Been, M., Jans, B., & De Weerd, P. (2011)
The Journal of Neuroscience, 31(42), 14944-14951.
Perceptual skills improve with daily practice (Fahle and Poggio, 2002; Fine and Jacobs, 2002). Practice induces plasticity in task-relevant brain regions during an “offline” consolidation period thought to last several hours, during which initially fragile memory traces become stable (Karni, 1996; Dudai, 2004). Impaired retention of a task if followed by training in another task is considered evidence for the instability of memory traces during consolidation (Dudai, 2004). However, it remains unknown when after training memory traces become stable and resistant against interference, where in the brain the neuronal mechanisms responsible for interference are localized, and how these mechanisms produce interference. Here, we show in human participants a strong interference between two visual skill-learning tasks for surprisingly long time intervals between training periods (up to 24 h). Interference occurred during asymptotic learning, but only when stimuli were similar between tasks. This supports a strong contribution to interference of low-level visual cortical areas (Karni and Bertini, 1997; Ahissar and Hochstein, 2004), where similar stimuli recruit overlapping neuronal populations. Our finding of stimulus-dependent and time-independent interference reveals a fundamental limit in cortical plasticity that constrains the simultaneous representation of multiple skills in a single neuronal population, rather than a time-limited consolidation process.
Contribution to the field
This probably is our most under-appreciated paper. It questions the dominant interpretation in older studies that a lack of interference between learning an initial task and learning a second task after a number of hours reveals a time-limited consolidation process. Our data rather suggest that interference was related to the amount of overlap in populations recruited in early visual cortex by the two tasks, and not to the time interval between training the two tasks. In our study, we had control over this overlap as we studied interference on asymptotic levels of orientation discrimination performance, which is known to induce plasticity in early visual cortex with sufficient training. Moreover, the two competing tasks were designed to require different network connectivity in highly overlapping neural populations in early visual cortex. Our study involved alternating between the two tasks over a long series of sessions, to push towards a level of skilled performance that would make low-level visual contributions more dominant. In agreement with that, we indeed found the interference only during asymptotic performance (late in learning, when improvement in performance only progresses in small steps from session to session). Many prior interference studies considered interference between only a single session of training in two tasks. In the context of early learning, it is quite possible that representations of a skill shift between different networks over time, so that a lack of interference may relate to that representational shift, rather than to a time-limited process of consolidation. In our study, a plausible interpretation is that connectivity remains malleable at all times, so that alternating experience with one and the other skill leads to interference. Open questions however are whether consolidation may simply take longer than 24h, and whether the alternation between tasks prevented full consolidation. These questions are being pursued in ongoing research.
Another question for which this paper is relevant is the extent to which performance gains in visual skill learning are related to plastic changes in low-level visual cortex (such as the primary visual cortex, or area V1), or to improved read-out of information that is already present in low-level visual cortex. The specific characteristics of the interference effects, such as its dependence on the orientation difference between the competing stimuli, are more easily understood in the context of plasticity in low-level visual areas than in the context of a read-out theoretical framework.