1. We recorded and tested quantitatively 65 middle temporal (MT) and 82 middle superior temporal (MST) cells in paralyzed and anesthetized monkeys. 2. Responses to the three elementary optic flow components (EFCs)--rotation, deformation, and expansion/contraction--and to translation (in the display) were compared after optimization of stimulus direction, speed, size, and position. As a control responses to flicker were measured. 3. Response windows were adapted in correspondence with our finding that latencies of MT and MST cells decrease with increasing speed for all types of motion. 4. There was a response continuum in MT as well as in MST cells. Compared with translation, MST cells responded significantly more to rotation but less to flicker than MT cells. MST cells were significantly more direction selective for expansion/contraction than MT cells. 5. MST cells generally responded to fewer motion types than MT cells. 6. Position invariance of EFC direction selectivity was tested over a region of the visual field centered on the translation receptive field (RF). Direction selectivity for an EFC was not position invariant in MT cells but it was invariant in 40% of the MST cells tested. These cells were considered EFC selective. 7. Most EFC-selective MST cells were selective for a single EFC, possibly combined with translation. Few of them were selective for deformation. 8. EFC selectivity was also speed invariant and EFC-selective MST cells usually had RFs summating inputs over wide portions of the visual field. 9. EFC-selective MST cells with similar selectivities were clustered.

Perceptual learning is an instance of adult plasticity whereby training in a sensory (e.g., a visual task) results in neuronal changes leading to an improved ability to perform the task. Yet studies in primary visual cortex have found that changes in neuronal response properties were relatively modest. The present study examines the effects of training in an orientation discrimination task on the response properties of V4 neurons in awake rhesus monkeys. Results indicate that the changes induced in V4 are indeed larger than those in V1. Nonspecific effects of training included a decrease in response variance, and an increase in overall orientation selectivity in V4. The orientation-specific changes involved a local steepening in the orientation tuning curve around the trained orientation that selectively improved orientation discriminability at the trained orientation. Moreover, these changes were largely confined to the population of neurons whose orientation tuning was optimal for signaling small differences in orientation at the trained orientation. Finally, the modifications were restricted to the part of the tuning curve close to the trained orientation. Thus, we conclude that it is the most informative V4 neurons, those most directly involved in the discrimination, that are specifically modified by perceptual learning.

The spatial organization of receptive fields in the middle temporal (MT) area of anaesthetized and paralysed macaque monkeys was studied. In all, 288 neurons were successfully recorded. The size and shape of the receptive field (RF) was mapped with small patches of translating random dots and the resulting data were fitted with a generalized Gaussian. Results show that the RF area increases with eccentricity, and is larger in lamina 5 than in other layers. Most of these RFs are elongated, and the axis of elongation tends to be orthogonal to the preferred direction of motion. The direction selectivity is maintained in all positions in the RF, but layer 5 cells are less direction-selective than cells in other layers. In a second series of experiments, radial dimensions of the classical RF and the antagonistic surround were estimated from area summation tests. These data were fitted with the difference of the integrals of two Gaussians. Surrounds were weakest in layer 4 and strongest in layer 2. Optimal stimulus diameters, also estimated from the area summation curve, were larger in the infragranular layers than in the other layers. The maximum sensitivity of the surround was clearly displaced from the classical RF (CRF) centre, indicating that the surround is not concentric with the CRF. This radial offset and the extent of the surround were largest in layers 2 and 5 and smallest in 3a. The extent of the surround half-height equalled, on average, 3-4 times that of the CRF. These results suggest that antagonistic surrounds are constructed in MT, probably through horizontal connections, and that a strong vertical organization exists in area MT, as has been shown for V1.

1. We tested quantitatively the responses of 147 middle temporal (MT) cells to light and dark bars moving at different speeds ranging over a 1,000-fold range (0.5-512 deg/s). 2. We derived the following quantities from the speed-response (SR) curves obtained for opposite directions of motion. Speed selectivity was characterized by the maximum response, optimum speed, upper cutoff speed, response to slow movement, and tuning width. Direction selectivity was characterized by the direction index (DI) averaged over speeds yielding significant responses (MDI) and by the direction index at optimal speed (PDI). 3. There was an excellent correlation between speed characteristics for light and dark bars. These correlations were stronger than the correlations between direction indexes. The strongest correlations were obtained for maximum response and upper cutoff. 4. SR curves were classified into three groups: low pass (25%), tuned (43%), and broadband (28%), leaving 4% unclassified. 5. In the majority (75%) of MT cells, there was an agreement between the typology of speed selectivity for light and dark bars. Cells were classified as tuned (33%), low pass (22%), broadband (19%), and mixed (22%), leaving 4% unclassified. In addition to differences in speed characteristics, these groups also differed in response level, direction selectivity, and distribution of preferred directions. 6. For tuned cells, there was a very tight correlation of most speed characteristics for light and dark bars. 7. Direction selectivity depended on stimulus speed in most neurons, yielding a tuned average speed-DI curve. 8. Speed characteristics, proportions of speed selectivity types, and direction selectivity indexes showed little dependence on laminar position. 9. Speed characteristics and direction selectivity indexes were not dependent on eccentricity. Proportion of speed selectivity types however, changed dramatically with eccentricity: low-pass cells dominated foveally, tuned cells parafoveally, and broadband cells peripherally. 10. There were also small eccentricity effects on the range of optimal speeds shown by tuned cells and on the speed at which direction selectivity decreases in the slow speed range.

Optical flow is a rich source of information about the three-dimensional motion and structure of the visual environment. Little is known of how the brain derives this information. One possibility is that it analyzes first-order elementary components of optical flow, such as expansion, rotation, and shear. Using a combination of physiological recordings and modeling techniques, we investigated the contribution of the middle superior temporal area (MST), a third-order cortical area in the dorsal visual pathway that receives inputs from the medial temporal area (MT Optical flow, which can be defined as the apparent motion of the image brightness on the retina, has long been considered as a useful representation of visual motion information (1). According to a fundamental theorem of kinematics, or Helmholtz theorem (2), optical flow, within a small area of the visual field, can be seen as the sum of a translation with four elementary flow components (EFCs): a rotation (circular motion), an expansion (radial motion), and two components of shear (deformations) (3). This result has been successfully used in computer and computational vision for the analysis of motion (3-7). In addition, EFCs have been shown to be biologically relevant (8,9). Therefore, as suggested by recent studies in which middle superior temporal area (MST) cells of the monkey brain were reported to be selective for rotation and expansion (10-15), one may propose that the brain explicitly represents EFCs. MST is a third-order cortical area in the dorsal pathway leading to the parietal cortex (16). It receives inputs from the middle temporal area (MT) (17, 18), which has been implicated in motion analysis (19)(20)(21)(22), and is mainly driven by magnocellular input (23). However, there is disagreement about the degree of selectivity of MST cells: MST cells have been reported to be selective for a single EFC (10, 24) but also for several components (12, 13). Furthermore, there is no information about whether or not complex motion patterns are decomposed into the (linear) superposition of EFCs. Finally, it is not clear how the MST selectivity for EFCs is generated.Therefore, we quantitatively compared responses of cortical cells to translation and EFCs in both areas (MST and MT). To clearly distinguish between responses to local flow vectors and to global arrangements of vectors characteristic of EFCs, we systematically tested for the position invariance of the selectivity. Once selectivity for an EFC was established, we then studied the responses of MST cells to a combination of components, keeping one component constant. The results of the physiological study were used to model the transformations effected by the MST cells on MT input. METHODSWe have recorded from single MST and MT neurons of anesthetized and paralyzed monkeys (Macaca fascicularis) to obtain stable recordings over several hours and precisely localize the recorded units with respect to area and layer. The monkeys were anesthetized with sufentanil (5 pg-kg-1lh-1) and paralyze...