David C. Bradley

Stimulus onset quenches neural variability: a widespread cortical phenomenon

Neural responses are typically characterized by computing the mean firing rate, but response variability can exist across trials. Many studies have examined the effect of a stimulus on the mean response, but few have examined the effect on response variability. We measured neural variability in 13 extracellularly recorded datasets and one intracellularly recorded dataset from seven areas spanning the four cortical lobes in monkeys and cats. In every case, stimulus onset caused a decline in neural variability. This occurred even when the stimulus produced little change in mean firing rate. The variability decline was observed in membrane potential recordings, in the spiking of individual neurons and in correlated spiking variability measured with implanted 96-electrode arrays. The variability decline was observed for all stimuli tested, regardless of whether the animal was awake, behaving or anaesthetized. This widespread variability decline suggests a rather general property of cortex, that its state is stabilized by an input.

Encoding of three-dimensional structure-from-motion by primate area MT neurons

We see the world as three-dimensional, but because the retinal image is flat, we must derive the third dimension, depth, from two-dimensional cues. Image movement provides one of the most potent cues for depth. For example, the shadow of a contorted wire appears flat when the wire is stationary, but rotating the wire causes motion in the shadow, which suddenly appears three-dimensional. The neural mechanism of this effect, known as ‘structure-from-motion’, has not been discovered. Here we study cortical area MT, a primate region that is involved in visual motion perception. Two rhesus monkeys were trained to fixate their gaze while viewing two-dimensional projections of transparent, revolving cylinders. These stimuli appear to be three-dimensional, but the surface order perceived (front as opposed to back) tends to reverse spontaneously. These reversals occur because the stimulus does not specify which surface is in front or at the back. Monkeys reported which surface order they perceived after viewing the stimulus. In many of the neurons tested, there was a reproducible change in activity that coincided with reversals of the perceived surface order, even though the stimulus remained identical. This suggests that area MT has a basic role in structure-from-motion perception.

Mechanisms of Heading Perception in Primate Visual Cortex

When we move forward while walking or driving, what we see appears to expand. The center or focus of this expansion tells us our direction of self-motion, or heading, as long as our eyes are still. However, if our eyes move, as when tracking a nearby object on the ground, the retinal image is disrupted and the focus is shifted away from the heading. Neurons in primate dorso-medial superior temporal area responded selectively to an expansion focus in a certain part of the visual field, and this selective region shifted during tracking eye movements in a way that compensated for the retinal focus shift. Therefore, these neurons account for the effect of eye movements on what we see as we travel forward through the world.

Velocity computation in the primate visual system

Computational neuroscience combines theory and experiment to shed light on the principles and mechanisms of neural computation. This approach has been highly fruitful in the ongoing effort to understand velocity computation by the primate visual system. This Review describes the success of spatiotemporal-energy models in representing local-velocity detection. It shows why local-velocity measurements tend to differ from the velocity of the object as a whole. Certain cells in the middle temporal area are thought to solve this problem by combining local-velocity estimates to compute the overall pattern velocity. The Review discusses different models for how this might occur and experiments that test these models. Although no model is yet firmly established, evidence suggests that computing pattern velocity from local-velocity estimates involves simple operations in the spatiotemporal frequency domain.

Perception of three-dimensional structure from motion

The ability to perceive the 3-D shape of objects solely from motion cues is referred to as structure-from-motion perception. Recent experiments indicate how this remarkable perceptual attribute is computed by the brains of primates. This computation proceeds in at least two stages, one in which motion measurements are made and another in which moving surfaces are reconstructed. The middle temporal area (MT) in the macaque monkey appears to play a pivotal role in the latter step and suggests a previously unappreciated function for this well-known cortical region, which had previously been thought to play a more rudimentary role in simply signaling the direction of motion of images.

The Contributions of Vestibular Signals to the Representations of Space in the Posterior Parietal Cortex

Abstract: Vestibular signals play an important role in spatial orientation, perception of object location, and control of self‐motion. Prior physiological research on vestibular information processing has focused on brainstem mechanisms; relatively little is known about the processing of vestibular information at the level of the cerebral cortex. Recent electrophysiological experiments examining the use of vestibular canal signals in two different perceptual tasks are described: computation of self motion and localization of visual stimuli in a world‐centered reference frame. These two perceptual functions are mediated by different parts wof the posterior parietal cortex, the former in the dorsal aspect of the medial superior temporal area (MSTd) and the latter in area 7a.

Intracortical Visual Prosthesis Research - Approach and Progress

Following the early work of Brindley in the late 1960s, the NIH began intramural and extramural funding for stimulation of the primary visual cortex using fine-wire electrodes that are inserted into area V1 for the purpose of restoring vision in individuals with blindness. More recently researchers with experience in this project became part of our multi-institutional team with the intention to identify and close technological gaps so that the intracortical approach might be tested in humans on a chronic basis. Our team has formulated an approach for testing a prototype system in a human volunteer. Here, we describe our progress and expectations

Neural mechanisms for heading and structure-from-motion perception.

Two of the most important perceptual functions of the visual motion system are to compute our direction of heading as we move through the environment, and to deduce the three-dimensional structure of objects and the environment from motion cues. Below, we review experiments that provide insights into how these perceptual phenomena are constructed by the brain. Understanding how the motion system performs these analyses will likely have general applicability to other perceptual functions, both within and outside the motion pathway. For instance, understanding how motion signals are perceived as spatially constant despite eye movements, an important prerequisite for determining heading direction, may lead to a general understanding of spatial-perceptual constancy. Likewise, understanding how three-dimensional form is processed from motion cues in the dorsal visual pathway may provide important suggestions as to how form is derived from other visual cues in the ventral visual pathway.

Neural mechanisms for self-motion perception in area MST.

Research on the neural circuitry responsible for perception of self-motion has focused on the medial superior temporal area, particularly the dorsal division (MSTd). Cells in this area are selective for the location of the focus of expansion and to pursuit eye movements, two signals necessary for recovering the direction of self-motion (Gibson, 1950). Research reviewed here shows many interesting correlates between the perception of self-motion and the activity of MST neurons. In particular, the focus tuning curves of these cells adjust to take into account motions during eye movements using extra-retinal signals, similar to the results of human perceptional experiments. Eye rotations due to head movements are also compensated for perceptually, and the focus tuning of MST neurons are also compensated for during head-generated eye rotations. Finally, the focus tuning curves compensate for both the direction and the speed of eye rotations, similar to that found in psychophysical studies. However, there are also several aspects of MSTd activity that do not completely mesh with the perception of self-motion; these differences suggest that area MSTd is not the final stage or the only locus of brain activity which accounts for this percept. Finally we offer a “gain field” model, which explains how area MSTd neurons can compensate for gaze rotations.

Multichannel cortical stimulation for restoration of vision

Development of an intracortical visual prosthesis for restoration of vision, has been, and continues to be an elusive goal of neural prosthesis researchers. Our multi-institutional team has tested the feasibility of implanting and evaluating large numbers of stimulation/recording electrodes in an animal model. Using a combination of 8-electrode arrays and individual electrodes, 152 activated iridium microelectrodes were implanted in area V1 of a macaque. Visual stimuli were used to define a retinotopic map. Spatial coordinates for each electrode were used to train the animal to use electrical stimulation in performing a visual psychophysical task.