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Phase-sensitive optical coherence tomography (OCT) is emerging as an imaging modality that detects functional changes in the retina. Besides imaging photoreceptor function, recently, functional changes in the inner plexiform layer (IPL) have been detected using full-field swept-source OCT. The IPL connects neuronal cells which are dedicated for processing different aspects of the visual information, such as edges in the image or temporal changes. A characteristic of signal processing in the IPL is that different aspects of the visual impression are only processed in very specific depths. Here, we present an investigation of these functional signals for different depths in the IPL with the aim to separate different properties of the visual signal processing. Therefore, we investigate the phase changes of three different sub-layers. Whereas the first two depths, closest to the ganglion cell layer, exhibit an increase in the optical path length, the third depth, closest to the bipolar cell layer, exhibits a decrease in the optical path length. Additionally, we found that the second or middle depth is sensitive to temporal changes, showing a maximum increase of the optical path length at a stimulation frequency of around 10 Hz. The results suggest that the responses from different cell types, which are sensitive to different features of the stimulation signal, can be distinguished by phase-sensitive OCT.
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