A possible neurobiological basis for the ''oblique effect'' is linked to the finding that more neural machinery is devoted to processing cardinal vs. oblique orientations in primary visual cortex (V1). We used optical imaging to determine whether more territory is devoted to processing horizontal and vertical orientations than oblique orientations in owl monkey middle temporal visual area (MT), a visual area highly sensitive to moving stimuli. We found that more of MT was devoted to representing cardinal than oblique orientations, and that the anisotropy was more prominent in parts of MT representing central vision (<10°). Neural responses to orientations of 0°and 90°were also greater than those to 45°and 135°. In comparison, an overrepresentation of cardinal orientations in the representation of central vision in owl monkey V1 was relatively small and inconsistent. Our data could explain the greater sensitivity to motion discrimination when stimuli are moved along cardinal meridians and suggest that the neural machinery necessary to explain the motion oblique effect either originates in MT or is enhanced at this level.oblique effect ͉ optical imaging ͉ orientation preference ͉ owl monkey ͉ visuotopic maps P rimary visual cortex (V1) of primates and a number of other mammals contains a retinotopic map of visual space with a map of stimulus orientations superimposed. Neurons selective for similar orientations are clustered together in regions devoted to a portion of visual space. One of the unexpected observations is that more cortical machinery is devoted to representing vertical and horizontal (cardinal) than oblique orientations, because more neurons are selective for cardinal orientations, and cardinal orientations produce a greater neuronal response (1-5). Although the functional consequences of greater representation of cardinal orientations are uncertain, a popular proposal is that such an anisotropy in human visual cortex underlies the ability of humans to better discriminate gratings and other visual stimuli with horizontal and vertical rather than oblique orientations (6-8). This psychophysical observation is known as the ''oblique effect,'' and the effect can be eccentricity-dependent (9, 10). Another proposal suggests that the greater representation promotes the stability of orientation tuning for neurons most sensitive to cardinal orientations, because stimulus adaptation can alter the tuning properties of cortical neurons (11). In either case, differences in the representation of stimulus features in V1 are expected to have significant perceptual consequences.Surprisingly, there have been no reports about anisotropies in the representation of orientation in primate visual areas outside V1, although few studies have ever investigated this issue beyond V1 (3,12,13). Yet there are reasons for believing that orientation anisotropies might be found in other visual cortical areas, especially the middle temporal visual area (MT; also known as V5), because MT receives direct and indirect inputs from V1, and...

Optical imaging of intrinsic cortical responses to visual stimuli was used to characterize the organization of the middle temporal visual area (MT) of a prosimian primate, the bush baby (Otolemur garnetti). Stimulation with moving gratings revealed a patchwork of oval-like domains in MT. These orientation domains could, in turn, be subdivided into zones selective to directional movements that were mainly orthogonal to the preferred orientation. Similar, but not identical, zones were activated by movements of random dots in the preferred direction. Orientation domains shifted in preference systematically either around a center to form pinwheels or as gradual linear shifts. Stimuli presented in different portions of the visual field demonstrated a global representation of visual space in MT. As optical imaging has revealed similar features in MT of New World monkeys, MT appears to have retained these basic features of organization for at least the 60 million years since the divergence of prosimian and simian primates.

The middle temporal area (MT) is a visual area in primates with direct and indirect inputs from the primary visual cortex (V1), a role in visual motion perception, and a suggested role in ''blindsight.'' When V1 is deactivated, some studies report continued activation of MT neurons, which has been attributed to an indirect pathway to MT from the superior colliculus. Here we used muscimol to deactivate V1 while optically imaging visually evoked activity in MT in two primates, owl monkeys and galagos, where MT is exposed on the brain surface. The partial loss of V1 inputs abolished all or nearly all evoked activity in the retinotopically matched part of MT. Low levels of activation that persisted in portions of MT that were unstimulated or retinotopically congruent with the blocked portion of V1 appeared to reflect the spread of activity from stimulated to unstimulated parts of MT. Thus, a significant pathway based on the superior colliculus was not demonstrated.V1 lesion ͉ blindsight ͉ muscimol ͉ intrinsic connections T he middle temporal area (MT, also known as V5) is a visual area in the upper temporal lobe of primates that contains a very high proportion of neurons selective for the direction of motion (1, 2) and the orientation of moving gratings (3, 4) and clearly inf luences motion perception (5). Although major inputs to MT are from the primary visual cortex (V1) and areas dependent on V1 for visual activation, such as V2 (6, 7), the effects of lesions or cooling of V1 on the responsiveness of neurons in MT have been inconsistent and difficult to interpret. In early experiments in macaque monkeys, lesions of V1 appeared to only partially inactivate MT (8); an additional lesion of the superior colliculus rendered MT neurons unresponsive to visual stimulation (9). Similarly, a cooling block of part of V1 failed to eliminate all visually evoked activity in the retinotopically matched portion of MT (10). In addition, functional MRI of the visual cortex in a well studied patient with V1 damage that is known for blindsight suggested that visually evoked activity in MT can be independent of V1 (11, 12). Thus, a possible source of blindsight (13), the ability to respond to visual information without awareness of the stimuli, is a relay of visual information from the superior colliculus to the pulvinar and then to extrastriate cortex (8 -10, 14).In other experiments on New World owl monkeys, MT was found to be completely dependent on V1 for visual activation, whether the lesion was recent (15) or long standing (16). In contrast, recordings from MT of the marmoset, another New World monkey, suggested that some neurons do not depend on V1 (14; for contrast, see ref. 16). These varying results suggest that species differ in dependencies of MT on V1 or that visually evoked responses in deprived parts of MT have been incorrectly attributed to an extrastriate (non-V1) source of activation.The present experiments allowed us to visualize global patterns of visual activation in MT after a block of relayed information from t...

Coincident spikes have been implicated in vision-related processes such as feature binding, gain modulation, and long-distance communication. The source of these spike-time correlations is unknown. Although several studies have proposed that cortical spikes are correlated based on stimulus structure, others have suggested that spike-time correlations reflect ongoing cortical activity present even in the absence of a coherent visual stimulus. To examine this issue, we collected single-unit recordings from primary visual cortex (V1) of the anesthetized and paralyzed prosimian bush baby using a 100-electrode array. Spike-time correlations for pairs of cells were compared under three conditions: a moving grating at the cells' preferred orientation, an equiluminant blank screen, and a dark condition with eyes covered. The amplitudes, lags, and widths of cross-correlation histograms (CCHs) were strongly correlated between these conditions although for the blank stimulus and dark condition, the CCHs were broader with peaks lower in amplitude. In both preferred stimulus and blank conditions, the CCH amplitudes were greater when the cells within the pair had overlapping receptive fields and preferred similar orientations rather than nonoverlapping receptive fields and different orientations. These data suggest that spike-time correlations present in evoked activity are generated by mechanisms common to those operating in spontaneous conditions.