Integration of visual features in the early visual system of the mouse
Authors:   , Cazemier Johanna Leopoldine 
Published: August 2024

This thesis focused on two brain areas in the visual system of the mouse: superior colliculus (SC) and the primary visual cortex (V1). Both these areas belong to the early visual system and encode features like luminance contrast and orientation. The ability to encode such simple features is highly relevant: first of all, this information is necessary to compute representations of more complex visual features. Secondly, even simple features can trigger behaviours. In this thesis, we aimed to answer some fundamental questions regarding feature integration in the early visual system.

Figure-ground modulation is the phenomenon where visual neurons respond more strongly to visual patches that have a different orientation than their surround. Although SC shows figure-ground modulation, its role in object detection is not quite clear. In Chapter 2, we investigated the role of the superior colliculus in object detection in mice. When we inhibited sSC by activating its inhibitory neurons, the object detection performance of mice was decreased, indicating a causal role for the sSC. Electrophysiological recordings of sSC during task performance showed 1) an increased firing rate for contrast- and orientation-defined figures; 2) statistically significant decoding of the orientation- and phase-defined stimulus from the population firing rates; 3) higher discriminability of the neural code during correct vs. incorrect trials.

Superior colliculus is also known to be involved in visually-guided behaviours like freezing and fleeing. The brain mechanisms underlying habituation of visually-guided behaviours are still under investigation. In Chapter 3, we investigated the responses of SC upon repeated exposure to a moving dot inside and outside of the receptive field of the recorded neurons. We discovered a novel type of feature integration in sSC, where neurons show adaptation to the out-of-RF dots. When we inhibited the wide-field neurons in SC during the adaptation protocol, the adaptation was strongly reduced, indicating that the wide-field neurons mediate the adaptation.

In Chapter 4, we turn to investigate feature integration in V1. Frequently, the tuning of neurons to certain (combinations of) features is non-linear. Such non-linear tuning can often be described by divisive normalization. It has been hypothesized that short-term depression of thalamocortical synapses may be responsible for some forms of divisive normalization in V1. To put this hypothesis to the test, we used a new in vivo knockout model for increased short-term depression. We assessed contrast tuning and cross-orientation suppression, and found no differences in between the wild-type and heterozygous knockout mice. We therefore conclude that short-term depression is unlikely to be responsible for these types of divisive normalization.

This thesis has explored feature integration in the early visual system of mice. The results concerning superior colliculus are in line with other recent studies that support a role for superior colliculus in complex information encoding and behaviour. V1 and SC work together with many other visual and non-visual brain areas. How all these processes are integrated is the big question of neuroscience.