Steven Raiguel

Shape and Spatial Distribution of Receptive Fields and Antagonistic Motion Surrounds in the Middle Temporal Area (V5) of the Macaque

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.

Response latencies of visual cells in macaque areas V1, V2 and V5

The response to moving light and dark slits was recorded from a total of 94 cells in V1, V2, and V5 (MT) in 9 anesthetized and paralyzed macaque monkeys (M. fascicularis). Using the spatial lag method2, response latencies were calculated for each cell. We obtained median latencies of 85, 96, and 94 ms for cells in areas V1, V2, and V5, respectively. The higher median latencies of V2 and V5 cells compared to V1 are commensurate with later stages of information processing, and are predictable from the anatomy of the interconnections. In addition, a distinct, second population of high-latency cells is present in all 3 regions, but is most abundant in lamina 4 of V5. These may represent either external feedback from other regions or ongoing processing. Extensive overlap of latencies in all 3 regions at both the high and low ends of their respective ranges indicates a considerable degree of parallel interaction between striate and extrastriate cortex.

Learning to see the difference specifically alters the most informative V4 neurons

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.

SELECTIVITY OF MACAQUE MT/V5 NEURONS FOR SURFACE ORIENTATION IN DEPTH SPECIFIED BY MOTION

Area MT/V5 in the macaque brain is one of the major cortical regions involved in the analysis of retinal image motion. The majority of the neurons in this cortical area have non‐uniform antagonistic surrounds as components of their receptive field complexes. Theoretical studies indicate that such asymmetrical surrounds should enable neurons to extract orientation in depth from motion. Here we show that nearly half of the MT/V5 neurons encode the tilt component of the orientation in depth of a plane specified by motion. Furthermore, we show that such selectivity for depth from motion depends on the presence of an asymmetrical surround and on the speed tuning of those asymmetrical surround influences.

Size and shape of receptive fields in the medial superior temporal area (MST) of the macaque

NINETY-ONE single units were recorded in area MSTd of anesthetized and paralyzed macaques. Receptive fields (RFs) were mapped quantitatively using small patches of moving random dots in 25 different positions (the two-dimensional position test, or P2D). The dimensions of the receptive fields (RFs) were estimated by fitting P2D data with a generalized Gaussian function. The half-height areas of the RFs in MSTd were found to average 1085 deg2 and were not dependent upon eccentricity, in contrast to those in MT/V5 (n = 295) which averaged 31 deg2 at the fovea but at the periphery approached the RFs of MSTd in size. The RFs of some MSTd neurons extended 30–40° into the ipsilateral hemifield. In comparison, the overlap was only 10–15° in area MT/V5.

Influence of stimulus speed upon the antagonistic surrounds of area Mt/v5 neurons

WE investigated the effect of stimulus speed upon surround antagonism in macaque MT/V5 neurons, using probe stimuli placed at different positions in the surround. Their speed was varied, while the stimulation of the excitatory receptive field (RF) was held at optimal speed. Most Surrounds proved asymmetric, arising from a single region on one side of RF, although bilaterally and circularly symmetric surrounds were occasionally observed. Surround organization was generally retained at faster or slower surround speeds. Speed-dependent changes usually entailed diminished position dependence of surround influence, consequent to reduced surround effect at the position producing maximum inhibition. The effect of a stimulus covering the entire surround was much less dependent upon motion speed. Results show that surround non-uniformity is a robust finding in MT/V5 and endows neurons with multiple mechanisms for extracting surface orientation in depth.

Effects of Visual Cortex Lesions on Orientation Discrimination of Illusory Contours in the Cat

We have trained five cats in orientation discrimination using different contours, and compared the deficits caused by lesions of cortical areas 17 and 18 (tier I) to the deficits induced by removal of those areas receiving afferents originating in areas 17 and 18 (tier II). As contour stimuli we used two types of illusory contours and a luminance bar. The two illusory contours were defined by opposed line‐ends. One of them coincided with a luminance gradient whereas the other did not. Tier I lesions destroyed the capacity to discriminate the orientation of both illusory contours, and also caused an important, though less severe, deficit in bar orientation discrimination. The deficits induced by tier I lesions were permanent. Tier II lesions also caused significant deficits in orientation discrimination of illusory contours, but only a negligible deficit in bar orientation discrimination, and this result was not a mere consequence of a difference in difficulty between the tasks involving bars and illusory contours. In addition, tier II lesions differentiated between illusory contour types, the deficit being more pronounced for the illusory contour without luminance gradient than for the one with luminance gradient. In contrast to tier I lesions, tier II lesions allowed significant recovery, leading to small final deficits for all contour types tested.

The Selectivity of Neurons in the Macaque Fundus of the Superior Temporal Area for Three-Dimensional Structure from Motion

Motion is a potent cue for the perception of three-dimensional (3D) shape in primates, but little is known about its underlying neural mechanisms. Guided by recent functional magnetic resonance imaging results, we tested neurons in the fundus of the superior temporal sulcus (FST) area of two macaque monkeys (Macaca mulatta, one male) using motion-defined surface patches with various 3D shapes such as slanted planes, saddles, or cylinders. The majority of the FST neurons (>80%) were selective for stimuli depicting specific shapes, and all the surfaces tested were represented among the selective FST neurons. Importantly, this selectivity tolerated changes in speed, position, size, or between binocular and monocular presentations. This tolerance demonstrates that the 3D structure-from-motion (3D-SFM) selectivity of FST neurons is a higher-order selectivity, which cannot be reduced to a lower-order speed selectivity. The 3D-SFM selectivity of FST neurons was unaffected by removal of the opposed-motion cue that supplemented the speed gradient cue in the standard stimuli. When tested with the same standard stimuli, fewer neurons in the middle temporal/visual 5 (MT/V5) area were selective than FST neurons. In addition, selective MT/V5 neurons represented fewer types of surfaces and were less tolerant of stimulus changes than FST neurons. Overall, these results indicate that FST neurons code motion-defined 3D shape fragments, underscoring the central role of FST in processing 3D-SFM.