Takahisa M. Sanada

Representation of 3-D surface orientation by velocity and disparity gradient cues in area MT

Neural coding of the three-dimensional (3-D) orientation of planar surface patches may be an important intermediate step in constructing representations of complex 3-D surface structure. Spatial gradients of binocular disparity, image velocity, and texture provide potent cues to the 3-D orientation (tilt and slant) of planar surfaces. Previous studies have described neurons in both dorsal and ventral stream areas that are selective for surface tilt based on one or more of these gradient cues. However, relatively little is known about whether single neurons provide consistent information about surface orientation from multiple gradient cues. Moreover, it is unclear how neural responses to combinations of surface orientation cues are related to responses to the individual cues. We measured responses of middle temporal (MT) neurons to random dot stimuli that simulated planar surfaces at a variety of tilts and slants. Four cue conditions were tested: disparity, velocity, and texture gradients alone, as well as all three gradient cues combined. Many neurons showed robust tuning for surface tilt based on disparity and velocity gradients, with relatively little selectivity for texture gradients. Some neurons showed consistent tilt preferences for disparity and velocity cues, whereas others showed large discrepancies. Responses to the combined stimulus were generally well described as a weighted linear sum of responses to the individual cues, even when disparity and velocity preferences were discrepant. These findings suggest that area MT contains a rudimentary representation of 3-D surface orientation based on multiple cues, with single neurons implementing a simple cue integration rule.

Contributions of excitation and suppression in shaping spatial frequency selectivity of V1 neurons as revealed by binocular measurements

Neurons in the early visual cortex are generally highly sensitive to stimuli presented to the two eyes. However, the majority of studies on spatial and temporal aspects of neural responses were based on monocular measurements. To study neurons under more natural, i.e., binocular, conditions, we presented sinusoidal gratings of a variety of spatial frequencies (SF) dichoptically in rapid sequential flashes and analyzed the data using a binocular reverse correlation technique for neurons in cat area 17. The resulting set of data represents a frequency-domain binocular receptive field from which detailed selectivities, both monocular and binocular, could be obtained. Consistent with previous studies, the responses could generally be explained by linear summation of inputs from the two eyes. Suppressive responses were also observed and were delayed typically by 5-15 ms relative to excitatory responses. However, we have found more diverse nature of suppressive responses than those reported previously. The optimal suppressive frequency could be either higher or lower than that of the excitatory responses. The bandwidth of SF tuning of the suppressive responses was usually broader than that of the excitatory responses. Cells with lower optimal SFs for suppression tended to show high optimal SFs and sharp tuning curves. The dynamic shift of optimal SF from low to high SF was accompanied by suppression with earlier onset and higher peak SF or later onset and lower peak SF than excitation. These results suggest that the suppression plays an essential role in generating the temporal dynamics of SF selectivity.

Supranormal orientation selectivity of visual neurons in orientation-restricted animals.

Altered sensory experience in early life often leads to remarkable adaptations so that humans and animals can make the best use of the available information in a particular environment. By restricting visual input to a limited range of orientations in young animals, this investigation shows that stimulus selectivity, e.g., the sharpness of tuning of single neurons in the primary visual cortex, is modified to match a particular environment. Specifically, neurons tuned to an experienced orientation in orientation-restricted animals show sharper orientation tuning than neurons in normal animals, whereas the opposite was true for neurons tuned to non-experienced orientations. This sharpened tuning appears to be due to elongated receptive fields. Our results demonstrate that restricted sensory experiences can sculpt the supranormal functions of single neurons tailored for a particular environment. The above findings, in addition to the minimal population response to orientations close to the experienced one, agree with the predictions of a sparse coding hypothesis in which information is represented efficiently by a small number of activated neurons. This suggests that early brain areas adopt an efficient strategy for coding information even when animals are raised in a severely limited visual environment where sensory inputs have an unnatural statistical structure.

Receptive field structures of transcallosally connected neurons in the cat's visual cortex

It is known that when we search for a target object in a complex scene, we tend to make saccades to the visual features that stand out from the background (exogenous saccades). However, it seems less likely that we find the target object solely based on exogenous saccades during visual search. To investigate additional factors (endogenous components) involved in the saccades, we asked human subjects to perform visual search tasks in which a target object was chosen from one of three object categories that were familiarized to the subject beforehand. We then calculated the category specific information content of each local feature, centered on the saccade end-points. We found that the end points of saccades tend to provide more information about the category of the target object than randomly selected features. Thus, a determining factor for saccades during visual search could be visual features specific to the target object category.