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We recorded from single neurons in the parvocellular layers of the lateral geniculate body of anesthetized monkeys. Spectral response curves of parvocellular neurons depended on the luminance ratio between the chromatic stimuli and achromatic background. From response/intensity curves, we determined the relative luminance between a coloured and an achromatic (white) light at which a given cell became non-responsive (critical luminance ratio, CLR). The spectral dependence of the CLRs of narrow (N) and wide band (W) cells with opponent receptor input showed characteristic differences. The activity of W-cells increased with luminance increase of a white light and of a coloured light in the specific spectral region of the cell (yellow-red for the long wave length sensitive WL-, and yellow-green-blue for the short wave length sensitive WS-cells), while N-cells were activated by their specific spectral light (blue for NS-cells, red for NL-cells) and by a luminance decrease of achromatic white. N-cells discriminate best between their characteristic colour and white at luminance ratios below their respective CLR, while W-cells distinguish best between a light of their characteristic colour and white at chromatic/achromatic luminance ratios above their respective CLR. Yellow sensitive W-cells with a narrow spectral sensitivity peaking around 570 nm and with only a small or no response to white light, could enable distinction between white and yellow of similar luminance. The findings are consistent with the opponency model of spectrally sensitive cells in the LGB. We discuss their implications for colour coding by parvocellular cells. N- and W-cells appear to behave complementary with respect to luminance information (N-cells may be compared to the cat's off-cells, W-cells to on-cells). S- and L-cells are complementary with respect to colour. The yellow sensitive WM-cells are critical for the discrimination of yellow and white, while cells with excitatory cone input from blue and red cones (W-SL-cells) may aid the perception of purple. The fact that, at different relative luminance ratios between a chromatic stimulus and a white background, the whole family of parvocellular cells is involved differently in coding for colour, may explain the different appearance of colours against a white background at different luminance ratios and the perception of induced colours.

Macaques were trained to respond to electrical excitation applied through electrodes permanently implanted within or upon striate cortex. Threshold current for the animal to detect this stimulation was highly consistent from day to day and, in the absence of tissue encapsulation of the electrodes or deliberately inflicted damage, remained stable indefinitely, 38 months in the longest case so far. Stimulating continuously for 1-8h, however, produces an elevation of threshold, which may be permanent or temporary, depending upon a variety of conditions. A major cause of such injury is the hydrolysis commonly occurring consequent to passage of low-level currents between solutions and metal electrodes. Even when the hydrolytic reaction is eliminated by restricting the level of electrode polarization or by using capacitative stimulation with tantalum pentoxide electrodes, a rise in threshold often still occurs with protracted stimulation. With proper control in some instances, however, effective stimulation at 2-10 times the threshold level could be maintained indefinitely without apparent injury, e.g. in a blind monkey having a threshold of 290 muA that could respond immediately to an 80-muA diminution in 580-muA, 0.2-msec stimulus pulses which had been applied steadily for 1 h at 50 Hz.

Coloured surfaces in the normal environment may be brighter or dimmer than the mean adaptation level. Changes in the firing rate of cells of the parvocellular layers of macaque lateral geniculate nucleus were studied with such stimuli; chromatic mixtures briefly replaced a white adaptation field. This paradigm is therefore one of successive contrast. Families of intensity-response curves for different wavelengths were measured. When taking sections at different luminance ratios through these families of curves, strongly opponent cells displayed spectrally selective responses at low luminance ratios, while weakly opponent cells had higher chromatic thresholds and responded well to stimuli at higher luminance ratios, brighter than the adaptation field. Strength of cone opponency, defined as the weight of the inhibitory cone mechanism relative to the excitatory one, was thus related to the range of intensity in which cells appeared to operate most effectively. S-cone inputs, as tested with lights lying along tritanopic confusion lines, could either be excitatory or inhibitory. Families of curves for different wavelengths can be simulated mathematically for a given cell by a simple model by using known cone absorption spectra. Hyperbolic response functions relate cone absorption to the output signals of the three cone mechanisms, which are assumed to interact linearly. Parameters from the simulation provided estimates of strength of cone opponency and cone sensitivity which were shown to be continuously distributed. Cell activity can be related to cone excitation in a trichromatic colour space with the help of the model, to give an indication of suprathreshold coding of colour and lightness.

Thirty renal transplant recipients were studied prospectively to evaluate the relationship of cytomegalovirus-specific cytotoxic lymphocyte responses to clinical outcome during cytomegalovirus infection. Cytomegalovirus infection developed in 20 patients; of these 20, 14 had cytomegalovirus-specific cytotoxic lymphocyte responses whereas six did not. Clinical findings (fever, leukopenia, thrombocytopenia, or elevations in serum transaminase levels) were significantly more frequent among patients without responses than among patients with responses (p less than 0.001), and prolonged viremia and complications of infection including superinfection, interstitial pneumonitis, pancreatitis, and death occurred exclusively among patients without responses. Acute allograft dysfunction during infection was experienced by four patients without responses but by only one patient with response (p = 0.02), indicating that the virus-specific cytotoxic response did not result in a renal immunopathologic condition, and may have protected against virus-induced injury to the graft. In seven of nine patients with responses who shed virus, cytotoxic responses occurred within one week of detection of activation of virus shedding. Absence of cytotoxic responses correlated with prior high-dose, intravenous methylprednisolone treatment, and apparently resulted from inhibition of cytotoxic T cell precursors. Immunosuppressive treatment to inhibit graft rejection should be minimized, and methods should be developed that do not inhibit cytomegalovirus-specific cytotoxic T cell responses.

1. Heterochromatic flicker photometry is a way of measuring the spectral sensitivity of the human eye. Two lights of different colour are sinusoidally alternated at, typically, 10-20 Hz, and their relative intensities adjusted by the observer until the sensation of flicker is minimized. This technique has been used to define the human photopic luminosity, or V lambda, function on which photometry is based. 2. We have studied the responses of macaque retinal ganglion cells using this stimulus paradigm. The responses of the phasic ganglion cells go through a minimum at relative radiances very similar to that predicted from the V lambda function. At this point, defined as equal luminance, an abrupt change in response phase was observed. A small residual response at twice the flicker frequency was apparent under some conditions. 3. The spectral sensitivity of parafoveal phasic cells measured in this way corresponded very closely to that of human observers minimizing flicker on the same apparatus. 4. Minima in phasic cell activity were independent of flicker frequency, as is the case in the psychophysical task. 5. The response minima of phasic cells obey the laws of additivity and transitivity which are important characteristics of heterochromatic flicker photometry. 6. As the relative intensities of the lights were altered responses of tonic, spectrally opponent cells usually underwent a gradual phase change with vigorous responses at equal luminance. The responses of tonic cells treated individually or as a population could not be related to the V lambda function in any meaningful way. 7. We conclude that the phasic, magnocellular cell system of the primate visual pathway underlies performance in the psychophysical task of heterochromatic flicker photometry. It is likely that other tasks in which spectral sensitivity conforms to the V lambda function also rely on this cell system.

Neurons with strong inhibitory short-wavelength sensitive cone (S-cone) inputs have been identified in the macaque geniculate using a tritanopic confusion line test, i.e. by stimulation with equiluminous stimuli which leave excitations of long- and middle-wavelength sensitive cones (L- and M-cones) constant while differentially exciting S-cones. Mathematical simulation of the responses of these cells, using known spectral sensitivities of the cone receptors, demonstrates that they receive excitation from M-cones, inhibition from S-cones, and little or no inhibition from L-cones. Excitatory and inhibitory pools are largely spatially coextensive.

We have studied the responses of ganglion cells in the macaque retina to stimuli that alternate in color. With most color combinations, the phasic retinal ganglion cells, which sum input from M- and L-cones in both center and surround, showed a response with twice the alternation frequency at equal luminance. This frequency doubling was directly related to the degree to which the M- and L-cones were stimulated out-of-phase with one another, and thus varied with the wavelength combinations used. It was absent with wavelength combinations that lay along tritanopic confusion lines, when at equal luminance the M- and L-cones are not modulated. Such a frequency-doubled response is evidence for a nonlinearity at or before M- and L-cone summation. The effect became much smaller or was abolished when the receptive field center alone was stimulated, indicating that its mechanism lies in the surround or in a center-surround interaction. Also, it was much more marked at high luminance levels, being almost absent at retinal illuminances below 100 td. Its origin is not clear, but it seems to derive more from the L- than the M-cone. The results imply that phasic cells, through this nonlinearity, could respond to the red-green equal luminance borders used in some psychophysical experiments.

We studied the relationship between light intensity and cell response to various wavelengths and wavelength combinations in the dorsal, parvocellular layers of the macaque lateral geniculate nucleus. When response is plotted as a function of the logarithm of stimulus intensity, the slope and shape of curves depends on wavelength. For wavelengths near the crossover point between excitatory and suppressive responses, nonmonotonic curves are common. Consequently, the form of spectral-response functions depends on stimulus intensity. Responses to combined stimuli made up of wavelengths close together near one spectral extreme are approximately additive. If one wavelength is near the crossover point, responses are nonadditive so that a midspectral wavelength, only producing a weak excitatory response, is able to occlude more vigorous responses to wavelengths near the spectral ends. Responses of parvocellular layer cells are consistent with their being a result of linear interaction of opponent cone mechanisms, the response of each of which follows a modified hyperbolic tangent function (22). Responses to all wavelength combinations, even those showing strikingly nonadditive effects, could be predicted from the additive opponent model described above.

1. We have measured responses of macaque retinal ganglion cells to a uniform flickering field, with variation in luminance, chromaticity or both (heterochromatic flicker). 2. With heterochromatic flicker, as the luminance ratio of the flicker components was varied, phasic ganglion cell activity went through a minimum and an abrupt phase change close to equal luminance. Tonic ganglion cell responses underwent a gradual phase change without any minimum close to equal luminance. For red on-centre cells, when wavelengths above 570 nm were altered with white, a progressive phase advance occurred as luminance ratio (L lambda/LW) was increased. With wavelengths below 570 nm a progressive phase lag occurred. For green on-centre cells, the opposite pattern was found. For all tonic cells, the higher the temporal frequency, the more rapidly did such phase changes occur. A simple model incorporating a centre-surround delay of 3-8 ms could quantitatively account for these changes. 3. With luminance flicker of different dominant wavelengths, amplitudes and phase of responses of phasic ganglion cells were independent of wavelength at all frequencies. The amplitude and phase of the responses of tonic ganglion cells was very dependent on wavelength, as well as on flicker frequency. Their characteristics hardly ever resembled results from phasic cells. 4. For achromatic flicker, response phase of tonic cells at or above 10 Hz was variable, probably due to the centre-surround delay. Such variability was not seen among phasic cells. 5. An interesting implication of these results is that the ability of tonic ganglion cells to unambiguously signal rapid chromatic or spatial change is limited.

The responses of neurones in the lateral geniculate nucleus (LGN) were investigated in anaesthetised rhesus monkeys. A new classification for cells in the parvocellular layers (PCL) is proposed, based on their spectral response curve and their response to white stimuli: (A) narrow-band, short wavelength (NS) excited cells, activity suppressed by white stimuli; (B) wide-band, short-wavelength (WS) excited cells, excited by white stimuli; (C) wide-band, long-wavelength (WL) excited cells, (D) narrow-band, long-wavelength (NL) excited cells, activity suppressed by white stimuli; (E) light suppressed (LI) cells, activity suppressed by all wavelengths, usually with some concealed excitatory input at extreme short or long wavelengths. Responses to moving bars and to spots of various diameters (area response curves) were determined for various wavelengths. It was found that the receptive fields from which wavelength-dependent excitatory or suppressive effects could be elicited are concentrically superimposed. The spectral responsiveness of the excitatory inputs to individual cell types corresponds to the absorption curves of single cones (S-, M- or L-cone for NS, WS and WL cells respectively), the spectral distribution of the suppressive mechanisms of all cells was panchromatic and approximately fitted to a sum of all cones. The excitatory input to NL-cells cannot be related to any of the known cone absorption curves, and a simple (L-M) subtraction model is questioned. Neurones in the magnocellular layers (MCL) can be divided into on- and off-centre cells as in the cat's LGN and give qualitatively similar responses over the whole spectrum. In contrast to the tonic responses of PCL cells, MCL cells respond phasically to chromatic and white flashed spots, even with the smallest stimuli. Implications of these findings for colour processing in the LGN are discussed.

Response patterns to complex visual stimuli were further analysed. Patterns were correlated with linear or non-linear components of the stimulus pattern at various wavelengths. Resulting correlograms revealed the spatial and spectral structure of receptive fields; they showed peaks or troughs according to whether that wavelength was associated with an increase or decrease in cell firing. Spectral response curves as derived from linear correlograms were similar to those reported for monochromatic stimuli. Variability in responsiveness and crossover wavelengths was high between parvocellular layer (PCL) cells even of the same class. Spatial differences between excitatory and suppressive receptive field components, i.e. a centre-surround organisation, are not apparent in linear correlograms from PCL cells. In this respect, spectral response curves do not qualitatively change with stimulus size. Correlation in time and the derivation of impulse functions showed that, even in magnocellular layer (MCL) cells, responses to luminance steps are of mainly temporal origin and due to a transient component in the response. A description of cell responses based on linear processing accounted well for the response patterns obtained in our experiments. Of various non-linear interactions investigated, only some kind of non-linear spectral differentiation provided an improvement in the description of cell responses. This improvement, however, was only minor and not present in all cells.

The receptive fields of LGN cells were investigated with stationary light and dark spot and annulus stimuli. Stimulus size and background intensity were varied while stimulus/background contrast was kept constant. The speed of dark adaptation vaired considerably from cell to cell. Dark adaptation made responses more sustained in all neurones and eliminated the oscillatory on-responses evoked under some conditions in the light-adapted cells. Dark adaptation led also to a disappearance of early phasic inhibition in on-responses, and increased response rise time and latency. The power of surround responses to inhibit centre responses decreased slightly at low levels of light adaptation in LGN cells but much less than in retinal ganglion cells. Some other traces of changing retinal surround effects also appeared inthe LGN on dark adaptation. For example, the functional size of receptive fields increased at low levels of illuminance as has been observed in retinal ganglion cells and the receptive fields as estimated from response peaks were larger than those estimated from sustained components.

Responses of cells in the parvocellular (p.c.l.) and magnocellular (m.c.l.) layers of the macaque lateral geniculate nucleus to sine-wave gratings were studied. Both p.c.l. and m.c.l. cells responded best at a temporal frequency (drift rate) of 10-20 Hz. P.c.l. cells responded at temporal frequencies lower than 1 Hz; m.c.l. cells did not. With coloured- or white-black luminance-modulated gratings, responses of m.c.l. cells were weaker at low than at medium spatial frequencies. With coloured gratings, p.c.l. cell responses were not attenuated at low spatial frequencies. With white gratings a few p.c.l. cells did show such attenuation. Optimal responses from p.c.l. cells were obtained with coloured gratings; white gratings evoked weaker responses. With a grating of a colour causing suppression of a p.c.l. cell's activity, the modulation of firing was much less than with a grating of a colour excitatory for the cell. M.c.l. on- and off-centre cells responded equally well to moving gratings. The ability of p.c.l. cells to resolve fine gratings was dependent on cell type as well as on the colour of grating used. The ability of m.c.l. cells to resolve fine gratings was comparable to that of p.c.l. cells. The contrast sensitivity of m.c.l. cells was much higher than that of p.c.l. cells. This may account for their ability to resolve fine gratings, despite their larger centre size. In comparison with luminance-modulated gratings, chromatically modulated gratings could evoke larger or smaller responses, depending on p.c.l. cell type and the colours in the grating. M.c.l. cells responded poorly or not at all.

Responses of macaque lateral geniculate nucleus (LGN) cells to stimuli of different incremental intensities and wavelength compositions were studied at different levels of light adaptation from scotopic to low photopic levels. Stimuli were large in comparison with receptive-field size. Human increment thresholds were measured for comparison. The strength of responses grew in many cells from threshold up to a saturation level as a logarithmic function of incremental intensity. More complex intensity-response functions were also obtained, particularly from parvocellular layer (PCL) cells, but the shape and slope of the intensity-response function changed as a function of adaptation level only with chromatic backgrounds. As a function of adaptation level, the intensity-response functions shifted along the logarithmic abscissa but not sufficiently for a complete contrast constancy. Thus responses to any constant contrast became smaller when adaptation level decreased. The change from cone to rod responses, when possible, took place without noticeable change in shape of intensity-response functions, and much of the adaptive shift of the functions could be attributed to the change-over between rods and cones. Differences between different cells in light adaptation and dark-adapted sensitivity were large, mostly because of variation in the strength of rod input. The strongest excitatory rod inputs were found in PCL cells activated by short-wavelength light, so that the highest sensitivity at low levels of illumination occurred in blue- and blue-green-sensitive cells. The lowest increment thresholds based on cones matched the thresholds of macaque cone late receptor potentials recorded by Boynton and Whitten (3). They were also similar to human cone thresholds measured psychophysically but only for small stimulus sizes that may approximate the size of the receptive-field centers. Human sensitivity was much higher when measured with large stimulus sizes, indicating integration at post-geniculate neural levels. Light adaptation, as evaluated with respect to contrast constancy and Weber law behavior, was similarly incomplete in monkey single cells and human perception. A few cat LGN cells were studied in a control experiment; results agreed with previous findings. The light adaptation of cat cels was more complete and sensitivity higher than those observed under comparable conditions in macaque single cells and human. The maintained activity level of cells was little affected by the intensity of steady backgrounds. Thus, the steady-state hyper-polarisation of receptors was not transmitted to LGN cells.

The responses of neurones in laminae A and A1 of the cat lateral geniculate nucleus to moving stimuli were investigated at different background luminances. Moving bright slits, dark bars and edges were employed; the contrast of stimuli against the background was held constant. Background intensities varied from 10(-3) to 10(2) td. Responses as stimuli passed across the centres of LGN receptive fields became stronger with increasing levels of light adaptation up to 10(-1)-10(1) td and then remained constant. Responses as stimuli passed through surround regions altered qualitatively with adaptation level, generally increasing in strength and complexity with background luminance. As a bright slit for on-centre cells or dark bar for off-centre cells left the surround, in almost all units a strong secondary peak could be elicited by an appropriate selection of the adaptation conditions. Many features of the responses to moving stimuli could not be predicted from the responses to stationary stimuli under different adaptation conditions described in the previous paper.

We compared visual responses of cat lateral geniculate nucleus (l.g.n.) neurones with those of retinal ganglion cells providing their afferent inputs. Quantitative studies were made on twenty such pairs; eight X on-centre, seven Y on-centre, two X off-centre and three Y off-centre pairs. Receptive field centre locations of cell pairs with correlated activities were very closely superimposed, having a mean centre displacement of 1.6 minutes of arc for X cells and 11 minutes of arc for Y cells. With flashed spots and annuli, responses of l.g.n. cells were almost always smaller than those of their retinal afferents, with peaks and troughs in ganglion cell responses being faithfully followed in the geniculate neurones. This is consistent with almost all impulses from the l.g.n. cell being triggered by the afferent feeding its centre. With spots of different sizes and contrasts, modulation of responses by l.g.n. inhibition was obvious, but effects were complex. With moving bright-bar stimuli, although response histograms were clearly reshaped to some extent in the l.g.n., peak firing rates under different stimulus conditions were often merely attenuated by a constant factor for most l.g.n. cells in comparison with their retinal inputs. For velocity tuning curves, a few cell pairs showed selective attenuation at high speeds, while others showed it at low speeds. All the latter group appeared to have more than one major excitatory afferent. These changes in velocity tuning occurred across the X/Y classification, so that differences in velocity preference of the X and Y systems is more blurred in the l.g.n. than in the retina.

Neurones recording during penetrations through cat area 17 as near parallel to the radial fibre bundles as possible have been quantitatively tested as to their optimal orientation. Optimal orientation within any one penetration was similar though considerable variability was observed. Histological reconstruction and other considerations showed that this variability could not be attributed to poor penetration angle or limitations of the microelectrode technique. These results confirm that neurones with similar optimal orientations are found in all cortical layers at one cortical locus, but it is difficult to reconcile the variability observed with a mosaic-like distribution of orientation across the cortical surface. The findings were consistent, however, with the assumption of a continuous distribution of orientation sensitivity across the cortical surface with considerable superimposed scatter.

A living related kidney transplant recipient with normal renal function and severe hypertension secondary to renal artery stenosis, was treated with captopril and developed reversible renal failure requiring temporary hemodialysis. This complication of captopril, an angiotensin converting enzyme inhibitor, has been reported previously in hypertensive patients with renal artery stenosis with and without a kidney transplant. It is recommended that this drug be used with caution in this setting.

It is well recognized that in the visual cortex of the cat, some of the cells (hypercomplex) are sharply tuned for the length of a bar moving backwards and forwards across their receptive fields. Other cells (simple) exhibit no such tuning but appear to respond proportionately over a range of bar lengths. The tuning seen in hypercomplex cells is already observable to a lesser degree in retinal ganglion cells. Our experiments were carried out to determine the extent of this tuning in the lateral geniculate nucleus, which relays information from the retina to the cortex. Results show that geniculate cells have tuning properties intermediate between those of ganglion cells and hypercomplex cells. By adding together a linear array of geniculate cells, it is possible to model the characteristics of a simple cell and to demonstrate that while an elongated bar gives a minimal response in hypercomplex cells, it should have little effect on the response of simple cells.

1. We have studied the sensitivity of macaque retinal ganglion cells to sinusoidal flicker. Contrast thresholds were compared for stimuli which alternated only in luminance ('luminance flicker') or chromaticity ('chromatic flicker'), or which modulated only the middle- or long-wavelength-sensitive cones ('silent substitution'). 2. For luminance flicker, the lowest thresholds were those of phasic, non-opponent ganglion cells. Sensitivity was maximal near 10 Hz. 3. Tonic, cone-opponent ganglion cells were relatively insensitive to luminance flicker, especially at low temporal frequencies, but were sensitive to chromatic flicker, thresholds changing little from 1 to 20 Hz. Those with antagonistic input from middle- and long-wavelength-sensitive (M- and L-) cones had a low threshold to chromatic flicker between red and green lights. Those with input from short-wavelength-sensitive (S-) cones had a low threshold to chromatic flicker between blue and green. Expressed in terms of cone contrast, the S-cone inputs to blue on-centre cells had higher thresholds than M- and L-cone inputs to other cell types. 4. Phasic, non-opponent cells responded to high-contrast red-green chromatic flicker at twice the flicker frequency. This frequency-doubled response is due to a non-linearity of summation of M- and L-cone mechanisms. It was only apparent at cone contrasts which were above threshold for most tonic cells. 5. M- or L-cones were stimulated selectively using silent substitution. Thresholds of M- and L-cone inputs to both red and green on-centre cells were similar. This implies that these cells' sensitivity to chromatic flicker is derived in equal measure from centre and surround. Thresholds of the isolated cone inputs could be used to predict sensitivity to chromatic flicker. The high threshold of these cells to achromatic contrast is thus, at least in part, due to mutual cancellation by opponent inputs rather than intrinsically low sensitivity. 6. Thresholds of M- and L-cone inputs to phasic cells were similar at 10 Hz, and were comparable to those of tonic cells, suggesting that at 1400 td cone inputs to both cell groups are of similar strength. 7. The modulation transfer function of phasic cells to luminance flicker was similar to the detection sensitivity curve of human observers who viewed the same stimulus. For chromatic flicker, at low temporal frequencies thresholds of tonic cells (red or green on-centre cells in the case of red-green flicker or blue on-centre cells in the case of blue-green flicker) approached that of human observers. We propose the different cell types are the substrate of different channels which have been postulated on the basis of psychophysical experiments. 8. At frequencies of chromatic flicker above 2 Hz, human sensitivity falls off steeply whereas tonic cell sensitivity remained the same or increased. This implies that high-frequency signals in the chromatic, tonic cell pathway are not available to the central pathway respons

Steadily illuminated surrounds, remote from the receptive field centre, are shown to affect the responses of primate visual cells. Intensity-response curves of cells of the macaque lateral geniculate nucleus were measured using a successive contrast paradigm where chromatic or achromatic stimuli were presented in alternation with a white adaptation field of constant luminance. Adding white surround annuli around stimuli and adaptation field shifted the intensity-response curves to higher intensity ranges. Since response curves can be nonmonotonic, this remote surround effect can result in an increase or decrease in responsiveness (facilitation or suppression) dependent on stimulus intensity. Steady surrounds, remote from the receptive field centre, thus control cell sensitivity and responses by means of simultaneous contrast.