Scientific Understanding of Consciousness
Visual Space Compressed in Prefrontal Cortex
Nature 507, 504–507 (27 March 2014)
Visual space is compressed in prefrontal cortex before eye movements
Marc Zirnsak, Nicholas A. Steinmetz, Behrad Noudoost, Kitty Z. Xu & Tirin Moore
Department of Neurobiology, Stanford University School of Medicine, Stanford, California 94305, USA
Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, California 94305, USA
We experience the visual world through a series of saccadic eye movements, each one shifting our gaze to bring objects of interest to the fovea for further processing. Although such movements lead to frequent and substantial displacements of the retinal image, these displacements go unnoticed. It is widely assumed that a primary mechanism underlying this apparent stability is an anticipatory shifting of visual receptive fields (RFs) from their presaccadic to their postsaccadic locations before movement onset. Evidence of this predictive ‘remapping’ of RFs has been particularly apparent within brain structures involved in gaze control. However, critically absent among that evidence are detailed measurements of visual RFs before movement onset. Here we show that during saccade preparation, rather than remap, RFs of neurons in a prefrontal gaze control area massively converge towards the saccadic target. We mapped the visual RFs of prefrontal neurons during stable fixation and immediately before the onset of eye movements, using multi-electrode recordings in monkeys. Following movements from an initial fixation point to a target, RFs remained stationary in retinocentric space. However, in the period immediately before movement onset, RFs shifted by as much as 18 degrees of visual angle, and converged towards the target location. This convergence resulted in a threefold increase in the proportion of RFs responding to stimuli near the target region. In addition, like in human observers, the population of prefrontal neurons grossly mislocalized presaccadic stimuli as being closer to the target. Our results show that RF shifts do not predict the retinal displacements due to saccades, but instead reflect the overriding perception of target space during eye movements.
We recorded from neurons within the frontal eye field (FEF) of monkeys (Macaca mulatta) using linear electrode arrays. The FEF is an area of prefrontal cortex with a known involvement in gaze control7 and visual attention. Previous studies have found evidence that visual RFs of FEF neurons predictively remap before saccades. That is, this body of evidence suggests that FEF RFs shift from their presaccadic locations to their anticipated, postsaccadic locations before the onset of each saccade.
Consistent with this hypothesis is psychophysical evidence of enhanced perception at the saccade target before movement onset, as well as enhanced visual cortical signals. Furthermore, the perception of visual space is massively compressed before saccades. Our results reveal a neuronal correlate of these perceptual effects. In particular, we found that populations of FEF neurons grossly mislocalize stimuli as being closer to the target, resembling psychophysical compression. Thus, regardless of the role of the above perceptual phenomena in visual stability, the representation within the FEF mirrors them.
FEF neurons have been causally implicated in the control of visual attention, and the corresponding modulation of stimulus-driven activity in posterior visual cortex. Several recent studies suggest that the influence exerted by FEF neurons on visual cortex during attention originates from predominantly visual signals. Our results indicate that FEF visual signals conveyed to posterior areas before saccades grossly over-represent the space occupied by the target. Thus, before each eye movement, or during covert attention, feedback from FEF neurons may impose the same distortion onto visual cortex, and this biased representation of target space could result in the aforementioned attentional enhancement within that space.
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Nature 507, 434–435 (27 March 2014)
Neuroscience: Updating views of visual updating
John A. Assad
Center for Neuroscience and Cognitive Systems, Istituto Italiano di Tecnologia, Rovereto 38068, Italy, and the Department of Neurobiology, Harvard Medical School, Boston, Massachusetts.
Our brains create a stable view of the world even though our eyes dart around. Although scenes before our eyes seem vivid and detailed, the part of the eye dedicated to high-acuity vision, the fovea, can cover only a narrow sliver of visual space — little more than the breadth of a thumbnail held at arm's length. Vision seems so detailed because we constantly move our eyes to scan the high-acuity fovea across a scene. Saccades — quick, jerky movements of the eyes — occur several times per second, filling in the perceptual fogginess of the visual periphery.
The brain generates eye movements, and thus could perceptually compensate for saccades.
Parietal neurons, like other visual neurons, have a receptive field (RF), a circumscribed part of the visual field for which visual stimuli activate electrical responses. These RFs are defined relative to the fixation position (the point in visual space where the fovea is focused when eyes are stationary), and are usually considered spatially fixed relative to that position, reflecting the convergent hard-wired inputs to the neurons that ultimately stem from the retina. Thus, if the fixation position is displaced by a saccade, the RF should 'move' as well, in lock-step with the fixation position.
The researchers thus suggested that the RF was “updated” or “remapped” to the new position, perhaps to compensate perceptually for the upcoming eye movement. Similar spatial updating was subsequently found in other brain structures that have mixed visual and oculomotor (eye-movement) function, such as the frontal eye fields (FEFs) and the superior colliculus, suggesting a common compensatory mechanism.
When the full RFs were revealed, the authors found that these fields did not rigidly translate before the eye movement, but instead transiently shifted en masse towards the location of the upcoming saccade target. Some of the shifts of individual RFs were substantial, up to 18 degrees of visual angle.
The discovery that RFs collapse onto the saccade target is drastically different from the original spatial-updating hypothesis. In fact, the RFs of neurons in the FEF (and related visual–oculomotor structures) are quite large, so even if the RFs collapsed to the saccade target, the fringes of many of those RFs would have probably overlapped with the upcoming RF location.
RF shifts to a saccade target resemble the RF shifts that occur towards targets on which our attention is focused, even if the eyes do not move. The two phenomena are probably related, inasmuch as shifts of attention to targets of interest precede eye movements. In this perspective, visual stability during saccades could result from the fact that we effectively ignore those parts of the visual scene that are away from the saccade target.
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