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Abstract:
As most cortical neurons are broadly tuned to various stimulus parameters, it is inevitable that individual neurons participate in the representation of more than one visual object. We asked here whether the prefrontal representation of immediately preceding objects would interfere with the representation of subsequently processed object stimuli, supporting the idea that neuronal processes challenged by more input and compressed in time leads to a degradation of the quality of encoding. In the past, we analyzed simultaneously recorded multi- and single-unit signals derived from arrays of single-ended microelectrodes and tetrodes during a simple visual memory task (Waizel et al., SfN 20072008) and found that accurate representations of individual objects require the participation of large neuronal populations. Based on single trial firing rate values, we calculated one-way ANOVAs at 1 significance thresholds and performed subsequent posthoc comparisons (Scheffé) in order to detect stimulus selectivity and stimulus specificity for the activity at each single site, respectively. With tetrodes we were able to detect highly-specific units in PFC with a narrow band of stimulus preferences, which were remarkably stable throughout all stimulus comparisons. In order to increase the probability to find more of these specific units, we sharpened the impact and enhanced the temporal structure of the task. Two monkeys, who were trained to perform the basic task at ~80 performance, were ad hoc presented with a sequence of up to 4 objects that were shown consecutively within a fixed period of 900 ms. Not only the monkeys were able to impromptu generalize from a simple (Load 1) to a demanding task (Load 2-4) (Wildt et al., SfN 2008), they also showed highly selective sites (p< .009- p< 7 × 10-13) in all four load conditions, even for those last objects during load 4 (p<.006) which were presented for less than 250 ms. For all load conditions, highly specific sites could be found (118 pairwise comparisons with p<.01). One group of these sites kept their object preference throughout the entire sequence of all four objects, others responded position-dependent to different objects, but were still highly stable throughout all pairwise comparisons.
These results suggest that neuronal ensembles in primate PFC are capable of encoding up to 4 objects without interactions among the activity expressed in relation to other objects in the sequence. In addition, they are able to resolve even very shortly presented objects (<250 ms) showing strong selectivity uniquely for one of them and without superimposing this representation with signals evoked by more recently perceived objects.
