Tapaus Lysenko - tiedepolitiikan oppitunti

Lysenkolaisen biologian ylivalta Neuvostoliitossa 1940–1964 lienee viime vuosisadan kuuluisin esimerkki poliittisen ohjauksen turmiollisuudesta luonnontieteissä. Lähes neljännesvuosisadan ajan oli mahdotonta harjoittaa tieteellistä genetiikkaa maassa, jonka itsekuvaan kuului nimenomaan luonnontieteiden edistäminen ja hyödyntäminen. Osa toisinajattelevista biologeista eliminoitiin jopa fyysisesti: 1930-luvun lopussa ja 40-luvun alussa kymmeniä tutkijoita menehtyi vankileireillä

Temporal vision : measures, mechanisms and meaning

Time is largely a hidden variable in vision. It is the condition for seeing interesting things such as spatial forms and patterns, colours and movements in the external world, and yet is not meant to be noticed in itself. Temporal aspects of visual processing have received comparatively little attention in research. Temporal properties have been made explicit mainly in measurements of resolution and integration in simple tasks such as detection of spatially homogeneous flicker or light pulses of varying duration. Only through a mechanistic understanding of their basis in retinal photoreceptors and circuits can such measures guide modelling of natural vision in different species and illuminate functional and evolutionary trade-offs. Temporal vision research would benefit from bridging traditions that speak different languages. Towards that goal, I here review studies from the fields of human psychophysics, retinal physiology and neuroethology, with a focus on fundamental constraints set by early vision.Peer reviewe

Spectral and thermal properties of rhodopsins: closely related but not tightly coupled

Rhodopsins, the primary molecules of vision in all seeing animals, can be activated not only by photon energy (light) but also by thermal energy (heat). Spectral absorbance is evolutionarily tuned by critical residues in the amino acid sequence of the protein part (opsin), which affect the energy needed for 11-cis → all-trans isomerization of the covalently bound chromophore. Already in the 1940’s it was suggested that high sensitivity to long-wavelength light, being indicative of a low energy barrier for activation, should correlate with high probability for thermal activation, and that randomly occurring thermal activations would constitute an irreducible noise setting absolute constraints for the detection of weak light signals. This idea has received strong experimental as well as theoretical support over the last 40 years. Most of the experimental evidence comes from physiological studies of light responses and dark noise in the light-sensitive current of vertebrate photoreceptor cells. Here I review this work, which has firmly established the correlation of spectral sensitivity and thermal noise and led to new theoretical insights. On the other hand, there remains significant freedom for independent adjustment of the two variables by tinkering with the opsin. This is a question of fundamental evolutionary as well as practical interest.Peer reviewe

A frog’s eye view: Foundational revelations and future promises

From the mid-19th century until the 1980's, frogs and toads provided important research models for many fundamental questions in visual neuroscience. In the present century, they have been largely neglected. Yet they are animals with highly developed vision, a complex retina built on the basic vertebrate plan, an accessible brain, and an experimentally useful behavioural repertoire. They also offer a rich diversity of species and life histories on a reasonably restricted physiological and evolutionary background. We suggest that important insights may be gained from revisiting classical questions in anurans with state-of-the-art methods. At the input to the system, this especially concerns the molecular evolution of visual pigments and photoreceptors, at the output, the relation between retinal signals, brain processing and behavioural decision-making.Peer reviewe

Effects of Mean Luminance Changes on Human Contrast Perception : Contrast Dependence, Time-Course and Spatial Specificity

Background When we are viewing natural scenes, every saccade abruptly changes both the mean luminance and the contrast structure falling on any given retinal location. Thus it would be useful if the two were independently encoded by the visual system, even when they change simultaneously. Recordings from single neurons in the cat visual system have suggested that contrast information may be quite independently represented in neural responses to simultaneous changes in contrast and luminance. Here we test to what extent this is true in human perception. Methodology/Principal Findings Small contrast stimuli were presented together with a 7-fold upward or downward step of mean luminance (between 185 and 1295 Td, corresponding to 14 and 98 cd/m2), either simultaneously or with various delays (50–800 ms). The perceived contrast of the target under the different conditions was measured with an adaptive staircase method. Over the contrast range 0.1–0.45, mainly subtractive attenuation was found. Perceived contrast decreased by 0.052±0.021 (N = 3) when target onset was simultaneous with the luminance increase. The attenuation subsided within 400 ms, and even faster after luminance decreases, where the effect was also smaller. The main results were robust against differences in target types and the size of the field over which luminance changed. Conclusions/Significance Perceived contrast is attenuated mainly by a subtractive term when coincident with a luminance change. The effect is of ecologically relevant magnitude and duration; in other words, strict contrast constancy must often fail during normal human visual behaviour. Still, the relative robustness of the contrast signal is remarkable in view of the limited dynamic response range of retinal cones. We propose a conceptual model for how early retinal signalling may allow this.Peer reviewe

The effect of mean luminance change and grating pedestals on contrast perception: Model simulations suggest a common, retinal, origin

Peer reviewe

Eye spectral sensitivity in fresh- and brackish-water populations of three glacial-relict Mysis species (Crustacea) : physiology and genetics of differential tuning

Absorbance spectra of single rhabdoms were studied by microspectrophotometry (MSP) and spectral sensitivities of whole eyes by electroretinography (ERG) in three glacial-relict species of opossum shrimps (Mysis). Among eight populations from Fennoscandian fresh-water lakes (L) and seven populations from the brackish-water Baltic Sea (S), L spectra were systematically red-shifted by 20-30 nm compared with S spectra, save for one L and one S population. The difference holds across species and bears no consistent adaptive relation to the current light environments. In the most extensively studied L-S pair, two populations of M. relicta (L (p) and S (p)) separated for less than 10,000 years, no differences translating into amino acid substitutions have been found in the opsin genes, and the chromophore of the visual pigments as analyzed by HPLC is pure A1. However, MSP experiments with spectrally selective bleaching show the presence of two rhodopsins (lambda (max) a parts per thousand 525-530 nm, MWS, and 565-570 nm, LWS) expressed in different proportions. ERG recordings of responses to "red" and "blue" light linearly polarized at orthogonal angles indicate segregation of the pigments into different cells differing in polarization sensitivity. We propose that the pattern of development of LWS and MWS photoreceptors is governed by an ontogenetic switch responsive to some environmental signal(s) other than light that generally differ(s) between lakes and sea, and that this reaction norm is conserved from a common ancestor of all three species.Peer reviewe