A number of cortical structures are reported to have elevated single unit firing rates sustained throughout the memory period of a working memory task. How the nervous system forms and maintains these memories is unknown but reverberating neuronal network activity is thought to be important. We studied the temporal structure of single unit (SU) activity and simultaneously recorded local field potential (LFP) activity from area LIP in the inferior parietal lobe of two awake macaques during a memory-saccade task. Using multitaper techniques for spectral analysis, which play an important role in obtaining the present results, we find elevations in spectral power in a 50-90Hz (gamma) frequency band during the memory period in both SU and LFP activity. The activity is tuned to the direction of the saccade providing evidence for temporal structure that codes for movement plans during working memory. We also find SU and LFP activity are coherent during the memory period in the 50-90Hz gamma band and no consistent relation is present during simple fixation. Finally, we find organized LFP activity in a 15-25Hz frequency band that may be related to movement execution and preparatory aspects of the task. Neuronal activity could be used to control a neural prosthesis but SU activity can be hard to isolate with cortical implants. As the LFP is easier to acquire than SU activity, our finding of rich temporal structure in LFP activity related to movement planning and execution may accelerate the development of this medical application.Keywords: parietal, prosthesis, local field potential, gamma band, coherence, temporal structure.Pesaran et. al. 3Working memory is a brain system requiring the temporary storage and manipulation of information necessary for the performance of complex cognitive tasks (Baddeley, 1992). The neurophysiological basis of working memory is studied in non-human primates by recording neural activity during delayed-response tasks (Fuster, 1995). Cue-selective elevated single unit firing rates have been recorded during the delay period in many brain areas during different versions of the task (Fuster and Jervey, 1982;Bruce and Goldberg, 1985;Gnadt and Andersen, 1988;Miyashita and Chang, 1988;Funahashi et al., 1989;Koch and Fuster, 1989;Miller et al., 1996;Zhou and Fuster, 1996). How this neural activity is sustained is unknown but may be important to understanding the neural basis of working memory (Goldman-Rakic, 1995). Converging evidence points to the importance of a distributed recurrent neuronal network (Goldman-Rakic, 1988) and reverberating network activity has long been suggested as a possible mechanism for short-term memory (Lorente de No, 1938;Hebb, 1949;Amit, 1995;Seung, 1996;Wang, 1999).Measures with the potential to capture correlated neural activity on a millisecond time scale may be needed to resolve reverberating memory activity. The dynamical structure of neuronal activity has been the source of much interest as a temporal code (for a review see Singer and Gray (1995) ) however ...

Electrical stimulation of the visual system might serve as the foundation for a prosthetic device for the blind. We examined whether microstimulation of the dorsal lateral geniculate nucleus of the thalamus can generate localized visual percepts in alert monkeys. To assess electrically generated percepts, an eyemovement task was used with targets presented on a computer screen (optically) or through microstimulation of the lateral geniculate nucleus (electrically). Saccades (fast, direct eye movements) made to electrical targets were comparable to saccades made to optical targets. Gaze locations for electrical targets were well predicted by measured visual response maps of cells at the electrode tips. With two electrodes, two distinct targets could be independently created. A sequential saccade task verified that electrical targets were processed not in motor coordinates, but in visual spatial coordinates. Microstimulation produced predictable visual percepts, showing that this technique may be useful for a visual prosthesis.primate ͉ prosthesis ͉ tetrode

Electrical stimulation of the thalamus has been widely used to test for the existence of monosynaptic input to cortical neurons, typically with stimulation currents that evoke cortical spikes with high probability. We stimulated the lateral geniculate nucleus (LGN) of the thalamus and recorded monosynaptically evoked spikes from layer 4 neurons in visual cortex. We found that with moderate currents, cortical spikes were evoked with low to moderate probability and their occurrence was modulated by ongoing sensory (visual) input. Furthermore, when repeated at 8 -12 Hz, electrical stimulation of the thalamic afferents caused such profound inhibition that cortical spiking activity was suppressed, aside from electrically evoked monosynaptic spikes. Visual input to layer 4 cortical cells between electrical stimuli must therefore have derived exclusively from LGN afferents. We used white-noise visual stimuli to make a 2D map of the receptive field of each cortical simple cell during repetitive electrical stimulation in the LGN. The receptive field of electrically evoked monosynaptic spikes (and thus of the thalamic input alone) was significantly elongated. Its primary subfield was comparable to that of the control receptive field, but secondary (flanking) subfields were weaker. These findings extend previous results from intracellular recordings, but also demonstrate the effectiveness of an extracellular method of measuring subthreshold afferent input to cortex. O rientation selective neurons in primary visual cortex receive feed-forward input from lateral geniculate nucleus (LGN) cells that are themselves poorly oriented. Experiments that have examined the role of the feed-forward thalamocortical pathway in this receptive field transformation have yielded two independent findings. First, the thalamic input to cortical simple cells is highly specific; LGN cells make monosynaptic connections with simple cells predominantly when the pre-and postsynaptic receptive fields overlap and match in sign, size, and time course (1, 2). Second, when intracortical inputs are silenced by cooling or by electrical stimulation in cortex, intracellular recordings demonstrate that the summed thalamic input to a layer 4 simple cell is orientation selective (3, 4). Here, we present a technique that we have used to examine a related but hitherto untested hypothesis: that the spatial receptive field of a simple cell is very similar to the receptive field of its summed thalamic input. We have concentrated on two receptive-field parameters, the elongation of the strongest subfield and the relative strength of antagonistic, flanking subfields.The experimental approach that we took to examine these questions was based on two independent characteristics of electrical stimulation: (i) if electrical stimulation is not 100% effective in evoking cortical spikes, then the probability of evoking a spike will depend on the subthreshold activity of the cortical neuron at the time of the stimulus, and (ii) strong electrical stimulation leads to prolonged (...