The role of ocelli in cockroach optomotor performance

Abstract

Insect ocelli are relatively simple eyes that have been assigned various functions not related to pictorial vision. In some species they function as sensors of ambient light intensity, from which information is relayed to various parts of the nervous system, e.g., for the control of circadian rhythms. In this work we have investigated the possibility that the ocellar light stimulation changes the properties of the optomotor performance of the cockroach Periplaneta americana. We used a virtual reality environment where a panoramic moving image is presented to the cockroach while its movements are recorded with a trackball. Previously we have shown that the optomotor reaction of the cockroach persists down to the intensity of moonless night sky, equivalent to less than 0.1 photons/s being absorbed by each compound eye photoreceptor. By occluding the compound eyes, the ocelli, or both, we show that the ocellar stimulation can change the intensity dependence of the optomotor reaction, indicating involvement of the ocellar visual system in the information processing of movement. We also measured the cuticular transmission, which, although relatively large, is unlikely to contribute profoundly to ocellar function, but may be significant in determining the mean activity level of completely blinded cockroaches.

The effect of vertical extent of stimuli on cockroach optomotor response

“…Insects are known to move slower in dim light than in bright light (Baird et al, 2011;Reber et al, 2015;Honkanen et al, 2018), and our results support this observation. The more modest distance travelled in dim light (Fig.…”

“…When comparing Fig. 2C with the average velocities of unmanipulated cockroaches stimulated with full-screen grating (Honkanen et al, 2018), we can see that reducing the stimulus vertically to 90 deg reduced the average velocity at 500 lx by 30%, from 109 to 64 deg s −1 . Velocities obtained with the vertically shortest stimuli, 1 and 2 deg, are comparable to velocities of cockroaches whose compound eyes or compound eyes together with ocelli were covered, and which were most likely reacting to light reaching their photoreceptors through the cuticle (Honkanen et al, 2018).…”

“…Data from these experiments were not normally distributed (see also Honkanen et al, 2014Honkanen et al, , 2018, nor did they follow other assumptions of parametric tests. Therefore, the non-parametric Friedman test was used to inspect the statistical significance of stimulus effects.…”

“…The American cockroach (Periplaneta americana, Dictyoptera: Blattodea) has been a workhorse of vision research for decades. The structure of its apposition-type compound eyes is known (Butler, 1973a,b), its visually guided behaviour has been studied (Autrum and Stöcker, 1952;Roberts, 1965;Okada and Toh, 1998;Ye et al, 2003;Lent and Kwon, 2004), and special interest has been paid to its ability to see in low light (Heimonen et al, 2006(Heimonen et al, , 2012Salmela et al, 2012;Honkanen et al, 2014Honkanen et al, , 2018Immonen et al, 2014;Zhukovskaya et al, 2017).…”

“…In recent studies, we have used a spherical panoramic virtual reality (VR) setup (Takalo et al, 2012) to assess the visual capabilities of the American cockroach in dim light (Honkanen et al, 2014(Honkanen et al, , 2018. We have previously shown that their optomotor response persists down to light levels where each compound eye photoreceptor captures photons at a rate of 0.1 s −1 (Honkanen et al, 2014), and that ocelli improve the sensitivity of the optomotor response at low light intensities (Honkanen et al, 2018). Estimating the extent of spatial summation at different light intensities based on visual performance is possible when the average number of absorbed photons at each intensity is known.…”