Neural correlates of non-retinotopic visual processing and light adaptation

Abstract

In the main part of this thesis, I investigate the neural correlates of nonretinotopic processing. We might intuitively think of the visual representation of the world as retinotopic, as is the case for the organization of most visual brain areas, but in fact our perception is usually not retinotopic. Consider for example a bicycle passing by with a reflector on one of its wheels. The reflector appears to move in a circular or prolate cycloidal orbit. It’s true motion trajectory, however, is a curtate cycloid. It seems that we perceive the reflector motion relative to the bicycle. The bicycle thus serves as a moving reference system for computing the motion of its constituent parts. Only little is known about the neural correlates of this type of non-retinotopic processing. Here, using EEG, I investigate the temporal dynamics of the encoding of non-retinotopic motion relative to a moving reference system. The results indicate that the motion of the reference system is discounted from the motion of the part already from 120 ms after stimulus onset and that the non-retinotopic representation dominates throughout the rest of the visual processing. Further, I use fMRI to show that the earliest area in the visual hierarchy holding non-retinotopic representations is the human motion processing complex (hMT+). In the second part of the thesis, the EEG correlates of light adaptation are investigated. Light adaptation is crucial for coping with the large range of ambient luminance values in our environment. We first confirm the known adaptation effect; the overall response to light flashes decreases substantially after adaption to bright light as compared to dim light. Furthermore, this adaptation effect depends on the wavelength of the light, being most pronounced for red light as compared to white, green and blue light. In addition, at short latencies (less than 100 ms) responses to light flashes are stronger after bright than after dim light adaptation for all colors except red. Our results show that even very short-latency EEG responses are differentially affected by different wavelengths of light.