SYNCHRONIZATION OF NEURONAL RESPONSES IN PRIMARY VISUAL CORTEX OF MONKEYS VIEWING NATURAL IMAGES.

Pedro E Maldonado — 2008-06-25T10:26:41-00:00

Journal of neurophysiology (18 June 2008)

When inspecting visual scenes, primates perform on average four saccadic eye movements per second which implies that scene segmentation, feature binding and identification of image components is accomplished in less than 200ms. Thus, individual neurons can contribute only a small number of discharges for these complex computations, suggesting that information is encoded not only in the discharge rate but also in the timing of action potentials. While monkeys inspected natural scenes we registered with multi-electrodes from primary visual cortex, the discharges of simultaneously recorded neurons. Relating these signals to eye movements, revealed that discharge rates peaked around 90ms after fixation onset and then decreased to near baseline levels within 200ms. Unitary event analysis revealed that preceding this increase in firing, there was an episode of enhanced response synchronization during which discharges of spatially distributed cells coincided within 5ms windows significantly more often than predicted by the discharge rates. This episode started 30ms after fixation onset and ended by the time discharge rates had reached their maximum. When the animals scanned a blank screen a small change in firing rate but no excess synchronization was observed. The short latency of the stimulation related synchronization phenomena suggests a fast acting mechanism for the coordination of spike timing that may contribute to the basic operations of scene segmentation.

Neuronal activity in the primary visual cortex of the cat freely viewing natural images.

PE Maldonado — 2008-04-14T08:53:08-00:00

Neuroscience, Vol. 144, No. 4. (23 February 2007), pp. 1536-1543.

Many studies have now demonstrated that neurons in the visual cortex of cats and monkeys change their activity when stimuli are presented beyond their classical receptive field, and that these responses are not readily apparent from their receptive field properties. However few studies have been conducted to investigate the discharge properties of neurons in the visual cortex of animals when they are allow to freely view natural images. We employ tetrodes, which enable simultaneous and separable recordings of small numbers of neighboring neurons, to record 102 single units from 59 sites from areas 17 and 18 of two alert cats. While the animals viewed either natural images or black screens, they made frequent saccadic eye movements and gaze fixations. Fixations onto an image's location increased neuronal firing peaking at 80-100 ms after the fixation onset, to then decrease steadily with time despite continuous fixation. Saccades trigger a fast decrease in firing rate for both images and darkness. When we examined the incidence of correlated firing, we observed significant synchrony during the initial phases of visual fixations when the animals viewed natural scenes. Such synchrony was absent during saccadic eye movements and during eye movements in darkness. Our data revealed that scanning of natural scenes is associated with a rapid succession of distinct fixation-related activation patterns that included transient rate changes and excess coincident firing. The transient nature of these synchronization phenomena suggests a fast acting mechanism, which is in good agreement with the evidence that basic operations of scene analysis must be accomplished within a few tens of milliseconds in primary visual cortex.

CiteULike: j-ito's Maldonado

SYNCHRONIZATION OF NEURONAL RESPONSES IN PRIMARY VISUAL CORTEX OF MONKEYS VIEWING NATURAL IMAGES.

Neuronal activity in the primary visual cortex of the cat freely viewing natural images.