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We investigated the dynamics of neurons in the striate cortex (VI) and the lateral geniculate nucleus (LGN) to study the transformation in temporal-frequency tuning between the LGN and VI. Furthermore, we compared the temporal-frequency tuning of simple with that of complex cells and direction-selective cells with nondirection-selective cells, in order to determine whether there are significant differences in temporalfrequency tuning among distinct functional classes of cells within VI. In addition, we compared the cells in the primary input layers of VI (4a, 4ca, and 4c/3) with cells in the layers that are predominantly second and higher order (2, 3, 4b, 5, and 6). We measured temporal-frequency responses to drifting sinusoidal gratings. For LGN neurons and simple cells, we used the amplitude and phase of the fundamental response. For complex cells, the elevation of impulse rate (F0) to a drifting grating was the response measure. There is significant low-pass filtering between the LGN and the input layers of VI accompanied by a small, 3-ms increase in visual delay. There is further low-pass filtering between VI input layers and the second-and higher-order neurons in VI. This results in an average decrease in high cutoff temporal-frequency between the LGN and VI output layers of about 20 Hz and an increase in average visual latency of about 12-14 ms.One of the most salient results is the increased diversity of the dynamic properties seen in VI when compared to the cells of the lateral geniculate, possibly reflecting specialization of function among cells in VI. Simple and complex cells had distributions of temporal-frequency tuning properties that were similar to each other. Direction-selective and nondirection-selective cells had similar preferred and high cutoff temporal frequencies, but direction-selective cells were almost exclusively band-pass while nondirectionselective cells distributed equally between band-pass and low-pass categories. Integration time, a measure of visual delay, was about 10 ms longer for VI than LGN. In VI there was a relatively broad distribution of integration times from 40-80 ms for simple cells and 60-100 ms for complex cells while in the LGN the distribution was narrower.

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.

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 ofcells, low-frequency and high-frequency units, at a given eccentricity (perhaps corresponding to the magno-and 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 ofcells presumably covering the whole three-to 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 ...