Sabine Kastner

Neuronal correlates of pop-out in cat striate cortex

Neuronal responses to static and moving texture patterns were investigated in the striate cortex of anaesthetized and paralysed adults cats. Texture patterns were composed of a central light bar presented in the excitatory receptive field of a cell and an array of many similar elements in the surround. For the static condition, elements in the surround were either parallel or orthogonal to the centre line (orientation test). For the moving condition, centre and surround elements (all at same orientation) moved either in the same or in the opposite directions (motion test). Thirty-six percent (31/86) of the neurons tested for motion and 24% (24/99) of the neurons tested for orientation responded more strongly to the patterns displaying feature contrast than to the uniform patterns. These neurons may form a neural basis for visual pop-out of orientation and motion.

Evidence for asynchronous development of sleep in cortical areas

WE have recorded from extrastriate area V4 in monkeys performing a visual search task. When animals became tired or drowsy, responses to visual stimulation were often reduced or even completely blocked, and background activity changed to the burst-pause pattern typically seen in sleep. In spite of such neuronal sleep observed in V4, animals continued to perform the visual task, indicating that at least the primary visual cortex was still working. This observation shows that sleep does not develop simultaneously in all cortical areas but may affect some areas earlier than others. In particular conditions, local sleep of certain areas may be a stable and long-lasting phenomenon.

Neuronal responses to orientation and motion contrast in cat striate cortex

Responses of striate neurons to line textures were investigated in anesthetized and paralyzed adult cats. Light bars centered over the excitatory receptive field (RF) were presented with different texture surrounds composed of many similar bars. In two test series, responses of 169 neurons to textures with orientation contrast (surrounding bars orthogonal to the center bar) or motion contrast (surrounding bars moving opposite to the center bar) were compared to the responses to the corresponding uniform texture conditions (all lines parallel, coherent motion) and to the center bar alone. In the majority of neurons center bar responses were suppressed by the texture surrounds. Two main effects were found. Some neurons were generally suppressed by either texture surround. Other neurons were less suppressed by texture displaying orientation or motion (i.e. feature) contrast than by the respective uniform texture, so that their responses to orientation or motion contrast appeared to be relatively enhanced (preference for feature contrast). General suppression was obtained in 33% of neurons tested for orientation and in 19% of neurons tested for motion. Preference for orientation or motion contrast was obtained in 22% and 34% of the neurons, respectively, and was also seen in the mean response of the population. One hundred nineteen neurons were studied in both orientation and motion tests. General suppression was correlated across the orientation and motion dimension, but not preference for feature contrast. We also distinguished modulatory effects from end-zones and flanks using butterfly-configured texture patterns. Both regions contributed to the generally suppressive effects. Preference for orientation or motion contrast was not generated from either end-zones or flanks exclusively. Neurons with preference for feature contrast may form the physiological basis of the perceptual saliency of pop-out elements in line textures. If so, pop-out of motion and pop-out of orientation would be encoded in different pools of neurons at the level of striate cortex.

High-frequency repetitive transcranial magnetic stimulation delays rapid eye movement sleep.

REPETITIVE transcranial magnetic stimulation (rTMS) is a promising new treatment for patients with major depression. However, the mechanisms underlying the antidepressive action of rTMS are widely unclear. Rapid eye movement (REM) sleep has been shown to play an important role in the pathophysiology of depression. In the present study we demonstrate that rTMS delays the first REM sleep epoch on average by 17 min (102.6 ± 22.5 min vs 85.7 ± 18.8 min; p <0.02) and prolongs the nonREM-REM cycle length (109.1 ± 11.4 min vs 101.8 ± 13.2 min, p <0.012). These rTMS-induced changes in REM sleep variables correspond to findings observed after pharmacological and electroconvulsive treatment of depression. Therefore, it is likely that the capability of rTMS to affect circadian and ultradian biological rhythms contributes to its antidepressive action.

The neurophysiological correlates of colour and brightness contrast in lateral geniculate neurons

SummaryThe colour of an object is changed by surround colours so that the perceived colour is shifted in a direction complementary to the surround colour. To investigate the physiological mechanism underlying this phenomenon, we recorded from 260 neurons in the parvo-cellular lateral geniculate nucleus (P-LGN) of anaesthetized monkeys (Macaca fascicularis), and measured their responses to 1.0–2.0° diameter spots of equiluminant light of various spectral composition, centered over their receptive field (spectral response function, SRF). Five classes of colour opponent neurons and two groups of light inhibited cells were distinguished following the classification proposed by Creutzfeldt et al. (1979). In each cell we repeated the SRF measurement while an outer surround (inner diameter 5°, outer diameter 20°) was continuously illuminated with blue (452 nm) or red (664 nm) light of the same luminance as the center spots. The 1.0–1.5° gap between the center and the surround was illuminated with a dim white background light (0.5–1cd/m2). During blue surround illumination, neurons with an excitatory input from S-or M-cones (narrowand wide-band/short-wavelength sensitive cells, NSand WS-cells, respectively) showed a strong attenuation of responses to blue and green center spots, while their maintained discharge rate (MDR) increased. During red surround illumination the on-minus-off-responses of NS- and WS-cells showed a clear increment. L-cone excited WL-cells (wide-band/long-wavelength sensitive) showed a decrement of on-responses to red, yellow and green center spots during red surround illumination and, in the majority, also an increment of MDR. The response attenuation of narrow-band/long-wavelength sensitive (NL)-cellls was more variable, but their on-minus-off-responses were also clearly reduced in the average during red surrounds. Blue surround illumination affected WL-cell responses little and less consistently than those of NL-cells, but often broadened the SRF also in the WL-cells towards shorter wavelengths. The M-cone excited and S-cone suppressed WM-cells were strongly suppressed by blue but only little affected by red surround illumination. The changes of spectral responsiveness came out clearly in the group averages of the different cell classes, but snowed some variation between individual cells in each group. The zero-crossing wavelengths derived from on-minus-off-responses were also characteristically shifted towards wavelengths complementary to those of the surround. The direction of changes of spectral responsiveness of P-LGN-cells are thus consistent with psychophysical colour contrast and colour induction effects which imply that light of one spectral region in the surround reduces the contribution of light from that same spectral region in the (broad band or composite) object colour. Surrounds of any colour also decrease the brightness of a central coloured or achromatic light (darkness induction). We calculated the population response of P-LGN-units by summing the activity of all WS-, WM- and WL-cells and subtracting that of all NS- and NL-cells. The SRF of this population response closely resembled the spectral brightness function for equiluminous lights rather than the photopic luminosity function. With red or blue surrounds, this population SRF was lowered nearly parallel across the whole spectrum to about 0.7 of the amplitude of the control. In a psychophysical test on 4 observers we estimated the darkness induction of an equiluminous surround in a stimulus arrangement identical to the neurophysiological experiment, and found a brightness reduction for white, blue, green and red center stimuli to 0.5–0.7 of the brightness values without surround. This indicates that the neurophysiological results may be directly related to perception, and that P-LGN-cells not only signal for chroma but also for brightness, but in different combinations. The results indicate that both an additive (direct excitation or suppression of activity) and a multiplicative mechanism (change of gain control) must be involved in brightness and colour contrast perception. As mechanisms for the surround effects horizontal cell interactions appear not to be sufficient, and a direct adaptive effect on receptors feeding positive or negative (opponent) signals into the ganglion cells receptive fields by straylight from the surround must be seriously considered. This will be examined in the following companion paper. The results indicate that changes of spectral and brightness responses in a colour contrast situation sufficient to explain corresponding changes in perception are found already in geniculate neurons and their retinal afferents. This applies to mechanisms for colour constancy as well in as much as they are related to colour contrast.

A reversible system for chronic recordings in macaque monkeys

We propose a system for head fixation and neuronal recording that minimizes surgery for implantation. Fixation is obtained by posts which are attached to the opposite sides of the skull and are connected by a rigid frame around the animals head. As forces are counterbalanced and distributed around the head, the system does not need to be implanted into the skull, and thus allows for continuous adjustment to the growing skull in young animals. Except for small incisions for the posts, the skin over the skull is left intact. Recording is achieved through small bone holes which are easily reached by means of conical guide tubes. The system provides perfect stability of recording, allows flexible access to various areas of the brain and can be easily removed during longer pauses in experiments. The use of this system may also decrease the number of laboratory animals needed.

Neurons with large bilateral receptive fields in monkey prelunate gyrus

Abstract. In single-cell recordings from the dorsocaudal part of the prelunate gyrus of an alert monkey (Macacafascicularis) we found neurons with unexpectedly large receptive fields (RFs) that spread bilaterally into the contra- and ipsilateral visual fields. These neurons (n=82) appeared to be clustered in the periphery of V4. They were surrounded by neurons with relatively small (3–10°) and unilateral RFs in the contralateral field with properties similar to those previously described for neurons in area V4. Bilateral RFs extended over large parts of the lower visual field but always spared the fovea. Receptive fields typically revealed two foci of maximal responsiveness that were arranged symmetrically in the ipsi- and contralateral fields. Twenty-six cells did not respond to stimuli along the vertical meridian; these neurons had two distinct RFs. The preference for stimulus orientation, color, or motion was similar in all parts of these large RFs.

Neurophysiological Correlates of Colour Induction on White Surfaces.

Coloured light surrounding a white surface of about equal luminance makes the white surface appear illuminated with an unsaturated light of the complementary colour. In an attempt to discover the neurophysiological basis of such colour induction, we recorded from spectrally opponent cells of the parvocellular layers of the lateral geniculate nucleus (P‐LGN) of anaesthetized macaques. Only cells with wide‐band (W) spectral sensitivity in the short (S) or long wavelength (L) part of the spectrum (WS, WL) are excited by white spots of light centred on their receptive field. Cells with narrow‐band (N) spectral sensitivity (NS, NL) and light‐inhibited (L1) cells are inhibited by white light. Therefore, it is likely that the code for white is contained in a balanced excitation of the W cells. The effects of continuous illumination of remote surrounds with different wavelengths on the responses to achromatic light stimuli were investigated. Responses [on minus maintained discharge rate (MDR) or on‐minus‐off] were determined for white spots (1–3° diameter) flashed on the receptive field centre, presented either alone or in the presence of an annular surround of equal luminance (inner diameter 5°; outer diameter 20°). During red surround illumination the responses of WL cells to white spots tended to be reduced as were those of WS cells during blue surround illumination. Surround illumination with the opponent colour had more variable effects, neither WS nor WL cells showing a significant alteration of their mean response to white during surround illumination with opponent light. Response alterations were to a large extent due to changes in MDR, which increased in WS cells during blue surround illumination and in WL cells during red surround illumination. It is argued that the surround effects on centre responses are due to intraocular stray light rather than lateral connections in the retina. The surround effects also depended to some extent on the size of the test spot. L1 cells and the very rare parvocellular panchromatic on‐cells showed no chromatic response changes during coloured surround illumination. Inasmuch as the excitation of WS cells, either alone or in combination with NS cell activation, is involved in coding for green and blue, and that of WL cells, in combination with NL cell activation, is involved in coding for red and yellow in perception, the shift of excitation towards one or the other W cell group indicates relatively more red or green signals in the white response, consistent with and in the same direction as colour induction. In addition, the summed population response of WS and WL cells is decreased during surround illumination with any colour including white. This is related to brightness decrease during surround illumination in perception.

On Neurophysiological Correlates of Simultaneous Colour and Brightness Contrast as Demonstrated in P-LGN-Cells of the Macaque

The colour of objects depends not only on the spectral composition and intensity of light reflected from them, but also on the spectral composition and intensity of objects surrounding them. Set in a monochromatic environment, the colour of an object generally appears as to be shifted in a direction complementary to that of the environment and its brightness also changes (colour and brightness contrast). Although simultaneous colour and brightness contrast phenomena have been investigated extensively, little is known about its neuronal correlate and the level at which it arises within the visual pathway. Some follow Helmholtz (1866) and place it in the primary visual cortex or beyond (Zeki, 1983; Land et al., 1983; Livingston and Hubel, 1984), while others find evidence supporting Mach’s location of the mechanism in the retina (Poppel, 1986) or even at a prereceptoral level (Walraven, 1973).