Stimulus intensity affects cortical processing mechanisms of visual information. There have been reports about the effect on cortical response latency and gamma-band activation caused by stimulus intensity changes in mammals. However, gamma-band is still controversial both its frequency ranges and roles of early activation, this might because of the complexity of mammalian forebrain structure. A Brain of relatively simple structure of cerebrum but well developed visual pathway is found in turtles. In this study, we performed experiments to see whether stimulus intensity modifies turtle’s visual responses, and also elicits gamma-band activity. Then, we investigated same responses in the absence of gamma-band to confirm the significance of gamma-band in processing early sensory information. The result showed that gamma-band has little effect on cortical response latency. As a result, early gamma-band activation might be considered as a part of sensory coding process. This result might be helpful those who are working on the relation between early sensory processing and gamma-band activity in mammals and other reptiles.