given longstanding interest in visual signaling in this group. However, despite extensive research investigating the form and function of visual signals in the poeciliids, there remains a surprising paucity of research regarding poeciliid visual system function and evolution. To address this gap, my dissertation research sought to investigate: (1) how Trinidadian guppies (Poecilia reticulata) perceive visual stimuli that vary in spatial detail and contrast, (2) correlates of eye size and eye investment across P. reticulata populations that experience different threats from predation and, (3) visual signaling correlates of eye size across the family Poeciliidae.

The first chapter of this dissertation introduces the questions and the study system. In Chapter 2, I examine the ability of Trinidadian guppies (Poecilia reticulata) to perceive visual stimuli that vary in spatial frequency and contrast. Male P. reticulata bear complex body patterning made up of patches that vary in color, contrast, and size, and these visual signals that are known to be important in mate choice. However, the extent to which conspecifics are able to resolve the details of these patterns has historically been overlooked. I used an optomotor assay to measure the behavioral responses of eight individual P. reticulata (N = 4 males; 4 females) to rotating achromatic stimuli. Unsurprisingly, I found that P. reticulata are better able to perceive stimuli as they increase in contrast and decrease in spatial frequency. Moreover, I found that female P. reticulata may outperform males on optomotor tasks.

In Chapter 3, I investigate how predation environment contributes to eye size variation in P. reticulata. Eye size is an important predictor of visual abilities, and it varies widely across taxa. Moreover, eye size is known to be correlated with numerous ecological factors including habitat complexity, light availability, and predation risk. However, less is known about how differences in ecological parameters across populations influence variation in eye size within species. I measured the eye diameter and standard length of 45 females and 307 males from 21 populations of known geographic origin and predation environment. I found that eye diameter was correlated with predation environment after controlling for standard length, with fish from low-predation environments having eyes that are 5.5% - 7.9% larger in diameter than those from high-predation environments. I also found that sexual dimorphism in eye diameter appears to be driven by sexual dimorphism in standard length, as there was no significant effect of sex on eye diameter after accounting for standard length.

Finally, in Chapter 4 I examine variation in eye size across the Poeciliidae. The poeciliids are a diverse family of freshwater fishes to which Poecilia reticulata belongs, and the group exhibits substantial variation in the distribution and types of visual signals used in mate choice. I measured eye size and eye investment for 66 species of poeciliids and took a phylogenetic approach to test whether variation in eye morphology was correlated with aspects of visual signaling. I found that the presence of sexually selected visual signals was associated with greater eye investment and, in particular, that sexual dichromatism was associated with an approximately 6% increase in eye diameter investment compared to species without sexual dichromatism.

This dissertation research focused on the ways in which clear crustaceans with complex bodies (i.e. with hard cuticles, thick muscles, and other internal organs) maintain transparency across their entire body volume. I used transparent crustacean species that had relatively large (> 25 mm long and > 2 mm thick) bodies and that occupied physically different (pelagic vs. benthic reef) habitats. Studying these transparent crustaceans and making comparisons with closely related opaque crustaceans provided some of the first insights into the puzzling problem of the physical basis of transparency in whole organisms.

First, I examined the ultrastructure of the cuticle of hyperiid amphipods, the first surface to interact with light, to understand what features may minimize reflectance. I investigated the cuticle surfaces of seven species of mostly transparent hyperiids using scanning electron microscopy and found two previously undocumented features that reduced reflectance. I found that the legs of Cystisoma spp. were covered with an ordered array of nanoprotuberances that functioned optically as a gradient refractive index material to reduce reflections. Additionally, I found that Cystisoma and six other species of hyperiids were covered with a monolayer of homogenous nanospheres (approximately 50 nm to 350 nm in diameter) that were most likely bacteria. Optical modeling demonstrated that both the nanoprotuberances and the monolayers reduced reflectance by as much as 250-fold. Even though the models only considered surface reflectance and not internal light scattering, these models showed that the nanoprotuberances and spheres could improve crypsis in a featureless habitat where the smallest reflection could render an animal vulnerable to visual predation.

Second, I took a morphological approach to investigate how light scattering may be minimized internally. Using bright field microscopy, I explored whether there were any gross anatomical differences in the abdominal muscles between a transparent species of shrimp, Ancylomenes pedersoni, and a similarly sized opaque shrimp species, Lysmata wurdemanni. I found no differences in muscle fiber size or any other features. Using transmission electron microscopy (TEM) to visualize muscle ultrastructure, I found that the myofibrils of the transparent species were twice the diameter of the opaque species (mean values of 2.2 μm compared to 1.0 μm). Over a given distance of muscle, light passes through fewer myofibrils due to their larger diameter, with fewer opportunities for light to be scattered at the interfaces between the high-index myofibrillar lattice and the surrounding lower-index fluid-filled sarcoplasmic reticulum (SR). Additionally, because transparency is not always a static trait and can sometimes be disrupted after exercise or physiological stress, I compared the ultrastructure of muscle in transparent A. pedersoni shrimp with the ultrastructure of muscle in A. pedersoni that had temporarily turned opaque after exercise. I found that in this opacified tissue, the fluid-filled space around myofibrils had an increased thickness of 360 nm as compared to a normal thickness of 20 nm. While this could have been a fixation artifact, this result still suggests that opacified tissue had some change in osmolarity or increase in fluid. Models of light scattering across a range of thicknesses and possible refractive indices showed that this observed increase in fluid-filled space dramatically reduced transparency.

Third, I further investigated how exertion or physiological stress may disrupt transparency, what may occur in the tissues to cause this disruption, and what may explain the increased fluid-filled SR interface. I hypothesized that increased perfusion, or an increase in blood volume between muscle fibers, can disrupt the normal organization of tissue, resulting in increased light scattering. I measured pre- and post-exercise perfusion via the injection of a specific fluorescent stain (Alexa Fluor 594-labeled wheat germ agglutinin) that labeled the sarcolemmal areas in contact with hemolymph and the endothelial cells of the blood vessels, and found more open vessels and greater hemolymph perfusion around fibers post-exercise. Changing salinity in the shrimps’ tanks, wounding the shrimp, and injecting proctolin (a vasodilator) were also associated with increased opacity and perfusion. To visualize the shrimps’ overall muscle morphology, I used Diffusible Iodine-based Contrast-Enhanced Computed Tomography (DICECT) to scan one control (transparent) and one experimental (opaque) A. pedersoni. The resulting images added further support to my hypothesis that hemolymph volume in the muscle increases in post-exercise opacified A. pedersoni.

One of the central problems in neuroscience is that there is a lack of understanding of the diversity and functions of cell types in the brain. Even in brain areas that have been studied extensively, such as the retina, much remains to be learned about the diversity and functions of cell types. Morphological, functional and genetic studies have yet to converge on a consistent picture of cell type diversity in the retina, because the field lacks a standardized approach to classify cell types. A systematic classification approach is essential to provide an unambiguous appreciation of cell type diversity, and a better understanding of the organization and function of the retina. In the first portion of this dissertation, we present a novel approach that classifies retinal ganglion cells (RGCs) in a quantitative, verifiable and reproducible manner. We utilize diverse visual stimuli and a multi-electrode array, to record simultaneously from multiple RGCs, and show that there are at least 13 RGC types with distinct functional properties. In the second portion of the dissertation, we present a quantitative determination and comparison of the spatiotemporal receptive field (RF) structures and neural coding properties across these RGC types. Determining the RF structure of RGC types is important, because it constrains the computations performed by retinal circuits and identifies the signals available to retinal recipient areas. We find that RGC types exhibit functional asymmetries in terms of their RF size, temporal integration, and response nonlinearities. We also show that no RGC types exhibited RFs that were strictly independent in space and time. These results provide several new insights into the computations performed in the rodent retina, and highlight the importance of understanding cell type diversity to further our understanding of how the retina works and the role it plays in visual processing.

Scallops, a family of swimming bivalve mollusks, have dozens of eyes arrayed along the edges of their valves. Relatively little is known about the form and function of these unusual eyes. To learn more about them, we studied the visually influenced behavior of scallops, as well as the morphology and spectral sensitivity of their eyes. Of particular interest was whether or not the simple neural architecture of these animals constrains the number of visually-influenced behaviors they can perform. We were also interested to learn whether scallop eyes, despite providing relatively poor visual acuity, show optical refinements, such as corrections for spherical and chromatic aberration, that are known from the eyes of animals with better vision. In the following dissertation, Chapter 2 discusses the visually-influenced behaviors of scallops. It has been argued that bivalve mantle eyes only act as predator-detectors, but the behavioral trials described in this chapter suggest that vision may serve additional purposes in scallops. For example, it was found that visual cues relating to flow conditions may influence scallop feeding behavior. Chapter 3 presents a comparative study of scallop eye morphology. Here, it is found that eye morphology varies considerably between scallop species and that highly mobile scallops have better vision than less mobile or immobile species. Evidence is also presented that one of the two scallop retinas may perform tasks of similar importance to all species, such as predator detection, while the other retina may perform tasks more important to mobile species, such as those associated with the visual detection of preferred habitats. Chapter 4 investigates the spectral sensitivity of the two retinas in the mantle eyes of two scallop species. It is found that there is both inter- and intra-specific variation in scallop spectral sensitivity and that color perception in scallops may be influenced by both environmental light conditions and chromatic aberration caused by their lens. The research in this dissertation provides insight into how vision functions in animals that, like scallops, have a vast number of eyes, but a limited capacity for neural processing. Despite such limitations, it is evident that scallops display a wide range of visual behaviors and have eyes with highly-refined optics.

The dissertation work discussed here focuses on the behavioral and physiological aspects of visual sexual signaling in the blue crab, Callinectes sapidus. The blue crab has a pair of apposition compound eyes that are relatively acute (1.5 ° resolution) for an arthropod. The eyes have two photopigments sensitive to blue (λ_max = 440 nm) and green (λ _max=500 nm) light, allowing for simple color vision. Visual cues and signals are used during antagonistic and sexual communication and primarily involve claw-waving motions. A primary feature of the blue crab morphology is its sexually dimorphic claw coloration; males have blue and white claws and females have red claws. However, despite the potential for interesting color signaling, visual cues have typically been considered non-important, particularly in sexual communication where chemical cues have dominated blue crab signaling studies.

In a series of experiments designed to simultaneously test the role of visual cues in mating behavior and blue crab color vision, I tested males' responses to photographs of females with differently colored claws. I found that photographs of females elicited male courting behaviors. I also found that males preferred females with red claws over those with white or isoluminant (i.e. matched brightness) gray claws. The discrimination of red from isoluminant grey showed the use of color vision in male mate choice.

In natural populations, the claws of sexually mature females vary from light orange to deep red. To determine males' abilities discriminate between similar colors, I tested male color preferences for red against several shades of orange varying in brightness. Overall, males showed an innate preference for red-clawed females over those with variations of orange claws. However, in tests between red and orange shades similar in both brightness and hue, male blue crabs did not show a distinct preference, suggesting that males are either not able or not motivated to discriminate between these shades. Further, my results suggest that male blue crabs may use a mixture of chromatic and achromatic cues to discriminate between long-wavelength colors.

After confirming the use of color in mate choice, I focused on the role of claw color in intraspecific communication. To quantify claw coloration, I measured spectral reflectance of claws of a blue crab population in North Carolina. In both sexes, the color of the claw varied with reproductive maturity and may act as a cue of reproductive readiness. Additionally, there was individual variation in claw color which could indicate individual quality. I have modeled the appearance of claw coloration to the blue crab eye and found that these color differences are visible to the blue crab eye and potentially signal gender, reproductive readiness, and/or individual quality.

After investigating male mate choice, I began investigating visual aspects of female mating behavior. In the blue crab, like many crustaceans, courtship occurs during the female molting cycle and copulation takes place after the female has shed her exoskeleton. In crustaceans and other arthropods with compound eyes, the corneal lens of each facet is part of the exoskeleton and thus shed during molting. I used optomotor assays to evaluate the impact of molting on visual acuity (as measured by the minimum resolvable angle &alpha _min) in the female blue crab. I found that visual acuity decreases substantially in the days prior to molting and is gradually recovered after molting. Prior to molting, &alpha_min was 1.8 °, a value approximating the best possible acuity in this species. In the 24 hours before molting, &alpha _minincreased to a median of 15.0 ° (N=12), an eight-fold drop in visual acuity. Six days after molting, &alpha _minreturned to the pre-molting value. Micrographs of C. sapidus eyes showed that a gap between the corneal lens and the crystalline cone appeared approximately five days prior to shedding and increased in width the process progressed. This separation was likely responsible for the loss of visual acuity observed in behavioral tests. Since mating is limited to the female's pubertal molt, a reduction in acuity during this time may have an effect on the sensory cues used in female mate choice. These results may be broadly applicable to all arthropods that molt and have particular importance for crustaceans that molt multiple times in their lifetime or have mating cycles paired with molting.

Efficient coding theory provides a powerful framework for understanding the organization of the early visual system. Prior research has demonstrated that efficient coding theory can help account for a range of retinal ganglion cell (RGC) organizational features, including the center-surround spatial receptive fields and ON and OFF parallel pathways. Here, we use a machine learning-based computational framework for efficient coding and show that more functional architecture of visual processing can be explained on the basis of this principle. First, how should receptive fields (RFs) be arranged to best encode natural images? When the spatial RFs and contrast response functions are optimized in order to maximally encode natural stimuli given noise and firing rate constraints, the RFs form a pair of mosaics, one with ON RFs and one with OFF RFs, similar to those of mammalian retina, as an existing finding from previous research. Interestingly, the relative arrangement of the two mosaics transitions between alignment under high signal-to-noise conditions and anti-alignment under low signal-to-noise conditions. The next question we tackled is: how are the ON and OFF RF mosaics arranged in the mammalian retina? We examined the retina of rats and primates and confirmed that the ON and OFF mosaic pairs encoding the same visual feature are anti-aligned, indicating that the retina is optimized to handle dim or low-contrast stimuli. Finally, we dove into the question: how many cell types can be predicted by efficient coding theory? We examined encoding of natural videos, and found that, as the available channel capacity – the number of simulated RGCs available for encoding – increases, new cell types emerge that focus on higher temporal frequencies and larger spatial areas. Together, these studies advance our understanding of the relationships between efficient coding, retinal organization, and diversity of retinal cell types.

Any information represented in the brain, whether an individual is aware of it or not, holds the potential to affect behavior. The extent of visual perceptual processing that occurs in the absence of awareness is therefore a question of broad import and interest to the field of cognitive neuroscience. A useful approach for examining the extent and quality of visual processing that occurs in the absence of awareness is the dissociation paradigm. In this approach, experimenters track implicit measures of the visual process of interest across conditions of awareness modulated by visual presentation manipulations. Object-category discrimination by the visual system represents a relatively sophisticated level of representation that may or may not occur in the absence of awareness. Here, electrophysiological measures (scalp-recorded event-related potentials, or ERPs) of object-category discrimination by the brain (the face-specific N170 ERP component and the longer-latency face-specific negativity) were tracked across conditions of visual awareness as manipulated by multiple presentation paradigms (sandwich masking, object-substitution masking, the attentional blink, and motion-induced blindness). In addition, where possible, other related comparisons examining lower-level visual processes and higher-level attentional processes were employed to help delineate the specific level and mechanism by which awareness was disrupted in each case. The experiments implicated a unique set of mechanisms of reducing awareness for each method, while providing insight into the complex relationships between the various phases of visual processing in the human brain and awareness. Ultimately it was observed that neural indices of face-specific processing are differentially susceptible to disruption exerted by these various methods, and that there do in fact exist conditions in which awareness can be disrupted while leaving various facets and phases of face-specific processing intact. These findings help to establish object-category discrimination as a process that can occur in the absence of visual awareness, and contributes to our understanding of the neural factors that influence and determine behavior.