Temporal response properties to second-order visual stimuli in the LGN of cats

Visual stimuli occurring naturally are rich in instances of objects delineated from the backgrounds only by differences in luminance, which is called first-order stimuli, as well as those defined by differences of contrast or texture, referred to as second-order stimuli. The neuronal mechanism for processing second-order stimuli is still unclear. In this study, we compared the responses of cat LGN (lateral geniculate nucleus) cells to second-order stimuli at five temporal frequencies to their responses to first-order stimuli. Our results showed that most LGN cells can be evoked by second-order stimuli, and their firing rates to second-order stimuli decreased relative to first-order stimuli as temporal frequency increased from 0.5 to 8 Hz; moreover the ratio of a nonlinear to linear factor had a higher value in the responses to second-order stimuli than to first-order stimuli. We also found that the responses of Y-cells to second-order stimuli were significantly higher than the responses of X-cells, suggesting the Y-cells have a more important role in the processing of second-order stimuli. All these results reveal that first-order and second-order signals might be processed in separate 'streams' of the visual system.