Psychophysical indices of visual cortex functioning
Our thoughts, emotions and decision-making are all based on neurophysiological processes. The neuronal underpinnings of these complex psychological phenomena are, however, difficult to investigate. This is more feasible in the case of basic sensory functions. Psychophysical paradigms allow investigation of neurophysiological processes without measuring brain activity. In MEG-Center we implemented several psychophysical indices of visual cortex functioning that are listed below.
Motion direction discrimination: Spatial suppression and facilitation
Spatial Suppression (SS) and Spatial Facilitation (SF) are the phenomena of deteriorated (SS) or improved (SF) motion perception with increasing stimuli size (Fig.1). These two phenomena critically depend on E/I balance in the visual cortex. While SS dominates at high contrasts, SF is evident for the low contrast stimuli, due to the prevalence of inhibitory contextual modulations in the former, and excitatory ones in the latter case.
Fig.1 Stimuli used to study the phenomenon of Spatial Suppression.
We found that the SS was atypically reduced, while the SF was abnormally enhanced in children with ASD, pointing to increased E/I ratio in the visual cortex. In addition, the SF index in children with ASD correlated with severity of their autism symptoms and their poor registration abilities. A strong direct link between abnormally enhanced SF and autism symptoms suggests importance of the enhanced excitatory influences observed at the low levels of the visual processing in the ethiology of ASD (Sysoeva et al., 2017).
Line orientation discrimination: The ‘oblique effect’
People can discriminate the orientation of a line very precisely in relation to the cardinal (vertical and horizontal) axes, while their orientation discrimination sensitivity along the oblique axes is less refined. This difference in discrimination sensitivity along cardinal and oblique axes is called the “oblique effect.” The oblique effect is a basic feature of visual processing with an early developmental origin. It is to a great extent based on the experience-dependent tuning of orientation sensitive columns within the visual cortex. Local (lateral) inhibition in the visual cortex plays a major role in this tuning.
Fig.2 Stimuli used to study orientation discrimination and the ‘Oblique Effect’.
We found (Sysoeva et al., 2016), that oblique effect is reduced in children with Autism Spectrum disorder (ASD) as compared to their typically-developing peers. This reduction was explained by poor orientation sensitivity along the vertical axis in children with ASD, while their ability to discriminate line orientation along the oblique axis was unaffected. This deficit in sensitivity to vertical orientation may reflect disrupted mechanisms of early experience-dependent learning that takes place during the critical period for orientation selectivity. The disruption of such basic function in ASD might cause the problems in further steps of visual analysis and be indicative of general problems in inhibitory functions in ASD.