3. The mean discharge frequency of Y-cells (unlike that of X-cells) was greatly increased when grating patterns drifted across their receptive fields.4. In twenty-one X-cells the relation between the contrast and spatial frequency of drifting sinusoidal gratings which evoked the same small response was measured. In every case it was found that the reciprocal of this relation, the contrast sensitivity function, could be satisfactorily described by the difference of two Gaussian functions.5. This finding supports the hypothesis that the sensitivities of the antagonistic centre and surround summating regions of ganglion cell receptive fields fall off as Gaussian functions of the distance from the field centre.6. The way in which the sensitivity of an X-cell for a contrast-edge pattern varied with the distance of the edge from the receptive field centre was determined and found to be consistent with the cell's measured contrast sensitivity function.7. Reducing the retinal illumination produced changes in the contrast * On leave from the Physiological Laboratory, Cambridge.t Mailing address.

SUMMARY1. The contrast thresholds of a variety of grating patterns have been measured over a wide range of spatial frequencies.2. Contrast thresholds for the detection of gratings whose luminance profiles are sine, square, rectangular or saw-tooth waves can be simply related using Fourier theory.3. Over a wide range of spatial frequencies the contrast threshold of a grating is determined only by the amplitude of the fundamental Fourier component of its wave form.4. Gratings of complex wave form cannot be distinguished from sinewave gratings until their contrast has been raised to a level at which the higher harmonic components reach their independent threshold. 5. These findings can be explained by the existence within the nervous system of linearly operating independent mechanisms selectively sensitive to limited ranges of spatial frequencies.

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1. The response to an optimally oriented stimulus of both simple and complex cells in the cat's striate visual cortex (area 17) can be suppressed by the superposition of an orthogonally oriented drifting grating. This effect is referred to as cross-orientation suppression. We have examined the spatial organization and tuning characteristics of this suppressive effect with the use of extracellular recording techniques. 2. For a total of 75 neurons, we have measured the size of each cell's excitatory receptive field by use of rectangular patches of drifting sinusoidal gratings presented at the optimal orientation and spatial frequency. The length and width of these grating patches are varied independently. Receptive-field length and width are determined from the dimensions of the smallest grating patch required to elicit a maximal response. 3. The extent of the area from which cross-orientation suppression originates has been measured in an analogous manner. Each neuron is excited by a patch of drifting grating the same size as the receptive field. The response to this stimulus is modulated by a superimposed patch of grating having an orthogonal orientation. After selecting the spatial frequency that produces maximal suppression, the response of each cell is examined as a function of the length and width of the orthogonal (suppressive) grating patch. Results from 29 cells show that the dimensions of the orthogonal grating patch required to elicit maximal suppression are similar to, or smaller than, the dimensions of the excitatory receptive field. Thus cross-orientation suppression originates from within the receptive field. 4. For some cells the spatial frequency tuning of the suppressive effect is much broader than the spatial frequency tuning for excitation. In these cases it is possible to find a spatial frequency that produces suppression but not excitation. With the use of a suppressive stimulus having this spatial frequency, we examined the strength of suppression as a function of orientation for 11 cells. These tests show that suppression occurs at all orientations, including the preferred orientation for excitation. In some cases, suppression is somewhat stronger at the preferred orientation for excitation than at any other orientation. 5. For 12 cells we varied the relative spatial phase between the optimally oriented and orthogonal gratings. In all cases the magnitude of suppression is largely independent of the relative spatial phase. 6. For three binocular cells we examined whether the suppressive effect of a grating oriented orthogonal to the optimum could be mediated dichoptically.(ABSTRACT TRUNCATED AT 400 WORDS)

The flash electroretinogram (ERG) is a useful tool for assessing retinal function in both the laboratory and the clinic. It is a mass electrical potential that changes in a characteristic way in response to an increase in retinal illumination, and it can be recorded non-invasively using a corneal electrode. Recording non-invasively provides the opportunity for studying retinal function while maintaining an essentially normal physiological environment for the tissue. However, because it is a mass potential, the ERG represents the summed activity of all retinal cells. For specific information about retinal function, it is important to be able to separately analyse the contributions from the cells and circuits that combine to form the mass response.Much research on the flash ERG has focused on separating the various components of the response that correspond to different retinal cell types. As a result, it is well known that the initial waves of the dark-adapted (scotopic) ERG, the a-and b-waves, originate mainly from cells at early stages of retinal processing. Following an intense brief flash of light from darkness, the negative-going a-wave is generated by rod photocurrents (Penn & Hagins, 1969) and the positive-going b-wave by depolarizing bipolar-cell currents in combination with bipolar cell-dependent K + currents affecting Müller cells (Miller & Dowling, 1970;Stockton & Slaughter, 1989;Xu & Karwoski, 1994;Shiells & Falk, 1999;Robson & Frishman, 1995; for a review see Pugh et al. 1998). For weaker flashes, the a-wave is too small to be seen in the record, and the dominant component of the ERG is the b-wave.For very weak flashes from darkness, in several mammals, e.g. cats, monkeys, humans, and rats, a small post-

3. These Q cells appear to form a homogeneous group which is probably a subset of the tonic W cells (Stone & Fukuda, 1974) or sluggish centre-surround cells (Cleland & Levick, 1974).4. The over-all spatio-temporal frequency characteristics of cells showing linear spatial summation are not separable in space and time. The form of the spatial frequency responsivity function of these cells depends upon the temporal frequency at which it is measured while the temporal phase of their response measured at any constant temporal frequency depends upon the spatial frequency of the stimulus.5. The behaviour of X and Q cells is quite well explained by an extension of the model in which signals from centre and surround mechanisms with radially Gaussian weighting functions are summed to provide the drive to the retinal ganglion cell. While the general form of the temporal frequency response characteristics of these ganglion cells are probably provided by the characteristics of elements common to the centre and surround pathways, the spatio-temporal interactions can be explained by assuming that the surround signal is delayed relative to the centre signal by a few milliseconds.

The electroretinogram (ERG) of the dark-adapted cat eye in response to brief ganzfeld flashes of a wide range of intensities was recorded after intravitreal injection of n-methyl dl aspartate (NMdlA, cumulative intravitreal concentration of 1.3–3.9 mM) to suppress inner-retinal components, and after intravitreal dl or L-2-amino-4-phosphonobutyric acid (dl-APB, 1–3 mM; l-APB, 1.2 mM) and 6-cyano-7-nitroquinoxaline-2, 3 dione (CNQX, 40–60 µM), to suppress all post-receptoral neuronal responses. Rod PII, the ERG component arising from rod bipolar cells, was derived by subtracting records obtained after APB and CNQX from post-NMDLA records. When we measured the derived response at fixed times after the stimulus, we found that PII initially increased in proportion to stimulus intensity without any sign of a threshold. The leading edge of PII at early times after the stimulus, when the response was still small, was well described by V(t) = kI(t −td)5 where k is a constant, I is the intensity of the stimulus, and td is a brief delay of about 3 ms. Correspondingly, the time for the response to rise to an arbitrary small criterion voltage Vcrit was adequately fitted by tcrit = td + (Vcrit/kI)1/5. The time course of the leading edge of the PII response can be interpreted to indicate that the mechanism generating PII introduces three stages of temporal integration in addition to the three stages that are provided by the mechanism of the rod photoreceptors. This finding is consistent with the operation within the rod bipolar cell of a G-protein cascade similar to that in the rods.