Background changes delay information represented in macaque V1 neurons

Attention modulates subjective time perception across eye movements

Prior research has established that actions, such as eye movements, influence time perception. However, the relationship between pre-saccadic attention, which is often associated with eye movement, and subjective time perception is not explored. Our study examines the impact of pre-saccadic attention on the subjective experience of time during eye movements, particularly focusing on its influence on subjective time perception at the saccade target. Participants were presented with two clocks featuring spinning hands, positioned at distinct locations corresponding to fixation and the saccade target. They were required to report the perceived time of these clocks across the eye movements, enabling us to measure and compare both the perceived and actual timing at these specific clock locations. In Experiment 1, we observed that participants tended to report the timing of their eyes' arrival at the target location as occurring slightly ahead of the actual time. In contrast, in Experiment 2, when participants divert their attention to the fixation clock prior to the imperative saccade, this perceptual bias diminishes. These results indicate that subjective time perception is strongly impacted by attentional conditions across the two experiments. Together, these findings offer further evidence for the notion that stable time perception during eye movements is not solely an inherent property of the eye movement system but also encompasses other cognitive mechanisms, such as attention. STATEMENT OF RELEVANCE: While we often remain unaware of the frequent saccades (rapid eye movements) we make, they have a profound impact on our perception of the world and the flow of time. Nevertheless, the connection between pre-saccadic attention, often associated with eye movements, and our subjective perception of time remains largely unexplored. In our research, we investigated the relationship between attention and our subjective experience of time. Our findings revealed the crucial role of attention, serving as a bridge between the physical movements of our eyes and our internal sense of temporal continuity. In essence, although previous studies have demonstrated the impact of eye movements on time perception, our current study emphasizes the critical influence of attention during the preparatory phase of saccades on the subjective experience of time during eye movements.

Coding strategy for surface luminance switches in the primary visual cortex of the awake monkey

Both surface luminance and edge contrast of an object are essential features for object identification. However, cortical processing of surface luminance remains unclear. In this study, we aim to understand how the primary visual cortex (V1) processes surface luminance information across its different layers. We report that edge-driven responses are stronger than surface-driven responses in V1 input layers, but luminance information is coded more accurately by surface responses. In V1 output layers, the advantage of edge over surface responses increased eight times and luminance information was coded more accurately at edges. Further analysis of neural dynamics shows that such substantial changes for neural responses and luminance coding are mainly due to non-local cortical inhibition in V1’s output layers. Our results suggest that non-local cortical inhibition modulates the responses elicited by the surfaces and edges of objects, and that switching the coding strategy in V1 promotes efficient coding for luminance.

How brightness is encoded in the visual cortex remains incompletely understood. By recording from macaque V1, the authors revealed a switch from surface to edge encoding that is mediated by widespread inhibition in the output layers of the cortex.

Comparison of pop-out responses to luminance and motion contrasting stimuli of tectal neurons in pigeons

The emergence of visual saliency has been widely studied in the primary visual cortex and the superior colliculus (SC) in mammals. There are fewer studies on the pop-out response to motion direction contrasting stimuli taken in the optic tectum (OT, homologous to mammalian SC), and these are mainly of owls and fish. To our knowledge the influence of spatial luminance has not been reported. In this study, we have recorded multi-units in pigeon OT and analyzed the tectal response to spatial luminance contrasting, motion direction contrasting, and contrasting stimuli from both feature dimensions. The comparison results showed that 1) the tectal response would pop-out in either motion direction or spatial luminance contrasting conditions. 2) The modulation from motion direction contrasting was independent of the temporal luminance variation of the visual stimuli. 3) When both spatial luminance and motion direction were salient, the response of tectal neurons was modulated more intensely by motion direction than by spatial luminance. The phenomenon was consistent with the innate instinct of avians in their natural environment. This study will help to deepen the understanding of mechanisms involved in bottom-up visual information processing and selective attention in the avian.

Neural mechanism for sensing fast motion in dim light

Luminance is a fundamental property of visual scenes. A population of neurons in primary visual cortex (V1) is sensitive to uniform luminance. In natural vision, however, the retinal image often changes rapidly. Consequently the luminance signals visual cells receive are transiently varying. How V1 neurons respond to such luminance changes is unknown. By applying large static uniform stimuli or grating stimuli altering at 25 Hz that resemble the rapid luminance changes in the environment, we show that approximately 40% V1 cells responded to rapid luminance changes of uniform stimuli. Most of them strongly preferred luminance decrements. Importantly, when tested with drifting gratings, the preferred speeds of these cells were significantly higher than cells responsive to static grating stimuli but not to uniform stimuli. This responsiveness can be accounted for by the preferences for low spatial frequencies and high temporal frequencies. These luminance-sensitive cells subserve the detection of fast motion under the conditions of dim illumination.

Eye movements reset visual perception

Human vision uses saccadic eye movements to rapidly shift the sensitive foveal portion of our retina to objects of interest. For vision to function properly amidst these ballistic eye movements, a mechanism is needed to extract discrete percepts on each fixation from the continuous stream of neural activity that spans fixations. The speed of visual parsing is crucial because human behaviors ranging from reading to driving to sports rely on rapid visual analysis. We find that a brain signal associated with moving the eyes appears to play a role in resetting visual analysis on each fixation, a process that may aid in parsing the neural signal. We quantified the degree to which the perception of tilt is influenced by the tilt of a stimulus on a preceding fixation. Two key conditions were compared, one in which a saccade moved the eyes from one stimulus to the next and a second simulated saccade condition in which the stimuli moved in the same manner but the subjects did not move their eyes. We find that there is a brief period of time at the start of each fixation during which the tilt of the previous stimulus influences perception (in a direction opposite to the tilt aftereffect)--perception is not instantaneously reset when a fixation starts. Importantly, the results show that this perceptual bias is much greater, with nearly identical visual input, when saccades are simulated. This finding suggests that, in real-saccade conditions, some signal related to the eye movement may be involved in the reset phenomenon. While proprioceptive information from the extraocular muscles is conceivably a factor, the fast speed of the effect we observe suggests that a more likely mechanism is a corollary discharge signal associated with eye movement.

Macaque V1 representations in natural and reduced visual contexts: spatial and temporal properties and influence of saccadic eye movements

Vision in natural situations is different from the paradigms generally used to study vision in the laboratory. In natural vision, stimuli usually appear in a receptive field as the result of saccadic eye movements rather than suddenly flashing into view. The stimuli themselves are rich with meaningful and recognizable objects rather than simple abstract patterns. In this study we examined the sensitivity of neurons in macaque area V1 to saccades and to complex background contexts. Using a variety of visual conditions, we find that natural visual response patterns are unique. Compared with standard laboratory situations, in more natural vision V1 responses have longer latency, slower time course, delayed orientation selectivity, higher peak selectivity, and lower amplitude. Furthermore, the influences of saccades and background type (complex picture vs. uniform gray) interact to give a distinctive, and presumably more natural, response pattern. While in most of the experiments natural images were used as background, we find that similar synthetic unnatural background stimuli produce nearly identical responses (i.e., complexity matters more than "naturalness"). These findings have important implications for our understanding of vision in more natural situations. They suggest that with the saccades used to explore complex images, visual context ("surround effects") would have a far greater effect on perception than in standard experiments with stimuli flashed on a uniform background. Perceptual thresholds for contrast and orientation should also be significantly different in more natural situations.

Representation of surface luminance and contrast in primary visual cortex

In visual perception, object identification requires both the ability to define regions of uniform luminance and zones of luminance contrast. Neural processes underlying contrast detection have been well studied, while those defining luminance remain poorly understood and controversial. Partially because stimuli comprised of uniform luminance are relatively ineffective in driving responses of cortical neurons, little effort has been made to systematically compare responses of individual neurons to both uniform luminance and contrast. Using large static uniform luminance and contrast stimuli, modulated temporally in luminance or contrast, we found a continuum of responses ranging from a few cells modulated only by luminance (luminance-only), to many cells modulated by both luminance and contrast (luminance-contrast), and to many others modulated only by contrast (contrast-only) in primary visual cortex. Moreover, luminance-contrast cells had broader orientation tuning, larger receptive field (RF) and lower spatial frequency Preference, on average, than contrast-only cells. Contrast-only cells had contrast responses more linearly correlated to the spatial structure of their RFs than luminance-contrast cells. Taken together these results suggest that luminance and contrast are represented, to some degree, by independent mechanisms that may be shaped by different classes of subcortical and/or cortical inputs.

Relating information, encoding and adaptation: decoding the population firing rate in visual areas 17/18 in response to a stimulus transition

Neurons in the primary visual cortex typically reach their highest firing rate after an abrupt image transition. Since the mutual information between the firing rate and the currently presented image is largest during this early firing period it is tempting to conclude this early firing encodes the current image. This view is, however, made more complicated by the fact that the response to the current image is dependent on the preceding image. Therefore we hypothesize that neurons encode a combination of current and previous images, and that the strength of the current image relative to the previous image changes over time. The temporal encoding is interesting, first, because neurons are, at different time points, sensitive to different features such as luminance, edges and textures; second, because the temporal evolution provides temporal constraints for deciphering the instantaneous population activity. To study the temporal evolution of the encoding we presented a sequence of 250 ms stimulus patterns during multiunit recordings in areas 17 and 18 of the anaesthetized ferret. Using a novel method we decoded the pattern given the instantaneous population-firing rate. Following a stimulus transition from stimulus A to B the decoded stimulus during the first 90ms was more correlated with the difference between A and B (B-A) than with B alone. After 90ms the decoded stimulus was more correlated with stimulus B than with B-A. Finally we related our results to information measures of previous (B) and current stimulus (A). Despite that the initial transient conveys the majority of the stimulus-related information; we show that it actually encodes a difference image which can be independent of the stimulus. Only later on, spikes gradually encode the stimulus more exclusively.

Mapping of contextual modulation in the population response of primary visual cortex

We review the evidence of long-range contextual modulation in V1. Populations of neurons in V1 are activated by a wide variety of stimuli outside of their classical receptive fields (RF), well beyond their surround region. These effects generally involve extra-RF features with an orientation component. The population mapping of orientation preferences to the upper layers of V1 is well understood, as far as the classical RF properties are concerned, and involves organization into pinwheel-like structures. We introduce a novel hypothesis regarding the organization of V1's contextual response. We show that RF and extra-RF orientation preferences are mapped in related ways. Orientation pinwheels are the foci of both types of features. The mapping of contextual features onto the orientation pinwheel has a form that recapitulates the organization of the visual field: an iso-orientation patch within the pinwheel also responds to extra-RF stimuli of the same orientation. We hypothesize that the same form of mapping applies to other stimulus properties that are mapped out in V1, such as colour and contrast selectivity. A specific consequence is that fovea-like properties will be mapped in a systematic way to orientation pinwheels. We review the evidence that cytochrome oxidase blobs comprise the foci of this contextual remapping for colour and low contrasts. Neurodynamics and motion in the visual field are argued to play an important role in the shaping and maintenance of this type of mapping in V1.

A comparison of visuomotor cue integration strategies for object placement and prehension

Visual cue integration strategies are known to depend on cue reliability and how rapidly the visual system processes incoming information. We investigated whether these strategies also depend on differences in the information demands for different natural tasks. Using two common goal-oriented tasks, prehension and object placement, we determined whether monocular and binocular information influence estimates of three-dimensional (3D) orientation differently depending on task demands. Both tasks rely on accurate 3D orientation estimates, but 3D position is potentially more important for grasping. Subjects placed an object on or picked up a disc in a virtual environment. On some trials, the monocular cues (aspect ratio and texture compression) and binocular cues (e.g., binocular disparity) suggested slightly different 3D orientations for the disc; these conflicts either were present upon initial stimulus presentation or were introduced after movement initiation, which allowed us to quantify how information from the cues accumulated over time. We analyzed the time-varying orientations of subjects' fingers in the grasping task and those of the object in the object placement task to quantify how different visual cues influenced motor control. In the first experiment, different subjects performed each task, and those performing the grasping task relied on binocular information more when orienting their hands than those performing the object placement task. When subjects in the second experiment performed both tasks in interleaved sessions, binocular cues were still more influential during grasping than object placement, and the different cue integration strategies observed for each task in isolation were maintained. In both experiments, the temporal analyses showed that subjects processed binocular information faster than monocular information, but task demands did not affect the time course of cue processing. How one uses visual cues for motor control depends on the task being performed, although how quickly the information is processed appears to be task invariant.

V1 response timing and surface filling-in

There is ample evidence from demonstrations such as color induction and stabilized images that information from surface boundaries plays a special role in determining the perception of surface interiors. Surface interiors appear to "fill-in." Psychophysical experiments also show that surface perception involves a slow scale-dependent process distinct from mechanisms involved in contour perception. The present experiments aimed to test the hypothesis that surface perception is associated with relatively slow scale-dependent neural filling-in. We found that responses in macaque primary visual cortex (V1) are slower to surface interiors than responses to optimal bar stimuli. Moreover, we found that the response to a surface interior is delayed relative to the response to the surface's border and the extent of the delay is proportional to the distance between a receptive field and the border. These findings are consistent with some forms of neural filling-in and suggest that V1 may provide the neural substrate for perceptual filling-in.

Rebounding V1 activity and a new visual aftereffect

A serendipitous observation led to this study of V1 activity rebounds, which occur well after stimulus offset, and their relationship to visual aftereffects. We found that when a stimulus bar and background were simultaneously turned off, there was strong delayed rebounding activity (distinct from any off response). The neural rebound started 350-500 ms after stimulus offset, and its magnitude and duration were correlated with the prior visual response of the cell. In human psychophysical experiments, we found a delayed aftereffect that may be a perceptual correlate of the activity rebound. Both the rebound activity and the perceptual aftereffect disappeared if the stimulus bar and background were not extinguished together. The magnitude of the rebound varied with the spatial scale of the background even though background size had little effect on the visual response. It thus appeared that rebound magnitude was determined by a relatively large integration area. The aftereffect was not seen when the bar and background offsets were presented to different eyes, suggesting an early neural (monocular) basis for the aftereffect. Overall, we find a strong correlation between rebound activity and the perceived aftereffect. In addition to providing a possible explanation and neural correlate of a visual aftereffect, rebounding activity may provide new insight into the dynamics of early visual processing.

Luminance-evoked inhibition in primary visual cortex: a transient veto of simultaneous and ongoing response

Large-scale changes in luminance are known to exert a significant suppressive or masking effect on visual perception, but the neural substrate for this effect remains unclear. In this report, we describe the results of experiments using in vivo intracellular recording to explore the impact of luminance transients on the responses of orientation-selective neurons in layer 2/3 of tree shrew primary visual cortex. By measuring changes in excitatory and inhibitory conductances, we find that instantaneous changes in luminance evoke strong cortical inhibition. When combined with visual stimuli that would otherwise yield strong excitatory responses, luminance transients produce significant reductions in excitation as well as increases in inhibition. As a result, luminance transients significantly delay the emergence of orientation tuned cortical responses, and virtually eliminate ongoing responses to effective stimuli. We conclude that cortical inhibition is a critical factor in luminance-evoked cortical suppression and the likely substrate for luminance-induced visual masking phenomenon.

Lightness, filling-in, and the fundamental role of context in visual perception

Visual perception is defined by the unique spatial interactions that distinguish it from the point-to-point precision of a photometer. Over several decades, Lothar Spillmann has made key observations about the nature of these interactions and the role of context in perception. Our lab has explored the perceptual properties of spatial interactions and more generally the importance of visual context for neuronal responses and perception. Our investigations into the spatiotemporal dynamics of lightness provide insight into underlying mechanisms. For example, backward masking and luminance modulation experiments suggest that the representation of a uniformly luminous object develops first at the borders and, in some manner, the center fills in. The temporal dynamics of lightness induction are also consistent with a filling-in process. There is a slow cutoff temporal frequency above which surround luminance modulation will not elicit perceptual induction of a central area. The larger the central area, the lower the cutoff frequency for induction, perhaps indicating that an edge-based process requires more time to "complete" the larger area. In recordings from primary visual cortex we find that neurons respond in a manner surprisingly consistent with lightness perception and the spatial and temporal properties of induction. For example, the activity of V1 neurons can be modulated by light outside the receptive field and as the modulation rate is increased response modulation falls off more rapidly for large uniform areas than smaller areas. The conclusion we draw from these experiments is that lightness appears to be computed slowly on the basis of edge and context information. A possible role for the spatial interactions is lightness constancy, which is thought to depend on extensive spatial integration. We find not only that V1 responses are strongly context dependent, but that this dependence makes V1 lightness constant on average. The dependence of constancy on surround interactions underscores the fundamental role that context plays in perception. In more recent studies, further support has been found for the importance of context in experiments using natural scene stimuli.

Vision: a brake on the speed of sight

We move our eyes more often than our heart beats. Our brain seems to cope effortlessly with the consequences of these rapid visual alterations, but a new study shows that similar scene changes in the absence of eye movements delay the speed of information processing. So are there costs in constantly shifting our focus of gaze?