FROM STIMULUS-ENCODING TO FEATURE-EXTRACTION IN WEAKLY ELECTRIC FISH

ANIMALS acquire information about sensory stimuli around them and enco de it using an analogue or a pulse-based code. Behaviourally relevant features need to be extracted from this representation for further pro cessing. In the electrosensory system of weakly electric fish, single P-type electroreceptor afferents accurately encode the time course of random modulations in electric-field amplitude(1). We applied a stimul us estimation method(2) and a signal-detection method to both P-recept or afferents and their targets, the pyramidal cells in the electrosens ory lateral-line lobe. We found that although pyramidal cells do not a ccurately convey detailed information about the time course of the sti mulus, they reliably encode up- and downstrokes of random modulations in electric-field amplitude. The presence of such temporal features is best signalled by short bursts of spikes, probably caused by dendriti c processing, rather than by isolated spikes. Furthermore, pyramidal c ells outperform P-receptor afferents in signalling the presence of tem poral features in the stimulus waveform. We conclude that the sensory neurons are specialized to acquire information accurately with little processing, whereas the following stage extracts behaviourally relevan t features, thus performing a nonlinear pattern-recognition task.