Olfactory memory capacity of the cricket Gryllus bimaculatus

Abstract

Olfactory learning in insects is a useful model for studying neural mechanisms underlying learning and memory, but memory storage capacity for olfactory learning in insects has not been studied. We investigate whether crickets are capable of simultaneously memorizing seven odour pairs. Fourteen odours were grouped into seven A/B pairs, and crickets in one group were trained to associate A odours with water reward and B odours with saline punishment for all the seven pairs. Crickets in another group were trained with the opposite stimulus arrangement. Crickets in all the groups exhibited significantly greater preference for the odours associated with water reward for all the seven odour pairs. We conclude that crickets are capable of memorizing seven odour pairs at the same time.

Figure 1

The time schedule for training (Tr, black bars) and preference test (PT, white bars). Crickets were kept in 12 h light–dark cycle and they received training and preference tests in the photophase. The training session consists of two differential conditioning trials to associate A odour (hatched bar) with water (white square) and B odour (shaded bar) with saline (black square) (for group 1 crickets), or to associate B odour with water and A odour with saline (for group 2 crickets). The intervals between trials were 5 min. The conditioning trials were performed for seven odour pairs at intervals of 90 min (Tr-1–7). Training was performed on 4 consecutive days. One day after the cessation of 4 day training, odour preference was tested for A1/B1, A2/B2, A3/B3 and A4/B4 pairs (PT-1–4). On the next day, preference was tested for the remaining A5/B5, A6/B6 and A7/B7 pairs (PT-5–7). Sources of odorants (inset) were: Miyako Kosho (Tokyo, Japan) for peppermint essence; Kyoritsu Shokuhin (Tokyo) for vanilla essence; Narizuka (Tokyo) for banana, orange, melon and maple essences; Meijiya (Tokyo) for strawberry and lemon essences and Asaoka (Tokyo) for all other essences.

Figure 2

Odour preferences of (a) group 1 crickets (n=31) that received conditioning trials to associate A odours with water and B odours with saline and (b) those of group 2 crickets (n=28) that received conditioning trials to associate B odours with water and A odours with saline for seven A/B odour pairs. The time spent at the source of A odour (hatched bars) and that spent at the source of B odour (shaded bars) for each of the seven odour pairs are shown as means+s.e. These were significantly different (p<0.001, WCX test) for all the seven pairs in both groups.