The Interplay of Inhibitory and Excitatory Plasticity in Visual Cortex

Memory recall is a mystery of cognition. At will, or via an environmental trigger, past
events can be evoked in the mind's eye before again disappearing completely. One theory
is that during learning, a memory trace or "engram" may form in the brain, comprised of
interconnected excitatory neurons, which somehow encapsulates what has been learnt
and is reactivated during recall (Reijmers et al., 2007). After engram formation, there may
be potentiation of inputs from inhibitory neurons onto the engram cells, which keep the
engram quiescent, available for recall upon disinhibition (Barron et al., 2017). Such
potentiation of inhibitory inputs could arise homeostatically (Barron et al., 2017, Agnes
and Vogels, 2024), to maintain the firing rates of neurons within set ranges and sustain the
brain's excitatory/inhibitory balance. Notably, reversible inhibition of neuronal activity
when it is not useful has been experimentally demonstrated in various species in
paradigms of behavioural habituation (Barron et al., 2017, Trisal et al., 2022); this principle
could form the basic apparatus for broader memory storage and retrieval.
Stimulus-specific response potentiation is a phenomenon whereby visually-evoked local
field potentials (LFP) in mouse primary visual cortex (V1) potentiate when a particular
stimulus is repetitively presented over multiple days (Frenkel et al., 2006). Visually-evoked
potentials are thought to represent both excitatory and inhibitory synaptic currents, and
their potentiation may reflect mechanisms of basic implicit memory formation (Frenkel et
al., 2006). Gavornik et al. (2014) demonstrated that repetitive presentation of a short
sequence of visual stimuli led to enhanced evoked potentials in mouse V1 versus an
untrained sequence of the same stimuli. In an omission paradigm, LFP measurements
indicated that the mice developed a predictive representation of the upcoming (omitted)
element of a familiar sequence, suggestive of memory recall. We will perform experiments
on mice to investigate how visual experience drives plasticity in V1.