Patterns of Synchronous Spikes in Macaque Motor Cortex During an Instructed-Delay Reach-to-Grasp Task

Cell assemblies [1] exhibiting neuronal interactions at the millisecond time scale were suggested as building blocks of information processing in the brain [2,3]. Significant patterns of synchronous spikes inelectrophysiological recordings are considered as a signature of an active assembly. We recently developeda statistical method - the Spike Pattern Detection and Evaluation (SPADE) analysis [4] - to detect statisticallysignificant synchronous spike patterns in massively parallel spike data (MPST) on the order of 100 or moreneurons. The method deals with a) the combinatorial explosion of the number of patterns to consider byemploying a variant of the frequent item set mining technique [5, 6], and with b) the challenge of statisticallyassessing these patterns due to the multiple testing problem by using Monte-Carlo techniques.We applied the SPADE analysis [7] to electrophysiological data recorded from the motor cortex of twomonkeys (macaca mulatta), while they executed a delayed reach-to-grasp task. Monkeys were trained toreach to and grasp, pull and hold an object by using either a side grip or a precision grip and with highor low force. To allow the monkey to prepare the upcoming movement, the animal was instructed aboutthe grip type at the beginning of a delay period of 1s preceding the GO signal. MPST was recorded byusing a 100 electrodes Utah array, chronically implanted at the MI/PMd border [8]. We hypothesized thatdifferent cell assemblies, and in consequence different classes of spike patterns, occur depending on thebehavioral conditions and time points in the trial. To investigate this hypothesis, recordings of the sameset of neurons were analyzed for the occurrence of significant spike patterns and compared during the 4different behavioral conditions (combination of object load and grip type), and during 6 time epochs withineach trial that we defined based on different stages of the task protocol.We found for both monkeys a variety of significant patterns. We then analyzed the spatial organizationof these patterns on the recording array with respect to its cortical location, and found that pairs of neuronsinvolved in synchronous patterns were preferentially aligned along the medio-lateral orientation. We alsoassessed the specificity of the neuronal composition of patterns to different behavioral contexts and founda strong specificity to the grip type and the experimental epoch. These findings provide evidence for theexistence of higher-order spike patterns occurring in relation to behavior