Associative learning is a complex phenomenon, and some responses to compound stimuli that are commonly found in vertebrates undergoing associative learning are referred to in the literature as higher-order forms of associative learning: blocking, overshadowing, sensory pre-conditioning (SPC) and second-order conditioning (SOC). Blocking has been discussed above. In overshadowing, a stimulus that can normally condition a response when presented as a CS on its own will acquire a much weaker association with the US when presented with another, stronger, stimulus (e.g., a more intense tone or light). Thus one stimulus 'overshadows' the other. Sensory preconditioning means that reinforcement (i.e. an associated reward or punishment) of stimulus A after unreinforced exposure to a compound AB also leads to responses to stimulus B, while second-order conditioning means that reinforcement of stimulus A followed by unreinforced exposure to a compound AB also leads to responses to stimulus B.

In response to a query on whether blocking has been confirmed yet in any invertebrates, Brembs (personal email, 22 December 2002) wrote:

As far as I have heard, the jury is still out, whether there is blocking, although those that have found it still claim that there is no dispute about their data. There are a few finds, but alternative explanations have not been ruled out, yet. So far, blocking, if it is there, is definitely not as universal and general as in vertebrates, at the least.

In response to a query as to whether there had been any research that settled the question of whether blocking occurs in the nematode worm C. elegans, I received this reply from a neurobiologist who specialises in this area (Nuttley, personal email, 18 July 2003):

As for blocking, a student has been looking at that and although the trends are all encouraging and the data looks pretty good, we fall just short of making stats. A few changes to the protocol and I think we will be able to demonstrate blocking, but so far no.

Nuttley has investigated the occurrence of two forms of higher-order associative learning - sensory preconditioning and second-order conditioning - in the nematode worm C. elegans, with mixed success (personal email, 18 July 2003). Sensory preconditioning was observed, while second-order conditioning was not. Nuttley believes that the likely explanation is extinction - the lack of reinforcement following exposure to the compound AB extinguished the worms' conditioned response to a reinforcement of A, so they failed to respond to B as well.

The mixed results, coupled with the rapidity with which the original reinforced response was extinguished, hardly inspires confidence that we can talk about mindful behaviour here.