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Replacing a missing gene in adult colour-blind monkeys restores normal colour vision. How the new photoreceptor cells produced by this therapy lead to colour vision is a fascinating question.

Orientation tuning of neurons is one of the chief emergent characteristics of the primary visual cortex, VI (refs 1,2). Neurons of the lateral geniculate nucleus, which comprise the thalamic input to VI, are not orientation-tuned, but the majority of VI neurons are quite selective. How orientation tuning arises within VI is still controversial1,3–17. To study this problem, we measured how the orientation tuning of neurons evolves with time18–20 using a new method: reverse correlation in the orientation domain. Orientation tuning develops after a delay of 30–45 milliseconds and persists for 40–85 ms. Neurons in layers 4Cα or 4Cβ, which receive direct input from the thalamus, show a single orientation preference which remains unchanged throughout the response period. In contrast, the preferred orientations of output layer neurons (in layers 2,3,4B, 5 or 6) usually change with time, and in many cases the orientation tuning may have more than one peak. This difference in dynamics is accompanied by a change in the sharpness of orientation tuning; cells in the input layers are more broadly tuned than cells in the output layers. Many of these observed properties of output layer neurons cannot be explained by simple feedforward models1,3–6, whereas they arise naturally in feedback networks7–17. Our results indicate that VI is more than a bank of static oriented filters; the dynamics of output layer cells appear to be shaped by intracortical feedback.

WE describe here a new view of primary visual cortex (VI) based on measurements of neural responses in V1 to patterns called ‘illusory contours’ (Fig. la, b). Detection of an object's boundary contours is a fundamental visual task. Boundary contours are defined by discontinuities not only in luminance and colour, but also in texture1–3, disparity4 and motion5–7. Two theoretical approaches can account for illusory contour perception. The cognitive approach emphasizes top-down processes8,9. An alternative emphasizes bottom-up processing. This latter view is supported by (1) stimulus constraints for illusory contour perception10–14 and (2) the discovery by von der Heydt and Peterhans15–17 of neurons in extrastriate visual area V2 (but not in V1) of macaque monkeys that respond to illusory contours. Using stimuli different from those used previously15,16, we found illusory contour responses in about half the neurons studied in V1 of macaque monkeys. Therefore, there are neurons as early as V1 with the computational power to detect illusory contours and to help distinguish figure from ground.

Human colour vision depends on three classes of cone photoreceptors, those sensitive to short (S), medium (M) or long (L) wavelengths, and on how signals from these cones are combined by neurons in the retina and brain. Macaque monkey colour vision is similar to human, and the receptive fields of macaque visual neurons have been used as an animal model of human colour processing. P retinal ganglion cells and parvocellular neurons are colour-selective neurons in macaque retina and lateral geniculate nucleus. Interactions between cone signals feeding into these neurons are still unclear. On the basis of experimental results with chromatic adaptation, excitatory and inhibitory inputs from L and M cones onto P cells (and parvocellular neurons) were thought to be quite specific (Fig. 1a). But these experiments with spatially diffuse adaptation did not rule out the 'mixed-surround' hypothesis: that there might be one cone-specific mechanism, the receptive field centre, and a surround mechanism connected to all cone types indiscriminately (Fig. 1e). Recent work has tended to support the mixed-surround hypothesis. We report here the development of new stimuli to measure spatial maps of the linear L-, M- and S-cone inputs to test the hypothesis definitively. Our measurements contradict the mixed-surround hypothesis and imply cone specificity in both centre and surround.

We describe here a new view of primary visual cortex (V1) based on measurements of neural responses in V1 to patterns called 'illusory contours' (Fig. 1a, b). Detection of an object's boundary contours is a fundamental visual task. Boundary contours are defined by discontinuities not only in luminance and colour, but also in texture, disparity and motion. Two theoretical approaches can account for illusory contour perception. The cognitive approach emphasizes top-down processes. An alternative emphasizes bottom-up processing. This latter view is supported by (1) stimulus constraints for illusory contour perception and (2) the discovery by von der Heydt and Peterhans of neurons in extrastriate visual area V2 (but not in V1) of macaque monkeys that respond to illusory contours. Using stimuli different from those used previously, we found illusory contour responses in about half the neurons studied in V1 of macaque monkeys. Therefore, there are neurons as early as V1 with the computational power to detect illusory contours and to help distinguish figure from ground.

The primary visual cortex (V1) is the first stage in the cerebral cortex to analyse visual signals from the retina. Signals are relayed to V1 from the lateral geniculate nucleus, and a new study shows that this process involves elaborate neural computation. Rather than acting as static receptors, the visual cortical cells seem to behave dynamically, and cortico-cortical interactions are needed for monkeys, at least, to respond to visual scenes.

I. THROUGHOUT the known sidereal universe there is, among material bodies, an obvious associative tendency, which we see well illustrated in meteor showers, in satellite and planetary systems, in binary stars, and in larger stellar groups such as the Pleiades. These various products of gravitational ordering are clearly but parts of still greater systems, and one of the most fascinating of astronomical studies is to attempt to seek out the structure of an all-inclusive sidereal organisation.

A deterministic spike generating model accounts very well for the properties of the impulse rate fluctuations in Limulus eccentric cells.

VISUAL perception can be viewed as a transformation and a distortion of the spatial patterns which exist in the outside world. The transformation of external objects into neural images is accomplished by the summation of excitatory and inhibitory influences over the visual field of each neurone. Distortion occurs when the neural response is not simply proportional to the sum of excitation and inhibition because of nonlinear mechanisms in spatial summation. If the responses of all visual cells were distorted by such nonlinear summation, fine visual discriminations like those necessary for reading Nature would be impossible. Nonlinear mechanisms do however, seem to be important in one class of visual neurone, perhaps for signalling change or motion in the external world. We have attempted to understand the importance of the different kinds of spatial summation by studying single cells in the cat visual system.

(1) THE determination of the past duration of solar radiation, and, consequently, the problem of the age of the inhabitable earth, imposes upon theories of radiation a difficulty the magnitude and fundamental importance of which appear to be too infrequently considered. The difference in the time-scales derived from the gravitational theory of solar energy and from geological and astronomical observation is not one of a few per cent. (or less) of the basic quantities involved, as is generally the case with the discrepancies that have led to conspicuous modifications of radiation theories; the discrepancy is rather a matter of a hundred to one, or even of a thousand or more to one. A more glaring disagreement could scarcely be imagined between a generally accepted and thoroughly workable theory on one hand, and, on the other, a mass of observation now too extensive and varied to be denied and some equally formidable physical laws.

Study of parallel processing in the visual pathway1 of the cat has revealed several classes of retinal ganglion cells which are physiologically distinct and which project to various locations in the brain2,3. Two classes have been studied most extensively: X cells, which sum neural signals linearly over their receptive fields, and Y cells, in which the spatial summation is nonlinear1,4. In the cat's lateral geniculate nucleus (LGN) cells also can be classified as X or Y, a result of the parallel projection of retinal X and Y inputs to different geniculate neurones5–9. We report here our study of parallel signal processing in the LGN of the macaque monkey. We find that (1) monkey LGN cells can be classified as X or Y on the basis of spatial summation; (2) X-like cells are found in the four parvocellular and the two magnocellular laminae, whereas Y-like cells are found almost exclusively in the magnocellular laminae; and (3) the cells of the magnocellular laminae have high sensitivity and the parvocellular cells low sensitivity for homochromatic patterns. This implies that in macaque monkeys the magnocellular cells and their cortical projections may be the neural vehicle for contrast vision near threshold. The cells of the parvocellular laminae seem to be primarily concerned with wavelength discrimination and patterns of colour. As the human visual system is similar to that of the macaque in structure and behavioural performance, our findings are probably also applicable to man.

Light adaptation is the adjustment of retinal response properties to variations in ambient illumination. It enables the encoding of visual information over a millionfold intensity range, from moonlight to broad daylight, despite the relatively small dynamic range of response of visual neurones. We have studied the effects of light adaptation on the dynamics and sensitivity of visual responses of neurones in the turtle retina, by measuring the responses of horizontal cells in the retina to light which was modulated with a sinusoidal time course around various mean levels. As a quantitative measure of the transduction from light to neural signals, we calculated the gain of response at each frequency. Gain is defined as the amplitude of the modulated response component divided by the amplitude of light modulation. We report here that the gain (mV photon−1) at low temporal frequencies decreased as the mean light level increased. Over a 2 log-unit range of mean light levels, low-frequency gain was inversely proportional to the mean light level, as in Weber's law. However, at high temporal frequencies, the gain was almost independent of mean light level. Our results are reminiscent of Kelly's1 results on human temporal-frequency sensitivity in various states of light adaptation. We found that a family of horizontal-cell temporal frequency responses, measured at various mean light levels, could be accounted for by a negative feedback model in which the feedback strength is proportional to mean light level.

Continuing the trend towards relaxing controls on recombinant DNA research, the National Institutes of Health published revised guidelines in the Federal Register of 21 April 1982. Although the existing system of mandatory controls and institutional biosafety committees is retained, no class of experiments will be totally prohibited, and the guidelines dealing with containment levels have been greatly simplified.