The effect of several new stimulus parameters on the perception of a moving plaid pattern (the sum of two sine-wave gratings) were tested. It was found that: (i) the degree of perceived sliding is strongly influenced by the aperture configuration through which the plaid is viewed; (ii) the chromaticity of the sinusoidal components affects coherence in that more sliding is observed when the plaid components differ in hue, and there is less sliding when they are of the same hue; (iii) equiluminant plaids made of components equal in color almost never show any sliding; and (iv) sliding increases with viewing time.The coherence-sliding percept must therefore be influenced by color, by global interactions, and by adaptation or learning effects, thus suggesting a higher-level influence. These results are most easily modelled by separating the decision to carry out recombination from the process of recombination.

We have examined the human ability to determine the direction of movement of a variety of plaid patterns. The plaids were composed of two orthogonal sine-wave gratings. When the plaid components are of unequal spatial frequency or sometimes of unequal contrast, observers judge the direction of movement incorrectly. In terms of the two-stage model of Adelson and Movshon (1982), these errors may result from either a misjudgment in the perceived speeds of each of the components or a failure in the combination of one-dimensional component movements into a coherent direction of motion of the two-dimensional plaid pattern, or both. A comparison of the perceived direction of motion of plaids with the relative perceived speeds of the plaid component gratings suggests that both failures occur, but in different circumstances. The relative perceived speed of the plaid components was measured with a spatial and a temporal forced-choice technique, the former leading to larger differences. Our results support the notion that the visual system decomposes a moving plaid into oriented components and subsequently recombines the component motions.

Hubel and Wiesel (1962; Journal of Physiology, London, 160, 106-154) introduced the classification of cortical neurons as simple and complex on the basis of four tests of their receptive field structure. These tests are partly subjective and no one of them unequivocally places neurons into distinct classes. A simple, objective classification criterion based on the form of the response to drifting sinusoidal gratings has been used by several laboratories, although it has been criticized by others. We review published and unpublished evidence which indicates that this simple and objective criterion reliability divides neurons of the striate cortex in both cats and monkeys into two groups that correspond closely to the classically-described simple and complex classes.

When a plaid made by superimposing two orthogonal sine wave gratings is moved, the apparent direction of motion is dependent on the contrasts and the relative spatial frequencies of the component gratings. We measured the apparent direction of plaids with 1 and 1.5-cycle/deg components. Because these patterns partially slide (i.e., are not completely coherent) the task was to respond to the main direction of motion. When scaled for equal multiples of threshold (equal visibility), the lower-spatial-frequency component dominates the direction of motion, indicating a disproportionally larger influence of low spatial frequencies on the motion system. The directional shift can be eliminated or even reversed by increasing the contrast of the high-spatial-frequency component relative to the low. However, the addition of contrast to the high-spatial-frequency component has less effect when the average contrast of the plaid is increased, suggesting a compressive contrast nonlinearity in the motion system. The movement characteristics of a red-green color plaid are similar to those of a low- contrast luminance plaid. The motion of the color plaid may be determined primarily by the small luminance distortions that are inevitably present due to chromatic aberrations.

We measured the spatial-frequency tuning of cells at regular intervals along tangential probes through the monkey striate cortex and correlated the recording sites with the cortical cytochrome oxidase (CytOx) patterns to address three questions with regard to the cortical spatial-frequency organization. (') Is there a periodic anatomical arrangement of cells tuned to different spatial-frequency ranges? We found there is, because the spatial-frequency tuning of cells along tangential probes changed systematically, varying from a low frequency to a middle range to high frequencies and back again repeatedly over distances of about 0.6-0.7 mm. (it) Are there just two populations of cells, low-frequency and high-frequency units, at a given eccentricity (perhaps corresponding to the magnoand parvocellular geniculate pathways) or is there a continuum of spatial-frequency peaks? We found a continuum of peak tuning. Most cells are tuned to intermediate spatial frequencies and form a unimodal rather than a bimodal distribution of cell peaks. Furthermore, the cells with different peak frequencies were found to be continuously and smoothly distributed across a module. (iii) What is the relation between the physiological spatial-frequency organization and the regions of high CytOx concentration ("blobs")? We found a systematic correlation between the topographical variation in spatial-frequency tuning and the modular CytOx pattern, which also varied continuously in density. Low-frequency cells are at the center of the blobs, and cells tuned to increasingly higher spatial frequencies are at increasing radial distances. The primary transformation of visual information at the striate cortex level in cat and monkey appears to be the sharpening of the orientation and spatial-frequency tuning of cells (1-5). Retinal ganglion and lateral geniculate nucleus cells respond to a wide range of orientations and spatial frequencies, whereas striate cortex cells are usually much more narrowly tuned. In the striate cortex, most cells have both spatial frequency and orientation tuning and thus respond to only a limited two-dimensional spatial-frequency range, each acting in effect as a band-pass two-dimensional filter of patterns within a localized region of the visual field. Different cells within a cortical region respond to different two-dimensional frequency ranges, with the ensemble of cells presumably covering the whole threeto five-octave range of spatial frequencies visible at that eccentricity (5). This physiological evidence is in good agreement with considerable psychophysical evidence for the presence of multiple two-dimensional spatial-frequency channels underlying human spatial vision (6-9). Early studies of the anatomical arrangement of cells in macaque striate cortex found evidence for a modular pattern to the cortical organization (10, 11). The cells within a slab of cortex -1-1.5 mm on a side all process the visual input from one small retinal region. Half of the cells within such a module receive their primary input from one eye, and the other half from the other eye. Within each half-module are found cells tuned to all of the different orientations in a systematic order. These early studies did not examine the spatial-frequency organization. Staining the cortex for cytochrome oxidase (CytOx) reveals an anatomical pattern of CytOx-rich "blobs" (regions of high CytOx concentration) distributed across the whole striate cortex in a striking leopard-like pattern (12-14). CytOx blobs turn out to be systematically related to the columnar organization for ocular dominance, a single CytOx blob being at the center of each half-module (12, 15, 16). We have studied (17, 18) the arrangement of different functional groups of cells within the cortex, using the 2deoxy-D-[U-14C]glucose (2-dG) technique and found a systematic spatial-frequency organization. Low spatial-frequency stimulation produced 2-dG uptake restricted to the CytOx blobs, whereas high-frequency stimulation produced uptake restricted to the interblobs. Middle spatial frequencies produced an essentially uniform pattern of activation in blobs and interblobs (CytOx-poor areas interspersed between blobs). These 2-dG studies left certain unanswered questions, however, to which the present experiment is addressed. A principal question is whether there are more than just two classes of cells-those tuned to low spatial frequencies and those tuned to high ones-given that only two discrete anatomical patterns could be discerned in the 2-dG uptake. Such a bimodal spatialfrequency organization would appear quite unlikely, given the extensive psychophysical evidence for multiple (not just two) spatial-frequency channels, and is hardly demanded by the 2-dG evidence cited above, given the relatively poor spatial resolution of the technique. Nonetheless, a bimodal-frequency distribution is a possibility that must be considered, particularly since there is much recent evidence for two functionally separate projections through the visual system that are segregated at the lateral geniculate level into the magnoand the parvocellular laminae and in the striate cortex in the 4Ca and 4CP laminae. In addition, there is evidence that these two systems have somewhat different spatial-frequency tuning on the average. Thus, it is conceivable that the blob pattern seen with lowspatial-frequency stimulation reflects just magnocellularrelated activity, and the high-frequency interblob pattern reflects parvocellular activity rather than a true multiplespatial-frequency organization. Given that cells with a variety of peak spatial frequencies occur within a striate region, a related question is whether these cells are arranged in a regular order or randomly distributed throughout the module. To examine these questions, we have turned to microelectrode recording, with its finer spatial resolution. Abbreviations: CytOx, cytochrome oxidase; 2-dG, 2-deoxyD-[U-14C]glucose; c/deg, cycles per degree; V1 and V2, visual areas 1 and 2. *Present address: Central Institute for the Deaf, and Department of Anatomy and Neurobiology, Washington University School of Medicine, Saint Louis, MO 63110. 711 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. 712 Neurobiology: Silverman et al. MATERIALS AND METHODS We recorded from cells at regular sites along a long tangential probe through the striate cortex, quantitatively measuring the spatial-frequency tuning of the cells encountered at each recording locus. The recording techniques were similar to those described elsewhere (5, 19). The macaque monkey was held painlessly by a pedestal attached to the skull in a prior operation. Anesthesia was induced with ketamine, and the animal was maintained anesthetized throughout the experiment by continuous infusion of morphine (0.7-1 mg/kg of body weight per hr) supplemented by N20; eye movements were minimized by paralysis with pancuronium bromide (Pavulon). Contact lenses kept the eyes from drying, and appropriate lenses were placed before each eye to bring the experimental stimuli to a focus on the retina. We used electrodes with longer than usual tips (15-25 tum) to sample a small group of cells at each recording site. The electrode was inserted at a very acute angle with respect to the cortical surface, just posterior to the lunate sulcus, close to the border of visual areas 1 and 2 (Vi-V2 border). Thereby we were able to make tangential penetrations up to 3 mm through the upper layers of the cortex. At the termination of the experiment, the opercula were removed and flattened (20). The cortex was then sectioned parallel to the electrode tracks at 40 tum, and the sections were treated for CytOx (21). Histological examination of the electrode tracks, of several small lesions made along each probe, and of the CytOx pattern in that and adjacent sections enabled us to localize each recording site with respect to the CytOx blobs. Densitometry along the electrode path allowed us to specify quantitatively the CytOx density at each recording site. The stimuli consisted of luminance-varying achromatic grating patterns presented monocularly under computer control on a monitor located 114 cm from the animal. The receptive field was centered on the display by appropriate movement of the monitor and/or of the animal's head, which was held in a gimbal arrangement. On the main probes, recordings were made every 100 tum along the electrode track. At each site, the preferred orientation and the ocular dominance of the unit(s) were first determined with hand-held stimuli. Then the spatialfrequency tuning was quantitatively measured with optimally oriented gratings of various spatial frequencies presented in a random order, drifting across the receptive field at 2-4 Hz. The computer analyzed the windowed spike discharge. We then advanced the electrode to the next recording site.