Adaptation in temporal cortex: a combined optogenetic-electrophysiology study

A common feature of all sensory modalities is their ability to adapt. Adaptation is a crucial component of the nervous system, as it allows organisms to respond to changes in the environment. The work exposed in this dissertation will focus on one particular kind of rapid sensory adaptation, which is the decrease in neural response when a stimulus is repeated, commonly known as repetition suppression. Repetition suppression has aroused interest because of the widespread use of adaptation paradigms in human fMRI, especially in ventral visual stream areas, in normal and patient populations, and that makes it important to understand the neural mechanisms of adaptation in those areas. The manuscript consists of a collection of studies conducted in the visual system of macaques in the Laboratory for Neuro- and Psychophysiology of KU Leuven. Repetition suppression is ubiquitous in monkeys' inferior temporal cortex (IT) neurons, the end stage of the ventral visual stream. Therefore, we targeted the area with microelectrode recordings in the attempt to answer some of the (still many, despite the relevance of the topic) open questions on the mechanisms underlying the generation of this form of adaptation. The title we choose for the dissertation is "Adaptation in temporal cortex: a combined optogenetic-electrophysiology study" as it indicates the two different technical methodologies we combined in the course of our studies. Optogenetics was employed to allow the direct perturbation of neural activity with high temporal and spatial precision and, simultaneously, electrical artifact-free recordings of action potentials. Performing optogenetics in macaques offers great opportunities but, on the other hand, presents several challenges and technical limitations. In the first chapter of the manuscript, we described the state of the art of optogenetics in non-human primates. In chapter 2, we combined optogenetic stimulation with electrophysiological recordings to determine the contribution of a spike-frequency dependent adaptation mechanism to the generation of repetition suppression. In fact, by depolarizing directly IT neurons, bypassing bottom-up stages of visual processing, we could establish the role of this fatigue-related mechanism. The results demonstrated that spike-frequency dependent adaptation has only a minor, if any, role in the generation of repetition suppression. In chapter 3, we investigated a different possibility, which is the contribution to the adaptation of input from 'lower' visual areas, upstream of IT across the ventral visual pathway. To determine that, we benefit from one of the distinctive features of IT neurons: their large receptive field size compared to upstream areas. The results indicated that at least part of repetition suppression could not be inherited from bottom-up inputs and must therefore arise within the IT network, or from top-down modulation of the neural activity.

Publication year:2022

Accessibility:Open