ANALYZING NEURONAL PROCESSING LOCUS IN STIMULUS-RESPONSE ASSOCIATION TASKS

If a neuron is being recorded while a trained animal performs a 2 x 2 stimulus-response association task, how can we decide whether it is re lated more to the encoding and analysis of the sensory stimulus, to th e preparation and execution of the motor response, or to the animal's decision that associates the two? The difficulty arises because, withi n a single task, stimulus and response are intrinsically confounded pe r task instruction; it is only through proper analysis of errors in pe rformance (behavioral noise) and variance in recorded neural activity (neuronal noise) that one can identify the sensorimotor significance o f such activity. A quantitative technique is proposed here, based on t he framework of signal detection theory, to determine the sensorimotor ''locus'' of a neural process when recorded simultaneously with the a nimal's performance on a trial-by-trial basis. The premise is that a p ure sensory process should be influenced only by the nature of the sen sory stimulus regardless of the nature of the behavioral response, and vice versa for a pure motor process. From the recorded neural activit y, we calculate the prediction or discriminability ( by an ideal opera tor) for the stimulus categories a nd for the response categories. The se discriminability values are then compared with each other to infer whether the neural process is more related to stimulus or to response. An index is derived that quantitatively specifies the processing locu s of a given neural process along the sensorimotor continuum, with pur e sensory and pure motor processes at the two extremes. In between lie s the locus of decision-related processes whose activities allow equal (but not chance) prediction for stimulus and response categories. The technique is applied to single-unit activities recorded in monkey pri mary motor cortex (MI) while the monkey performed a simple go/nogo tas k involving visual stimulus and hand/wrist movement. We find that sens orimotor indices of MI neurons are widely distributed, with a preponde rance of motor-related units (that better predict go/nogo response tha n go/nogo stimulus) but also sensory-related ones (with predictabiliti es reversed). (C) 1997 Academic Press.