"Behavioral analyses of the visual functions of snakes: confronting the structural bases of the retina with patterns of reflexive and defensive responses."

• Grant number: 25/08162-7
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• Start date: July 01, 2025
• End date: June 30, 2028
• Field of knowledge: Humanities - Psychology - Physiological Psychology
• Principal Investigator: Dora Selma Fix Ventura
• Grantee: Juliana Hiromi Tashiro
• Host Institution: Instituto de Psicologia (IP). Universidade de São Paulo (USP). São Paulo , SP, Brazil
• Associated research grant: 22/00191-0 - Mechanisms, genetics and performance of the visual system: clinical research in humans and animal experiments, AP.TEM

Abstract

For over a decade, our research group has employed snakes as a model system to investigate the evolutionary strategies of the vertebrate visual system (Bittencourt et al., 2019; Hauzman et al., 2014, 2017, 2018, 2021; Hauzman, 2020; Tashiro et al., 2022). Our work centers on the analysis of opsin genes involved in color vision, as well as the morphological characteristics of the retina across various species, with an emphasis on the density and distribution of photoreceptors and ganglion cells. Retinal cell distribution is inherently non-uniform, and its topographic organization is partially preserved, thereby enabling spatial information from distinct regions of the visual field to be maintained from the retina to cortical processing areas (Udin & Fawcett, 1988; Triplett et al., 2009). Retinal specializations-regions of elevated cell density-represent areas of the visual field with heightened ecological importance. These specializations can be categorized as either an area centralis or a visual streak.In snakes, variation in the type and spatial configuration of retinal specializations has been correlated with habitat preferences (Hart et al., 2012; Hauzman et al., 2014; Tashiro et al., 2022), diel activity patterns, and hunting strategies (Hauzman et al., 2018; Tashiro et al., 2022). For example, Hauzman et al. (2014) suggested that a ventrally located area centralis in the terrestrial species Philodryas patagoniensis may facilitate the detection of aerial predators, whereas a horizontally aligned visual streak in the arboreal P. olfersii could enhance navigation through arboreal environments and improve the detection of prey or predators at the same vertical level. These hypotheses linking retinal topography to visual ecology have not yet been empirically tested in snakes. In rodents, such as mice-which possess a ventral area centralis-the presentation of looming stimuli from above reliably induces escape responses and shelter-seeking behavior (De Franceschi et al., 2016; Yang et al., 2020), while laterally moving stimuli typically elicit a freezing response (De Franceschi et al., 2016). The present project aims to determine whether similar correlations exist between retinal specializations and visually guided behavioral responses in snakes.Concurrently, we will assess optokinetic nystagmus (OKN) reflex responses to determine the visual acuity of various snake species and compare these results with acuity estimates derived from anatomical data. Previous studies have estimated visual acuity in snakes based on peak ganglion cell density and eye focal length (Hart et al., 2012; Hauzman et al., 2014, 2018; Tashiro et al., 2022). However, only two behavioral studies to date have measured snake visual acuity using OKN paradigms (Zamore et al., 2020; Gomez et al., 2023). By comparing OKN-based acuity measurements with those estimated from anatomical parameters in the same species, this project will evaluate methodological reliability and deepen our understanding of visual function in snakes.Objectives:1.To characterize behavioral response patterns of snakes to visual stimuli presented in the upper and lateral visual fields and correlate these patterns with retinal topography.2.To evaluate visual acuity in multiple snake species using optokinetic nystagmus (OKN) and compare the findings with anatomically derived acuity estimates based on eye and retinal morphology.