Effects of fixational saccades on response timing in macaque lateral geniculate nucleus

V1-bypassing suppression leads to direction-specific microsaccade modulation in visual coding and perception

Microsaccades play a critical role in refreshing visual information and have been shown to have direction-specific influences on human perception. However, the neural mechanisms underlying such direction-specific effects remains unknown. Here, we report the emergence of direction-specific microsaccade modulation in the middle layer of V2 but not in V1: responses of V2 neurons after microsaccades moved toward their receptive fields were stronger than those when microsaccades moved away. The decreased responses from V1 to V2, which are correlated with the amplitude of microsaccades away from receptive fields, suggest topographically location-specific suppression from an oculomotor source. Consistent with directional effects in V2, microsaccades function as a guide for monkeys' behavior in a peripheral detection task; both can be explained by a dynamic neural network. Our findings suggest a V1-bypassing suppressive circuit for direction-specific microsaccade modulation in V2 and its functional influence on visual sensitivity, which highlights the optimal sampling nature of microsaccades.

Perceptual enhancement and suppression correlate with V1 neural activity during active sensing

Perception in multiple sensory modalities is an active process that involves exploratory behaviors. In humans and other primates, vision results from sensory sampling guided by saccadic eye movements. Saccades are known to modulate visual perception, and a corollary discharge signal associated with saccades appears to establish a sense of visual stability. Neural recordings have shown that saccades also modulate activity widely across the brain. To investigate the neural basis of saccadic effects on perception, simultaneous recordings from multiple neurons in area V1 were made as animals performed a contrast detection task. Perceptual and neural measures were compared when the animal made real saccades that brought a stimulus into V1 receptive fields and when simulated saccades were made (identical retinal stimulation but no eye movement). When real saccades were made and low spatial frequency stimuli were presented, we observed a reduction in both perceptual sensitivity and neural activity compared with simulated saccades; conversely, with higher spatial frequency stimuli, saccades increased visual sensitivity and neural activity. The performance of neural decoders, which used the activity of the population of simultaneously recorded neurons, showed saccade effects on sensitivity that mirrored the frequency-dependent perceptual changes, suggesting that the V1 population activity could support the perceptual effects. A minority of V1 neurons had significant choice probabilities, and the saccades decreased both average choice probability and pairwise noise correlations. Taken together, the findings suggest that a signal related to saccadic eye movements alters V1 spiking to increase the independence of spiking neurons and bias the system toward processing higher spatial frequencies, presumably to enhance object recognition. The effects of saccades on visual perception and noise correlations appear to parallel effects observed in other sensory modalities, suggesting a general principle of active sensory processing.

Age effects on saccadic suppression of luminance and color

Saccadic eye movements modulate visual perception: they initiate and terminate high acuity vision at a certain location in space, but before and during their execution visual contrast sensitivity is strongly attenuated for 100 to 200 ms. Transient perisaccadic perceptual distortions are assumed to be an important mechanism to maintain visual stability. Little is known about age effects on saccadic suppression, even though for healthy adults other major age-related changes are well documented, like a decrease of visual contrast sensitivity for intermediate and high spatial frequencies or an increase of saccade latencies. Here, we tested saccadic suppression of luminance and isoluminant chromatic flashes in 100 participants from eight to 78 years. To estimate the effect of saccadic suppression on contrast sensitivity, we used a two-alternative forced choice (2AFC) design and an adaptive staircase procedure to modulate the luminance or chromatic contrast of a flashed detection target during fixation and 15 ms after saccade onset. The target was a single horizontal luminance or chromatic line flashed 2° above or below the fixation or saccade target. Compared to fixation, average perisaccadic contrast sensitivity decreased significantly by 66% for luminance and by 36% for color. A significant correlation was found for the strength of saccadic suppression of luminance and color. However, a small age effect was found only for the strength of saccadic suppression of luminance, which increased from 64% to 70% from young to old age. We conclude that saccadic suppression for luminance and color is present in most participants independent of their age and that mechanisms of suppression stay relatively stable during healthy aging.

Mapping the primate lateral geniculate nucleus: a review of experiments and methods

Mapping neuronal responses in the lateral geniculate nucleus (LGN) is key to understanding how visual information is processed in the brain. This paper focuses on our current knowledge of the dynamics the receptive field (RF) as broken down into the classical receptive field (CRF) and the extra-classical receptive field (ECRF) in primate LGN. CRFs in the LGN are known to be similar to those in the retinal ganglion cell layer in terms of both spatial and temporal characteristics, leading to the standard interpretation of the LGN as a relay center from retina to primary visual cortex. ECRFs have generally been found to be large and inhibitory, with some differences in magnitude between the magno-, parvo-, and koniocellular pathways. The specific contributions of the retina, thalamus, and visual cortex to LGN ECRF properties are presently unknown. Some reports suggest a retinal origin for extra-classical suppression based on latency arguments and other reports have suggested a thalamic origin for extra-classical suppression. This issue is complicated by the use of anesthetized animals, where cortical activity is likely to be altered. Thus further study of LGN ECRFs is warranted to reconcile these discrepancies. Producing descriptions of RF properties of LGN neurons could be enhanced by employing preferred naturalistic stimuli. Although there has been significant work in cats with natural scene stimuli and noise that statistically imitates natural scenes, we highlight a need for similar data from primates. Obtaining these data may be aided by recent advancements in experimental and analytical techniques that permit the efficient study of nonlinear RF characteristics in addition to traditional linear factors. In light of the reviewed topics, we conclude by suggesting experiments to more clearly elucidate the spatial and temporal structure of ECRFs of primate LGN neurons.

Human occipital cortices differentially exert saccadic suppression: Intracranial recording in children

By repeating saccades unconsciously, humans explore the surrounding world every day. Saccades inevitably move external visual images across the retina at high velocity; nonetheless, healthy humans don't perceive transient blurring of the visual scene during saccades. This perceptual stability is referred to as saccadic suppression. Functional suppression is believed to take place transiently in the visual systems, but it remains unknown how commonly or differentially the human occipital lobe activities are suppressed at the large-scale cortical network level. We determined the spatial-temporal dynamics of intracranially-recorded gamma activity at 80-150 Hz around spontaneous saccades under no-task conditions during wakefulness and those in darkness during REM sleep. Regardless of wakefulness or REM sleep, a small degree of attenuation of gamma activity was noted in the occipital regions during saccades, most extensively in the polar and least in the medial portions. Longer saccades were associated with more intense gamma-attenuation. Gamma-attenuation was subsequently followed by gamma-augmentation most extensively involving the medial and least involving the polar occipital region. Such gamma-augmentation was more intense during wakefulness and temporally locked to the offset of saccades. The polarities of initial peaks of perisaccadic event-related potentials (ERPs) were frequently positive in the medial and negative in the polar occipital regions. The present study, for the first time, provided the electrophysiological evidence that human occipital cortices differentially exert perisaccadic modulation. Transiently suppressed sensitivity of the primary visual cortex in the polar region may be an important neural basis for saccadic suppression. Presence of occipital gamma-attenuation even during REM sleep suggests that saccadic suppression might be exerted even without external visual inputs. The primary visual cortex in the medial region, compared to the polar region, may be more sensitive to an upcoming visual scene provided at the offset of each saccade.

Visual perception and saccadic eye movements

We use saccades several times per second to move the fovea between points of interest and build an understanding of our visual environment. Recent behavioral experiments show evidence for the integration of pre- and postsaccadic information (even subliminally), the modulation of visual sensitivity, and the rapid reallocation of attention. The recent physiological literature has identified a characteristic modulation of neural responsiveness-perisaccadic reduction followed by a postsaccadic increase-that is found in many visual areas, but whose source is as yet unknown. This modulation seems optimal for reducing sensitivity during and boosting sensitivity between saccades, but no study has yet established a direct causal link between neural and behavioral changes.

An equivalent noise investigation of saccadic suppression

Visual stimuli presented just before or during an eye movement are more difficult to detect than those same visual stimuli presented during fixation. This laboratory phenomenon--behavioral saccadic suppression--is thought to underlie the everyday experience of not perceiving the motion created by our own eye movements-saccadic omission. At the neural level, many cortical and subcortical areas respond differently to perisaccadic visual stimuli than to stimuli presented during fixation. Those neural response changes, however, are complex and the link to the behavioral phenomena of reduced detectability remains tentative. We used a well established model of human visual detection performance to provide a quantitative description of behavioral saccadic suppression and thereby allow a more focused search for its neural mechanisms. We used an equivalent noise method to distinguish between three mechanisms that could underlie saccadic suppression. The first hypothesized mechanism reduces the gain of the visual system, the second increases internal noise levels in a stimulus-dependent manner, and the third increases stimulus uncertainty. All three mechanisms predict that perisaccadic stimuli should be more difficult to detect, but each mechanism predicts a unique pattern of detectability as a function of the amount of external noise. Our experimental finding was that saccades increased detection thresholds at low external noise, but had little influence on thresholds at high levels of external noise. A formal analysis of these data in the equivalent noise analysis framework showed that the most parsimonious mechanism underlying saccadic suppression is a stimulus-independent reduction in response gain.