Attention can be focused volitionally by “top-down” signals derived from task demands and automatically by “bottom-up” signals from salient stimuli. The frontal and parietal cortices are involved, but their neural activity has not been directly compared. Therefore, we recorded from them simultaneously in monkeys. Prefrontal neurons reflected the target location first during top-down attention, whereas parietal neurons signaled it earlier during bottom-up attention. Synchrony between frontal and parietal areas was stronger in lower frequencies during top-down attention and in higher frequencies during bottom-up attention. This result indicates that top-down and bottom-up signals arise from the frontal and sensory cortex, respectively, and different modes of attention may emphasize synchrony at different frequencies.

Summary Working memory is thought to result from sustained neuron spiking. However, computational models suggest complex dynamics with discrete oscillatory bursts. We analyzed local field potential (LFP) and spiking from the prefrontal cortex (PFC) of monkeys performing a working memory task. There were brief bursts of narrow-band gamma oscillations (45-100 Hz), varied in time and frequency, accompanying encoding and re-activation of sensory information. They appeared at a minority of recording sites associated with spiking reflecting the to-be-remembered items. Beta oscillations (20-35 Hz) also occurred in brief, variable bursts but reflected a default state interrupted by encoding and decoding. Only activity of neurons reflecting encoding/decoding correlated with changes in gamma burst rate. Thus, gamma bursts could gate access to, and prevent sensory interference with, working memory. This supports the hypothesis that working memory is manifested by discrete oscillatory dynamics and spiking, not sustained activity.

Attention to a stimulus enhances both neuronal responses and gamma frequency synchrony in visual area V4, both of which should increase the impact of attended information on downstream neurons. To determine whether gamma synchrony is common throughout the ventral stream, we recorded from neurons in the superficial and deep layers of V1, V2, and V4 in two rhesus monkeys. We found an unexpected striking difference in gamma synchrony in the superficial vs. deep layers. In all three areas, spike-field coherence in the gamma (40-60 Hz) frequency range was largely confined to the superficial layers, whereas the deep layers showed maximal coherence at low frequencies (6-16 Hz), which included the alpha range. In the superficial layers of V2 and V4, gamma synchrony was enhanced by attention, whereas in the deep layers, alpha synchrony was reduced by attention. Unlike these major differences in synchrony, attentional effects on firing rates and noise correlation did not differ substantially between the superficial and deep layers. The results suggest that synchrony plays very different roles in feedback and feedforward projections. electrophysiology | macaque | oscillation A natomical and physiological studies have characterized the afferent inputs to and efferent inputs from neurons in different layers of visual cortical areas. However, physiological distinctions across layers, such as synchronous interactions, have not been fully identified. We first came across laminar differences in synchrony serendipitously. Gamma-band synchrony, measured either by spike-field or spike-spike interactions across multiple electrodes, is a prominent feature in visual cortex, and several studies have shown that attention enhances gamma-band synchrony in area V4 (1-5). In our first recordings in area V1, we also found prominent gamma-band synchrony, although the effects of attention, if any, were much smaller than what we previously found in V4 (1). However, in our first recordings in area V2 in the lunate sulcus, we were surprised to find hardly any gamma-band synchrony. We initially had no explanation for why V2 should be so different from V1 and V4. Probing at greater electrode depths led to the discovery that V2 cells do show gamma-band synchrony but only at those deeper electrode depths. Because V2 in the lunate sulcus bends under V1, layer 6 cells are closer to V1 on the occipital surface than are layer 1 cells. Thus, our deeper electrode recordings were actually located in the more superficial layers of V2. Because we typically studied the first responsive cells found in any penetration, this must have strongly biased our first recordings in V2 to the deep layers, and these deep layers apparently had little gamma-band synchrony. Conversely, the same tendency to sample the first responsive cells on a penetration would have resulted in a strong bias to record cells in the superficial layers of V1 and V4, from which we recorded directly on the cortical surface. This possibility led us to test whether the deep layers of V1 and V4 were...

During flexible behavior, multiple brain regions encode sensory inputs, the current task, and choices. It remains unclear how these signals evolve. We simultaneously recorded neuronal activity from six cortical regions (MT, V4, IT, LIP, PFC and FEF) of monkeys reporting the color or motion of stimuli. Following a transient bottom-up sweep, there was a top-down flow of sustained task information from frontoparietal to visual cortex. Sensory information flowed from visual to parietal and prefrontal cortex. Choice signals developed simultaneously in frontoparietal regions and travelled to FEF and sensory cortex. This suggests that flexible sensorimotor choices emerge in a frontoparietal network from the integration of opposite flows of sensory and task information.