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Krishnamoorthy, Vidhyasankar (2012): Encoding of saccadic scene changes in the mouse retina. Dissertation, LMU München: Faculty of Biology
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Abstract

The task of the visual system is to extract behaviourally relevant information from the visual scene. A common strategy for most animals ranging from insects to humans is to constantly reposition gaze by making saccades within the scene. This ‘fixate and saccade’ strategy seems to pose a challenge, as it introduces a highly blurred image on the retina during a saccade, but at the same time acquires a ‘snapshot’ of the world during every fixation. The visual signals on the retina are thus segmented into brief image fixations separated by global motion. What is the response of a ganglion cell to ‘motion blur’ caused by a saccade, and how does it influence the response to subsequent fixations? Also, how does the global motion signal influence the response dynamics of a ganglion cell? In this thesis, we addressed these questions by two complementary approaches. First, we analysed the retinal ganglion cell responses to simulated saccades. We analysed two important aspects of the response - 1) response during a saccade-like motion, 2) response to fixation images. For about half of the recorded cells, we found strong spiking activity during the saccade. This supports the idea that the retina actively encodes the saccade and may signal the abrupt scene change to downstream brain areas. Furthermore, we characterized the responses to the newly fixated image. While there appears to be only little influence of the preceding motion signal itself on these responses, the responses depended strongly on the image content during the fixation period prior to the saccade. Thus, saccadic vision may provide ‘temporal context’ to each fixation, and ganglion cells encode image transitions rather than currently fixated images. Based on this perspective, we classified retinal ganglion cells into five response types, suggesting that the retina encodes at least five parallel channels of information under saccadic visual stimulation. The five response types identified in this study are as follows: 1) Classical Encoders - Response only to preferred stimuli; 2) Offset Detectors - Response only to the saccade; 3) Indifferent Encoders - Response to all fixated images; 4) Change Detectors - Response only when the new image after the saccade differs from the previous image; 5) Similarity Detectors - Response only when the new image after the saccade is similar to the previous image. Second, we analysed the influence of global motion signals on the response of a retinal ganglion cell to the stimulus in its receptive field. The stimulus beyond the receptive field is designated as remote stimulus. We chose simple stimulus that represent various configurations used in earlier studies, thus allowing us to compare our results. We show that the remote stimulus both enhances and suppresses the mean firing rate, but only suppresses the evoked activity. Furthermore, we show that the remote stimulus decreases the contrast sensitivity and modifies the response gain. Thus, the ganglion cells encode the stimulus in relation to the whole scene, rather than purely respond to the stimulus in the receptive field. Our results suggest that the global motion signals provide ‘spatial context’ to the response of the stimulus within the receptive field.