Article Figures & Data
Figures
- Figure 1.
Correlation-based models of elementary motion detection. A, The HR-EMD correlates spatially separated luminance signals (S1 and S2) with one path delayed in time. The subtraction of a mirror symmetric unit provides opponent direction selectivity to the EMD. B, In the 2-detector EMD (Eichner et al., 2011), like channels are correlated with one another: ON with ON (L1) and OFF with OFF (L2). L1 and L2 refer to types of laminar monopolar cell. C, The 6-detector EMD (Clark et al., 2011) has more complex combinatorial interactions between channels, including correlation between spatially separated OFF and ON signals. D, In the ESTMD model, center-surround antagonism of ON and OFF channels is followed by the correlation of the delayed OFF with the undelayed ON signal.
- Figure 2.
Visual stimuli presented to the dragonfly during electrophysiological recordings from STMD neurons. Upper rows of each combination depict a snapshot in time of the 2 d display on which stimuli were presented (not to scale), whereas lower rows are space-time plots for the luminance change along the line that the feature moves. A, High contrast OFF and ON edges and a single black target are drifted from the bottom to the top of the display. B, Low contrast versions of both edges and target, with the addition of a “target” composed of two OFF edges. C, On a mean background, ON and OFF edges are composed from both mean background to light and mean background to dark transitions. Both light and dark targets are also presented.
- Figure 3.
Dark target selectivity in the dragonfly brain. A, An individual example of CSTMD1 responses to targets of varying contrast moving on a mid-luminance background (mean ± SEM, 5 trials). Responses strengthen as negative contrast polarity of the target increases (dark); however, positive contrast (light) targets elicit little response. Across 5 neurons (inset graph, mean ± SEM), CSTMD1 displays dark target selectivity, responding significantly more (p = 0.009) to high contrast black targets (CWeber = −1) than to high contrast white targets (CWeber = 1), which are indistinguishable from spontaneous activity. B, Four unidentified neurons in H. tau produce robust responses to targets of increasing dark contrast but not to light contrast (left ordinate: black squares and gray diamonds; right ordinate: gray triangles and white circles). These unidentified feature-selective neurons exhibit varying size selectivity (inset graph).
- Figure 4.
Target responses are more than the linear combination of responses to ON and OFF edges. A, Two examples of individual CSTMD1 responses to high contrast stimuli drifted through the receptive field. In the first example, CSTMD1 exhibits little response to either ON or OFF edges, and the response to targets is larger than a linear combination of the edge responses (mean of 20 trials). In the second example, CSTMD1 responds to high contrast ON and OFF edges, although both are weaker than target responses (mean of 16 trials). B, The response of an unidentified STMD neuron to the same stimuli exhibits a similar supralinearity (mean of 20 trials). C, Pooled CSTMD1 responses (68 trials >5 neurons) show that median target responses are larger than the linear combination of responses to ON and OFF edges (Target vs ON + OFF, p = 0.001, Mann–Whitney U). In some trials, edge responses evoke responses from CSTMD1, in particular to the OFF stimulus. D, CSTMD1 responses to lower contrast target and edge stimuli (CWeber = −0.6) and a “double” OFF edge stimulus (white to gray to black). An example CSTMD1 response to the lower contrast stimuli exhibits strong target responses and weak response to either ON or OFF edges. Furthermore, the double OFF/OFF stimulus elicits minimal response (average of 12). E, CSTMD1 target responses are greater than any linear combination of the ON and OFF edge responses (16 trials >2 neurons).
- Figure 5.
CSTMD1 responses to low contrast edge and target stimuli A, Regardless of the edge transition with respect to the background (i.e., ON1, dark to mean; or ON2, mean to light; OFF1, mean to dark; or OFF2, light to mean), CSTMD1 responses in a further two neurons (12 trials total) are much stronger to the dark target. These dark target responses are much larger than those to the light target or the linear combination of either of the ON and OFF edge combinations. B, CSTMD1 responses to low contrast dark targets are plotted against the linear sum of ON and OFF responses (matched at the same receptive field location). Solid line indicates a linear combination (slope of 1); dashed line indicates best fit (slope = 1.9 ± 0.4 [95% CI]).
- Figure 6.
An ESTMD model includes terms for separate ON and OFF channels, as well as a facilitatory correlation between the delayed OFF and undelayed ON signals. a · [ON] + b · [OFFdelayed] + c · [ON] · [OFFdelayed]. We varied a–c coefficients and examine their pooled effect on each distribution of ON edge, OFF edge, and target responses. All model variants predict supralinear target responses, A, High contrast stimuli and more sustaining high-pass filtering (τ = 200 ms) predict either or both ON and OFF edge responses. B, High contrast and more transient high-pass filtering (τ = 100 ms) produces larger OFF edge responses, as observed in experiments (compare B with Fig. 4C). C, For lower contrast stimuli, the ESTMD model predicts minimal ON or OFF edge responses, again matching the physiological data (compare C,D with Figs. 4E and 5A).
- Figure 7.
Edges (ON and OFF) and dark targets serve as input to model variants of the EMD as well as the ESTMD model. These include both the 2-detector EMD, which sums channels of ‘like” correlations (Eichner et al., 2011) and the 6-detector EMD, which includes terms between ON and OFF correlations (Clark et al., 2011). Each of these versions is modeled with and without strong surround antagonism. For each model variant, output is normalized to the sum of the OFF edge, ON edge, and target output values. A, Without surround antagonism, both the 2- and 6-detector models predict a sublinear response to targets rather than the supralinearity observed in CSTMD1 (Figs. 4C,E and 5). B, Model outputs do not change when surround antagonism is included in the model variants. C, In comparison, an STMD model matches the physiological data with target responses much larger than the linear combination of ON and OFF edge responses. D, EMD and STMD model output in response to targets of varying extent (in the direction of travel). The ESTMD model is more responsive to small targets, and the output of the OFF and ON correlation is inherently tuned to the separation of the OFF and ON edges in the direction of travel (i.e., velocity/width tuned).
