Transient Cortical Excitation at the Onset of Visual Fixation

Abstract

Primates actively examine the visual world by rapidly shifting gaze (fixation) over the elements in a scene. Despite this fact, we typically study vision by presenting stimuli with gaze held constant. To better understand the dynamics of natural vision, we examined how the onset of visual fixation affects ongoing neuronal activity in the absence of visual stimulation. We used multiunit activity and current source density measurements to index neuronal firing patterns and underlying synaptic processes in macaque V1. Initial averaging of neural activity synchronized to the onset of fixation suggested that a brief period of cortical excitation follows each fixation. Subsequent single-trial analyses revealed that 1) neuronal oscillation phase transits from random to a highly organized state just after the fixation onset, 2) this phase concentration is accompanied by increased spectral power in several frequency bands, and 3) visual response amplitude is enhanced at the specific oscillatory phase associated with fixation. We hypothesize that nonvisual inputs are used by the brain to increase cortical excitability at fixation onset, thus "priming" the system for new visual inputs generated at fixation. Despite remaining mechanistic questions, it appears that analysis of fixation-related responses may be useful in studying natural vision.

Oscillatory activity in the monkey hippocampus during visual exploration and memory formation

“…Modulation of the phase of theta may have important implications for cognition, as demonstrated in some studies of hippocampal long-term potentiation (36)(37)(38) and sensory processing (34,39). For example, neural activity in the hippocampus may be influenced by theta phase in such a way that resetting to an "ideal" phase on saccade initiation may set up the optimal conditions for plasticity, and thus memory formation.…”

Spontaneous EEG oscillations reveal periodic sampling of visual attention

“…Periodic sampling was quantified as the correlation between the phase of prestimulus EEG oscillations and the detection of these stimuli. Electrophysiological oscillations reflect periodic variations of membrane potentials in neuronal ensembles, which are associated with variations of neural excitability (15,16,20,32). Thus, the momentary phase of ongoing oscillations at the time a stimulus is presented can influence the strength of the neural response to the stimulus (19,(33)(34)(35)(36)(37) and the speed of manual reaction times (19,38,39).…”

“…It was found that covert shifts of attention (i.e., shifts that do not involve eye movements) were locked to oscillations in the frontal eye fields, such that each cycle of the oscillation corresponded to a distinct item. These oscillations occurred in the β frequency range (18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34), but the exact frequency was variable across trials and correlated with single-trial reaction times. Thus, the authors suggested that the speed of attentional shifts might be controlled by the frequency of the underlying oscillations (10).…”

“…Here, oscillations correspond to periodic fluctuations of the local electrical field and the intrinsic excitability of neuronal populations (15)(16)(17)(18)(19)(20)(21). Of course, these electrical neural oscillations could provide the physiological basis of periodic perceptual and attentional sampling phenomena (22).…”

Corticospinal Beta-Band Synchronization Entails Rhythmic Gain Modulation

“…Thus, physiological beta-band coherence leads to spikes optimally timed for impact onto the spinal cord. Related analyses of ongoing or evoked phases that lead to enhancement or suppression of neuronal responses have been performed previously in other systems and frequency bands (Kruglikov and Schiff, 2003;Lakatos et al, 2007Lakatos et al, , 2008Rajkai et al, 2008).…”