Bernardes, R. | bioRxiv Science

bioRxiv2024

Joint contribution of adaptation and neuronal population recruitment to response level in visual area MT: a computational model

Adaptation is a form of short-term plasticity triggered by prolonged exposure to a stimulus, often resulting in altered perceptual sensitivity to stimulus features through a reduction in neuronal firing rates. Experimental studies have explored adaptation to bistable stimuli, specifically a stimulus comprising inward-moving plaids that can be perceived as either a grating moving coherently downward or two plaids moving incoherently through each other. Functional magnetic resonance imaging (fMRI) recordings have shown higher activity during incoherent perception and lower activity during coherent stimulus perception. There are two potential explanations for the underlying neural mechanisms: a weaker coherent stimulus response may result from stronger adaptation to coherent versus incoherent motion, or a stronger incoherent stimulus response could stem from the involvement of more neural populations to represent motion in more directions. Here, we employ a computational model of visual neurons with and without firing rate adaptation to test these hypotheses. By simulating the mean activity of a network of thirty-two columnar populations of visual area MT, each tuned to one direction of motion, we investigate the impact of firing rate adaptation on the blood-oxygen-level-dependent (BOLD) signal generated by the network in response to coherent and incoherent stimuli. Our results replicate the experimental curves both during and after stimulus presentation only when the model includes adaptation, highlighting the importance of this mechanism. However, our findings reveal that the response to incoherent motion is larger than the response to coherent motion for a wide variety of stimulus parameters and adaptation regimes, suggesting that the observed reduced response to coherent stimuli is most likely due to the activation of smaller neuronal populations, in alignment with the second hypothesis. Hence, adaptation and differential neuronal recruitment work together to give rise to the observed hemodynamic responses. This computational work sheds light on experimental results and enriches our understanding of the mechanisms involved in neural adaptation, particularly in the context of heterogeneous neuronal populations.

neuroscience↗

bioRxiv2023

Subtractive adaptation is a more effective and general mechanism in binocular rivalry than divisive adaptation

The activity of neurons is influenced by random fluctuations and can be strongly modulated by firing rate adaptation, especially in sensory systems. Still, there is an ongoing debate about the characteristics of neuronal noise and the mechanisms of adaptation, and even less is known about how exactly they affect perception. Noise and adaptation play central roles in binocular rivalry, a visual phenomenon where two images compete for perceptual dominance. Here, we investigated the effects of different noise processes and adaptation mechanisms on visual perception by simulating a model of binocular rivalry with Gaussian white noise, Ornstein-Uhlenbeck noise, and pink noise, in variants with divisive adaptation, subtractive adaptation, and without adaptation. By simulating the nine versions of the model for a wide range of parameter values, we find that white noise only produces rivalry when paired with subtractive adaptation and that subtractive adaptation reduces the influence of noise intensity on rivalry strength and introduces convergence of the mean percept duration, an important metric of binocular rivalry, across all noise processes. In sum, our results show that white noise is an insufficient description of background activity in the brain and that subtractive adaptation is a stronger and more general switching mechanism in binocular rivalry than divisive adaptation, with important noise-filtering properties. Author SummaryVisual neurons adapt to the environment by reducing the number of spikes evoked by a constant stimulus. They are also susceptible to random spikes produced by nearby neurons. These two phenomena, adaptation and noise, are essential features of brain activity and affect how we perceive the world. Although we know a great deal about the visual system, our understanding of the properties and mechanisms of neuronal noise and adaptation is still piecemeal, and even less is known about how these microscopic processes affect macroscopic behaviors. We shed light on this question by studying a bistable visual phenomenon called binocular rivalry, where two images compete for perception and where noise and adaptation play important roles. We simulated the activity of neurons involved in binocular rivalry to test different hypotheses about the statistics of neuronal noise and the mechanisms of adaptation. Our results reveal important differences between subtractive and divisive adaptation, suggesting that subtractive adaptation is a stronger switching mechanism in binocular rivalry and an effective noise filter. Our simulations also show the fundamental distinction between noise with and without temporal correlation, supporting the correlated noise hypothesis.

neuroscience↗