Differential Processing Of Global And Local Features In Behaving Monkeys

An important issue of neuroscience is to understand how visual systems process retinal image signals to recognize objects. Over the past five decades, behavioral,neurophysiological and computational studies have led to a general model of visual information processing, which starts from extracting local features in the early visual pathways followed by feature binding processes to extract global features in the later visual pathways. However, this prevailing model has been challenged by accumulating evidence demonstrating that visual systems are more sensitive to global features than local features. For example, visual systems are more sensitive to the distinction between a ring and a circle in a global feature (a connected component with or without a hole)than the distinction between a square and a circle in local features (a connected component with or without vertices, edges and angles).The goal of the present study was to elucidate the neural mechanisms underlying processing global and local features in a non-human primate model. Monkeys were trained to make a saccade to an odd-ball target, which was different from the distractors(white circle) in color (red circle), local features (square), global features (ring) or their combinations. Saccade latencies were used to assess monkeys' sensitivity to distinctions in color, local features and the global feature. Four experiments were conducted to examine how visual systems process local and global features.In the first experiment, there are four small experiments, including one main experiment and three control experiments. In the main experiment, contrary to predictions based on the general model, we found that detecting the global features (ring)took much less time than detecting the local features (square), but took about the similar time as detecting color (red). These results showed that visual system might not get some global features by combining the local features, but directly extract the global feature from external information. In the first control experiment, we further knew that the results of main experiment in target eccentricity 4° were better than that in target eccentricity 7°, it indicated that visual system might process the global features more significantly in the fovea receptive field. The second control experiment told us that the faster processing of global features is not significantly related to the difference of luminous flux between oddball targets and distractor. In the last control experiment, it was just one special case that visual system processes the red information rapidly. Visual system processes other color information (green, blue and yellow) much more slowly than processing of red information.In the second experiment, we tried to study the impacts of target onset effect in main experiment of the first experiment. We found that target onset effect didn't affect the perception of global features, there was no significantly difference in saccade latencies to circle between target onset effect and target non-onset effect; while target onset effect affected the perception of red information clearly, visual system perceives much more slowly red information in target non-onset effect, it showed that fast perception of red information might be related to target onset effect; at the same time, there were complicate affects in the perception of local features, the performance of two monkeys were different.In the third experiment, we examined how distinctions in local and global features affect detecting distinction in color. A series of results indicated that detecting distinction in the global features facilitated detecting distinction in color information, but detecting distinction in the local features didn't affect it clearly. These results also showed that there was different ways and properties in processing of global and local features.At last, in the fourth experiment, we further examined the interactions between detecting distinctions in local and globe features. We found that, when taking ring as reference, if hole was changed to square-shape hole, saccade latencies to ring might increase and it indicated that processing of ring in visual system got slower; while if the outside of ring was changed to square, saccade latencies to ring might decrease and it indicated that processing of ring in visual system got faster. These results also showed that it was complicate that the interactions between detecting distinctions in local and globe features.Above the results, there are independent between processing of global (e.g. ring)and local (edges and angles, etc.) features in visual system, global feature might be not merely combined from processing of local features; at the same time, processing of global features is faster than local features; there are complicate interactions among processing of global, local and color features. Higher cortex might further combine global, local and color features from external information to form the cognition to subject and produce the selection and judgement.