Modelling the temporal response properties of an insect small target motion detector

Abstract

Insects are an excellent model system for investigating computational mechanisms evolved for the challenging task of visualising and tracking small moving targets. We examined a well categorised small target motion detector (STMD) neuron, the dragonfly centrifugal STMD 1 (CSTMD1). This neuron has an unusually slow response onset, with a time course in the order of hundreds of milliseconds. A parsimonious explanation for this slow onset would be temporal low-pass filtering. However other authors have dismissed this and instead proposed a facilitation mechanism derived from second order motion detectors. We tested the spatial locality of response to continuous motion on non-contiguous paths and found spatial discontinuities in otherwise continuous motion reset the neuronal response. We modelled an array of elementary motion detectors (EMDs) in the insect visual pathway. We found that whilst individual components of the response can be explained simply by modifying the properties of the EMDs, the neurons response considered as a whole requires further elaborations within the system such as the proposed second order motion pathway.

Correlation between OFF and ON Channels Underlies Dark Target Selectivity in an Insect Visual System

“…However, CSTMD1 is a neuron that responds very quickly in absolute terms, with a response latency Ͻ50 ms (Nordström et al, 2011) and tuning to high target velocity (Geurten et al, 2007;Dunbier et al, 2012). Our modeling of its response kinetics suggests that the facilitation must be the result of a higher-order property within the STMD pathway (e.g., attentional modulation) in response to slower moving features, rather than any inherent sluggishness in the time constants of the underlying motion pathway (Dunbier et al, 2011(Dunbier et al, , 2012. Thus, some caution is still in order in attributing the observed supralinear response characteristics of CSTMD1 to an underlying elementary correlation between local OFF and ON channels, versus a mechanism that upregulates attention for the highly salient target stimulus.…”

“…ter (Wiederman and O'Carroll, 2011). To account for some ON and OFF edge excitation, we include linear terms in our phenomenological model.…”

“…We implemented a model for an ESTMD in MATLAB as described in detail previously (Wiederman et al, , 2009(Wiederman et al, , 2010. This model accounts for the size and velocity tuning observed in insect STMDs and is robust in the presence of background clutter, even without relative motion cues , as observed in physiological responses of STMDs (Nordström and O'Carroll, 2006;Wiederman and O'Carroll, 2011). Parameters are based on fly physiology, with modification to match velocity tuning of the dragonfly (Geurten et al, 2007).…”

“…1A) have previously been used to model spatiotemporal tuning of STMD neurons (Geurten et al, 2007;Dunbier et al, 2011Dunbier et al, , 2012, these models lack the size selectivity that characterizes STMDs. We therefore previously proposed the ESTMD (Fig.…”

“…One such class, small target motion detector (STMD) neurons, has been characterized in the lobula and lateral midbrain of both dragonflies (O'Carroll, 1993;Geurten et al, 2007) and dipteran flies (Nordström and O'Carroll, 2006;Barnett et al, 2007). Typical STMDs respond robustly to small moving targets (subtending ϳ1-3°), even against background clutter (Nordström and O'Carroll, 2006;Wiederman and O'Carroll, 2011).…”