Transformation of visual signals at the synaptic terminals of retinal bipolar cells

For a long time, the retina has been thought to be a part of the brain that performs only simple computations before more complex processes within higher visual centres disentangle the relevant features of the stimulus from the irrelevant background information. However, since the first electrical recordings from retinal ganglion cells in the 1930s, more recent advances in genetic engineering, electrophysiology, and optics, have made it clear that the retina performs intricate feature-extracting computations. Central to these computations is the inner plexiform layer in which the terminals of bipolar cells make connections to the dendrites of ganglion- and amacrine cells. In this thesis, we describe two novel circuits within the inner plexiform layer of larval zebrafish. Chapter 1 comprises a general introduction. Chapter 2 describes the methods we used to study transgenic zebrafish larvae in vivo under two-photon illumination. Chapters 3 and 4 give insights into the computational power of individual bipolar cells. Given that most bipolar cells have multiple output synapses begs the question of whether each synapse can signal different properties of a stimulus. We show that the output synapses of many individual bipolar cells release glutamate in a heterogeneous manner in response to changes in light intensity: some terminals give an excitatory response whenever the light increases (ON) or decreases (OFF), whilst others signal both polarities. This finding contradicts the classical view that ON and OFF pathways are separated from each other before they converge at later stages in the visual system. Chapter 5 comprises an investigation into the emergence of retinal orientation and direction-selectivity. Previous studies have located these properties to the dendritic trees of retinal ganglion cells. However, we show that ~25% of bipolar cell terminals are tuned to the orientation but not the direction of a stimulus. Orientation selectivity could not be explained by asymmetric receptive fields but was removed by blocking inhibition. Chapter 6 encompasses a general discussion

VLSI analogs of neuronal visual processing: a synthesis of form and function

This thesis describes the development and testing of a simple visual system fabricated using complementary metal-oxide-semiconductor (CMOS) very large scale integration (VLSI) technology. This visual system is composed of three subsystems. A silicon retina, fabricated on a single chip, transduces light and performs signal processing in a manner similar to a simple vertebrate retina. A stereocorrespondence chip uses bilateral retinal input to estimate the location of objects in depth. A silicon optic nerve allows communication between chips by a method that preserves the idiom of action potential transmission in the nervous system. Each of these subsystems illuminates various aspects of the relationship between VLSI analogs and their neurobiological counterparts. The overall synthetic visual system demonstrates that analog VLSI can capture a significant portion of the function of neural structures at a systems level, and concomitantly, that incorporating neural architectures leads to new engineering approaches to computation in VLSI. The relationship between neural systems and VLSI is rooted in the shared limitations imposed by computing in similar physical media. The systems discussed in this text support the belief that the physical limitations imposed by the computational medium significantly affect the evolving algorithm. Since circuits are essentially physical structures, I advocate the use of analog VLSI as powerful medium of abstraction, suitable for understanding and expressing the function of real neural systems. The working chip elevates the circuit description to a kind of synthetic formalism. The behaving physical circuit provides a formal test of theories of function that can be expressed in the language of circuits

The limits of visual sensitivity and its circadian control

At the sensitivity limit of vision, the quantal fluctuations of light and neural noise in the retina and the brain limit the detection of light signals. The challenge for vision, as for all senses, lies in separating the weakest signals from the neural noise originating within the sensory system. In this thesis, I studied sparse signal detection in the vertebrate visual system (mouse and frog) at low light levels from single retinal neurons to behavioral performance. First, we determined the sensitivity limit of amphibian color vision at low light levels. Unlike most vertebrates, amphibians are potential dichromats even at night, with two spectrally distinct classes of rod photoreceptors: common vertebrate rods (peak sensitivity at 500 nm) and an additional class called “green rods” (peak sensitivity at 430 nm). We showed that frogs in a phototaxis experiment can distinguish blue from green down to their absolute visual threshold, meaning that they have wavelength discrimination as soon as they start seeing anything. Remarkably, the behavioral blue/green discrimination approached theoretical limits set by photon fluctuations and rod noise, highlighting the sensitivity of the system comparing signals from the two different photoreceptors. Additionally, we show that the amphibian threshold for color discrimination is task- and context-dependent, underlining that sensory discrimination is not universally driven to absolute physical limits, but depends on evolutionary trade-offs and flexible brain states. In the second paper, we studied the impact of the circadian rhythm on the sensitivity limit of mouse vision. The retina has its own intrinsic circadian rhythms, which has led to the hypothesis that the sensitivity limit of vision would be under circadian control. We used a simple photon detection task, which allowed us to link well-defined retinal output signals to visually guided behavior. We found that mice have strikingly better performance in the visual task at night, so that they can reliably detect 10-fold dimmer light in the night than in the day. Interestingly, and contrary to previous hypotheses, this sensitivity difference did not arise in the retina, as assessed by spike recordings from retinal ganglion cells. Instead, mice utilize a more efficient search strategy in the task during the night. They are even able to apply the more efficient strategy at day once they have first performed the task during the night. Measured differences in search strategy explain only part of the day/night difference, however. We hypothesize that in addition there are diurnal changes in the state of brain circuits reading out the retinal input and making decisions. In the third paper, we determined the sensitivity limit of decrement (shadow) detection of mouse vision. Compared with the question of ultimate limit for detecting light, the question of sensitivity limits for detecting light decrements (negative contrast) has been remarkably neglected. We recorded the OFF responses of the most sensitive retinal ganglion cells at dim background light levels and correlated the thresholds to visually guided behavior in tightly matched conditions. We show that compared with an ideal-observer model most of the losses happen in the retina and remarkably, the behavioral performance is very close to an optimal read-out of the retinal ganglion cells. I have shown across visual tasks and in two different species how closely behavior in specific conditions can approach the performance limit set by physical constraints, rejecting noise and making use of every available photon. However, the actual performance strongly depends on the behavioral context and relevance of the task and state of the brain.Näön herkkyyden rajalla valokvanttien vähäinen määrä asettaa erityisen suuren haasteen näköjärjestelmän toiminnalle. Tällöin näköjärjestelmän on kyettävä erottamaan heikoimmatkin signaalit aistijärjestelmän sisäisestä kohinasta. Väitöskirjassani kysyn miten biologiset mekanismit vastaavat näihin haasteisiin. Selvitän hermoverkkojen signaalinkäsittelykykyä käyttäytymistä määrittävänä tekijänä, käyttäen mallina selkärankaisen (sammakko, hiiri) silmän verkkokalvon suorituskykyä lähellä näönherkkyyden rajaa. Tutkin näköaistin suorituskykyä yksittäisten verkkokalvon hermosolujen tasolta koko eläimen näönvaraiseen käyttäytymiseen. Ensimmäisessä osaprojektissa määritin heikoimman valointensiteetin missä sammakko pystyy erottamaan värejä. Toisin kuin muilla selkärankaisilla, sammakkoeläimillä on kaksi erityyppistä hämäränäköön erikoistunutta sauva-valoreseptoria: vihreän valon aallonpituutta parhaiten absorboiva viherherkkä sauvareseptori (kuten meillä) sekä lyhyempiä, sinisiä aallonpituuksia absorboiva siniherkkä sauva. Osoitimme, että sammakot pystyvät erottamaan värejä valaistuksessa, missä ihmiset eivät pysty. Tämä sini/viher-erotuskyky lähestyi teoreettisia fysikaalisia raja-arvoja. Lisäksi näytimme, että sammakkoeläinten värinäön kynnysherkkyys riippuu käyttäytymistehtävästä, korostaen kuinka aistinvarainen erotuskyky riippuu myös evolutiivisista valinnoista ja eläimen käyttäytymistilasta. Toisessa osatyössä tutkin vuorokausirytmin vaikutusta hiiren näönherkkyyteen. On oletettu, että verkkokalvon sopeutuu ennakoivasti valtaviin valon intensiteettimuutoksiin yön ja päivän välillä. Tässä työssä käytimme yksinkertaista, himmeiden valojen havaitsemistehtävää, joka mahdollisti verkkokalvon aivoihin lähettämän signaalin yhdistämisen näönvaraiseen käyttäytymiseen tiukan kvantitatiivisesti. Havaitsimme, että yöllä hiiret erottivat hyvin heikkoja valoja pimeässä jopa kymmenen kertaa paremmin kuin päivällä. Yllätykseksemme tämä ei johtunut muutoksista verkkokalvon maksimaalisessa herkkyydessä, vaan parempi suorituskyky yöllä johtui tehokkaammasta käyttäytymisstrategiasta sekä tarkemmasta näkötiedon käsittelystä aivoissa. Näytimme lisäksi, että hiiret pystyvät hyödyntämään tehokkaampaa strategiaa myös päiväsaikaan, jos ovat ensin suorittaneet tehtävän yöllä. Näkeminen vaatii sekä valojen että varjojen havaitsemista. Kolmannessa osaprojektissa määritin kuinka paljon fotoneja pitää poistaa heikosta taustavalosta, jotta hiiri havaitsee eron. Tämä kysymys on jäänyt verrattain huomiotta, verrattuna kymmenien vuosien tutkimukseen pienimmästä havaittavasta valomäärästä. Vertasimme herkimpien verkkokalvosolujen suorituskykyä näönvaraiseen käyttäytymiseen vastaavissa olosuhteissa. Näytimme verkkokalvon prosessointiin perustuvaa mallinnusta hyväksi käyttäen, että verrattuna teoreettisesti täydelliseen suorituskykyyn suurin osa informaationmenetyksistä tapahtuu verkkokalvolla fotonien pyydystämisessä ja verkkokalvon signaalinkäsittelyssä. Käyttäytyminen sen sijaan on huomattavan lähellä tilannetta, jossa aivot lukevat verkkokalvon lähettämää signaalia lähes täydellisesti. Väitöskirjassani näytän kahdella selkärankaisella, kuinka näkösuoritus pääsee useassa tehtävässä erittäin lähelle fysikaalisten reunaehtojen määräämiä raja-arvoja. Suorituskyky riippuu kuitenkin kontekstista ja käyttäytymistehtävän merkityksestä eläimelle sekä aivojen sen hetkisestä tilasta

Linear and Nonlinear Components of the Human Multifocal Electroretinogram in Normals and in Glaucoma

Aim. To investigate the retinal origins of the linear and nonlinear components of the human multifocal electroretinogram. Method. Linear and nonlinear multifocal electroretinograms of human subjects were recorded with the VERIS system. Normal subjects (n=34) with an age range of 26-76 years were studied, and subjects with glaucoma (n=23). The temporal and spatial characteristics of VERIS responses were analysed. Comparisons were formed between VERIS responses and published data on retinal neurone densities. The affects of ageing and of primary open angle glaucoma on VERIS responses were studied. Results. a-Wave amplitude (linear response) shows close similarity with cone densities in the central retina, but not above 3 degrees. Temporal analysis of second order kernel slices reveals that three corneal positive components and one corneal negative component contribute to the nonlinear response. A close approximation of the nonlinear response can be achieved using Gaussian functions modelled on these four components. A further component of the nonlinear response may be extracted using the method of Sutter and Bearse (1995, 1999). The amplitude of the a- and b-waves are unaffected by ageing (P>0.05), except in the central retina (<1.55 degrees). The latency of the a- and b-waves increase with ageing at every eccentricity at between 0.05 and 0.10 ms/year (P<0.05). The amplitude of the nonlinear response is unaffected by ageing, but above 6.5 degrees the latency decreases at 0.05- 0.06 ms/year. Glaucoma causes a reduction in the amplitude of a positive component of the nonlinear response (P<0.05) in scotomatous areas of retina only, whilst the linear response is everywhere unchanged. ROC curves indicate that VERIS is as efficient as the PERG at discriminating between scotomatous and normal retina. Conclusions. The a-wave contains a significant cone response, but at eccentricities>3 degrees other responses intrude. Cone function in ageing may be reduced due to impaired phagocytotic reactions. Age related changes in the nonlinear response may be associated with an achromatic component that is only present in peripheral responses and has an enhanced susceptibility to degeneration. Alterations to the outer retina in glaucoma do not register in the linear response. The nonlinear response contains a component rooted in inner retinal neurone function, but VERIS may not be capable of detecting neurone loss in glaucoma that is hidden using standard perimetric methods

Neuronal circuitry of the pigeon retina (Columba livia) : the morphological classification and organization of various neuronal types

The three studies presented in this thesis were conducted to advance our understanding of the retinal circuitry that contributes to processing high visual acuity in the pigeon. In the first study, the topographic density changes and degree of photoreceptor (PR) to retinal ganglion cell (RGC) convergence in the pigeon retina was determined. DAPI or Propidium iodide labelled PRs and RGCs were counted in the retina. Rod density was quantified by counting anti-rod opsin stained outer segments. The fovea and the red field contained significantly higher cone and RGC densities compared with the yellow field. Rods were missing from the fovea, but not in the red field, which suggests that a rod circuitry may be present in this area. The ratio of cones to RGCs was lower in both the fovea and red field, which is consistent with the higher visual acuities that have been reported in these regions. The second study classified the types of DiO-Iabelled bipolar cells in the fovea, central red and yellow fields. Eight bipolar cell types were classified in the retina using a modification of Mariani's (1987) classification scheme. Eight BC types (B1 - B8) had similar dendritic morphology as the ones described by Mariani. Two bipolar cell types, B7 and B8, had comparatively smaller dendritic fields than the other types. It was estimated to receive input from possibly one photoreceptor in the fovea and the central red field. Based on the small dendritic field size, B7 and B8 may be good candidates for being the midget-like BCs in the pigeon retina. The third study classified the RGC groups in the pigeon retina. Classification of RGCs labelled with DiI/DiO in the pigeon retina was based on the dendritic stratification pattern in the inner plexiform layer (IPL). Five morphological RGC groups were identified, the unstratified, monostratified, bistratified, tristratified and tetrastratified. The unstratified group was characterised by vertically oriented dendrites occupying a thick portion of the IPL, whereas the other groups had horizontally oriented dendrites stratifying at narrow portion of the IPL. The unstratified RGC had the narrowest dendritic field (diameter >- 18.5 urn). Based on the unstratified RGC's dendritic field size, it is a good candidate for a 'midget-like' RGC in the pigeon retina. However, it has a different morphology from the primate midget RGC. Further work is required to determine the physiology and differential distribution of the different types of bipolar and ganglion cells in the pigeon retina

Central and Peripheral Visual Function: Effects of Age and Disease

* The overall aim of this study was to assess how the processing of different stimulus attributes in human vision is affected by ageing and disease. Both foveal and paracentral regions of the retina were investigated, with emphasis on both pregeniculate and postgeniculate impairments. Isolation of different stimulus attributes and the assessment of visual performance were carried out using a series of Advanced Vision and Optometric Tests (AVOT) developed at City University. A total of 133 normal controls and 59 patients participated in the study. Contrast detection (CT) and contrast acuity (CA) thresholds were assessed using Landolt ring stimuli. First-order motion was examined using moving stimuli embedded in static luminance contrast noise. Red/green (RG) and yellow/blue (YB) colour sensitivity were investigated using dynamic luminance contrast noise, a technique that isolates the use of colour signals. The effects of ageing and loss of visual function caused by disease were examined at the fovea and at each of four paracentral locations. Visual performance was assessed to establish how ageing and disease affect the thresholds for detection of stimulus structure, motion and colour. For each of the AVOT tests, non-parametric limits were established based on data from normal subjects to allow differentiation of the effects of ageing and disease in the patient group. All attributes tested were influenced by ageing, degeneration and disease. The results demonstrate the benefots of parafoveal and peripheral testing. Some conditions are reflected in the periphery first, encroaching on foveal vision with progressing disease. Stimulus attributes tested parafoveally enabled discovery of disease earlier than at the foveal location. * Ageing affects contrast detection thresholds (CT) differently for foveal and parafoveal locations. Data were more variable for the foveal location across the sample, indicating larger intersubject differences compared to the parafovea. In general, the ageing effects on visual performance can be described by a weak linear upwards trend in threshold as age increases. Additionally, some presumed subclinical cases were included in the sample. These subjects exhibited no signs of abnormality on standard examination but had larger CT thresholds, especially in the older age groups. * The effect of age on contrast acuity thresholds (CA) was similar for foveal and parafoveal locations. The appropriate choice of target size scaling ensured similar results with no statistically significant differences between foveal and paracentral thresholds. Data were less variable at the fovea indicating larger intersubject differences at parafoveal locations across the sample. In general, the ageing effect was accounted for by a weak linear upwards trend of CA thresholds with age, although the thresholds for presumed subclinical cases in the older subject groups departed significantly from the linear trend and increased rapidly with age. * Motion sensitivity thresholds are influenced by age beyond the 5th decade. Statistical analysis revealed that motion data can be separated into a younger (20-49.9) and an older (50-79.9) age group. Within each of these age bands motion thresholds showed no statistically significant differences. Within each group, the thresholds were similar for the five locations tested, a finding facilitated by the larger target size employed paracentrally. * Chromatic sensitivity thresholds were influenced by age beyond the 6th decade. Statistical analysis revealed that RG and YB colour data can again be separated into a younger (20-59.9) and an older (60-79.9) age band. Within each of these age bands, age effects are not statistically signficant. Foveal thresholds were, however, statistically significantly different from parafoveal locations for both RG and YB discrimination. Ageing had a greater effect on YB thresholds than on RG thresholds. RG thresholds for subjects within the older age band were significantly larger with a similar increment for all five locations tested. * The following findings were established from studies in 36 patients with pregeniculate lesions (23 with Glaucoma, 6 retinal conditions, 6 optic nerve conditions and 1 chiasmal lesion): In general, loss was usually diffuse and corresponded with the location of the visual field defect. The majority of pregeniculate patients exhibited impairment of all functions tested: CT, CA and motion were all substantially impaired and chromatic discrimination was also affected symmetrically in one or both channels (RG, YB). This pattern of loss was present within the area identified by visual field loss, where visual attributes were often not seen at the phosphor limits of the display. Some pregeniculate patients also exhibited substantial loss of all visual functions in areas where perimetric loss was largely absent. In most pregeniculate patients all quadrants revealed similar loss. In some patients the least affected quadrant exhibited normal CT or motion thresholds, but CA and colour vision were always affected to some degree. In pregeniculate patients, loss in both CT and CA was a marker of profound loss in both colour and motion. Chromatic sensitivity loss was always symmetric, frequently the RG channel was more affected than YB. The YB channel was affected more in patients with early glaucoma with more advanced disease, the RG channel was a®ected most, and finally further progression of the disease resulted in large thresholds limited by the phosphor limits of the visual display. * In 23 patients with postgeniculate lesions: the majority of those with pre-striate damage exhibited loss of all tested visual functions with symmetric chromatic impairment. Some pre-striate lesions were associated only with CA and colour loss, and in these cases the chromatic loss was symmetric and affected either one or both channels. Striate or extra-striate lesions tended to exhibit loss of CT, CA, motion and colour vision within the area identified by visual field testing. More specific loss tended to be associated with less impaired areas that were often normal on perimetric testing. Some striate or extra-striate lesions were only associated with CA and colour loss. In such cases chromatic loss was asymmetric for one or more colour categories. When striate or extra-striate lesions were also accompanied by underlying pre-striate damage, chromatic sensitivity loss was always symmetric and sometimes accompanied by CT loss. Motion was affected least in postgeniculate conditions and was always correlated with the area of densest visual field loss. The findings from this study show that the loss of chromatic sensitivity in cerebral achromatopsia varies considerably with location in the visual field. The same subject can exhibit loss of chromatic sensitivity that is either colour channel or colour category specific and such losses often affect only restricted areas of the visual field. * The findings from this investigation show how ageing processes affect the most important aspects of visual performance and provide the statistical limits needed to differentiate ageing effects from disease. The study also reveals how specific, localised damage to visual pathways can produce selective loss of visual function and how the latter varies with retinal topography. The observed variation in the processing of the same stimulus attribute with retinal location as well as the differences measured for different stimulus attributes at the same location illustrate the importance of the testing paradigm employed to reveal early onset of disease