Investigating the comparative effects of adaptation on supra and infragranular layers with visual and acoustic stimulation in cat’s visual cortex

In the cat primary visual cortex (V1 or area17), neurons fundamentally respond to orientations of the objects in the outside world. Neurons responding to specific orientations form the orientation columns in V1. A neuron responding to a horizontal orientation will get optimally excited towards the outline of a horizontal object. This feature of the visual cortex known as orientation selectivity has been continuously explored to study the effects of adaptation. Following a training paradigm called adaptation, the same neuron that was inherently responding to the horizontal orientation will respond to an oblique orientation. In this thesis, we seek to examine the orientation tuning properties of individual neurons in superficial and deeper layers of V1 in different adaptation environments. Due to the extensive interconnectivity between V1 neurons, we hypothesize that not only do individual neurons get affected by adaptation paradigm, but the whole cortex is reprogramed.  To this aim, extracellular recordings were performed in conventionally prepared anesthetized cats. Neural activities were recorded simultaneously from layer 2/3 and layer 5/6 using a tungsten multichannel electrode. Neurons were adapted with a visual adapter (visual adaptation) and a repetitive sound (sound adaptation) in two different settings. Both types of adaptations were performed uninterrupted for 12 minutes. In both settings, sine-wave drifting gratings were presented to evoke responses in V1 and generate tuning curves from the recorded multiunit activity. The functional connectivity between the recorded neurons was revealed by computing cross-correlation between individual neuron pairs. In response to visual adaptation, layer 2/3 and 5/6 neurons displayed classical attractive and repulsive shifts. On comparing the behaviour of the neurons in either layer, an equivalent tendency was observed. Cross-correlograms between the spike trains of neurons in layers 2/3 and 5/6 revealed synchronized firing between the neurons suggesting coordinated dynamics of the co-active neurons and their functional connections. During sound adaptation, where the visual adapter was completely absent, shifts in the tuning curves were observed in either layer indicating a novel orientation selectivity. However, it is noteworthy that cells in both layers shifted in opposite directions indicating independent behaviour. V1 neurons might have an additional role besides processing visual stimuli. The visual neurons may have demonstrated multisensory properties when stimulated indirectly through neighbouring sensory regions. Our results indicate that primary visual neurons can be evoked by direct or indirect stimulation. The difference in the responses of layer 2/3 and layer 5/6 neurons towards the different adaptation environments indicate that neurons in V1 may behave similar or different towards the different sensory stimulus. This suggests that V1 responses are stimulus dependent. Additionally, the synchronized firing of layer 2/3 and layer 5/6 neurons towards visual adapter signify an existence of functional connectivity between the neuron pairs. Together, it can be summarised that visual neurons undergo an alteration of selectivity by building new orientation maps that ultimately potentiates plasticity within sensory regions that are highly suggestive of entire cortex being multisensory.

Keyphrases: adaptation, corrélation, Visual cortex, Plasticity, Orientation selectivity, Multisensory, Functional connectivity