During natural vision, the brain efficiently processes views of the external world as the eyes actively scan the environment. To better understand the neural mechanisms underlying this process, we recorded the activity of individual temporal cortical neurons while monkeys looked for and identified familiar targets embedded in natural scenes. We found a group of visual neurons that exhibited stimulus-selective neuronal bursts just before the monkey’s response. Most of these cells showed similar selectivity whether effective targets were viewed in isolation or encountered in the course of exploring complex scenes. In addition, by embedding target stimuli in natural scenes, we could examine the activity of these stimulus-selective cells during visual search and at the time targets were fixated and Convergent evidence from behavioral, neuropsychological, and neurophysiological experiments indicates that, in the primate

How does the saccadic movement system select a target when visual, auditory, and planned movement commands differ? How do retinal, head-centered, and motor error coordinates interact during the selection process? Recent data on superior colliculus (SC) reveal a spreading wave of activation across buildup cells the peak activity of which covaries with the current gaze error. In contrast, the locus of peak activity remains constant at burst cells, whereas their activity level decays with residual gaze error. A neural model answers these questions and simulates burst and buildup responses in visual, overlap, memory, and gap tasks. The model also simulates data on multimodal enhancement and suppression of activity in the deeper SC layers and suggests a functional role for NMDA receptors in this region. In particular, the model suggests how auditory and planned saccadic target positions become aligned and compete

ABSTRACT: The brain cannot monitor or react towards the entire world at a given time. Instead, using the process of attention, it selects objects in the world for further analysis. Neuronal activity in the monkey intraparietal area has the properties appropriate for a neuronal substrate of attention: instead of all objects being represented in the parietal cortex, only salient objects are. Such objects can be salient because of their physical properties (recently flashed objects or moving objects) or because they can be made important to the animal by virtue of a task. Although lateral intraparietal area (LIP) neurons respond through the delay period of a memory-guided saccade, they also respond in an enhanced manner to distractors flashed during the delay period of a memoryguided saccade being generated to a position outside the receptive field. This activity parallels the monkey’s psychophysical attentional process: attention is ordinarily pinned at the goal of a memory-guided saccade, but it shifts briefly to the locus of a task-irrelevant distractor flashed briefly during the delay period and then returns to the goal. Although neurons in LIP have been implicated as being directly involved in the generation of saccadic eye movements, their activity does not predict where, when, or if a saccade will occur. The ensemble of activity in LIP, however, does accurately describe the locus of attention.

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The influential “premotor theory of attention” proposes that developing oculomotor commands mediate covert visual spatial attention. A likely source of this attentional bias is the frontal eye field (FEF), an area of the frontal cortex involved in converting visual information into saccade commands. We investigated the link between FEF activity and covert spatial attention by recording from FEF visual and saccade-related neurons in monkeys performing covert visual search tasks without eye movements. Here we show that the source of attention signals in the FEF is enhanced activity of visually responsive neurons. At the time attention is allocated to the visual search target, nonvisually responsive saccade-related movement neurons are inhibited. Therefore, in the FEF, spatial attention signals are independent of explicit saccade command signals. We propose that spatially selective activity in FEF visually responsive neurons corresponds to the mental spotlight of attention via modulation of ongoing visual processing.

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ts; line bisection; attention; hemianopia Abbreviation: ANOVA 5 analysis of variance Introduction Hemi-neglect is a condition in which patients with cerebral lesions ignore or fail to explore all, or part, of the space contralateral to the side of their lesion. It is more frequent and severe after lesions of the right hemisphere (Albert, 1973; Chain et al., 1979; Weintraub and Mesulam, 1987). While it is classically described with parietal lesions (Brain, 1941), it can occur with damage elsewhere, including the frontal lobe (Heilman and Valenstein, 1972; Damasio et al., 1980; Liu et al., 1992; Maeshima et al., 1994), thalamus (Watson and Heilman, 1979; Watson et al., 1981) and basal ganglia (Hier et al., 1977; Damasio et al., 1980). Hemineglect arises not from defects in early visual processing (Riddoch and Humphreys, 1987), but from impaired attentional p

Abstract. Some visual search tasks require to memorize the location of stimuli that have been previously scanned. Considerations about the eye movements raise the question of how we are able to maintain a coherent memory, despite the frequent drastically changes in the perception. In this article, we present a computational model that is able to anticipate the consequences of the eye movements on the visual perception in order to update a spatial memory. 1

Expectations about the environment influence motor behavior. In simple tasks, for example, prior knowledge about which stimulus event will likely occur or which response will likely be rewarded induces a tendency to take the favored action (i.e., a motor or response bias), especially when sensory information is sparse or ambiguous. Models of choice behavior account for this bias by weighting decision alternatives unequally, either at an early sensory-input stage or at a downstream motor-output stage. These two alternatives can be distinguished empirically; the former predicts an altered percept that correlates with motor bias, the latter predicts no perceptual effect. By varying the prior probability of target or reward location, we induced biased oculomotor responses in a brightness selection task with human subjects. We found that the induced motor bias was correlated with an amplification of both the sensory signals and internal noise underlying brightness perception, without a systematic change in perceived overall brightness. We also found that the magnitude of the sensory amplification was correlated with the amount of noise in the brightness percept, consistent with a multiplicative weighting factor located downstream from the limiting internal sensory noise. Our data demonstrate that prior knowledge (about target location or reward) shapes visual signals for perception and action in parallel but does not improve the quality (i.e., signal-to-noise ratio) of sensory processing. Key words: saccade; attention; basal ganglia; superior colliculus; frontal eye field; lateral intraparietal area

Our ability to explore our surroundings requires a combination of high-resolution vision and frequent rotations of the visual axis toward objects of interest. Such gaze shifts are themselves a source of powerful retinal stimulation, and so the visual system appears to have evolved mechanisms to maintain perceptual stability during movements of the eyes in space. The mechanisms underlying this perceptual stability can be probed in the laboratory by briefly presenting a stimulus around the time of a saccadic eye movement and asking subjects to report its position. Under such conditions, there is a systematic misperception of the probes toward the saccade end point. This perisaccadic compression of visual space has been the subject of much research, but few studies have attempted to relate it to specific brain mechanisms. Here, we show that the magnitude of perceptual compression for a wide variety of probe stimuli and saccade amplitudes is quantitatively predicted by a simple heuristic model based on the geometry of retinotopic representations in the primate brain. Specifically, we propose that perisaccadic compression is determined by the distance between the probe and saccade end point on a map that has a logarithmic representation of visual space, similar to those found in numerous cortical and subcortical visual structures. Under this assumption, the psychophysical data on perisaccadic compression can be appreciated intuitively by imagining that, around the time of a saccade, the brain confounds nearby oculomotor and sensory signals while attempting to localize the position of objects in visual space.