Thesis title: Description of Visual and Non-Visual Components of Saccade and Blink Responses of Retinal Ganglion Cells in Awake Mice
Saccades and Blinks have been studied for a long time and of specific interest is
the fact that both are not consciously perceived. This is especially surprising when
considering the strong perceptual impact of comparable external events such as
a camera video quickly sweeping from one point to another or a brief flicker of
electric light during a short power drop. Absence of these perceptual impacts are
attributed to suppression mechanisms at different levels of the hierarchical visual
processing chain. I will use the term suppression to describe the attenuated sen-
sory responses and omission to describe the (conscious) perceptual suppression of
these events. Both phenomena have been studied in great detail mainly in the case
of saccades. Here, saccadic suppression is thought to be mainly driven by visual
phenomena such as visual masking at the early visual processing stages while mo-
tor signals, generally called corollary discharge, are integrated at higher cortical ar-
eas. Both visual and motor components combined are then causing the omission of
saccades from conscious perception. A series of recent publications has described
a potential mechanism causing saccadic visual masking based on the interaction
of cone and bipolar cells and described a similarity of these dynamics with the
dynamics for saccadic omission in human psychophysics experiments, suggesting
that visual masking in the retina is the main driving factor of saccadic omission.
Another publication recently found that non-visual inputs from the pulvinar al-
low for distinction between internally and externally caused saccadic-like motion
at the level of mouse primary visual cortex (V1), showing that motor signals are in-
corporated into the saccade response earlier than previously thought. We are still
lacking a description of what is happening during saccades and blinks at the level
of the retina in the awake in-vivo condition. Previous studies have indicated that
RGC activity in-vivo can differ significantly from what has been described ex-vivo.
In this thesis I am describing the direct impacts of saccades and blinks on the
activity of mouse retinal ganglion cells (RGCs) in-vivo. I found that saccade re-
sponses have similar dynamics regardless of the visual context, be it different forms
of direct visual stimulation or passive contexts such as saccades crossing high-
contrast edges in the visual scene. Furthermore many RGCs exhibited a strong
selectivity regarding the direction of saccades that is not explained by visual direc-
tion selectivity to stimuli of moving gratings. The responses to saccades differed
significantly from externally simulated saccades (pseudo-saccades). These find-
ings suggest that there are non-visual factors driving saccade responses in RGCs
in the awake mouse. In comparison I found that blink responses match the ex-
pected behavior of RGCs responding to a transient OFF stimulus. Blinks but not
saccades thus cause responses that can be explained by the visual properties of
RGCs. My results pose the question if motor-related signals influence the visual re-
sponse already at the level of RGCs and suggest that RGCs can distinguish between
internally and externally generated motion in-vivo. Our experimental paradigm
also lets us shine light on the question of saccadic suppression at the level of the
retina: Unless visual masking is completely independent of the initial response
to saccades, visual context-independence of saccade responses suggest that retinal
saccadic suppression can not be fully described by visual responses alone.