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  • 1
    Electronic Resource
    Electronic Resource
    Characterisation of columnar neurons and visual signal processing in the medulla of the locust optic lobe by system identification techniques (1996)
    Springer
    Journal of comparative physiology 178 (1996), S. 183-199 
    Details
    ISSN: 1432-1351
    Keywords: Compound eye ; Locust ; Insect ; Vision ; Lamina
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract We describe visual responses of seventeen physiological classes of columnar neuron from the retina, lamina and medulla of the locust (Locusta migratoria) optic lobe. Many of these neurons were anatomically identified by neurobiotin injection. Characterisation of neuronal responses was made by moving and flash stimuli, and by two system identification techniques: 1. The first-order spatiotemporal kernel was estimated from response to a spatiotemporal white-noise stimulus; 2. A set of kernels to second order was derived by the maximal-length shift register (M-sequence) technique, describing the system response to a two-channel centre-surround stimulus. Most cells have small receptive fields, usually with a centre diameter of about 1.5°, which is similar to that of a single receptor in the compound eye. Linear response components show varying spatial and temporal tuning, although lateral inhibition is generally fairly weak. Second-order nonlinearities often have a simple form consistent with a static nonlinear transformation of the input from the large monopolar cells of the lamina followed by further linear filtering.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00188161
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  • 2
    Electronic Resource
    Electronic Resource
    Tetrachromacy, oil droplets and bird plumage colours (1998)
    Springer
    Journal of comparative physiology 183 (1998), S. 621-633 
    Details
    ISSN: 1432-1351
    Keywords: Key words Bird ; Colour ; Vision ; UV ; Plumage
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract There is a growing body of data on avian eyes, including measurements of visual pigment and oil droplet spectral absorption, and of receptor densities and their distributions across the retina. These data are sufficient to predict psychophysical colour discrimination thresholds for light-adapted eyes, and hence provide a basis for relating eye design to visual needs. We examine the advantages of coloured oil droplets, UV vision and tetrachromacy for discriminating a diverse set of avian plumage spectra under natural illumination. Discriminability is enhanced both by tetrachromacy and coloured oil droplets. Oil droplets may also improve colour constancy. Comparison of the performance of a pigeon's eye, where the shortest wavelength receptor peak is at 410 nm, with that of the passerine Leiothrix, where the ultraviolet-sensitive peak is at 365 nm, generally shows a small advantage to the latter, but this advantage depends critically on the noise level in the sensitivity mechanism and on the set of spectra being viewed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003590050286
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  • 3
    Electronic Resource
    Electronic Resource
    Directionally selective cells in the locust medulla (1986)
    Springer
    Journal of comparative physiology 159 (1986), S. 841-847 
    Details
    ISSN: 1432-1351
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary Intracellular recordings of a distinctive class of directionally selective cell from the medulla of the locust,Locusta migratoria, optic lobe are described. Dye marking shows that these cells arborize in the distal part of the medulla, and project through the lobula complex. The cells are excited by upward movement and have receptive fields of about 20° in diameter. They are sensitive to a wide range of angular velocities from 0.02°/s to over 200°/s. The cells are sensitive to stationary flicker and have different latencies to dimming and brightening. Evidence is presented which suggests that directional computation depends, at least in part, on an inhibitory interaction between flicker sensitive channels.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00603737
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  • 4
    Electronic Resource
    Electronic Resource
    Spectral responses and chromatic processing in the dragonfly lamina (1996)
    Springer
    Journal of comparative physiology 178 (1996), S. 543-550 
    Details
    ISSN: 1432-1351
    Keywords: Compound-eye ; Dragonfly ; Lamina ; Colour vision ; Spectral sensitivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract The dragonfly Hemicordulia tau has five spectral classes of photoreceptor which drive five lamina monopolars, m1-m5. The monopolars encode spectral information. Here, spectral coding by m2, m4 and m5 are described. m2 is the most sensitive to dim light. m4 and m5 are less sensitive than m2, and so we call them photopic cells. The effects of selective adaptation of receptor inputs to m4 and m5 are unusual. For example, in m4 adaptation of the green receptor suppresses the UV input, but green sensitivity is unaffected, while green adaptation of m5 increases its green sensitivity. In m5 the dark adapted spectral tuning resembles the 520 nm receptor, but on adaptation to 430 nm light this narrows markedly. Adaptation either of green or of UV receptor input to m2 and m4 modifies the time course of responses both to green and to UV, implying that changes in the time courses of monopolar responses with adaptation state do not directly reflect receptor responses (Fig. 2). Finally, the antagonistic surround of m2 is UV sensitive, and of m4 green sensitive (Fig. 3).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00190184
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  • 5
    Electronic Resource
    Electronic Resource
    Local feedback mediated via amacrine cells in the insect optic lobe (1992)
    Springer
    Journal of comparative physiology 171 (1992), S. 447-455 
    Details
    ISSN: 1432-1351
    Keywords: Insect vision ; Amacrine cell ; Visual processing ; Neural circuitry ; Neurobiotin ; Synaptology
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary 1. The proximal part of the medulla of the locust (Locusta migratoria) optic lobe contains a small number of tangential amacrine cells. Using the recently developed intracellular label, Neurobiotin, we have combined physiological characterisations with structural descriptions of the cells at light and at electron microscopic levels. 2. Each of these tangential medulla amacrine (TMA) cells arborises over a large portion of the visual field (Fig. 1), with strongly beaded dendrites restricted to the layer of the medulla immediately proximal to the large serpentine layer that divides the ganglion. There is a second, more sparse and finer arborisation in the most proximal layer of the medulla. 3. Using our own modification of Neurobiotin histochemistry for transmission electron microscopy, we investigated the synaptology of the TMA cells. In the principal layer of dendrites, TMA cells make both input and output synapses with the same cells. Thus the TMA cells might act to connect one (or more) classes of columnar cells, providing a substrate for lateral interactions between retinotopic afferent pathways. The “beads” seen at the LM level are due to aggregations of mitochondria and not to synaptic terminals. 4. Physiologically these TMA cells are transient (on/ off) units. Whilst their arborisation is extensive, the “receptive field” measured in a single recording is only about 20° across. 5. These TMA cells appear suited to mediating the inhibitory interaction between the columnar inputs to the lobula giant movement detector (LGMD) that accounts for the preference of the latter cell for small targets.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00194577
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  • 6
    Electronic Resource
    Electronic Resource
    Relationships between pupil working range and habitat luminance in flies and butterflies (1997)
    Springer
    Journal of comparative physiology 182 (1997), S. 1-9 
    Details
    ISSN: 1432-1351
    Keywords: Key words Fly ; Butterfly ; Compound eye ; Pupil ; Visual ecology
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract The luminance range over which the pupil mechanism operates was measured with pupil reflectometry in 11 species of butterflies and 13 species of dipteran flies. The different species were selected to be as different as possible regarding the range of ambient luminances in which they are active. Habitat luminance ranges were also measured and correlated to luminances in the experimental situation. The pupil mechanism in butterflies operates in the centre of the luminance range in which the different species are active. Three distinct groups of butterflies with pupil sensitivities matched to their specific types of activity pattern were identified: species active only in direct sunlight, species active also in shaded places and species extending their activity into dawn and dusk. Quite differently, the pupil mechanisms of dipteran flies operate in the upper end of the ambient luminances, and in some species well above the luminances normally encountered by the animal. All fly pupils start to close roughly at the same luminance, irrespective of the luminances in which the species are active. The results suggest that the most important role for the pupil mechanism in many of the butterfly species is to maximize acuity over a wide range of luminances, whereas in flies it is to avoid saturation of transduction units and thereby maximize the photoreceptor's signal-to-noise ratio at high light intensities.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003590050152
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  • 7
    Electronic Resource
    Electronic Resource
    Human cone-pigment spectral sensitivities and the reflectances of natural surfaces (1992)
    Springer
    Biological cybernetics 67 (1992), S. 217-222 
    Details
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract The evolution of visual pigment spectral sensitivities is probably influenced by the reflectance spectra of surfaces in the animal's environment. These reflectances, we conjecture, fall into three main classes: i. Most inorganic and many organic surfaces, including tree bark, dead leaves and animal melanin pigmentation, whose reflectance increases gradually as a function of wavelength, ii. Living leaves, which contain chlorophyll, have a sharp reflectance peak at about 555 nm. iii. Flowers, fruit and other signaling colours that have co-evolved with animal vision typically do not reflect strongly at the same wavelength as leaves, and present a colour contrast against a leafy background. These three spectral functions we call ‘grey-red’, ‘leaf-green’ and ‘leaf-contrast’ respectively. This simple categorisation allows us to interpret the spectral tuning of human cone pigments in a way that might not seem possible given the wide variety of colours present in nature. In particular L-(red) cones will capture the highest possible proportion of photons reflected by leaves, and M-(green) cones will capture about 10% fewer photons both from leaves and from ‘grey-red’ surfaces. These observations have some clear implications for our understanding of the evolution of trichomacy and the trade-off between chromatic and luminance vision in Old-World Primates.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00204394
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  • 8
    Electronic Resource
    Electronic Resource
    The temporal properties of non-linear, transient cells in the locust medulla (1987)
    Springer
    Journal of comparative physiology 161 (1987), S. 431-440 
    Details
    ISSN: 1432-1351
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary 1. Some visual interneurons in the medulla of the locust (Locusta migratoria) optic lobe, give highly phasic responses, typically a single spike, to any suprathreshold intensity change. Although forming a distinctive class, these cells vary in receptive field area and their relative sensitivities to intensity increments and decrements. 2. The timing of responses is quite precise. Typically the standard deviation of the spike latency is 2 ms. The responses to intensity increments and decrements are very similar in waveform and latency, so the cells not signal stimulus polarity. Spike timing depends upon stimulus contrast, below contrasts of about 0.4. 3. Spiking responses adapt so that cells do not give a steady state response to stimulus frequencies of over 10 Hz. Graded potential responses to sinusoidal flicker exhibit a powerful second harmonic component. 4. A description is given of a simple way in which the responses of this group of cells can be obtained by linear and nonlinear operations on the photoreceptor input.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00603968
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  • 9
    Electronic Resource
    Electronic Resource
    Temporal and spectral properties of sustaining cells in the medulla of the locust (1987)
    Springer
    Journal of comparative physiology 161 (1987), S. 441-448 
    Details
    ISSN: 1432-1351
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary 1. Intracellular recordings were made from of cells in the optic medulla of the locust which give opposite responses to dimming and brightening, and are insensitive to movement within their receptive field. They resemble the sustaining cells described in decapod optic lobe (Wiersma et al. 1982). 2. The cells' receptive field areas vary from 2° to 30° in diameter. They receive inputs from both green and ultraviolet sensitive photoreceptors. 3. The responses to stationary flicker are approximately linear, although the cells are more responsive to depolarizing than to hyperpolarizing stimuli. 4. Cells are tuned to a range of temporal frequencies between 5 Hz and 30 Hz, and bandwidths vary from 1.5–3.5 octaves. Thus there may be range-fractionation of temporal frequencies. 5. One set of cells which have similar temporal properties but a variety of spectral properties and receptive field areas are examined in more detail. They have a phasotonic response, and their response amplitude and phase can be modelled as an addition of two linear filters. These two inputs may differ in spectral sensitivity, allowing dissection of a complex response into its components.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00603969
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  • 10
    Electronic Resource
    Electronic Resource
    Visual Ecology and Perception of Coloration Patterns by Domestic Chicks (1999)
    Springer
    Evolutionary ecology 13 (1999), S. 673-689 
    Details
    ISSN: 1573-8477
    Keywords: chicken ; colour ; learning ; object recognition ; visual environment
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract This article suggests how we might understand the way potential predators see coloration patterns used in aposematism and visual mimicry. We start by briefly reviewing work on evolutionary function of eyes and neural mechanisms of vision. Often mechanisms used for achromatic vision are accurately modeled as adaptations for detection and recognition of the generality of optical stimuli, rather than specific stimuli such as biological signals. Colour vision is less well understood, but for photoreceptor spectral sensitivities of birds and hymenopterans there is no evidence for adaptations to species-specific stimuli, such as those of food or mates. Turning to experimental work, we investigate how achromatic and chromatic stimuli are used for object recognition by foraging domestic chicks (Gallus gallus). Chicks use chromatic and achromatic signals in different ways: discrimination of large targets uses (chromatic) colour differences, and chicks remember chromatic signals accurately. However, detection of small targets, and discrimination of visual textures requires achromatic contrast. The different roles of chromatic and achromatic information probably reflect their utility for object recognition in nature. Achromatic (intensity) variation exceeds chromatic variation, and hence is more informative about change in reflectance – for example, object borders, while chromatic signals yield more information about surface reflectance (object colour) under variable illumination.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1011059715610
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