Temporal dynamics of sensitivity control at high and low spatial frequencies

Introduction: It is widely accepted that the Magnocellular and Parvocellular pathways in the visual system have different functional properties. Presumably, the transient response of the Magno pathway and the sustained response of the Parvo pathway underlie the fast and slow responses to low and high spatial frequency patterns respectively. However, a fast response does not necessarily mean a fast sensitivity adjustment. In this study, we tested whether high spatial frequency channels (supported by the Parvo pathway) adjust their sensitivity slower than low spatial frequency channels following orientation-specific adaptation. Methods: Two observers adapted to gratings either at 1 cpd or 15 cpd at 80% contrast for 5 minutes. The adapting grating was oriented either at a 45 or 135 degree tilt in different sessions. Immediately after the 5-minute adaptation, test gratings were presented at the same spatial frequency as the adapting grating, but alternated between the adapted orientation and the orthogonal orientation. Over a three-minute period following adaptation, subjects were instructed to make as many threshold measurements as possible. These threshold measurements give a dynamic picture of the sensitivity change over time. Results and conclusion: Contrast thresholds were raised immediately after adaptation, and much more so for test gratings with the same orientation as the adapting grating. The ratio between the same orientation and the orthogonal orientation test threshold measurements can be taken as the index of the adaptation effect. Results show that the adaptation effect decays much slower at 15 cpd than that at 1 cpd. In other words, it takes longer for the high spatial frequency channel to recover its sensitivity following adaptation. We conclude that the Magno pathway not only has a more transient response than the Parvo pathway, but it also has a faster sensitivity adjustment mechanism.